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Journal of Membrane Science 336 (2009) 86ndash100
Contents lists available at ScienceDirect
Journal of Membrane Science
journa l homepage wwwe lsev ier com locate memsci
Triazine retention by nanofiltration in the presence of organic matterThe role of humic substance characteristics
KV Plakas AJ Karabelas lowast
Department of Chemical Engineering Aristotle University of Thessaloniki and Chemical Process Engineering Research Institute
Centre for Research and Technology - Hellas PO Box 60361 6th km Charilaou-Thermi Road Thermi Thessaloniki GR 570-01 Greece
a r t i c l e i n f o
Article history
Received 25 November 2008
Received in revised form 24 February 2009
Accepted 15 March 2009
Available online 27 March 2009
Keywords
Atrazine
Prometryn
Humic substances
Nanofiltration
Herbicidendashorganic matter interactions
a b s t r a c t
The influence of different types of humic substances (HS) is studied in the presence or absence of calcium
ions on the retention of two triazines atrazine and prometryn To correlate the retention of triazines
with the characteristic chemical parameters of the humic substances three standard humic materials
from the International Humic Substances Society (IHSS) as well as a NOM surrogate tannic acid are
employed Differences are observed in the triazine retention behaviour between organic-free triazine
solutions and those of mixed HSndashtriazines The results suggest that physicochemical interactions alone
between triazines and humic substances cannot fully account for the observed triazine retention by
nanofiltration The HS acidity (total carboxylic and phenolic) exhibits a weak correlation with triazine
rejection However the conformation of humic species appears to play a more significant role The pres-
ence of calcium ions improves both HSndashtriazine association and membrane rejection of the HSndashtriazine
complexes In support of the membrane filtration results gel permeation chromatography shows that
the hydrophobic interactions can explain the different triazine absorbing capacities of the HS used In
general the results illustrate the complexity of the adsorption and retention mechanisms of triazines
in the presence of humic substances and confirm the important role of membrane fouling on process
efficiency
copy 2009 Elsevier BV All rights reserved
1 Introduction
There is growing concern in recent years regarding the risks to
human health posed by toxic pesticides of various types Numerous
studies have shown the presence of herbicides in drinking water
raising questions about health effects to humans and especially to
children Among herbicides triazine compounds are considered
to be important environmental contaminants furthermore the
potential carcinogenic effects of s-triazines are of growing concern
in water quality management [1] Atrazine and prometryn two of
the most widely applied herbicides have been detected in Euro-
pean drinking water sources with great frequency [23] which is
attributed to their persistence in water and their mobility in soil
Because of their significant solubility in water these compounds
can be leached into the ground water and be transported in surface
runoff [4] Consequently there is a need to develop advanced treat-
ment methods in order to ensure the complete removal of these
and other pesticides from water and therefore to protect human
health
lowast Corresponding author Tel +30 2310 498181 fax +30 2310 498189
E-mail address karabajcpericerthgr (AJ Karabelas)
Membrane processes such as reverse osmosis (RO) and nanofil-
tration (NF) are considered promising candidates for the removal
of low molecular weight organic compounds of environmental
concern like pesticides An increasing number of research papers
have appeared in recent years focused on membrane rejection
mechanisms and factors affecting rejection Comprehensive liter-
ature reviews on pesticide removal by membranes are reported
in recent papers [56] Based on these findings there are a num-
ber of key solute parameters (molecular weight molecular size
log Kow polarity) and membrane properties (molecular weight cut-
off surface charge hydrophobicityhydrophilicity and roughness)
that primarily affect pesticide retention Moreover feed water com-
position especially the presence of inorganic and organic matter
is identified as a significant factor influencing pesticide reten-
tion Indeed a number of studies performed with either NFRO
membranes [7ndash11] or dialysis membranes [1213] have shown
that the retention of pesticides is significantly influenced by the
presence of natural organic matter (NOM) in water This fact
is of considerable importance since a large percentage of pesti-
cide residues is present in surface and ground waters together
with organic matter (humic and fulvic acids polysaccharides etc)
[14]
In general humic substances (HS) are a ubiquitous component
of natural water systems that may function as an auxiliary phase
0376-7388$ ndash see front matter copy 2009 Elsevier BV All rights reserved
doi101016jmemsci200903020
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 87
to alter the speciation and transport behaviour of other xenobiotic
compounds present in water [15] Thus organic micropollutants
like pesticides may exist either as free dissolved species or as a
complex with HS [16] In a study performed by Barriuso et al [17]
15ndash40 of the initial atrazine concentration in soil columns was
still present after 6 months and the residues were mainly asso-
ciated with humic compounds In an earlier study [18] bound
atrazine residues were in the range 10ndash40 of the applied atrazine
depending on the presence or absence of a micro-flora capable of
mineralizing the triazinic ring between 13 and 30 of such bound
residues was associated with humic acids It is evident therefore
that the interactions between pesticides and natural organic mat-
ter may significantly affect their retention during membrane water
treatment
A literature review on the effect of NOM on pesticide reten-
tion by membranes suggests that there is a dependence on the
type of NOM present in the water NOM is composed of an
extremely diverse group of compounds including carbohydrates
alcohols amino acids carboxylic acids lignins and pigments
whose origin greatly influences its character and behaviour Though
various fractions of NOM can be defined by special procedures for
separating organic matter from water a general structural char-
acterization is practically impossible to obtain since the exact
NOM composition varies greatly as a function of NOM source
aqueous solution conditions season etc [12] It is therefore rea-
sonable to assume that NOMndashpesticide interactions may vary as
a function of properties of different NOM fractions and origins
For example the study by Sposito et al [19] revealed that the
weak interactions between s-triazines and humic acid functional
groups include proton transfer hydrogen bonding and electron-
transfer The actual interactions depend on the basicity of s-triazine
molecule functional groups of humic substances and the pH of the
medium They suggested that proton transfer more likely occurs
where humic substances have high acidic functional group content
and the s-triazines have low basicity however electron-transfer
mechanisms are more likely in cases humic substances have low
acidity functional groups and the s-triazines have a high basicity
[19]
The majority of the published works agree on the fact that
the retention of pesticides in membrane-based systems tends to
increase in the presence of NOM [79ndash13] which is generally
attributed to a variety of factors eg the size shape and surface
chemistry of compounds involved On the other hand the use by
various researchers of NOM of different origins and the inadequate
information on their physicochemical properties (elemental anal-
ysis functional groups) do not allow comparison of experimental
results as well as the correlation of the pesticidendashNOM retention
by membranes with the characteristic properties of the naturally
occurring organics in water Most of the literature is concerned with
adsorption and rejection properties of herbicides but usually fails
to appraise the variability introduced by the different properties
of humic substances Therefore it is of particular interest to study
the relation between the characteristic physicochemical proper-
ties of the humic substances that are present in drinking water
sources with the herbicide retention performance of NFULPRO
membranes To this end the use of well-defined NOM will aid the
better understanding of mechanisms of NOMndashpesticide retention
by membranes
In the present work pressure-driven dead-end filtration is
employed to study membrane retention of triazines (atrazine and
prometryn) in the presence of different types of natural organic
matter the latter are well-characterized HS purchased from the
International Humic Substances Society (IHSS) Additionally com-
mercially available tannic acid (TA) is used as a HS surrogate
in order to compare the results of the present work with those
obtained by previous researchers
2 Experimental work
21 Materials and methods
211 Membranes
Three kinds of flat sheet type commercial membranes (Dow
Filmtec) denoted as NF90 NF270 and XLE were used in this
study NF90 and NF270 are nanofiltration membranes while XLE
is described by the manufacturer as extra low energy membrane
(ultra-low pressure RO-ULPRO) The membrane coupons used for
the experiments were stored at 4 C in a 075 Na2S2O5 aqueous
solution (to suppress development of micro-organisms) which was
regularly replaced The characteristic properties of the membranes
used were described elsewhere [20] The differences observed
regarding their hydrophobicity and surface charge properties as
well as their specific flux and salt rejection may be attributed
to different reactants employed in their production and to the
membrane surface treatment More specifically it is reported that
NF270 membrane is composed of a semi-aromatic piperazine-
based polyamide layer [21] whereas NF90 and XLE membranes
are prepared by interfacial polymerization of m-phenylenediamine
[22] Each membrane is reinforced with a polyester non-woven
backing layer covered with a polysulfone porous layer The NF270
membrane is quite hydrophilic exhibiting the highest permeability
among the three membranes tested as well as fairly high retention
of charged ions On the other hand the NF90 and XLE membranes
exhibit similar permeability values and surface hydrophobicity
Moreover all three membranes are negatively charged at neutral
pH with NF270 and NF90 membranes characterized by a much
higher negative charge than XLE [20] In this study in addition to
other measurements made scanning electron microscopy (SEM)
images were obtained (using a JEOL JSM-6300 Scanning Micro-
scope) to examine the surface condition of the virgin and the treated
with HS membranes
212 Herbicides
Two herbicides belonging in the triazine family ie atrazine
and prometryn were selected for this study Both triazines are
traditional selective pre- and post-emergence herbicides that are
extensively used mainly for the protection of various crops orna-
mental plants and forest trees Herbicide analytical standards were
purchased from Riedel de-Haeumln (SigmandashAldrich) atrazine (purity
974) prometryn (purity 992) The molecular structure and
physicochemical properties of the tested herbicides are presented
in Table 1 Both triazines are considered as non-ionic hydropho-
bic (log Kow gt 2) compounds that are moderately soluble in water
Between the two prometryn is the largest molecule due to its
branched structure whereas its greater pKa value indicates a
greater basicity in comparison to atrazine [20] As far as polarity
is concerned it is uncertain whether the symmetry characteris-
ing the prometryn molecule (in enhancing its polar character) is
more important than the highly polar heteroatom (ndashCl) included
in atrazine [23] Nevertheless these two triazines are generally
considered in the literature as weak polar compounds
213 Humic substances
The use of well-characterized standard humic substances makes
it possible for researchers to check the results and compare with
similar experimental data For this purpose four different types
of well-characterized HS were used in this investigation Three of
them are typical water born HS purchased from the International
Humic Substances Society (IHSS University of Minnesota SWC
Dept) whereas the fourth TA is a HS surrogate purchased from
SigmandashAldrich Tannic acid is a lower molecular mass compound
considered to be a representative surrogate for NOM frequently
used as such in membrane-related research
88 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
Table 1
Properties of the herbicides used in this work [24]
Herbicide Atrazine Prometryn
Chemical structure
Molecular formula C8H14ClN5 C10H19N5S
Chemical class Cl-triazine S-triazine
Molecular weight (Da) 21569 24135
Molecular sizea (nm) 0788 ndash
Log Kow 268 308
Aqueous solubility (mg Lminus1) 33 33
Dipole momentb (debye) 2460 ndash
pKa (20 C) 17 409
a Obtained from Ref [25]b Obtained from Ref [26]
Table 2
Acidity and elemental composition of HS used in this study [27]
Humic substance Cat No Acidity (mequiv gminus1 C) Elemental composition
Carboxylic Phenolic Total Ca H2Ob
HA IHSS 2S101H 913 372 1285 5263 204
FA IHSS 2S101F 1187 284 1401 5234 169
NOM IHSS 1R101N 985 394 1379 4880 815
TAc SigmandashAldrich 188 955 114 5000 58
a Elemental composition in (ww) of a dry ash-free sampleb (ww) of H2O in the air-equilibrated sample (a function of relative humidity)c Values reported in Ref [28]
The three IHSS standards are denoted as Suwannee River Humic
Acid (HA) Suwannee River Fulvic Acid (FA) and Suwannee River
NOM The IHSS humic and fulvic acids contain only hydrophobic
organic acids while the NOM sample contains not only hydropho-
bic and hydrophilic acids but also other soluble substances that
are present in natural waters [27] On the other hand tannic
acid is a polyphenol representative of relatively hydrophilic com-
pounds of medium molar mass It is reported by the supplier to
have a mean molecular mass of 170118 g molminus1 with an empiri-
cal formula C76H52O46 but in fact it contains a mixture of related
compounds The characteristic chemical parameters of the four sub-
stances employed are summarized in Table 2 the average molecular
masses of the IHSS standards reported in the literature are pre-
sented in Table 3
214 Feed solutions
In the filtration experiments three different types of solu-
tions were used ie solutions of atrazinendashprometryn in ultra-
pure water atrazinendashprometryn in a model HS solution and
atrazinendashprometryn in a model HS solution in the presence of
calcium ions The model solutions of the four selected humic
substances were prepared at neutral pH in order to simulate
waters with different chemical characteristics Since humic sub-
stances concentrations in natural waters usually fall in the range
2ndash40 mg Lminus1 [32] all the solutions were prepared with ultra-pure
water and with a concentration of 5ndash10 mg Lminus1 humic substance
The HS were obtained in powder form and used without further
purification as the bound iron and ash contents were very low The
calcium ion content in the corresponding experiments was fixed
at 40 mg Lminus1 by adding calcium chloride (111 mg Lminus1 JT Baker)
In the case of the two triazines separate standard stock solutions
were prepared in high-performance liquid chromatography-grade
methanol and stored at 4 C with concentration 100 mg Lminus1 The
feed triazine solutions were prepared from ultra-pure water by
dilution of stock solutions at a level of 10ndash30 g Lminus1 for each
triazine The ultra-pure water used (resistivity gt 18 M cm) for
Table 3
Molecular mass data of IHSS organics employed in this study
Humic substance Technique [Ref] Mna Mw
b MwMnc
Suwannee River HA FFF [29] 1580 4390 278
Ultracentrifugation [30] ndash 4260 plusmn 280 ndash
Suwannee River FA FFF [29] 1150 1910 166
Ultracentrifugation [30] ndash 1460 plusmn 80 ndash
Suwannee River NOM HPSEC [31] 1760 2360 134
a Number average molecular weightb Weight average molecular weightc Polydispersivity index Suwannee River HA has a much larger polydispersivity compared to FA
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 89
solution preparation was obtained from a Milli-Q purification
system (Millipore Milford MA USA) Similar to the protocol
used by Devitt et al [9] once a model solution of HS and
atrazinendashprometryn was prepared in the presence or not of cal-
cium it was placed in a foil-covered container (to prevent atrazine
and prometryn degradation by exposure to light) and stirred for
more than 24 h after which it was assumed to be in equilibrium
215 Analytical techniques
2151 Herbicide analysis Off-line solid phase extraction (SPE)
with gas chromatography employing a mass-selective detector
(GC-MSD) was used to achieve the sensitivity required for the
tested herbicides The SPE procedure performed prior to the
chromatographic analysis is described in a previous paper [33]
Herbicide analyses were performed in an Agilent Technologies
Model 7890A gas chromatograph system interfaced with an Agilent
Technologies Model 5975C mass-selective detector (MSD) The gas
chromatograph was equipped with a 30 m times 250 m id times 025 m
film thickness fused silica capillary column (Agilent 19091S-
433) The chromatographic conditions were as follows injection
splitless injector temperature 180 C transfer line temperature
220 C injection volume 1 L carrier gas He flow 1 mL minminus1
oven temperature programmed from an initial temperature of 70 C
held for 3 min to 150 C at a rate of 30 C minminus1 held for 1 min next
to 200 C at a rate of 10 C minminus1 and finally to 260 C at a rate of
5 C minminus1 held for 1 min The MSD was operated in the selective ion
monitoring (SIM) mode Three characteristic ions were monitored
for each herbicide analyzed The most abundant ion for each herbi-
cide was used for quantification while the other two ions were used
to confirm the presence of the herbicide The minimum detectable
quantity (MDQ) for the analysis with a 1-L injection volume was
approximately 1 ng for all herbicides analyzed
Simazine (Riedel de-Haeumln purity 999) a triazine compound
similar to atrazine and prometryn was used as internal standard in
order to assess the recovery of the two triazines in the SPE-GC-MSD
analyses With the selected SUPELCO SPE cartridges recoveries
from the model humic substance solutions ranged from 35 to 98
and in general were lower than the recoveries obtained in pure
water or permeate samples of the triazinendashHS filtration (75ndash128)
The analysis of the concentrate samples which were characterized
by high dissolved carbon content resulted in many cases in even
lower recoveries This is attributed to the saturation of the sorption
sites of the SPE cartridges by the humic substances which renders
them inefficient in extracting the two triazines associated with HS
Therefore triazine concentrations were determined only in the feed
and permeate samples
2152 Organic and inorganic matter analysis Measurement of the
HS concentration (and more specifically the aromaticity) was per-
formed in a UVndashvis spectrophotometer (UV-1700 Shimadzu Japan)
at 254 nm in the case of Suwannee River HA and Suwannee River
NOM and at 275 nm in the case of Suwannee River FA A standard
colorimetric method (5550) [34] was adopted for the preparation
of tannic acid samples prior to their spectrophotometric measure-
ment at 700 nm The quality control of the data was achieved by
means of calibration and repeated analyses Specifically two cal-
ibration curves were created for each HS one corresponding to
the permeate samples (standards of similar concentrations to the
one expected in the permeates) and a second one corresponding
to the higher concentration level of the feed and retentate sam-
ples This procedure resulted in high R2 values (gt0999) reflecting
the high accuracy and sensitivity of the UVvis method for all sam-
ples treated The use of two different Quartz cuvettes (1 and 10 cm)
enhanced this accuracy (measurement of absorption in a range
01ndash15 unitsmdashas proposed in the literature) Further the accuracy
of the analytical results was ensured by repeatedly measuring the
concentration of a sample (more than four times) All measure-
ments in the case of higher concentrations (feed and concentrate)
resulted in RSD values smaller than 1 while in the case of the
lower concentrations the RSD values did not exceeded 2
The determination of calcium ion content in the water samples
was performed by an ion selective electrode connected to a 692
pHIon Meter Metrohm (Herisau Switzerland) The conductivity
and pH of the samples were measured by a conductometer 712
and pH meter 744 both by Metrohm
2153 Gel permeation chromatography (GPC) Gel permeation
chromatography (or size exclusion chromatography SEC) is a rela-
tively rapid and convenient method for estimating the partitioning
or binding of organic compounds to humic substances in solution
[35] In particular GPC can give information on the liability of the
interaction itself The solutions tested in the present work were (a)
HS (b) HSndashtriazine (atrazine prometryn or both) (c) HSndashcalcium
and (d) HSndashcalciumndashatrazinendashprometryn All solutions were stirred
for more than 24 h in foil-covered containers prior to their injection
into the GPC column The concentrations of all the materials used
were the same as those applied in the filtration experiments Tannic
acid was excluded from the GPC tests due to the possible interac-
tions that may occur between the chemicals of the colorimetric
method used (folin phenol Na2CO3ndashNa2C4H4O6middot2H2O solution)
and the GPC column which in turn may significantly alter the tannic
acid elution behaviour
The accuracy of the elution profiles was ensured by apply-
ing identical chemical conditions for the GPC investigation
This was achieved by using feed waters of the same source
thus all tested solutions (HS alone HSndashtriazines HAndashcalcium
HAndashcalciumndashtriazines) were prepared from the same HS stock solu-
tion This led to a negligible experimental uncertainty which was
evident in the repeatable GPC measurements (three to four injec-
tions) for each solution Indeed the spectrum of each HS was
very uniform whereas the shapes of the GPCSEC elution curves
recorded at the same wavelength were essentially identical
The GPC tests were carried out in a Shimadzu-10 AVP system The
GPC column was a PL aquagel-OH Mixed 8 m 300 mm times 75 mm
(Polymer Laboratories Essex UK) guarded by a PL aquagel-OH
Guard 8 m 50 mm times 75 mm column PL aquagel-OH columns
are packed with macroporous copolymer beads with extremely
hydrophilic polyhydroxyl functionality offering high resolution
over a wide range of molecular weights (up to 104 kDa) Aqueous
samples (100 L) were pipetted directly onto the gel and passed
through the column Phosphate buffer (01 M KH2PO4 at pH 70
corrected with 2 M NaOH) was used as the eluent with a flow
rate of 1 mL minminus1 Dissolved humic substance (HA FA and NOM)
was measured with a Shimadzu-10 AVP UV detector by monitor-
ing ultraviolet light absorbance at a wavelength of 254 275 and
254 nm respectively Calibration of the column to determine the
exact molecular sizes of the HS was not performed in this work
22 Nanofiltration tests and filtration protocol
Dead-end filtration experiments were performed by using an
experimental set-up consisting of three high pressure stirred cells
described elsewhere [20] The three test cells were operated con-
currently to assess the reproducibility and accuracy of the results
A new membrane coupon was used in each filtration test in order
to avoid employing membranes with adsorbed amounts of humic
materials and herbicides
The filtration protocol involves a sequence of the following six
steps (1) membrane preconditioning with Milli-Q water under
pressure of 5 bar for several hours to ensure that the removal of
preservatives from the new membrane coupon is complete [20]
(2) membrane compaction for 1 h at 10 bar (3) measurement of the
90 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
pure water flux for 30 min at 5 bar (4) filtration of triazines in the
absence of humic materials (ldquofree triazine solutionsrdquo) at 5 bar (5)
filtration of the model HSndashtriazines solution at 5 bar and (6) mea-
surement of the pure water flux for 30 min at 5 bar Compaction
(step 2) is crucial in every membrane filtration protocol as it may
change both the active layer and its support thus affecting the flux
and the rejection properties of the membrane To eliminate this
impact membranes are often subjected to at a higher pressure (here
10 bar) than the operating pressure (5 bar) to ensure flux stability
during experiments [36] Preliminary experiments with the three
membranes tested showed that 1 h of 10 bar pressure application
was more than adequate to achieve membrane stability
All filtration steps were performed with a feed solution of
300 mL while the tests performed with the herbicides were car-
ried out until 150 mL of permeate were collected (50 recovery)
Feed and three permeate samples (50 mL each) were retained for
subsequent analysis in order to determine the evolution of con-
taminant concentration in some cases retentate samples were also
removed for chromatographic analysis Similar to previous studies
[2033] the triazine feed solutions were stirred in the cells for 1 h
without pressure In this way herbicide adsorption on the mem-
brane surface was assumed to reach equilibrium After termination
of the filtration experiments the membrane coupons were dried
for subsequent analysis (SEM) while the test cells were thoroughly
washed with acetone and repeatedly rinsed with Milli-Q water
23 Data treatment
The performance of nanofiltration can be expressed in terms of
the percentage removal of the organic and inorganic substances
The determination of the retention was calculated based on the
method described in our previous study [20] which proved to be
appropriate in the case of batch filtration experiments since the
membrane performance depends on the retentate concentration
which increases with time The concentration of the two triazines
was measured for the feed solution and for all the permeate samples
while retentate samples were removed for subsequent analysis of
the humic and inorganic compounds The retention was evaluated
based on the so-called ldquostablerdquo permeate samples where the water
recovery rate was between 17 and 50 The amount of deposited
humic substance on the membrane surface was determined from
mass balance as follows
A () = 100
(
1 minus
sumj
i=1Vpi
Cpi+ Crj
Vr
Cf Vf
)
(1)
where Cpj Cr Cf are the concentrations of a humic substance in
the permeate sample j retentate and feed respectively while Vpj
Vr Vf are the respective volumes of permeate sample j retentate
and feed Reduction of the pure water flux (FRPW) which may be
considered a measure of the adsorbed humic substances effect on
flux was determined as a percentage of clean water flux Jw before
and after membrane operation as previously described [20]
3 Results and discussion
31 Pure triazine solutions
Prior to the combined filtration of triazines with humic materi-
als pure water solutions with atrazine and prometryn were filtered
at 5 bar in concentrations ranging between 10 and 30 g Lminus1 The
performance of the membranes in terms of triazine retention is
illustrated in Fig 1 The accuracy of experimental results is indi-
cated by the bars which represent the scatter of data obtained from
six replicate experiments for each membrane
Fig 1 Atrazine and prometryn retention from free triazine solutions by the three
selected NFULPRO membranes mean retention values for 50 recovery of feed
volume feed concentration 10ndash30 g Lminus1
As in our previous studies [2033] prometryn the molecule
with the larger molecular weight and size displays the great-
est retention with all three membranes used The performance
of membranes regarding retention of the two triazines follows
the order XLE ge NF90 gt NF270 which is consistent with the results
obtained in our previous work [20] Comparing the experimen-
tal results for double solute mixtures of atrazine and prometryn
in the case of high (600ndash700 g Lminus1) [20] and low feed concen-
trations (10ndash30 g Lminus1) the differences in retention values vary
between 7 and 13 In general the retention results with pure tri-
azine solutions are in agreement with observations made by other
researchers [71125] in that herbicide concentration does not sig-
nificantly affect their retention The fact that the filtration of lower
feed concentrations leads to a slight reduction of triazine retention
(especially in the case of XLE membrane) could be attributed to the
amount of triazines adsorbed on the selected membranes Based
on the mass balances summarized in Table 4 the lower feed tri-
azine concentration is accompanied in general by a slightly lower
adsorption in comparison to the results obtained with higher feed
concentrations [20] something that is more pronounced in the case
of the less tight NF270 membrane
32 Aqueous solutions of mixed humic substances and triazines
321 GPC results on triazinesndashhumic substances interactions
The UVndashvis spectra of fractions collected during GPC chromatog-
raphy of HS alone and HS with atrazine andor prometryn in the
absence or presence of calcium ions are depicted in Figs 2ndash4
The comparison of the chromatographic behaviour of the solu-
tions used in qualitative terms allows an assessment of possible
interactions between humic substances and the two triazines [37]
The elution profiles display a maximum for all three IHSS organ-
ics under the same test conditions at retention time 185ndash195 min
In the case of the low molecular weight triazines the elution pro-
file maximum at a wavelength of 225 nm and concentration of
10 mg Lminus1 (high concentrations used in order to be measurable
by UV absorption) appear at 110 min (data not presented here)
The longer retention time of atrazine and prometryn is due to the
fact that small molecules can penetrate into the pores of the gel
and follow a torturous path through the column On the other
hand large molecules such as humic substances are excluded
from internal pore spaces in the gel and elute faster from the col-
umn Co-elution of small triazine molecules with the much larger
humic molecules indicates their association with the humic frac-
tion Humic-bound triazines appear to display the behaviour of the
larger humic molecules as they are also excluded from the internal
pores of the gel [37] This is reflected in the reduced GPC areas of
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 91
Table 4
Adsorption data of triazines on tested membranes
Membrane Feed mass of
triazinesa (g)
Quantity adsorbed
(g)
adsorption adsorption
[19]b
NF270 45 plusmn 27(A) 048 plusmn 027 127 plusmn 18 165
53 plusmn 28(P) 020 plusmn 008 38 plusmn 13 275
NF90 52 plusmn 28(A) 122 plusmn 083 248 plusmn 66 305
55 plusmn 13(P) 105 plusmn 033 250 plusmn 67 262
XLE 66 plusmn 10(A) 114 plusmn 012 219 plusmn 12 230
74 plusmn 11(P) 084 plusmn 008 171 plusmn 09 295
a (A) and (P) designate atrazine and prometryn respectivelyb Adsorption values for higher triazine (atrazine and prometryn) concentrations (600ndash700 g Lminus1 each)
Fig 2 GPC elution profiles of Suwannee River humic acid (HA) in the absence or presence of triazines andor calcium (detection by UV absorption at 254 nm)
the humic materials when treated together with the two triazines
(Table 5) In summary aquatic humic substances from the same
water source (Suwannee River) show differences in their GPC elu-
tion behaviour when treated together with calcium andor with
either of the two triazines
The peaks of the GPC chromatograms reflect differences in spe-
cific absorption Distinct differences between HS and atrazine or
prometryn are found in the case of FA and NOM In contrast HA
show a rather small variation on elution behaviour This is also in
accord with the values presented in Table 5 Complexation with
the NOM and FA appears to occur for both atrazine and prometryn
molecules the interactions with NOM indicate greater affinity to
atrazine On the other hand FA interactions appear to be stronger
with prometryn In Figs 2ndash4 and Table 5 one can observe that
among the three HS used FA seems to interact more with the two
triazines The weakest interaction is observed in the case of humic
acids which is reflected in the almost identical elution profiles and
the low GPC reduction areas
The presence of calcium in the solutions of humic substances
leads to a sharp reduction of HS concentrations (Table 5) indi-
cating the formation of calcium humate and fulvate complexes
According to the literature when negatively charged humic colloids
Table 5
Percentage () reduction of GPC integration areas of humic substances due to the presence of triazines andor calcium
Humic substances combined with Percentage GPC area reduction ()
HA FA NOM
40 mg Lminus1 Ca2+ 26 17 138
10 g Lminus1 atrazine 06 64 27
10 g Lminus1 prometryn 02 81 lt01
10 g Lminus1 atrazine + 10 g Lminus1 prometryn 23 80 30
10 g Lminus1 atrazine + 10 g Lminus1 prometryn + 40 mg Lminus1 Ca2+ 151 221 279
92 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
Fig 3 GPC elution profiles of Suwannee River fulvic acid (FA) in the absence or presence of triazines andor calcium (detection by UV absorption at 275 nm)
come in contact with positively charged metal ions (like calcium)
several binding mechanisms become effective The bond strengths
vary from a weak physical adsorption to strong chemical bond-
ing involving chelation reactions [38] In the case of herbicides
the partially chelated calcium may serve here as a bridging ion
and act as a possible site for the adsorption of a triazine molecule
by displacing water of hydration Indeed the addition of calcium
enhances the interaction between the humic substances and the
two triazines something that is manifested by the higher GPC area
reductions and the lower elution profiles Browman and Chester
[39] also concluded that cations on the surface of humic material
can act as binding bridges between synthetic chemicals and humic
compounds Similarly Helal et al [38] reported that the exchange
reaction between the pesticides and the humic compounds takes
place on the same sites at which binding of humic compound to the
metal occurs Moreover these authors claim that the pesticidendashHA
Fig 4 GPC elution profiles of Suwannee River NOM in the absence or presence of triazines andor calcium (detection by UV absorption at 254 nm)
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 93
Fig 5 Retention of atrazine (A) and prometryn (P) by the three NFULPRO membranes in the absence or presence of humic substances (HA FA NOM TA) andor calcium
ions
or ndashFA complex is stronger than the metalndashhumate or ndashfulvate
complex The mechanisms involved in the interactions between
triazines and calciumndashHS complexes may be described by either
a physical adsorption of the herbicide on the surface of metal
humatefulvate complex or a ligand exchange in which the metal
serves as a bridging ion and acts as a possible site for adsorption of
the herbicide molecule by displacing water of hydration [38]
322 Effect of different types of humic materials on triazine
retention
Retention results of atrazine and prometryn during the com-
bined nanofiltration with the four selected humic substances are
depicted in Fig 5 Results of triazine retention from organic solu-
tions with elevated calcium ion content are also included in Fig 5
and discussed in Section 324 Depending on the type of humic
material present increased retention of the two triazines may be
negligible or significant Specifically the use of HA results in reten-
tion values that are comparable to those obtained in the absence
of organics An exception is observed in the case of XLE membrane
where the combined filtration of triazines and HA leads to relatively
smaller retention values (7 and 13 for atrazine and prometryn
respectively) On the other hand the presence of tannic acid results
in a significant increase of triazine retention for all three mem-
branes tested In particular the addition of TA at a concentration of
10 mg Lminus1 (yielding a triazineTA ratio of 1 g1 mg) results in 14ndash21
and 20ndash29 increase of prometryn and atrazine retention respec-
tively The enhancement of pesticide removal in the presence of
the NOM surrogate tannic acid has been also observed by previ-
ous researchers [91213] According to Devitt and Wiesner [12] the
increased retention of a triazine molecule like atrazine in the pres-
ence of TA is due to the branched structure of TA which traps the
triazine within the macromolecule Moreover these authors sug-
gest that molecular conformations and weak interactions between
functional groups may combine to produce physical entrapment of
triazines through a low multiplicative probability of escape result-
ing from a large number of encounters with low-energy sites on the
tannic acid molecule (eg hydrogen bonding)
In the case of the combined nanofiltration of atrazine and prom-
etryn with FA or NOM a modest increase of retention in comparison
to the pure triazine solutions was recorded In particular depending
on the membrane used the presence of FA resulted in an increase
of 8ndash15 of atrazine retention and 4ndash14 of prometryn retention
whereas nanofiltration in the presence of NOM led to 12ndash17 and
3ndash15 increase of atrazine and prometryn retention respectively
The interactions between atrazine and prometryn with the
humic substances observed in the GPC experiments seem to agree
(up to a point) with the trends in triazine retention by the three
NFULPRO membranes Indeed the negligible interactions observed
between the two herbicides and the humic acids are in accord with
the slight differences observed in atrazine and prometryn retention
by the NF270 and NF90 membranes On the other hand the some-
what reduced retention observed in the case of XLE membrane
may be not only due to the weak interactions between triazines
and humic acids but also to the hydrophobic interactions between
HA and the membrane surface which tends to affect atrazine and
prometryn retention This is in accord with the observed higher
adsorption rates of HA on the XLE membrane (Table 8) which in turn
may affect the surface properties of the membrane and therefore
triazine retention
The GPC results in the case of FA and NOM are in accord with
the retention results of triazines for all three membranes FA and
NOM interact with the two herbicides to form complexes of various
stabilities The formation of these pseudo-complexes may explain
the higher retention of atrazine and prometryn as a result of the
94 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
Table 6
Average retention values of atrazine and prometryn at two different humic substance concentrations by the three NFULPRO membranes
Humic substance Cfeed (mg Lminus1) NF270 NF90 XLE
Ratrazine () Rprometryn () Ratrazine () Rprometryn () Ratrazine () Rprometryn ()
HA 5 782 804 873 883 83 852
10 776 798 838 846 769 744
FA 5 784 829 828 870 848 913
10 816 842 937 933 906 977
NOM 5 813 859 843 838 879 922
10 862 901 894 857 956 968
TA 5 898 880 876 903 905 963
10 969 976 96 957 100 100
following mechanisms that may take place (a) increase of the
steric congestion or reduction of the diffusivity of the FAndashtriazine
or NOMndashtriazine pseudo-complex (b) increase of the density of
the pseudo-complexrsquos negative charge which enhances the elec-
trostatic repulsion with the negatively charged membranes and (c)
increase of the adsorption capability of the pseudo-complex on the
surface of the membrane in contrast to the hydrophobic nature of
the humic materials The validity of the above mechanism hypothe-
ses adopted by previous researchers [7] depends on the type of
the organic material and consequently on the characteristic chem-
ical properties of both HS (acidity functional groups) and triazines
(basicity hydrophobicity charge) The correlation between humic
material properties and triazine retention is examined in a subse-
quent paragraph
The greater extent of FAndashtriazines pseudo-complexes in com-
parison to NOMndashtriazines pseudo-complexes manifested in the
GPC results is not accompanied by a greater increase of triazine
retention More specifically the increasing order of triazine reten-
tion in the presence of a specific humic material follows the order
TA gt NOM gt FA gt HA in the case of NF270 and XLE membranes
whereas for NF90 the order is TA ge FA gt NOM gt HA Therefore the
interactions between the humic materials and triazines alone
cannot explain the retention performance of the NFULPRO mem-
branes This fact indicates that interactions between the naturally
occurring organic substances and the membranes may also play a
significant role in triazine retention as previously observed [33] In
view of these observations dead-end filtration tests using the three
IHSS organic materials and the selected NFULPRO membranes are
carried out in this laboratory in order to distinguish the effects of
organic fouling and of HSndashtriazines interactions on triazine perme-
ation during nanofiltration
323 Effect of humic substance concentration on triazine
retention
Surveying the literature on the complicated influence of organic
matter on retention of pesticides by NFRO membranes reveals a
considerable variability regarding the importance of the organic
matter concentration Experiments in a nanofiltration pilot unit on
the removal of atrazine and simazine have demonstrated the sig-
nificance of the organic load in the feed water since the removal
efficiency increased from 50 to 90ndash100 when dissolved organic
carbon varied between 04 and 36 mg Lminus1 [7] In contrast Berg et
al [8] reported no increase of pesticide retention (atrazine and
simazine among them) by several NF membranes with increas-
ing organic matter concentration Along the same lines Zhang et
al [11] showed comparable simazine retention between tap water
and river water although the latter had a high concentration of
NOM whereas tap water had a low NOM concentration In an effort
to understand the reasons for the above inconsistency in the lit-
erature Nghiem et al [40] took into consideration the effect of
ionic strength Experiments with hormone mimicking trace organic
contaminants revealed the significance of solution ionic strength
in influencing NOMndashtrace organic interactions which is probably
the cause for the ambiguity of literature data regarding the role
of background organic matter in trace organic rejection The mag-
nitude of ionic strength in terms of calcium ion content on the
NOMndashtriazines interaction has been also recorded through the GPC
tests described previously and is subsequently examined Although
ionic strength can strongly influence the HSndashtriazines interactions
the effect of the type of the humic substance seems to play also a
significant role on triazine retention This is evident in the results
included in Table 6 regarding retention experiments in the pres-
ence of a medium (5 mg Lminus1) and relatively high (10 mg Lminus1) HS
concentration in the absence of ionic environment
Table 6 shows a clear increase of triazine retention with increas-
ing humic material concentration More specifically by doubling
the concentration of FA NOM and TA in the feed solutions atrazine
and prometryn retention increases 4ndash13 and 2ndash10 respectively
depending on the membrane and the humic material used An
exception is observed again in the case of humic acids since the
increased organic load results in negligible or slightly negative
effects of atrazine and prometryn retention Regarding the XLE
membrane the use of medium HA concentration results in similar
retention performance as in organic-free solutions On the other
hand higher HA concentration results in a notable reduction of
triazine retention ranging up to 74 and 127 for atrazine and prom-
etryn respectively As previously discussed this is attributed to the
increased HA adsorption on the membrane surface (Table 8) which
in turn affects triazine adsorption on XLE membrane and therefore
retention This trend is not observed in the case of FA NOM and
TA despite their adsorptive capacity on the membrane materials
This finding is probably related to fouling phenomena which tend
to modify the membrane properties (ie surface charge hydropho-
bicity morphology) and therefore triazine adsorption Although it
is difficult to use mass balances for calculating triazine adsorption
during the combined filtration with humic substances (due to ana-
lytical problems related to the concentrate samples) the changes
of triazine concentration in the permeate samples provide a good
indication of the potential adsorption of atrazine and prometryn
on the membrane surface Furthermore the increased passage of
triazines in the permeate side as a result of HA adsorption on the
XLE membrane is in accord with the increase of triazine adsorption
on the membrane and therefore with the reduced retention values
324 Effect of calcium ion on triazine retention
From Fig 5 it is evident that the presence of calcium acts
positively in both atrazine and prometryn retention during the
combined nanofiltration with all four types of humic substances
Furthermore the higher interactions between the humic organ-
ics and triazines manifested in the GPC tests are in agreement
with the increased retention performance of atrazine and prom-
etryn by the three membranes used Additionally the suggestion
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 95
of ligand exchange in which calcium serves as a bridging ion and
acts as a possible site for higher adsorption of triazine molecules
on humic substances [38] seems to be in accord with the forma-
tion of an increased number of pseudo-complexes between humic
matter and triazines which explain the increase of herbicide reten-
tion
Among the four HS used tannic acid is associated with the high-
est triazine retention since both triazines are fully retained in the
presence of calcium by all three membranes tested Moreover the
enhanced interaction between humic acids and triazines as a result
of the calcium addition to the feed solutions is accompanied by
a significant increase of atrazine and prometryn retention In this
case calcium ions favour the binding between HA and triazines
which in other cases would not have taken place this is in agree-
ment with the preceding GPC and NFULPRO results
Prometryn exhibits again the highest retention values in com-
parison to atrazine which may be attributed to the relatively higher
prometryn hydrophobicity and basicity (Table 1) The former prop-
erty likely favours hydrophobic interactions between prometryn
and the HSndashcalcium complexes whereas the latter renders prom-
etryn highly effective in mechanisms involving electron-transfer
with the HSndashcalcium complexes [19] This mechanism is due to the
lower acidic functional group content of the HS as a result of the
complexation with calcium ions in the feed solutions Sposito et
al [19] suggested that atrazine itself does not readily participate
in electron-transfer reactions with humic substances However
they demonstrated that hydroxyl-atrazine does react through an
electron-transfer mechanism with HA and FA It is noted that
atrazine is readily converted to hydroxyl-atrazine even in labo-
ratory samples at low water content and this may explain the
presence of some of the electron transfer products detected in stud-
ies of atrazinendashHA interactions [41]
Surveying the literature however it is noted that some stud-
ies either with dialysis membranes [1213] or NF membranes [9]
have shown reduced values of atrazine retention when divalent
calcium is present together with natural organic matter includ-
ing the NOM surrogate tannic acid According to Devitt et al [9]
Devitt and Wiesner [12] this is due to the reduced association of
atrazine and NOM as a result of the occupation of interaction sites
by calcium andor the reduced access of atrazine to NOM sites due
to changes in molecular conformation The GPC results in this study
have shown that this is not the case since the presence of calcium
tends to increase the interaction of humic substances with triazine
compounds like atrazine
The conflicting results between the previous studies and the
present work may be attributed to the different types of membranes
and filtration techniques used More specifically the use of cellulose
ester membranes as well as the experimentation with batch dialy-
sis systems where concentration and osmotic pressure difference
serve as the driving force for solute transport (ie in the absence of
hydrodynamic forces as in the present study) may explain the dif-
ferences regarding the calcium effect on triazine retention Runge
et al [42] claim that the SpectraPor cellulose ester membranes
used in their studies (having a molecular weight cut-off of 100 Da)
Fig 6 Retention of atrazine (a) and prometryn (b) as a function of humic material total aciditymdashcalculations based on the values presented in Table 2 taking into consideration
the carbon composition and the HS concentration
Fig 7 Retention of atrazine (a) and prometryn (b) as a function of humic material carboxylic aciditymdashcalculations based on the values presented in Table 2 taking into
consideration the carbon composition and the HS concentration
96 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
present a significant incompatibility with distilled water whereas
a significant membrane potential can build up with certain salts
which can inhibit or enhance diffusion depending on the size of
the solutes diffusing and the polarity of the membrane potential
generated In this case an increased diffusion of hydrated calcium
ions to the permeate side in order to balance the concentrations
between the two sides of the cellulose ester membrane may result
in a pore blockage or a positive charge on the membrane surface
which in turn could lead to an increased passage of free dissolved
atrazine to the permeate side
Another possible explanation for this inconsistency is the time
period of the filtration tests According to the protocol followed
by previous researchers [1213] the dialysis time period ranged
between 1 and 5 days at the end of which concentration measure-
ments were performed On the other hand the filtration periods
in the case of the present dead-end nanofiltration experiments
ranged between 2 and 5 h depending on the membrane used To
this period one more hour should be added in which the feed solu-
tion was stirred in the cell without pressurization Even though
triazine adsorption on the membrane surface was observed to sta-
bilize within the relatively short period of nanofiltration there is
a possibility that longer filtration periods may have resulted in
higher adsorption rates which in turn could lead to greater pas-
sage of triazine to the permeate side and therefore to reduced
retention values This explanation is in agreement with the pilot
tests performed by Boussahel et al [43] who report that the
increasing adsorption of humic matter on the membranes leads
to an increase of pesticide adsorption on the membranes which
facilitates the diffusion of pesticides in the direction of perme-
ate
33 Correlation between humic material properties and triazine
retention
The correlation of atrazine and prometryn retention with the
total carboxylic and phenolic acidity of the humic substances used
in the present study as well as the role of the molecular mass of HS
on triazine retention are depicted in Figs 6ndash9 These are results of
nanofiltration experiments performed in the absence of calcium
Triazine removal by the three NFULPRO membranes seems
to poorly correlate with the total acidity of the humic material
This is evident especially in the case of NF270 and NF90 mem-
branes (Fig 6) The XLE membrane on the other hand presents a
reduced tendency for atrazine and prometryn removal when acid-
ity increases This is attributed to the characteristic low negative
charge of the XLE membrane surface which causes weak repulsions
with the negatively charged humic organics This leads to a greater
adsorption of humic compounds on the membrane surface (Table 8)
and consequently to greater triazine adsorption which favours the
diffusion of the triazines to the permeate side On the other hand
the strong negative charge of the NF270 and NF90 membranes [20]
promotes a strong repulsion between HS and the membranes The
greater the negative charge of the humic organic the higher the
repulsion which in turn results in higher concentration of HS in
the bulk Consequently the interactions between HS and triazines
in the bulk are increased leading to an improved triazine removal
Fig 8 Retention of atrazine (a) and prometryn (b) as a function of humic material phenolic aciditymdashcalculations based on the values presented in Table 2 taking into
consideration the carbon composition and the HS concentration
Fig 9 Retention of atrazine (a) and prometryn (b) as a function of humic substance molecular mass (based on the values presented in Table 3)
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 97
This is more evident in the case of the wide pore NF270 membrane
since the NF90 membrane seems to retain atrazine and prometryn
to a higher degree in the presence of TA which is characterized by
a lower total acidity among the four HS used in this study
Humic compounds contain a variety of functional groups
such as aliphatic and aromatic carboxyls phenolic hydroxyls
alcoholic hydroxyls carbonyls quinones methoxyls and amino
groups These functional groups are the reactive sites in the humic
molecules As carboxylate protonation seems to be critical for the
capacity of triazine binding [44] it is of interest to correlate the
effect of carboxylic acidity on herbicide removal by nanofiltration
The effect of carboxylic acidity on atrazine and prometryn retention
by the three NFULPRO membranes is depicted in Fig 7
The results show a negative trend of triazine retention with
an increased carboxylic acidity of humic substances This is clear
in the case of NF270 and XLE membranes while NF90 does not
seem to be significantly affected by the changes in the number of
carboxyl groups of a humic molecule Remarkably the smaller TA
molecules containing fewer carboxyl groups per unit mass (Table 2)
exhibit the highest triazine retention (sim100) as a result of the even
higher triazine binding capacity in contrast to the IHSS organics
which contain a higher carboxylic group content Moreover experi-
ments in batch filtration mode result in HS concentration increasing
with time which in turn leads to a reduced number of carboxylic
binding sites available to the two triazines This is probably due
to the self ldquocompetingrdquo character of the HS used since an increase
in concentration andor HS aggregation tend to reduce the avail-
able binding sites [44] This phenomenon led Wang et al [44] to
propose a model in which hydrogen bonding was considered to be
the primary interaction resulting in triazine binding Therefore the
increase in triazine retention observed in the case of elevated HS
concentration may not be directly related with the carboxylic acid-
ity of the HS but rather with other mechanisms such as hydrogen
bonding
Regarding the phenolic acidity of the humic substances there
does not seem to be a sufficient correlation with atrazine or prom-
etryn retention by the three NFULPRO membranes (Fig 8) Actually
there is a contradictory behaviour between the three IHSS organ-
ics and the NOM surrogate TA The high phenolic content of TA
is related to high retention values of the two triazines while
the smaller and relatively similar phenolic acidity of the IHSS
compounds leads to smaller and different retention values These
results imply that there are other factors influencing the association
between humic organics and herbicides and therefore their reten-
tion by nanofiltration In this respect the molecular structure of the
humic materials as well as the interactions taking place between
the organics and the membranes seem to explain the above incon-
sistencies This is evident in the results depicted in Fig 9 and the
adsorption of humic substances subsequently described
The rather strong relation of triazine retention with the molec-
ular mass of the organics used in this study (Fig 9) leads to the
suggestion that structural features of HS are important for the
removal capacity of triazines Up to now there is no consensus
concerning both the binding mechanism of triazines to humic sub-
stances and the structural features responsible for this binding
Nevertheless the preferential binding with low molecular weight
fractions of humic compounds (especially of FA and TA) deduced
from the GPC and filtration tests results in higher retention val-
Table 7
Humic substance and calcium retention performance of the three NFULPRO membranes (mean retention values for 50 recovery of feed volume as permeate)
Membrane Humic substancea Humic substances retention () Ca2+ retention ()
HSs alone HSs with triazines HSs with triazines and Ca2+ In the presence of HSs
NF270 HA 990 994 1000 949
FA 967 983 991 951
NOM 957 997 1000 951
TA 972 992 997 957
NF90 HA 972 980 1000 994
FA 953 984 1000 990
NOM 985 993 1000 991
TA 967 998 998 994
XLE HA 992 996 1000 994
FA 862 915 994 997
NOM 976 1000 1000 988
TA 923 945 1000 996
a HA FA NOM and TA designate the four humic substances used in this study
Table 8
Adsorption data of humic substances on tested membranes
Membrane Humic substancea Feed mass of humic substance (g) Quantity adsorbed (g) Adsorption ()
Alone With Ca2+ Alone With Ca2+ Alone With Ca2+
NF270 HA 3480 2630 244 2314 07 88
FA 3570 2840 71 1335 02 47
NOM 3390 2660 34 2633 01 99
TA 4050 3300 2025 7161 50 217
NF90 HA 3220 2690 354 3658 11 136
FA 2930 2450 1494 857 51 35
NOM 3000 2910 120 437 04 15
TA 2820 2730 649 3631 23 133
XLE HA 3100 2680 1643 3457 53 129
FA 3050 2810 1830 2866 60 102
NOM 2650 2770 610 1219 23 44
TA 3120 2940 2184 2970 70 141
a HA FA NOM and TA designate the four humic substances used in this study
98 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
ues for the two triazines Thus the interaction with small branched
molecules like FA and TA seems to favour a physical entrapment
mechanism such as the one suggested by Devitt and Wiesner [12]
34 Retention of humic species and calcium ion by the NFULPRO
membranes
The performance of nanofiltration on the humic substances and
calcium removal is summarized in Table 7 Regarding humic sub-
stances the retention is compared for three different feed solutions
(a) solutions of HS alone (b) solutions of HS with the two triazines
and (c) solutions of HS with the two triazines in the presence of cal-
cium In general HS retention is high for all three membranes used
According to the UV measurements IHSS HA exhibits the highest
retention values followed by NOM TA and FA which is apparently
related to their relative molecular masses (Table 3) Complexation
or interaction with triazines as evidenced by the GPC tests appar-
ently plays also a significant role in the relative permeability of the
organic species More specifically the retention of the four organic
compounds examined is increased in the presence of the two her-
bicides which verifies also the binding of atrazine and prometryn
with the four humic substances Thus a comparison between the
retention values of the HS in the absence or presence of triazines
gives also a good insight into the HSndashtriazine interactions The slight
increase (lt1) of HA retention in the presence of the two triazines
is in accord with the relatively low HA binding capacity observed
in the GPC tests On the other hand the increase of 2ndash6 in FA
NOM and TA retention is in agreement with the observed increased
interactions between those substances and the two triazines
In the case of calcium addition in the organicndashtriazine solu-
tions the UV absorbing species of the four organics examined are
retained almost completely by all three membranes used This is
again in agreement with the results obtained with gel chromatogra-
phy According to the literature [12] the addition of calcium results
in charge shielding and neutralization of the organic matter charged
functional groups and can shrink the humic matrix The formation
of aggregates of larger apparent molecular weight due to the sub-
stantial increase in the hydrophobicity of the organic molecules
tends to reduce their permeability through membranes and there-
fore to increase their retention
Apart from the increased retention of triazines and humic sub-
stances there is also a significant improvement of calcium rejection
when the latter is present in the solution In comparison to pure
water solutions of elevated calcium ion content [20] the rejection of
calcium in humic solutions is almost complete This is attributed to
size exclusion mechanisms related to the binding tendency of diva-
lent ions like calcium to the negatively charged humic organics
Among the four humic substances used calcium is rejected more in
the presence of the polyphenol tannic acid which is accompanied
by a higher organic adsorption on all three NFULPRO membranes
tested (Table 8)
From the data summarized in Table 8 it can be seen that the
adsorption of humic substances onto the membrane surfaces is
markedly enhanced in the presence of calcium ions due to the
formation of HA-complexed divalent cations This is also clearly
depicted in the SEM images of virgin and fouled membranes in
the absence or presence of calcium ions Characteristic SEM images
in the case of the Suwannee River NOM filtration with the three
NFULPRO membranes are included in Fig 10 these images show
the severe fouling on the three NFULPRO membranes as a result
of the combined filtration of NOM with CaCl2 salt It is possible
that the divalent cations such as Ca2+ act as a bridge between the
membrane surface and the negatively charged humic molecules
that tend to be repelled by the negatively charged membrane As a
result the electrical repulsion between the membrane surface and
the humic molecules is weakened thus rendering the membrane
Fig 10 SEM images of virgin fouled with NOM and fouled with NOM + calcium membranes (a) NF270 (b) NF90 and (c) XLE membrane
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 99
fouling process more severe [45] On the other hand the relatively
smaller adsorption rates of NOM in the absence of calcium result
in less fouled membranes A similar trend is also observed in the
case of the other three humic substances used in this study Among
the three membranes used XLE exhibits in the absence of calcium
the highest adsorption rates for all four humic substances This is
explained by the relatively high hydrophobicity and the low surface
charge that characterize the XLE membrane surface as previously
explained
4 Concluding remarks
The influence of humic substances on retention of two tri-
azines (atrazine and prometryn) by NFULPRO membranes is rather
complicated and involves combinations of solutendashmembrane and
solutendashsolute interactions In summary the retention of triazines
depends on the type of the humic material present in the feed
water In most cases the combined nanofiltration of triazines and
naturally occurring humic substances facilitates the formation of
complexes with triazines which in turn enhance their removal by
means of membrane filtration This complexation appears to be
related not to the characteristic acidity (phenolic carboxylic) of
the HS used but rather to their molecular conformation More
specifically a preferential binding is observed between triazines
and low molecular weight fractions of humic compounds (espe-
cially of FA and TA) which results in higher retention values for the
two triazines Under all conditions tannic acid exhibits the greatest
effect on triazine retention among the four standard HS compounds
used leading to an almost complete removal of the two triazines
(95ndash100) for all three membranes tested
The removal of triazines is improved in the presence of calcium
which tends to enhance the interaction between HS and triazines
Moreover triazine retention is increased with increasing HS con-
centration to a degree depending on the type of HS Additionally
triazine retention is apparently also affected by the interactions
between the organics and the membrane surface which in turn
may alter triazine retention These results illustrate the complex-
ity of the triazine adsorption and retention mechanisms in the
presence of humic substances that needs further investigation for
improved understanding Moreover regarding triazine removal by
nanofiltration it is of particular interest to distinguish the effect
of the HSndashtriazine interactions from that of membrane HS fouling
Research in this direction is carried out in this laboratory
References
[1] DP Birardar AL Rayburn Chromosomal damage induced by herbicide con-tamination at concentrations observed in public water supplies J Environ Qual24 (1995) 1222ndash1225
[2] IK Konstantinou DG Hela TA Albanis The status of pesticide pollution insurface waters (rivers and lakes) of Greece Part I Review on occurrence andlevels Environ Pollut 141 (3) (2006) 555ndash570
[3] European Environment Agency (EEA) Groundwater quality and quantity inEurope Report Copenhagen June 1999 available in httpreportseeaeuropaeugroundwater07012000enenviassrp199903
[4] EM Thurman DA Goolsby MT Meyer MS Mills ML Pomes DW Kolpin AReconnaissance study of herbicides and their metabolites in surface water of theMidwestern United States using immunoassay and gas chromatographymassspectrometry Environ Sci Technol 26 (1992) 2440ndash2447
[5] C Bellona JE Drewes P Xu G Amy Factors affecting the rejection of organicsolutes during NFRO treatmentmdasha literature review Water Res 38 (2004)2795ndash2809
[6] A Bhattacharya Remediation of pesticide-polluted waters through mem-branes Sep Purif Rev 35 (2006) 1ndash38
[7] KM Agbekodo B Legube S Dard Atrazine and simazine removal mechanismsby nanofiltration influence of natural organic matter concentration Water Res30 (1996) 2535ndash2542
[8] P Berg G Hagmeyer R Gimbel Removal of pesticides and other micropollu-tants by nanofiltration Desalination 113 (1997) 205ndash208
[9] EC Devitt F Ducellier P Coumlteacute MR Wiesner Effects of natural organic mat-ter and the raw water matrix on the rejection of atrazine by pressure-drivenmembranes Water Res 32 (1998) 2563ndash2568
[10] R Boussahel S Bouland KM Moussaoui A Montiel Removal of pesticideresidues in water using the nanofiltration process Desalination 132 (2000)205ndash209
[11] Y Zhang B Van der Bruggen GX Chen L Braeken C Vandecasteele Removalof pesticides by nanofiltration effect of the water matrix Sep Purif Technol38 (2004) 163ndash172
[12] EC Devitt MR Wiesner Dialysis investigations of atrazinendashorganic matterinteractions and the role of a divalent metal Environ Sci Technol 32 (1998)232ndash237
[13] SK Dalton JA Brant MR Wiesner Chemical interactions between dissolvedorganic matter and low-molecular weight organic compounds impacts onmembrane separation J Membr Sci 266 (2005) 30ndash39
[14] NA Kulikova IV Perminova Binding of atrazine to humic substances fromsoil peat and coal related to their structure Environ Sci Technol 36 (2002)3720ndash3724
[15] RL Wershaw The importance of humic substancendashmineral particle complexesin the modeling of contaminant transport in sedimentndashwater systems in RABaker (Ed) Organic Substances and Sediments in Water Humics and SoilsLewis Publishers Chelsea MI 1991 pp 23ndash34
[16] SM Schrap A Opperhuizen Relationship between bioavailability andhydrophobicity reduction of the uptake of organic chemicals by fish due tothe sorption on particles Environ Toxicol Chem 9 (1990) 715ndash724
[17] E Barriuso M Schiavon F Andreux JM Portal Localization of atrazine non-extractable (bound) residues in soil size fractions Chemosphere 12 (1991)1131ndash1140
[18] L Loiseau E Barriuso Characterization of the atrazinersquos bound (nonextractable)residues using fractionation techniques for soil organic matter Environ SciTechnol 36 (2002) 683ndash689
[19] G Sposito L Martin-Neto A Yang Atrazine complexation by soil humic acidsJ Environ Qual 25 (1996) 1203ndash1209
[20] KV Plakas AJ Karabelas Membrane retention of herbicides from single andmulti-solute media the effect of ionic environment J Membr Sci 320 (2008)325ndash334
[21] V Freger J Gilron S Belfer TFC polyamide membranes modified by graftingof hydrophilic polymers an FT-IRAFMTEM study J Membr Sci 209 (2002)283ndash292
[22] P Xu JE Drewes Viability of nanofiltration and ultra-low pressure reverseosmosis membranes for multi-beneficial use of methane produced water SepPurif Technol 52 (2006) 67ndash76
[23] Z Knez A Rizner-Hras K Kokot D Bauman Solubility of some solid triazineherbicides in supercritical carbon dioxide Fluid Phase Equilibria 152 (1998)95ndash108
[24] Weed Science Society of America (WSSA) Herbicide Handbook 7th ed 1994[25] B Van der Bruggen J Schaep W Maes D Wilms C Vandecasteele Nanofiltra-
tion as a treatment method for the removal of pesticides from ground watersDesalination 117 (1998) 139ndash147
[26] KN Reddy MA Locke Molecular properties as descriptors of octanol-waterpartition coefficients of herbicides Water Air Soil Pollut 86 (1996) 389ndash405
[27] IHSS website httpwwwihssgatechedu[28] F Flores-Ceacutespedes M Fernaacutendez-Peacuterez M Villafranca-Saacutenchez E Gonzaacutelez-
Pradas Cosorption study of organic pollutants and dissolved organic matter ina soil Environ Pollut 142 (2006) 449ndash456
[29] R Beckett Z Jue JC Giddings Determination of molecular weight distribu-tions of fulvic and humic acids using flow field-flow fractionation Environ SciTechnol 21 (1987) 289ndash295
[30] PM Reid AE Wilkinson E Tipping MN Jones Determination of molecularweights of humic substances by analytical (UV scanning) ultracentrifugationGeochim Cosmochim Acta 54 (1990) 131ndash138
[31] S Lee B Kwon M Sun J Cho Characterizations of NOM included in NF and UFmembrane permeates Desalination 173 (2005) 131ndash142
[32] MN Jones ND Bryan Colloidal properties of humic substances Adv ColloidInterf Sci 78 (1998) 1ndash48
[33] KV Plakas AJ Karabelas T Wintgens T Melin A study of selected herbicidesretention by nanofiltration membranesmdashthe role of organic fouling J MembrSci 284 (2006) 291ndash300
[34] APHA-AWWA-WPCF Standard Methods for the Examination of Water andWastewater 17th ed Port City Press Baltimore MD 1989 pp 567ndash569
[35] R Artinger G Buckau JI Kim S Geyer Characterization of groundwater humicand fulvic acids of different origin by GPC with UVVis and fluorescence detec-tion Fresenius J Anal Chem 364 (1999) 737ndash745
[36] AI Schaumlfer N Andritsos AJ Karabelas EMV Hoek R Schneider M Nys-troumlm Chapter 8 Fouling in nanofiltration in AI Schaumlfer AG Fane TDWaite (Eds) Nanofiltration Principles and Applications Elsevier Oxford UK2005 pp 173ndash174
[37] I Franco L Catalano M Contin M De Nobili Interaction between organicmodel compounds and pesticides with water-soluble soil humic substancesActa Hydrochim Hydrobiol 29 (2001) 88ndash99
[38] AA Helal DM Imam SM Khalifa HF Aly Interaction of pesticides with humiccompounds and their metal complexes Radiochemistry 48 (4) (2006) 419ndash425
[39] MG Browman G Chester Fate of Pollutants in the Air and Water EnvironmentWiley New York 1977 p 50
[40] LD Nghiem AI Schaefer M Elimelech Nanofiltration of hormone mimickingtrace organic contaminants Sep Sci Technol 40 (2005) 2633ndash2649
[41] R Celis J Cornejo MC Hermosin WC Koskinen Sorptionndashdesorption ofatrazine and simazine by model soil colloidal components Soil Sci Soc AmJ 61 (1997) 436ndash443
100 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
[42] SW Runge KR Shelton SA Melton WM Moran Maintaining the ionic per-meability of a cellulose ester membrane J Biochem Biophys Methods 64(2005) 200ndash206
[43] R Boussahel A Montiel M Baudu Effects of organic and inorganic matter onpesticide rejection by nanofiltration Desalination 145 (2002) 109ndash114
[44] Z Wang DS Gamble CH Langford Interaction of atrazine with Laurentianhumic acid Anal Chim Acta 244 (1991) 135ndash143
[45] C Jucker MM Clark Adsorption of aquatic humic substances on hydrophobicultrafiltration membranes J Membr Sci 97 (1994) 37ndash52
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 87
to alter the speciation and transport behaviour of other xenobiotic
compounds present in water [15] Thus organic micropollutants
like pesticides may exist either as free dissolved species or as a
complex with HS [16] In a study performed by Barriuso et al [17]
15ndash40 of the initial atrazine concentration in soil columns was
still present after 6 months and the residues were mainly asso-
ciated with humic compounds In an earlier study [18] bound
atrazine residues were in the range 10ndash40 of the applied atrazine
depending on the presence or absence of a micro-flora capable of
mineralizing the triazinic ring between 13 and 30 of such bound
residues was associated with humic acids It is evident therefore
that the interactions between pesticides and natural organic mat-
ter may significantly affect their retention during membrane water
treatment
A literature review on the effect of NOM on pesticide reten-
tion by membranes suggests that there is a dependence on the
type of NOM present in the water NOM is composed of an
extremely diverse group of compounds including carbohydrates
alcohols amino acids carboxylic acids lignins and pigments
whose origin greatly influences its character and behaviour Though
various fractions of NOM can be defined by special procedures for
separating organic matter from water a general structural char-
acterization is practically impossible to obtain since the exact
NOM composition varies greatly as a function of NOM source
aqueous solution conditions season etc [12] It is therefore rea-
sonable to assume that NOMndashpesticide interactions may vary as
a function of properties of different NOM fractions and origins
For example the study by Sposito et al [19] revealed that the
weak interactions between s-triazines and humic acid functional
groups include proton transfer hydrogen bonding and electron-
transfer The actual interactions depend on the basicity of s-triazine
molecule functional groups of humic substances and the pH of the
medium They suggested that proton transfer more likely occurs
where humic substances have high acidic functional group content
and the s-triazines have low basicity however electron-transfer
mechanisms are more likely in cases humic substances have low
acidity functional groups and the s-triazines have a high basicity
[19]
The majority of the published works agree on the fact that
the retention of pesticides in membrane-based systems tends to
increase in the presence of NOM [79ndash13] which is generally
attributed to a variety of factors eg the size shape and surface
chemistry of compounds involved On the other hand the use by
various researchers of NOM of different origins and the inadequate
information on their physicochemical properties (elemental anal-
ysis functional groups) do not allow comparison of experimental
results as well as the correlation of the pesticidendashNOM retention
by membranes with the characteristic properties of the naturally
occurring organics in water Most of the literature is concerned with
adsorption and rejection properties of herbicides but usually fails
to appraise the variability introduced by the different properties
of humic substances Therefore it is of particular interest to study
the relation between the characteristic physicochemical proper-
ties of the humic substances that are present in drinking water
sources with the herbicide retention performance of NFULPRO
membranes To this end the use of well-defined NOM will aid the
better understanding of mechanisms of NOMndashpesticide retention
by membranes
In the present work pressure-driven dead-end filtration is
employed to study membrane retention of triazines (atrazine and
prometryn) in the presence of different types of natural organic
matter the latter are well-characterized HS purchased from the
International Humic Substances Society (IHSS) Additionally com-
mercially available tannic acid (TA) is used as a HS surrogate
in order to compare the results of the present work with those
obtained by previous researchers
2 Experimental work
21 Materials and methods
211 Membranes
Three kinds of flat sheet type commercial membranes (Dow
Filmtec) denoted as NF90 NF270 and XLE were used in this
study NF90 and NF270 are nanofiltration membranes while XLE
is described by the manufacturer as extra low energy membrane
(ultra-low pressure RO-ULPRO) The membrane coupons used for
the experiments were stored at 4 C in a 075 Na2S2O5 aqueous
solution (to suppress development of micro-organisms) which was
regularly replaced The characteristic properties of the membranes
used were described elsewhere [20] The differences observed
regarding their hydrophobicity and surface charge properties as
well as their specific flux and salt rejection may be attributed
to different reactants employed in their production and to the
membrane surface treatment More specifically it is reported that
NF270 membrane is composed of a semi-aromatic piperazine-
based polyamide layer [21] whereas NF90 and XLE membranes
are prepared by interfacial polymerization of m-phenylenediamine
[22] Each membrane is reinforced with a polyester non-woven
backing layer covered with a polysulfone porous layer The NF270
membrane is quite hydrophilic exhibiting the highest permeability
among the three membranes tested as well as fairly high retention
of charged ions On the other hand the NF90 and XLE membranes
exhibit similar permeability values and surface hydrophobicity
Moreover all three membranes are negatively charged at neutral
pH with NF270 and NF90 membranes characterized by a much
higher negative charge than XLE [20] In this study in addition to
other measurements made scanning electron microscopy (SEM)
images were obtained (using a JEOL JSM-6300 Scanning Micro-
scope) to examine the surface condition of the virgin and the treated
with HS membranes
212 Herbicides
Two herbicides belonging in the triazine family ie atrazine
and prometryn were selected for this study Both triazines are
traditional selective pre- and post-emergence herbicides that are
extensively used mainly for the protection of various crops orna-
mental plants and forest trees Herbicide analytical standards were
purchased from Riedel de-Haeumln (SigmandashAldrich) atrazine (purity
974) prometryn (purity 992) The molecular structure and
physicochemical properties of the tested herbicides are presented
in Table 1 Both triazines are considered as non-ionic hydropho-
bic (log Kow gt 2) compounds that are moderately soluble in water
Between the two prometryn is the largest molecule due to its
branched structure whereas its greater pKa value indicates a
greater basicity in comparison to atrazine [20] As far as polarity
is concerned it is uncertain whether the symmetry characteris-
ing the prometryn molecule (in enhancing its polar character) is
more important than the highly polar heteroatom (ndashCl) included
in atrazine [23] Nevertheless these two triazines are generally
considered in the literature as weak polar compounds
213 Humic substances
The use of well-characterized standard humic substances makes
it possible for researchers to check the results and compare with
similar experimental data For this purpose four different types
of well-characterized HS were used in this investigation Three of
them are typical water born HS purchased from the International
Humic Substances Society (IHSS University of Minnesota SWC
Dept) whereas the fourth TA is a HS surrogate purchased from
SigmandashAldrich Tannic acid is a lower molecular mass compound
considered to be a representative surrogate for NOM frequently
used as such in membrane-related research
88 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
Table 1
Properties of the herbicides used in this work [24]
Herbicide Atrazine Prometryn
Chemical structure
Molecular formula C8H14ClN5 C10H19N5S
Chemical class Cl-triazine S-triazine
Molecular weight (Da) 21569 24135
Molecular sizea (nm) 0788 ndash
Log Kow 268 308
Aqueous solubility (mg Lminus1) 33 33
Dipole momentb (debye) 2460 ndash
pKa (20 C) 17 409
a Obtained from Ref [25]b Obtained from Ref [26]
Table 2
Acidity and elemental composition of HS used in this study [27]
Humic substance Cat No Acidity (mequiv gminus1 C) Elemental composition
Carboxylic Phenolic Total Ca H2Ob
HA IHSS 2S101H 913 372 1285 5263 204
FA IHSS 2S101F 1187 284 1401 5234 169
NOM IHSS 1R101N 985 394 1379 4880 815
TAc SigmandashAldrich 188 955 114 5000 58
a Elemental composition in (ww) of a dry ash-free sampleb (ww) of H2O in the air-equilibrated sample (a function of relative humidity)c Values reported in Ref [28]
The three IHSS standards are denoted as Suwannee River Humic
Acid (HA) Suwannee River Fulvic Acid (FA) and Suwannee River
NOM The IHSS humic and fulvic acids contain only hydrophobic
organic acids while the NOM sample contains not only hydropho-
bic and hydrophilic acids but also other soluble substances that
are present in natural waters [27] On the other hand tannic
acid is a polyphenol representative of relatively hydrophilic com-
pounds of medium molar mass It is reported by the supplier to
have a mean molecular mass of 170118 g molminus1 with an empiri-
cal formula C76H52O46 but in fact it contains a mixture of related
compounds The characteristic chemical parameters of the four sub-
stances employed are summarized in Table 2 the average molecular
masses of the IHSS standards reported in the literature are pre-
sented in Table 3
214 Feed solutions
In the filtration experiments three different types of solu-
tions were used ie solutions of atrazinendashprometryn in ultra-
pure water atrazinendashprometryn in a model HS solution and
atrazinendashprometryn in a model HS solution in the presence of
calcium ions The model solutions of the four selected humic
substances were prepared at neutral pH in order to simulate
waters with different chemical characteristics Since humic sub-
stances concentrations in natural waters usually fall in the range
2ndash40 mg Lminus1 [32] all the solutions were prepared with ultra-pure
water and with a concentration of 5ndash10 mg Lminus1 humic substance
The HS were obtained in powder form and used without further
purification as the bound iron and ash contents were very low The
calcium ion content in the corresponding experiments was fixed
at 40 mg Lminus1 by adding calcium chloride (111 mg Lminus1 JT Baker)
In the case of the two triazines separate standard stock solutions
were prepared in high-performance liquid chromatography-grade
methanol and stored at 4 C with concentration 100 mg Lminus1 The
feed triazine solutions were prepared from ultra-pure water by
dilution of stock solutions at a level of 10ndash30 g Lminus1 for each
triazine The ultra-pure water used (resistivity gt 18 M cm) for
Table 3
Molecular mass data of IHSS organics employed in this study
Humic substance Technique [Ref] Mna Mw
b MwMnc
Suwannee River HA FFF [29] 1580 4390 278
Ultracentrifugation [30] ndash 4260 plusmn 280 ndash
Suwannee River FA FFF [29] 1150 1910 166
Ultracentrifugation [30] ndash 1460 plusmn 80 ndash
Suwannee River NOM HPSEC [31] 1760 2360 134
a Number average molecular weightb Weight average molecular weightc Polydispersivity index Suwannee River HA has a much larger polydispersivity compared to FA
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 89
solution preparation was obtained from a Milli-Q purification
system (Millipore Milford MA USA) Similar to the protocol
used by Devitt et al [9] once a model solution of HS and
atrazinendashprometryn was prepared in the presence or not of cal-
cium it was placed in a foil-covered container (to prevent atrazine
and prometryn degradation by exposure to light) and stirred for
more than 24 h after which it was assumed to be in equilibrium
215 Analytical techniques
2151 Herbicide analysis Off-line solid phase extraction (SPE)
with gas chromatography employing a mass-selective detector
(GC-MSD) was used to achieve the sensitivity required for the
tested herbicides The SPE procedure performed prior to the
chromatographic analysis is described in a previous paper [33]
Herbicide analyses were performed in an Agilent Technologies
Model 7890A gas chromatograph system interfaced with an Agilent
Technologies Model 5975C mass-selective detector (MSD) The gas
chromatograph was equipped with a 30 m times 250 m id times 025 m
film thickness fused silica capillary column (Agilent 19091S-
433) The chromatographic conditions were as follows injection
splitless injector temperature 180 C transfer line temperature
220 C injection volume 1 L carrier gas He flow 1 mL minminus1
oven temperature programmed from an initial temperature of 70 C
held for 3 min to 150 C at a rate of 30 C minminus1 held for 1 min next
to 200 C at a rate of 10 C minminus1 and finally to 260 C at a rate of
5 C minminus1 held for 1 min The MSD was operated in the selective ion
monitoring (SIM) mode Three characteristic ions were monitored
for each herbicide analyzed The most abundant ion for each herbi-
cide was used for quantification while the other two ions were used
to confirm the presence of the herbicide The minimum detectable
quantity (MDQ) for the analysis with a 1-L injection volume was
approximately 1 ng for all herbicides analyzed
Simazine (Riedel de-Haeumln purity 999) a triazine compound
similar to atrazine and prometryn was used as internal standard in
order to assess the recovery of the two triazines in the SPE-GC-MSD
analyses With the selected SUPELCO SPE cartridges recoveries
from the model humic substance solutions ranged from 35 to 98
and in general were lower than the recoveries obtained in pure
water or permeate samples of the triazinendashHS filtration (75ndash128)
The analysis of the concentrate samples which were characterized
by high dissolved carbon content resulted in many cases in even
lower recoveries This is attributed to the saturation of the sorption
sites of the SPE cartridges by the humic substances which renders
them inefficient in extracting the two triazines associated with HS
Therefore triazine concentrations were determined only in the feed
and permeate samples
2152 Organic and inorganic matter analysis Measurement of the
HS concentration (and more specifically the aromaticity) was per-
formed in a UVndashvis spectrophotometer (UV-1700 Shimadzu Japan)
at 254 nm in the case of Suwannee River HA and Suwannee River
NOM and at 275 nm in the case of Suwannee River FA A standard
colorimetric method (5550) [34] was adopted for the preparation
of tannic acid samples prior to their spectrophotometric measure-
ment at 700 nm The quality control of the data was achieved by
means of calibration and repeated analyses Specifically two cal-
ibration curves were created for each HS one corresponding to
the permeate samples (standards of similar concentrations to the
one expected in the permeates) and a second one corresponding
to the higher concentration level of the feed and retentate sam-
ples This procedure resulted in high R2 values (gt0999) reflecting
the high accuracy and sensitivity of the UVvis method for all sam-
ples treated The use of two different Quartz cuvettes (1 and 10 cm)
enhanced this accuracy (measurement of absorption in a range
01ndash15 unitsmdashas proposed in the literature) Further the accuracy
of the analytical results was ensured by repeatedly measuring the
concentration of a sample (more than four times) All measure-
ments in the case of higher concentrations (feed and concentrate)
resulted in RSD values smaller than 1 while in the case of the
lower concentrations the RSD values did not exceeded 2
The determination of calcium ion content in the water samples
was performed by an ion selective electrode connected to a 692
pHIon Meter Metrohm (Herisau Switzerland) The conductivity
and pH of the samples were measured by a conductometer 712
and pH meter 744 both by Metrohm
2153 Gel permeation chromatography (GPC) Gel permeation
chromatography (or size exclusion chromatography SEC) is a rela-
tively rapid and convenient method for estimating the partitioning
or binding of organic compounds to humic substances in solution
[35] In particular GPC can give information on the liability of the
interaction itself The solutions tested in the present work were (a)
HS (b) HSndashtriazine (atrazine prometryn or both) (c) HSndashcalcium
and (d) HSndashcalciumndashatrazinendashprometryn All solutions were stirred
for more than 24 h in foil-covered containers prior to their injection
into the GPC column The concentrations of all the materials used
were the same as those applied in the filtration experiments Tannic
acid was excluded from the GPC tests due to the possible interac-
tions that may occur between the chemicals of the colorimetric
method used (folin phenol Na2CO3ndashNa2C4H4O6middot2H2O solution)
and the GPC column which in turn may significantly alter the tannic
acid elution behaviour
The accuracy of the elution profiles was ensured by apply-
ing identical chemical conditions for the GPC investigation
This was achieved by using feed waters of the same source
thus all tested solutions (HS alone HSndashtriazines HAndashcalcium
HAndashcalciumndashtriazines) were prepared from the same HS stock solu-
tion This led to a negligible experimental uncertainty which was
evident in the repeatable GPC measurements (three to four injec-
tions) for each solution Indeed the spectrum of each HS was
very uniform whereas the shapes of the GPCSEC elution curves
recorded at the same wavelength were essentially identical
The GPC tests were carried out in a Shimadzu-10 AVP system The
GPC column was a PL aquagel-OH Mixed 8 m 300 mm times 75 mm
(Polymer Laboratories Essex UK) guarded by a PL aquagel-OH
Guard 8 m 50 mm times 75 mm column PL aquagel-OH columns
are packed with macroporous copolymer beads with extremely
hydrophilic polyhydroxyl functionality offering high resolution
over a wide range of molecular weights (up to 104 kDa) Aqueous
samples (100 L) were pipetted directly onto the gel and passed
through the column Phosphate buffer (01 M KH2PO4 at pH 70
corrected with 2 M NaOH) was used as the eluent with a flow
rate of 1 mL minminus1 Dissolved humic substance (HA FA and NOM)
was measured with a Shimadzu-10 AVP UV detector by monitor-
ing ultraviolet light absorbance at a wavelength of 254 275 and
254 nm respectively Calibration of the column to determine the
exact molecular sizes of the HS was not performed in this work
22 Nanofiltration tests and filtration protocol
Dead-end filtration experiments were performed by using an
experimental set-up consisting of three high pressure stirred cells
described elsewhere [20] The three test cells were operated con-
currently to assess the reproducibility and accuracy of the results
A new membrane coupon was used in each filtration test in order
to avoid employing membranes with adsorbed amounts of humic
materials and herbicides
The filtration protocol involves a sequence of the following six
steps (1) membrane preconditioning with Milli-Q water under
pressure of 5 bar for several hours to ensure that the removal of
preservatives from the new membrane coupon is complete [20]
(2) membrane compaction for 1 h at 10 bar (3) measurement of the
90 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
pure water flux for 30 min at 5 bar (4) filtration of triazines in the
absence of humic materials (ldquofree triazine solutionsrdquo) at 5 bar (5)
filtration of the model HSndashtriazines solution at 5 bar and (6) mea-
surement of the pure water flux for 30 min at 5 bar Compaction
(step 2) is crucial in every membrane filtration protocol as it may
change both the active layer and its support thus affecting the flux
and the rejection properties of the membrane To eliminate this
impact membranes are often subjected to at a higher pressure (here
10 bar) than the operating pressure (5 bar) to ensure flux stability
during experiments [36] Preliminary experiments with the three
membranes tested showed that 1 h of 10 bar pressure application
was more than adequate to achieve membrane stability
All filtration steps were performed with a feed solution of
300 mL while the tests performed with the herbicides were car-
ried out until 150 mL of permeate were collected (50 recovery)
Feed and three permeate samples (50 mL each) were retained for
subsequent analysis in order to determine the evolution of con-
taminant concentration in some cases retentate samples were also
removed for chromatographic analysis Similar to previous studies
[2033] the triazine feed solutions were stirred in the cells for 1 h
without pressure In this way herbicide adsorption on the mem-
brane surface was assumed to reach equilibrium After termination
of the filtration experiments the membrane coupons were dried
for subsequent analysis (SEM) while the test cells were thoroughly
washed with acetone and repeatedly rinsed with Milli-Q water
23 Data treatment
The performance of nanofiltration can be expressed in terms of
the percentage removal of the organic and inorganic substances
The determination of the retention was calculated based on the
method described in our previous study [20] which proved to be
appropriate in the case of batch filtration experiments since the
membrane performance depends on the retentate concentration
which increases with time The concentration of the two triazines
was measured for the feed solution and for all the permeate samples
while retentate samples were removed for subsequent analysis of
the humic and inorganic compounds The retention was evaluated
based on the so-called ldquostablerdquo permeate samples where the water
recovery rate was between 17 and 50 The amount of deposited
humic substance on the membrane surface was determined from
mass balance as follows
A () = 100
(
1 minus
sumj
i=1Vpi
Cpi+ Crj
Vr
Cf Vf
)
(1)
where Cpj Cr Cf are the concentrations of a humic substance in
the permeate sample j retentate and feed respectively while Vpj
Vr Vf are the respective volumes of permeate sample j retentate
and feed Reduction of the pure water flux (FRPW) which may be
considered a measure of the adsorbed humic substances effect on
flux was determined as a percentage of clean water flux Jw before
and after membrane operation as previously described [20]
3 Results and discussion
31 Pure triazine solutions
Prior to the combined filtration of triazines with humic materi-
als pure water solutions with atrazine and prometryn were filtered
at 5 bar in concentrations ranging between 10 and 30 g Lminus1 The
performance of the membranes in terms of triazine retention is
illustrated in Fig 1 The accuracy of experimental results is indi-
cated by the bars which represent the scatter of data obtained from
six replicate experiments for each membrane
Fig 1 Atrazine and prometryn retention from free triazine solutions by the three
selected NFULPRO membranes mean retention values for 50 recovery of feed
volume feed concentration 10ndash30 g Lminus1
As in our previous studies [2033] prometryn the molecule
with the larger molecular weight and size displays the great-
est retention with all three membranes used The performance
of membranes regarding retention of the two triazines follows
the order XLE ge NF90 gt NF270 which is consistent with the results
obtained in our previous work [20] Comparing the experimen-
tal results for double solute mixtures of atrazine and prometryn
in the case of high (600ndash700 g Lminus1) [20] and low feed concen-
trations (10ndash30 g Lminus1) the differences in retention values vary
between 7 and 13 In general the retention results with pure tri-
azine solutions are in agreement with observations made by other
researchers [71125] in that herbicide concentration does not sig-
nificantly affect their retention The fact that the filtration of lower
feed concentrations leads to a slight reduction of triazine retention
(especially in the case of XLE membrane) could be attributed to the
amount of triazines adsorbed on the selected membranes Based
on the mass balances summarized in Table 4 the lower feed tri-
azine concentration is accompanied in general by a slightly lower
adsorption in comparison to the results obtained with higher feed
concentrations [20] something that is more pronounced in the case
of the less tight NF270 membrane
32 Aqueous solutions of mixed humic substances and triazines
321 GPC results on triazinesndashhumic substances interactions
The UVndashvis spectra of fractions collected during GPC chromatog-
raphy of HS alone and HS with atrazine andor prometryn in the
absence or presence of calcium ions are depicted in Figs 2ndash4
The comparison of the chromatographic behaviour of the solu-
tions used in qualitative terms allows an assessment of possible
interactions between humic substances and the two triazines [37]
The elution profiles display a maximum for all three IHSS organ-
ics under the same test conditions at retention time 185ndash195 min
In the case of the low molecular weight triazines the elution pro-
file maximum at a wavelength of 225 nm and concentration of
10 mg Lminus1 (high concentrations used in order to be measurable
by UV absorption) appear at 110 min (data not presented here)
The longer retention time of atrazine and prometryn is due to the
fact that small molecules can penetrate into the pores of the gel
and follow a torturous path through the column On the other
hand large molecules such as humic substances are excluded
from internal pore spaces in the gel and elute faster from the col-
umn Co-elution of small triazine molecules with the much larger
humic molecules indicates their association with the humic frac-
tion Humic-bound triazines appear to display the behaviour of the
larger humic molecules as they are also excluded from the internal
pores of the gel [37] This is reflected in the reduced GPC areas of
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 91
Table 4
Adsorption data of triazines on tested membranes
Membrane Feed mass of
triazinesa (g)
Quantity adsorbed
(g)
adsorption adsorption
[19]b
NF270 45 plusmn 27(A) 048 plusmn 027 127 plusmn 18 165
53 plusmn 28(P) 020 plusmn 008 38 plusmn 13 275
NF90 52 plusmn 28(A) 122 plusmn 083 248 plusmn 66 305
55 plusmn 13(P) 105 plusmn 033 250 plusmn 67 262
XLE 66 plusmn 10(A) 114 plusmn 012 219 plusmn 12 230
74 plusmn 11(P) 084 plusmn 008 171 plusmn 09 295
a (A) and (P) designate atrazine and prometryn respectivelyb Adsorption values for higher triazine (atrazine and prometryn) concentrations (600ndash700 g Lminus1 each)
Fig 2 GPC elution profiles of Suwannee River humic acid (HA) in the absence or presence of triazines andor calcium (detection by UV absorption at 254 nm)
the humic materials when treated together with the two triazines
(Table 5) In summary aquatic humic substances from the same
water source (Suwannee River) show differences in their GPC elu-
tion behaviour when treated together with calcium andor with
either of the two triazines
The peaks of the GPC chromatograms reflect differences in spe-
cific absorption Distinct differences between HS and atrazine or
prometryn are found in the case of FA and NOM In contrast HA
show a rather small variation on elution behaviour This is also in
accord with the values presented in Table 5 Complexation with
the NOM and FA appears to occur for both atrazine and prometryn
molecules the interactions with NOM indicate greater affinity to
atrazine On the other hand FA interactions appear to be stronger
with prometryn In Figs 2ndash4 and Table 5 one can observe that
among the three HS used FA seems to interact more with the two
triazines The weakest interaction is observed in the case of humic
acids which is reflected in the almost identical elution profiles and
the low GPC reduction areas
The presence of calcium in the solutions of humic substances
leads to a sharp reduction of HS concentrations (Table 5) indi-
cating the formation of calcium humate and fulvate complexes
According to the literature when negatively charged humic colloids
Table 5
Percentage () reduction of GPC integration areas of humic substances due to the presence of triazines andor calcium
Humic substances combined with Percentage GPC area reduction ()
HA FA NOM
40 mg Lminus1 Ca2+ 26 17 138
10 g Lminus1 atrazine 06 64 27
10 g Lminus1 prometryn 02 81 lt01
10 g Lminus1 atrazine + 10 g Lminus1 prometryn 23 80 30
10 g Lminus1 atrazine + 10 g Lminus1 prometryn + 40 mg Lminus1 Ca2+ 151 221 279
92 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
Fig 3 GPC elution profiles of Suwannee River fulvic acid (FA) in the absence or presence of triazines andor calcium (detection by UV absorption at 275 nm)
come in contact with positively charged metal ions (like calcium)
several binding mechanisms become effective The bond strengths
vary from a weak physical adsorption to strong chemical bond-
ing involving chelation reactions [38] In the case of herbicides
the partially chelated calcium may serve here as a bridging ion
and act as a possible site for the adsorption of a triazine molecule
by displacing water of hydration Indeed the addition of calcium
enhances the interaction between the humic substances and the
two triazines something that is manifested by the higher GPC area
reductions and the lower elution profiles Browman and Chester
[39] also concluded that cations on the surface of humic material
can act as binding bridges between synthetic chemicals and humic
compounds Similarly Helal et al [38] reported that the exchange
reaction between the pesticides and the humic compounds takes
place on the same sites at which binding of humic compound to the
metal occurs Moreover these authors claim that the pesticidendashHA
Fig 4 GPC elution profiles of Suwannee River NOM in the absence or presence of triazines andor calcium (detection by UV absorption at 254 nm)
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 93
Fig 5 Retention of atrazine (A) and prometryn (P) by the three NFULPRO membranes in the absence or presence of humic substances (HA FA NOM TA) andor calcium
ions
or ndashFA complex is stronger than the metalndashhumate or ndashfulvate
complex The mechanisms involved in the interactions between
triazines and calciumndashHS complexes may be described by either
a physical adsorption of the herbicide on the surface of metal
humatefulvate complex or a ligand exchange in which the metal
serves as a bridging ion and acts as a possible site for adsorption of
the herbicide molecule by displacing water of hydration [38]
322 Effect of different types of humic materials on triazine
retention
Retention results of atrazine and prometryn during the com-
bined nanofiltration with the four selected humic substances are
depicted in Fig 5 Results of triazine retention from organic solu-
tions with elevated calcium ion content are also included in Fig 5
and discussed in Section 324 Depending on the type of humic
material present increased retention of the two triazines may be
negligible or significant Specifically the use of HA results in reten-
tion values that are comparable to those obtained in the absence
of organics An exception is observed in the case of XLE membrane
where the combined filtration of triazines and HA leads to relatively
smaller retention values (7 and 13 for atrazine and prometryn
respectively) On the other hand the presence of tannic acid results
in a significant increase of triazine retention for all three mem-
branes tested In particular the addition of TA at a concentration of
10 mg Lminus1 (yielding a triazineTA ratio of 1 g1 mg) results in 14ndash21
and 20ndash29 increase of prometryn and atrazine retention respec-
tively The enhancement of pesticide removal in the presence of
the NOM surrogate tannic acid has been also observed by previ-
ous researchers [91213] According to Devitt and Wiesner [12] the
increased retention of a triazine molecule like atrazine in the pres-
ence of TA is due to the branched structure of TA which traps the
triazine within the macromolecule Moreover these authors sug-
gest that molecular conformations and weak interactions between
functional groups may combine to produce physical entrapment of
triazines through a low multiplicative probability of escape result-
ing from a large number of encounters with low-energy sites on the
tannic acid molecule (eg hydrogen bonding)
In the case of the combined nanofiltration of atrazine and prom-
etryn with FA or NOM a modest increase of retention in comparison
to the pure triazine solutions was recorded In particular depending
on the membrane used the presence of FA resulted in an increase
of 8ndash15 of atrazine retention and 4ndash14 of prometryn retention
whereas nanofiltration in the presence of NOM led to 12ndash17 and
3ndash15 increase of atrazine and prometryn retention respectively
The interactions between atrazine and prometryn with the
humic substances observed in the GPC experiments seem to agree
(up to a point) with the trends in triazine retention by the three
NFULPRO membranes Indeed the negligible interactions observed
between the two herbicides and the humic acids are in accord with
the slight differences observed in atrazine and prometryn retention
by the NF270 and NF90 membranes On the other hand the some-
what reduced retention observed in the case of XLE membrane
may be not only due to the weak interactions between triazines
and humic acids but also to the hydrophobic interactions between
HA and the membrane surface which tends to affect atrazine and
prometryn retention This is in accord with the observed higher
adsorption rates of HA on the XLE membrane (Table 8) which in turn
may affect the surface properties of the membrane and therefore
triazine retention
The GPC results in the case of FA and NOM are in accord with
the retention results of triazines for all three membranes FA and
NOM interact with the two herbicides to form complexes of various
stabilities The formation of these pseudo-complexes may explain
the higher retention of atrazine and prometryn as a result of the
94 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
Table 6
Average retention values of atrazine and prometryn at two different humic substance concentrations by the three NFULPRO membranes
Humic substance Cfeed (mg Lminus1) NF270 NF90 XLE
Ratrazine () Rprometryn () Ratrazine () Rprometryn () Ratrazine () Rprometryn ()
HA 5 782 804 873 883 83 852
10 776 798 838 846 769 744
FA 5 784 829 828 870 848 913
10 816 842 937 933 906 977
NOM 5 813 859 843 838 879 922
10 862 901 894 857 956 968
TA 5 898 880 876 903 905 963
10 969 976 96 957 100 100
following mechanisms that may take place (a) increase of the
steric congestion or reduction of the diffusivity of the FAndashtriazine
or NOMndashtriazine pseudo-complex (b) increase of the density of
the pseudo-complexrsquos negative charge which enhances the elec-
trostatic repulsion with the negatively charged membranes and (c)
increase of the adsorption capability of the pseudo-complex on the
surface of the membrane in contrast to the hydrophobic nature of
the humic materials The validity of the above mechanism hypothe-
ses adopted by previous researchers [7] depends on the type of
the organic material and consequently on the characteristic chem-
ical properties of both HS (acidity functional groups) and triazines
(basicity hydrophobicity charge) The correlation between humic
material properties and triazine retention is examined in a subse-
quent paragraph
The greater extent of FAndashtriazines pseudo-complexes in com-
parison to NOMndashtriazines pseudo-complexes manifested in the
GPC results is not accompanied by a greater increase of triazine
retention More specifically the increasing order of triazine reten-
tion in the presence of a specific humic material follows the order
TA gt NOM gt FA gt HA in the case of NF270 and XLE membranes
whereas for NF90 the order is TA ge FA gt NOM gt HA Therefore the
interactions between the humic materials and triazines alone
cannot explain the retention performance of the NFULPRO mem-
branes This fact indicates that interactions between the naturally
occurring organic substances and the membranes may also play a
significant role in triazine retention as previously observed [33] In
view of these observations dead-end filtration tests using the three
IHSS organic materials and the selected NFULPRO membranes are
carried out in this laboratory in order to distinguish the effects of
organic fouling and of HSndashtriazines interactions on triazine perme-
ation during nanofiltration
323 Effect of humic substance concentration on triazine
retention
Surveying the literature on the complicated influence of organic
matter on retention of pesticides by NFRO membranes reveals a
considerable variability regarding the importance of the organic
matter concentration Experiments in a nanofiltration pilot unit on
the removal of atrazine and simazine have demonstrated the sig-
nificance of the organic load in the feed water since the removal
efficiency increased from 50 to 90ndash100 when dissolved organic
carbon varied between 04 and 36 mg Lminus1 [7] In contrast Berg et
al [8] reported no increase of pesticide retention (atrazine and
simazine among them) by several NF membranes with increas-
ing organic matter concentration Along the same lines Zhang et
al [11] showed comparable simazine retention between tap water
and river water although the latter had a high concentration of
NOM whereas tap water had a low NOM concentration In an effort
to understand the reasons for the above inconsistency in the lit-
erature Nghiem et al [40] took into consideration the effect of
ionic strength Experiments with hormone mimicking trace organic
contaminants revealed the significance of solution ionic strength
in influencing NOMndashtrace organic interactions which is probably
the cause for the ambiguity of literature data regarding the role
of background organic matter in trace organic rejection The mag-
nitude of ionic strength in terms of calcium ion content on the
NOMndashtriazines interaction has been also recorded through the GPC
tests described previously and is subsequently examined Although
ionic strength can strongly influence the HSndashtriazines interactions
the effect of the type of the humic substance seems to play also a
significant role on triazine retention This is evident in the results
included in Table 6 regarding retention experiments in the pres-
ence of a medium (5 mg Lminus1) and relatively high (10 mg Lminus1) HS
concentration in the absence of ionic environment
Table 6 shows a clear increase of triazine retention with increas-
ing humic material concentration More specifically by doubling
the concentration of FA NOM and TA in the feed solutions atrazine
and prometryn retention increases 4ndash13 and 2ndash10 respectively
depending on the membrane and the humic material used An
exception is observed again in the case of humic acids since the
increased organic load results in negligible or slightly negative
effects of atrazine and prometryn retention Regarding the XLE
membrane the use of medium HA concentration results in similar
retention performance as in organic-free solutions On the other
hand higher HA concentration results in a notable reduction of
triazine retention ranging up to 74 and 127 for atrazine and prom-
etryn respectively As previously discussed this is attributed to the
increased HA adsorption on the membrane surface (Table 8) which
in turn affects triazine adsorption on XLE membrane and therefore
retention This trend is not observed in the case of FA NOM and
TA despite their adsorptive capacity on the membrane materials
This finding is probably related to fouling phenomena which tend
to modify the membrane properties (ie surface charge hydropho-
bicity morphology) and therefore triazine adsorption Although it
is difficult to use mass balances for calculating triazine adsorption
during the combined filtration with humic substances (due to ana-
lytical problems related to the concentrate samples) the changes
of triazine concentration in the permeate samples provide a good
indication of the potential adsorption of atrazine and prometryn
on the membrane surface Furthermore the increased passage of
triazines in the permeate side as a result of HA adsorption on the
XLE membrane is in accord with the increase of triazine adsorption
on the membrane and therefore with the reduced retention values
324 Effect of calcium ion on triazine retention
From Fig 5 it is evident that the presence of calcium acts
positively in both atrazine and prometryn retention during the
combined nanofiltration with all four types of humic substances
Furthermore the higher interactions between the humic organ-
ics and triazines manifested in the GPC tests are in agreement
with the increased retention performance of atrazine and prom-
etryn by the three membranes used Additionally the suggestion
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 95
of ligand exchange in which calcium serves as a bridging ion and
acts as a possible site for higher adsorption of triazine molecules
on humic substances [38] seems to be in accord with the forma-
tion of an increased number of pseudo-complexes between humic
matter and triazines which explain the increase of herbicide reten-
tion
Among the four HS used tannic acid is associated with the high-
est triazine retention since both triazines are fully retained in the
presence of calcium by all three membranes tested Moreover the
enhanced interaction between humic acids and triazines as a result
of the calcium addition to the feed solutions is accompanied by
a significant increase of atrazine and prometryn retention In this
case calcium ions favour the binding between HA and triazines
which in other cases would not have taken place this is in agree-
ment with the preceding GPC and NFULPRO results
Prometryn exhibits again the highest retention values in com-
parison to atrazine which may be attributed to the relatively higher
prometryn hydrophobicity and basicity (Table 1) The former prop-
erty likely favours hydrophobic interactions between prometryn
and the HSndashcalcium complexes whereas the latter renders prom-
etryn highly effective in mechanisms involving electron-transfer
with the HSndashcalcium complexes [19] This mechanism is due to the
lower acidic functional group content of the HS as a result of the
complexation with calcium ions in the feed solutions Sposito et
al [19] suggested that atrazine itself does not readily participate
in electron-transfer reactions with humic substances However
they demonstrated that hydroxyl-atrazine does react through an
electron-transfer mechanism with HA and FA It is noted that
atrazine is readily converted to hydroxyl-atrazine even in labo-
ratory samples at low water content and this may explain the
presence of some of the electron transfer products detected in stud-
ies of atrazinendashHA interactions [41]
Surveying the literature however it is noted that some stud-
ies either with dialysis membranes [1213] or NF membranes [9]
have shown reduced values of atrazine retention when divalent
calcium is present together with natural organic matter includ-
ing the NOM surrogate tannic acid According to Devitt et al [9]
Devitt and Wiesner [12] this is due to the reduced association of
atrazine and NOM as a result of the occupation of interaction sites
by calcium andor the reduced access of atrazine to NOM sites due
to changes in molecular conformation The GPC results in this study
have shown that this is not the case since the presence of calcium
tends to increase the interaction of humic substances with triazine
compounds like atrazine
The conflicting results between the previous studies and the
present work may be attributed to the different types of membranes
and filtration techniques used More specifically the use of cellulose
ester membranes as well as the experimentation with batch dialy-
sis systems where concentration and osmotic pressure difference
serve as the driving force for solute transport (ie in the absence of
hydrodynamic forces as in the present study) may explain the dif-
ferences regarding the calcium effect on triazine retention Runge
et al [42] claim that the SpectraPor cellulose ester membranes
used in their studies (having a molecular weight cut-off of 100 Da)
Fig 6 Retention of atrazine (a) and prometryn (b) as a function of humic material total aciditymdashcalculations based on the values presented in Table 2 taking into consideration
the carbon composition and the HS concentration
Fig 7 Retention of atrazine (a) and prometryn (b) as a function of humic material carboxylic aciditymdashcalculations based on the values presented in Table 2 taking into
consideration the carbon composition and the HS concentration
96 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
present a significant incompatibility with distilled water whereas
a significant membrane potential can build up with certain salts
which can inhibit or enhance diffusion depending on the size of
the solutes diffusing and the polarity of the membrane potential
generated In this case an increased diffusion of hydrated calcium
ions to the permeate side in order to balance the concentrations
between the two sides of the cellulose ester membrane may result
in a pore blockage or a positive charge on the membrane surface
which in turn could lead to an increased passage of free dissolved
atrazine to the permeate side
Another possible explanation for this inconsistency is the time
period of the filtration tests According to the protocol followed
by previous researchers [1213] the dialysis time period ranged
between 1 and 5 days at the end of which concentration measure-
ments were performed On the other hand the filtration periods
in the case of the present dead-end nanofiltration experiments
ranged between 2 and 5 h depending on the membrane used To
this period one more hour should be added in which the feed solu-
tion was stirred in the cell without pressurization Even though
triazine adsorption on the membrane surface was observed to sta-
bilize within the relatively short period of nanofiltration there is
a possibility that longer filtration periods may have resulted in
higher adsorption rates which in turn could lead to greater pas-
sage of triazine to the permeate side and therefore to reduced
retention values This explanation is in agreement with the pilot
tests performed by Boussahel et al [43] who report that the
increasing adsorption of humic matter on the membranes leads
to an increase of pesticide adsorption on the membranes which
facilitates the diffusion of pesticides in the direction of perme-
ate
33 Correlation between humic material properties and triazine
retention
The correlation of atrazine and prometryn retention with the
total carboxylic and phenolic acidity of the humic substances used
in the present study as well as the role of the molecular mass of HS
on triazine retention are depicted in Figs 6ndash9 These are results of
nanofiltration experiments performed in the absence of calcium
Triazine removal by the three NFULPRO membranes seems
to poorly correlate with the total acidity of the humic material
This is evident especially in the case of NF270 and NF90 mem-
branes (Fig 6) The XLE membrane on the other hand presents a
reduced tendency for atrazine and prometryn removal when acid-
ity increases This is attributed to the characteristic low negative
charge of the XLE membrane surface which causes weak repulsions
with the negatively charged humic organics This leads to a greater
adsorption of humic compounds on the membrane surface (Table 8)
and consequently to greater triazine adsorption which favours the
diffusion of the triazines to the permeate side On the other hand
the strong negative charge of the NF270 and NF90 membranes [20]
promotes a strong repulsion between HS and the membranes The
greater the negative charge of the humic organic the higher the
repulsion which in turn results in higher concentration of HS in
the bulk Consequently the interactions between HS and triazines
in the bulk are increased leading to an improved triazine removal
Fig 8 Retention of atrazine (a) and prometryn (b) as a function of humic material phenolic aciditymdashcalculations based on the values presented in Table 2 taking into
consideration the carbon composition and the HS concentration
Fig 9 Retention of atrazine (a) and prometryn (b) as a function of humic substance molecular mass (based on the values presented in Table 3)
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 97
This is more evident in the case of the wide pore NF270 membrane
since the NF90 membrane seems to retain atrazine and prometryn
to a higher degree in the presence of TA which is characterized by
a lower total acidity among the four HS used in this study
Humic compounds contain a variety of functional groups
such as aliphatic and aromatic carboxyls phenolic hydroxyls
alcoholic hydroxyls carbonyls quinones methoxyls and amino
groups These functional groups are the reactive sites in the humic
molecules As carboxylate protonation seems to be critical for the
capacity of triazine binding [44] it is of interest to correlate the
effect of carboxylic acidity on herbicide removal by nanofiltration
The effect of carboxylic acidity on atrazine and prometryn retention
by the three NFULPRO membranes is depicted in Fig 7
The results show a negative trend of triazine retention with
an increased carboxylic acidity of humic substances This is clear
in the case of NF270 and XLE membranes while NF90 does not
seem to be significantly affected by the changes in the number of
carboxyl groups of a humic molecule Remarkably the smaller TA
molecules containing fewer carboxyl groups per unit mass (Table 2)
exhibit the highest triazine retention (sim100) as a result of the even
higher triazine binding capacity in contrast to the IHSS organics
which contain a higher carboxylic group content Moreover experi-
ments in batch filtration mode result in HS concentration increasing
with time which in turn leads to a reduced number of carboxylic
binding sites available to the two triazines This is probably due
to the self ldquocompetingrdquo character of the HS used since an increase
in concentration andor HS aggregation tend to reduce the avail-
able binding sites [44] This phenomenon led Wang et al [44] to
propose a model in which hydrogen bonding was considered to be
the primary interaction resulting in triazine binding Therefore the
increase in triazine retention observed in the case of elevated HS
concentration may not be directly related with the carboxylic acid-
ity of the HS but rather with other mechanisms such as hydrogen
bonding
Regarding the phenolic acidity of the humic substances there
does not seem to be a sufficient correlation with atrazine or prom-
etryn retention by the three NFULPRO membranes (Fig 8) Actually
there is a contradictory behaviour between the three IHSS organ-
ics and the NOM surrogate TA The high phenolic content of TA
is related to high retention values of the two triazines while
the smaller and relatively similar phenolic acidity of the IHSS
compounds leads to smaller and different retention values These
results imply that there are other factors influencing the association
between humic organics and herbicides and therefore their reten-
tion by nanofiltration In this respect the molecular structure of the
humic materials as well as the interactions taking place between
the organics and the membranes seem to explain the above incon-
sistencies This is evident in the results depicted in Fig 9 and the
adsorption of humic substances subsequently described
The rather strong relation of triazine retention with the molec-
ular mass of the organics used in this study (Fig 9) leads to the
suggestion that structural features of HS are important for the
removal capacity of triazines Up to now there is no consensus
concerning both the binding mechanism of triazines to humic sub-
stances and the structural features responsible for this binding
Nevertheless the preferential binding with low molecular weight
fractions of humic compounds (especially of FA and TA) deduced
from the GPC and filtration tests results in higher retention val-
Table 7
Humic substance and calcium retention performance of the three NFULPRO membranes (mean retention values for 50 recovery of feed volume as permeate)
Membrane Humic substancea Humic substances retention () Ca2+ retention ()
HSs alone HSs with triazines HSs with triazines and Ca2+ In the presence of HSs
NF270 HA 990 994 1000 949
FA 967 983 991 951
NOM 957 997 1000 951
TA 972 992 997 957
NF90 HA 972 980 1000 994
FA 953 984 1000 990
NOM 985 993 1000 991
TA 967 998 998 994
XLE HA 992 996 1000 994
FA 862 915 994 997
NOM 976 1000 1000 988
TA 923 945 1000 996
a HA FA NOM and TA designate the four humic substances used in this study
Table 8
Adsorption data of humic substances on tested membranes
Membrane Humic substancea Feed mass of humic substance (g) Quantity adsorbed (g) Adsorption ()
Alone With Ca2+ Alone With Ca2+ Alone With Ca2+
NF270 HA 3480 2630 244 2314 07 88
FA 3570 2840 71 1335 02 47
NOM 3390 2660 34 2633 01 99
TA 4050 3300 2025 7161 50 217
NF90 HA 3220 2690 354 3658 11 136
FA 2930 2450 1494 857 51 35
NOM 3000 2910 120 437 04 15
TA 2820 2730 649 3631 23 133
XLE HA 3100 2680 1643 3457 53 129
FA 3050 2810 1830 2866 60 102
NOM 2650 2770 610 1219 23 44
TA 3120 2940 2184 2970 70 141
a HA FA NOM and TA designate the four humic substances used in this study
98 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
ues for the two triazines Thus the interaction with small branched
molecules like FA and TA seems to favour a physical entrapment
mechanism such as the one suggested by Devitt and Wiesner [12]
34 Retention of humic species and calcium ion by the NFULPRO
membranes
The performance of nanofiltration on the humic substances and
calcium removal is summarized in Table 7 Regarding humic sub-
stances the retention is compared for three different feed solutions
(a) solutions of HS alone (b) solutions of HS with the two triazines
and (c) solutions of HS with the two triazines in the presence of cal-
cium In general HS retention is high for all three membranes used
According to the UV measurements IHSS HA exhibits the highest
retention values followed by NOM TA and FA which is apparently
related to their relative molecular masses (Table 3) Complexation
or interaction with triazines as evidenced by the GPC tests appar-
ently plays also a significant role in the relative permeability of the
organic species More specifically the retention of the four organic
compounds examined is increased in the presence of the two her-
bicides which verifies also the binding of atrazine and prometryn
with the four humic substances Thus a comparison between the
retention values of the HS in the absence or presence of triazines
gives also a good insight into the HSndashtriazine interactions The slight
increase (lt1) of HA retention in the presence of the two triazines
is in accord with the relatively low HA binding capacity observed
in the GPC tests On the other hand the increase of 2ndash6 in FA
NOM and TA retention is in agreement with the observed increased
interactions between those substances and the two triazines
In the case of calcium addition in the organicndashtriazine solu-
tions the UV absorbing species of the four organics examined are
retained almost completely by all three membranes used This is
again in agreement with the results obtained with gel chromatogra-
phy According to the literature [12] the addition of calcium results
in charge shielding and neutralization of the organic matter charged
functional groups and can shrink the humic matrix The formation
of aggregates of larger apparent molecular weight due to the sub-
stantial increase in the hydrophobicity of the organic molecules
tends to reduce their permeability through membranes and there-
fore to increase their retention
Apart from the increased retention of triazines and humic sub-
stances there is also a significant improvement of calcium rejection
when the latter is present in the solution In comparison to pure
water solutions of elevated calcium ion content [20] the rejection of
calcium in humic solutions is almost complete This is attributed to
size exclusion mechanisms related to the binding tendency of diva-
lent ions like calcium to the negatively charged humic organics
Among the four humic substances used calcium is rejected more in
the presence of the polyphenol tannic acid which is accompanied
by a higher organic adsorption on all three NFULPRO membranes
tested (Table 8)
From the data summarized in Table 8 it can be seen that the
adsorption of humic substances onto the membrane surfaces is
markedly enhanced in the presence of calcium ions due to the
formation of HA-complexed divalent cations This is also clearly
depicted in the SEM images of virgin and fouled membranes in
the absence or presence of calcium ions Characteristic SEM images
in the case of the Suwannee River NOM filtration with the three
NFULPRO membranes are included in Fig 10 these images show
the severe fouling on the three NFULPRO membranes as a result
of the combined filtration of NOM with CaCl2 salt It is possible
that the divalent cations such as Ca2+ act as a bridge between the
membrane surface and the negatively charged humic molecules
that tend to be repelled by the negatively charged membrane As a
result the electrical repulsion between the membrane surface and
the humic molecules is weakened thus rendering the membrane
Fig 10 SEM images of virgin fouled with NOM and fouled with NOM + calcium membranes (a) NF270 (b) NF90 and (c) XLE membrane
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 99
fouling process more severe [45] On the other hand the relatively
smaller adsorption rates of NOM in the absence of calcium result
in less fouled membranes A similar trend is also observed in the
case of the other three humic substances used in this study Among
the three membranes used XLE exhibits in the absence of calcium
the highest adsorption rates for all four humic substances This is
explained by the relatively high hydrophobicity and the low surface
charge that characterize the XLE membrane surface as previously
explained
4 Concluding remarks
The influence of humic substances on retention of two tri-
azines (atrazine and prometryn) by NFULPRO membranes is rather
complicated and involves combinations of solutendashmembrane and
solutendashsolute interactions In summary the retention of triazines
depends on the type of the humic material present in the feed
water In most cases the combined nanofiltration of triazines and
naturally occurring humic substances facilitates the formation of
complexes with triazines which in turn enhance their removal by
means of membrane filtration This complexation appears to be
related not to the characteristic acidity (phenolic carboxylic) of
the HS used but rather to their molecular conformation More
specifically a preferential binding is observed between triazines
and low molecular weight fractions of humic compounds (espe-
cially of FA and TA) which results in higher retention values for the
two triazines Under all conditions tannic acid exhibits the greatest
effect on triazine retention among the four standard HS compounds
used leading to an almost complete removal of the two triazines
(95ndash100) for all three membranes tested
The removal of triazines is improved in the presence of calcium
which tends to enhance the interaction between HS and triazines
Moreover triazine retention is increased with increasing HS con-
centration to a degree depending on the type of HS Additionally
triazine retention is apparently also affected by the interactions
between the organics and the membrane surface which in turn
may alter triazine retention These results illustrate the complex-
ity of the triazine adsorption and retention mechanisms in the
presence of humic substances that needs further investigation for
improved understanding Moreover regarding triazine removal by
nanofiltration it is of particular interest to distinguish the effect
of the HSndashtriazine interactions from that of membrane HS fouling
Research in this direction is carried out in this laboratory
References
[1] DP Birardar AL Rayburn Chromosomal damage induced by herbicide con-tamination at concentrations observed in public water supplies J Environ Qual24 (1995) 1222ndash1225
[2] IK Konstantinou DG Hela TA Albanis The status of pesticide pollution insurface waters (rivers and lakes) of Greece Part I Review on occurrence andlevels Environ Pollut 141 (3) (2006) 555ndash570
[3] European Environment Agency (EEA) Groundwater quality and quantity inEurope Report Copenhagen June 1999 available in httpreportseeaeuropaeugroundwater07012000enenviassrp199903
[4] EM Thurman DA Goolsby MT Meyer MS Mills ML Pomes DW Kolpin AReconnaissance study of herbicides and their metabolites in surface water of theMidwestern United States using immunoassay and gas chromatographymassspectrometry Environ Sci Technol 26 (1992) 2440ndash2447
[5] C Bellona JE Drewes P Xu G Amy Factors affecting the rejection of organicsolutes during NFRO treatmentmdasha literature review Water Res 38 (2004)2795ndash2809
[6] A Bhattacharya Remediation of pesticide-polluted waters through mem-branes Sep Purif Rev 35 (2006) 1ndash38
[7] KM Agbekodo B Legube S Dard Atrazine and simazine removal mechanismsby nanofiltration influence of natural organic matter concentration Water Res30 (1996) 2535ndash2542
[8] P Berg G Hagmeyer R Gimbel Removal of pesticides and other micropollu-tants by nanofiltration Desalination 113 (1997) 205ndash208
[9] EC Devitt F Ducellier P Coumlteacute MR Wiesner Effects of natural organic mat-ter and the raw water matrix on the rejection of atrazine by pressure-drivenmembranes Water Res 32 (1998) 2563ndash2568
[10] R Boussahel S Bouland KM Moussaoui A Montiel Removal of pesticideresidues in water using the nanofiltration process Desalination 132 (2000)205ndash209
[11] Y Zhang B Van der Bruggen GX Chen L Braeken C Vandecasteele Removalof pesticides by nanofiltration effect of the water matrix Sep Purif Technol38 (2004) 163ndash172
[12] EC Devitt MR Wiesner Dialysis investigations of atrazinendashorganic matterinteractions and the role of a divalent metal Environ Sci Technol 32 (1998)232ndash237
[13] SK Dalton JA Brant MR Wiesner Chemical interactions between dissolvedorganic matter and low-molecular weight organic compounds impacts onmembrane separation J Membr Sci 266 (2005) 30ndash39
[14] NA Kulikova IV Perminova Binding of atrazine to humic substances fromsoil peat and coal related to their structure Environ Sci Technol 36 (2002)3720ndash3724
[15] RL Wershaw The importance of humic substancendashmineral particle complexesin the modeling of contaminant transport in sedimentndashwater systems in RABaker (Ed) Organic Substances and Sediments in Water Humics and SoilsLewis Publishers Chelsea MI 1991 pp 23ndash34
[16] SM Schrap A Opperhuizen Relationship between bioavailability andhydrophobicity reduction of the uptake of organic chemicals by fish due tothe sorption on particles Environ Toxicol Chem 9 (1990) 715ndash724
[17] E Barriuso M Schiavon F Andreux JM Portal Localization of atrazine non-extractable (bound) residues in soil size fractions Chemosphere 12 (1991)1131ndash1140
[18] L Loiseau E Barriuso Characterization of the atrazinersquos bound (nonextractable)residues using fractionation techniques for soil organic matter Environ SciTechnol 36 (2002) 683ndash689
[19] G Sposito L Martin-Neto A Yang Atrazine complexation by soil humic acidsJ Environ Qual 25 (1996) 1203ndash1209
[20] KV Plakas AJ Karabelas Membrane retention of herbicides from single andmulti-solute media the effect of ionic environment J Membr Sci 320 (2008)325ndash334
[21] V Freger J Gilron S Belfer TFC polyamide membranes modified by graftingof hydrophilic polymers an FT-IRAFMTEM study J Membr Sci 209 (2002)283ndash292
[22] P Xu JE Drewes Viability of nanofiltration and ultra-low pressure reverseosmosis membranes for multi-beneficial use of methane produced water SepPurif Technol 52 (2006) 67ndash76
[23] Z Knez A Rizner-Hras K Kokot D Bauman Solubility of some solid triazineherbicides in supercritical carbon dioxide Fluid Phase Equilibria 152 (1998)95ndash108
[24] Weed Science Society of America (WSSA) Herbicide Handbook 7th ed 1994[25] B Van der Bruggen J Schaep W Maes D Wilms C Vandecasteele Nanofiltra-
tion as a treatment method for the removal of pesticides from ground watersDesalination 117 (1998) 139ndash147
[26] KN Reddy MA Locke Molecular properties as descriptors of octanol-waterpartition coefficients of herbicides Water Air Soil Pollut 86 (1996) 389ndash405
[27] IHSS website httpwwwihssgatechedu[28] F Flores-Ceacutespedes M Fernaacutendez-Peacuterez M Villafranca-Saacutenchez E Gonzaacutelez-
Pradas Cosorption study of organic pollutants and dissolved organic matter ina soil Environ Pollut 142 (2006) 449ndash456
[29] R Beckett Z Jue JC Giddings Determination of molecular weight distribu-tions of fulvic and humic acids using flow field-flow fractionation Environ SciTechnol 21 (1987) 289ndash295
[30] PM Reid AE Wilkinson E Tipping MN Jones Determination of molecularweights of humic substances by analytical (UV scanning) ultracentrifugationGeochim Cosmochim Acta 54 (1990) 131ndash138
[31] S Lee B Kwon M Sun J Cho Characterizations of NOM included in NF and UFmembrane permeates Desalination 173 (2005) 131ndash142
[32] MN Jones ND Bryan Colloidal properties of humic substances Adv ColloidInterf Sci 78 (1998) 1ndash48
[33] KV Plakas AJ Karabelas T Wintgens T Melin A study of selected herbicidesretention by nanofiltration membranesmdashthe role of organic fouling J MembrSci 284 (2006) 291ndash300
[34] APHA-AWWA-WPCF Standard Methods for the Examination of Water andWastewater 17th ed Port City Press Baltimore MD 1989 pp 567ndash569
[35] R Artinger G Buckau JI Kim S Geyer Characterization of groundwater humicand fulvic acids of different origin by GPC with UVVis and fluorescence detec-tion Fresenius J Anal Chem 364 (1999) 737ndash745
[36] AI Schaumlfer N Andritsos AJ Karabelas EMV Hoek R Schneider M Nys-troumlm Chapter 8 Fouling in nanofiltration in AI Schaumlfer AG Fane TDWaite (Eds) Nanofiltration Principles and Applications Elsevier Oxford UK2005 pp 173ndash174
[37] I Franco L Catalano M Contin M De Nobili Interaction between organicmodel compounds and pesticides with water-soluble soil humic substancesActa Hydrochim Hydrobiol 29 (2001) 88ndash99
[38] AA Helal DM Imam SM Khalifa HF Aly Interaction of pesticides with humiccompounds and their metal complexes Radiochemistry 48 (4) (2006) 419ndash425
[39] MG Browman G Chester Fate of Pollutants in the Air and Water EnvironmentWiley New York 1977 p 50
[40] LD Nghiem AI Schaefer M Elimelech Nanofiltration of hormone mimickingtrace organic contaminants Sep Sci Technol 40 (2005) 2633ndash2649
[41] R Celis J Cornejo MC Hermosin WC Koskinen Sorptionndashdesorption ofatrazine and simazine by model soil colloidal components Soil Sci Soc AmJ 61 (1997) 436ndash443
100 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
[42] SW Runge KR Shelton SA Melton WM Moran Maintaining the ionic per-meability of a cellulose ester membrane J Biochem Biophys Methods 64(2005) 200ndash206
[43] R Boussahel A Montiel M Baudu Effects of organic and inorganic matter onpesticide rejection by nanofiltration Desalination 145 (2002) 109ndash114
[44] Z Wang DS Gamble CH Langford Interaction of atrazine with Laurentianhumic acid Anal Chim Acta 244 (1991) 135ndash143
[45] C Jucker MM Clark Adsorption of aquatic humic substances on hydrophobicultrafiltration membranes J Membr Sci 97 (1994) 37ndash52
88 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
Table 1
Properties of the herbicides used in this work [24]
Herbicide Atrazine Prometryn
Chemical structure
Molecular formula C8H14ClN5 C10H19N5S
Chemical class Cl-triazine S-triazine
Molecular weight (Da) 21569 24135
Molecular sizea (nm) 0788 ndash
Log Kow 268 308
Aqueous solubility (mg Lminus1) 33 33
Dipole momentb (debye) 2460 ndash
pKa (20 C) 17 409
a Obtained from Ref [25]b Obtained from Ref [26]
Table 2
Acidity and elemental composition of HS used in this study [27]
Humic substance Cat No Acidity (mequiv gminus1 C) Elemental composition
Carboxylic Phenolic Total Ca H2Ob
HA IHSS 2S101H 913 372 1285 5263 204
FA IHSS 2S101F 1187 284 1401 5234 169
NOM IHSS 1R101N 985 394 1379 4880 815
TAc SigmandashAldrich 188 955 114 5000 58
a Elemental composition in (ww) of a dry ash-free sampleb (ww) of H2O in the air-equilibrated sample (a function of relative humidity)c Values reported in Ref [28]
The three IHSS standards are denoted as Suwannee River Humic
Acid (HA) Suwannee River Fulvic Acid (FA) and Suwannee River
NOM The IHSS humic and fulvic acids contain only hydrophobic
organic acids while the NOM sample contains not only hydropho-
bic and hydrophilic acids but also other soluble substances that
are present in natural waters [27] On the other hand tannic
acid is a polyphenol representative of relatively hydrophilic com-
pounds of medium molar mass It is reported by the supplier to
have a mean molecular mass of 170118 g molminus1 with an empiri-
cal formula C76H52O46 but in fact it contains a mixture of related
compounds The characteristic chemical parameters of the four sub-
stances employed are summarized in Table 2 the average molecular
masses of the IHSS standards reported in the literature are pre-
sented in Table 3
214 Feed solutions
In the filtration experiments three different types of solu-
tions were used ie solutions of atrazinendashprometryn in ultra-
pure water atrazinendashprometryn in a model HS solution and
atrazinendashprometryn in a model HS solution in the presence of
calcium ions The model solutions of the four selected humic
substances were prepared at neutral pH in order to simulate
waters with different chemical characteristics Since humic sub-
stances concentrations in natural waters usually fall in the range
2ndash40 mg Lminus1 [32] all the solutions were prepared with ultra-pure
water and with a concentration of 5ndash10 mg Lminus1 humic substance
The HS were obtained in powder form and used without further
purification as the bound iron and ash contents were very low The
calcium ion content in the corresponding experiments was fixed
at 40 mg Lminus1 by adding calcium chloride (111 mg Lminus1 JT Baker)
In the case of the two triazines separate standard stock solutions
were prepared in high-performance liquid chromatography-grade
methanol and stored at 4 C with concentration 100 mg Lminus1 The
feed triazine solutions were prepared from ultra-pure water by
dilution of stock solutions at a level of 10ndash30 g Lminus1 for each
triazine The ultra-pure water used (resistivity gt 18 M cm) for
Table 3
Molecular mass data of IHSS organics employed in this study
Humic substance Technique [Ref] Mna Mw
b MwMnc
Suwannee River HA FFF [29] 1580 4390 278
Ultracentrifugation [30] ndash 4260 plusmn 280 ndash
Suwannee River FA FFF [29] 1150 1910 166
Ultracentrifugation [30] ndash 1460 plusmn 80 ndash
Suwannee River NOM HPSEC [31] 1760 2360 134
a Number average molecular weightb Weight average molecular weightc Polydispersivity index Suwannee River HA has a much larger polydispersivity compared to FA
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 89
solution preparation was obtained from a Milli-Q purification
system (Millipore Milford MA USA) Similar to the protocol
used by Devitt et al [9] once a model solution of HS and
atrazinendashprometryn was prepared in the presence or not of cal-
cium it was placed in a foil-covered container (to prevent atrazine
and prometryn degradation by exposure to light) and stirred for
more than 24 h after which it was assumed to be in equilibrium
215 Analytical techniques
2151 Herbicide analysis Off-line solid phase extraction (SPE)
with gas chromatography employing a mass-selective detector
(GC-MSD) was used to achieve the sensitivity required for the
tested herbicides The SPE procedure performed prior to the
chromatographic analysis is described in a previous paper [33]
Herbicide analyses were performed in an Agilent Technologies
Model 7890A gas chromatograph system interfaced with an Agilent
Technologies Model 5975C mass-selective detector (MSD) The gas
chromatograph was equipped with a 30 m times 250 m id times 025 m
film thickness fused silica capillary column (Agilent 19091S-
433) The chromatographic conditions were as follows injection
splitless injector temperature 180 C transfer line temperature
220 C injection volume 1 L carrier gas He flow 1 mL minminus1
oven temperature programmed from an initial temperature of 70 C
held for 3 min to 150 C at a rate of 30 C minminus1 held for 1 min next
to 200 C at a rate of 10 C minminus1 and finally to 260 C at a rate of
5 C minminus1 held for 1 min The MSD was operated in the selective ion
monitoring (SIM) mode Three characteristic ions were monitored
for each herbicide analyzed The most abundant ion for each herbi-
cide was used for quantification while the other two ions were used
to confirm the presence of the herbicide The minimum detectable
quantity (MDQ) for the analysis with a 1-L injection volume was
approximately 1 ng for all herbicides analyzed
Simazine (Riedel de-Haeumln purity 999) a triazine compound
similar to atrazine and prometryn was used as internal standard in
order to assess the recovery of the two triazines in the SPE-GC-MSD
analyses With the selected SUPELCO SPE cartridges recoveries
from the model humic substance solutions ranged from 35 to 98
and in general were lower than the recoveries obtained in pure
water or permeate samples of the triazinendashHS filtration (75ndash128)
The analysis of the concentrate samples which were characterized
by high dissolved carbon content resulted in many cases in even
lower recoveries This is attributed to the saturation of the sorption
sites of the SPE cartridges by the humic substances which renders
them inefficient in extracting the two triazines associated with HS
Therefore triazine concentrations were determined only in the feed
and permeate samples
2152 Organic and inorganic matter analysis Measurement of the
HS concentration (and more specifically the aromaticity) was per-
formed in a UVndashvis spectrophotometer (UV-1700 Shimadzu Japan)
at 254 nm in the case of Suwannee River HA and Suwannee River
NOM and at 275 nm in the case of Suwannee River FA A standard
colorimetric method (5550) [34] was adopted for the preparation
of tannic acid samples prior to their spectrophotometric measure-
ment at 700 nm The quality control of the data was achieved by
means of calibration and repeated analyses Specifically two cal-
ibration curves were created for each HS one corresponding to
the permeate samples (standards of similar concentrations to the
one expected in the permeates) and a second one corresponding
to the higher concentration level of the feed and retentate sam-
ples This procedure resulted in high R2 values (gt0999) reflecting
the high accuracy and sensitivity of the UVvis method for all sam-
ples treated The use of two different Quartz cuvettes (1 and 10 cm)
enhanced this accuracy (measurement of absorption in a range
01ndash15 unitsmdashas proposed in the literature) Further the accuracy
of the analytical results was ensured by repeatedly measuring the
concentration of a sample (more than four times) All measure-
ments in the case of higher concentrations (feed and concentrate)
resulted in RSD values smaller than 1 while in the case of the
lower concentrations the RSD values did not exceeded 2
The determination of calcium ion content in the water samples
was performed by an ion selective electrode connected to a 692
pHIon Meter Metrohm (Herisau Switzerland) The conductivity
and pH of the samples were measured by a conductometer 712
and pH meter 744 both by Metrohm
2153 Gel permeation chromatography (GPC) Gel permeation
chromatography (or size exclusion chromatography SEC) is a rela-
tively rapid and convenient method for estimating the partitioning
or binding of organic compounds to humic substances in solution
[35] In particular GPC can give information on the liability of the
interaction itself The solutions tested in the present work were (a)
HS (b) HSndashtriazine (atrazine prometryn or both) (c) HSndashcalcium
and (d) HSndashcalciumndashatrazinendashprometryn All solutions were stirred
for more than 24 h in foil-covered containers prior to their injection
into the GPC column The concentrations of all the materials used
were the same as those applied in the filtration experiments Tannic
acid was excluded from the GPC tests due to the possible interac-
tions that may occur between the chemicals of the colorimetric
method used (folin phenol Na2CO3ndashNa2C4H4O6middot2H2O solution)
and the GPC column which in turn may significantly alter the tannic
acid elution behaviour
The accuracy of the elution profiles was ensured by apply-
ing identical chemical conditions for the GPC investigation
This was achieved by using feed waters of the same source
thus all tested solutions (HS alone HSndashtriazines HAndashcalcium
HAndashcalciumndashtriazines) were prepared from the same HS stock solu-
tion This led to a negligible experimental uncertainty which was
evident in the repeatable GPC measurements (three to four injec-
tions) for each solution Indeed the spectrum of each HS was
very uniform whereas the shapes of the GPCSEC elution curves
recorded at the same wavelength were essentially identical
The GPC tests were carried out in a Shimadzu-10 AVP system The
GPC column was a PL aquagel-OH Mixed 8 m 300 mm times 75 mm
(Polymer Laboratories Essex UK) guarded by a PL aquagel-OH
Guard 8 m 50 mm times 75 mm column PL aquagel-OH columns
are packed with macroporous copolymer beads with extremely
hydrophilic polyhydroxyl functionality offering high resolution
over a wide range of molecular weights (up to 104 kDa) Aqueous
samples (100 L) were pipetted directly onto the gel and passed
through the column Phosphate buffer (01 M KH2PO4 at pH 70
corrected with 2 M NaOH) was used as the eluent with a flow
rate of 1 mL minminus1 Dissolved humic substance (HA FA and NOM)
was measured with a Shimadzu-10 AVP UV detector by monitor-
ing ultraviolet light absorbance at a wavelength of 254 275 and
254 nm respectively Calibration of the column to determine the
exact molecular sizes of the HS was not performed in this work
22 Nanofiltration tests and filtration protocol
Dead-end filtration experiments were performed by using an
experimental set-up consisting of three high pressure stirred cells
described elsewhere [20] The three test cells were operated con-
currently to assess the reproducibility and accuracy of the results
A new membrane coupon was used in each filtration test in order
to avoid employing membranes with adsorbed amounts of humic
materials and herbicides
The filtration protocol involves a sequence of the following six
steps (1) membrane preconditioning with Milli-Q water under
pressure of 5 bar for several hours to ensure that the removal of
preservatives from the new membrane coupon is complete [20]
(2) membrane compaction for 1 h at 10 bar (3) measurement of the
90 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
pure water flux for 30 min at 5 bar (4) filtration of triazines in the
absence of humic materials (ldquofree triazine solutionsrdquo) at 5 bar (5)
filtration of the model HSndashtriazines solution at 5 bar and (6) mea-
surement of the pure water flux for 30 min at 5 bar Compaction
(step 2) is crucial in every membrane filtration protocol as it may
change both the active layer and its support thus affecting the flux
and the rejection properties of the membrane To eliminate this
impact membranes are often subjected to at a higher pressure (here
10 bar) than the operating pressure (5 bar) to ensure flux stability
during experiments [36] Preliminary experiments with the three
membranes tested showed that 1 h of 10 bar pressure application
was more than adequate to achieve membrane stability
All filtration steps were performed with a feed solution of
300 mL while the tests performed with the herbicides were car-
ried out until 150 mL of permeate were collected (50 recovery)
Feed and three permeate samples (50 mL each) were retained for
subsequent analysis in order to determine the evolution of con-
taminant concentration in some cases retentate samples were also
removed for chromatographic analysis Similar to previous studies
[2033] the triazine feed solutions were stirred in the cells for 1 h
without pressure In this way herbicide adsorption on the mem-
brane surface was assumed to reach equilibrium After termination
of the filtration experiments the membrane coupons were dried
for subsequent analysis (SEM) while the test cells were thoroughly
washed with acetone and repeatedly rinsed with Milli-Q water
23 Data treatment
The performance of nanofiltration can be expressed in terms of
the percentage removal of the organic and inorganic substances
The determination of the retention was calculated based on the
method described in our previous study [20] which proved to be
appropriate in the case of batch filtration experiments since the
membrane performance depends on the retentate concentration
which increases with time The concentration of the two triazines
was measured for the feed solution and for all the permeate samples
while retentate samples were removed for subsequent analysis of
the humic and inorganic compounds The retention was evaluated
based on the so-called ldquostablerdquo permeate samples where the water
recovery rate was between 17 and 50 The amount of deposited
humic substance on the membrane surface was determined from
mass balance as follows
A () = 100
(
1 minus
sumj
i=1Vpi
Cpi+ Crj
Vr
Cf Vf
)
(1)
where Cpj Cr Cf are the concentrations of a humic substance in
the permeate sample j retentate and feed respectively while Vpj
Vr Vf are the respective volumes of permeate sample j retentate
and feed Reduction of the pure water flux (FRPW) which may be
considered a measure of the adsorbed humic substances effect on
flux was determined as a percentage of clean water flux Jw before
and after membrane operation as previously described [20]
3 Results and discussion
31 Pure triazine solutions
Prior to the combined filtration of triazines with humic materi-
als pure water solutions with atrazine and prometryn were filtered
at 5 bar in concentrations ranging between 10 and 30 g Lminus1 The
performance of the membranes in terms of triazine retention is
illustrated in Fig 1 The accuracy of experimental results is indi-
cated by the bars which represent the scatter of data obtained from
six replicate experiments for each membrane
Fig 1 Atrazine and prometryn retention from free triazine solutions by the three
selected NFULPRO membranes mean retention values for 50 recovery of feed
volume feed concentration 10ndash30 g Lminus1
As in our previous studies [2033] prometryn the molecule
with the larger molecular weight and size displays the great-
est retention with all three membranes used The performance
of membranes regarding retention of the two triazines follows
the order XLE ge NF90 gt NF270 which is consistent with the results
obtained in our previous work [20] Comparing the experimen-
tal results for double solute mixtures of atrazine and prometryn
in the case of high (600ndash700 g Lminus1) [20] and low feed concen-
trations (10ndash30 g Lminus1) the differences in retention values vary
between 7 and 13 In general the retention results with pure tri-
azine solutions are in agreement with observations made by other
researchers [71125] in that herbicide concentration does not sig-
nificantly affect their retention The fact that the filtration of lower
feed concentrations leads to a slight reduction of triazine retention
(especially in the case of XLE membrane) could be attributed to the
amount of triazines adsorbed on the selected membranes Based
on the mass balances summarized in Table 4 the lower feed tri-
azine concentration is accompanied in general by a slightly lower
adsorption in comparison to the results obtained with higher feed
concentrations [20] something that is more pronounced in the case
of the less tight NF270 membrane
32 Aqueous solutions of mixed humic substances and triazines
321 GPC results on triazinesndashhumic substances interactions
The UVndashvis spectra of fractions collected during GPC chromatog-
raphy of HS alone and HS with atrazine andor prometryn in the
absence or presence of calcium ions are depicted in Figs 2ndash4
The comparison of the chromatographic behaviour of the solu-
tions used in qualitative terms allows an assessment of possible
interactions between humic substances and the two triazines [37]
The elution profiles display a maximum for all three IHSS organ-
ics under the same test conditions at retention time 185ndash195 min
In the case of the low molecular weight triazines the elution pro-
file maximum at a wavelength of 225 nm and concentration of
10 mg Lminus1 (high concentrations used in order to be measurable
by UV absorption) appear at 110 min (data not presented here)
The longer retention time of atrazine and prometryn is due to the
fact that small molecules can penetrate into the pores of the gel
and follow a torturous path through the column On the other
hand large molecules such as humic substances are excluded
from internal pore spaces in the gel and elute faster from the col-
umn Co-elution of small triazine molecules with the much larger
humic molecules indicates their association with the humic frac-
tion Humic-bound triazines appear to display the behaviour of the
larger humic molecules as they are also excluded from the internal
pores of the gel [37] This is reflected in the reduced GPC areas of
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 91
Table 4
Adsorption data of triazines on tested membranes
Membrane Feed mass of
triazinesa (g)
Quantity adsorbed
(g)
adsorption adsorption
[19]b
NF270 45 plusmn 27(A) 048 plusmn 027 127 plusmn 18 165
53 plusmn 28(P) 020 plusmn 008 38 plusmn 13 275
NF90 52 plusmn 28(A) 122 plusmn 083 248 plusmn 66 305
55 plusmn 13(P) 105 plusmn 033 250 plusmn 67 262
XLE 66 plusmn 10(A) 114 plusmn 012 219 plusmn 12 230
74 plusmn 11(P) 084 plusmn 008 171 plusmn 09 295
a (A) and (P) designate atrazine and prometryn respectivelyb Adsorption values for higher triazine (atrazine and prometryn) concentrations (600ndash700 g Lminus1 each)
Fig 2 GPC elution profiles of Suwannee River humic acid (HA) in the absence or presence of triazines andor calcium (detection by UV absorption at 254 nm)
the humic materials when treated together with the two triazines
(Table 5) In summary aquatic humic substances from the same
water source (Suwannee River) show differences in their GPC elu-
tion behaviour when treated together with calcium andor with
either of the two triazines
The peaks of the GPC chromatograms reflect differences in spe-
cific absorption Distinct differences between HS and atrazine or
prometryn are found in the case of FA and NOM In contrast HA
show a rather small variation on elution behaviour This is also in
accord with the values presented in Table 5 Complexation with
the NOM and FA appears to occur for both atrazine and prometryn
molecules the interactions with NOM indicate greater affinity to
atrazine On the other hand FA interactions appear to be stronger
with prometryn In Figs 2ndash4 and Table 5 one can observe that
among the three HS used FA seems to interact more with the two
triazines The weakest interaction is observed in the case of humic
acids which is reflected in the almost identical elution profiles and
the low GPC reduction areas
The presence of calcium in the solutions of humic substances
leads to a sharp reduction of HS concentrations (Table 5) indi-
cating the formation of calcium humate and fulvate complexes
According to the literature when negatively charged humic colloids
Table 5
Percentage () reduction of GPC integration areas of humic substances due to the presence of triazines andor calcium
Humic substances combined with Percentage GPC area reduction ()
HA FA NOM
40 mg Lminus1 Ca2+ 26 17 138
10 g Lminus1 atrazine 06 64 27
10 g Lminus1 prometryn 02 81 lt01
10 g Lminus1 atrazine + 10 g Lminus1 prometryn 23 80 30
10 g Lminus1 atrazine + 10 g Lminus1 prometryn + 40 mg Lminus1 Ca2+ 151 221 279
92 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
Fig 3 GPC elution profiles of Suwannee River fulvic acid (FA) in the absence or presence of triazines andor calcium (detection by UV absorption at 275 nm)
come in contact with positively charged metal ions (like calcium)
several binding mechanisms become effective The bond strengths
vary from a weak physical adsorption to strong chemical bond-
ing involving chelation reactions [38] In the case of herbicides
the partially chelated calcium may serve here as a bridging ion
and act as a possible site for the adsorption of a triazine molecule
by displacing water of hydration Indeed the addition of calcium
enhances the interaction between the humic substances and the
two triazines something that is manifested by the higher GPC area
reductions and the lower elution profiles Browman and Chester
[39] also concluded that cations on the surface of humic material
can act as binding bridges between synthetic chemicals and humic
compounds Similarly Helal et al [38] reported that the exchange
reaction between the pesticides and the humic compounds takes
place on the same sites at which binding of humic compound to the
metal occurs Moreover these authors claim that the pesticidendashHA
Fig 4 GPC elution profiles of Suwannee River NOM in the absence or presence of triazines andor calcium (detection by UV absorption at 254 nm)
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 93
Fig 5 Retention of atrazine (A) and prometryn (P) by the three NFULPRO membranes in the absence or presence of humic substances (HA FA NOM TA) andor calcium
ions
or ndashFA complex is stronger than the metalndashhumate or ndashfulvate
complex The mechanisms involved in the interactions between
triazines and calciumndashHS complexes may be described by either
a physical adsorption of the herbicide on the surface of metal
humatefulvate complex or a ligand exchange in which the metal
serves as a bridging ion and acts as a possible site for adsorption of
the herbicide molecule by displacing water of hydration [38]
322 Effect of different types of humic materials on triazine
retention
Retention results of atrazine and prometryn during the com-
bined nanofiltration with the four selected humic substances are
depicted in Fig 5 Results of triazine retention from organic solu-
tions with elevated calcium ion content are also included in Fig 5
and discussed in Section 324 Depending on the type of humic
material present increased retention of the two triazines may be
negligible or significant Specifically the use of HA results in reten-
tion values that are comparable to those obtained in the absence
of organics An exception is observed in the case of XLE membrane
where the combined filtration of triazines and HA leads to relatively
smaller retention values (7 and 13 for atrazine and prometryn
respectively) On the other hand the presence of tannic acid results
in a significant increase of triazine retention for all three mem-
branes tested In particular the addition of TA at a concentration of
10 mg Lminus1 (yielding a triazineTA ratio of 1 g1 mg) results in 14ndash21
and 20ndash29 increase of prometryn and atrazine retention respec-
tively The enhancement of pesticide removal in the presence of
the NOM surrogate tannic acid has been also observed by previ-
ous researchers [91213] According to Devitt and Wiesner [12] the
increased retention of a triazine molecule like atrazine in the pres-
ence of TA is due to the branched structure of TA which traps the
triazine within the macromolecule Moreover these authors sug-
gest that molecular conformations and weak interactions between
functional groups may combine to produce physical entrapment of
triazines through a low multiplicative probability of escape result-
ing from a large number of encounters with low-energy sites on the
tannic acid molecule (eg hydrogen bonding)
In the case of the combined nanofiltration of atrazine and prom-
etryn with FA or NOM a modest increase of retention in comparison
to the pure triazine solutions was recorded In particular depending
on the membrane used the presence of FA resulted in an increase
of 8ndash15 of atrazine retention and 4ndash14 of prometryn retention
whereas nanofiltration in the presence of NOM led to 12ndash17 and
3ndash15 increase of atrazine and prometryn retention respectively
The interactions between atrazine and prometryn with the
humic substances observed in the GPC experiments seem to agree
(up to a point) with the trends in triazine retention by the three
NFULPRO membranes Indeed the negligible interactions observed
between the two herbicides and the humic acids are in accord with
the slight differences observed in atrazine and prometryn retention
by the NF270 and NF90 membranes On the other hand the some-
what reduced retention observed in the case of XLE membrane
may be not only due to the weak interactions between triazines
and humic acids but also to the hydrophobic interactions between
HA and the membrane surface which tends to affect atrazine and
prometryn retention This is in accord with the observed higher
adsorption rates of HA on the XLE membrane (Table 8) which in turn
may affect the surface properties of the membrane and therefore
triazine retention
The GPC results in the case of FA and NOM are in accord with
the retention results of triazines for all three membranes FA and
NOM interact with the two herbicides to form complexes of various
stabilities The formation of these pseudo-complexes may explain
the higher retention of atrazine and prometryn as a result of the
94 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
Table 6
Average retention values of atrazine and prometryn at two different humic substance concentrations by the three NFULPRO membranes
Humic substance Cfeed (mg Lminus1) NF270 NF90 XLE
Ratrazine () Rprometryn () Ratrazine () Rprometryn () Ratrazine () Rprometryn ()
HA 5 782 804 873 883 83 852
10 776 798 838 846 769 744
FA 5 784 829 828 870 848 913
10 816 842 937 933 906 977
NOM 5 813 859 843 838 879 922
10 862 901 894 857 956 968
TA 5 898 880 876 903 905 963
10 969 976 96 957 100 100
following mechanisms that may take place (a) increase of the
steric congestion or reduction of the diffusivity of the FAndashtriazine
or NOMndashtriazine pseudo-complex (b) increase of the density of
the pseudo-complexrsquos negative charge which enhances the elec-
trostatic repulsion with the negatively charged membranes and (c)
increase of the adsorption capability of the pseudo-complex on the
surface of the membrane in contrast to the hydrophobic nature of
the humic materials The validity of the above mechanism hypothe-
ses adopted by previous researchers [7] depends on the type of
the organic material and consequently on the characteristic chem-
ical properties of both HS (acidity functional groups) and triazines
(basicity hydrophobicity charge) The correlation between humic
material properties and triazine retention is examined in a subse-
quent paragraph
The greater extent of FAndashtriazines pseudo-complexes in com-
parison to NOMndashtriazines pseudo-complexes manifested in the
GPC results is not accompanied by a greater increase of triazine
retention More specifically the increasing order of triazine reten-
tion in the presence of a specific humic material follows the order
TA gt NOM gt FA gt HA in the case of NF270 and XLE membranes
whereas for NF90 the order is TA ge FA gt NOM gt HA Therefore the
interactions between the humic materials and triazines alone
cannot explain the retention performance of the NFULPRO mem-
branes This fact indicates that interactions between the naturally
occurring organic substances and the membranes may also play a
significant role in triazine retention as previously observed [33] In
view of these observations dead-end filtration tests using the three
IHSS organic materials and the selected NFULPRO membranes are
carried out in this laboratory in order to distinguish the effects of
organic fouling and of HSndashtriazines interactions on triazine perme-
ation during nanofiltration
323 Effect of humic substance concentration on triazine
retention
Surveying the literature on the complicated influence of organic
matter on retention of pesticides by NFRO membranes reveals a
considerable variability regarding the importance of the organic
matter concentration Experiments in a nanofiltration pilot unit on
the removal of atrazine and simazine have demonstrated the sig-
nificance of the organic load in the feed water since the removal
efficiency increased from 50 to 90ndash100 when dissolved organic
carbon varied between 04 and 36 mg Lminus1 [7] In contrast Berg et
al [8] reported no increase of pesticide retention (atrazine and
simazine among them) by several NF membranes with increas-
ing organic matter concentration Along the same lines Zhang et
al [11] showed comparable simazine retention between tap water
and river water although the latter had a high concentration of
NOM whereas tap water had a low NOM concentration In an effort
to understand the reasons for the above inconsistency in the lit-
erature Nghiem et al [40] took into consideration the effect of
ionic strength Experiments with hormone mimicking trace organic
contaminants revealed the significance of solution ionic strength
in influencing NOMndashtrace organic interactions which is probably
the cause for the ambiguity of literature data regarding the role
of background organic matter in trace organic rejection The mag-
nitude of ionic strength in terms of calcium ion content on the
NOMndashtriazines interaction has been also recorded through the GPC
tests described previously and is subsequently examined Although
ionic strength can strongly influence the HSndashtriazines interactions
the effect of the type of the humic substance seems to play also a
significant role on triazine retention This is evident in the results
included in Table 6 regarding retention experiments in the pres-
ence of a medium (5 mg Lminus1) and relatively high (10 mg Lminus1) HS
concentration in the absence of ionic environment
Table 6 shows a clear increase of triazine retention with increas-
ing humic material concentration More specifically by doubling
the concentration of FA NOM and TA in the feed solutions atrazine
and prometryn retention increases 4ndash13 and 2ndash10 respectively
depending on the membrane and the humic material used An
exception is observed again in the case of humic acids since the
increased organic load results in negligible or slightly negative
effects of atrazine and prometryn retention Regarding the XLE
membrane the use of medium HA concentration results in similar
retention performance as in organic-free solutions On the other
hand higher HA concentration results in a notable reduction of
triazine retention ranging up to 74 and 127 for atrazine and prom-
etryn respectively As previously discussed this is attributed to the
increased HA adsorption on the membrane surface (Table 8) which
in turn affects triazine adsorption on XLE membrane and therefore
retention This trend is not observed in the case of FA NOM and
TA despite their adsorptive capacity on the membrane materials
This finding is probably related to fouling phenomena which tend
to modify the membrane properties (ie surface charge hydropho-
bicity morphology) and therefore triazine adsorption Although it
is difficult to use mass balances for calculating triazine adsorption
during the combined filtration with humic substances (due to ana-
lytical problems related to the concentrate samples) the changes
of triazine concentration in the permeate samples provide a good
indication of the potential adsorption of atrazine and prometryn
on the membrane surface Furthermore the increased passage of
triazines in the permeate side as a result of HA adsorption on the
XLE membrane is in accord with the increase of triazine adsorption
on the membrane and therefore with the reduced retention values
324 Effect of calcium ion on triazine retention
From Fig 5 it is evident that the presence of calcium acts
positively in both atrazine and prometryn retention during the
combined nanofiltration with all four types of humic substances
Furthermore the higher interactions between the humic organ-
ics and triazines manifested in the GPC tests are in agreement
with the increased retention performance of atrazine and prom-
etryn by the three membranes used Additionally the suggestion
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 95
of ligand exchange in which calcium serves as a bridging ion and
acts as a possible site for higher adsorption of triazine molecules
on humic substances [38] seems to be in accord with the forma-
tion of an increased number of pseudo-complexes between humic
matter and triazines which explain the increase of herbicide reten-
tion
Among the four HS used tannic acid is associated with the high-
est triazine retention since both triazines are fully retained in the
presence of calcium by all three membranes tested Moreover the
enhanced interaction between humic acids and triazines as a result
of the calcium addition to the feed solutions is accompanied by
a significant increase of atrazine and prometryn retention In this
case calcium ions favour the binding between HA and triazines
which in other cases would not have taken place this is in agree-
ment with the preceding GPC and NFULPRO results
Prometryn exhibits again the highest retention values in com-
parison to atrazine which may be attributed to the relatively higher
prometryn hydrophobicity and basicity (Table 1) The former prop-
erty likely favours hydrophobic interactions between prometryn
and the HSndashcalcium complexes whereas the latter renders prom-
etryn highly effective in mechanisms involving electron-transfer
with the HSndashcalcium complexes [19] This mechanism is due to the
lower acidic functional group content of the HS as a result of the
complexation with calcium ions in the feed solutions Sposito et
al [19] suggested that atrazine itself does not readily participate
in electron-transfer reactions with humic substances However
they demonstrated that hydroxyl-atrazine does react through an
electron-transfer mechanism with HA and FA It is noted that
atrazine is readily converted to hydroxyl-atrazine even in labo-
ratory samples at low water content and this may explain the
presence of some of the electron transfer products detected in stud-
ies of atrazinendashHA interactions [41]
Surveying the literature however it is noted that some stud-
ies either with dialysis membranes [1213] or NF membranes [9]
have shown reduced values of atrazine retention when divalent
calcium is present together with natural organic matter includ-
ing the NOM surrogate tannic acid According to Devitt et al [9]
Devitt and Wiesner [12] this is due to the reduced association of
atrazine and NOM as a result of the occupation of interaction sites
by calcium andor the reduced access of atrazine to NOM sites due
to changes in molecular conformation The GPC results in this study
have shown that this is not the case since the presence of calcium
tends to increase the interaction of humic substances with triazine
compounds like atrazine
The conflicting results between the previous studies and the
present work may be attributed to the different types of membranes
and filtration techniques used More specifically the use of cellulose
ester membranes as well as the experimentation with batch dialy-
sis systems where concentration and osmotic pressure difference
serve as the driving force for solute transport (ie in the absence of
hydrodynamic forces as in the present study) may explain the dif-
ferences regarding the calcium effect on triazine retention Runge
et al [42] claim that the SpectraPor cellulose ester membranes
used in their studies (having a molecular weight cut-off of 100 Da)
Fig 6 Retention of atrazine (a) and prometryn (b) as a function of humic material total aciditymdashcalculations based on the values presented in Table 2 taking into consideration
the carbon composition and the HS concentration
Fig 7 Retention of atrazine (a) and prometryn (b) as a function of humic material carboxylic aciditymdashcalculations based on the values presented in Table 2 taking into
consideration the carbon composition and the HS concentration
96 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
present a significant incompatibility with distilled water whereas
a significant membrane potential can build up with certain salts
which can inhibit or enhance diffusion depending on the size of
the solutes diffusing and the polarity of the membrane potential
generated In this case an increased diffusion of hydrated calcium
ions to the permeate side in order to balance the concentrations
between the two sides of the cellulose ester membrane may result
in a pore blockage or a positive charge on the membrane surface
which in turn could lead to an increased passage of free dissolved
atrazine to the permeate side
Another possible explanation for this inconsistency is the time
period of the filtration tests According to the protocol followed
by previous researchers [1213] the dialysis time period ranged
between 1 and 5 days at the end of which concentration measure-
ments were performed On the other hand the filtration periods
in the case of the present dead-end nanofiltration experiments
ranged between 2 and 5 h depending on the membrane used To
this period one more hour should be added in which the feed solu-
tion was stirred in the cell without pressurization Even though
triazine adsorption on the membrane surface was observed to sta-
bilize within the relatively short period of nanofiltration there is
a possibility that longer filtration periods may have resulted in
higher adsorption rates which in turn could lead to greater pas-
sage of triazine to the permeate side and therefore to reduced
retention values This explanation is in agreement with the pilot
tests performed by Boussahel et al [43] who report that the
increasing adsorption of humic matter on the membranes leads
to an increase of pesticide adsorption on the membranes which
facilitates the diffusion of pesticides in the direction of perme-
ate
33 Correlation between humic material properties and triazine
retention
The correlation of atrazine and prometryn retention with the
total carboxylic and phenolic acidity of the humic substances used
in the present study as well as the role of the molecular mass of HS
on triazine retention are depicted in Figs 6ndash9 These are results of
nanofiltration experiments performed in the absence of calcium
Triazine removal by the three NFULPRO membranes seems
to poorly correlate with the total acidity of the humic material
This is evident especially in the case of NF270 and NF90 mem-
branes (Fig 6) The XLE membrane on the other hand presents a
reduced tendency for atrazine and prometryn removal when acid-
ity increases This is attributed to the characteristic low negative
charge of the XLE membrane surface which causes weak repulsions
with the negatively charged humic organics This leads to a greater
adsorption of humic compounds on the membrane surface (Table 8)
and consequently to greater triazine adsorption which favours the
diffusion of the triazines to the permeate side On the other hand
the strong negative charge of the NF270 and NF90 membranes [20]
promotes a strong repulsion between HS and the membranes The
greater the negative charge of the humic organic the higher the
repulsion which in turn results in higher concentration of HS in
the bulk Consequently the interactions between HS and triazines
in the bulk are increased leading to an improved triazine removal
Fig 8 Retention of atrazine (a) and prometryn (b) as a function of humic material phenolic aciditymdashcalculations based on the values presented in Table 2 taking into
consideration the carbon composition and the HS concentration
Fig 9 Retention of atrazine (a) and prometryn (b) as a function of humic substance molecular mass (based on the values presented in Table 3)
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 97
This is more evident in the case of the wide pore NF270 membrane
since the NF90 membrane seems to retain atrazine and prometryn
to a higher degree in the presence of TA which is characterized by
a lower total acidity among the four HS used in this study
Humic compounds contain a variety of functional groups
such as aliphatic and aromatic carboxyls phenolic hydroxyls
alcoholic hydroxyls carbonyls quinones methoxyls and amino
groups These functional groups are the reactive sites in the humic
molecules As carboxylate protonation seems to be critical for the
capacity of triazine binding [44] it is of interest to correlate the
effect of carboxylic acidity on herbicide removal by nanofiltration
The effect of carboxylic acidity on atrazine and prometryn retention
by the three NFULPRO membranes is depicted in Fig 7
The results show a negative trend of triazine retention with
an increased carboxylic acidity of humic substances This is clear
in the case of NF270 and XLE membranes while NF90 does not
seem to be significantly affected by the changes in the number of
carboxyl groups of a humic molecule Remarkably the smaller TA
molecules containing fewer carboxyl groups per unit mass (Table 2)
exhibit the highest triazine retention (sim100) as a result of the even
higher triazine binding capacity in contrast to the IHSS organics
which contain a higher carboxylic group content Moreover experi-
ments in batch filtration mode result in HS concentration increasing
with time which in turn leads to a reduced number of carboxylic
binding sites available to the two triazines This is probably due
to the self ldquocompetingrdquo character of the HS used since an increase
in concentration andor HS aggregation tend to reduce the avail-
able binding sites [44] This phenomenon led Wang et al [44] to
propose a model in which hydrogen bonding was considered to be
the primary interaction resulting in triazine binding Therefore the
increase in triazine retention observed in the case of elevated HS
concentration may not be directly related with the carboxylic acid-
ity of the HS but rather with other mechanisms such as hydrogen
bonding
Regarding the phenolic acidity of the humic substances there
does not seem to be a sufficient correlation with atrazine or prom-
etryn retention by the three NFULPRO membranes (Fig 8) Actually
there is a contradictory behaviour between the three IHSS organ-
ics and the NOM surrogate TA The high phenolic content of TA
is related to high retention values of the two triazines while
the smaller and relatively similar phenolic acidity of the IHSS
compounds leads to smaller and different retention values These
results imply that there are other factors influencing the association
between humic organics and herbicides and therefore their reten-
tion by nanofiltration In this respect the molecular structure of the
humic materials as well as the interactions taking place between
the organics and the membranes seem to explain the above incon-
sistencies This is evident in the results depicted in Fig 9 and the
adsorption of humic substances subsequently described
The rather strong relation of triazine retention with the molec-
ular mass of the organics used in this study (Fig 9) leads to the
suggestion that structural features of HS are important for the
removal capacity of triazines Up to now there is no consensus
concerning both the binding mechanism of triazines to humic sub-
stances and the structural features responsible for this binding
Nevertheless the preferential binding with low molecular weight
fractions of humic compounds (especially of FA and TA) deduced
from the GPC and filtration tests results in higher retention val-
Table 7
Humic substance and calcium retention performance of the three NFULPRO membranes (mean retention values for 50 recovery of feed volume as permeate)
Membrane Humic substancea Humic substances retention () Ca2+ retention ()
HSs alone HSs with triazines HSs with triazines and Ca2+ In the presence of HSs
NF270 HA 990 994 1000 949
FA 967 983 991 951
NOM 957 997 1000 951
TA 972 992 997 957
NF90 HA 972 980 1000 994
FA 953 984 1000 990
NOM 985 993 1000 991
TA 967 998 998 994
XLE HA 992 996 1000 994
FA 862 915 994 997
NOM 976 1000 1000 988
TA 923 945 1000 996
a HA FA NOM and TA designate the four humic substances used in this study
Table 8
Adsorption data of humic substances on tested membranes
Membrane Humic substancea Feed mass of humic substance (g) Quantity adsorbed (g) Adsorption ()
Alone With Ca2+ Alone With Ca2+ Alone With Ca2+
NF270 HA 3480 2630 244 2314 07 88
FA 3570 2840 71 1335 02 47
NOM 3390 2660 34 2633 01 99
TA 4050 3300 2025 7161 50 217
NF90 HA 3220 2690 354 3658 11 136
FA 2930 2450 1494 857 51 35
NOM 3000 2910 120 437 04 15
TA 2820 2730 649 3631 23 133
XLE HA 3100 2680 1643 3457 53 129
FA 3050 2810 1830 2866 60 102
NOM 2650 2770 610 1219 23 44
TA 3120 2940 2184 2970 70 141
a HA FA NOM and TA designate the four humic substances used in this study
98 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
ues for the two triazines Thus the interaction with small branched
molecules like FA and TA seems to favour a physical entrapment
mechanism such as the one suggested by Devitt and Wiesner [12]
34 Retention of humic species and calcium ion by the NFULPRO
membranes
The performance of nanofiltration on the humic substances and
calcium removal is summarized in Table 7 Regarding humic sub-
stances the retention is compared for three different feed solutions
(a) solutions of HS alone (b) solutions of HS with the two triazines
and (c) solutions of HS with the two triazines in the presence of cal-
cium In general HS retention is high for all three membranes used
According to the UV measurements IHSS HA exhibits the highest
retention values followed by NOM TA and FA which is apparently
related to their relative molecular masses (Table 3) Complexation
or interaction with triazines as evidenced by the GPC tests appar-
ently plays also a significant role in the relative permeability of the
organic species More specifically the retention of the four organic
compounds examined is increased in the presence of the two her-
bicides which verifies also the binding of atrazine and prometryn
with the four humic substances Thus a comparison between the
retention values of the HS in the absence or presence of triazines
gives also a good insight into the HSndashtriazine interactions The slight
increase (lt1) of HA retention in the presence of the two triazines
is in accord with the relatively low HA binding capacity observed
in the GPC tests On the other hand the increase of 2ndash6 in FA
NOM and TA retention is in agreement with the observed increased
interactions between those substances and the two triazines
In the case of calcium addition in the organicndashtriazine solu-
tions the UV absorbing species of the four organics examined are
retained almost completely by all three membranes used This is
again in agreement with the results obtained with gel chromatogra-
phy According to the literature [12] the addition of calcium results
in charge shielding and neutralization of the organic matter charged
functional groups and can shrink the humic matrix The formation
of aggregates of larger apparent molecular weight due to the sub-
stantial increase in the hydrophobicity of the organic molecules
tends to reduce their permeability through membranes and there-
fore to increase their retention
Apart from the increased retention of triazines and humic sub-
stances there is also a significant improvement of calcium rejection
when the latter is present in the solution In comparison to pure
water solutions of elevated calcium ion content [20] the rejection of
calcium in humic solutions is almost complete This is attributed to
size exclusion mechanisms related to the binding tendency of diva-
lent ions like calcium to the negatively charged humic organics
Among the four humic substances used calcium is rejected more in
the presence of the polyphenol tannic acid which is accompanied
by a higher organic adsorption on all three NFULPRO membranes
tested (Table 8)
From the data summarized in Table 8 it can be seen that the
adsorption of humic substances onto the membrane surfaces is
markedly enhanced in the presence of calcium ions due to the
formation of HA-complexed divalent cations This is also clearly
depicted in the SEM images of virgin and fouled membranes in
the absence or presence of calcium ions Characteristic SEM images
in the case of the Suwannee River NOM filtration with the three
NFULPRO membranes are included in Fig 10 these images show
the severe fouling on the three NFULPRO membranes as a result
of the combined filtration of NOM with CaCl2 salt It is possible
that the divalent cations such as Ca2+ act as a bridge between the
membrane surface and the negatively charged humic molecules
that tend to be repelled by the negatively charged membrane As a
result the electrical repulsion between the membrane surface and
the humic molecules is weakened thus rendering the membrane
Fig 10 SEM images of virgin fouled with NOM and fouled with NOM + calcium membranes (a) NF270 (b) NF90 and (c) XLE membrane
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 99
fouling process more severe [45] On the other hand the relatively
smaller adsorption rates of NOM in the absence of calcium result
in less fouled membranes A similar trend is also observed in the
case of the other three humic substances used in this study Among
the three membranes used XLE exhibits in the absence of calcium
the highest adsorption rates for all four humic substances This is
explained by the relatively high hydrophobicity and the low surface
charge that characterize the XLE membrane surface as previously
explained
4 Concluding remarks
The influence of humic substances on retention of two tri-
azines (atrazine and prometryn) by NFULPRO membranes is rather
complicated and involves combinations of solutendashmembrane and
solutendashsolute interactions In summary the retention of triazines
depends on the type of the humic material present in the feed
water In most cases the combined nanofiltration of triazines and
naturally occurring humic substances facilitates the formation of
complexes with triazines which in turn enhance their removal by
means of membrane filtration This complexation appears to be
related not to the characteristic acidity (phenolic carboxylic) of
the HS used but rather to their molecular conformation More
specifically a preferential binding is observed between triazines
and low molecular weight fractions of humic compounds (espe-
cially of FA and TA) which results in higher retention values for the
two triazines Under all conditions tannic acid exhibits the greatest
effect on triazine retention among the four standard HS compounds
used leading to an almost complete removal of the two triazines
(95ndash100) for all three membranes tested
The removal of triazines is improved in the presence of calcium
which tends to enhance the interaction between HS and triazines
Moreover triazine retention is increased with increasing HS con-
centration to a degree depending on the type of HS Additionally
triazine retention is apparently also affected by the interactions
between the organics and the membrane surface which in turn
may alter triazine retention These results illustrate the complex-
ity of the triazine adsorption and retention mechanisms in the
presence of humic substances that needs further investigation for
improved understanding Moreover regarding triazine removal by
nanofiltration it is of particular interest to distinguish the effect
of the HSndashtriazine interactions from that of membrane HS fouling
Research in this direction is carried out in this laboratory
References
[1] DP Birardar AL Rayburn Chromosomal damage induced by herbicide con-tamination at concentrations observed in public water supplies J Environ Qual24 (1995) 1222ndash1225
[2] IK Konstantinou DG Hela TA Albanis The status of pesticide pollution insurface waters (rivers and lakes) of Greece Part I Review on occurrence andlevels Environ Pollut 141 (3) (2006) 555ndash570
[3] European Environment Agency (EEA) Groundwater quality and quantity inEurope Report Copenhagen June 1999 available in httpreportseeaeuropaeugroundwater07012000enenviassrp199903
[4] EM Thurman DA Goolsby MT Meyer MS Mills ML Pomes DW Kolpin AReconnaissance study of herbicides and their metabolites in surface water of theMidwestern United States using immunoassay and gas chromatographymassspectrometry Environ Sci Technol 26 (1992) 2440ndash2447
[5] C Bellona JE Drewes P Xu G Amy Factors affecting the rejection of organicsolutes during NFRO treatmentmdasha literature review Water Res 38 (2004)2795ndash2809
[6] A Bhattacharya Remediation of pesticide-polluted waters through mem-branes Sep Purif Rev 35 (2006) 1ndash38
[7] KM Agbekodo B Legube S Dard Atrazine and simazine removal mechanismsby nanofiltration influence of natural organic matter concentration Water Res30 (1996) 2535ndash2542
[8] P Berg G Hagmeyer R Gimbel Removal of pesticides and other micropollu-tants by nanofiltration Desalination 113 (1997) 205ndash208
[9] EC Devitt F Ducellier P Coumlteacute MR Wiesner Effects of natural organic mat-ter and the raw water matrix on the rejection of atrazine by pressure-drivenmembranes Water Res 32 (1998) 2563ndash2568
[10] R Boussahel S Bouland KM Moussaoui A Montiel Removal of pesticideresidues in water using the nanofiltration process Desalination 132 (2000)205ndash209
[11] Y Zhang B Van der Bruggen GX Chen L Braeken C Vandecasteele Removalof pesticides by nanofiltration effect of the water matrix Sep Purif Technol38 (2004) 163ndash172
[12] EC Devitt MR Wiesner Dialysis investigations of atrazinendashorganic matterinteractions and the role of a divalent metal Environ Sci Technol 32 (1998)232ndash237
[13] SK Dalton JA Brant MR Wiesner Chemical interactions between dissolvedorganic matter and low-molecular weight organic compounds impacts onmembrane separation J Membr Sci 266 (2005) 30ndash39
[14] NA Kulikova IV Perminova Binding of atrazine to humic substances fromsoil peat and coal related to their structure Environ Sci Technol 36 (2002)3720ndash3724
[15] RL Wershaw The importance of humic substancendashmineral particle complexesin the modeling of contaminant transport in sedimentndashwater systems in RABaker (Ed) Organic Substances and Sediments in Water Humics and SoilsLewis Publishers Chelsea MI 1991 pp 23ndash34
[16] SM Schrap A Opperhuizen Relationship between bioavailability andhydrophobicity reduction of the uptake of organic chemicals by fish due tothe sorption on particles Environ Toxicol Chem 9 (1990) 715ndash724
[17] E Barriuso M Schiavon F Andreux JM Portal Localization of atrazine non-extractable (bound) residues in soil size fractions Chemosphere 12 (1991)1131ndash1140
[18] L Loiseau E Barriuso Characterization of the atrazinersquos bound (nonextractable)residues using fractionation techniques for soil organic matter Environ SciTechnol 36 (2002) 683ndash689
[19] G Sposito L Martin-Neto A Yang Atrazine complexation by soil humic acidsJ Environ Qual 25 (1996) 1203ndash1209
[20] KV Plakas AJ Karabelas Membrane retention of herbicides from single andmulti-solute media the effect of ionic environment J Membr Sci 320 (2008)325ndash334
[21] V Freger J Gilron S Belfer TFC polyamide membranes modified by graftingof hydrophilic polymers an FT-IRAFMTEM study J Membr Sci 209 (2002)283ndash292
[22] P Xu JE Drewes Viability of nanofiltration and ultra-low pressure reverseosmosis membranes for multi-beneficial use of methane produced water SepPurif Technol 52 (2006) 67ndash76
[23] Z Knez A Rizner-Hras K Kokot D Bauman Solubility of some solid triazineherbicides in supercritical carbon dioxide Fluid Phase Equilibria 152 (1998)95ndash108
[24] Weed Science Society of America (WSSA) Herbicide Handbook 7th ed 1994[25] B Van der Bruggen J Schaep W Maes D Wilms C Vandecasteele Nanofiltra-
tion as a treatment method for the removal of pesticides from ground watersDesalination 117 (1998) 139ndash147
[26] KN Reddy MA Locke Molecular properties as descriptors of octanol-waterpartition coefficients of herbicides Water Air Soil Pollut 86 (1996) 389ndash405
[27] IHSS website httpwwwihssgatechedu[28] F Flores-Ceacutespedes M Fernaacutendez-Peacuterez M Villafranca-Saacutenchez E Gonzaacutelez-
Pradas Cosorption study of organic pollutants and dissolved organic matter ina soil Environ Pollut 142 (2006) 449ndash456
[29] R Beckett Z Jue JC Giddings Determination of molecular weight distribu-tions of fulvic and humic acids using flow field-flow fractionation Environ SciTechnol 21 (1987) 289ndash295
[30] PM Reid AE Wilkinson E Tipping MN Jones Determination of molecularweights of humic substances by analytical (UV scanning) ultracentrifugationGeochim Cosmochim Acta 54 (1990) 131ndash138
[31] S Lee B Kwon M Sun J Cho Characterizations of NOM included in NF and UFmembrane permeates Desalination 173 (2005) 131ndash142
[32] MN Jones ND Bryan Colloidal properties of humic substances Adv ColloidInterf Sci 78 (1998) 1ndash48
[33] KV Plakas AJ Karabelas T Wintgens T Melin A study of selected herbicidesretention by nanofiltration membranesmdashthe role of organic fouling J MembrSci 284 (2006) 291ndash300
[34] APHA-AWWA-WPCF Standard Methods for the Examination of Water andWastewater 17th ed Port City Press Baltimore MD 1989 pp 567ndash569
[35] R Artinger G Buckau JI Kim S Geyer Characterization of groundwater humicand fulvic acids of different origin by GPC with UVVis and fluorescence detec-tion Fresenius J Anal Chem 364 (1999) 737ndash745
[36] AI Schaumlfer N Andritsos AJ Karabelas EMV Hoek R Schneider M Nys-troumlm Chapter 8 Fouling in nanofiltration in AI Schaumlfer AG Fane TDWaite (Eds) Nanofiltration Principles and Applications Elsevier Oxford UK2005 pp 173ndash174
[37] I Franco L Catalano M Contin M De Nobili Interaction between organicmodel compounds and pesticides with water-soluble soil humic substancesActa Hydrochim Hydrobiol 29 (2001) 88ndash99
[38] AA Helal DM Imam SM Khalifa HF Aly Interaction of pesticides with humiccompounds and their metal complexes Radiochemistry 48 (4) (2006) 419ndash425
[39] MG Browman G Chester Fate of Pollutants in the Air and Water EnvironmentWiley New York 1977 p 50
[40] LD Nghiem AI Schaefer M Elimelech Nanofiltration of hormone mimickingtrace organic contaminants Sep Sci Technol 40 (2005) 2633ndash2649
[41] R Celis J Cornejo MC Hermosin WC Koskinen Sorptionndashdesorption ofatrazine and simazine by model soil colloidal components Soil Sci Soc AmJ 61 (1997) 436ndash443
100 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
[42] SW Runge KR Shelton SA Melton WM Moran Maintaining the ionic per-meability of a cellulose ester membrane J Biochem Biophys Methods 64(2005) 200ndash206
[43] R Boussahel A Montiel M Baudu Effects of organic and inorganic matter onpesticide rejection by nanofiltration Desalination 145 (2002) 109ndash114
[44] Z Wang DS Gamble CH Langford Interaction of atrazine with Laurentianhumic acid Anal Chim Acta 244 (1991) 135ndash143
[45] C Jucker MM Clark Adsorption of aquatic humic substances on hydrophobicultrafiltration membranes J Membr Sci 97 (1994) 37ndash52
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 89
solution preparation was obtained from a Milli-Q purification
system (Millipore Milford MA USA) Similar to the protocol
used by Devitt et al [9] once a model solution of HS and
atrazinendashprometryn was prepared in the presence or not of cal-
cium it was placed in a foil-covered container (to prevent atrazine
and prometryn degradation by exposure to light) and stirred for
more than 24 h after which it was assumed to be in equilibrium
215 Analytical techniques
2151 Herbicide analysis Off-line solid phase extraction (SPE)
with gas chromatography employing a mass-selective detector
(GC-MSD) was used to achieve the sensitivity required for the
tested herbicides The SPE procedure performed prior to the
chromatographic analysis is described in a previous paper [33]
Herbicide analyses were performed in an Agilent Technologies
Model 7890A gas chromatograph system interfaced with an Agilent
Technologies Model 5975C mass-selective detector (MSD) The gas
chromatograph was equipped with a 30 m times 250 m id times 025 m
film thickness fused silica capillary column (Agilent 19091S-
433) The chromatographic conditions were as follows injection
splitless injector temperature 180 C transfer line temperature
220 C injection volume 1 L carrier gas He flow 1 mL minminus1
oven temperature programmed from an initial temperature of 70 C
held for 3 min to 150 C at a rate of 30 C minminus1 held for 1 min next
to 200 C at a rate of 10 C minminus1 and finally to 260 C at a rate of
5 C minminus1 held for 1 min The MSD was operated in the selective ion
monitoring (SIM) mode Three characteristic ions were monitored
for each herbicide analyzed The most abundant ion for each herbi-
cide was used for quantification while the other two ions were used
to confirm the presence of the herbicide The minimum detectable
quantity (MDQ) for the analysis with a 1-L injection volume was
approximately 1 ng for all herbicides analyzed
Simazine (Riedel de-Haeumln purity 999) a triazine compound
similar to atrazine and prometryn was used as internal standard in
order to assess the recovery of the two triazines in the SPE-GC-MSD
analyses With the selected SUPELCO SPE cartridges recoveries
from the model humic substance solutions ranged from 35 to 98
and in general were lower than the recoveries obtained in pure
water or permeate samples of the triazinendashHS filtration (75ndash128)
The analysis of the concentrate samples which were characterized
by high dissolved carbon content resulted in many cases in even
lower recoveries This is attributed to the saturation of the sorption
sites of the SPE cartridges by the humic substances which renders
them inefficient in extracting the two triazines associated with HS
Therefore triazine concentrations were determined only in the feed
and permeate samples
2152 Organic and inorganic matter analysis Measurement of the
HS concentration (and more specifically the aromaticity) was per-
formed in a UVndashvis spectrophotometer (UV-1700 Shimadzu Japan)
at 254 nm in the case of Suwannee River HA and Suwannee River
NOM and at 275 nm in the case of Suwannee River FA A standard
colorimetric method (5550) [34] was adopted for the preparation
of tannic acid samples prior to their spectrophotometric measure-
ment at 700 nm The quality control of the data was achieved by
means of calibration and repeated analyses Specifically two cal-
ibration curves were created for each HS one corresponding to
the permeate samples (standards of similar concentrations to the
one expected in the permeates) and a second one corresponding
to the higher concentration level of the feed and retentate sam-
ples This procedure resulted in high R2 values (gt0999) reflecting
the high accuracy and sensitivity of the UVvis method for all sam-
ples treated The use of two different Quartz cuvettes (1 and 10 cm)
enhanced this accuracy (measurement of absorption in a range
01ndash15 unitsmdashas proposed in the literature) Further the accuracy
of the analytical results was ensured by repeatedly measuring the
concentration of a sample (more than four times) All measure-
ments in the case of higher concentrations (feed and concentrate)
resulted in RSD values smaller than 1 while in the case of the
lower concentrations the RSD values did not exceeded 2
The determination of calcium ion content in the water samples
was performed by an ion selective electrode connected to a 692
pHIon Meter Metrohm (Herisau Switzerland) The conductivity
and pH of the samples were measured by a conductometer 712
and pH meter 744 both by Metrohm
2153 Gel permeation chromatography (GPC) Gel permeation
chromatography (or size exclusion chromatography SEC) is a rela-
tively rapid and convenient method for estimating the partitioning
or binding of organic compounds to humic substances in solution
[35] In particular GPC can give information on the liability of the
interaction itself The solutions tested in the present work were (a)
HS (b) HSndashtriazine (atrazine prometryn or both) (c) HSndashcalcium
and (d) HSndashcalciumndashatrazinendashprometryn All solutions were stirred
for more than 24 h in foil-covered containers prior to their injection
into the GPC column The concentrations of all the materials used
were the same as those applied in the filtration experiments Tannic
acid was excluded from the GPC tests due to the possible interac-
tions that may occur between the chemicals of the colorimetric
method used (folin phenol Na2CO3ndashNa2C4H4O6middot2H2O solution)
and the GPC column which in turn may significantly alter the tannic
acid elution behaviour
The accuracy of the elution profiles was ensured by apply-
ing identical chemical conditions for the GPC investigation
This was achieved by using feed waters of the same source
thus all tested solutions (HS alone HSndashtriazines HAndashcalcium
HAndashcalciumndashtriazines) were prepared from the same HS stock solu-
tion This led to a negligible experimental uncertainty which was
evident in the repeatable GPC measurements (three to four injec-
tions) for each solution Indeed the spectrum of each HS was
very uniform whereas the shapes of the GPCSEC elution curves
recorded at the same wavelength were essentially identical
The GPC tests were carried out in a Shimadzu-10 AVP system The
GPC column was a PL aquagel-OH Mixed 8 m 300 mm times 75 mm
(Polymer Laboratories Essex UK) guarded by a PL aquagel-OH
Guard 8 m 50 mm times 75 mm column PL aquagel-OH columns
are packed with macroporous copolymer beads with extremely
hydrophilic polyhydroxyl functionality offering high resolution
over a wide range of molecular weights (up to 104 kDa) Aqueous
samples (100 L) were pipetted directly onto the gel and passed
through the column Phosphate buffer (01 M KH2PO4 at pH 70
corrected with 2 M NaOH) was used as the eluent with a flow
rate of 1 mL minminus1 Dissolved humic substance (HA FA and NOM)
was measured with a Shimadzu-10 AVP UV detector by monitor-
ing ultraviolet light absorbance at a wavelength of 254 275 and
254 nm respectively Calibration of the column to determine the
exact molecular sizes of the HS was not performed in this work
22 Nanofiltration tests and filtration protocol
Dead-end filtration experiments were performed by using an
experimental set-up consisting of three high pressure stirred cells
described elsewhere [20] The three test cells were operated con-
currently to assess the reproducibility and accuracy of the results
A new membrane coupon was used in each filtration test in order
to avoid employing membranes with adsorbed amounts of humic
materials and herbicides
The filtration protocol involves a sequence of the following six
steps (1) membrane preconditioning with Milli-Q water under
pressure of 5 bar for several hours to ensure that the removal of
preservatives from the new membrane coupon is complete [20]
(2) membrane compaction for 1 h at 10 bar (3) measurement of the
90 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
pure water flux for 30 min at 5 bar (4) filtration of triazines in the
absence of humic materials (ldquofree triazine solutionsrdquo) at 5 bar (5)
filtration of the model HSndashtriazines solution at 5 bar and (6) mea-
surement of the pure water flux for 30 min at 5 bar Compaction
(step 2) is crucial in every membrane filtration protocol as it may
change both the active layer and its support thus affecting the flux
and the rejection properties of the membrane To eliminate this
impact membranes are often subjected to at a higher pressure (here
10 bar) than the operating pressure (5 bar) to ensure flux stability
during experiments [36] Preliminary experiments with the three
membranes tested showed that 1 h of 10 bar pressure application
was more than adequate to achieve membrane stability
All filtration steps were performed with a feed solution of
300 mL while the tests performed with the herbicides were car-
ried out until 150 mL of permeate were collected (50 recovery)
Feed and three permeate samples (50 mL each) were retained for
subsequent analysis in order to determine the evolution of con-
taminant concentration in some cases retentate samples were also
removed for chromatographic analysis Similar to previous studies
[2033] the triazine feed solutions were stirred in the cells for 1 h
without pressure In this way herbicide adsorption on the mem-
brane surface was assumed to reach equilibrium After termination
of the filtration experiments the membrane coupons were dried
for subsequent analysis (SEM) while the test cells were thoroughly
washed with acetone and repeatedly rinsed with Milli-Q water
23 Data treatment
The performance of nanofiltration can be expressed in terms of
the percentage removal of the organic and inorganic substances
The determination of the retention was calculated based on the
method described in our previous study [20] which proved to be
appropriate in the case of batch filtration experiments since the
membrane performance depends on the retentate concentration
which increases with time The concentration of the two triazines
was measured for the feed solution and for all the permeate samples
while retentate samples were removed for subsequent analysis of
the humic and inorganic compounds The retention was evaluated
based on the so-called ldquostablerdquo permeate samples where the water
recovery rate was between 17 and 50 The amount of deposited
humic substance on the membrane surface was determined from
mass balance as follows
A () = 100
(
1 minus
sumj
i=1Vpi
Cpi+ Crj
Vr
Cf Vf
)
(1)
where Cpj Cr Cf are the concentrations of a humic substance in
the permeate sample j retentate and feed respectively while Vpj
Vr Vf are the respective volumes of permeate sample j retentate
and feed Reduction of the pure water flux (FRPW) which may be
considered a measure of the adsorbed humic substances effect on
flux was determined as a percentage of clean water flux Jw before
and after membrane operation as previously described [20]
3 Results and discussion
31 Pure triazine solutions
Prior to the combined filtration of triazines with humic materi-
als pure water solutions with atrazine and prometryn were filtered
at 5 bar in concentrations ranging between 10 and 30 g Lminus1 The
performance of the membranes in terms of triazine retention is
illustrated in Fig 1 The accuracy of experimental results is indi-
cated by the bars which represent the scatter of data obtained from
six replicate experiments for each membrane
Fig 1 Atrazine and prometryn retention from free triazine solutions by the three
selected NFULPRO membranes mean retention values for 50 recovery of feed
volume feed concentration 10ndash30 g Lminus1
As in our previous studies [2033] prometryn the molecule
with the larger molecular weight and size displays the great-
est retention with all three membranes used The performance
of membranes regarding retention of the two triazines follows
the order XLE ge NF90 gt NF270 which is consistent with the results
obtained in our previous work [20] Comparing the experimen-
tal results for double solute mixtures of atrazine and prometryn
in the case of high (600ndash700 g Lminus1) [20] and low feed concen-
trations (10ndash30 g Lminus1) the differences in retention values vary
between 7 and 13 In general the retention results with pure tri-
azine solutions are in agreement with observations made by other
researchers [71125] in that herbicide concentration does not sig-
nificantly affect their retention The fact that the filtration of lower
feed concentrations leads to a slight reduction of triazine retention
(especially in the case of XLE membrane) could be attributed to the
amount of triazines adsorbed on the selected membranes Based
on the mass balances summarized in Table 4 the lower feed tri-
azine concentration is accompanied in general by a slightly lower
adsorption in comparison to the results obtained with higher feed
concentrations [20] something that is more pronounced in the case
of the less tight NF270 membrane
32 Aqueous solutions of mixed humic substances and triazines
321 GPC results on triazinesndashhumic substances interactions
The UVndashvis spectra of fractions collected during GPC chromatog-
raphy of HS alone and HS with atrazine andor prometryn in the
absence or presence of calcium ions are depicted in Figs 2ndash4
The comparison of the chromatographic behaviour of the solu-
tions used in qualitative terms allows an assessment of possible
interactions between humic substances and the two triazines [37]
The elution profiles display a maximum for all three IHSS organ-
ics under the same test conditions at retention time 185ndash195 min
In the case of the low molecular weight triazines the elution pro-
file maximum at a wavelength of 225 nm and concentration of
10 mg Lminus1 (high concentrations used in order to be measurable
by UV absorption) appear at 110 min (data not presented here)
The longer retention time of atrazine and prometryn is due to the
fact that small molecules can penetrate into the pores of the gel
and follow a torturous path through the column On the other
hand large molecules such as humic substances are excluded
from internal pore spaces in the gel and elute faster from the col-
umn Co-elution of small triazine molecules with the much larger
humic molecules indicates their association with the humic frac-
tion Humic-bound triazines appear to display the behaviour of the
larger humic molecules as they are also excluded from the internal
pores of the gel [37] This is reflected in the reduced GPC areas of
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 91
Table 4
Adsorption data of triazines on tested membranes
Membrane Feed mass of
triazinesa (g)
Quantity adsorbed
(g)
adsorption adsorption
[19]b
NF270 45 plusmn 27(A) 048 plusmn 027 127 plusmn 18 165
53 plusmn 28(P) 020 plusmn 008 38 plusmn 13 275
NF90 52 plusmn 28(A) 122 plusmn 083 248 plusmn 66 305
55 plusmn 13(P) 105 plusmn 033 250 plusmn 67 262
XLE 66 plusmn 10(A) 114 plusmn 012 219 plusmn 12 230
74 plusmn 11(P) 084 plusmn 008 171 plusmn 09 295
a (A) and (P) designate atrazine and prometryn respectivelyb Adsorption values for higher triazine (atrazine and prometryn) concentrations (600ndash700 g Lminus1 each)
Fig 2 GPC elution profiles of Suwannee River humic acid (HA) in the absence or presence of triazines andor calcium (detection by UV absorption at 254 nm)
the humic materials when treated together with the two triazines
(Table 5) In summary aquatic humic substances from the same
water source (Suwannee River) show differences in their GPC elu-
tion behaviour when treated together with calcium andor with
either of the two triazines
The peaks of the GPC chromatograms reflect differences in spe-
cific absorption Distinct differences between HS and atrazine or
prometryn are found in the case of FA and NOM In contrast HA
show a rather small variation on elution behaviour This is also in
accord with the values presented in Table 5 Complexation with
the NOM and FA appears to occur for both atrazine and prometryn
molecules the interactions with NOM indicate greater affinity to
atrazine On the other hand FA interactions appear to be stronger
with prometryn In Figs 2ndash4 and Table 5 one can observe that
among the three HS used FA seems to interact more with the two
triazines The weakest interaction is observed in the case of humic
acids which is reflected in the almost identical elution profiles and
the low GPC reduction areas
The presence of calcium in the solutions of humic substances
leads to a sharp reduction of HS concentrations (Table 5) indi-
cating the formation of calcium humate and fulvate complexes
According to the literature when negatively charged humic colloids
Table 5
Percentage () reduction of GPC integration areas of humic substances due to the presence of triazines andor calcium
Humic substances combined with Percentage GPC area reduction ()
HA FA NOM
40 mg Lminus1 Ca2+ 26 17 138
10 g Lminus1 atrazine 06 64 27
10 g Lminus1 prometryn 02 81 lt01
10 g Lminus1 atrazine + 10 g Lminus1 prometryn 23 80 30
10 g Lminus1 atrazine + 10 g Lminus1 prometryn + 40 mg Lminus1 Ca2+ 151 221 279
92 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
Fig 3 GPC elution profiles of Suwannee River fulvic acid (FA) in the absence or presence of triazines andor calcium (detection by UV absorption at 275 nm)
come in contact with positively charged metal ions (like calcium)
several binding mechanisms become effective The bond strengths
vary from a weak physical adsorption to strong chemical bond-
ing involving chelation reactions [38] In the case of herbicides
the partially chelated calcium may serve here as a bridging ion
and act as a possible site for the adsorption of a triazine molecule
by displacing water of hydration Indeed the addition of calcium
enhances the interaction between the humic substances and the
two triazines something that is manifested by the higher GPC area
reductions and the lower elution profiles Browman and Chester
[39] also concluded that cations on the surface of humic material
can act as binding bridges between synthetic chemicals and humic
compounds Similarly Helal et al [38] reported that the exchange
reaction between the pesticides and the humic compounds takes
place on the same sites at which binding of humic compound to the
metal occurs Moreover these authors claim that the pesticidendashHA
Fig 4 GPC elution profiles of Suwannee River NOM in the absence or presence of triazines andor calcium (detection by UV absorption at 254 nm)
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 93
Fig 5 Retention of atrazine (A) and prometryn (P) by the three NFULPRO membranes in the absence or presence of humic substances (HA FA NOM TA) andor calcium
ions
or ndashFA complex is stronger than the metalndashhumate or ndashfulvate
complex The mechanisms involved in the interactions between
triazines and calciumndashHS complexes may be described by either
a physical adsorption of the herbicide on the surface of metal
humatefulvate complex or a ligand exchange in which the metal
serves as a bridging ion and acts as a possible site for adsorption of
the herbicide molecule by displacing water of hydration [38]
322 Effect of different types of humic materials on triazine
retention
Retention results of atrazine and prometryn during the com-
bined nanofiltration with the four selected humic substances are
depicted in Fig 5 Results of triazine retention from organic solu-
tions with elevated calcium ion content are also included in Fig 5
and discussed in Section 324 Depending on the type of humic
material present increased retention of the two triazines may be
negligible or significant Specifically the use of HA results in reten-
tion values that are comparable to those obtained in the absence
of organics An exception is observed in the case of XLE membrane
where the combined filtration of triazines and HA leads to relatively
smaller retention values (7 and 13 for atrazine and prometryn
respectively) On the other hand the presence of tannic acid results
in a significant increase of triazine retention for all three mem-
branes tested In particular the addition of TA at a concentration of
10 mg Lminus1 (yielding a triazineTA ratio of 1 g1 mg) results in 14ndash21
and 20ndash29 increase of prometryn and atrazine retention respec-
tively The enhancement of pesticide removal in the presence of
the NOM surrogate tannic acid has been also observed by previ-
ous researchers [91213] According to Devitt and Wiesner [12] the
increased retention of a triazine molecule like atrazine in the pres-
ence of TA is due to the branched structure of TA which traps the
triazine within the macromolecule Moreover these authors sug-
gest that molecular conformations and weak interactions between
functional groups may combine to produce physical entrapment of
triazines through a low multiplicative probability of escape result-
ing from a large number of encounters with low-energy sites on the
tannic acid molecule (eg hydrogen bonding)
In the case of the combined nanofiltration of atrazine and prom-
etryn with FA or NOM a modest increase of retention in comparison
to the pure triazine solutions was recorded In particular depending
on the membrane used the presence of FA resulted in an increase
of 8ndash15 of atrazine retention and 4ndash14 of prometryn retention
whereas nanofiltration in the presence of NOM led to 12ndash17 and
3ndash15 increase of atrazine and prometryn retention respectively
The interactions between atrazine and prometryn with the
humic substances observed in the GPC experiments seem to agree
(up to a point) with the trends in triazine retention by the three
NFULPRO membranes Indeed the negligible interactions observed
between the two herbicides and the humic acids are in accord with
the slight differences observed in atrazine and prometryn retention
by the NF270 and NF90 membranes On the other hand the some-
what reduced retention observed in the case of XLE membrane
may be not only due to the weak interactions between triazines
and humic acids but also to the hydrophobic interactions between
HA and the membrane surface which tends to affect atrazine and
prometryn retention This is in accord with the observed higher
adsorption rates of HA on the XLE membrane (Table 8) which in turn
may affect the surface properties of the membrane and therefore
triazine retention
The GPC results in the case of FA and NOM are in accord with
the retention results of triazines for all three membranes FA and
NOM interact with the two herbicides to form complexes of various
stabilities The formation of these pseudo-complexes may explain
the higher retention of atrazine and prometryn as a result of the
94 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
Table 6
Average retention values of atrazine and prometryn at two different humic substance concentrations by the three NFULPRO membranes
Humic substance Cfeed (mg Lminus1) NF270 NF90 XLE
Ratrazine () Rprometryn () Ratrazine () Rprometryn () Ratrazine () Rprometryn ()
HA 5 782 804 873 883 83 852
10 776 798 838 846 769 744
FA 5 784 829 828 870 848 913
10 816 842 937 933 906 977
NOM 5 813 859 843 838 879 922
10 862 901 894 857 956 968
TA 5 898 880 876 903 905 963
10 969 976 96 957 100 100
following mechanisms that may take place (a) increase of the
steric congestion or reduction of the diffusivity of the FAndashtriazine
or NOMndashtriazine pseudo-complex (b) increase of the density of
the pseudo-complexrsquos negative charge which enhances the elec-
trostatic repulsion with the negatively charged membranes and (c)
increase of the adsorption capability of the pseudo-complex on the
surface of the membrane in contrast to the hydrophobic nature of
the humic materials The validity of the above mechanism hypothe-
ses adopted by previous researchers [7] depends on the type of
the organic material and consequently on the characteristic chem-
ical properties of both HS (acidity functional groups) and triazines
(basicity hydrophobicity charge) The correlation between humic
material properties and triazine retention is examined in a subse-
quent paragraph
The greater extent of FAndashtriazines pseudo-complexes in com-
parison to NOMndashtriazines pseudo-complexes manifested in the
GPC results is not accompanied by a greater increase of triazine
retention More specifically the increasing order of triazine reten-
tion in the presence of a specific humic material follows the order
TA gt NOM gt FA gt HA in the case of NF270 and XLE membranes
whereas for NF90 the order is TA ge FA gt NOM gt HA Therefore the
interactions between the humic materials and triazines alone
cannot explain the retention performance of the NFULPRO mem-
branes This fact indicates that interactions between the naturally
occurring organic substances and the membranes may also play a
significant role in triazine retention as previously observed [33] In
view of these observations dead-end filtration tests using the three
IHSS organic materials and the selected NFULPRO membranes are
carried out in this laboratory in order to distinguish the effects of
organic fouling and of HSndashtriazines interactions on triazine perme-
ation during nanofiltration
323 Effect of humic substance concentration on triazine
retention
Surveying the literature on the complicated influence of organic
matter on retention of pesticides by NFRO membranes reveals a
considerable variability regarding the importance of the organic
matter concentration Experiments in a nanofiltration pilot unit on
the removal of atrazine and simazine have demonstrated the sig-
nificance of the organic load in the feed water since the removal
efficiency increased from 50 to 90ndash100 when dissolved organic
carbon varied between 04 and 36 mg Lminus1 [7] In contrast Berg et
al [8] reported no increase of pesticide retention (atrazine and
simazine among them) by several NF membranes with increas-
ing organic matter concentration Along the same lines Zhang et
al [11] showed comparable simazine retention between tap water
and river water although the latter had a high concentration of
NOM whereas tap water had a low NOM concentration In an effort
to understand the reasons for the above inconsistency in the lit-
erature Nghiem et al [40] took into consideration the effect of
ionic strength Experiments with hormone mimicking trace organic
contaminants revealed the significance of solution ionic strength
in influencing NOMndashtrace organic interactions which is probably
the cause for the ambiguity of literature data regarding the role
of background organic matter in trace organic rejection The mag-
nitude of ionic strength in terms of calcium ion content on the
NOMndashtriazines interaction has been also recorded through the GPC
tests described previously and is subsequently examined Although
ionic strength can strongly influence the HSndashtriazines interactions
the effect of the type of the humic substance seems to play also a
significant role on triazine retention This is evident in the results
included in Table 6 regarding retention experiments in the pres-
ence of a medium (5 mg Lminus1) and relatively high (10 mg Lminus1) HS
concentration in the absence of ionic environment
Table 6 shows a clear increase of triazine retention with increas-
ing humic material concentration More specifically by doubling
the concentration of FA NOM and TA in the feed solutions atrazine
and prometryn retention increases 4ndash13 and 2ndash10 respectively
depending on the membrane and the humic material used An
exception is observed again in the case of humic acids since the
increased organic load results in negligible or slightly negative
effects of atrazine and prometryn retention Regarding the XLE
membrane the use of medium HA concentration results in similar
retention performance as in organic-free solutions On the other
hand higher HA concentration results in a notable reduction of
triazine retention ranging up to 74 and 127 for atrazine and prom-
etryn respectively As previously discussed this is attributed to the
increased HA adsorption on the membrane surface (Table 8) which
in turn affects triazine adsorption on XLE membrane and therefore
retention This trend is not observed in the case of FA NOM and
TA despite their adsorptive capacity on the membrane materials
This finding is probably related to fouling phenomena which tend
to modify the membrane properties (ie surface charge hydropho-
bicity morphology) and therefore triazine adsorption Although it
is difficult to use mass balances for calculating triazine adsorption
during the combined filtration with humic substances (due to ana-
lytical problems related to the concentrate samples) the changes
of triazine concentration in the permeate samples provide a good
indication of the potential adsorption of atrazine and prometryn
on the membrane surface Furthermore the increased passage of
triazines in the permeate side as a result of HA adsorption on the
XLE membrane is in accord with the increase of triazine adsorption
on the membrane and therefore with the reduced retention values
324 Effect of calcium ion on triazine retention
From Fig 5 it is evident that the presence of calcium acts
positively in both atrazine and prometryn retention during the
combined nanofiltration with all four types of humic substances
Furthermore the higher interactions between the humic organ-
ics and triazines manifested in the GPC tests are in agreement
with the increased retention performance of atrazine and prom-
etryn by the three membranes used Additionally the suggestion
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 95
of ligand exchange in which calcium serves as a bridging ion and
acts as a possible site for higher adsorption of triazine molecules
on humic substances [38] seems to be in accord with the forma-
tion of an increased number of pseudo-complexes between humic
matter and triazines which explain the increase of herbicide reten-
tion
Among the four HS used tannic acid is associated with the high-
est triazine retention since both triazines are fully retained in the
presence of calcium by all three membranes tested Moreover the
enhanced interaction between humic acids and triazines as a result
of the calcium addition to the feed solutions is accompanied by
a significant increase of atrazine and prometryn retention In this
case calcium ions favour the binding between HA and triazines
which in other cases would not have taken place this is in agree-
ment with the preceding GPC and NFULPRO results
Prometryn exhibits again the highest retention values in com-
parison to atrazine which may be attributed to the relatively higher
prometryn hydrophobicity and basicity (Table 1) The former prop-
erty likely favours hydrophobic interactions between prometryn
and the HSndashcalcium complexes whereas the latter renders prom-
etryn highly effective in mechanisms involving electron-transfer
with the HSndashcalcium complexes [19] This mechanism is due to the
lower acidic functional group content of the HS as a result of the
complexation with calcium ions in the feed solutions Sposito et
al [19] suggested that atrazine itself does not readily participate
in electron-transfer reactions with humic substances However
they demonstrated that hydroxyl-atrazine does react through an
electron-transfer mechanism with HA and FA It is noted that
atrazine is readily converted to hydroxyl-atrazine even in labo-
ratory samples at low water content and this may explain the
presence of some of the electron transfer products detected in stud-
ies of atrazinendashHA interactions [41]
Surveying the literature however it is noted that some stud-
ies either with dialysis membranes [1213] or NF membranes [9]
have shown reduced values of atrazine retention when divalent
calcium is present together with natural organic matter includ-
ing the NOM surrogate tannic acid According to Devitt et al [9]
Devitt and Wiesner [12] this is due to the reduced association of
atrazine and NOM as a result of the occupation of interaction sites
by calcium andor the reduced access of atrazine to NOM sites due
to changes in molecular conformation The GPC results in this study
have shown that this is not the case since the presence of calcium
tends to increase the interaction of humic substances with triazine
compounds like atrazine
The conflicting results between the previous studies and the
present work may be attributed to the different types of membranes
and filtration techniques used More specifically the use of cellulose
ester membranes as well as the experimentation with batch dialy-
sis systems where concentration and osmotic pressure difference
serve as the driving force for solute transport (ie in the absence of
hydrodynamic forces as in the present study) may explain the dif-
ferences regarding the calcium effect on triazine retention Runge
et al [42] claim that the SpectraPor cellulose ester membranes
used in their studies (having a molecular weight cut-off of 100 Da)
Fig 6 Retention of atrazine (a) and prometryn (b) as a function of humic material total aciditymdashcalculations based on the values presented in Table 2 taking into consideration
the carbon composition and the HS concentration
Fig 7 Retention of atrazine (a) and prometryn (b) as a function of humic material carboxylic aciditymdashcalculations based on the values presented in Table 2 taking into
consideration the carbon composition and the HS concentration
96 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
present a significant incompatibility with distilled water whereas
a significant membrane potential can build up with certain salts
which can inhibit or enhance diffusion depending on the size of
the solutes diffusing and the polarity of the membrane potential
generated In this case an increased diffusion of hydrated calcium
ions to the permeate side in order to balance the concentrations
between the two sides of the cellulose ester membrane may result
in a pore blockage or a positive charge on the membrane surface
which in turn could lead to an increased passage of free dissolved
atrazine to the permeate side
Another possible explanation for this inconsistency is the time
period of the filtration tests According to the protocol followed
by previous researchers [1213] the dialysis time period ranged
between 1 and 5 days at the end of which concentration measure-
ments were performed On the other hand the filtration periods
in the case of the present dead-end nanofiltration experiments
ranged between 2 and 5 h depending on the membrane used To
this period one more hour should be added in which the feed solu-
tion was stirred in the cell without pressurization Even though
triazine adsorption on the membrane surface was observed to sta-
bilize within the relatively short period of nanofiltration there is
a possibility that longer filtration periods may have resulted in
higher adsorption rates which in turn could lead to greater pas-
sage of triazine to the permeate side and therefore to reduced
retention values This explanation is in agreement with the pilot
tests performed by Boussahel et al [43] who report that the
increasing adsorption of humic matter on the membranes leads
to an increase of pesticide adsorption on the membranes which
facilitates the diffusion of pesticides in the direction of perme-
ate
33 Correlation between humic material properties and triazine
retention
The correlation of atrazine and prometryn retention with the
total carboxylic and phenolic acidity of the humic substances used
in the present study as well as the role of the molecular mass of HS
on triazine retention are depicted in Figs 6ndash9 These are results of
nanofiltration experiments performed in the absence of calcium
Triazine removal by the three NFULPRO membranes seems
to poorly correlate with the total acidity of the humic material
This is evident especially in the case of NF270 and NF90 mem-
branes (Fig 6) The XLE membrane on the other hand presents a
reduced tendency for atrazine and prometryn removal when acid-
ity increases This is attributed to the characteristic low negative
charge of the XLE membrane surface which causes weak repulsions
with the negatively charged humic organics This leads to a greater
adsorption of humic compounds on the membrane surface (Table 8)
and consequently to greater triazine adsorption which favours the
diffusion of the triazines to the permeate side On the other hand
the strong negative charge of the NF270 and NF90 membranes [20]
promotes a strong repulsion between HS and the membranes The
greater the negative charge of the humic organic the higher the
repulsion which in turn results in higher concentration of HS in
the bulk Consequently the interactions between HS and triazines
in the bulk are increased leading to an improved triazine removal
Fig 8 Retention of atrazine (a) and prometryn (b) as a function of humic material phenolic aciditymdashcalculations based on the values presented in Table 2 taking into
consideration the carbon composition and the HS concentration
Fig 9 Retention of atrazine (a) and prometryn (b) as a function of humic substance molecular mass (based on the values presented in Table 3)
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 97
This is more evident in the case of the wide pore NF270 membrane
since the NF90 membrane seems to retain atrazine and prometryn
to a higher degree in the presence of TA which is characterized by
a lower total acidity among the four HS used in this study
Humic compounds contain a variety of functional groups
such as aliphatic and aromatic carboxyls phenolic hydroxyls
alcoholic hydroxyls carbonyls quinones methoxyls and amino
groups These functional groups are the reactive sites in the humic
molecules As carboxylate protonation seems to be critical for the
capacity of triazine binding [44] it is of interest to correlate the
effect of carboxylic acidity on herbicide removal by nanofiltration
The effect of carboxylic acidity on atrazine and prometryn retention
by the three NFULPRO membranes is depicted in Fig 7
The results show a negative trend of triazine retention with
an increased carboxylic acidity of humic substances This is clear
in the case of NF270 and XLE membranes while NF90 does not
seem to be significantly affected by the changes in the number of
carboxyl groups of a humic molecule Remarkably the smaller TA
molecules containing fewer carboxyl groups per unit mass (Table 2)
exhibit the highest triazine retention (sim100) as a result of the even
higher triazine binding capacity in contrast to the IHSS organics
which contain a higher carboxylic group content Moreover experi-
ments in batch filtration mode result in HS concentration increasing
with time which in turn leads to a reduced number of carboxylic
binding sites available to the two triazines This is probably due
to the self ldquocompetingrdquo character of the HS used since an increase
in concentration andor HS aggregation tend to reduce the avail-
able binding sites [44] This phenomenon led Wang et al [44] to
propose a model in which hydrogen bonding was considered to be
the primary interaction resulting in triazine binding Therefore the
increase in triazine retention observed in the case of elevated HS
concentration may not be directly related with the carboxylic acid-
ity of the HS but rather with other mechanisms such as hydrogen
bonding
Regarding the phenolic acidity of the humic substances there
does not seem to be a sufficient correlation with atrazine or prom-
etryn retention by the three NFULPRO membranes (Fig 8) Actually
there is a contradictory behaviour between the three IHSS organ-
ics and the NOM surrogate TA The high phenolic content of TA
is related to high retention values of the two triazines while
the smaller and relatively similar phenolic acidity of the IHSS
compounds leads to smaller and different retention values These
results imply that there are other factors influencing the association
between humic organics and herbicides and therefore their reten-
tion by nanofiltration In this respect the molecular structure of the
humic materials as well as the interactions taking place between
the organics and the membranes seem to explain the above incon-
sistencies This is evident in the results depicted in Fig 9 and the
adsorption of humic substances subsequently described
The rather strong relation of triazine retention with the molec-
ular mass of the organics used in this study (Fig 9) leads to the
suggestion that structural features of HS are important for the
removal capacity of triazines Up to now there is no consensus
concerning both the binding mechanism of triazines to humic sub-
stances and the structural features responsible for this binding
Nevertheless the preferential binding with low molecular weight
fractions of humic compounds (especially of FA and TA) deduced
from the GPC and filtration tests results in higher retention val-
Table 7
Humic substance and calcium retention performance of the three NFULPRO membranes (mean retention values for 50 recovery of feed volume as permeate)
Membrane Humic substancea Humic substances retention () Ca2+ retention ()
HSs alone HSs with triazines HSs with triazines and Ca2+ In the presence of HSs
NF270 HA 990 994 1000 949
FA 967 983 991 951
NOM 957 997 1000 951
TA 972 992 997 957
NF90 HA 972 980 1000 994
FA 953 984 1000 990
NOM 985 993 1000 991
TA 967 998 998 994
XLE HA 992 996 1000 994
FA 862 915 994 997
NOM 976 1000 1000 988
TA 923 945 1000 996
a HA FA NOM and TA designate the four humic substances used in this study
Table 8
Adsorption data of humic substances on tested membranes
Membrane Humic substancea Feed mass of humic substance (g) Quantity adsorbed (g) Adsorption ()
Alone With Ca2+ Alone With Ca2+ Alone With Ca2+
NF270 HA 3480 2630 244 2314 07 88
FA 3570 2840 71 1335 02 47
NOM 3390 2660 34 2633 01 99
TA 4050 3300 2025 7161 50 217
NF90 HA 3220 2690 354 3658 11 136
FA 2930 2450 1494 857 51 35
NOM 3000 2910 120 437 04 15
TA 2820 2730 649 3631 23 133
XLE HA 3100 2680 1643 3457 53 129
FA 3050 2810 1830 2866 60 102
NOM 2650 2770 610 1219 23 44
TA 3120 2940 2184 2970 70 141
a HA FA NOM and TA designate the four humic substances used in this study
98 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
ues for the two triazines Thus the interaction with small branched
molecules like FA and TA seems to favour a physical entrapment
mechanism such as the one suggested by Devitt and Wiesner [12]
34 Retention of humic species and calcium ion by the NFULPRO
membranes
The performance of nanofiltration on the humic substances and
calcium removal is summarized in Table 7 Regarding humic sub-
stances the retention is compared for three different feed solutions
(a) solutions of HS alone (b) solutions of HS with the two triazines
and (c) solutions of HS with the two triazines in the presence of cal-
cium In general HS retention is high for all three membranes used
According to the UV measurements IHSS HA exhibits the highest
retention values followed by NOM TA and FA which is apparently
related to their relative molecular masses (Table 3) Complexation
or interaction with triazines as evidenced by the GPC tests appar-
ently plays also a significant role in the relative permeability of the
organic species More specifically the retention of the four organic
compounds examined is increased in the presence of the two her-
bicides which verifies also the binding of atrazine and prometryn
with the four humic substances Thus a comparison between the
retention values of the HS in the absence or presence of triazines
gives also a good insight into the HSndashtriazine interactions The slight
increase (lt1) of HA retention in the presence of the two triazines
is in accord with the relatively low HA binding capacity observed
in the GPC tests On the other hand the increase of 2ndash6 in FA
NOM and TA retention is in agreement with the observed increased
interactions between those substances and the two triazines
In the case of calcium addition in the organicndashtriazine solu-
tions the UV absorbing species of the four organics examined are
retained almost completely by all three membranes used This is
again in agreement with the results obtained with gel chromatogra-
phy According to the literature [12] the addition of calcium results
in charge shielding and neutralization of the organic matter charged
functional groups and can shrink the humic matrix The formation
of aggregates of larger apparent molecular weight due to the sub-
stantial increase in the hydrophobicity of the organic molecules
tends to reduce their permeability through membranes and there-
fore to increase their retention
Apart from the increased retention of triazines and humic sub-
stances there is also a significant improvement of calcium rejection
when the latter is present in the solution In comparison to pure
water solutions of elevated calcium ion content [20] the rejection of
calcium in humic solutions is almost complete This is attributed to
size exclusion mechanisms related to the binding tendency of diva-
lent ions like calcium to the negatively charged humic organics
Among the four humic substances used calcium is rejected more in
the presence of the polyphenol tannic acid which is accompanied
by a higher organic adsorption on all three NFULPRO membranes
tested (Table 8)
From the data summarized in Table 8 it can be seen that the
adsorption of humic substances onto the membrane surfaces is
markedly enhanced in the presence of calcium ions due to the
formation of HA-complexed divalent cations This is also clearly
depicted in the SEM images of virgin and fouled membranes in
the absence or presence of calcium ions Characteristic SEM images
in the case of the Suwannee River NOM filtration with the three
NFULPRO membranes are included in Fig 10 these images show
the severe fouling on the three NFULPRO membranes as a result
of the combined filtration of NOM with CaCl2 salt It is possible
that the divalent cations such as Ca2+ act as a bridge between the
membrane surface and the negatively charged humic molecules
that tend to be repelled by the negatively charged membrane As a
result the electrical repulsion between the membrane surface and
the humic molecules is weakened thus rendering the membrane
Fig 10 SEM images of virgin fouled with NOM and fouled with NOM + calcium membranes (a) NF270 (b) NF90 and (c) XLE membrane
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 99
fouling process more severe [45] On the other hand the relatively
smaller adsorption rates of NOM in the absence of calcium result
in less fouled membranes A similar trend is also observed in the
case of the other three humic substances used in this study Among
the three membranes used XLE exhibits in the absence of calcium
the highest adsorption rates for all four humic substances This is
explained by the relatively high hydrophobicity and the low surface
charge that characterize the XLE membrane surface as previously
explained
4 Concluding remarks
The influence of humic substances on retention of two tri-
azines (atrazine and prometryn) by NFULPRO membranes is rather
complicated and involves combinations of solutendashmembrane and
solutendashsolute interactions In summary the retention of triazines
depends on the type of the humic material present in the feed
water In most cases the combined nanofiltration of triazines and
naturally occurring humic substances facilitates the formation of
complexes with triazines which in turn enhance their removal by
means of membrane filtration This complexation appears to be
related not to the characteristic acidity (phenolic carboxylic) of
the HS used but rather to their molecular conformation More
specifically a preferential binding is observed between triazines
and low molecular weight fractions of humic compounds (espe-
cially of FA and TA) which results in higher retention values for the
two triazines Under all conditions tannic acid exhibits the greatest
effect on triazine retention among the four standard HS compounds
used leading to an almost complete removal of the two triazines
(95ndash100) for all three membranes tested
The removal of triazines is improved in the presence of calcium
which tends to enhance the interaction between HS and triazines
Moreover triazine retention is increased with increasing HS con-
centration to a degree depending on the type of HS Additionally
triazine retention is apparently also affected by the interactions
between the organics and the membrane surface which in turn
may alter triazine retention These results illustrate the complex-
ity of the triazine adsorption and retention mechanisms in the
presence of humic substances that needs further investigation for
improved understanding Moreover regarding triazine removal by
nanofiltration it is of particular interest to distinguish the effect
of the HSndashtriazine interactions from that of membrane HS fouling
Research in this direction is carried out in this laboratory
References
[1] DP Birardar AL Rayburn Chromosomal damage induced by herbicide con-tamination at concentrations observed in public water supplies J Environ Qual24 (1995) 1222ndash1225
[2] IK Konstantinou DG Hela TA Albanis The status of pesticide pollution insurface waters (rivers and lakes) of Greece Part I Review on occurrence andlevels Environ Pollut 141 (3) (2006) 555ndash570
[3] European Environment Agency (EEA) Groundwater quality and quantity inEurope Report Copenhagen June 1999 available in httpreportseeaeuropaeugroundwater07012000enenviassrp199903
[4] EM Thurman DA Goolsby MT Meyer MS Mills ML Pomes DW Kolpin AReconnaissance study of herbicides and their metabolites in surface water of theMidwestern United States using immunoassay and gas chromatographymassspectrometry Environ Sci Technol 26 (1992) 2440ndash2447
[5] C Bellona JE Drewes P Xu G Amy Factors affecting the rejection of organicsolutes during NFRO treatmentmdasha literature review Water Res 38 (2004)2795ndash2809
[6] A Bhattacharya Remediation of pesticide-polluted waters through mem-branes Sep Purif Rev 35 (2006) 1ndash38
[7] KM Agbekodo B Legube S Dard Atrazine and simazine removal mechanismsby nanofiltration influence of natural organic matter concentration Water Res30 (1996) 2535ndash2542
[8] P Berg G Hagmeyer R Gimbel Removal of pesticides and other micropollu-tants by nanofiltration Desalination 113 (1997) 205ndash208
[9] EC Devitt F Ducellier P Coumlteacute MR Wiesner Effects of natural organic mat-ter and the raw water matrix on the rejection of atrazine by pressure-drivenmembranes Water Res 32 (1998) 2563ndash2568
[10] R Boussahel S Bouland KM Moussaoui A Montiel Removal of pesticideresidues in water using the nanofiltration process Desalination 132 (2000)205ndash209
[11] Y Zhang B Van der Bruggen GX Chen L Braeken C Vandecasteele Removalof pesticides by nanofiltration effect of the water matrix Sep Purif Technol38 (2004) 163ndash172
[12] EC Devitt MR Wiesner Dialysis investigations of atrazinendashorganic matterinteractions and the role of a divalent metal Environ Sci Technol 32 (1998)232ndash237
[13] SK Dalton JA Brant MR Wiesner Chemical interactions between dissolvedorganic matter and low-molecular weight organic compounds impacts onmembrane separation J Membr Sci 266 (2005) 30ndash39
[14] NA Kulikova IV Perminova Binding of atrazine to humic substances fromsoil peat and coal related to their structure Environ Sci Technol 36 (2002)3720ndash3724
[15] RL Wershaw The importance of humic substancendashmineral particle complexesin the modeling of contaminant transport in sedimentndashwater systems in RABaker (Ed) Organic Substances and Sediments in Water Humics and SoilsLewis Publishers Chelsea MI 1991 pp 23ndash34
[16] SM Schrap A Opperhuizen Relationship between bioavailability andhydrophobicity reduction of the uptake of organic chemicals by fish due tothe sorption on particles Environ Toxicol Chem 9 (1990) 715ndash724
[17] E Barriuso M Schiavon F Andreux JM Portal Localization of atrazine non-extractable (bound) residues in soil size fractions Chemosphere 12 (1991)1131ndash1140
[18] L Loiseau E Barriuso Characterization of the atrazinersquos bound (nonextractable)residues using fractionation techniques for soil organic matter Environ SciTechnol 36 (2002) 683ndash689
[19] G Sposito L Martin-Neto A Yang Atrazine complexation by soil humic acidsJ Environ Qual 25 (1996) 1203ndash1209
[20] KV Plakas AJ Karabelas Membrane retention of herbicides from single andmulti-solute media the effect of ionic environment J Membr Sci 320 (2008)325ndash334
[21] V Freger J Gilron S Belfer TFC polyamide membranes modified by graftingof hydrophilic polymers an FT-IRAFMTEM study J Membr Sci 209 (2002)283ndash292
[22] P Xu JE Drewes Viability of nanofiltration and ultra-low pressure reverseosmosis membranes for multi-beneficial use of methane produced water SepPurif Technol 52 (2006) 67ndash76
[23] Z Knez A Rizner-Hras K Kokot D Bauman Solubility of some solid triazineherbicides in supercritical carbon dioxide Fluid Phase Equilibria 152 (1998)95ndash108
[24] Weed Science Society of America (WSSA) Herbicide Handbook 7th ed 1994[25] B Van der Bruggen J Schaep W Maes D Wilms C Vandecasteele Nanofiltra-
tion as a treatment method for the removal of pesticides from ground watersDesalination 117 (1998) 139ndash147
[26] KN Reddy MA Locke Molecular properties as descriptors of octanol-waterpartition coefficients of herbicides Water Air Soil Pollut 86 (1996) 389ndash405
[27] IHSS website httpwwwihssgatechedu[28] F Flores-Ceacutespedes M Fernaacutendez-Peacuterez M Villafranca-Saacutenchez E Gonzaacutelez-
Pradas Cosorption study of organic pollutants and dissolved organic matter ina soil Environ Pollut 142 (2006) 449ndash456
[29] R Beckett Z Jue JC Giddings Determination of molecular weight distribu-tions of fulvic and humic acids using flow field-flow fractionation Environ SciTechnol 21 (1987) 289ndash295
[30] PM Reid AE Wilkinson E Tipping MN Jones Determination of molecularweights of humic substances by analytical (UV scanning) ultracentrifugationGeochim Cosmochim Acta 54 (1990) 131ndash138
[31] S Lee B Kwon M Sun J Cho Characterizations of NOM included in NF and UFmembrane permeates Desalination 173 (2005) 131ndash142
[32] MN Jones ND Bryan Colloidal properties of humic substances Adv ColloidInterf Sci 78 (1998) 1ndash48
[33] KV Plakas AJ Karabelas T Wintgens T Melin A study of selected herbicidesretention by nanofiltration membranesmdashthe role of organic fouling J MembrSci 284 (2006) 291ndash300
[34] APHA-AWWA-WPCF Standard Methods for the Examination of Water andWastewater 17th ed Port City Press Baltimore MD 1989 pp 567ndash569
[35] R Artinger G Buckau JI Kim S Geyer Characterization of groundwater humicand fulvic acids of different origin by GPC with UVVis and fluorescence detec-tion Fresenius J Anal Chem 364 (1999) 737ndash745
[36] AI Schaumlfer N Andritsos AJ Karabelas EMV Hoek R Schneider M Nys-troumlm Chapter 8 Fouling in nanofiltration in AI Schaumlfer AG Fane TDWaite (Eds) Nanofiltration Principles and Applications Elsevier Oxford UK2005 pp 173ndash174
[37] I Franco L Catalano M Contin M De Nobili Interaction between organicmodel compounds and pesticides with water-soluble soil humic substancesActa Hydrochim Hydrobiol 29 (2001) 88ndash99
[38] AA Helal DM Imam SM Khalifa HF Aly Interaction of pesticides with humiccompounds and their metal complexes Radiochemistry 48 (4) (2006) 419ndash425
[39] MG Browman G Chester Fate of Pollutants in the Air and Water EnvironmentWiley New York 1977 p 50
[40] LD Nghiem AI Schaefer M Elimelech Nanofiltration of hormone mimickingtrace organic contaminants Sep Sci Technol 40 (2005) 2633ndash2649
[41] R Celis J Cornejo MC Hermosin WC Koskinen Sorptionndashdesorption ofatrazine and simazine by model soil colloidal components Soil Sci Soc AmJ 61 (1997) 436ndash443
100 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
[42] SW Runge KR Shelton SA Melton WM Moran Maintaining the ionic per-meability of a cellulose ester membrane J Biochem Biophys Methods 64(2005) 200ndash206
[43] R Boussahel A Montiel M Baudu Effects of organic and inorganic matter onpesticide rejection by nanofiltration Desalination 145 (2002) 109ndash114
[44] Z Wang DS Gamble CH Langford Interaction of atrazine with Laurentianhumic acid Anal Chim Acta 244 (1991) 135ndash143
[45] C Jucker MM Clark Adsorption of aquatic humic substances on hydrophobicultrafiltration membranes J Membr Sci 97 (1994) 37ndash52
90 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
pure water flux for 30 min at 5 bar (4) filtration of triazines in the
absence of humic materials (ldquofree triazine solutionsrdquo) at 5 bar (5)
filtration of the model HSndashtriazines solution at 5 bar and (6) mea-
surement of the pure water flux for 30 min at 5 bar Compaction
(step 2) is crucial in every membrane filtration protocol as it may
change both the active layer and its support thus affecting the flux
and the rejection properties of the membrane To eliminate this
impact membranes are often subjected to at a higher pressure (here
10 bar) than the operating pressure (5 bar) to ensure flux stability
during experiments [36] Preliminary experiments with the three
membranes tested showed that 1 h of 10 bar pressure application
was more than adequate to achieve membrane stability
All filtration steps were performed with a feed solution of
300 mL while the tests performed with the herbicides were car-
ried out until 150 mL of permeate were collected (50 recovery)
Feed and three permeate samples (50 mL each) were retained for
subsequent analysis in order to determine the evolution of con-
taminant concentration in some cases retentate samples were also
removed for chromatographic analysis Similar to previous studies
[2033] the triazine feed solutions were stirred in the cells for 1 h
without pressure In this way herbicide adsorption on the mem-
brane surface was assumed to reach equilibrium After termination
of the filtration experiments the membrane coupons were dried
for subsequent analysis (SEM) while the test cells were thoroughly
washed with acetone and repeatedly rinsed with Milli-Q water
23 Data treatment
The performance of nanofiltration can be expressed in terms of
the percentage removal of the organic and inorganic substances
The determination of the retention was calculated based on the
method described in our previous study [20] which proved to be
appropriate in the case of batch filtration experiments since the
membrane performance depends on the retentate concentration
which increases with time The concentration of the two triazines
was measured for the feed solution and for all the permeate samples
while retentate samples were removed for subsequent analysis of
the humic and inorganic compounds The retention was evaluated
based on the so-called ldquostablerdquo permeate samples where the water
recovery rate was between 17 and 50 The amount of deposited
humic substance on the membrane surface was determined from
mass balance as follows
A () = 100
(
1 minus
sumj
i=1Vpi
Cpi+ Crj
Vr
Cf Vf
)
(1)
where Cpj Cr Cf are the concentrations of a humic substance in
the permeate sample j retentate and feed respectively while Vpj
Vr Vf are the respective volumes of permeate sample j retentate
and feed Reduction of the pure water flux (FRPW) which may be
considered a measure of the adsorbed humic substances effect on
flux was determined as a percentage of clean water flux Jw before
and after membrane operation as previously described [20]
3 Results and discussion
31 Pure triazine solutions
Prior to the combined filtration of triazines with humic materi-
als pure water solutions with atrazine and prometryn were filtered
at 5 bar in concentrations ranging between 10 and 30 g Lminus1 The
performance of the membranes in terms of triazine retention is
illustrated in Fig 1 The accuracy of experimental results is indi-
cated by the bars which represent the scatter of data obtained from
six replicate experiments for each membrane
Fig 1 Atrazine and prometryn retention from free triazine solutions by the three
selected NFULPRO membranes mean retention values for 50 recovery of feed
volume feed concentration 10ndash30 g Lminus1
As in our previous studies [2033] prometryn the molecule
with the larger molecular weight and size displays the great-
est retention with all three membranes used The performance
of membranes regarding retention of the two triazines follows
the order XLE ge NF90 gt NF270 which is consistent with the results
obtained in our previous work [20] Comparing the experimen-
tal results for double solute mixtures of atrazine and prometryn
in the case of high (600ndash700 g Lminus1) [20] and low feed concen-
trations (10ndash30 g Lminus1) the differences in retention values vary
between 7 and 13 In general the retention results with pure tri-
azine solutions are in agreement with observations made by other
researchers [71125] in that herbicide concentration does not sig-
nificantly affect their retention The fact that the filtration of lower
feed concentrations leads to a slight reduction of triazine retention
(especially in the case of XLE membrane) could be attributed to the
amount of triazines adsorbed on the selected membranes Based
on the mass balances summarized in Table 4 the lower feed tri-
azine concentration is accompanied in general by a slightly lower
adsorption in comparison to the results obtained with higher feed
concentrations [20] something that is more pronounced in the case
of the less tight NF270 membrane
32 Aqueous solutions of mixed humic substances and triazines
321 GPC results on triazinesndashhumic substances interactions
The UVndashvis spectra of fractions collected during GPC chromatog-
raphy of HS alone and HS with atrazine andor prometryn in the
absence or presence of calcium ions are depicted in Figs 2ndash4
The comparison of the chromatographic behaviour of the solu-
tions used in qualitative terms allows an assessment of possible
interactions between humic substances and the two triazines [37]
The elution profiles display a maximum for all three IHSS organ-
ics under the same test conditions at retention time 185ndash195 min
In the case of the low molecular weight triazines the elution pro-
file maximum at a wavelength of 225 nm and concentration of
10 mg Lminus1 (high concentrations used in order to be measurable
by UV absorption) appear at 110 min (data not presented here)
The longer retention time of atrazine and prometryn is due to the
fact that small molecules can penetrate into the pores of the gel
and follow a torturous path through the column On the other
hand large molecules such as humic substances are excluded
from internal pore spaces in the gel and elute faster from the col-
umn Co-elution of small triazine molecules with the much larger
humic molecules indicates their association with the humic frac-
tion Humic-bound triazines appear to display the behaviour of the
larger humic molecules as they are also excluded from the internal
pores of the gel [37] This is reflected in the reduced GPC areas of
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 91
Table 4
Adsorption data of triazines on tested membranes
Membrane Feed mass of
triazinesa (g)
Quantity adsorbed
(g)
adsorption adsorption
[19]b
NF270 45 plusmn 27(A) 048 plusmn 027 127 plusmn 18 165
53 plusmn 28(P) 020 plusmn 008 38 plusmn 13 275
NF90 52 plusmn 28(A) 122 plusmn 083 248 plusmn 66 305
55 plusmn 13(P) 105 plusmn 033 250 plusmn 67 262
XLE 66 plusmn 10(A) 114 plusmn 012 219 plusmn 12 230
74 plusmn 11(P) 084 plusmn 008 171 plusmn 09 295
a (A) and (P) designate atrazine and prometryn respectivelyb Adsorption values for higher triazine (atrazine and prometryn) concentrations (600ndash700 g Lminus1 each)
Fig 2 GPC elution profiles of Suwannee River humic acid (HA) in the absence or presence of triazines andor calcium (detection by UV absorption at 254 nm)
the humic materials when treated together with the two triazines
(Table 5) In summary aquatic humic substances from the same
water source (Suwannee River) show differences in their GPC elu-
tion behaviour when treated together with calcium andor with
either of the two triazines
The peaks of the GPC chromatograms reflect differences in spe-
cific absorption Distinct differences between HS and atrazine or
prometryn are found in the case of FA and NOM In contrast HA
show a rather small variation on elution behaviour This is also in
accord with the values presented in Table 5 Complexation with
the NOM and FA appears to occur for both atrazine and prometryn
molecules the interactions with NOM indicate greater affinity to
atrazine On the other hand FA interactions appear to be stronger
with prometryn In Figs 2ndash4 and Table 5 one can observe that
among the three HS used FA seems to interact more with the two
triazines The weakest interaction is observed in the case of humic
acids which is reflected in the almost identical elution profiles and
the low GPC reduction areas
The presence of calcium in the solutions of humic substances
leads to a sharp reduction of HS concentrations (Table 5) indi-
cating the formation of calcium humate and fulvate complexes
According to the literature when negatively charged humic colloids
Table 5
Percentage () reduction of GPC integration areas of humic substances due to the presence of triazines andor calcium
Humic substances combined with Percentage GPC area reduction ()
HA FA NOM
40 mg Lminus1 Ca2+ 26 17 138
10 g Lminus1 atrazine 06 64 27
10 g Lminus1 prometryn 02 81 lt01
10 g Lminus1 atrazine + 10 g Lminus1 prometryn 23 80 30
10 g Lminus1 atrazine + 10 g Lminus1 prometryn + 40 mg Lminus1 Ca2+ 151 221 279
92 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
Fig 3 GPC elution profiles of Suwannee River fulvic acid (FA) in the absence or presence of triazines andor calcium (detection by UV absorption at 275 nm)
come in contact with positively charged metal ions (like calcium)
several binding mechanisms become effective The bond strengths
vary from a weak physical adsorption to strong chemical bond-
ing involving chelation reactions [38] In the case of herbicides
the partially chelated calcium may serve here as a bridging ion
and act as a possible site for the adsorption of a triazine molecule
by displacing water of hydration Indeed the addition of calcium
enhances the interaction between the humic substances and the
two triazines something that is manifested by the higher GPC area
reductions and the lower elution profiles Browman and Chester
[39] also concluded that cations on the surface of humic material
can act as binding bridges between synthetic chemicals and humic
compounds Similarly Helal et al [38] reported that the exchange
reaction between the pesticides and the humic compounds takes
place on the same sites at which binding of humic compound to the
metal occurs Moreover these authors claim that the pesticidendashHA
Fig 4 GPC elution profiles of Suwannee River NOM in the absence or presence of triazines andor calcium (detection by UV absorption at 254 nm)
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 93
Fig 5 Retention of atrazine (A) and prometryn (P) by the three NFULPRO membranes in the absence or presence of humic substances (HA FA NOM TA) andor calcium
ions
or ndashFA complex is stronger than the metalndashhumate or ndashfulvate
complex The mechanisms involved in the interactions between
triazines and calciumndashHS complexes may be described by either
a physical adsorption of the herbicide on the surface of metal
humatefulvate complex or a ligand exchange in which the metal
serves as a bridging ion and acts as a possible site for adsorption of
the herbicide molecule by displacing water of hydration [38]
322 Effect of different types of humic materials on triazine
retention
Retention results of atrazine and prometryn during the com-
bined nanofiltration with the four selected humic substances are
depicted in Fig 5 Results of triazine retention from organic solu-
tions with elevated calcium ion content are also included in Fig 5
and discussed in Section 324 Depending on the type of humic
material present increased retention of the two triazines may be
negligible or significant Specifically the use of HA results in reten-
tion values that are comparable to those obtained in the absence
of organics An exception is observed in the case of XLE membrane
where the combined filtration of triazines and HA leads to relatively
smaller retention values (7 and 13 for atrazine and prometryn
respectively) On the other hand the presence of tannic acid results
in a significant increase of triazine retention for all three mem-
branes tested In particular the addition of TA at a concentration of
10 mg Lminus1 (yielding a triazineTA ratio of 1 g1 mg) results in 14ndash21
and 20ndash29 increase of prometryn and atrazine retention respec-
tively The enhancement of pesticide removal in the presence of
the NOM surrogate tannic acid has been also observed by previ-
ous researchers [91213] According to Devitt and Wiesner [12] the
increased retention of a triazine molecule like atrazine in the pres-
ence of TA is due to the branched structure of TA which traps the
triazine within the macromolecule Moreover these authors sug-
gest that molecular conformations and weak interactions between
functional groups may combine to produce physical entrapment of
triazines through a low multiplicative probability of escape result-
ing from a large number of encounters with low-energy sites on the
tannic acid molecule (eg hydrogen bonding)
In the case of the combined nanofiltration of atrazine and prom-
etryn with FA or NOM a modest increase of retention in comparison
to the pure triazine solutions was recorded In particular depending
on the membrane used the presence of FA resulted in an increase
of 8ndash15 of atrazine retention and 4ndash14 of prometryn retention
whereas nanofiltration in the presence of NOM led to 12ndash17 and
3ndash15 increase of atrazine and prometryn retention respectively
The interactions between atrazine and prometryn with the
humic substances observed in the GPC experiments seem to agree
(up to a point) with the trends in triazine retention by the three
NFULPRO membranes Indeed the negligible interactions observed
between the two herbicides and the humic acids are in accord with
the slight differences observed in atrazine and prometryn retention
by the NF270 and NF90 membranes On the other hand the some-
what reduced retention observed in the case of XLE membrane
may be not only due to the weak interactions between triazines
and humic acids but also to the hydrophobic interactions between
HA and the membrane surface which tends to affect atrazine and
prometryn retention This is in accord with the observed higher
adsorption rates of HA on the XLE membrane (Table 8) which in turn
may affect the surface properties of the membrane and therefore
triazine retention
The GPC results in the case of FA and NOM are in accord with
the retention results of triazines for all three membranes FA and
NOM interact with the two herbicides to form complexes of various
stabilities The formation of these pseudo-complexes may explain
the higher retention of atrazine and prometryn as a result of the
94 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
Table 6
Average retention values of atrazine and prometryn at two different humic substance concentrations by the three NFULPRO membranes
Humic substance Cfeed (mg Lminus1) NF270 NF90 XLE
Ratrazine () Rprometryn () Ratrazine () Rprometryn () Ratrazine () Rprometryn ()
HA 5 782 804 873 883 83 852
10 776 798 838 846 769 744
FA 5 784 829 828 870 848 913
10 816 842 937 933 906 977
NOM 5 813 859 843 838 879 922
10 862 901 894 857 956 968
TA 5 898 880 876 903 905 963
10 969 976 96 957 100 100
following mechanisms that may take place (a) increase of the
steric congestion or reduction of the diffusivity of the FAndashtriazine
or NOMndashtriazine pseudo-complex (b) increase of the density of
the pseudo-complexrsquos negative charge which enhances the elec-
trostatic repulsion with the negatively charged membranes and (c)
increase of the adsorption capability of the pseudo-complex on the
surface of the membrane in contrast to the hydrophobic nature of
the humic materials The validity of the above mechanism hypothe-
ses adopted by previous researchers [7] depends on the type of
the organic material and consequently on the characteristic chem-
ical properties of both HS (acidity functional groups) and triazines
(basicity hydrophobicity charge) The correlation between humic
material properties and triazine retention is examined in a subse-
quent paragraph
The greater extent of FAndashtriazines pseudo-complexes in com-
parison to NOMndashtriazines pseudo-complexes manifested in the
GPC results is not accompanied by a greater increase of triazine
retention More specifically the increasing order of triazine reten-
tion in the presence of a specific humic material follows the order
TA gt NOM gt FA gt HA in the case of NF270 and XLE membranes
whereas for NF90 the order is TA ge FA gt NOM gt HA Therefore the
interactions between the humic materials and triazines alone
cannot explain the retention performance of the NFULPRO mem-
branes This fact indicates that interactions between the naturally
occurring organic substances and the membranes may also play a
significant role in triazine retention as previously observed [33] In
view of these observations dead-end filtration tests using the three
IHSS organic materials and the selected NFULPRO membranes are
carried out in this laboratory in order to distinguish the effects of
organic fouling and of HSndashtriazines interactions on triazine perme-
ation during nanofiltration
323 Effect of humic substance concentration on triazine
retention
Surveying the literature on the complicated influence of organic
matter on retention of pesticides by NFRO membranes reveals a
considerable variability regarding the importance of the organic
matter concentration Experiments in a nanofiltration pilot unit on
the removal of atrazine and simazine have demonstrated the sig-
nificance of the organic load in the feed water since the removal
efficiency increased from 50 to 90ndash100 when dissolved organic
carbon varied between 04 and 36 mg Lminus1 [7] In contrast Berg et
al [8] reported no increase of pesticide retention (atrazine and
simazine among them) by several NF membranes with increas-
ing organic matter concentration Along the same lines Zhang et
al [11] showed comparable simazine retention between tap water
and river water although the latter had a high concentration of
NOM whereas tap water had a low NOM concentration In an effort
to understand the reasons for the above inconsistency in the lit-
erature Nghiem et al [40] took into consideration the effect of
ionic strength Experiments with hormone mimicking trace organic
contaminants revealed the significance of solution ionic strength
in influencing NOMndashtrace organic interactions which is probably
the cause for the ambiguity of literature data regarding the role
of background organic matter in trace organic rejection The mag-
nitude of ionic strength in terms of calcium ion content on the
NOMndashtriazines interaction has been also recorded through the GPC
tests described previously and is subsequently examined Although
ionic strength can strongly influence the HSndashtriazines interactions
the effect of the type of the humic substance seems to play also a
significant role on triazine retention This is evident in the results
included in Table 6 regarding retention experiments in the pres-
ence of a medium (5 mg Lminus1) and relatively high (10 mg Lminus1) HS
concentration in the absence of ionic environment
Table 6 shows a clear increase of triazine retention with increas-
ing humic material concentration More specifically by doubling
the concentration of FA NOM and TA in the feed solutions atrazine
and prometryn retention increases 4ndash13 and 2ndash10 respectively
depending on the membrane and the humic material used An
exception is observed again in the case of humic acids since the
increased organic load results in negligible or slightly negative
effects of atrazine and prometryn retention Regarding the XLE
membrane the use of medium HA concentration results in similar
retention performance as in organic-free solutions On the other
hand higher HA concentration results in a notable reduction of
triazine retention ranging up to 74 and 127 for atrazine and prom-
etryn respectively As previously discussed this is attributed to the
increased HA adsorption on the membrane surface (Table 8) which
in turn affects triazine adsorption on XLE membrane and therefore
retention This trend is not observed in the case of FA NOM and
TA despite their adsorptive capacity on the membrane materials
This finding is probably related to fouling phenomena which tend
to modify the membrane properties (ie surface charge hydropho-
bicity morphology) and therefore triazine adsorption Although it
is difficult to use mass balances for calculating triazine adsorption
during the combined filtration with humic substances (due to ana-
lytical problems related to the concentrate samples) the changes
of triazine concentration in the permeate samples provide a good
indication of the potential adsorption of atrazine and prometryn
on the membrane surface Furthermore the increased passage of
triazines in the permeate side as a result of HA adsorption on the
XLE membrane is in accord with the increase of triazine adsorption
on the membrane and therefore with the reduced retention values
324 Effect of calcium ion on triazine retention
From Fig 5 it is evident that the presence of calcium acts
positively in both atrazine and prometryn retention during the
combined nanofiltration with all four types of humic substances
Furthermore the higher interactions between the humic organ-
ics and triazines manifested in the GPC tests are in agreement
with the increased retention performance of atrazine and prom-
etryn by the three membranes used Additionally the suggestion
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 95
of ligand exchange in which calcium serves as a bridging ion and
acts as a possible site for higher adsorption of triazine molecules
on humic substances [38] seems to be in accord with the forma-
tion of an increased number of pseudo-complexes between humic
matter and triazines which explain the increase of herbicide reten-
tion
Among the four HS used tannic acid is associated with the high-
est triazine retention since both triazines are fully retained in the
presence of calcium by all three membranes tested Moreover the
enhanced interaction between humic acids and triazines as a result
of the calcium addition to the feed solutions is accompanied by
a significant increase of atrazine and prometryn retention In this
case calcium ions favour the binding between HA and triazines
which in other cases would not have taken place this is in agree-
ment with the preceding GPC and NFULPRO results
Prometryn exhibits again the highest retention values in com-
parison to atrazine which may be attributed to the relatively higher
prometryn hydrophobicity and basicity (Table 1) The former prop-
erty likely favours hydrophobic interactions between prometryn
and the HSndashcalcium complexes whereas the latter renders prom-
etryn highly effective in mechanisms involving electron-transfer
with the HSndashcalcium complexes [19] This mechanism is due to the
lower acidic functional group content of the HS as a result of the
complexation with calcium ions in the feed solutions Sposito et
al [19] suggested that atrazine itself does not readily participate
in electron-transfer reactions with humic substances However
they demonstrated that hydroxyl-atrazine does react through an
electron-transfer mechanism with HA and FA It is noted that
atrazine is readily converted to hydroxyl-atrazine even in labo-
ratory samples at low water content and this may explain the
presence of some of the electron transfer products detected in stud-
ies of atrazinendashHA interactions [41]
Surveying the literature however it is noted that some stud-
ies either with dialysis membranes [1213] or NF membranes [9]
have shown reduced values of atrazine retention when divalent
calcium is present together with natural organic matter includ-
ing the NOM surrogate tannic acid According to Devitt et al [9]
Devitt and Wiesner [12] this is due to the reduced association of
atrazine and NOM as a result of the occupation of interaction sites
by calcium andor the reduced access of atrazine to NOM sites due
to changes in molecular conformation The GPC results in this study
have shown that this is not the case since the presence of calcium
tends to increase the interaction of humic substances with triazine
compounds like atrazine
The conflicting results between the previous studies and the
present work may be attributed to the different types of membranes
and filtration techniques used More specifically the use of cellulose
ester membranes as well as the experimentation with batch dialy-
sis systems where concentration and osmotic pressure difference
serve as the driving force for solute transport (ie in the absence of
hydrodynamic forces as in the present study) may explain the dif-
ferences regarding the calcium effect on triazine retention Runge
et al [42] claim that the SpectraPor cellulose ester membranes
used in their studies (having a molecular weight cut-off of 100 Da)
Fig 6 Retention of atrazine (a) and prometryn (b) as a function of humic material total aciditymdashcalculations based on the values presented in Table 2 taking into consideration
the carbon composition and the HS concentration
Fig 7 Retention of atrazine (a) and prometryn (b) as a function of humic material carboxylic aciditymdashcalculations based on the values presented in Table 2 taking into
consideration the carbon composition and the HS concentration
96 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
present a significant incompatibility with distilled water whereas
a significant membrane potential can build up with certain salts
which can inhibit or enhance diffusion depending on the size of
the solutes diffusing and the polarity of the membrane potential
generated In this case an increased diffusion of hydrated calcium
ions to the permeate side in order to balance the concentrations
between the two sides of the cellulose ester membrane may result
in a pore blockage or a positive charge on the membrane surface
which in turn could lead to an increased passage of free dissolved
atrazine to the permeate side
Another possible explanation for this inconsistency is the time
period of the filtration tests According to the protocol followed
by previous researchers [1213] the dialysis time period ranged
between 1 and 5 days at the end of which concentration measure-
ments were performed On the other hand the filtration periods
in the case of the present dead-end nanofiltration experiments
ranged between 2 and 5 h depending on the membrane used To
this period one more hour should be added in which the feed solu-
tion was stirred in the cell without pressurization Even though
triazine adsorption on the membrane surface was observed to sta-
bilize within the relatively short period of nanofiltration there is
a possibility that longer filtration periods may have resulted in
higher adsorption rates which in turn could lead to greater pas-
sage of triazine to the permeate side and therefore to reduced
retention values This explanation is in agreement with the pilot
tests performed by Boussahel et al [43] who report that the
increasing adsorption of humic matter on the membranes leads
to an increase of pesticide adsorption on the membranes which
facilitates the diffusion of pesticides in the direction of perme-
ate
33 Correlation between humic material properties and triazine
retention
The correlation of atrazine and prometryn retention with the
total carboxylic and phenolic acidity of the humic substances used
in the present study as well as the role of the molecular mass of HS
on triazine retention are depicted in Figs 6ndash9 These are results of
nanofiltration experiments performed in the absence of calcium
Triazine removal by the three NFULPRO membranes seems
to poorly correlate with the total acidity of the humic material
This is evident especially in the case of NF270 and NF90 mem-
branes (Fig 6) The XLE membrane on the other hand presents a
reduced tendency for atrazine and prometryn removal when acid-
ity increases This is attributed to the characteristic low negative
charge of the XLE membrane surface which causes weak repulsions
with the negatively charged humic organics This leads to a greater
adsorption of humic compounds on the membrane surface (Table 8)
and consequently to greater triazine adsorption which favours the
diffusion of the triazines to the permeate side On the other hand
the strong negative charge of the NF270 and NF90 membranes [20]
promotes a strong repulsion between HS and the membranes The
greater the negative charge of the humic organic the higher the
repulsion which in turn results in higher concentration of HS in
the bulk Consequently the interactions between HS and triazines
in the bulk are increased leading to an improved triazine removal
Fig 8 Retention of atrazine (a) and prometryn (b) as a function of humic material phenolic aciditymdashcalculations based on the values presented in Table 2 taking into
consideration the carbon composition and the HS concentration
Fig 9 Retention of atrazine (a) and prometryn (b) as a function of humic substance molecular mass (based on the values presented in Table 3)
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 97
This is more evident in the case of the wide pore NF270 membrane
since the NF90 membrane seems to retain atrazine and prometryn
to a higher degree in the presence of TA which is characterized by
a lower total acidity among the four HS used in this study
Humic compounds contain a variety of functional groups
such as aliphatic and aromatic carboxyls phenolic hydroxyls
alcoholic hydroxyls carbonyls quinones methoxyls and amino
groups These functional groups are the reactive sites in the humic
molecules As carboxylate protonation seems to be critical for the
capacity of triazine binding [44] it is of interest to correlate the
effect of carboxylic acidity on herbicide removal by nanofiltration
The effect of carboxylic acidity on atrazine and prometryn retention
by the three NFULPRO membranes is depicted in Fig 7
The results show a negative trend of triazine retention with
an increased carboxylic acidity of humic substances This is clear
in the case of NF270 and XLE membranes while NF90 does not
seem to be significantly affected by the changes in the number of
carboxyl groups of a humic molecule Remarkably the smaller TA
molecules containing fewer carboxyl groups per unit mass (Table 2)
exhibit the highest triazine retention (sim100) as a result of the even
higher triazine binding capacity in contrast to the IHSS organics
which contain a higher carboxylic group content Moreover experi-
ments in batch filtration mode result in HS concentration increasing
with time which in turn leads to a reduced number of carboxylic
binding sites available to the two triazines This is probably due
to the self ldquocompetingrdquo character of the HS used since an increase
in concentration andor HS aggregation tend to reduce the avail-
able binding sites [44] This phenomenon led Wang et al [44] to
propose a model in which hydrogen bonding was considered to be
the primary interaction resulting in triazine binding Therefore the
increase in triazine retention observed in the case of elevated HS
concentration may not be directly related with the carboxylic acid-
ity of the HS but rather with other mechanisms such as hydrogen
bonding
Regarding the phenolic acidity of the humic substances there
does not seem to be a sufficient correlation with atrazine or prom-
etryn retention by the three NFULPRO membranes (Fig 8) Actually
there is a contradictory behaviour between the three IHSS organ-
ics and the NOM surrogate TA The high phenolic content of TA
is related to high retention values of the two triazines while
the smaller and relatively similar phenolic acidity of the IHSS
compounds leads to smaller and different retention values These
results imply that there are other factors influencing the association
between humic organics and herbicides and therefore their reten-
tion by nanofiltration In this respect the molecular structure of the
humic materials as well as the interactions taking place between
the organics and the membranes seem to explain the above incon-
sistencies This is evident in the results depicted in Fig 9 and the
adsorption of humic substances subsequently described
The rather strong relation of triazine retention with the molec-
ular mass of the organics used in this study (Fig 9) leads to the
suggestion that structural features of HS are important for the
removal capacity of triazines Up to now there is no consensus
concerning both the binding mechanism of triazines to humic sub-
stances and the structural features responsible for this binding
Nevertheless the preferential binding with low molecular weight
fractions of humic compounds (especially of FA and TA) deduced
from the GPC and filtration tests results in higher retention val-
Table 7
Humic substance and calcium retention performance of the three NFULPRO membranes (mean retention values for 50 recovery of feed volume as permeate)
Membrane Humic substancea Humic substances retention () Ca2+ retention ()
HSs alone HSs with triazines HSs with triazines and Ca2+ In the presence of HSs
NF270 HA 990 994 1000 949
FA 967 983 991 951
NOM 957 997 1000 951
TA 972 992 997 957
NF90 HA 972 980 1000 994
FA 953 984 1000 990
NOM 985 993 1000 991
TA 967 998 998 994
XLE HA 992 996 1000 994
FA 862 915 994 997
NOM 976 1000 1000 988
TA 923 945 1000 996
a HA FA NOM and TA designate the four humic substances used in this study
Table 8
Adsorption data of humic substances on tested membranes
Membrane Humic substancea Feed mass of humic substance (g) Quantity adsorbed (g) Adsorption ()
Alone With Ca2+ Alone With Ca2+ Alone With Ca2+
NF270 HA 3480 2630 244 2314 07 88
FA 3570 2840 71 1335 02 47
NOM 3390 2660 34 2633 01 99
TA 4050 3300 2025 7161 50 217
NF90 HA 3220 2690 354 3658 11 136
FA 2930 2450 1494 857 51 35
NOM 3000 2910 120 437 04 15
TA 2820 2730 649 3631 23 133
XLE HA 3100 2680 1643 3457 53 129
FA 3050 2810 1830 2866 60 102
NOM 2650 2770 610 1219 23 44
TA 3120 2940 2184 2970 70 141
a HA FA NOM and TA designate the four humic substances used in this study
98 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
ues for the two triazines Thus the interaction with small branched
molecules like FA and TA seems to favour a physical entrapment
mechanism such as the one suggested by Devitt and Wiesner [12]
34 Retention of humic species and calcium ion by the NFULPRO
membranes
The performance of nanofiltration on the humic substances and
calcium removal is summarized in Table 7 Regarding humic sub-
stances the retention is compared for three different feed solutions
(a) solutions of HS alone (b) solutions of HS with the two triazines
and (c) solutions of HS with the two triazines in the presence of cal-
cium In general HS retention is high for all three membranes used
According to the UV measurements IHSS HA exhibits the highest
retention values followed by NOM TA and FA which is apparently
related to their relative molecular masses (Table 3) Complexation
or interaction with triazines as evidenced by the GPC tests appar-
ently plays also a significant role in the relative permeability of the
organic species More specifically the retention of the four organic
compounds examined is increased in the presence of the two her-
bicides which verifies also the binding of atrazine and prometryn
with the four humic substances Thus a comparison between the
retention values of the HS in the absence or presence of triazines
gives also a good insight into the HSndashtriazine interactions The slight
increase (lt1) of HA retention in the presence of the two triazines
is in accord with the relatively low HA binding capacity observed
in the GPC tests On the other hand the increase of 2ndash6 in FA
NOM and TA retention is in agreement with the observed increased
interactions between those substances and the two triazines
In the case of calcium addition in the organicndashtriazine solu-
tions the UV absorbing species of the four organics examined are
retained almost completely by all three membranes used This is
again in agreement with the results obtained with gel chromatogra-
phy According to the literature [12] the addition of calcium results
in charge shielding and neutralization of the organic matter charged
functional groups and can shrink the humic matrix The formation
of aggregates of larger apparent molecular weight due to the sub-
stantial increase in the hydrophobicity of the organic molecules
tends to reduce their permeability through membranes and there-
fore to increase their retention
Apart from the increased retention of triazines and humic sub-
stances there is also a significant improvement of calcium rejection
when the latter is present in the solution In comparison to pure
water solutions of elevated calcium ion content [20] the rejection of
calcium in humic solutions is almost complete This is attributed to
size exclusion mechanisms related to the binding tendency of diva-
lent ions like calcium to the negatively charged humic organics
Among the four humic substances used calcium is rejected more in
the presence of the polyphenol tannic acid which is accompanied
by a higher organic adsorption on all three NFULPRO membranes
tested (Table 8)
From the data summarized in Table 8 it can be seen that the
adsorption of humic substances onto the membrane surfaces is
markedly enhanced in the presence of calcium ions due to the
formation of HA-complexed divalent cations This is also clearly
depicted in the SEM images of virgin and fouled membranes in
the absence or presence of calcium ions Characteristic SEM images
in the case of the Suwannee River NOM filtration with the three
NFULPRO membranes are included in Fig 10 these images show
the severe fouling on the three NFULPRO membranes as a result
of the combined filtration of NOM with CaCl2 salt It is possible
that the divalent cations such as Ca2+ act as a bridge between the
membrane surface and the negatively charged humic molecules
that tend to be repelled by the negatively charged membrane As a
result the electrical repulsion between the membrane surface and
the humic molecules is weakened thus rendering the membrane
Fig 10 SEM images of virgin fouled with NOM and fouled with NOM + calcium membranes (a) NF270 (b) NF90 and (c) XLE membrane
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 99
fouling process more severe [45] On the other hand the relatively
smaller adsorption rates of NOM in the absence of calcium result
in less fouled membranes A similar trend is also observed in the
case of the other three humic substances used in this study Among
the three membranes used XLE exhibits in the absence of calcium
the highest adsorption rates for all four humic substances This is
explained by the relatively high hydrophobicity and the low surface
charge that characterize the XLE membrane surface as previously
explained
4 Concluding remarks
The influence of humic substances on retention of two tri-
azines (atrazine and prometryn) by NFULPRO membranes is rather
complicated and involves combinations of solutendashmembrane and
solutendashsolute interactions In summary the retention of triazines
depends on the type of the humic material present in the feed
water In most cases the combined nanofiltration of triazines and
naturally occurring humic substances facilitates the formation of
complexes with triazines which in turn enhance their removal by
means of membrane filtration This complexation appears to be
related not to the characteristic acidity (phenolic carboxylic) of
the HS used but rather to their molecular conformation More
specifically a preferential binding is observed between triazines
and low molecular weight fractions of humic compounds (espe-
cially of FA and TA) which results in higher retention values for the
two triazines Under all conditions tannic acid exhibits the greatest
effect on triazine retention among the four standard HS compounds
used leading to an almost complete removal of the two triazines
(95ndash100) for all three membranes tested
The removal of triazines is improved in the presence of calcium
which tends to enhance the interaction between HS and triazines
Moreover triazine retention is increased with increasing HS con-
centration to a degree depending on the type of HS Additionally
triazine retention is apparently also affected by the interactions
between the organics and the membrane surface which in turn
may alter triazine retention These results illustrate the complex-
ity of the triazine adsorption and retention mechanisms in the
presence of humic substances that needs further investigation for
improved understanding Moreover regarding triazine removal by
nanofiltration it is of particular interest to distinguish the effect
of the HSndashtriazine interactions from that of membrane HS fouling
Research in this direction is carried out in this laboratory
References
[1] DP Birardar AL Rayburn Chromosomal damage induced by herbicide con-tamination at concentrations observed in public water supplies J Environ Qual24 (1995) 1222ndash1225
[2] IK Konstantinou DG Hela TA Albanis The status of pesticide pollution insurface waters (rivers and lakes) of Greece Part I Review on occurrence andlevels Environ Pollut 141 (3) (2006) 555ndash570
[3] European Environment Agency (EEA) Groundwater quality and quantity inEurope Report Copenhagen June 1999 available in httpreportseeaeuropaeugroundwater07012000enenviassrp199903
[4] EM Thurman DA Goolsby MT Meyer MS Mills ML Pomes DW Kolpin AReconnaissance study of herbicides and their metabolites in surface water of theMidwestern United States using immunoassay and gas chromatographymassspectrometry Environ Sci Technol 26 (1992) 2440ndash2447
[5] C Bellona JE Drewes P Xu G Amy Factors affecting the rejection of organicsolutes during NFRO treatmentmdasha literature review Water Res 38 (2004)2795ndash2809
[6] A Bhattacharya Remediation of pesticide-polluted waters through mem-branes Sep Purif Rev 35 (2006) 1ndash38
[7] KM Agbekodo B Legube S Dard Atrazine and simazine removal mechanismsby nanofiltration influence of natural organic matter concentration Water Res30 (1996) 2535ndash2542
[8] P Berg G Hagmeyer R Gimbel Removal of pesticides and other micropollu-tants by nanofiltration Desalination 113 (1997) 205ndash208
[9] EC Devitt F Ducellier P Coumlteacute MR Wiesner Effects of natural organic mat-ter and the raw water matrix on the rejection of atrazine by pressure-drivenmembranes Water Res 32 (1998) 2563ndash2568
[10] R Boussahel S Bouland KM Moussaoui A Montiel Removal of pesticideresidues in water using the nanofiltration process Desalination 132 (2000)205ndash209
[11] Y Zhang B Van der Bruggen GX Chen L Braeken C Vandecasteele Removalof pesticides by nanofiltration effect of the water matrix Sep Purif Technol38 (2004) 163ndash172
[12] EC Devitt MR Wiesner Dialysis investigations of atrazinendashorganic matterinteractions and the role of a divalent metal Environ Sci Technol 32 (1998)232ndash237
[13] SK Dalton JA Brant MR Wiesner Chemical interactions between dissolvedorganic matter and low-molecular weight organic compounds impacts onmembrane separation J Membr Sci 266 (2005) 30ndash39
[14] NA Kulikova IV Perminova Binding of atrazine to humic substances fromsoil peat and coal related to their structure Environ Sci Technol 36 (2002)3720ndash3724
[15] RL Wershaw The importance of humic substancendashmineral particle complexesin the modeling of contaminant transport in sedimentndashwater systems in RABaker (Ed) Organic Substances and Sediments in Water Humics and SoilsLewis Publishers Chelsea MI 1991 pp 23ndash34
[16] SM Schrap A Opperhuizen Relationship between bioavailability andhydrophobicity reduction of the uptake of organic chemicals by fish due tothe sorption on particles Environ Toxicol Chem 9 (1990) 715ndash724
[17] E Barriuso M Schiavon F Andreux JM Portal Localization of atrazine non-extractable (bound) residues in soil size fractions Chemosphere 12 (1991)1131ndash1140
[18] L Loiseau E Barriuso Characterization of the atrazinersquos bound (nonextractable)residues using fractionation techniques for soil organic matter Environ SciTechnol 36 (2002) 683ndash689
[19] G Sposito L Martin-Neto A Yang Atrazine complexation by soil humic acidsJ Environ Qual 25 (1996) 1203ndash1209
[20] KV Plakas AJ Karabelas Membrane retention of herbicides from single andmulti-solute media the effect of ionic environment J Membr Sci 320 (2008)325ndash334
[21] V Freger J Gilron S Belfer TFC polyamide membranes modified by graftingof hydrophilic polymers an FT-IRAFMTEM study J Membr Sci 209 (2002)283ndash292
[22] P Xu JE Drewes Viability of nanofiltration and ultra-low pressure reverseosmosis membranes for multi-beneficial use of methane produced water SepPurif Technol 52 (2006) 67ndash76
[23] Z Knez A Rizner-Hras K Kokot D Bauman Solubility of some solid triazineherbicides in supercritical carbon dioxide Fluid Phase Equilibria 152 (1998)95ndash108
[24] Weed Science Society of America (WSSA) Herbicide Handbook 7th ed 1994[25] B Van der Bruggen J Schaep W Maes D Wilms C Vandecasteele Nanofiltra-
tion as a treatment method for the removal of pesticides from ground watersDesalination 117 (1998) 139ndash147
[26] KN Reddy MA Locke Molecular properties as descriptors of octanol-waterpartition coefficients of herbicides Water Air Soil Pollut 86 (1996) 389ndash405
[27] IHSS website httpwwwihssgatechedu[28] F Flores-Ceacutespedes M Fernaacutendez-Peacuterez M Villafranca-Saacutenchez E Gonzaacutelez-
Pradas Cosorption study of organic pollutants and dissolved organic matter ina soil Environ Pollut 142 (2006) 449ndash456
[29] R Beckett Z Jue JC Giddings Determination of molecular weight distribu-tions of fulvic and humic acids using flow field-flow fractionation Environ SciTechnol 21 (1987) 289ndash295
[30] PM Reid AE Wilkinson E Tipping MN Jones Determination of molecularweights of humic substances by analytical (UV scanning) ultracentrifugationGeochim Cosmochim Acta 54 (1990) 131ndash138
[31] S Lee B Kwon M Sun J Cho Characterizations of NOM included in NF and UFmembrane permeates Desalination 173 (2005) 131ndash142
[32] MN Jones ND Bryan Colloidal properties of humic substances Adv ColloidInterf Sci 78 (1998) 1ndash48
[33] KV Plakas AJ Karabelas T Wintgens T Melin A study of selected herbicidesretention by nanofiltration membranesmdashthe role of organic fouling J MembrSci 284 (2006) 291ndash300
[34] APHA-AWWA-WPCF Standard Methods for the Examination of Water andWastewater 17th ed Port City Press Baltimore MD 1989 pp 567ndash569
[35] R Artinger G Buckau JI Kim S Geyer Characterization of groundwater humicand fulvic acids of different origin by GPC with UVVis and fluorescence detec-tion Fresenius J Anal Chem 364 (1999) 737ndash745
[36] AI Schaumlfer N Andritsos AJ Karabelas EMV Hoek R Schneider M Nys-troumlm Chapter 8 Fouling in nanofiltration in AI Schaumlfer AG Fane TDWaite (Eds) Nanofiltration Principles and Applications Elsevier Oxford UK2005 pp 173ndash174
[37] I Franco L Catalano M Contin M De Nobili Interaction between organicmodel compounds and pesticides with water-soluble soil humic substancesActa Hydrochim Hydrobiol 29 (2001) 88ndash99
[38] AA Helal DM Imam SM Khalifa HF Aly Interaction of pesticides with humiccompounds and their metal complexes Radiochemistry 48 (4) (2006) 419ndash425
[39] MG Browman G Chester Fate of Pollutants in the Air and Water EnvironmentWiley New York 1977 p 50
[40] LD Nghiem AI Schaefer M Elimelech Nanofiltration of hormone mimickingtrace organic contaminants Sep Sci Technol 40 (2005) 2633ndash2649
[41] R Celis J Cornejo MC Hermosin WC Koskinen Sorptionndashdesorption ofatrazine and simazine by model soil colloidal components Soil Sci Soc AmJ 61 (1997) 436ndash443
100 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
[42] SW Runge KR Shelton SA Melton WM Moran Maintaining the ionic per-meability of a cellulose ester membrane J Biochem Biophys Methods 64(2005) 200ndash206
[43] R Boussahel A Montiel M Baudu Effects of organic and inorganic matter onpesticide rejection by nanofiltration Desalination 145 (2002) 109ndash114
[44] Z Wang DS Gamble CH Langford Interaction of atrazine with Laurentianhumic acid Anal Chim Acta 244 (1991) 135ndash143
[45] C Jucker MM Clark Adsorption of aquatic humic substances on hydrophobicultrafiltration membranes J Membr Sci 97 (1994) 37ndash52
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 91
Table 4
Adsorption data of triazines on tested membranes
Membrane Feed mass of
triazinesa (g)
Quantity adsorbed
(g)
adsorption adsorption
[19]b
NF270 45 plusmn 27(A) 048 plusmn 027 127 plusmn 18 165
53 plusmn 28(P) 020 plusmn 008 38 plusmn 13 275
NF90 52 plusmn 28(A) 122 plusmn 083 248 plusmn 66 305
55 plusmn 13(P) 105 plusmn 033 250 plusmn 67 262
XLE 66 plusmn 10(A) 114 plusmn 012 219 plusmn 12 230
74 plusmn 11(P) 084 plusmn 008 171 plusmn 09 295
a (A) and (P) designate atrazine and prometryn respectivelyb Adsorption values for higher triazine (atrazine and prometryn) concentrations (600ndash700 g Lminus1 each)
Fig 2 GPC elution profiles of Suwannee River humic acid (HA) in the absence or presence of triazines andor calcium (detection by UV absorption at 254 nm)
the humic materials when treated together with the two triazines
(Table 5) In summary aquatic humic substances from the same
water source (Suwannee River) show differences in their GPC elu-
tion behaviour when treated together with calcium andor with
either of the two triazines
The peaks of the GPC chromatograms reflect differences in spe-
cific absorption Distinct differences between HS and atrazine or
prometryn are found in the case of FA and NOM In contrast HA
show a rather small variation on elution behaviour This is also in
accord with the values presented in Table 5 Complexation with
the NOM and FA appears to occur for both atrazine and prometryn
molecules the interactions with NOM indicate greater affinity to
atrazine On the other hand FA interactions appear to be stronger
with prometryn In Figs 2ndash4 and Table 5 one can observe that
among the three HS used FA seems to interact more with the two
triazines The weakest interaction is observed in the case of humic
acids which is reflected in the almost identical elution profiles and
the low GPC reduction areas
The presence of calcium in the solutions of humic substances
leads to a sharp reduction of HS concentrations (Table 5) indi-
cating the formation of calcium humate and fulvate complexes
According to the literature when negatively charged humic colloids
Table 5
Percentage () reduction of GPC integration areas of humic substances due to the presence of triazines andor calcium
Humic substances combined with Percentage GPC area reduction ()
HA FA NOM
40 mg Lminus1 Ca2+ 26 17 138
10 g Lminus1 atrazine 06 64 27
10 g Lminus1 prometryn 02 81 lt01
10 g Lminus1 atrazine + 10 g Lminus1 prometryn 23 80 30
10 g Lminus1 atrazine + 10 g Lminus1 prometryn + 40 mg Lminus1 Ca2+ 151 221 279
92 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
Fig 3 GPC elution profiles of Suwannee River fulvic acid (FA) in the absence or presence of triazines andor calcium (detection by UV absorption at 275 nm)
come in contact with positively charged metal ions (like calcium)
several binding mechanisms become effective The bond strengths
vary from a weak physical adsorption to strong chemical bond-
ing involving chelation reactions [38] In the case of herbicides
the partially chelated calcium may serve here as a bridging ion
and act as a possible site for the adsorption of a triazine molecule
by displacing water of hydration Indeed the addition of calcium
enhances the interaction between the humic substances and the
two triazines something that is manifested by the higher GPC area
reductions and the lower elution profiles Browman and Chester
[39] also concluded that cations on the surface of humic material
can act as binding bridges between synthetic chemicals and humic
compounds Similarly Helal et al [38] reported that the exchange
reaction between the pesticides and the humic compounds takes
place on the same sites at which binding of humic compound to the
metal occurs Moreover these authors claim that the pesticidendashHA
Fig 4 GPC elution profiles of Suwannee River NOM in the absence or presence of triazines andor calcium (detection by UV absorption at 254 nm)
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 93
Fig 5 Retention of atrazine (A) and prometryn (P) by the three NFULPRO membranes in the absence or presence of humic substances (HA FA NOM TA) andor calcium
ions
or ndashFA complex is stronger than the metalndashhumate or ndashfulvate
complex The mechanisms involved in the interactions between
triazines and calciumndashHS complexes may be described by either
a physical adsorption of the herbicide on the surface of metal
humatefulvate complex or a ligand exchange in which the metal
serves as a bridging ion and acts as a possible site for adsorption of
the herbicide molecule by displacing water of hydration [38]
322 Effect of different types of humic materials on triazine
retention
Retention results of atrazine and prometryn during the com-
bined nanofiltration with the four selected humic substances are
depicted in Fig 5 Results of triazine retention from organic solu-
tions with elevated calcium ion content are also included in Fig 5
and discussed in Section 324 Depending on the type of humic
material present increased retention of the two triazines may be
negligible or significant Specifically the use of HA results in reten-
tion values that are comparable to those obtained in the absence
of organics An exception is observed in the case of XLE membrane
where the combined filtration of triazines and HA leads to relatively
smaller retention values (7 and 13 for atrazine and prometryn
respectively) On the other hand the presence of tannic acid results
in a significant increase of triazine retention for all three mem-
branes tested In particular the addition of TA at a concentration of
10 mg Lminus1 (yielding a triazineTA ratio of 1 g1 mg) results in 14ndash21
and 20ndash29 increase of prometryn and atrazine retention respec-
tively The enhancement of pesticide removal in the presence of
the NOM surrogate tannic acid has been also observed by previ-
ous researchers [91213] According to Devitt and Wiesner [12] the
increased retention of a triazine molecule like atrazine in the pres-
ence of TA is due to the branched structure of TA which traps the
triazine within the macromolecule Moreover these authors sug-
gest that molecular conformations and weak interactions between
functional groups may combine to produce physical entrapment of
triazines through a low multiplicative probability of escape result-
ing from a large number of encounters with low-energy sites on the
tannic acid molecule (eg hydrogen bonding)
In the case of the combined nanofiltration of atrazine and prom-
etryn with FA or NOM a modest increase of retention in comparison
to the pure triazine solutions was recorded In particular depending
on the membrane used the presence of FA resulted in an increase
of 8ndash15 of atrazine retention and 4ndash14 of prometryn retention
whereas nanofiltration in the presence of NOM led to 12ndash17 and
3ndash15 increase of atrazine and prometryn retention respectively
The interactions between atrazine and prometryn with the
humic substances observed in the GPC experiments seem to agree
(up to a point) with the trends in triazine retention by the three
NFULPRO membranes Indeed the negligible interactions observed
between the two herbicides and the humic acids are in accord with
the slight differences observed in atrazine and prometryn retention
by the NF270 and NF90 membranes On the other hand the some-
what reduced retention observed in the case of XLE membrane
may be not only due to the weak interactions between triazines
and humic acids but also to the hydrophobic interactions between
HA and the membrane surface which tends to affect atrazine and
prometryn retention This is in accord with the observed higher
adsorption rates of HA on the XLE membrane (Table 8) which in turn
may affect the surface properties of the membrane and therefore
triazine retention
The GPC results in the case of FA and NOM are in accord with
the retention results of triazines for all three membranes FA and
NOM interact with the two herbicides to form complexes of various
stabilities The formation of these pseudo-complexes may explain
the higher retention of atrazine and prometryn as a result of the
94 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
Table 6
Average retention values of atrazine and prometryn at two different humic substance concentrations by the three NFULPRO membranes
Humic substance Cfeed (mg Lminus1) NF270 NF90 XLE
Ratrazine () Rprometryn () Ratrazine () Rprometryn () Ratrazine () Rprometryn ()
HA 5 782 804 873 883 83 852
10 776 798 838 846 769 744
FA 5 784 829 828 870 848 913
10 816 842 937 933 906 977
NOM 5 813 859 843 838 879 922
10 862 901 894 857 956 968
TA 5 898 880 876 903 905 963
10 969 976 96 957 100 100
following mechanisms that may take place (a) increase of the
steric congestion or reduction of the diffusivity of the FAndashtriazine
or NOMndashtriazine pseudo-complex (b) increase of the density of
the pseudo-complexrsquos negative charge which enhances the elec-
trostatic repulsion with the negatively charged membranes and (c)
increase of the adsorption capability of the pseudo-complex on the
surface of the membrane in contrast to the hydrophobic nature of
the humic materials The validity of the above mechanism hypothe-
ses adopted by previous researchers [7] depends on the type of
the organic material and consequently on the characteristic chem-
ical properties of both HS (acidity functional groups) and triazines
(basicity hydrophobicity charge) The correlation between humic
material properties and triazine retention is examined in a subse-
quent paragraph
The greater extent of FAndashtriazines pseudo-complexes in com-
parison to NOMndashtriazines pseudo-complexes manifested in the
GPC results is not accompanied by a greater increase of triazine
retention More specifically the increasing order of triazine reten-
tion in the presence of a specific humic material follows the order
TA gt NOM gt FA gt HA in the case of NF270 and XLE membranes
whereas for NF90 the order is TA ge FA gt NOM gt HA Therefore the
interactions between the humic materials and triazines alone
cannot explain the retention performance of the NFULPRO mem-
branes This fact indicates that interactions between the naturally
occurring organic substances and the membranes may also play a
significant role in triazine retention as previously observed [33] In
view of these observations dead-end filtration tests using the three
IHSS organic materials and the selected NFULPRO membranes are
carried out in this laboratory in order to distinguish the effects of
organic fouling and of HSndashtriazines interactions on triazine perme-
ation during nanofiltration
323 Effect of humic substance concentration on triazine
retention
Surveying the literature on the complicated influence of organic
matter on retention of pesticides by NFRO membranes reveals a
considerable variability regarding the importance of the organic
matter concentration Experiments in a nanofiltration pilot unit on
the removal of atrazine and simazine have demonstrated the sig-
nificance of the organic load in the feed water since the removal
efficiency increased from 50 to 90ndash100 when dissolved organic
carbon varied between 04 and 36 mg Lminus1 [7] In contrast Berg et
al [8] reported no increase of pesticide retention (atrazine and
simazine among them) by several NF membranes with increas-
ing organic matter concentration Along the same lines Zhang et
al [11] showed comparable simazine retention between tap water
and river water although the latter had a high concentration of
NOM whereas tap water had a low NOM concentration In an effort
to understand the reasons for the above inconsistency in the lit-
erature Nghiem et al [40] took into consideration the effect of
ionic strength Experiments with hormone mimicking trace organic
contaminants revealed the significance of solution ionic strength
in influencing NOMndashtrace organic interactions which is probably
the cause for the ambiguity of literature data regarding the role
of background organic matter in trace organic rejection The mag-
nitude of ionic strength in terms of calcium ion content on the
NOMndashtriazines interaction has been also recorded through the GPC
tests described previously and is subsequently examined Although
ionic strength can strongly influence the HSndashtriazines interactions
the effect of the type of the humic substance seems to play also a
significant role on triazine retention This is evident in the results
included in Table 6 regarding retention experiments in the pres-
ence of a medium (5 mg Lminus1) and relatively high (10 mg Lminus1) HS
concentration in the absence of ionic environment
Table 6 shows a clear increase of triazine retention with increas-
ing humic material concentration More specifically by doubling
the concentration of FA NOM and TA in the feed solutions atrazine
and prometryn retention increases 4ndash13 and 2ndash10 respectively
depending on the membrane and the humic material used An
exception is observed again in the case of humic acids since the
increased organic load results in negligible or slightly negative
effects of atrazine and prometryn retention Regarding the XLE
membrane the use of medium HA concentration results in similar
retention performance as in organic-free solutions On the other
hand higher HA concentration results in a notable reduction of
triazine retention ranging up to 74 and 127 for atrazine and prom-
etryn respectively As previously discussed this is attributed to the
increased HA adsorption on the membrane surface (Table 8) which
in turn affects triazine adsorption on XLE membrane and therefore
retention This trend is not observed in the case of FA NOM and
TA despite their adsorptive capacity on the membrane materials
This finding is probably related to fouling phenomena which tend
to modify the membrane properties (ie surface charge hydropho-
bicity morphology) and therefore triazine adsorption Although it
is difficult to use mass balances for calculating triazine adsorption
during the combined filtration with humic substances (due to ana-
lytical problems related to the concentrate samples) the changes
of triazine concentration in the permeate samples provide a good
indication of the potential adsorption of atrazine and prometryn
on the membrane surface Furthermore the increased passage of
triazines in the permeate side as a result of HA adsorption on the
XLE membrane is in accord with the increase of triazine adsorption
on the membrane and therefore with the reduced retention values
324 Effect of calcium ion on triazine retention
From Fig 5 it is evident that the presence of calcium acts
positively in both atrazine and prometryn retention during the
combined nanofiltration with all four types of humic substances
Furthermore the higher interactions between the humic organ-
ics and triazines manifested in the GPC tests are in agreement
with the increased retention performance of atrazine and prom-
etryn by the three membranes used Additionally the suggestion
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 95
of ligand exchange in which calcium serves as a bridging ion and
acts as a possible site for higher adsorption of triazine molecules
on humic substances [38] seems to be in accord with the forma-
tion of an increased number of pseudo-complexes between humic
matter and triazines which explain the increase of herbicide reten-
tion
Among the four HS used tannic acid is associated with the high-
est triazine retention since both triazines are fully retained in the
presence of calcium by all three membranes tested Moreover the
enhanced interaction between humic acids and triazines as a result
of the calcium addition to the feed solutions is accompanied by
a significant increase of atrazine and prometryn retention In this
case calcium ions favour the binding between HA and triazines
which in other cases would not have taken place this is in agree-
ment with the preceding GPC and NFULPRO results
Prometryn exhibits again the highest retention values in com-
parison to atrazine which may be attributed to the relatively higher
prometryn hydrophobicity and basicity (Table 1) The former prop-
erty likely favours hydrophobic interactions between prometryn
and the HSndashcalcium complexes whereas the latter renders prom-
etryn highly effective in mechanisms involving electron-transfer
with the HSndashcalcium complexes [19] This mechanism is due to the
lower acidic functional group content of the HS as a result of the
complexation with calcium ions in the feed solutions Sposito et
al [19] suggested that atrazine itself does not readily participate
in electron-transfer reactions with humic substances However
they demonstrated that hydroxyl-atrazine does react through an
electron-transfer mechanism with HA and FA It is noted that
atrazine is readily converted to hydroxyl-atrazine even in labo-
ratory samples at low water content and this may explain the
presence of some of the electron transfer products detected in stud-
ies of atrazinendashHA interactions [41]
Surveying the literature however it is noted that some stud-
ies either with dialysis membranes [1213] or NF membranes [9]
have shown reduced values of atrazine retention when divalent
calcium is present together with natural organic matter includ-
ing the NOM surrogate tannic acid According to Devitt et al [9]
Devitt and Wiesner [12] this is due to the reduced association of
atrazine and NOM as a result of the occupation of interaction sites
by calcium andor the reduced access of atrazine to NOM sites due
to changes in molecular conformation The GPC results in this study
have shown that this is not the case since the presence of calcium
tends to increase the interaction of humic substances with triazine
compounds like atrazine
The conflicting results between the previous studies and the
present work may be attributed to the different types of membranes
and filtration techniques used More specifically the use of cellulose
ester membranes as well as the experimentation with batch dialy-
sis systems where concentration and osmotic pressure difference
serve as the driving force for solute transport (ie in the absence of
hydrodynamic forces as in the present study) may explain the dif-
ferences regarding the calcium effect on triazine retention Runge
et al [42] claim that the SpectraPor cellulose ester membranes
used in their studies (having a molecular weight cut-off of 100 Da)
Fig 6 Retention of atrazine (a) and prometryn (b) as a function of humic material total aciditymdashcalculations based on the values presented in Table 2 taking into consideration
the carbon composition and the HS concentration
Fig 7 Retention of atrazine (a) and prometryn (b) as a function of humic material carboxylic aciditymdashcalculations based on the values presented in Table 2 taking into
consideration the carbon composition and the HS concentration
96 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
present a significant incompatibility with distilled water whereas
a significant membrane potential can build up with certain salts
which can inhibit or enhance diffusion depending on the size of
the solutes diffusing and the polarity of the membrane potential
generated In this case an increased diffusion of hydrated calcium
ions to the permeate side in order to balance the concentrations
between the two sides of the cellulose ester membrane may result
in a pore blockage or a positive charge on the membrane surface
which in turn could lead to an increased passage of free dissolved
atrazine to the permeate side
Another possible explanation for this inconsistency is the time
period of the filtration tests According to the protocol followed
by previous researchers [1213] the dialysis time period ranged
between 1 and 5 days at the end of which concentration measure-
ments were performed On the other hand the filtration periods
in the case of the present dead-end nanofiltration experiments
ranged between 2 and 5 h depending on the membrane used To
this period one more hour should be added in which the feed solu-
tion was stirred in the cell without pressurization Even though
triazine adsorption on the membrane surface was observed to sta-
bilize within the relatively short period of nanofiltration there is
a possibility that longer filtration periods may have resulted in
higher adsorption rates which in turn could lead to greater pas-
sage of triazine to the permeate side and therefore to reduced
retention values This explanation is in agreement with the pilot
tests performed by Boussahel et al [43] who report that the
increasing adsorption of humic matter on the membranes leads
to an increase of pesticide adsorption on the membranes which
facilitates the diffusion of pesticides in the direction of perme-
ate
33 Correlation between humic material properties and triazine
retention
The correlation of atrazine and prometryn retention with the
total carboxylic and phenolic acidity of the humic substances used
in the present study as well as the role of the molecular mass of HS
on triazine retention are depicted in Figs 6ndash9 These are results of
nanofiltration experiments performed in the absence of calcium
Triazine removal by the three NFULPRO membranes seems
to poorly correlate with the total acidity of the humic material
This is evident especially in the case of NF270 and NF90 mem-
branes (Fig 6) The XLE membrane on the other hand presents a
reduced tendency for atrazine and prometryn removal when acid-
ity increases This is attributed to the characteristic low negative
charge of the XLE membrane surface which causes weak repulsions
with the negatively charged humic organics This leads to a greater
adsorption of humic compounds on the membrane surface (Table 8)
and consequently to greater triazine adsorption which favours the
diffusion of the triazines to the permeate side On the other hand
the strong negative charge of the NF270 and NF90 membranes [20]
promotes a strong repulsion between HS and the membranes The
greater the negative charge of the humic organic the higher the
repulsion which in turn results in higher concentration of HS in
the bulk Consequently the interactions between HS and triazines
in the bulk are increased leading to an improved triazine removal
Fig 8 Retention of atrazine (a) and prometryn (b) as a function of humic material phenolic aciditymdashcalculations based on the values presented in Table 2 taking into
consideration the carbon composition and the HS concentration
Fig 9 Retention of atrazine (a) and prometryn (b) as a function of humic substance molecular mass (based on the values presented in Table 3)
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 97
This is more evident in the case of the wide pore NF270 membrane
since the NF90 membrane seems to retain atrazine and prometryn
to a higher degree in the presence of TA which is characterized by
a lower total acidity among the four HS used in this study
Humic compounds contain a variety of functional groups
such as aliphatic and aromatic carboxyls phenolic hydroxyls
alcoholic hydroxyls carbonyls quinones methoxyls and amino
groups These functional groups are the reactive sites in the humic
molecules As carboxylate protonation seems to be critical for the
capacity of triazine binding [44] it is of interest to correlate the
effect of carboxylic acidity on herbicide removal by nanofiltration
The effect of carboxylic acidity on atrazine and prometryn retention
by the three NFULPRO membranes is depicted in Fig 7
The results show a negative trend of triazine retention with
an increased carboxylic acidity of humic substances This is clear
in the case of NF270 and XLE membranes while NF90 does not
seem to be significantly affected by the changes in the number of
carboxyl groups of a humic molecule Remarkably the smaller TA
molecules containing fewer carboxyl groups per unit mass (Table 2)
exhibit the highest triazine retention (sim100) as a result of the even
higher triazine binding capacity in contrast to the IHSS organics
which contain a higher carboxylic group content Moreover experi-
ments in batch filtration mode result in HS concentration increasing
with time which in turn leads to a reduced number of carboxylic
binding sites available to the two triazines This is probably due
to the self ldquocompetingrdquo character of the HS used since an increase
in concentration andor HS aggregation tend to reduce the avail-
able binding sites [44] This phenomenon led Wang et al [44] to
propose a model in which hydrogen bonding was considered to be
the primary interaction resulting in triazine binding Therefore the
increase in triazine retention observed in the case of elevated HS
concentration may not be directly related with the carboxylic acid-
ity of the HS but rather with other mechanisms such as hydrogen
bonding
Regarding the phenolic acidity of the humic substances there
does not seem to be a sufficient correlation with atrazine or prom-
etryn retention by the three NFULPRO membranes (Fig 8) Actually
there is a contradictory behaviour between the three IHSS organ-
ics and the NOM surrogate TA The high phenolic content of TA
is related to high retention values of the two triazines while
the smaller and relatively similar phenolic acidity of the IHSS
compounds leads to smaller and different retention values These
results imply that there are other factors influencing the association
between humic organics and herbicides and therefore their reten-
tion by nanofiltration In this respect the molecular structure of the
humic materials as well as the interactions taking place between
the organics and the membranes seem to explain the above incon-
sistencies This is evident in the results depicted in Fig 9 and the
adsorption of humic substances subsequently described
The rather strong relation of triazine retention with the molec-
ular mass of the organics used in this study (Fig 9) leads to the
suggestion that structural features of HS are important for the
removal capacity of triazines Up to now there is no consensus
concerning both the binding mechanism of triazines to humic sub-
stances and the structural features responsible for this binding
Nevertheless the preferential binding with low molecular weight
fractions of humic compounds (especially of FA and TA) deduced
from the GPC and filtration tests results in higher retention val-
Table 7
Humic substance and calcium retention performance of the three NFULPRO membranes (mean retention values for 50 recovery of feed volume as permeate)
Membrane Humic substancea Humic substances retention () Ca2+ retention ()
HSs alone HSs with triazines HSs with triazines and Ca2+ In the presence of HSs
NF270 HA 990 994 1000 949
FA 967 983 991 951
NOM 957 997 1000 951
TA 972 992 997 957
NF90 HA 972 980 1000 994
FA 953 984 1000 990
NOM 985 993 1000 991
TA 967 998 998 994
XLE HA 992 996 1000 994
FA 862 915 994 997
NOM 976 1000 1000 988
TA 923 945 1000 996
a HA FA NOM and TA designate the four humic substances used in this study
Table 8
Adsorption data of humic substances on tested membranes
Membrane Humic substancea Feed mass of humic substance (g) Quantity adsorbed (g) Adsorption ()
Alone With Ca2+ Alone With Ca2+ Alone With Ca2+
NF270 HA 3480 2630 244 2314 07 88
FA 3570 2840 71 1335 02 47
NOM 3390 2660 34 2633 01 99
TA 4050 3300 2025 7161 50 217
NF90 HA 3220 2690 354 3658 11 136
FA 2930 2450 1494 857 51 35
NOM 3000 2910 120 437 04 15
TA 2820 2730 649 3631 23 133
XLE HA 3100 2680 1643 3457 53 129
FA 3050 2810 1830 2866 60 102
NOM 2650 2770 610 1219 23 44
TA 3120 2940 2184 2970 70 141
a HA FA NOM and TA designate the four humic substances used in this study
98 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
ues for the two triazines Thus the interaction with small branched
molecules like FA and TA seems to favour a physical entrapment
mechanism such as the one suggested by Devitt and Wiesner [12]
34 Retention of humic species and calcium ion by the NFULPRO
membranes
The performance of nanofiltration on the humic substances and
calcium removal is summarized in Table 7 Regarding humic sub-
stances the retention is compared for three different feed solutions
(a) solutions of HS alone (b) solutions of HS with the two triazines
and (c) solutions of HS with the two triazines in the presence of cal-
cium In general HS retention is high for all three membranes used
According to the UV measurements IHSS HA exhibits the highest
retention values followed by NOM TA and FA which is apparently
related to their relative molecular masses (Table 3) Complexation
or interaction with triazines as evidenced by the GPC tests appar-
ently plays also a significant role in the relative permeability of the
organic species More specifically the retention of the four organic
compounds examined is increased in the presence of the two her-
bicides which verifies also the binding of atrazine and prometryn
with the four humic substances Thus a comparison between the
retention values of the HS in the absence or presence of triazines
gives also a good insight into the HSndashtriazine interactions The slight
increase (lt1) of HA retention in the presence of the two triazines
is in accord with the relatively low HA binding capacity observed
in the GPC tests On the other hand the increase of 2ndash6 in FA
NOM and TA retention is in agreement with the observed increased
interactions between those substances and the two triazines
In the case of calcium addition in the organicndashtriazine solu-
tions the UV absorbing species of the four organics examined are
retained almost completely by all three membranes used This is
again in agreement with the results obtained with gel chromatogra-
phy According to the literature [12] the addition of calcium results
in charge shielding and neutralization of the organic matter charged
functional groups and can shrink the humic matrix The formation
of aggregates of larger apparent molecular weight due to the sub-
stantial increase in the hydrophobicity of the organic molecules
tends to reduce their permeability through membranes and there-
fore to increase their retention
Apart from the increased retention of triazines and humic sub-
stances there is also a significant improvement of calcium rejection
when the latter is present in the solution In comparison to pure
water solutions of elevated calcium ion content [20] the rejection of
calcium in humic solutions is almost complete This is attributed to
size exclusion mechanisms related to the binding tendency of diva-
lent ions like calcium to the negatively charged humic organics
Among the four humic substances used calcium is rejected more in
the presence of the polyphenol tannic acid which is accompanied
by a higher organic adsorption on all three NFULPRO membranes
tested (Table 8)
From the data summarized in Table 8 it can be seen that the
adsorption of humic substances onto the membrane surfaces is
markedly enhanced in the presence of calcium ions due to the
formation of HA-complexed divalent cations This is also clearly
depicted in the SEM images of virgin and fouled membranes in
the absence or presence of calcium ions Characteristic SEM images
in the case of the Suwannee River NOM filtration with the three
NFULPRO membranes are included in Fig 10 these images show
the severe fouling on the three NFULPRO membranes as a result
of the combined filtration of NOM with CaCl2 salt It is possible
that the divalent cations such as Ca2+ act as a bridge between the
membrane surface and the negatively charged humic molecules
that tend to be repelled by the negatively charged membrane As a
result the electrical repulsion between the membrane surface and
the humic molecules is weakened thus rendering the membrane
Fig 10 SEM images of virgin fouled with NOM and fouled with NOM + calcium membranes (a) NF270 (b) NF90 and (c) XLE membrane
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 99
fouling process more severe [45] On the other hand the relatively
smaller adsorption rates of NOM in the absence of calcium result
in less fouled membranes A similar trend is also observed in the
case of the other three humic substances used in this study Among
the three membranes used XLE exhibits in the absence of calcium
the highest adsorption rates for all four humic substances This is
explained by the relatively high hydrophobicity and the low surface
charge that characterize the XLE membrane surface as previously
explained
4 Concluding remarks
The influence of humic substances on retention of two tri-
azines (atrazine and prometryn) by NFULPRO membranes is rather
complicated and involves combinations of solutendashmembrane and
solutendashsolute interactions In summary the retention of triazines
depends on the type of the humic material present in the feed
water In most cases the combined nanofiltration of triazines and
naturally occurring humic substances facilitates the formation of
complexes with triazines which in turn enhance their removal by
means of membrane filtration This complexation appears to be
related not to the characteristic acidity (phenolic carboxylic) of
the HS used but rather to their molecular conformation More
specifically a preferential binding is observed between triazines
and low molecular weight fractions of humic compounds (espe-
cially of FA and TA) which results in higher retention values for the
two triazines Under all conditions tannic acid exhibits the greatest
effect on triazine retention among the four standard HS compounds
used leading to an almost complete removal of the two triazines
(95ndash100) for all three membranes tested
The removal of triazines is improved in the presence of calcium
which tends to enhance the interaction between HS and triazines
Moreover triazine retention is increased with increasing HS con-
centration to a degree depending on the type of HS Additionally
triazine retention is apparently also affected by the interactions
between the organics and the membrane surface which in turn
may alter triazine retention These results illustrate the complex-
ity of the triazine adsorption and retention mechanisms in the
presence of humic substances that needs further investigation for
improved understanding Moreover regarding triazine removal by
nanofiltration it is of particular interest to distinguish the effect
of the HSndashtriazine interactions from that of membrane HS fouling
Research in this direction is carried out in this laboratory
References
[1] DP Birardar AL Rayburn Chromosomal damage induced by herbicide con-tamination at concentrations observed in public water supplies J Environ Qual24 (1995) 1222ndash1225
[2] IK Konstantinou DG Hela TA Albanis The status of pesticide pollution insurface waters (rivers and lakes) of Greece Part I Review on occurrence andlevels Environ Pollut 141 (3) (2006) 555ndash570
[3] European Environment Agency (EEA) Groundwater quality and quantity inEurope Report Copenhagen June 1999 available in httpreportseeaeuropaeugroundwater07012000enenviassrp199903
[4] EM Thurman DA Goolsby MT Meyer MS Mills ML Pomes DW Kolpin AReconnaissance study of herbicides and their metabolites in surface water of theMidwestern United States using immunoassay and gas chromatographymassspectrometry Environ Sci Technol 26 (1992) 2440ndash2447
[5] C Bellona JE Drewes P Xu G Amy Factors affecting the rejection of organicsolutes during NFRO treatmentmdasha literature review Water Res 38 (2004)2795ndash2809
[6] A Bhattacharya Remediation of pesticide-polluted waters through mem-branes Sep Purif Rev 35 (2006) 1ndash38
[7] KM Agbekodo B Legube S Dard Atrazine and simazine removal mechanismsby nanofiltration influence of natural organic matter concentration Water Res30 (1996) 2535ndash2542
[8] P Berg G Hagmeyer R Gimbel Removal of pesticides and other micropollu-tants by nanofiltration Desalination 113 (1997) 205ndash208
[9] EC Devitt F Ducellier P Coumlteacute MR Wiesner Effects of natural organic mat-ter and the raw water matrix on the rejection of atrazine by pressure-drivenmembranes Water Res 32 (1998) 2563ndash2568
[10] R Boussahel S Bouland KM Moussaoui A Montiel Removal of pesticideresidues in water using the nanofiltration process Desalination 132 (2000)205ndash209
[11] Y Zhang B Van der Bruggen GX Chen L Braeken C Vandecasteele Removalof pesticides by nanofiltration effect of the water matrix Sep Purif Technol38 (2004) 163ndash172
[12] EC Devitt MR Wiesner Dialysis investigations of atrazinendashorganic matterinteractions and the role of a divalent metal Environ Sci Technol 32 (1998)232ndash237
[13] SK Dalton JA Brant MR Wiesner Chemical interactions between dissolvedorganic matter and low-molecular weight organic compounds impacts onmembrane separation J Membr Sci 266 (2005) 30ndash39
[14] NA Kulikova IV Perminova Binding of atrazine to humic substances fromsoil peat and coal related to their structure Environ Sci Technol 36 (2002)3720ndash3724
[15] RL Wershaw The importance of humic substancendashmineral particle complexesin the modeling of contaminant transport in sedimentndashwater systems in RABaker (Ed) Organic Substances and Sediments in Water Humics and SoilsLewis Publishers Chelsea MI 1991 pp 23ndash34
[16] SM Schrap A Opperhuizen Relationship between bioavailability andhydrophobicity reduction of the uptake of organic chemicals by fish due tothe sorption on particles Environ Toxicol Chem 9 (1990) 715ndash724
[17] E Barriuso M Schiavon F Andreux JM Portal Localization of atrazine non-extractable (bound) residues in soil size fractions Chemosphere 12 (1991)1131ndash1140
[18] L Loiseau E Barriuso Characterization of the atrazinersquos bound (nonextractable)residues using fractionation techniques for soil organic matter Environ SciTechnol 36 (2002) 683ndash689
[19] G Sposito L Martin-Neto A Yang Atrazine complexation by soil humic acidsJ Environ Qual 25 (1996) 1203ndash1209
[20] KV Plakas AJ Karabelas Membrane retention of herbicides from single andmulti-solute media the effect of ionic environment J Membr Sci 320 (2008)325ndash334
[21] V Freger J Gilron S Belfer TFC polyamide membranes modified by graftingof hydrophilic polymers an FT-IRAFMTEM study J Membr Sci 209 (2002)283ndash292
[22] P Xu JE Drewes Viability of nanofiltration and ultra-low pressure reverseosmosis membranes for multi-beneficial use of methane produced water SepPurif Technol 52 (2006) 67ndash76
[23] Z Knez A Rizner-Hras K Kokot D Bauman Solubility of some solid triazineherbicides in supercritical carbon dioxide Fluid Phase Equilibria 152 (1998)95ndash108
[24] Weed Science Society of America (WSSA) Herbicide Handbook 7th ed 1994[25] B Van der Bruggen J Schaep W Maes D Wilms C Vandecasteele Nanofiltra-
tion as a treatment method for the removal of pesticides from ground watersDesalination 117 (1998) 139ndash147
[26] KN Reddy MA Locke Molecular properties as descriptors of octanol-waterpartition coefficients of herbicides Water Air Soil Pollut 86 (1996) 389ndash405
[27] IHSS website httpwwwihssgatechedu[28] F Flores-Ceacutespedes M Fernaacutendez-Peacuterez M Villafranca-Saacutenchez E Gonzaacutelez-
Pradas Cosorption study of organic pollutants and dissolved organic matter ina soil Environ Pollut 142 (2006) 449ndash456
[29] R Beckett Z Jue JC Giddings Determination of molecular weight distribu-tions of fulvic and humic acids using flow field-flow fractionation Environ SciTechnol 21 (1987) 289ndash295
[30] PM Reid AE Wilkinson E Tipping MN Jones Determination of molecularweights of humic substances by analytical (UV scanning) ultracentrifugationGeochim Cosmochim Acta 54 (1990) 131ndash138
[31] S Lee B Kwon M Sun J Cho Characterizations of NOM included in NF and UFmembrane permeates Desalination 173 (2005) 131ndash142
[32] MN Jones ND Bryan Colloidal properties of humic substances Adv ColloidInterf Sci 78 (1998) 1ndash48
[33] KV Plakas AJ Karabelas T Wintgens T Melin A study of selected herbicidesretention by nanofiltration membranesmdashthe role of organic fouling J MembrSci 284 (2006) 291ndash300
[34] APHA-AWWA-WPCF Standard Methods for the Examination of Water andWastewater 17th ed Port City Press Baltimore MD 1989 pp 567ndash569
[35] R Artinger G Buckau JI Kim S Geyer Characterization of groundwater humicand fulvic acids of different origin by GPC with UVVis and fluorescence detec-tion Fresenius J Anal Chem 364 (1999) 737ndash745
[36] AI Schaumlfer N Andritsos AJ Karabelas EMV Hoek R Schneider M Nys-troumlm Chapter 8 Fouling in nanofiltration in AI Schaumlfer AG Fane TDWaite (Eds) Nanofiltration Principles and Applications Elsevier Oxford UK2005 pp 173ndash174
[37] I Franco L Catalano M Contin M De Nobili Interaction between organicmodel compounds and pesticides with water-soluble soil humic substancesActa Hydrochim Hydrobiol 29 (2001) 88ndash99
[38] AA Helal DM Imam SM Khalifa HF Aly Interaction of pesticides with humiccompounds and their metal complexes Radiochemistry 48 (4) (2006) 419ndash425
[39] MG Browman G Chester Fate of Pollutants in the Air and Water EnvironmentWiley New York 1977 p 50
[40] LD Nghiem AI Schaefer M Elimelech Nanofiltration of hormone mimickingtrace organic contaminants Sep Sci Technol 40 (2005) 2633ndash2649
[41] R Celis J Cornejo MC Hermosin WC Koskinen Sorptionndashdesorption ofatrazine and simazine by model soil colloidal components Soil Sci Soc AmJ 61 (1997) 436ndash443
100 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
[42] SW Runge KR Shelton SA Melton WM Moran Maintaining the ionic per-meability of a cellulose ester membrane J Biochem Biophys Methods 64(2005) 200ndash206
[43] R Boussahel A Montiel M Baudu Effects of organic and inorganic matter onpesticide rejection by nanofiltration Desalination 145 (2002) 109ndash114
[44] Z Wang DS Gamble CH Langford Interaction of atrazine with Laurentianhumic acid Anal Chim Acta 244 (1991) 135ndash143
[45] C Jucker MM Clark Adsorption of aquatic humic substances on hydrophobicultrafiltration membranes J Membr Sci 97 (1994) 37ndash52
92 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
Fig 3 GPC elution profiles of Suwannee River fulvic acid (FA) in the absence or presence of triazines andor calcium (detection by UV absorption at 275 nm)
come in contact with positively charged metal ions (like calcium)
several binding mechanisms become effective The bond strengths
vary from a weak physical adsorption to strong chemical bond-
ing involving chelation reactions [38] In the case of herbicides
the partially chelated calcium may serve here as a bridging ion
and act as a possible site for the adsorption of a triazine molecule
by displacing water of hydration Indeed the addition of calcium
enhances the interaction between the humic substances and the
two triazines something that is manifested by the higher GPC area
reductions and the lower elution profiles Browman and Chester
[39] also concluded that cations on the surface of humic material
can act as binding bridges between synthetic chemicals and humic
compounds Similarly Helal et al [38] reported that the exchange
reaction between the pesticides and the humic compounds takes
place on the same sites at which binding of humic compound to the
metal occurs Moreover these authors claim that the pesticidendashHA
Fig 4 GPC elution profiles of Suwannee River NOM in the absence or presence of triazines andor calcium (detection by UV absorption at 254 nm)
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 93
Fig 5 Retention of atrazine (A) and prometryn (P) by the three NFULPRO membranes in the absence or presence of humic substances (HA FA NOM TA) andor calcium
ions
or ndashFA complex is stronger than the metalndashhumate or ndashfulvate
complex The mechanisms involved in the interactions between
triazines and calciumndashHS complexes may be described by either
a physical adsorption of the herbicide on the surface of metal
humatefulvate complex or a ligand exchange in which the metal
serves as a bridging ion and acts as a possible site for adsorption of
the herbicide molecule by displacing water of hydration [38]
322 Effect of different types of humic materials on triazine
retention
Retention results of atrazine and prometryn during the com-
bined nanofiltration with the four selected humic substances are
depicted in Fig 5 Results of triazine retention from organic solu-
tions with elevated calcium ion content are also included in Fig 5
and discussed in Section 324 Depending on the type of humic
material present increased retention of the two triazines may be
negligible or significant Specifically the use of HA results in reten-
tion values that are comparable to those obtained in the absence
of organics An exception is observed in the case of XLE membrane
where the combined filtration of triazines and HA leads to relatively
smaller retention values (7 and 13 for atrazine and prometryn
respectively) On the other hand the presence of tannic acid results
in a significant increase of triazine retention for all three mem-
branes tested In particular the addition of TA at a concentration of
10 mg Lminus1 (yielding a triazineTA ratio of 1 g1 mg) results in 14ndash21
and 20ndash29 increase of prometryn and atrazine retention respec-
tively The enhancement of pesticide removal in the presence of
the NOM surrogate tannic acid has been also observed by previ-
ous researchers [91213] According to Devitt and Wiesner [12] the
increased retention of a triazine molecule like atrazine in the pres-
ence of TA is due to the branched structure of TA which traps the
triazine within the macromolecule Moreover these authors sug-
gest that molecular conformations and weak interactions between
functional groups may combine to produce physical entrapment of
triazines through a low multiplicative probability of escape result-
ing from a large number of encounters with low-energy sites on the
tannic acid molecule (eg hydrogen bonding)
In the case of the combined nanofiltration of atrazine and prom-
etryn with FA or NOM a modest increase of retention in comparison
to the pure triazine solutions was recorded In particular depending
on the membrane used the presence of FA resulted in an increase
of 8ndash15 of atrazine retention and 4ndash14 of prometryn retention
whereas nanofiltration in the presence of NOM led to 12ndash17 and
3ndash15 increase of atrazine and prometryn retention respectively
The interactions between atrazine and prometryn with the
humic substances observed in the GPC experiments seem to agree
(up to a point) with the trends in triazine retention by the three
NFULPRO membranes Indeed the negligible interactions observed
between the two herbicides and the humic acids are in accord with
the slight differences observed in atrazine and prometryn retention
by the NF270 and NF90 membranes On the other hand the some-
what reduced retention observed in the case of XLE membrane
may be not only due to the weak interactions between triazines
and humic acids but also to the hydrophobic interactions between
HA and the membrane surface which tends to affect atrazine and
prometryn retention This is in accord with the observed higher
adsorption rates of HA on the XLE membrane (Table 8) which in turn
may affect the surface properties of the membrane and therefore
triazine retention
The GPC results in the case of FA and NOM are in accord with
the retention results of triazines for all three membranes FA and
NOM interact with the two herbicides to form complexes of various
stabilities The formation of these pseudo-complexes may explain
the higher retention of atrazine and prometryn as a result of the
94 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
Table 6
Average retention values of atrazine and prometryn at two different humic substance concentrations by the three NFULPRO membranes
Humic substance Cfeed (mg Lminus1) NF270 NF90 XLE
Ratrazine () Rprometryn () Ratrazine () Rprometryn () Ratrazine () Rprometryn ()
HA 5 782 804 873 883 83 852
10 776 798 838 846 769 744
FA 5 784 829 828 870 848 913
10 816 842 937 933 906 977
NOM 5 813 859 843 838 879 922
10 862 901 894 857 956 968
TA 5 898 880 876 903 905 963
10 969 976 96 957 100 100
following mechanisms that may take place (a) increase of the
steric congestion or reduction of the diffusivity of the FAndashtriazine
or NOMndashtriazine pseudo-complex (b) increase of the density of
the pseudo-complexrsquos negative charge which enhances the elec-
trostatic repulsion with the negatively charged membranes and (c)
increase of the adsorption capability of the pseudo-complex on the
surface of the membrane in contrast to the hydrophobic nature of
the humic materials The validity of the above mechanism hypothe-
ses adopted by previous researchers [7] depends on the type of
the organic material and consequently on the characteristic chem-
ical properties of both HS (acidity functional groups) and triazines
(basicity hydrophobicity charge) The correlation between humic
material properties and triazine retention is examined in a subse-
quent paragraph
The greater extent of FAndashtriazines pseudo-complexes in com-
parison to NOMndashtriazines pseudo-complexes manifested in the
GPC results is not accompanied by a greater increase of triazine
retention More specifically the increasing order of triazine reten-
tion in the presence of a specific humic material follows the order
TA gt NOM gt FA gt HA in the case of NF270 and XLE membranes
whereas for NF90 the order is TA ge FA gt NOM gt HA Therefore the
interactions between the humic materials and triazines alone
cannot explain the retention performance of the NFULPRO mem-
branes This fact indicates that interactions between the naturally
occurring organic substances and the membranes may also play a
significant role in triazine retention as previously observed [33] In
view of these observations dead-end filtration tests using the three
IHSS organic materials and the selected NFULPRO membranes are
carried out in this laboratory in order to distinguish the effects of
organic fouling and of HSndashtriazines interactions on triazine perme-
ation during nanofiltration
323 Effect of humic substance concentration on triazine
retention
Surveying the literature on the complicated influence of organic
matter on retention of pesticides by NFRO membranes reveals a
considerable variability regarding the importance of the organic
matter concentration Experiments in a nanofiltration pilot unit on
the removal of atrazine and simazine have demonstrated the sig-
nificance of the organic load in the feed water since the removal
efficiency increased from 50 to 90ndash100 when dissolved organic
carbon varied between 04 and 36 mg Lminus1 [7] In contrast Berg et
al [8] reported no increase of pesticide retention (atrazine and
simazine among them) by several NF membranes with increas-
ing organic matter concentration Along the same lines Zhang et
al [11] showed comparable simazine retention between tap water
and river water although the latter had a high concentration of
NOM whereas tap water had a low NOM concentration In an effort
to understand the reasons for the above inconsistency in the lit-
erature Nghiem et al [40] took into consideration the effect of
ionic strength Experiments with hormone mimicking trace organic
contaminants revealed the significance of solution ionic strength
in influencing NOMndashtrace organic interactions which is probably
the cause for the ambiguity of literature data regarding the role
of background organic matter in trace organic rejection The mag-
nitude of ionic strength in terms of calcium ion content on the
NOMndashtriazines interaction has been also recorded through the GPC
tests described previously and is subsequently examined Although
ionic strength can strongly influence the HSndashtriazines interactions
the effect of the type of the humic substance seems to play also a
significant role on triazine retention This is evident in the results
included in Table 6 regarding retention experiments in the pres-
ence of a medium (5 mg Lminus1) and relatively high (10 mg Lminus1) HS
concentration in the absence of ionic environment
Table 6 shows a clear increase of triazine retention with increas-
ing humic material concentration More specifically by doubling
the concentration of FA NOM and TA in the feed solutions atrazine
and prometryn retention increases 4ndash13 and 2ndash10 respectively
depending on the membrane and the humic material used An
exception is observed again in the case of humic acids since the
increased organic load results in negligible or slightly negative
effects of atrazine and prometryn retention Regarding the XLE
membrane the use of medium HA concentration results in similar
retention performance as in organic-free solutions On the other
hand higher HA concentration results in a notable reduction of
triazine retention ranging up to 74 and 127 for atrazine and prom-
etryn respectively As previously discussed this is attributed to the
increased HA adsorption on the membrane surface (Table 8) which
in turn affects triazine adsorption on XLE membrane and therefore
retention This trend is not observed in the case of FA NOM and
TA despite their adsorptive capacity on the membrane materials
This finding is probably related to fouling phenomena which tend
to modify the membrane properties (ie surface charge hydropho-
bicity morphology) and therefore triazine adsorption Although it
is difficult to use mass balances for calculating triazine adsorption
during the combined filtration with humic substances (due to ana-
lytical problems related to the concentrate samples) the changes
of triazine concentration in the permeate samples provide a good
indication of the potential adsorption of atrazine and prometryn
on the membrane surface Furthermore the increased passage of
triazines in the permeate side as a result of HA adsorption on the
XLE membrane is in accord with the increase of triazine adsorption
on the membrane and therefore with the reduced retention values
324 Effect of calcium ion on triazine retention
From Fig 5 it is evident that the presence of calcium acts
positively in both atrazine and prometryn retention during the
combined nanofiltration with all four types of humic substances
Furthermore the higher interactions between the humic organ-
ics and triazines manifested in the GPC tests are in agreement
with the increased retention performance of atrazine and prom-
etryn by the three membranes used Additionally the suggestion
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 95
of ligand exchange in which calcium serves as a bridging ion and
acts as a possible site for higher adsorption of triazine molecules
on humic substances [38] seems to be in accord with the forma-
tion of an increased number of pseudo-complexes between humic
matter and triazines which explain the increase of herbicide reten-
tion
Among the four HS used tannic acid is associated with the high-
est triazine retention since both triazines are fully retained in the
presence of calcium by all three membranes tested Moreover the
enhanced interaction between humic acids and triazines as a result
of the calcium addition to the feed solutions is accompanied by
a significant increase of atrazine and prometryn retention In this
case calcium ions favour the binding between HA and triazines
which in other cases would not have taken place this is in agree-
ment with the preceding GPC and NFULPRO results
Prometryn exhibits again the highest retention values in com-
parison to atrazine which may be attributed to the relatively higher
prometryn hydrophobicity and basicity (Table 1) The former prop-
erty likely favours hydrophobic interactions between prometryn
and the HSndashcalcium complexes whereas the latter renders prom-
etryn highly effective in mechanisms involving electron-transfer
with the HSndashcalcium complexes [19] This mechanism is due to the
lower acidic functional group content of the HS as a result of the
complexation with calcium ions in the feed solutions Sposito et
al [19] suggested that atrazine itself does not readily participate
in electron-transfer reactions with humic substances However
they demonstrated that hydroxyl-atrazine does react through an
electron-transfer mechanism with HA and FA It is noted that
atrazine is readily converted to hydroxyl-atrazine even in labo-
ratory samples at low water content and this may explain the
presence of some of the electron transfer products detected in stud-
ies of atrazinendashHA interactions [41]
Surveying the literature however it is noted that some stud-
ies either with dialysis membranes [1213] or NF membranes [9]
have shown reduced values of atrazine retention when divalent
calcium is present together with natural organic matter includ-
ing the NOM surrogate tannic acid According to Devitt et al [9]
Devitt and Wiesner [12] this is due to the reduced association of
atrazine and NOM as a result of the occupation of interaction sites
by calcium andor the reduced access of atrazine to NOM sites due
to changes in molecular conformation The GPC results in this study
have shown that this is not the case since the presence of calcium
tends to increase the interaction of humic substances with triazine
compounds like atrazine
The conflicting results between the previous studies and the
present work may be attributed to the different types of membranes
and filtration techniques used More specifically the use of cellulose
ester membranes as well as the experimentation with batch dialy-
sis systems where concentration and osmotic pressure difference
serve as the driving force for solute transport (ie in the absence of
hydrodynamic forces as in the present study) may explain the dif-
ferences regarding the calcium effect on triazine retention Runge
et al [42] claim that the SpectraPor cellulose ester membranes
used in their studies (having a molecular weight cut-off of 100 Da)
Fig 6 Retention of atrazine (a) and prometryn (b) as a function of humic material total aciditymdashcalculations based on the values presented in Table 2 taking into consideration
the carbon composition and the HS concentration
Fig 7 Retention of atrazine (a) and prometryn (b) as a function of humic material carboxylic aciditymdashcalculations based on the values presented in Table 2 taking into
consideration the carbon composition and the HS concentration
96 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
present a significant incompatibility with distilled water whereas
a significant membrane potential can build up with certain salts
which can inhibit or enhance diffusion depending on the size of
the solutes diffusing and the polarity of the membrane potential
generated In this case an increased diffusion of hydrated calcium
ions to the permeate side in order to balance the concentrations
between the two sides of the cellulose ester membrane may result
in a pore blockage or a positive charge on the membrane surface
which in turn could lead to an increased passage of free dissolved
atrazine to the permeate side
Another possible explanation for this inconsistency is the time
period of the filtration tests According to the protocol followed
by previous researchers [1213] the dialysis time period ranged
between 1 and 5 days at the end of which concentration measure-
ments were performed On the other hand the filtration periods
in the case of the present dead-end nanofiltration experiments
ranged between 2 and 5 h depending on the membrane used To
this period one more hour should be added in which the feed solu-
tion was stirred in the cell without pressurization Even though
triazine adsorption on the membrane surface was observed to sta-
bilize within the relatively short period of nanofiltration there is
a possibility that longer filtration periods may have resulted in
higher adsorption rates which in turn could lead to greater pas-
sage of triazine to the permeate side and therefore to reduced
retention values This explanation is in agreement with the pilot
tests performed by Boussahel et al [43] who report that the
increasing adsorption of humic matter on the membranes leads
to an increase of pesticide adsorption on the membranes which
facilitates the diffusion of pesticides in the direction of perme-
ate
33 Correlation between humic material properties and triazine
retention
The correlation of atrazine and prometryn retention with the
total carboxylic and phenolic acidity of the humic substances used
in the present study as well as the role of the molecular mass of HS
on triazine retention are depicted in Figs 6ndash9 These are results of
nanofiltration experiments performed in the absence of calcium
Triazine removal by the three NFULPRO membranes seems
to poorly correlate with the total acidity of the humic material
This is evident especially in the case of NF270 and NF90 mem-
branes (Fig 6) The XLE membrane on the other hand presents a
reduced tendency for atrazine and prometryn removal when acid-
ity increases This is attributed to the characteristic low negative
charge of the XLE membrane surface which causes weak repulsions
with the negatively charged humic organics This leads to a greater
adsorption of humic compounds on the membrane surface (Table 8)
and consequently to greater triazine adsorption which favours the
diffusion of the triazines to the permeate side On the other hand
the strong negative charge of the NF270 and NF90 membranes [20]
promotes a strong repulsion between HS and the membranes The
greater the negative charge of the humic organic the higher the
repulsion which in turn results in higher concentration of HS in
the bulk Consequently the interactions between HS and triazines
in the bulk are increased leading to an improved triazine removal
Fig 8 Retention of atrazine (a) and prometryn (b) as a function of humic material phenolic aciditymdashcalculations based on the values presented in Table 2 taking into
consideration the carbon composition and the HS concentration
Fig 9 Retention of atrazine (a) and prometryn (b) as a function of humic substance molecular mass (based on the values presented in Table 3)
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 97
This is more evident in the case of the wide pore NF270 membrane
since the NF90 membrane seems to retain atrazine and prometryn
to a higher degree in the presence of TA which is characterized by
a lower total acidity among the four HS used in this study
Humic compounds contain a variety of functional groups
such as aliphatic and aromatic carboxyls phenolic hydroxyls
alcoholic hydroxyls carbonyls quinones methoxyls and amino
groups These functional groups are the reactive sites in the humic
molecules As carboxylate protonation seems to be critical for the
capacity of triazine binding [44] it is of interest to correlate the
effect of carboxylic acidity on herbicide removal by nanofiltration
The effect of carboxylic acidity on atrazine and prometryn retention
by the three NFULPRO membranes is depicted in Fig 7
The results show a negative trend of triazine retention with
an increased carboxylic acidity of humic substances This is clear
in the case of NF270 and XLE membranes while NF90 does not
seem to be significantly affected by the changes in the number of
carboxyl groups of a humic molecule Remarkably the smaller TA
molecules containing fewer carboxyl groups per unit mass (Table 2)
exhibit the highest triazine retention (sim100) as a result of the even
higher triazine binding capacity in contrast to the IHSS organics
which contain a higher carboxylic group content Moreover experi-
ments in batch filtration mode result in HS concentration increasing
with time which in turn leads to a reduced number of carboxylic
binding sites available to the two triazines This is probably due
to the self ldquocompetingrdquo character of the HS used since an increase
in concentration andor HS aggregation tend to reduce the avail-
able binding sites [44] This phenomenon led Wang et al [44] to
propose a model in which hydrogen bonding was considered to be
the primary interaction resulting in triazine binding Therefore the
increase in triazine retention observed in the case of elevated HS
concentration may not be directly related with the carboxylic acid-
ity of the HS but rather with other mechanisms such as hydrogen
bonding
Regarding the phenolic acidity of the humic substances there
does not seem to be a sufficient correlation with atrazine or prom-
etryn retention by the three NFULPRO membranes (Fig 8) Actually
there is a contradictory behaviour between the three IHSS organ-
ics and the NOM surrogate TA The high phenolic content of TA
is related to high retention values of the two triazines while
the smaller and relatively similar phenolic acidity of the IHSS
compounds leads to smaller and different retention values These
results imply that there are other factors influencing the association
between humic organics and herbicides and therefore their reten-
tion by nanofiltration In this respect the molecular structure of the
humic materials as well as the interactions taking place between
the organics and the membranes seem to explain the above incon-
sistencies This is evident in the results depicted in Fig 9 and the
adsorption of humic substances subsequently described
The rather strong relation of triazine retention with the molec-
ular mass of the organics used in this study (Fig 9) leads to the
suggestion that structural features of HS are important for the
removal capacity of triazines Up to now there is no consensus
concerning both the binding mechanism of triazines to humic sub-
stances and the structural features responsible for this binding
Nevertheless the preferential binding with low molecular weight
fractions of humic compounds (especially of FA and TA) deduced
from the GPC and filtration tests results in higher retention val-
Table 7
Humic substance and calcium retention performance of the three NFULPRO membranes (mean retention values for 50 recovery of feed volume as permeate)
Membrane Humic substancea Humic substances retention () Ca2+ retention ()
HSs alone HSs with triazines HSs with triazines and Ca2+ In the presence of HSs
NF270 HA 990 994 1000 949
FA 967 983 991 951
NOM 957 997 1000 951
TA 972 992 997 957
NF90 HA 972 980 1000 994
FA 953 984 1000 990
NOM 985 993 1000 991
TA 967 998 998 994
XLE HA 992 996 1000 994
FA 862 915 994 997
NOM 976 1000 1000 988
TA 923 945 1000 996
a HA FA NOM and TA designate the four humic substances used in this study
Table 8
Adsorption data of humic substances on tested membranes
Membrane Humic substancea Feed mass of humic substance (g) Quantity adsorbed (g) Adsorption ()
Alone With Ca2+ Alone With Ca2+ Alone With Ca2+
NF270 HA 3480 2630 244 2314 07 88
FA 3570 2840 71 1335 02 47
NOM 3390 2660 34 2633 01 99
TA 4050 3300 2025 7161 50 217
NF90 HA 3220 2690 354 3658 11 136
FA 2930 2450 1494 857 51 35
NOM 3000 2910 120 437 04 15
TA 2820 2730 649 3631 23 133
XLE HA 3100 2680 1643 3457 53 129
FA 3050 2810 1830 2866 60 102
NOM 2650 2770 610 1219 23 44
TA 3120 2940 2184 2970 70 141
a HA FA NOM and TA designate the four humic substances used in this study
98 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
ues for the two triazines Thus the interaction with small branched
molecules like FA and TA seems to favour a physical entrapment
mechanism such as the one suggested by Devitt and Wiesner [12]
34 Retention of humic species and calcium ion by the NFULPRO
membranes
The performance of nanofiltration on the humic substances and
calcium removal is summarized in Table 7 Regarding humic sub-
stances the retention is compared for three different feed solutions
(a) solutions of HS alone (b) solutions of HS with the two triazines
and (c) solutions of HS with the two triazines in the presence of cal-
cium In general HS retention is high for all three membranes used
According to the UV measurements IHSS HA exhibits the highest
retention values followed by NOM TA and FA which is apparently
related to their relative molecular masses (Table 3) Complexation
or interaction with triazines as evidenced by the GPC tests appar-
ently plays also a significant role in the relative permeability of the
organic species More specifically the retention of the four organic
compounds examined is increased in the presence of the two her-
bicides which verifies also the binding of atrazine and prometryn
with the four humic substances Thus a comparison between the
retention values of the HS in the absence or presence of triazines
gives also a good insight into the HSndashtriazine interactions The slight
increase (lt1) of HA retention in the presence of the two triazines
is in accord with the relatively low HA binding capacity observed
in the GPC tests On the other hand the increase of 2ndash6 in FA
NOM and TA retention is in agreement with the observed increased
interactions between those substances and the two triazines
In the case of calcium addition in the organicndashtriazine solu-
tions the UV absorbing species of the four organics examined are
retained almost completely by all three membranes used This is
again in agreement with the results obtained with gel chromatogra-
phy According to the literature [12] the addition of calcium results
in charge shielding and neutralization of the organic matter charged
functional groups and can shrink the humic matrix The formation
of aggregates of larger apparent molecular weight due to the sub-
stantial increase in the hydrophobicity of the organic molecules
tends to reduce their permeability through membranes and there-
fore to increase their retention
Apart from the increased retention of triazines and humic sub-
stances there is also a significant improvement of calcium rejection
when the latter is present in the solution In comparison to pure
water solutions of elevated calcium ion content [20] the rejection of
calcium in humic solutions is almost complete This is attributed to
size exclusion mechanisms related to the binding tendency of diva-
lent ions like calcium to the negatively charged humic organics
Among the four humic substances used calcium is rejected more in
the presence of the polyphenol tannic acid which is accompanied
by a higher organic adsorption on all three NFULPRO membranes
tested (Table 8)
From the data summarized in Table 8 it can be seen that the
adsorption of humic substances onto the membrane surfaces is
markedly enhanced in the presence of calcium ions due to the
formation of HA-complexed divalent cations This is also clearly
depicted in the SEM images of virgin and fouled membranes in
the absence or presence of calcium ions Characteristic SEM images
in the case of the Suwannee River NOM filtration with the three
NFULPRO membranes are included in Fig 10 these images show
the severe fouling on the three NFULPRO membranes as a result
of the combined filtration of NOM with CaCl2 salt It is possible
that the divalent cations such as Ca2+ act as a bridge between the
membrane surface and the negatively charged humic molecules
that tend to be repelled by the negatively charged membrane As a
result the electrical repulsion between the membrane surface and
the humic molecules is weakened thus rendering the membrane
Fig 10 SEM images of virgin fouled with NOM and fouled with NOM + calcium membranes (a) NF270 (b) NF90 and (c) XLE membrane
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 99
fouling process more severe [45] On the other hand the relatively
smaller adsorption rates of NOM in the absence of calcium result
in less fouled membranes A similar trend is also observed in the
case of the other three humic substances used in this study Among
the three membranes used XLE exhibits in the absence of calcium
the highest adsorption rates for all four humic substances This is
explained by the relatively high hydrophobicity and the low surface
charge that characterize the XLE membrane surface as previously
explained
4 Concluding remarks
The influence of humic substances on retention of two tri-
azines (atrazine and prometryn) by NFULPRO membranes is rather
complicated and involves combinations of solutendashmembrane and
solutendashsolute interactions In summary the retention of triazines
depends on the type of the humic material present in the feed
water In most cases the combined nanofiltration of triazines and
naturally occurring humic substances facilitates the formation of
complexes with triazines which in turn enhance their removal by
means of membrane filtration This complexation appears to be
related not to the characteristic acidity (phenolic carboxylic) of
the HS used but rather to their molecular conformation More
specifically a preferential binding is observed between triazines
and low molecular weight fractions of humic compounds (espe-
cially of FA and TA) which results in higher retention values for the
two triazines Under all conditions tannic acid exhibits the greatest
effect on triazine retention among the four standard HS compounds
used leading to an almost complete removal of the two triazines
(95ndash100) for all three membranes tested
The removal of triazines is improved in the presence of calcium
which tends to enhance the interaction between HS and triazines
Moreover triazine retention is increased with increasing HS con-
centration to a degree depending on the type of HS Additionally
triazine retention is apparently also affected by the interactions
between the organics and the membrane surface which in turn
may alter triazine retention These results illustrate the complex-
ity of the triazine adsorption and retention mechanisms in the
presence of humic substances that needs further investigation for
improved understanding Moreover regarding triazine removal by
nanofiltration it is of particular interest to distinguish the effect
of the HSndashtriazine interactions from that of membrane HS fouling
Research in this direction is carried out in this laboratory
References
[1] DP Birardar AL Rayburn Chromosomal damage induced by herbicide con-tamination at concentrations observed in public water supplies J Environ Qual24 (1995) 1222ndash1225
[2] IK Konstantinou DG Hela TA Albanis The status of pesticide pollution insurface waters (rivers and lakes) of Greece Part I Review on occurrence andlevels Environ Pollut 141 (3) (2006) 555ndash570
[3] European Environment Agency (EEA) Groundwater quality and quantity inEurope Report Copenhagen June 1999 available in httpreportseeaeuropaeugroundwater07012000enenviassrp199903
[4] EM Thurman DA Goolsby MT Meyer MS Mills ML Pomes DW Kolpin AReconnaissance study of herbicides and their metabolites in surface water of theMidwestern United States using immunoassay and gas chromatographymassspectrometry Environ Sci Technol 26 (1992) 2440ndash2447
[5] C Bellona JE Drewes P Xu G Amy Factors affecting the rejection of organicsolutes during NFRO treatmentmdasha literature review Water Res 38 (2004)2795ndash2809
[6] A Bhattacharya Remediation of pesticide-polluted waters through mem-branes Sep Purif Rev 35 (2006) 1ndash38
[7] KM Agbekodo B Legube S Dard Atrazine and simazine removal mechanismsby nanofiltration influence of natural organic matter concentration Water Res30 (1996) 2535ndash2542
[8] P Berg G Hagmeyer R Gimbel Removal of pesticides and other micropollu-tants by nanofiltration Desalination 113 (1997) 205ndash208
[9] EC Devitt F Ducellier P Coumlteacute MR Wiesner Effects of natural organic mat-ter and the raw water matrix on the rejection of atrazine by pressure-drivenmembranes Water Res 32 (1998) 2563ndash2568
[10] R Boussahel S Bouland KM Moussaoui A Montiel Removal of pesticideresidues in water using the nanofiltration process Desalination 132 (2000)205ndash209
[11] Y Zhang B Van der Bruggen GX Chen L Braeken C Vandecasteele Removalof pesticides by nanofiltration effect of the water matrix Sep Purif Technol38 (2004) 163ndash172
[12] EC Devitt MR Wiesner Dialysis investigations of atrazinendashorganic matterinteractions and the role of a divalent metal Environ Sci Technol 32 (1998)232ndash237
[13] SK Dalton JA Brant MR Wiesner Chemical interactions between dissolvedorganic matter and low-molecular weight organic compounds impacts onmembrane separation J Membr Sci 266 (2005) 30ndash39
[14] NA Kulikova IV Perminova Binding of atrazine to humic substances fromsoil peat and coal related to their structure Environ Sci Technol 36 (2002)3720ndash3724
[15] RL Wershaw The importance of humic substancendashmineral particle complexesin the modeling of contaminant transport in sedimentndashwater systems in RABaker (Ed) Organic Substances and Sediments in Water Humics and SoilsLewis Publishers Chelsea MI 1991 pp 23ndash34
[16] SM Schrap A Opperhuizen Relationship between bioavailability andhydrophobicity reduction of the uptake of organic chemicals by fish due tothe sorption on particles Environ Toxicol Chem 9 (1990) 715ndash724
[17] E Barriuso M Schiavon F Andreux JM Portal Localization of atrazine non-extractable (bound) residues in soil size fractions Chemosphere 12 (1991)1131ndash1140
[18] L Loiseau E Barriuso Characterization of the atrazinersquos bound (nonextractable)residues using fractionation techniques for soil organic matter Environ SciTechnol 36 (2002) 683ndash689
[19] G Sposito L Martin-Neto A Yang Atrazine complexation by soil humic acidsJ Environ Qual 25 (1996) 1203ndash1209
[20] KV Plakas AJ Karabelas Membrane retention of herbicides from single andmulti-solute media the effect of ionic environment J Membr Sci 320 (2008)325ndash334
[21] V Freger J Gilron S Belfer TFC polyamide membranes modified by graftingof hydrophilic polymers an FT-IRAFMTEM study J Membr Sci 209 (2002)283ndash292
[22] P Xu JE Drewes Viability of nanofiltration and ultra-low pressure reverseosmosis membranes for multi-beneficial use of methane produced water SepPurif Technol 52 (2006) 67ndash76
[23] Z Knez A Rizner-Hras K Kokot D Bauman Solubility of some solid triazineherbicides in supercritical carbon dioxide Fluid Phase Equilibria 152 (1998)95ndash108
[24] Weed Science Society of America (WSSA) Herbicide Handbook 7th ed 1994[25] B Van der Bruggen J Schaep W Maes D Wilms C Vandecasteele Nanofiltra-
tion as a treatment method for the removal of pesticides from ground watersDesalination 117 (1998) 139ndash147
[26] KN Reddy MA Locke Molecular properties as descriptors of octanol-waterpartition coefficients of herbicides Water Air Soil Pollut 86 (1996) 389ndash405
[27] IHSS website httpwwwihssgatechedu[28] F Flores-Ceacutespedes M Fernaacutendez-Peacuterez M Villafranca-Saacutenchez E Gonzaacutelez-
Pradas Cosorption study of organic pollutants and dissolved organic matter ina soil Environ Pollut 142 (2006) 449ndash456
[29] R Beckett Z Jue JC Giddings Determination of molecular weight distribu-tions of fulvic and humic acids using flow field-flow fractionation Environ SciTechnol 21 (1987) 289ndash295
[30] PM Reid AE Wilkinson E Tipping MN Jones Determination of molecularweights of humic substances by analytical (UV scanning) ultracentrifugationGeochim Cosmochim Acta 54 (1990) 131ndash138
[31] S Lee B Kwon M Sun J Cho Characterizations of NOM included in NF and UFmembrane permeates Desalination 173 (2005) 131ndash142
[32] MN Jones ND Bryan Colloidal properties of humic substances Adv ColloidInterf Sci 78 (1998) 1ndash48
[33] KV Plakas AJ Karabelas T Wintgens T Melin A study of selected herbicidesretention by nanofiltration membranesmdashthe role of organic fouling J MembrSci 284 (2006) 291ndash300
[34] APHA-AWWA-WPCF Standard Methods for the Examination of Water andWastewater 17th ed Port City Press Baltimore MD 1989 pp 567ndash569
[35] R Artinger G Buckau JI Kim S Geyer Characterization of groundwater humicand fulvic acids of different origin by GPC with UVVis and fluorescence detec-tion Fresenius J Anal Chem 364 (1999) 737ndash745
[36] AI Schaumlfer N Andritsos AJ Karabelas EMV Hoek R Schneider M Nys-troumlm Chapter 8 Fouling in nanofiltration in AI Schaumlfer AG Fane TDWaite (Eds) Nanofiltration Principles and Applications Elsevier Oxford UK2005 pp 173ndash174
[37] I Franco L Catalano M Contin M De Nobili Interaction between organicmodel compounds and pesticides with water-soluble soil humic substancesActa Hydrochim Hydrobiol 29 (2001) 88ndash99
[38] AA Helal DM Imam SM Khalifa HF Aly Interaction of pesticides with humiccompounds and their metal complexes Radiochemistry 48 (4) (2006) 419ndash425
[39] MG Browman G Chester Fate of Pollutants in the Air and Water EnvironmentWiley New York 1977 p 50
[40] LD Nghiem AI Schaefer M Elimelech Nanofiltration of hormone mimickingtrace organic contaminants Sep Sci Technol 40 (2005) 2633ndash2649
[41] R Celis J Cornejo MC Hermosin WC Koskinen Sorptionndashdesorption ofatrazine and simazine by model soil colloidal components Soil Sci Soc AmJ 61 (1997) 436ndash443
100 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
[42] SW Runge KR Shelton SA Melton WM Moran Maintaining the ionic per-meability of a cellulose ester membrane J Biochem Biophys Methods 64(2005) 200ndash206
[43] R Boussahel A Montiel M Baudu Effects of organic and inorganic matter onpesticide rejection by nanofiltration Desalination 145 (2002) 109ndash114
[44] Z Wang DS Gamble CH Langford Interaction of atrazine with Laurentianhumic acid Anal Chim Acta 244 (1991) 135ndash143
[45] C Jucker MM Clark Adsorption of aquatic humic substances on hydrophobicultrafiltration membranes J Membr Sci 97 (1994) 37ndash52
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 93
Fig 5 Retention of atrazine (A) and prometryn (P) by the three NFULPRO membranes in the absence or presence of humic substances (HA FA NOM TA) andor calcium
ions
or ndashFA complex is stronger than the metalndashhumate or ndashfulvate
complex The mechanisms involved in the interactions between
triazines and calciumndashHS complexes may be described by either
a physical adsorption of the herbicide on the surface of metal
humatefulvate complex or a ligand exchange in which the metal
serves as a bridging ion and acts as a possible site for adsorption of
the herbicide molecule by displacing water of hydration [38]
322 Effect of different types of humic materials on triazine
retention
Retention results of atrazine and prometryn during the com-
bined nanofiltration with the four selected humic substances are
depicted in Fig 5 Results of triazine retention from organic solu-
tions with elevated calcium ion content are also included in Fig 5
and discussed in Section 324 Depending on the type of humic
material present increased retention of the two triazines may be
negligible or significant Specifically the use of HA results in reten-
tion values that are comparable to those obtained in the absence
of organics An exception is observed in the case of XLE membrane
where the combined filtration of triazines and HA leads to relatively
smaller retention values (7 and 13 for atrazine and prometryn
respectively) On the other hand the presence of tannic acid results
in a significant increase of triazine retention for all three mem-
branes tested In particular the addition of TA at a concentration of
10 mg Lminus1 (yielding a triazineTA ratio of 1 g1 mg) results in 14ndash21
and 20ndash29 increase of prometryn and atrazine retention respec-
tively The enhancement of pesticide removal in the presence of
the NOM surrogate tannic acid has been also observed by previ-
ous researchers [91213] According to Devitt and Wiesner [12] the
increased retention of a triazine molecule like atrazine in the pres-
ence of TA is due to the branched structure of TA which traps the
triazine within the macromolecule Moreover these authors sug-
gest that molecular conformations and weak interactions between
functional groups may combine to produce physical entrapment of
triazines through a low multiplicative probability of escape result-
ing from a large number of encounters with low-energy sites on the
tannic acid molecule (eg hydrogen bonding)
In the case of the combined nanofiltration of atrazine and prom-
etryn with FA or NOM a modest increase of retention in comparison
to the pure triazine solutions was recorded In particular depending
on the membrane used the presence of FA resulted in an increase
of 8ndash15 of atrazine retention and 4ndash14 of prometryn retention
whereas nanofiltration in the presence of NOM led to 12ndash17 and
3ndash15 increase of atrazine and prometryn retention respectively
The interactions between atrazine and prometryn with the
humic substances observed in the GPC experiments seem to agree
(up to a point) with the trends in triazine retention by the three
NFULPRO membranes Indeed the negligible interactions observed
between the two herbicides and the humic acids are in accord with
the slight differences observed in atrazine and prometryn retention
by the NF270 and NF90 membranes On the other hand the some-
what reduced retention observed in the case of XLE membrane
may be not only due to the weak interactions between triazines
and humic acids but also to the hydrophobic interactions between
HA and the membrane surface which tends to affect atrazine and
prometryn retention This is in accord with the observed higher
adsorption rates of HA on the XLE membrane (Table 8) which in turn
may affect the surface properties of the membrane and therefore
triazine retention
The GPC results in the case of FA and NOM are in accord with
the retention results of triazines for all three membranes FA and
NOM interact with the two herbicides to form complexes of various
stabilities The formation of these pseudo-complexes may explain
the higher retention of atrazine and prometryn as a result of the
94 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
Table 6
Average retention values of atrazine and prometryn at two different humic substance concentrations by the three NFULPRO membranes
Humic substance Cfeed (mg Lminus1) NF270 NF90 XLE
Ratrazine () Rprometryn () Ratrazine () Rprometryn () Ratrazine () Rprometryn ()
HA 5 782 804 873 883 83 852
10 776 798 838 846 769 744
FA 5 784 829 828 870 848 913
10 816 842 937 933 906 977
NOM 5 813 859 843 838 879 922
10 862 901 894 857 956 968
TA 5 898 880 876 903 905 963
10 969 976 96 957 100 100
following mechanisms that may take place (a) increase of the
steric congestion or reduction of the diffusivity of the FAndashtriazine
or NOMndashtriazine pseudo-complex (b) increase of the density of
the pseudo-complexrsquos negative charge which enhances the elec-
trostatic repulsion with the negatively charged membranes and (c)
increase of the adsorption capability of the pseudo-complex on the
surface of the membrane in contrast to the hydrophobic nature of
the humic materials The validity of the above mechanism hypothe-
ses adopted by previous researchers [7] depends on the type of
the organic material and consequently on the characteristic chem-
ical properties of both HS (acidity functional groups) and triazines
(basicity hydrophobicity charge) The correlation between humic
material properties and triazine retention is examined in a subse-
quent paragraph
The greater extent of FAndashtriazines pseudo-complexes in com-
parison to NOMndashtriazines pseudo-complexes manifested in the
GPC results is not accompanied by a greater increase of triazine
retention More specifically the increasing order of triazine reten-
tion in the presence of a specific humic material follows the order
TA gt NOM gt FA gt HA in the case of NF270 and XLE membranes
whereas for NF90 the order is TA ge FA gt NOM gt HA Therefore the
interactions between the humic materials and triazines alone
cannot explain the retention performance of the NFULPRO mem-
branes This fact indicates that interactions between the naturally
occurring organic substances and the membranes may also play a
significant role in triazine retention as previously observed [33] In
view of these observations dead-end filtration tests using the three
IHSS organic materials and the selected NFULPRO membranes are
carried out in this laboratory in order to distinguish the effects of
organic fouling and of HSndashtriazines interactions on triazine perme-
ation during nanofiltration
323 Effect of humic substance concentration on triazine
retention
Surveying the literature on the complicated influence of organic
matter on retention of pesticides by NFRO membranes reveals a
considerable variability regarding the importance of the organic
matter concentration Experiments in a nanofiltration pilot unit on
the removal of atrazine and simazine have demonstrated the sig-
nificance of the organic load in the feed water since the removal
efficiency increased from 50 to 90ndash100 when dissolved organic
carbon varied between 04 and 36 mg Lminus1 [7] In contrast Berg et
al [8] reported no increase of pesticide retention (atrazine and
simazine among them) by several NF membranes with increas-
ing organic matter concentration Along the same lines Zhang et
al [11] showed comparable simazine retention between tap water
and river water although the latter had a high concentration of
NOM whereas tap water had a low NOM concentration In an effort
to understand the reasons for the above inconsistency in the lit-
erature Nghiem et al [40] took into consideration the effect of
ionic strength Experiments with hormone mimicking trace organic
contaminants revealed the significance of solution ionic strength
in influencing NOMndashtrace organic interactions which is probably
the cause for the ambiguity of literature data regarding the role
of background organic matter in trace organic rejection The mag-
nitude of ionic strength in terms of calcium ion content on the
NOMndashtriazines interaction has been also recorded through the GPC
tests described previously and is subsequently examined Although
ionic strength can strongly influence the HSndashtriazines interactions
the effect of the type of the humic substance seems to play also a
significant role on triazine retention This is evident in the results
included in Table 6 regarding retention experiments in the pres-
ence of a medium (5 mg Lminus1) and relatively high (10 mg Lminus1) HS
concentration in the absence of ionic environment
Table 6 shows a clear increase of triazine retention with increas-
ing humic material concentration More specifically by doubling
the concentration of FA NOM and TA in the feed solutions atrazine
and prometryn retention increases 4ndash13 and 2ndash10 respectively
depending on the membrane and the humic material used An
exception is observed again in the case of humic acids since the
increased organic load results in negligible or slightly negative
effects of atrazine and prometryn retention Regarding the XLE
membrane the use of medium HA concentration results in similar
retention performance as in organic-free solutions On the other
hand higher HA concentration results in a notable reduction of
triazine retention ranging up to 74 and 127 for atrazine and prom-
etryn respectively As previously discussed this is attributed to the
increased HA adsorption on the membrane surface (Table 8) which
in turn affects triazine adsorption on XLE membrane and therefore
retention This trend is not observed in the case of FA NOM and
TA despite their adsorptive capacity on the membrane materials
This finding is probably related to fouling phenomena which tend
to modify the membrane properties (ie surface charge hydropho-
bicity morphology) and therefore triazine adsorption Although it
is difficult to use mass balances for calculating triazine adsorption
during the combined filtration with humic substances (due to ana-
lytical problems related to the concentrate samples) the changes
of triazine concentration in the permeate samples provide a good
indication of the potential adsorption of atrazine and prometryn
on the membrane surface Furthermore the increased passage of
triazines in the permeate side as a result of HA adsorption on the
XLE membrane is in accord with the increase of triazine adsorption
on the membrane and therefore with the reduced retention values
324 Effect of calcium ion on triazine retention
From Fig 5 it is evident that the presence of calcium acts
positively in both atrazine and prometryn retention during the
combined nanofiltration with all four types of humic substances
Furthermore the higher interactions between the humic organ-
ics and triazines manifested in the GPC tests are in agreement
with the increased retention performance of atrazine and prom-
etryn by the three membranes used Additionally the suggestion
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 95
of ligand exchange in which calcium serves as a bridging ion and
acts as a possible site for higher adsorption of triazine molecules
on humic substances [38] seems to be in accord with the forma-
tion of an increased number of pseudo-complexes between humic
matter and triazines which explain the increase of herbicide reten-
tion
Among the four HS used tannic acid is associated with the high-
est triazine retention since both triazines are fully retained in the
presence of calcium by all three membranes tested Moreover the
enhanced interaction between humic acids and triazines as a result
of the calcium addition to the feed solutions is accompanied by
a significant increase of atrazine and prometryn retention In this
case calcium ions favour the binding between HA and triazines
which in other cases would not have taken place this is in agree-
ment with the preceding GPC and NFULPRO results
Prometryn exhibits again the highest retention values in com-
parison to atrazine which may be attributed to the relatively higher
prometryn hydrophobicity and basicity (Table 1) The former prop-
erty likely favours hydrophobic interactions between prometryn
and the HSndashcalcium complexes whereas the latter renders prom-
etryn highly effective in mechanisms involving electron-transfer
with the HSndashcalcium complexes [19] This mechanism is due to the
lower acidic functional group content of the HS as a result of the
complexation with calcium ions in the feed solutions Sposito et
al [19] suggested that atrazine itself does not readily participate
in electron-transfer reactions with humic substances However
they demonstrated that hydroxyl-atrazine does react through an
electron-transfer mechanism with HA and FA It is noted that
atrazine is readily converted to hydroxyl-atrazine even in labo-
ratory samples at low water content and this may explain the
presence of some of the electron transfer products detected in stud-
ies of atrazinendashHA interactions [41]
Surveying the literature however it is noted that some stud-
ies either with dialysis membranes [1213] or NF membranes [9]
have shown reduced values of atrazine retention when divalent
calcium is present together with natural organic matter includ-
ing the NOM surrogate tannic acid According to Devitt et al [9]
Devitt and Wiesner [12] this is due to the reduced association of
atrazine and NOM as a result of the occupation of interaction sites
by calcium andor the reduced access of atrazine to NOM sites due
to changes in molecular conformation The GPC results in this study
have shown that this is not the case since the presence of calcium
tends to increase the interaction of humic substances with triazine
compounds like atrazine
The conflicting results between the previous studies and the
present work may be attributed to the different types of membranes
and filtration techniques used More specifically the use of cellulose
ester membranes as well as the experimentation with batch dialy-
sis systems where concentration and osmotic pressure difference
serve as the driving force for solute transport (ie in the absence of
hydrodynamic forces as in the present study) may explain the dif-
ferences regarding the calcium effect on triazine retention Runge
et al [42] claim that the SpectraPor cellulose ester membranes
used in their studies (having a molecular weight cut-off of 100 Da)
Fig 6 Retention of atrazine (a) and prometryn (b) as a function of humic material total aciditymdashcalculations based on the values presented in Table 2 taking into consideration
the carbon composition and the HS concentration
Fig 7 Retention of atrazine (a) and prometryn (b) as a function of humic material carboxylic aciditymdashcalculations based on the values presented in Table 2 taking into
consideration the carbon composition and the HS concentration
96 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
present a significant incompatibility with distilled water whereas
a significant membrane potential can build up with certain salts
which can inhibit or enhance diffusion depending on the size of
the solutes diffusing and the polarity of the membrane potential
generated In this case an increased diffusion of hydrated calcium
ions to the permeate side in order to balance the concentrations
between the two sides of the cellulose ester membrane may result
in a pore blockage or a positive charge on the membrane surface
which in turn could lead to an increased passage of free dissolved
atrazine to the permeate side
Another possible explanation for this inconsistency is the time
period of the filtration tests According to the protocol followed
by previous researchers [1213] the dialysis time period ranged
between 1 and 5 days at the end of which concentration measure-
ments were performed On the other hand the filtration periods
in the case of the present dead-end nanofiltration experiments
ranged between 2 and 5 h depending on the membrane used To
this period one more hour should be added in which the feed solu-
tion was stirred in the cell without pressurization Even though
triazine adsorption on the membrane surface was observed to sta-
bilize within the relatively short period of nanofiltration there is
a possibility that longer filtration periods may have resulted in
higher adsorption rates which in turn could lead to greater pas-
sage of triazine to the permeate side and therefore to reduced
retention values This explanation is in agreement with the pilot
tests performed by Boussahel et al [43] who report that the
increasing adsorption of humic matter on the membranes leads
to an increase of pesticide adsorption on the membranes which
facilitates the diffusion of pesticides in the direction of perme-
ate
33 Correlation between humic material properties and triazine
retention
The correlation of atrazine and prometryn retention with the
total carboxylic and phenolic acidity of the humic substances used
in the present study as well as the role of the molecular mass of HS
on triazine retention are depicted in Figs 6ndash9 These are results of
nanofiltration experiments performed in the absence of calcium
Triazine removal by the three NFULPRO membranes seems
to poorly correlate with the total acidity of the humic material
This is evident especially in the case of NF270 and NF90 mem-
branes (Fig 6) The XLE membrane on the other hand presents a
reduced tendency for atrazine and prometryn removal when acid-
ity increases This is attributed to the characteristic low negative
charge of the XLE membrane surface which causes weak repulsions
with the negatively charged humic organics This leads to a greater
adsorption of humic compounds on the membrane surface (Table 8)
and consequently to greater triazine adsorption which favours the
diffusion of the triazines to the permeate side On the other hand
the strong negative charge of the NF270 and NF90 membranes [20]
promotes a strong repulsion between HS and the membranes The
greater the negative charge of the humic organic the higher the
repulsion which in turn results in higher concentration of HS in
the bulk Consequently the interactions between HS and triazines
in the bulk are increased leading to an improved triazine removal
Fig 8 Retention of atrazine (a) and prometryn (b) as a function of humic material phenolic aciditymdashcalculations based on the values presented in Table 2 taking into
consideration the carbon composition and the HS concentration
Fig 9 Retention of atrazine (a) and prometryn (b) as a function of humic substance molecular mass (based on the values presented in Table 3)
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 97
This is more evident in the case of the wide pore NF270 membrane
since the NF90 membrane seems to retain atrazine and prometryn
to a higher degree in the presence of TA which is characterized by
a lower total acidity among the four HS used in this study
Humic compounds contain a variety of functional groups
such as aliphatic and aromatic carboxyls phenolic hydroxyls
alcoholic hydroxyls carbonyls quinones methoxyls and amino
groups These functional groups are the reactive sites in the humic
molecules As carboxylate protonation seems to be critical for the
capacity of triazine binding [44] it is of interest to correlate the
effect of carboxylic acidity on herbicide removal by nanofiltration
The effect of carboxylic acidity on atrazine and prometryn retention
by the three NFULPRO membranes is depicted in Fig 7
The results show a negative trend of triazine retention with
an increased carboxylic acidity of humic substances This is clear
in the case of NF270 and XLE membranes while NF90 does not
seem to be significantly affected by the changes in the number of
carboxyl groups of a humic molecule Remarkably the smaller TA
molecules containing fewer carboxyl groups per unit mass (Table 2)
exhibit the highest triazine retention (sim100) as a result of the even
higher triazine binding capacity in contrast to the IHSS organics
which contain a higher carboxylic group content Moreover experi-
ments in batch filtration mode result in HS concentration increasing
with time which in turn leads to a reduced number of carboxylic
binding sites available to the two triazines This is probably due
to the self ldquocompetingrdquo character of the HS used since an increase
in concentration andor HS aggregation tend to reduce the avail-
able binding sites [44] This phenomenon led Wang et al [44] to
propose a model in which hydrogen bonding was considered to be
the primary interaction resulting in triazine binding Therefore the
increase in triazine retention observed in the case of elevated HS
concentration may not be directly related with the carboxylic acid-
ity of the HS but rather with other mechanisms such as hydrogen
bonding
Regarding the phenolic acidity of the humic substances there
does not seem to be a sufficient correlation with atrazine or prom-
etryn retention by the three NFULPRO membranes (Fig 8) Actually
there is a contradictory behaviour between the three IHSS organ-
ics and the NOM surrogate TA The high phenolic content of TA
is related to high retention values of the two triazines while
the smaller and relatively similar phenolic acidity of the IHSS
compounds leads to smaller and different retention values These
results imply that there are other factors influencing the association
between humic organics and herbicides and therefore their reten-
tion by nanofiltration In this respect the molecular structure of the
humic materials as well as the interactions taking place between
the organics and the membranes seem to explain the above incon-
sistencies This is evident in the results depicted in Fig 9 and the
adsorption of humic substances subsequently described
The rather strong relation of triazine retention with the molec-
ular mass of the organics used in this study (Fig 9) leads to the
suggestion that structural features of HS are important for the
removal capacity of triazines Up to now there is no consensus
concerning both the binding mechanism of triazines to humic sub-
stances and the structural features responsible for this binding
Nevertheless the preferential binding with low molecular weight
fractions of humic compounds (especially of FA and TA) deduced
from the GPC and filtration tests results in higher retention val-
Table 7
Humic substance and calcium retention performance of the three NFULPRO membranes (mean retention values for 50 recovery of feed volume as permeate)
Membrane Humic substancea Humic substances retention () Ca2+ retention ()
HSs alone HSs with triazines HSs with triazines and Ca2+ In the presence of HSs
NF270 HA 990 994 1000 949
FA 967 983 991 951
NOM 957 997 1000 951
TA 972 992 997 957
NF90 HA 972 980 1000 994
FA 953 984 1000 990
NOM 985 993 1000 991
TA 967 998 998 994
XLE HA 992 996 1000 994
FA 862 915 994 997
NOM 976 1000 1000 988
TA 923 945 1000 996
a HA FA NOM and TA designate the four humic substances used in this study
Table 8
Adsorption data of humic substances on tested membranes
Membrane Humic substancea Feed mass of humic substance (g) Quantity adsorbed (g) Adsorption ()
Alone With Ca2+ Alone With Ca2+ Alone With Ca2+
NF270 HA 3480 2630 244 2314 07 88
FA 3570 2840 71 1335 02 47
NOM 3390 2660 34 2633 01 99
TA 4050 3300 2025 7161 50 217
NF90 HA 3220 2690 354 3658 11 136
FA 2930 2450 1494 857 51 35
NOM 3000 2910 120 437 04 15
TA 2820 2730 649 3631 23 133
XLE HA 3100 2680 1643 3457 53 129
FA 3050 2810 1830 2866 60 102
NOM 2650 2770 610 1219 23 44
TA 3120 2940 2184 2970 70 141
a HA FA NOM and TA designate the four humic substances used in this study
98 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
ues for the two triazines Thus the interaction with small branched
molecules like FA and TA seems to favour a physical entrapment
mechanism such as the one suggested by Devitt and Wiesner [12]
34 Retention of humic species and calcium ion by the NFULPRO
membranes
The performance of nanofiltration on the humic substances and
calcium removal is summarized in Table 7 Regarding humic sub-
stances the retention is compared for three different feed solutions
(a) solutions of HS alone (b) solutions of HS with the two triazines
and (c) solutions of HS with the two triazines in the presence of cal-
cium In general HS retention is high for all three membranes used
According to the UV measurements IHSS HA exhibits the highest
retention values followed by NOM TA and FA which is apparently
related to their relative molecular masses (Table 3) Complexation
or interaction with triazines as evidenced by the GPC tests appar-
ently plays also a significant role in the relative permeability of the
organic species More specifically the retention of the four organic
compounds examined is increased in the presence of the two her-
bicides which verifies also the binding of atrazine and prometryn
with the four humic substances Thus a comparison between the
retention values of the HS in the absence or presence of triazines
gives also a good insight into the HSndashtriazine interactions The slight
increase (lt1) of HA retention in the presence of the two triazines
is in accord with the relatively low HA binding capacity observed
in the GPC tests On the other hand the increase of 2ndash6 in FA
NOM and TA retention is in agreement with the observed increased
interactions between those substances and the two triazines
In the case of calcium addition in the organicndashtriazine solu-
tions the UV absorbing species of the four organics examined are
retained almost completely by all three membranes used This is
again in agreement with the results obtained with gel chromatogra-
phy According to the literature [12] the addition of calcium results
in charge shielding and neutralization of the organic matter charged
functional groups and can shrink the humic matrix The formation
of aggregates of larger apparent molecular weight due to the sub-
stantial increase in the hydrophobicity of the organic molecules
tends to reduce their permeability through membranes and there-
fore to increase their retention
Apart from the increased retention of triazines and humic sub-
stances there is also a significant improvement of calcium rejection
when the latter is present in the solution In comparison to pure
water solutions of elevated calcium ion content [20] the rejection of
calcium in humic solutions is almost complete This is attributed to
size exclusion mechanisms related to the binding tendency of diva-
lent ions like calcium to the negatively charged humic organics
Among the four humic substances used calcium is rejected more in
the presence of the polyphenol tannic acid which is accompanied
by a higher organic adsorption on all three NFULPRO membranes
tested (Table 8)
From the data summarized in Table 8 it can be seen that the
adsorption of humic substances onto the membrane surfaces is
markedly enhanced in the presence of calcium ions due to the
formation of HA-complexed divalent cations This is also clearly
depicted in the SEM images of virgin and fouled membranes in
the absence or presence of calcium ions Characteristic SEM images
in the case of the Suwannee River NOM filtration with the three
NFULPRO membranes are included in Fig 10 these images show
the severe fouling on the three NFULPRO membranes as a result
of the combined filtration of NOM with CaCl2 salt It is possible
that the divalent cations such as Ca2+ act as a bridge between the
membrane surface and the negatively charged humic molecules
that tend to be repelled by the negatively charged membrane As a
result the electrical repulsion between the membrane surface and
the humic molecules is weakened thus rendering the membrane
Fig 10 SEM images of virgin fouled with NOM and fouled with NOM + calcium membranes (a) NF270 (b) NF90 and (c) XLE membrane
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 99
fouling process more severe [45] On the other hand the relatively
smaller adsorption rates of NOM in the absence of calcium result
in less fouled membranes A similar trend is also observed in the
case of the other three humic substances used in this study Among
the three membranes used XLE exhibits in the absence of calcium
the highest adsorption rates for all four humic substances This is
explained by the relatively high hydrophobicity and the low surface
charge that characterize the XLE membrane surface as previously
explained
4 Concluding remarks
The influence of humic substances on retention of two tri-
azines (atrazine and prometryn) by NFULPRO membranes is rather
complicated and involves combinations of solutendashmembrane and
solutendashsolute interactions In summary the retention of triazines
depends on the type of the humic material present in the feed
water In most cases the combined nanofiltration of triazines and
naturally occurring humic substances facilitates the formation of
complexes with triazines which in turn enhance their removal by
means of membrane filtration This complexation appears to be
related not to the characteristic acidity (phenolic carboxylic) of
the HS used but rather to their molecular conformation More
specifically a preferential binding is observed between triazines
and low molecular weight fractions of humic compounds (espe-
cially of FA and TA) which results in higher retention values for the
two triazines Under all conditions tannic acid exhibits the greatest
effect on triazine retention among the four standard HS compounds
used leading to an almost complete removal of the two triazines
(95ndash100) for all three membranes tested
The removal of triazines is improved in the presence of calcium
which tends to enhance the interaction between HS and triazines
Moreover triazine retention is increased with increasing HS con-
centration to a degree depending on the type of HS Additionally
triazine retention is apparently also affected by the interactions
between the organics and the membrane surface which in turn
may alter triazine retention These results illustrate the complex-
ity of the triazine adsorption and retention mechanisms in the
presence of humic substances that needs further investigation for
improved understanding Moreover regarding triazine removal by
nanofiltration it is of particular interest to distinguish the effect
of the HSndashtriazine interactions from that of membrane HS fouling
Research in this direction is carried out in this laboratory
References
[1] DP Birardar AL Rayburn Chromosomal damage induced by herbicide con-tamination at concentrations observed in public water supplies J Environ Qual24 (1995) 1222ndash1225
[2] IK Konstantinou DG Hela TA Albanis The status of pesticide pollution insurface waters (rivers and lakes) of Greece Part I Review on occurrence andlevels Environ Pollut 141 (3) (2006) 555ndash570
[3] European Environment Agency (EEA) Groundwater quality and quantity inEurope Report Copenhagen June 1999 available in httpreportseeaeuropaeugroundwater07012000enenviassrp199903
[4] EM Thurman DA Goolsby MT Meyer MS Mills ML Pomes DW Kolpin AReconnaissance study of herbicides and their metabolites in surface water of theMidwestern United States using immunoassay and gas chromatographymassspectrometry Environ Sci Technol 26 (1992) 2440ndash2447
[5] C Bellona JE Drewes P Xu G Amy Factors affecting the rejection of organicsolutes during NFRO treatmentmdasha literature review Water Res 38 (2004)2795ndash2809
[6] A Bhattacharya Remediation of pesticide-polluted waters through mem-branes Sep Purif Rev 35 (2006) 1ndash38
[7] KM Agbekodo B Legube S Dard Atrazine and simazine removal mechanismsby nanofiltration influence of natural organic matter concentration Water Res30 (1996) 2535ndash2542
[8] P Berg G Hagmeyer R Gimbel Removal of pesticides and other micropollu-tants by nanofiltration Desalination 113 (1997) 205ndash208
[9] EC Devitt F Ducellier P Coumlteacute MR Wiesner Effects of natural organic mat-ter and the raw water matrix on the rejection of atrazine by pressure-drivenmembranes Water Res 32 (1998) 2563ndash2568
[10] R Boussahel S Bouland KM Moussaoui A Montiel Removal of pesticideresidues in water using the nanofiltration process Desalination 132 (2000)205ndash209
[11] Y Zhang B Van der Bruggen GX Chen L Braeken C Vandecasteele Removalof pesticides by nanofiltration effect of the water matrix Sep Purif Technol38 (2004) 163ndash172
[12] EC Devitt MR Wiesner Dialysis investigations of atrazinendashorganic matterinteractions and the role of a divalent metal Environ Sci Technol 32 (1998)232ndash237
[13] SK Dalton JA Brant MR Wiesner Chemical interactions between dissolvedorganic matter and low-molecular weight organic compounds impacts onmembrane separation J Membr Sci 266 (2005) 30ndash39
[14] NA Kulikova IV Perminova Binding of atrazine to humic substances fromsoil peat and coal related to their structure Environ Sci Technol 36 (2002)3720ndash3724
[15] RL Wershaw The importance of humic substancendashmineral particle complexesin the modeling of contaminant transport in sedimentndashwater systems in RABaker (Ed) Organic Substances and Sediments in Water Humics and SoilsLewis Publishers Chelsea MI 1991 pp 23ndash34
[16] SM Schrap A Opperhuizen Relationship between bioavailability andhydrophobicity reduction of the uptake of organic chemicals by fish due tothe sorption on particles Environ Toxicol Chem 9 (1990) 715ndash724
[17] E Barriuso M Schiavon F Andreux JM Portal Localization of atrazine non-extractable (bound) residues in soil size fractions Chemosphere 12 (1991)1131ndash1140
[18] L Loiseau E Barriuso Characterization of the atrazinersquos bound (nonextractable)residues using fractionation techniques for soil organic matter Environ SciTechnol 36 (2002) 683ndash689
[19] G Sposito L Martin-Neto A Yang Atrazine complexation by soil humic acidsJ Environ Qual 25 (1996) 1203ndash1209
[20] KV Plakas AJ Karabelas Membrane retention of herbicides from single andmulti-solute media the effect of ionic environment J Membr Sci 320 (2008)325ndash334
[21] V Freger J Gilron S Belfer TFC polyamide membranes modified by graftingof hydrophilic polymers an FT-IRAFMTEM study J Membr Sci 209 (2002)283ndash292
[22] P Xu JE Drewes Viability of nanofiltration and ultra-low pressure reverseosmosis membranes for multi-beneficial use of methane produced water SepPurif Technol 52 (2006) 67ndash76
[23] Z Knez A Rizner-Hras K Kokot D Bauman Solubility of some solid triazineherbicides in supercritical carbon dioxide Fluid Phase Equilibria 152 (1998)95ndash108
[24] Weed Science Society of America (WSSA) Herbicide Handbook 7th ed 1994[25] B Van der Bruggen J Schaep W Maes D Wilms C Vandecasteele Nanofiltra-
tion as a treatment method for the removal of pesticides from ground watersDesalination 117 (1998) 139ndash147
[26] KN Reddy MA Locke Molecular properties as descriptors of octanol-waterpartition coefficients of herbicides Water Air Soil Pollut 86 (1996) 389ndash405
[27] IHSS website httpwwwihssgatechedu[28] F Flores-Ceacutespedes M Fernaacutendez-Peacuterez M Villafranca-Saacutenchez E Gonzaacutelez-
Pradas Cosorption study of organic pollutants and dissolved organic matter ina soil Environ Pollut 142 (2006) 449ndash456
[29] R Beckett Z Jue JC Giddings Determination of molecular weight distribu-tions of fulvic and humic acids using flow field-flow fractionation Environ SciTechnol 21 (1987) 289ndash295
[30] PM Reid AE Wilkinson E Tipping MN Jones Determination of molecularweights of humic substances by analytical (UV scanning) ultracentrifugationGeochim Cosmochim Acta 54 (1990) 131ndash138
[31] S Lee B Kwon M Sun J Cho Characterizations of NOM included in NF and UFmembrane permeates Desalination 173 (2005) 131ndash142
[32] MN Jones ND Bryan Colloidal properties of humic substances Adv ColloidInterf Sci 78 (1998) 1ndash48
[33] KV Plakas AJ Karabelas T Wintgens T Melin A study of selected herbicidesretention by nanofiltration membranesmdashthe role of organic fouling J MembrSci 284 (2006) 291ndash300
[34] APHA-AWWA-WPCF Standard Methods for the Examination of Water andWastewater 17th ed Port City Press Baltimore MD 1989 pp 567ndash569
[35] R Artinger G Buckau JI Kim S Geyer Characterization of groundwater humicand fulvic acids of different origin by GPC with UVVis and fluorescence detec-tion Fresenius J Anal Chem 364 (1999) 737ndash745
[36] AI Schaumlfer N Andritsos AJ Karabelas EMV Hoek R Schneider M Nys-troumlm Chapter 8 Fouling in nanofiltration in AI Schaumlfer AG Fane TDWaite (Eds) Nanofiltration Principles and Applications Elsevier Oxford UK2005 pp 173ndash174
[37] I Franco L Catalano M Contin M De Nobili Interaction between organicmodel compounds and pesticides with water-soluble soil humic substancesActa Hydrochim Hydrobiol 29 (2001) 88ndash99
[38] AA Helal DM Imam SM Khalifa HF Aly Interaction of pesticides with humiccompounds and their metal complexes Radiochemistry 48 (4) (2006) 419ndash425
[39] MG Browman G Chester Fate of Pollutants in the Air and Water EnvironmentWiley New York 1977 p 50
[40] LD Nghiem AI Schaefer M Elimelech Nanofiltration of hormone mimickingtrace organic contaminants Sep Sci Technol 40 (2005) 2633ndash2649
[41] R Celis J Cornejo MC Hermosin WC Koskinen Sorptionndashdesorption ofatrazine and simazine by model soil colloidal components Soil Sci Soc AmJ 61 (1997) 436ndash443
100 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
[42] SW Runge KR Shelton SA Melton WM Moran Maintaining the ionic per-meability of a cellulose ester membrane J Biochem Biophys Methods 64(2005) 200ndash206
[43] R Boussahel A Montiel M Baudu Effects of organic and inorganic matter onpesticide rejection by nanofiltration Desalination 145 (2002) 109ndash114
[44] Z Wang DS Gamble CH Langford Interaction of atrazine with Laurentianhumic acid Anal Chim Acta 244 (1991) 135ndash143
[45] C Jucker MM Clark Adsorption of aquatic humic substances on hydrophobicultrafiltration membranes J Membr Sci 97 (1994) 37ndash52
94 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
Table 6
Average retention values of atrazine and prometryn at two different humic substance concentrations by the three NFULPRO membranes
Humic substance Cfeed (mg Lminus1) NF270 NF90 XLE
Ratrazine () Rprometryn () Ratrazine () Rprometryn () Ratrazine () Rprometryn ()
HA 5 782 804 873 883 83 852
10 776 798 838 846 769 744
FA 5 784 829 828 870 848 913
10 816 842 937 933 906 977
NOM 5 813 859 843 838 879 922
10 862 901 894 857 956 968
TA 5 898 880 876 903 905 963
10 969 976 96 957 100 100
following mechanisms that may take place (a) increase of the
steric congestion or reduction of the diffusivity of the FAndashtriazine
or NOMndashtriazine pseudo-complex (b) increase of the density of
the pseudo-complexrsquos negative charge which enhances the elec-
trostatic repulsion with the negatively charged membranes and (c)
increase of the adsorption capability of the pseudo-complex on the
surface of the membrane in contrast to the hydrophobic nature of
the humic materials The validity of the above mechanism hypothe-
ses adopted by previous researchers [7] depends on the type of
the organic material and consequently on the characteristic chem-
ical properties of both HS (acidity functional groups) and triazines
(basicity hydrophobicity charge) The correlation between humic
material properties and triazine retention is examined in a subse-
quent paragraph
The greater extent of FAndashtriazines pseudo-complexes in com-
parison to NOMndashtriazines pseudo-complexes manifested in the
GPC results is not accompanied by a greater increase of triazine
retention More specifically the increasing order of triazine reten-
tion in the presence of a specific humic material follows the order
TA gt NOM gt FA gt HA in the case of NF270 and XLE membranes
whereas for NF90 the order is TA ge FA gt NOM gt HA Therefore the
interactions between the humic materials and triazines alone
cannot explain the retention performance of the NFULPRO mem-
branes This fact indicates that interactions between the naturally
occurring organic substances and the membranes may also play a
significant role in triazine retention as previously observed [33] In
view of these observations dead-end filtration tests using the three
IHSS organic materials and the selected NFULPRO membranes are
carried out in this laboratory in order to distinguish the effects of
organic fouling and of HSndashtriazines interactions on triazine perme-
ation during nanofiltration
323 Effect of humic substance concentration on triazine
retention
Surveying the literature on the complicated influence of organic
matter on retention of pesticides by NFRO membranes reveals a
considerable variability regarding the importance of the organic
matter concentration Experiments in a nanofiltration pilot unit on
the removal of atrazine and simazine have demonstrated the sig-
nificance of the organic load in the feed water since the removal
efficiency increased from 50 to 90ndash100 when dissolved organic
carbon varied between 04 and 36 mg Lminus1 [7] In contrast Berg et
al [8] reported no increase of pesticide retention (atrazine and
simazine among them) by several NF membranes with increas-
ing organic matter concentration Along the same lines Zhang et
al [11] showed comparable simazine retention between tap water
and river water although the latter had a high concentration of
NOM whereas tap water had a low NOM concentration In an effort
to understand the reasons for the above inconsistency in the lit-
erature Nghiem et al [40] took into consideration the effect of
ionic strength Experiments with hormone mimicking trace organic
contaminants revealed the significance of solution ionic strength
in influencing NOMndashtrace organic interactions which is probably
the cause for the ambiguity of literature data regarding the role
of background organic matter in trace organic rejection The mag-
nitude of ionic strength in terms of calcium ion content on the
NOMndashtriazines interaction has been also recorded through the GPC
tests described previously and is subsequently examined Although
ionic strength can strongly influence the HSndashtriazines interactions
the effect of the type of the humic substance seems to play also a
significant role on triazine retention This is evident in the results
included in Table 6 regarding retention experiments in the pres-
ence of a medium (5 mg Lminus1) and relatively high (10 mg Lminus1) HS
concentration in the absence of ionic environment
Table 6 shows a clear increase of triazine retention with increas-
ing humic material concentration More specifically by doubling
the concentration of FA NOM and TA in the feed solutions atrazine
and prometryn retention increases 4ndash13 and 2ndash10 respectively
depending on the membrane and the humic material used An
exception is observed again in the case of humic acids since the
increased organic load results in negligible or slightly negative
effects of atrazine and prometryn retention Regarding the XLE
membrane the use of medium HA concentration results in similar
retention performance as in organic-free solutions On the other
hand higher HA concentration results in a notable reduction of
triazine retention ranging up to 74 and 127 for atrazine and prom-
etryn respectively As previously discussed this is attributed to the
increased HA adsorption on the membrane surface (Table 8) which
in turn affects triazine adsorption on XLE membrane and therefore
retention This trend is not observed in the case of FA NOM and
TA despite their adsorptive capacity on the membrane materials
This finding is probably related to fouling phenomena which tend
to modify the membrane properties (ie surface charge hydropho-
bicity morphology) and therefore triazine adsorption Although it
is difficult to use mass balances for calculating triazine adsorption
during the combined filtration with humic substances (due to ana-
lytical problems related to the concentrate samples) the changes
of triazine concentration in the permeate samples provide a good
indication of the potential adsorption of atrazine and prometryn
on the membrane surface Furthermore the increased passage of
triazines in the permeate side as a result of HA adsorption on the
XLE membrane is in accord with the increase of triazine adsorption
on the membrane and therefore with the reduced retention values
324 Effect of calcium ion on triazine retention
From Fig 5 it is evident that the presence of calcium acts
positively in both atrazine and prometryn retention during the
combined nanofiltration with all four types of humic substances
Furthermore the higher interactions between the humic organ-
ics and triazines manifested in the GPC tests are in agreement
with the increased retention performance of atrazine and prom-
etryn by the three membranes used Additionally the suggestion
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 95
of ligand exchange in which calcium serves as a bridging ion and
acts as a possible site for higher adsorption of triazine molecules
on humic substances [38] seems to be in accord with the forma-
tion of an increased number of pseudo-complexes between humic
matter and triazines which explain the increase of herbicide reten-
tion
Among the four HS used tannic acid is associated with the high-
est triazine retention since both triazines are fully retained in the
presence of calcium by all three membranes tested Moreover the
enhanced interaction between humic acids and triazines as a result
of the calcium addition to the feed solutions is accompanied by
a significant increase of atrazine and prometryn retention In this
case calcium ions favour the binding between HA and triazines
which in other cases would not have taken place this is in agree-
ment with the preceding GPC and NFULPRO results
Prometryn exhibits again the highest retention values in com-
parison to atrazine which may be attributed to the relatively higher
prometryn hydrophobicity and basicity (Table 1) The former prop-
erty likely favours hydrophobic interactions between prometryn
and the HSndashcalcium complexes whereas the latter renders prom-
etryn highly effective in mechanisms involving electron-transfer
with the HSndashcalcium complexes [19] This mechanism is due to the
lower acidic functional group content of the HS as a result of the
complexation with calcium ions in the feed solutions Sposito et
al [19] suggested that atrazine itself does not readily participate
in electron-transfer reactions with humic substances However
they demonstrated that hydroxyl-atrazine does react through an
electron-transfer mechanism with HA and FA It is noted that
atrazine is readily converted to hydroxyl-atrazine even in labo-
ratory samples at low water content and this may explain the
presence of some of the electron transfer products detected in stud-
ies of atrazinendashHA interactions [41]
Surveying the literature however it is noted that some stud-
ies either with dialysis membranes [1213] or NF membranes [9]
have shown reduced values of atrazine retention when divalent
calcium is present together with natural organic matter includ-
ing the NOM surrogate tannic acid According to Devitt et al [9]
Devitt and Wiesner [12] this is due to the reduced association of
atrazine and NOM as a result of the occupation of interaction sites
by calcium andor the reduced access of atrazine to NOM sites due
to changes in molecular conformation The GPC results in this study
have shown that this is not the case since the presence of calcium
tends to increase the interaction of humic substances with triazine
compounds like atrazine
The conflicting results between the previous studies and the
present work may be attributed to the different types of membranes
and filtration techniques used More specifically the use of cellulose
ester membranes as well as the experimentation with batch dialy-
sis systems where concentration and osmotic pressure difference
serve as the driving force for solute transport (ie in the absence of
hydrodynamic forces as in the present study) may explain the dif-
ferences regarding the calcium effect on triazine retention Runge
et al [42] claim that the SpectraPor cellulose ester membranes
used in their studies (having a molecular weight cut-off of 100 Da)
Fig 6 Retention of atrazine (a) and prometryn (b) as a function of humic material total aciditymdashcalculations based on the values presented in Table 2 taking into consideration
the carbon composition and the HS concentration
Fig 7 Retention of atrazine (a) and prometryn (b) as a function of humic material carboxylic aciditymdashcalculations based on the values presented in Table 2 taking into
consideration the carbon composition and the HS concentration
96 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
present a significant incompatibility with distilled water whereas
a significant membrane potential can build up with certain salts
which can inhibit or enhance diffusion depending on the size of
the solutes diffusing and the polarity of the membrane potential
generated In this case an increased diffusion of hydrated calcium
ions to the permeate side in order to balance the concentrations
between the two sides of the cellulose ester membrane may result
in a pore blockage or a positive charge on the membrane surface
which in turn could lead to an increased passage of free dissolved
atrazine to the permeate side
Another possible explanation for this inconsistency is the time
period of the filtration tests According to the protocol followed
by previous researchers [1213] the dialysis time period ranged
between 1 and 5 days at the end of which concentration measure-
ments were performed On the other hand the filtration periods
in the case of the present dead-end nanofiltration experiments
ranged between 2 and 5 h depending on the membrane used To
this period one more hour should be added in which the feed solu-
tion was stirred in the cell without pressurization Even though
triazine adsorption on the membrane surface was observed to sta-
bilize within the relatively short period of nanofiltration there is
a possibility that longer filtration periods may have resulted in
higher adsorption rates which in turn could lead to greater pas-
sage of triazine to the permeate side and therefore to reduced
retention values This explanation is in agreement with the pilot
tests performed by Boussahel et al [43] who report that the
increasing adsorption of humic matter on the membranes leads
to an increase of pesticide adsorption on the membranes which
facilitates the diffusion of pesticides in the direction of perme-
ate
33 Correlation between humic material properties and triazine
retention
The correlation of atrazine and prometryn retention with the
total carboxylic and phenolic acidity of the humic substances used
in the present study as well as the role of the molecular mass of HS
on triazine retention are depicted in Figs 6ndash9 These are results of
nanofiltration experiments performed in the absence of calcium
Triazine removal by the three NFULPRO membranes seems
to poorly correlate with the total acidity of the humic material
This is evident especially in the case of NF270 and NF90 mem-
branes (Fig 6) The XLE membrane on the other hand presents a
reduced tendency for atrazine and prometryn removal when acid-
ity increases This is attributed to the characteristic low negative
charge of the XLE membrane surface which causes weak repulsions
with the negatively charged humic organics This leads to a greater
adsorption of humic compounds on the membrane surface (Table 8)
and consequently to greater triazine adsorption which favours the
diffusion of the triazines to the permeate side On the other hand
the strong negative charge of the NF270 and NF90 membranes [20]
promotes a strong repulsion between HS and the membranes The
greater the negative charge of the humic organic the higher the
repulsion which in turn results in higher concentration of HS in
the bulk Consequently the interactions between HS and triazines
in the bulk are increased leading to an improved triazine removal
Fig 8 Retention of atrazine (a) and prometryn (b) as a function of humic material phenolic aciditymdashcalculations based on the values presented in Table 2 taking into
consideration the carbon composition and the HS concentration
Fig 9 Retention of atrazine (a) and prometryn (b) as a function of humic substance molecular mass (based on the values presented in Table 3)
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 97
This is more evident in the case of the wide pore NF270 membrane
since the NF90 membrane seems to retain atrazine and prometryn
to a higher degree in the presence of TA which is characterized by
a lower total acidity among the four HS used in this study
Humic compounds contain a variety of functional groups
such as aliphatic and aromatic carboxyls phenolic hydroxyls
alcoholic hydroxyls carbonyls quinones methoxyls and amino
groups These functional groups are the reactive sites in the humic
molecules As carboxylate protonation seems to be critical for the
capacity of triazine binding [44] it is of interest to correlate the
effect of carboxylic acidity on herbicide removal by nanofiltration
The effect of carboxylic acidity on atrazine and prometryn retention
by the three NFULPRO membranes is depicted in Fig 7
The results show a negative trend of triazine retention with
an increased carboxylic acidity of humic substances This is clear
in the case of NF270 and XLE membranes while NF90 does not
seem to be significantly affected by the changes in the number of
carboxyl groups of a humic molecule Remarkably the smaller TA
molecules containing fewer carboxyl groups per unit mass (Table 2)
exhibit the highest triazine retention (sim100) as a result of the even
higher triazine binding capacity in contrast to the IHSS organics
which contain a higher carboxylic group content Moreover experi-
ments in batch filtration mode result in HS concentration increasing
with time which in turn leads to a reduced number of carboxylic
binding sites available to the two triazines This is probably due
to the self ldquocompetingrdquo character of the HS used since an increase
in concentration andor HS aggregation tend to reduce the avail-
able binding sites [44] This phenomenon led Wang et al [44] to
propose a model in which hydrogen bonding was considered to be
the primary interaction resulting in triazine binding Therefore the
increase in triazine retention observed in the case of elevated HS
concentration may not be directly related with the carboxylic acid-
ity of the HS but rather with other mechanisms such as hydrogen
bonding
Regarding the phenolic acidity of the humic substances there
does not seem to be a sufficient correlation with atrazine or prom-
etryn retention by the three NFULPRO membranes (Fig 8) Actually
there is a contradictory behaviour between the three IHSS organ-
ics and the NOM surrogate TA The high phenolic content of TA
is related to high retention values of the two triazines while
the smaller and relatively similar phenolic acidity of the IHSS
compounds leads to smaller and different retention values These
results imply that there are other factors influencing the association
between humic organics and herbicides and therefore their reten-
tion by nanofiltration In this respect the molecular structure of the
humic materials as well as the interactions taking place between
the organics and the membranes seem to explain the above incon-
sistencies This is evident in the results depicted in Fig 9 and the
adsorption of humic substances subsequently described
The rather strong relation of triazine retention with the molec-
ular mass of the organics used in this study (Fig 9) leads to the
suggestion that structural features of HS are important for the
removal capacity of triazines Up to now there is no consensus
concerning both the binding mechanism of triazines to humic sub-
stances and the structural features responsible for this binding
Nevertheless the preferential binding with low molecular weight
fractions of humic compounds (especially of FA and TA) deduced
from the GPC and filtration tests results in higher retention val-
Table 7
Humic substance and calcium retention performance of the three NFULPRO membranes (mean retention values for 50 recovery of feed volume as permeate)
Membrane Humic substancea Humic substances retention () Ca2+ retention ()
HSs alone HSs with triazines HSs with triazines and Ca2+ In the presence of HSs
NF270 HA 990 994 1000 949
FA 967 983 991 951
NOM 957 997 1000 951
TA 972 992 997 957
NF90 HA 972 980 1000 994
FA 953 984 1000 990
NOM 985 993 1000 991
TA 967 998 998 994
XLE HA 992 996 1000 994
FA 862 915 994 997
NOM 976 1000 1000 988
TA 923 945 1000 996
a HA FA NOM and TA designate the four humic substances used in this study
Table 8
Adsorption data of humic substances on tested membranes
Membrane Humic substancea Feed mass of humic substance (g) Quantity adsorbed (g) Adsorption ()
Alone With Ca2+ Alone With Ca2+ Alone With Ca2+
NF270 HA 3480 2630 244 2314 07 88
FA 3570 2840 71 1335 02 47
NOM 3390 2660 34 2633 01 99
TA 4050 3300 2025 7161 50 217
NF90 HA 3220 2690 354 3658 11 136
FA 2930 2450 1494 857 51 35
NOM 3000 2910 120 437 04 15
TA 2820 2730 649 3631 23 133
XLE HA 3100 2680 1643 3457 53 129
FA 3050 2810 1830 2866 60 102
NOM 2650 2770 610 1219 23 44
TA 3120 2940 2184 2970 70 141
a HA FA NOM and TA designate the four humic substances used in this study
98 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
ues for the two triazines Thus the interaction with small branched
molecules like FA and TA seems to favour a physical entrapment
mechanism such as the one suggested by Devitt and Wiesner [12]
34 Retention of humic species and calcium ion by the NFULPRO
membranes
The performance of nanofiltration on the humic substances and
calcium removal is summarized in Table 7 Regarding humic sub-
stances the retention is compared for three different feed solutions
(a) solutions of HS alone (b) solutions of HS with the two triazines
and (c) solutions of HS with the two triazines in the presence of cal-
cium In general HS retention is high for all three membranes used
According to the UV measurements IHSS HA exhibits the highest
retention values followed by NOM TA and FA which is apparently
related to their relative molecular masses (Table 3) Complexation
or interaction with triazines as evidenced by the GPC tests appar-
ently plays also a significant role in the relative permeability of the
organic species More specifically the retention of the four organic
compounds examined is increased in the presence of the two her-
bicides which verifies also the binding of atrazine and prometryn
with the four humic substances Thus a comparison between the
retention values of the HS in the absence or presence of triazines
gives also a good insight into the HSndashtriazine interactions The slight
increase (lt1) of HA retention in the presence of the two triazines
is in accord with the relatively low HA binding capacity observed
in the GPC tests On the other hand the increase of 2ndash6 in FA
NOM and TA retention is in agreement with the observed increased
interactions between those substances and the two triazines
In the case of calcium addition in the organicndashtriazine solu-
tions the UV absorbing species of the four organics examined are
retained almost completely by all three membranes used This is
again in agreement with the results obtained with gel chromatogra-
phy According to the literature [12] the addition of calcium results
in charge shielding and neutralization of the organic matter charged
functional groups and can shrink the humic matrix The formation
of aggregates of larger apparent molecular weight due to the sub-
stantial increase in the hydrophobicity of the organic molecules
tends to reduce their permeability through membranes and there-
fore to increase their retention
Apart from the increased retention of triazines and humic sub-
stances there is also a significant improvement of calcium rejection
when the latter is present in the solution In comparison to pure
water solutions of elevated calcium ion content [20] the rejection of
calcium in humic solutions is almost complete This is attributed to
size exclusion mechanisms related to the binding tendency of diva-
lent ions like calcium to the negatively charged humic organics
Among the four humic substances used calcium is rejected more in
the presence of the polyphenol tannic acid which is accompanied
by a higher organic adsorption on all three NFULPRO membranes
tested (Table 8)
From the data summarized in Table 8 it can be seen that the
adsorption of humic substances onto the membrane surfaces is
markedly enhanced in the presence of calcium ions due to the
formation of HA-complexed divalent cations This is also clearly
depicted in the SEM images of virgin and fouled membranes in
the absence or presence of calcium ions Characteristic SEM images
in the case of the Suwannee River NOM filtration with the three
NFULPRO membranes are included in Fig 10 these images show
the severe fouling on the three NFULPRO membranes as a result
of the combined filtration of NOM with CaCl2 salt It is possible
that the divalent cations such as Ca2+ act as a bridge between the
membrane surface and the negatively charged humic molecules
that tend to be repelled by the negatively charged membrane As a
result the electrical repulsion between the membrane surface and
the humic molecules is weakened thus rendering the membrane
Fig 10 SEM images of virgin fouled with NOM and fouled with NOM + calcium membranes (a) NF270 (b) NF90 and (c) XLE membrane
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 99
fouling process more severe [45] On the other hand the relatively
smaller adsorption rates of NOM in the absence of calcium result
in less fouled membranes A similar trend is also observed in the
case of the other three humic substances used in this study Among
the three membranes used XLE exhibits in the absence of calcium
the highest adsorption rates for all four humic substances This is
explained by the relatively high hydrophobicity and the low surface
charge that characterize the XLE membrane surface as previously
explained
4 Concluding remarks
The influence of humic substances on retention of two tri-
azines (atrazine and prometryn) by NFULPRO membranes is rather
complicated and involves combinations of solutendashmembrane and
solutendashsolute interactions In summary the retention of triazines
depends on the type of the humic material present in the feed
water In most cases the combined nanofiltration of triazines and
naturally occurring humic substances facilitates the formation of
complexes with triazines which in turn enhance their removal by
means of membrane filtration This complexation appears to be
related not to the characteristic acidity (phenolic carboxylic) of
the HS used but rather to their molecular conformation More
specifically a preferential binding is observed between triazines
and low molecular weight fractions of humic compounds (espe-
cially of FA and TA) which results in higher retention values for the
two triazines Under all conditions tannic acid exhibits the greatest
effect on triazine retention among the four standard HS compounds
used leading to an almost complete removal of the two triazines
(95ndash100) for all three membranes tested
The removal of triazines is improved in the presence of calcium
which tends to enhance the interaction between HS and triazines
Moreover triazine retention is increased with increasing HS con-
centration to a degree depending on the type of HS Additionally
triazine retention is apparently also affected by the interactions
between the organics and the membrane surface which in turn
may alter triazine retention These results illustrate the complex-
ity of the triazine adsorption and retention mechanisms in the
presence of humic substances that needs further investigation for
improved understanding Moreover regarding triazine removal by
nanofiltration it is of particular interest to distinguish the effect
of the HSndashtriazine interactions from that of membrane HS fouling
Research in this direction is carried out in this laboratory
References
[1] DP Birardar AL Rayburn Chromosomal damage induced by herbicide con-tamination at concentrations observed in public water supplies J Environ Qual24 (1995) 1222ndash1225
[2] IK Konstantinou DG Hela TA Albanis The status of pesticide pollution insurface waters (rivers and lakes) of Greece Part I Review on occurrence andlevels Environ Pollut 141 (3) (2006) 555ndash570
[3] European Environment Agency (EEA) Groundwater quality and quantity inEurope Report Copenhagen June 1999 available in httpreportseeaeuropaeugroundwater07012000enenviassrp199903
[4] EM Thurman DA Goolsby MT Meyer MS Mills ML Pomes DW Kolpin AReconnaissance study of herbicides and their metabolites in surface water of theMidwestern United States using immunoassay and gas chromatographymassspectrometry Environ Sci Technol 26 (1992) 2440ndash2447
[5] C Bellona JE Drewes P Xu G Amy Factors affecting the rejection of organicsolutes during NFRO treatmentmdasha literature review Water Res 38 (2004)2795ndash2809
[6] A Bhattacharya Remediation of pesticide-polluted waters through mem-branes Sep Purif Rev 35 (2006) 1ndash38
[7] KM Agbekodo B Legube S Dard Atrazine and simazine removal mechanismsby nanofiltration influence of natural organic matter concentration Water Res30 (1996) 2535ndash2542
[8] P Berg G Hagmeyer R Gimbel Removal of pesticides and other micropollu-tants by nanofiltration Desalination 113 (1997) 205ndash208
[9] EC Devitt F Ducellier P Coumlteacute MR Wiesner Effects of natural organic mat-ter and the raw water matrix on the rejection of atrazine by pressure-drivenmembranes Water Res 32 (1998) 2563ndash2568
[10] R Boussahel S Bouland KM Moussaoui A Montiel Removal of pesticideresidues in water using the nanofiltration process Desalination 132 (2000)205ndash209
[11] Y Zhang B Van der Bruggen GX Chen L Braeken C Vandecasteele Removalof pesticides by nanofiltration effect of the water matrix Sep Purif Technol38 (2004) 163ndash172
[12] EC Devitt MR Wiesner Dialysis investigations of atrazinendashorganic matterinteractions and the role of a divalent metal Environ Sci Technol 32 (1998)232ndash237
[13] SK Dalton JA Brant MR Wiesner Chemical interactions between dissolvedorganic matter and low-molecular weight organic compounds impacts onmembrane separation J Membr Sci 266 (2005) 30ndash39
[14] NA Kulikova IV Perminova Binding of atrazine to humic substances fromsoil peat and coal related to their structure Environ Sci Technol 36 (2002)3720ndash3724
[15] RL Wershaw The importance of humic substancendashmineral particle complexesin the modeling of contaminant transport in sedimentndashwater systems in RABaker (Ed) Organic Substances and Sediments in Water Humics and SoilsLewis Publishers Chelsea MI 1991 pp 23ndash34
[16] SM Schrap A Opperhuizen Relationship between bioavailability andhydrophobicity reduction of the uptake of organic chemicals by fish due tothe sorption on particles Environ Toxicol Chem 9 (1990) 715ndash724
[17] E Barriuso M Schiavon F Andreux JM Portal Localization of atrazine non-extractable (bound) residues in soil size fractions Chemosphere 12 (1991)1131ndash1140
[18] L Loiseau E Barriuso Characterization of the atrazinersquos bound (nonextractable)residues using fractionation techniques for soil organic matter Environ SciTechnol 36 (2002) 683ndash689
[19] G Sposito L Martin-Neto A Yang Atrazine complexation by soil humic acidsJ Environ Qual 25 (1996) 1203ndash1209
[20] KV Plakas AJ Karabelas Membrane retention of herbicides from single andmulti-solute media the effect of ionic environment J Membr Sci 320 (2008)325ndash334
[21] V Freger J Gilron S Belfer TFC polyamide membranes modified by graftingof hydrophilic polymers an FT-IRAFMTEM study J Membr Sci 209 (2002)283ndash292
[22] P Xu JE Drewes Viability of nanofiltration and ultra-low pressure reverseosmosis membranes for multi-beneficial use of methane produced water SepPurif Technol 52 (2006) 67ndash76
[23] Z Knez A Rizner-Hras K Kokot D Bauman Solubility of some solid triazineherbicides in supercritical carbon dioxide Fluid Phase Equilibria 152 (1998)95ndash108
[24] Weed Science Society of America (WSSA) Herbicide Handbook 7th ed 1994[25] B Van der Bruggen J Schaep W Maes D Wilms C Vandecasteele Nanofiltra-
tion as a treatment method for the removal of pesticides from ground watersDesalination 117 (1998) 139ndash147
[26] KN Reddy MA Locke Molecular properties as descriptors of octanol-waterpartition coefficients of herbicides Water Air Soil Pollut 86 (1996) 389ndash405
[27] IHSS website httpwwwihssgatechedu[28] F Flores-Ceacutespedes M Fernaacutendez-Peacuterez M Villafranca-Saacutenchez E Gonzaacutelez-
Pradas Cosorption study of organic pollutants and dissolved organic matter ina soil Environ Pollut 142 (2006) 449ndash456
[29] R Beckett Z Jue JC Giddings Determination of molecular weight distribu-tions of fulvic and humic acids using flow field-flow fractionation Environ SciTechnol 21 (1987) 289ndash295
[30] PM Reid AE Wilkinson E Tipping MN Jones Determination of molecularweights of humic substances by analytical (UV scanning) ultracentrifugationGeochim Cosmochim Acta 54 (1990) 131ndash138
[31] S Lee B Kwon M Sun J Cho Characterizations of NOM included in NF and UFmembrane permeates Desalination 173 (2005) 131ndash142
[32] MN Jones ND Bryan Colloidal properties of humic substances Adv ColloidInterf Sci 78 (1998) 1ndash48
[33] KV Plakas AJ Karabelas T Wintgens T Melin A study of selected herbicidesretention by nanofiltration membranesmdashthe role of organic fouling J MembrSci 284 (2006) 291ndash300
[34] APHA-AWWA-WPCF Standard Methods for the Examination of Water andWastewater 17th ed Port City Press Baltimore MD 1989 pp 567ndash569
[35] R Artinger G Buckau JI Kim S Geyer Characterization of groundwater humicand fulvic acids of different origin by GPC with UVVis and fluorescence detec-tion Fresenius J Anal Chem 364 (1999) 737ndash745
[36] AI Schaumlfer N Andritsos AJ Karabelas EMV Hoek R Schneider M Nys-troumlm Chapter 8 Fouling in nanofiltration in AI Schaumlfer AG Fane TDWaite (Eds) Nanofiltration Principles and Applications Elsevier Oxford UK2005 pp 173ndash174
[37] I Franco L Catalano M Contin M De Nobili Interaction between organicmodel compounds and pesticides with water-soluble soil humic substancesActa Hydrochim Hydrobiol 29 (2001) 88ndash99
[38] AA Helal DM Imam SM Khalifa HF Aly Interaction of pesticides with humiccompounds and their metal complexes Radiochemistry 48 (4) (2006) 419ndash425
[39] MG Browman G Chester Fate of Pollutants in the Air and Water EnvironmentWiley New York 1977 p 50
[40] LD Nghiem AI Schaefer M Elimelech Nanofiltration of hormone mimickingtrace organic contaminants Sep Sci Technol 40 (2005) 2633ndash2649
[41] R Celis J Cornejo MC Hermosin WC Koskinen Sorptionndashdesorption ofatrazine and simazine by model soil colloidal components Soil Sci Soc AmJ 61 (1997) 436ndash443
100 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
[42] SW Runge KR Shelton SA Melton WM Moran Maintaining the ionic per-meability of a cellulose ester membrane J Biochem Biophys Methods 64(2005) 200ndash206
[43] R Boussahel A Montiel M Baudu Effects of organic and inorganic matter onpesticide rejection by nanofiltration Desalination 145 (2002) 109ndash114
[44] Z Wang DS Gamble CH Langford Interaction of atrazine with Laurentianhumic acid Anal Chim Acta 244 (1991) 135ndash143
[45] C Jucker MM Clark Adsorption of aquatic humic substances on hydrophobicultrafiltration membranes J Membr Sci 97 (1994) 37ndash52
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 95
of ligand exchange in which calcium serves as a bridging ion and
acts as a possible site for higher adsorption of triazine molecules
on humic substances [38] seems to be in accord with the forma-
tion of an increased number of pseudo-complexes between humic
matter and triazines which explain the increase of herbicide reten-
tion
Among the four HS used tannic acid is associated with the high-
est triazine retention since both triazines are fully retained in the
presence of calcium by all three membranes tested Moreover the
enhanced interaction between humic acids and triazines as a result
of the calcium addition to the feed solutions is accompanied by
a significant increase of atrazine and prometryn retention In this
case calcium ions favour the binding between HA and triazines
which in other cases would not have taken place this is in agree-
ment with the preceding GPC and NFULPRO results
Prometryn exhibits again the highest retention values in com-
parison to atrazine which may be attributed to the relatively higher
prometryn hydrophobicity and basicity (Table 1) The former prop-
erty likely favours hydrophobic interactions between prometryn
and the HSndashcalcium complexes whereas the latter renders prom-
etryn highly effective in mechanisms involving electron-transfer
with the HSndashcalcium complexes [19] This mechanism is due to the
lower acidic functional group content of the HS as a result of the
complexation with calcium ions in the feed solutions Sposito et
al [19] suggested that atrazine itself does not readily participate
in electron-transfer reactions with humic substances However
they demonstrated that hydroxyl-atrazine does react through an
electron-transfer mechanism with HA and FA It is noted that
atrazine is readily converted to hydroxyl-atrazine even in labo-
ratory samples at low water content and this may explain the
presence of some of the electron transfer products detected in stud-
ies of atrazinendashHA interactions [41]
Surveying the literature however it is noted that some stud-
ies either with dialysis membranes [1213] or NF membranes [9]
have shown reduced values of atrazine retention when divalent
calcium is present together with natural organic matter includ-
ing the NOM surrogate tannic acid According to Devitt et al [9]
Devitt and Wiesner [12] this is due to the reduced association of
atrazine and NOM as a result of the occupation of interaction sites
by calcium andor the reduced access of atrazine to NOM sites due
to changes in molecular conformation The GPC results in this study
have shown that this is not the case since the presence of calcium
tends to increase the interaction of humic substances with triazine
compounds like atrazine
The conflicting results between the previous studies and the
present work may be attributed to the different types of membranes
and filtration techniques used More specifically the use of cellulose
ester membranes as well as the experimentation with batch dialy-
sis systems where concentration and osmotic pressure difference
serve as the driving force for solute transport (ie in the absence of
hydrodynamic forces as in the present study) may explain the dif-
ferences regarding the calcium effect on triazine retention Runge
et al [42] claim that the SpectraPor cellulose ester membranes
used in their studies (having a molecular weight cut-off of 100 Da)
Fig 6 Retention of atrazine (a) and prometryn (b) as a function of humic material total aciditymdashcalculations based on the values presented in Table 2 taking into consideration
the carbon composition and the HS concentration
Fig 7 Retention of atrazine (a) and prometryn (b) as a function of humic material carboxylic aciditymdashcalculations based on the values presented in Table 2 taking into
consideration the carbon composition and the HS concentration
96 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
present a significant incompatibility with distilled water whereas
a significant membrane potential can build up with certain salts
which can inhibit or enhance diffusion depending on the size of
the solutes diffusing and the polarity of the membrane potential
generated In this case an increased diffusion of hydrated calcium
ions to the permeate side in order to balance the concentrations
between the two sides of the cellulose ester membrane may result
in a pore blockage or a positive charge on the membrane surface
which in turn could lead to an increased passage of free dissolved
atrazine to the permeate side
Another possible explanation for this inconsistency is the time
period of the filtration tests According to the protocol followed
by previous researchers [1213] the dialysis time period ranged
between 1 and 5 days at the end of which concentration measure-
ments were performed On the other hand the filtration periods
in the case of the present dead-end nanofiltration experiments
ranged between 2 and 5 h depending on the membrane used To
this period one more hour should be added in which the feed solu-
tion was stirred in the cell without pressurization Even though
triazine adsorption on the membrane surface was observed to sta-
bilize within the relatively short period of nanofiltration there is
a possibility that longer filtration periods may have resulted in
higher adsorption rates which in turn could lead to greater pas-
sage of triazine to the permeate side and therefore to reduced
retention values This explanation is in agreement with the pilot
tests performed by Boussahel et al [43] who report that the
increasing adsorption of humic matter on the membranes leads
to an increase of pesticide adsorption on the membranes which
facilitates the diffusion of pesticides in the direction of perme-
ate
33 Correlation between humic material properties and triazine
retention
The correlation of atrazine and prometryn retention with the
total carboxylic and phenolic acidity of the humic substances used
in the present study as well as the role of the molecular mass of HS
on triazine retention are depicted in Figs 6ndash9 These are results of
nanofiltration experiments performed in the absence of calcium
Triazine removal by the three NFULPRO membranes seems
to poorly correlate with the total acidity of the humic material
This is evident especially in the case of NF270 and NF90 mem-
branes (Fig 6) The XLE membrane on the other hand presents a
reduced tendency for atrazine and prometryn removal when acid-
ity increases This is attributed to the characteristic low negative
charge of the XLE membrane surface which causes weak repulsions
with the negatively charged humic organics This leads to a greater
adsorption of humic compounds on the membrane surface (Table 8)
and consequently to greater triazine adsorption which favours the
diffusion of the triazines to the permeate side On the other hand
the strong negative charge of the NF270 and NF90 membranes [20]
promotes a strong repulsion between HS and the membranes The
greater the negative charge of the humic organic the higher the
repulsion which in turn results in higher concentration of HS in
the bulk Consequently the interactions between HS and triazines
in the bulk are increased leading to an improved triazine removal
Fig 8 Retention of atrazine (a) and prometryn (b) as a function of humic material phenolic aciditymdashcalculations based on the values presented in Table 2 taking into
consideration the carbon composition and the HS concentration
Fig 9 Retention of atrazine (a) and prometryn (b) as a function of humic substance molecular mass (based on the values presented in Table 3)
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 97
This is more evident in the case of the wide pore NF270 membrane
since the NF90 membrane seems to retain atrazine and prometryn
to a higher degree in the presence of TA which is characterized by
a lower total acidity among the four HS used in this study
Humic compounds contain a variety of functional groups
such as aliphatic and aromatic carboxyls phenolic hydroxyls
alcoholic hydroxyls carbonyls quinones methoxyls and amino
groups These functional groups are the reactive sites in the humic
molecules As carboxylate protonation seems to be critical for the
capacity of triazine binding [44] it is of interest to correlate the
effect of carboxylic acidity on herbicide removal by nanofiltration
The effect of carboxylic acidity on atrazine and prometryn retention
by the three NFULPRO membranes is depicted in Fig 7
The results show a negative trend of triazine retention with
an increased carboxylic acidity of humic substances This is clear
in the case of NF270 and XLE membranes while NF90 does not
seem to be significantly affected by the changes in the number of
carboxyl groups of a humic molecule Remarkably the smaller TA
molecules containing fewer carboxyl groups per unit mass (Table 2)
exhibit the highest triazine retention (sim100) as a result of the even
higher triazine binding capacity in contrast to the IHSS organics
which contain a higher carboxylic group content Moreover experi-
ments in batch filtration mode result in HS concentration increasing
with time which in turn leads to a reduced number of carboxylic
binding sites available to the two triazines This is probably due
to the self ldquocompetingrdquo character of the HS used since an increase
in concentration andor HS aggregation tend to reduce the avail-
able binding sites [44] This phenomenon led Wang et al [44] to
propose a model in which hydrogen bonding was considered to be
the primary interaction resulting in triazine binding Therefore the
increase in triazine retention observed in the case of elevated HS
concentration may not be directly related with the carboxylic acid-
ity of the HS but rather with other mechanisms such as hydrogen
bonding
Regarding the phenolic acidity of the humic substances there
does not seem to be a sufficient correlation with atrazine or prom-
etryn retention by the three NFULPRO membranes (Fig 8) Actually
there is a contradictory behaviour between the three IHSS organ-
ics and the NOM surrogate TA The high phenolic content of TA
is related to high retention values of the two triazines while
the smaller and relatively similar phenolic acidity of the IHSS
compounds leads to smaller and different retention values These
results imply that there are other factors influencing the association
between humic organics and herbicides and therefore their reten-
tion by nanofiltration In this respect the molecular structure of the
humic materials as well as the interactions taking place between
the organics and the membranes seem to explain the above incon-
sistencies This is evident in the results depicted in Fig 9 and the
adsorption of humic substances subsequently described
The rather strong relation of triazine retention with the molec-
ular mass of the organics used in this study (Fig 9) leads to the
suggestion that structural features of HS are important for the
removal capacity of triazines Up to now there is no consensus
concerning both the binding mechanism of triazines to humic sub-
stances and the structural features responsible for this binding
Nevertheless the preferential binding with low molecular weight
fractions of humic compounds (especially of FA and TA) deduced
from the GPC and filtration tests results in higher retention val-
Table 7
Humic substance and calcium retention performance of the three NFULPRO membranes (mean retention values for 50 recovery of feed volume as permeate)
Membrane Humic substancea Humic substances retention () Ca2+ retention ()
HSs alone HSs with triazines HSs with triazines and Ca2+ In the presence of HSs
NF270 HA 990 994 1000 949
FA 967 983 991 951
NOM 957 997 1000 951
TA 972 992 997 957
NF90 HA 972 980 1000 994
FA 953 984 1000 990
NOM 985 993 1000 991
TA 967 998 998 994
XLE HA 992 996 1000 994
FA 862 915 994 997
NOM 976 1000 1000 988
TA 923 945 1000 996
a HA FA NOM and TA designate the four humic substances used in this study
Table 8
Adsorption data of humic substances on tested membranes
Membrane Humic substancea Feed mass of humic substance (g) Quantity adsorbed (g) Adsorption ()
Alone With Ca2+ Alone With Ca2+ Alone With Ca2+
NF270 HA 3480 2630 244 2314 07 88
FA 3570 2840 71 1335 02 47
NOM 3390 2660 34 2633 01 99
TA 4050 3300 2025 7161 50 217
NF90 HA 3220 2690 354 3658 11 136
FA 2930 2450 1494 857 51 35
NOM 3000 2910 120 437 04 15
TA 2820 2730 649 3631 23 133
XLE HA 3100 2680 1643 3457 53 129
FA 3050 2810 1830 2866 60 102
NOM 2650 2770 610 1219 23 44
TA 3120 2940 2184 2970 70 141
a HA FA NOM and TA designate the four humic substances used in this study
98 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
ues for the two triazines Thus the interaction with small branched
molecules like FA and TA seems to favour a physical entrapment
mechanism such as the one suggested by Devitt and Wiesner [12]
34 Retention of humic species and calcium ion by the NFULPRO
membranes
The performance of nanofiltration on the humic substances and
calcium removal is summarized in Table 7 Regarding humic sub-
stances the retention is compared for three different feed solutions
(a) solutions of HS alone (b) solutions of HS with the two triazines
and (c) solutions of HS with the two triazines in the presence of cal-
cium In general HS retention is high for all three membranes used
According to the UV measurements IHSS HA exhibits the highest
retention values followed by NOM TA and FA which is apparently
related to their relative molecular masses (Table 3) Complexation
or interaction with triazines as evidenced by the GPC tests appar-
ently plays also a significant role in the relative permeability of the
organic species More specifically the retention of the four organic
compounds examined is increased in the presence of the two her-
bicides which verifies also the binding of atrazine and prometryn
with the four humic substances Thus a comparison between the
retention values of the HS in the absence or presence of triazines
gives also a good insight into the HSndashtriazine interactions The slight
increase (lt1) of HA retention in the presence of the two triazines
is in accord with the relatively low HA binding capacity observed
in the GPC tests On the other hand the increase of 2ndash6 in FA
NOM and TA retention is in agreement with the observed increased
interactions between those substances and the two triazines
In the case of calcium addition in the organicndashtriazine solu-
tions the UV absorbing species of the four organics examined are
retained almost completely by all three membranes used This is
again in agreement with the results obtained with gel chromatogra-
phy According to the literature [12] the addition of calcium results
in charge shielding and neutralization of the organic matter charged
functional groups and can shrink the humic matrix The formation
of aggregates of larger apparent molecular weight due to the sub-
stantial increase in the hydrophobicity of the organic molecules
tends to reduce their permeability through membranes and there-
fore to increase their retention
Apart from the increased retention of triazines and humic sub-
stances there is also a significant improvement of calcium rejection
when the latter is present in the solution In comparison to pure
water solutions of elevated calcium ion content [20] the rejection of
calcium in humic solutions is almost complete This is attributed to
size exclusion mechanisms related to the binding tendency of diva-
lent ions like calcium to the negatively charged humic organics
Among the four humic substances used calcium is rejected more in
the presence of the polyphenol tannic acid which is accompanied
by a higher organic adsorption on all three NFULPRO membranes
tested (Table 8)
From the data summarized in Table 8 it can be seen that the
adsorption of humic substances onto the membrane surfaces is
markedly enhanced in the presence of calcium ions due to the
formation of HA-complexed divalent cations This is also clearly
depicted in the SEM images of virgin and fouled membranes in
the absence or presence of calcium ions Characteristic SEM images
in the case of the Suwannee River NOM filtration with the three
NFULPRO membranes are included in Fig 10 these images show
the severe fouling on the three NFULPRO membranes as a result
of the combined filtration of NOM with CaCl2 salt It is possible
that the divalent cations such as Ca2+ act as a bridge between the
membrane surface and the negatively charged humic molecules
that tend to be repelled by the negatively charged membrane As a
result the electrical repulsion between the membrane surface and
the humic molecules is weakened thus rendering the membrane
Fig 10 SEM images of virgin fouled with NOM and fouled with NOM + calcium membranes (a) NF270 (b) NF90 and (c) XLE membrane
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 99
fouling process more severe [45] On the other hand the relatively
smaller adsorption rates of NOM in the absence of calcium result
in less fouled membranes A similar trend is also observed in the
case of the other three humic substances used in this study Among
the three membranes used XLE exhibits in the absence of calcium
the highest adsorption rates for all four humic substances This is
explained by the relatively high hydrophobicity and the low surface
charge that characterize the XLE membrane surface as previously
explained
4 Concluding remarks
The influence of humic substances on retention of two tri-
azines (atrazine and prometryn) by NFULPRO membranes is rather
complicated and involves combinations of solutendashmembrane and
solutendashsolute interactions In summary the retention of triazines
depends on the type of the humic material present in the feed
water In most cases the combined nanofiltration of triazines and
naturally occurring humic substances facilitates the formation of
complexes with triazines which in turn enhance their removal by
means of membrane filtration This complexation appears to be
related not to the characteristic acidity (phenolic carboxylic) of
the HS used but rather to their molecular conformation More
specifically a preferential binding is observed between triazines
and low molecular weight fractions of humic compounds (espe-
cially of FA and TA) which results in higher retention values for the
two triazines Under all conditions tannic acid exhibits the greatest
effect on triazine retention among the four standard HS compounds
used leading to an almost complete removal of the two triazines
(95ndash100) for all three membranes tested
The removal of triazines is improved in the presence of calcium
which tends to enhance the interaction between HS and triazines
Moreover triazine retention is increased with increasing HS con-
centration to a degree depending on the type of HS Additionally
triazine retention is apparently also affected by the interactions
between the organics and the membrane surface which in turn
may alter triazine retention These results illustrate the complex-
ity of the triazine adsorption and retention mechanisms in the
presence of humic substances that needs further investigation for
improved understanding Moreover regarding triazine removal by
nanofiltration it is of particular interest to distinguish the effect
of the HSndashtriazine interactions from that of membrane HS fouling
Research in this direction is carried out in this laboratory
References
[1] DP Birardar AL Rayburn Chromosomal damage induced by herbicide con-tamination at concentrations observed in public water supplies J Environ Qual24 (1995) 1222ndash1225
[2] IK Konstantinou DG Hela TA Albanis The status of pesticide pollution insurface waters (rivers and lakes) of Greece Part I Review on occurrence andlevels Environ Pollut 141 (3) (2006) 555ndash570
[3] European Environment Agency (EEA) Groundwater quality and quantity inEurope Report Copenhagen June 1999 available in httpreportseeaeuropaeugroundwater07012000enenviassrp199903
[4] EM Thurman DA Goolsby MT Meyer MS Mills ML Pomes DW Kolpin AReconnaissance study of herbicides and their metabolites in surface water of theMidwestern United States using immunoassay and gas chromatographymassspectrometry Environ Sci Technol 26 (1992) 2440ndash2447
[5] C Bellona JE Drewes P Xu G Amy Factors affecting the rejection of organicsolutes during NFRO treatmentmdasha literature review Water Res 38 (2004)2795ndash2809
[6] A Bhattacharya Remediation of pesticide-polluted waters through mem-branes Sep Purif Rev 35 (2006) 1ndash38
[7] KM Agbekodo B Legube S Dard Atrazine and simazine removal mechanismsby nanofiltration influence of natural organic matter concentration Water Res30 (1996) 2535ndash2542
[8] P Berg G Hagmeyer R Gimbel Removal of pesticides and other micropollu-tants by nanofiltration Desalination 113 (1997) 205ndash208
[9] EC Devitt F Ducellier P Coumlteacute MR Wiesner Effects of natural organic mat-ter and the raw water matrix on the rejection of atrazine by pressure-drivenmembranes Water Res 32 (1998) 2563ndash2568
[10] R Boussahel S Bouland KM Moussaoui A Montiel Removal of pesticideresidues in water using the nanofiltration process Desalination 132 (2000)205ndash209
[11] Y Zhang B Van der Bruggen GX Chen L Braeken C Vandecasteele Removalof pesticides by nanofiltration effect of the water matrix Sep Purif Technol38 (2004) 163ndash172
[12] EC Devitt MR Wiesner Dialysis investigations of atrazinendashorganic matterinteractions and the role of a divalent metal Environ Sci Technol 32 (1998)232ndash237
[13] SK Dalton JA Brant MR Wiesner Chemical interactions between dissolvedorganic matter and low-molecular weight organic compounds impacts onmembrane separation J Membr Sci 266 (2005) 30ndash39
[14] NA Kulikova IV Perminova Binding of atrazine to humic substances fromsoil peat and coal related to their structure Environ Sci Technol 36 (2002)3720ndash3724
[15] RL Wershaw The importance of humic substancendashmineral particle complexesin the modeling of contaminant transport in sedimentndashwater systems in RABaker (Ed) Organic Substances and Sediments in Water Humics and SoilsLewis Publishers Chelsea MI 1991 pp 23ndash34
[16] SM Schrap A Opperhuizen Relationship between bioavailability andhydrophobicity reduction of the uptake of organic chemicals by fish due tothe sorption on particles Environ Toxicol Chem 9 (1990) 715ndash724
[17] E Barriuso M Schiavon F Andreux JM Portal Localization of atrazine non-extractable (bound) residues in soil size fractions Chemosphere 12 (1991)1131ndash1140
[18] L Loiseau E Barriuso Characterization of the atrazinersquos bound (nonextractable)residues using fractionation techniques for soil organic matter Environ SciTechnol 36 (2002) 683ndash689
[19] G Sposito L Martin-Neto A Yang Atrazine complexation by soil humic acidsJ Environ Qual 25 (1996) 1203ndash1209
[20] KV Plakas AJ Karabelas Membrane retention of herbicides from single andmulti-solute media the effect of ionic environment J Membr Sci 320 (2008)325ndash334
[21] V Freger J Gilron S Belfer TFC polyamide membranes modified by graftingof hydrophilic polymers an FT-IRAFMTEM study J Membr Sci 209 (2002)283ndash292
[22] P Xu JE Drewes Viability of nanofiltration and ultra-low pressure reverseosmosis membranes for multi-beneficial use of methane produced water SepPurif Technol 52 (2006) 67ndash76
[23] Z Knez A Rizner-Hras K Kokot D Bauman Solubility of some solid triazineherbicides in supercritical carbon dioxide Fluid Phase Equilibria 152 (1998)95ndash108
[24] Weed Science Society of America (WSSA) Herbicide Handbook 7th ed 1994[25] B Van der Bruggen J Schaep W Maes D Wilms C Vandecasteele Nanofiltra-
tion as a treatment method for the removal of pesticides from ground watersDesalination 117 (1998) 139ndash147
[26] KN Reddy MA Locke Molecular properties as descriptors of octanol-waterpartition coefficients of herbicides Water Air Soil Pollut 86 (1996) 389ndash405
[27] IHSS website httpwwwihssgatechedu[28] F Flores-Ceacutespedes M Fernaacutendez-Peacuterez M Villafranca-Saacutenchez E Gonzaacutelez-
Pradas Cosorption study of organic pollutants and dissolved organic matter ina soil Environ Pollut 142 (2006) 449ndash456
[29] R Beckett Z Jue JC Giddings Determination of molecular weight distribu-tions of fulvic and humic acids using flow field-flow fractionation Environ SciTechnol 21 (1987) 289ndash295
[30] PM Reid AE Wilkinson E Tipping MN Jones Determination of molecularweights of humic substances by analytical (UV scanning) ultracentrifugationGeochim Cosmochim Acta 54 (1990) 131ndash138
[31] S Lee B Kwon M Sun J Cho Characterizations of NOM included in NF and UFmembrane permeates Desalination 173 (2005) 131ndash142
[32] MN Jones ND Bryan Colloidal properties of humic substances Adv ColloidInterf Sci 78 (1998) 1ndash48
[33] KV Plakas AJ Karabelas T Wintgens T Melin A study of selected herbicidesretention by nanofiltration membranesmdashthe role of organic fouling J MembrSci 284 (2006) 291ndash300
[34] APHA-AWWA-WPCF Standard Methods for the Examination of Water andWastewater 17th ed Port City Press Baltimore MD 1989 pp 567ndash569
[35] R Artinger G Buckau JI Kim S Geyer Characterization of groundwater humicand fulvic acids of different origin by GPC with UVVis and fluorescence detec-tion Fresenius J Anal Chem 364 (1999) 737ndash745
[36] AI Schaumlfer N Andritsos AJ Karabelas EMV Hoek R Schneider M Nys-troumlm Chapter 8 Fouling in nanofiltration in AI Schaumlfer AG Fane TDWaite (Eds) Nanofiltration Principles and Applications Elsevier Oxford UK2005 pp 173ndash174
[37] I Franco L Catalano M Contin M De Nobili Interaction between organicmodel compounds and pesticides with water-soluble soil humic substancesActa Hydrochim Hydrobiol 29 (2001) 88ndash99
[38] AA Helal DM Imam SM Khalifa HF Aly Interaction of pesticides with humiccompounds and their metal complexes Radiochemistry 48 (4) (2006) 419ndash425
[39] MG Browman G Chester Fate of Pollutants in the Air and Water EnvironmentWiley New York 1977 p 50
[40] LD Nghiem AI Schaefer M Elimelech Nanofiltration of hormone mimickingtrace organic contaminants Sep Sci Technol 40 (2005) 2633ndash2649
[41] R Celis J Cornejo MC Hermosin WC Koskinen Sorptionndashdesorption ofatrazine and simazine by model soil colloidal components Soil Sci Soc AmJ 61 (1997) 436ndash443
100 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
[42] SW Runge KR Shelton SA Melton WM Moran Maintaining the ionic per-meability of a cellulose ester membrane J Biochem Biophys Methods 64(2005) 200ndash206
[43] R Boussahel A Montiel M Baudu Effects of organic and inorganic matter onpesticide rejection by nanofiltration Desalination 145 (2002) 109ndash114
[44] Z Wang DS Gamble CH Langford Interaction of atrazine with Laurentianhumic acid Anal Chim Acta 244 (1991) 135ndash143
[45] C Jucker MM Clark Adsorption of aquatic humic substances on hydrophobicultrafiltration membranes J Membr Sci 97 (1994) 37ndash52
96 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
present a significant incompatibility with distilled water whereas
a significant membrane potential can build up with certain salts
which can inhibit or enhance diffusion depending on the size of
the solutes diffusing and the polarity of the membrane potential
generated In this case an increased diffusion of hydrated calcium
ions to the permeate side in order to balance the concentrations
between the two sides of the cellulose ester membrane may result
in a pore blockage or a positive charge on the membrane surface
which in turn could lead to an increased passage of free dissolved
atrazine to the permeate side
Another possible explanation for this inconsistency is the time
period of the filtration tests According to the protocol followed
by previous researchers [1213] the dialysis time period ranged
between 1 and 5 days at the end of which concentration measure-
ments were performed On the other hand the filtration periods
in the case of the present dead-end nanofiltration experiments
ranged between 2 and 5 h depending on the membrane used To
this period one more hour should be added in which the feed solu-
tion was stirred in the cell without pressurization Even though
triazine adsorption on the membrane surface was observed to sta-
bilize within the relatively short period of nanofiltration there is
a possibility that longer filtration periods may have resulted in
higher adsorption rates which in turn could lead to greater pas-
sage of triazine to the permeate side and therefore to reduced
retention values This explanation is in agreement with the pilot
tests performed by Boussahel et al [43] who report that the
increasing adsorption of humic matter on the membranes leads
to an increase of pesticide adsorption on the membranes which
facilitates the diffusion of pesticides in the direction of perme-
ate
33 Correlation between humic material properties and triazine
retention
The correlation of atrazine and prometryn retention with the
total carboxylic and phenolic acidity of the humic substances used
in the present study as well as the role of the molecular mass of HS
on triazine retention are depicted in Figs 6ndash9 These are results of
nanofiltration experiments performed in the absence of calcium
Triazine removal by the three NFULPRO membranes seems
to poorly correlate with the total acidity of the humic material
This is evident especially in the case of NF270 and NF90 mem-
branes (Fig 6) The XLE membrane on the other hand presents a
reduced tendency for atrazine and prometryn removal when acid-
ity increases This is attributed to the characteristic low negative
charge of the XLE membrane surface which causes weak repulsions
with the negatively charged humic organics This leads to a greater
adsorption of humic compounds on the membrane surface (Table 8)
and consequently to greater triazine adsorption which favours the
diffusion of the triazines to the permeate side On the other hand
the strong negative charge of the NF270 and NF90 membranes [20]
promotes a strong repulsion between HS and the membranes The
greater the negative charge of the humic organic the higher the
repulsion which in turn results in higher concentration of HS in
the bulk Consequently the interactions between HS and triazines
in the bulk are increased leading to an improved triazine removal
Fig 8 Retention of atrazine (a) and prometryn (b) as a function of humic material phenolic aciditymdashcalculations based on the values presented in Table 2 taking into
consideration the carbon composition and the HS concentration
Fig 9 Retention of atrazine (a) and prometryn (b) as a function of humic substance molecular mass (based on the values presented in Table 3)
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 97
This is more evident in the case of the wide pore NF270 membrane
since the NF90 membrane seems to retain atrazine and prometryn
to a higher degree in the presence of TA which is characterized by
a lower total acidity among the four HS used in this study
Humic compounds contain a variety of functional groups
such as aliphatic and aromatic carboxyls phenolic hydroxyls
alcoholic hydroxyls carbonyls quinones methoxyls and amino
groups These functional groups are the reactive sites in the humic
molecules As carboxylate protonation seems to be critical for the
capacity of triazine binding [44] it is of interest to correlate the
effect of carboxylic acidity on herbicide removal by nanofiltration
The effect of carboxylic acidity on atrazine and prometryn retention
by the three NFULPRO membranes is depicted in Fig 7
The results show a negative trend of triazine retention with
an increased carboxylic acidity of humic substances This is clear
in the case of NF270 and XLE membranes while NF90 does not
seem to be significantly affected by the changes in the number of
carboxyl groups of a humic molecule Remarkably the smaller TA
molecules containing fewer carboxyl groups per unit mass (Table 2)
exhibit the highest triazine retention (sim100) as a result of the even
higher triazine binding capacity in contrast to the IHSS organics
which contain a higher carboxylic group content Moreover experi-
ments in batch filtration mode result in HS concentration increasing
with time which in turn leads to a reduced number of carboxylic
binding sites available to the two triazines This is probably due
to the self ldquocompetingrdquo character of the HS used since an increase
in concentration andor HS aggregation tend to reduce the avail-
able binding sites [44] This phenomenon led Wang et al [44] to
propose a model in which hydrogen bonding was considered to be
the primary interaction resulting in triazine binding Therefore the
increase in triazine retention observed in the case of elevated HS
concentration may not be directly related with the carboxylic acid-
ity of the HS but rather with other mechanisms such as hydrogen
bonding
Regarding the phenolic acidity of the humic substances there
does not seem to be a sufficient correlation with atrazine or prom-
etryn retention by the three NFULPRO membranes (Fig 8) Actually
there is a contradictory behaviour between the three IHSS organ-
ics and the NOM surrogate TA The high phenolic content of TA
is related to high retention values of the two triazines while
the smaller and relatively similar phenolic acidity of the IHSS
compounds leads to smaller and different retention values These
results imply that there are other factors influencing the association
between humic organics and herbicides and therefore their reten-
tion by nanofiltration In this respect the molecular structure of the
humic materials as well as the interactions taking place between
the organics and the membranes seem to explain the above incon-
sistencies This is evident in the results depicted in Fig 9 and the
adsorption of humic substances subsequently described
The rather strong relation of triazine retention with the molec-
ular mass of the organics used in this study (Fig 9) leads to the
suggestion that structural features of HS are important for the
removal capacity of triazines Up to now there is no consensus
concerning both the binding mechanism of triazines to humic sub-
stances and the structural features responsible for this binding
Nevertheless the preferential binding with low molecular weight
fractions of humic compounds (especially of FA and TA) deduced
from the GPC and filtration tests results in higher retention val-
Table 7
Humic substance and calcium retention performance of the three NFULPRO membranes (mean retention values for 50 recovery of feed volume as permeate)
Membrane Humic substancea Humic substances retention () Ca2+ retention ()
HSs alone HSs with triazines HSs with triazines and Ca2+ In the presence of HSs
NF270 HA 990 994 1000 949
FA 967 983 991 951
NOM 957 997 1000 951
TA 972 992 997 957
NF90 HA 972 980 1000 994
FA 953 984 1000 990
NOM 985 993 1000 991
TA 967 998 998 994
XLE HA 992 996 1000 994
FA 862 915 994 997
NOM 976 1000 1000 988
TA 923 945 1000 996
a HA FA NOM and TA designate the four humic substances used in this study
Table 8
Adsorption data of humic substances on tested membranes
Membrane Humic substancea Feed mass of humic substance (g) Quantity adsorbed (g) Adsorption ()
Alone With Ca2+ Alone With Ca2+ Alone With Ca2+
NF270 HA 3480 2630 244 2314 07 88
FA 3570 2840 71 1335 02 47
NOM 3390 2660 34 2633 01 99
TA 4050 3300 2025 7161 50 217
NF90 HA 3220 2690 354 3658 11 136
FA 2930 2450 1494 857 51 35
NOM 3000 2910 120 437 04 15
TA 2820 2730 649 3631 23 133
XLE HA 3100 2680 1643 3457 53 129
FA 3050 2810 1830 2866 60 102
NOM 2650 2770 610 1219 23 44
TA 3120 2940 2184 2970 70 141
a HA FA NOM and TA designate the four humic substances used in this study
98 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
ues for the two triazines Thus the interaction with small branched
molecules like FA and TA seems to favour a physical entrapment
mechanism such as the one suggested by Devitt and Wiesner [12]
34 Retention of humic species and calcium ion by the NFULPRO
membranes
The performance of nanofiltration on the humic substances and
calcium removal is summarized in Table 7 Regarding humic sub-
stances the retention is compared for three different feed solutions
(a) solutions of HS alone (b) solutions of HS with the two triazines
and (c) solutions of HS with the two triazines in the presence of cal-
cium In general HS retention is high for all three membranes used
According to the UV measurements IHSS HA exhibits the highest
retention values followed by NOM TA and FA which is apparently
related to their relative molecular masses (Table 3) Complexation
or interaction with triazines as evidenced by the GPC tests appar-
ently plays also a significant role in the relative permeability of the
organic species More specifically the retention of the four organic
compounds examined is increased in the presence of the two her-
bicides which verifies also the binding of atrazine and prometryn
with the four humic substances Thus a comparison between the
retention values of the HS in the absence or presence of triazines
gives also a good insight into the HSndashtriazine interactions The slight
increase (lt1) of HA retention in the presence of the two triazines
is in accord with the relatively low HA binding capacity observed
in the GPC tests On the other hand the increase of 2ndash6 in FA
NOM and TA retention is in agreement with the observed increased
interactions between those substances and the two triazines
In the case of calcium addition in the organicndashtriazine solu-
tions the UV absorbing species of the four organics examined are
retained almost completely by all three membranes used This is
again in agreement with the results obtained with gel chromatogra-
phy According to the literature [12] the addition of calcium results
in charge shielding and neutralization of the organic matter charged
functional groups and can shrink the humic matrix The formation
of aggregates of larger apparent molecular weight due to the sub-
stantial increase in the hydrophobicity of the organic molecules
tends to reduce their permeability through membranes and there-
fore to increase their retention
Apart from the increased retention of triazines and humic sub-
stances there is also a significant improvement of calcium rejection
when the latter is present in the solution In comparison to pure
water solutions of elevated calcium ion content [20] the rejection of
calcium in humic solutions is almost complete This is attributed to
size exclusion mechanisms related to the binding tendency of diva-
lent ions like calcium to the negatively charged humic organics
Among the four humic substances used calcium is rejected more in
the presence of the polyphenol tannic acid which is accompanied
by a higher organic adsorption on all three NFULPRO membranes
tested (Table 8)
From the data summarized in Table 8 it can be seen that the
adsorption of humic substances onto the membrane surfaces is
markedly enhanced in the presence of calcium ions due to the
formation of HA-complexed divalent cations This is also clearly
depicted in the SEM images of virgin and fouled membranes in
the absence or presence of calcium ions Characteristic SEM images
in the case of the Suwannee River NOM filtration with the three
NFULPRO membranes are included in Fig 10 these images show
the severe fouling on the three NFULPRO membranes as a result
of the combined filtration of NOM with CaCl2 salt It is possible
that the divalent cations such as Ca2+ act as a bridge between the
membrane surface and the negatively charged humic molecules
that tend to be repelled by the negatively charged membrane As a
result the electrical repulsion between the membrane surface and
the humic molecules is weakened thus rendering the membrane
Fig 10 SEM images of virgin fouled with NOM and fouled with NOM + calcium membranes (a) NF270 (b) NF90 and (c) XLE membrane
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 99
fouling process more severe [45] On the other hand the relatively
smaller adsorption rates of NOM in the absence of calcium result
in less fouled membranes A similar trend is also observed in the
case of the other three humic substances used in this study Among
the three membranes used XLE exhibits in the absence of calcium
the highest adsorption rates for all four humic substances This is
explained by the relatively high hydrophobicity and the low surface
charge that characterize the XLE membrane surface as previously
explained
4 Concluding remarks
The influence of humic substances on retention of two tri-
azines (atrazine and prometryn) by NFULPRO membranes is rather
complicated and involves combinations of solutendashmembrane and
solutendashsolute interactions In summary the retention of triazines
depends on the type of the humic material present in the feed
water In most cases the combined nanofiltration of triazines and
naturally occurring humic substances facilitates the formation of
complexes with triazines which in turn enhance their removal by
means of membrane filtration This complexation appears to be
related not to the characteristic acidity (phenolic carboxylic) of
the HS used but rather to their molecular conformation More
specifically a preferential binding is observed between triazines
and low molecular weight fractions of humic compounds (espe-
cially of FA and TA) which results in higher retention values for the
two triazines Under all conditions tannic acid exhibits the greatest
effect on triazine retention among the four standard HS compounds
used leading to an almost complete removal of the two triazines
(95ndash100) for all three membranes tested
The removal of triazines is improved in the presence of calcium
which tends to enhance the interaction between HS and triazines
Moreover triazine retention is increased with increasing HS con-
centration to a degree depending on the type of HS Additionally
triazine retention is apparently also affected by the interactions
between the organics and the membrane surface which in turn
may alter triazine retention These results illustrate the complex-
ity of the triazine adsorption and retention mechanisms in the
presence of humic substances that needs further investigation for
improved understanding Moreover regarding triazine removal by
nanofiltration it is of particular interest to distinguish the effect
of the HSndashtriazine interactions from that of membrane HS fouling
Research in this direction is carried out in this laboratory
References
[1] DP Birardar AL Rayburn Chromosomal damage induced by herbicide con-tamination at concentrations observed in public water supplies J Environ Qual24 (1995) 1222ndash1225
[2] IK Konstantinou DG Hela TA Albanis The status of pesticide pollution insurface waters (rivers and lakes) of Greece Part I Review on occurrence andlevels Environ Pollut 141 (3) (2006) 555ndash570
[3] European Environment Agency (EEA) Groundwater quality and quantity inEurope Report Copenhagen June 1999 available in httpreportseeaeuropaeugroundwater07012000enenviassrp199903
[4] EM Thurman DA Goolsby MT Meyer MS Mills ML Pomes DW Kolpin AReconnaissance study of herbicides and their metabolites in surface water of theMidwestern United States using immunoassay and gas chromatographymassspectrometry Environ Sci Technol 26 (1992) 2440ndash2447
[5] C Bellona JE Drewes P Xu G Amy Factors affecting the rejection of organicsolutes during NFRO treatmentmdasha literature review Water Res 38 (2004)2795ndash2809
[6] A Bhattacharya Remediation of pesticide-polluted waters through mem-branes Sep Purif Rev 35 (2006) 1ndash38
[7] KM Agbekodo B Legube S Dard Atrazine and simazine removal mechanismsby nanofiltration influence of natural organic matter concentration Water Res30 (1996) 2535ndash2542
[8] P Berg G Hagmeyer R Gimbel Removal of pesticides and other micropollu-tants by nanofiltration Desalination 113 (1997) 205ndash208
[9] EC Devitt F Ducellier P Coumlteacute MR Wiesner Effects of natural organic mat-ter and the raw water matrix on the rejection of atrazine by pressure-drivenmembranes Water Res 32 (1998) 2563ndash2568
[10] R Boussahel S Bouland KM Moussaoui A Montiel Removal of pesticideresidues in water using the nanofiltration process Desalination 132 (2000)205ndash209
[11] Y Zhang B Van der Bruggen GX Chen L Braeken C Vandecasteele Removalof pesticides by nanofiltration effect of the water matrix Sep Purif Technol38 (2004) 163ndash172
[12] EC Devitt MR Wiesner Dialysis investigations of atrazinendashorganic matterinteractions and the role of a divalent metal Environ Sci Technol 32 (1998)232ndash237
[13] SK Dalton JA Brant MR Wiesner Chemical interactions between dissolvedorganic matter and low-molecular weight organic compounds impacts onmembrane separation J Membr Sci 266 (2005) 30ndash39
[14] NA Kulikova IV Perminova Binding of atrazine to humic substances fromsoil peat and coal related to their structure Environ Sci Technol 36 (2002)3720ndash3724
[15] RL Wershaw The importance of humic substancendashmineral particle complexesin the modeling of contaminant transport in sedimentndashwater systems in RABaker (Ed) Organic Substances and Sediments in Water Humics and SoilsLewis Publishers Chelsea MI 1991 pp 23ndash34
[16] SM Schrap A Opperhuizen Relationship between bioavailability andhydrophobicity reduction of the uptake of organic chemicals by fish due tothe sorption on particles Environ Toxicol Chem 9 (1990) 715ndash724
[17] E Barriuso M Schiavon F Andreux JM Portal Localization of atrazine non-extractable (bound) residues in soil size fractions Chemosphere 12 (1991)1131ndash1140
[18] L Loiseau E Barriuso Characterization of the atrazinersquos bound (nonextractable)residues using fractionation techniques for soil organic matter Environ SciTechnol 36 (2002) 683ndash689
[19] G Sposito L Martin-Neto A Yang Atrazine complexation by soil humic acidsJ Environ Qual 25 (1996) 1203ndash1209
[20] KV Plakas AJ Karabelas Membrane retention of herbicides from single andmulti-solute media the effect of ionic environment J Membr Sci 320 (2008)325ndash334
[21] V Freger J Gilron S Belfer TFC polyamide membranes modified by graftingof hydrophilic polymers an FT-IRAFMTEM study J Membr Sci 209 (2002)283ndash292
[22] P Xu JE Drewes Viability of nanofiltration and ultra-low pressure reverseosmosis membranes for multi-beneficial use of methane produced water SepPurif Technol 52 (2006) 67ndash76
[23] Z Knez A Rizner-Hras K Kokot D Bauman Solubility of some solid triazineherbicides in supercritical carbon dioxide Fluid Phase Equilibria 152 (1998)95ndash108
[24] Weed Science Society of America (WSSA) Herbicide Handbook 7th ed 1994[25] B Van der Bruggen J Schaep W Maes D Wilms C Vandecasteele Nanofiltra-
tion as a treatment method for the removal of pesticides from ground watersDesalination 117 (1998) 139ndash147
[26] KN Reddy MA Locke Molecular properties as descriptors of octanol-waterpartition coefficients of herbicides Water Air Soil Pollut 86 (1996) 389ndash405
[27] IHSS website httpwwwihssgatechedu[28] F Flores-Ceacutespedes M Fernaacutendez-Peacuterez M Villafranca-Saacutenchez E Gonzaacutelez-
Pradas Cosorption study of organic pollutants and dissolved organic matter ina soil Environ Pollut 142 (2006) 449ndash456
[29] R Beckett Z Jue JC Giddings Determination of molecular weight distribu-tions of fulvic and humic acids using flow field-flow fractionation Environ SciTechnol 21 (1987) 289ndash295
[30] PM Reid AE Wilkinson E Tipping MN Jones Determination of molecularweights of humic substances by analytical (UV scanning) ultracentrifugationGeochim Cosmochim Acta 54 (1990) 131ndash138
[31] S Lee B Kwon M Sun J Cho Characterizations of NOM included in NF and UFmembrane permeates Desalination 173 (2005) 131ndash142
[32] MN Jones ND Bryan Colloidal properties of humic substances Adv ColloidInterf Sci 78 (1998) 1ndash48
[33] KV Plakas AJ Karabelas T Wintgens T Melin A study of selected herbicidesretention by nanofiltration membranesmdashthe role of organic fouling J MembrSci 284 (2006) 291ndash300
[34] APHA-AWWA-WPCF Standard Methods for the Examination of Water andWastewater 17th ed Port City Press Baltimore MD 1989 pp 567ndash569
[35] R Artinger G Buckau JI Kim S Geyer Characterization of groundwater humicand fulvic acids of different origin by GPC with UVVis and fluorescence detec-tion Fresenius J Anal Chem 364 (1999) 737ndash745
[36] AI Schaumlfer N Andritsos AJ Karabelas EMV Hoek R Schneider M Nys-troumlm Chapter 8 Fouling in nanofiltration in AI Schaumlfer AG Fane TDWaite (Eds) Nanofiltration Principles and Applications Elsevier Oxford UK2005 pp 173ndash174
[37] I Franco L Catalano M Contin M De Nobili Interaction between organicmodel compounds and pesticides with water-soluble soil humic substancesActa Hydrochim Hydrobiol 29 (2001) 88ndash99
[38] AA Helal DM Imam SM Khalifa HF Aly Interaction of pesticides with humiccompounds and their metal complexes Radiochemistry 48 (4) (2006) 419ndash425
[39] MG Browman G Chester Fate of Pollutants in the Air and Water EnvironmentWiley New York 1977 p 50
[40] LD Nghiem AI Schaefer M Elimelech Nanofiltration of hormone mimickingtrace organic contaminants Sep Sci Technol 40 (2005) 2633ndash2649
[41] R Celis J Cornejo MC Hermosin WC Koskinen Sorptionndashdesorption ofatrazine and simazine by model soil colloidal components Soil Sci Soc AmJ 61 (1997) 436ndash443
100 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
[42] SW Runge KR Shelton SA Melton WM Moran Maintaining the ionic per-meability of a cellulose ester membrane J Biochem Biophys Methods 64(2005) 200ndash206
[43] R Boussahel A Montiel M Baudu Effects of organic and inorganic matter onpesticide rejection by nanofiltration Desalination 145 (2002) 109ndash114
[44] Z Wang DS Gamble CH Langford Interaction of atrazine with Laurentianhumic acid Anal Chim Acta 244 (1991) 135ndash143
[45] C Jucker MM Clark Adsorption of aquatic humic substances on hydrophobicultrafiltration membranes J Membr Sci 97 (1994) 37ndash52
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 97
This is more evident in the case of the wide pore NF270 membrane
since the NF90 membrane seems to retain atrazine and prometryn
to a higher degree in the presence of TA which is characterized by
a lower total acidity among the four HS used in this study
Humic compounds contain a variety of functional groups
such as aliphatic and aromatic carboxyls phenolic hydroxyls
alcoholic hydroxyls carbonyls quinones methoxyls and amino
groups These functional groups are the reactive sites in the humic
molecules As carboxylate protonation seems to be critical for the
capacity of triazine binding [44] it is of interest to correlate the
effect of carboxylic acidity on herbicide removal by nanofiltration
The effect of carboxylic acidity on atrazine and prometryn retention
by the three NFULPRO membranes is depicted in Fig 7
The results show a negative trend of triazine retention with
an increased carboxylic acidity of humic substances This is clear
in the case of NF270 and XLE membranes while NF90 does not
seem to be significantly affected by the changes in the number of
carboxyl groups of a humic molecule Remarkably the smaller TA
molecules containing fewer carboxyl groups per unit mass (Table 2)
exhibit the highest triazine retention (sim100) as a result of the even
higher triazine binding capacity in contrast to the IHSS organics
which contain a higher carboxylic group content Moreover experi-
ments in batch filtration mode result in HS concentration increasing
with time which in turn leads to a reduced number of carboxylic
binding sites available to the two triazines This is probably due
to the self ldquocompetingrdquo character of the HS used since an increase
in concentration andor HS aggregation tend to reduce the avail-
able binding sites [44] This phenomenon led Wang et al [44] to
propose a model in which hydrogen bonding was considered to be
the primary interaction resulting in triazine binding Therefore the
increase in triazine retention observed in the case of elevated HS
concentration may not be directly related with the carboxylic acid-
ity of the HS but rather with other mechanisms such as hydrogen
bonding
Regarding the phenolic acidity of the humic substances there
does not seem to be a sufficient correlation with atrazine or prom-
etryn retention by the three NFULPRO membranes (Fig 8) Actually
there is a contradictory behaviour between the three IHSS organ-
ics and the NOM surrogate TA The high phenolic content of TA
is related to high retention values of the two triazines while
the smaller and relatively similar phenolic acidity of the IHSS
compounds leads to smaller and different retention values These
results imply that there are other factors influencing the association
between humic organics and herbicides and therefore their reten-
tion by nanofiltration In this respect the molecular structure of the
humic materials as well as the interactions taking place between
the organics and the membranes seem to explain the above incon-
sistencies This is evident in the results depicted in Fig 9 and the
adsorption of humic substances subsequently described
The rather strong relation of triazine retention with the molec-
ular mass of the organics used in this study (Fig 9) leads to the
suggestion that structural features of HS are important for the
removal capacity of triazines Up to now there is no consensus
concerning both the binding mechanism of triazines to humic sub-
stances and the structural features responsible for this binding
Nevertheless the preferential binding with low molecular weight
fractions of humic compounds (especially of FA and TA) deduced
from the GPC and filtration tests results in higher retention val-
Table 7
Humic substance and calcium retention performance of the three NFULPRO membranes (mean retention values for 50 recovery of feed volume as permeate)
Membrane Humic substancea Humic substances retention () Ca2+ retention ()
HSs alone HSs with triazines HSs with triazines and Ca2+ In the presence of HSs
NF270 HA 990 994 1000 949
FA 967 983 991 951
NOM 957 997 1000 951
TA 972 992 997 957
NF90 HA 972 980 1000 994
FA 953 984 1000 990
NOM 985 993 1000 991
TA 967 998 998 994
XLE HA 992 996 1000 994
FA 862 915 994 997
NOM 976 1000 1000 988
TA 923 945 1000 996
a HA FA NOM and TA designate the four humic substances used in this study
Table 8
Adsorption data of humic substances on tested membranes
Membrane Humic substancea Feed mass of humic substance (g) Quantity adsorbed (g) Adsorption ()
Alone With Ca2+ Alone With Ca2+ Alone With Ca2+
NF270 HA 3480 2630 244 2314 07 88
FA 3570 2840 71 1335 02 47
NOM 3390 2660 34 2633 01 99
TA 4050 3300 2025 7161 50 217
NF90 HA 3220 2690 354 3658 11 136
FA 2930 2450 1494 857 51 35
NOM 3000 2910 120 437 04 15
TA 2820 2730 649 3631 23 133
XLE HA 3100 2680 1643 3457 53 129
FA 3050 2810 1830 2866 60 102
NOM 2650 2770 610 1219 23 44
TA 3120 2940 2184 2970 70 141
a HA FA NOM and TA designate the four humic substances used in this study
98 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
ues for the two triazines Thus the interaction with small branched
molecules like FA and TA seems to favour a physical entrapment
mechanism such as the one suggested by Devitt and Wiesner [12]
34 Retention of humic species and calcium ion by the NFULPRO
membranes
The performance of nanofiltration on the humic substances and
calcium removal is summarized in Table 7 Regarding humic sub-
stances the retention is compared for three different feed solutions
(a) solutions of HS alone (b) solutions of HS with the two triazines
and (c) solutions of HS with the two triazines in the presence of cal-
cium In general HS retention is high for all three membranes used
According to the UV measurements IHSS HA exhibits the highest
retention values followed by NOM TA and FA which is apparently
related to their relative molecular masses (Table 3) Complexation
or interaction with triazines as evidenced by the GPC tests appar-
ently plays also a significant role in the relative permeability of the
organic species More specifically the retention of the four organic
compounds examined is increased in the presence of the two her-
bicides which verifies also the binding of atrazine and prometryn
with the four humic substances Thus a comparison between the
retention values of the HS in the absence or presence of triazines
gives also a good insight into the HSndashtriazine interactions The slight
increase (lt1) of HA retention in the presence of the two triazines
is in accord with the relatively low HA binding capacity observed
in the GPC tests On the other hand the increase of 2ndash6 in FA
NOM and TA retention is in agreement with the observed increased
interactions between those substances and the two triazines
In the case of calcium addition in the organicndashtriazine solu-
tions the UV absorbing species of the four organics examined are
retained almost completely by all three membranes used This is
again in agreement with the results obtained with gel chromatogra-
phy According to the literature [12] the addition of calcium results
in charge shielding and neutralization of the organic matter charged
functional groups and can shrink the humic matrix The formation
of aggregates of larger apparent molecular weight due to the sub-
stantial increase in the hydrophobicity of the organic molecules
tends to reduce their permeability through membranes and there-
fore to increase their retention
Apart from the increased retention of triazines and humic sub-
stances there is also a significant improvement of calcium rejection
when the latter is present in the solution In comparison to pure
water solutions of elevated calcium ion content [20] the rejection of
calcium in humic solutions is almost complete This is attributed to
size exclusion mechanisms related to the binding tendency of diva-
lent ions like calcium to the negatively charged humic organics
Among the four humic substances used calcium is rejected more in
the presence of the polyphenol tannic acid which is accompanied
by a higher organic adsorption on all three NFULPRO membranes
tested (Table 8)
From the data summarized in Table 8 it can be seen that the
adsorption of humic substances onto the membrane surfaces is
markedly enhanced in the presence of calcium ions due to the
formation of HA-complexed divalent cations This is also clearly
depicted in the SEM images of virgin and fouled membranes in
the absence or presence of calcium ions Characteristic SEM images
in the case of the Suwannee River NOM filtration with the three
NFULPRO membranes are included in Fig 10 these images show
the severe fouling on the three NFULPRO membranes as a result
of the combined filtration of NOM with CaCl2 salt It is possible
that the divalent cations such as Ca2+ act as a bridge between the
membrane surface and the negatively charged humic molecules
that tend to be repelled by the negatively charged membrane As a
result the electrical repulsion between the membrane surface and
the humic molecules is weakened thus rendering the membrane
Fig 10 SEM images of virgin fouled with NOM and fouled with NOM + calcium membranes (a) NF270 (b) NF90 and (c) XLE membrane
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 99
fouling process more severe [45] On the other hand the relatively
smaller adsorption rates of NOM in the absence of calcium result
in less fouled membranes A similar trend is also observed in the
case of the other three humic substances used in this study Among
the three membranes used XLE exhibits in the absence of calcium
the highest adsorption rates for all four humic substances This is
explained by the relatively high hydrophobicity and the low surface
charge that characterize the XLE membrane surface as previously
explained
4 Concluding remarks
The influence of humic substances on retention of two tri-
azines (atrazine and prometryn) by NFULPRO membranes is rather
complicated and involves combinations of solutendashmembrane and
solutendashsolute interactions In summary the retention of triazines
depends on the type of the humic material present in the feed
water In most cases the combined nanofiltration of triazines and
naturally occurring humic substances facilitates the formation of
complexes with triazines which in turn enhance their removal by
means of membrane filtration This complexation appears to be
related not to the characteristic acidity (phenolic carboxylic) of
the HS used but rather to their molecular conformation More
specifically a preferential binding is observed between triazines
and low molecular weight fractions of humic compounds (espe-
cially of FA and TA) which results in higher retention values for the
two triazines Under all conditions tannic acid exhibits the greatest
effect on triazine retention among the four standard HS compounds
used leading to an almost complete removal of the two triazines
(95ndash100) for all three membranes tested
The removal of triazines is improved in the presence of calcium
which tends to enhance the interaction between HS and triazines
Moreover triazine retention is increased with increasing HS con-
centration to a degree depending on the type of HS Additionally
triazine retention is apparently also affected by the interactions
between the organics and the membrane surface which in turn
may alter triazine retention These results illustrate the complex-
ity of the triazine adsorption and retention mechanisms in the
presence of humic substances that needs further investigation for
improved understanding Moreover regarding triazine removal by
nanofiltration it is of particular interest to distinguish the effect
of the HSndashtriazine interactions from that of membrane HS fouling
Research in this direction is carried out in this laboratory
References
[1] DP Birardar AL Rayburn Chromosomal damage induced by herbicide con-tamination at concentrations observed in public water supplies J Environ Qual24 (1995) 1222ndash1225
[2] IK Konstantinou DG Hela TA Albanis The status of pesticide pollution insurface waters (rivers and lakes) of Greece Part I Review on occurrence andlevels Environ Pollut 141 (3) (2006) 555ndash570
[3] European Environment Agency (EEA) Groundwater quality and quantity inEurope Report Copenhagen June 1999 available in httpreportseeaeuropaeugroundwater07012000enenviassrp199903
[4] EM Thurman DA Goolsby MT Meyer MS Mills ML Pomes DW Kolpin AReconnaissance study of herbicides and their metabolites in surface water of theMidwestern United States using immunoassay and gas chromatographymassspectrometry Environ Sci Technol 26 (1992) 2440ndash2447
[5] C Bellona JE Drewes P Xu G Amy Factors affecting the rejection of organicsolutes during NFRO treatmentmdasha literature review Water Res 38 (2004)2795ndash2809
[6] A Bhattacharya Remediation of pesticide-polluted waters through mem-branes Sep Purif Rev 35 (2006) 1ndash38
[7] KM Agbekodo B Legube S Dard Atrazine and simazine removal mechanismsby nanofiltration influence of natural organic matter concentration Water Res30 (1996) 2535ndash2542
[8] P Berg G Hagmeyer R Gimbel Removal of pesticides and other micropollu-tants by nanofiltration Desalination 113 (1997) 205ndash208
[9] EC Devitt F Ducellier P Coumlteacute MR Wiesner Effects of natural organic mat-ter and the raw water matrix on the rejection of atrazine by pressure-drivenmembranes Water Res 32 (1998) 2563ndash2568
[10] R Boussahel S Bouland KM Moussaoui A Montiel Removal of pesticideresidues in water using the nanofiltration process Desalination 132 (2000)205ndash209
[11] Y Zhang B Van der Bruggen GX Chen L Braeken C Vandecasteele Removalof pesticides by nanofiltration effect of the water matrix Sep Purif Technol38 (2004) 163ndash172
[12] EC Devitt MR Wiesner Dialysis investigations of atrazinendashorganic matterinteractions and the role of a divalent metal Environ Sci Technol 32 (1998)232ndash237
[13] SK Dalton JA Brant MR Wiesner Chemical interactions between dissolvedorganic matter and low-molecular weight organic compounds impacts onmembrane separation J Membr Sci 266 (2005) 30ndash39
[14] NA Kulikova IV Perminova Binding of atrazine to humic substances fromsoil peat and coal related to their structure Environ Sci Technol 36 (2002)3720ndash3724
[15] RL Wershaw The importance of humic substancendashmineral particle complexesin the modeling of contaminant transport in sedimentndashwater systems in RABaker (Ed) Organic Substances and Sediments in Water Humics and SoilsLewis Publishers Chelsea MI 1991 pp 23ndash34
[16] SM Schrap A Opperhuizen Relationship between bioavailability andhydrophobicity reduction of the uptake of organic chemicals by fish due tothe sorption on particles Environ Toxicol Chem 9 (1990) 715ndash724
[17] E Barriuso M Schiavon F Andreux JM Portal Localization of atrazine non-extractable (bound) residues in soil size fractions Chemosphere 12 (1991)1131ndash1140
[18] L Loiseau E Barriuso Characterization of the atrazinersquos bound (nonextractable)residues using fractionation techniques for soil organic matter Environ SciTechnol 36 (2002) 683ndash689
[19] G Sposito L Martin-Neto A Yang Atrazine complexation by soil humic acidsJ Environ Qual 25 (1996) 1203ndash1209
[20] KV Plakas AJ Karabelas Membrane retention of herbicides from single andmulti-solute media the effect of ionic environment J Membr Sci 320 (2008)325ndash334
[21] V Freger J Gilron S Belfer TFC polyamide membranes modified by graftingof hydrophilic polymers an FT-IRAFMTEM study J Membr Sci 209 (2002)283ndash292
[22] P Xu JE Drewes Viability of nanofiltration and ultra-low pressure reverseosmosis membranes for multi-beneficial use of methane produced water SepPurif Technol 52 (2006) 67ndash76
[23] Z Knez A Rizner-Hras K Kokot D Bauman Solubility of some solid triazineherbicides in supercritical carbon dioxide Fluid Phase Equilibria 152 (1998)95ndash108
[24] Weed Science Society of America (WSSA) Herbicide Handbook 7th ed 1994[25] B Van der Bruggen J Schaep W Maes D Wilms C Vandecasteele Nanofiltra-
tion as a treatment method for the removal of pesticides from ground watersDesalination 117 (1998) 139ndash147
[26] KN Reddy MA Locke Molecular properties as descriptors of octanol-waterpartition coefficients of herbicides Water Air Soil Pollut 86 (1996) 389ndash405
[27] IHSS website httpwwwihssgatechedu[28] F Flores-Ceacutespedes M Fernaacutendez-Peacuterez M Villafranca-Saacutenchez E Gonzaacutelez-
Pradas Cosorption study of organic pollutants and dissolved organic matter ina soil Environ Pollut 142 (2006) 449ndash456
[29] R Beckett Z Jue JC Giddings Determination of molecular weight distribu-tions of fulvic and humic acids using flow field-flow fractionation Environ SciTechnol 21 (1987) 289ndash295
[30] PM Reid AE Wilkinson E Tipping MN Jones Determination of molecularweights of humic substances by analytical (UV scanning) ultracentrifugationGeochim Cosmochim Acta 54 (1990) 131ndash138
[31] S Lee B Kwon M Sun J Cho Characterizations of NOM included in NF and UFmembrane permeates Desalination 173 (2005) 131ndash142
[32] MN Jones ND Bryan Colloidal properties of humic substances Adv ColloidInterf Sci 78 (1998) 1ndash48
[33] KV Plakas AJ Karabelas T Wintgens T Melin A study of selected herbicidesretention by nanofiltration membranesmdashthe role of organic fouling J MembrSci 284 (2006) 291ndash300
[34] APHA-AWWA-WPCF Standard Methods for the Examination of Water andWastewater 17th ed Port City Press Baltimore MD 1989 pp 567ndash569
[35] R Artinger G Buckau JI Kim S Geyer Characterization of groundwater humicand fulvic acids of different origin by GPC with UVVis and fluorescence detec-tion Fresenius J Anal Chem 364 (1999) 737ndash745
[36] AI Schaumlfer N Andritsos AJ Karabelas EMV Hoek R Schneider M Nys-troumlm Chapter 8 Fouling in nanofiltration in AI Schaumlfer AG Fane TDWaite (Eds) Nanofiltration Principles and Applications Elsevier Oxford UK2005 pp 173ndash174
[37] I Franco L Catalano M Contin M De Nobili Interaction between organicmodel compounds and pesticides with water-soluble soil humic substancesActa Hydrochim Hydrobiol 29 (2001) 88ndash99
[38] AA Helal DM Imam SM Khalifa HF Aly Interaction of pesticides with humiccompounds and their metal complexes Radiochemistry 48 (4) (2006) 419ndash425
[39] MG Browman G Chester Fate of Pollutants in the Air and Water EnvironmentWiley New York 1977 p 50
[40] LD Nghiem AI Schaefer M Elimelech Nanofiltration of hormone mimickingtrace organic contaminants Sep Sci Technol 40 (2005) 2633ndash2649
[41] R Celis J Cornejo MC Hermosin WC Koskinen Sorptionndashdesorption ofatrazine and simazine by model soil colloidal components Soil Sci Soc AmJ 61 (1997) 436ndash443
100 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
[42] SW Runge KR Shelton SA Melton WM Moran Maintaining the ionic per-meability of a cellulose ester membrane J Biochem Biophys Methods 64(2005) 200ndash206
[43] R Boussahel A Montiel M Baudu Effects of organic and inorganic matter onpesticide rejection by nanofiltration Desalination 145 (2002) 109ndash114
[44] Z Wang DS Gamble CH Langford Interaction of atrazine with Laurentianhumic acid Anal Chim Acta 244 (1991) 135ndash143
[45] C Jucker MM Clark Adsorption of aquatic humic substances on hydrophobicultrafiltration membranes J Membr Sci 97 (1994) 37ndash52
98 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
ues for the two triazines Thus the interaction with small branched
molecules like FA and TA seems to favour a physical entrapment
mechanism such as the one suggested by Devitt and Wiesner [12]
34 Retention of humic species and calcium ion by the NFULPRO
membranes
The performance of nanofiltration on the humic substances and
calcium removal is summarized in Table 7 Regarding humic sub-
stances the retention is compared for three different feed solutions
(a) solutions of HS alone (b) solutions of HS with the two triazines
and (c) solutions of HS with the two triazines in the presence of cal-
cium In general HS retention is high for all three membranes used
According to the UV measurements IHSS HA exhibits the highest
retention values followed by NOM TA and FA which is apparently
related to their relative molecular masses (Table 3) Complexation
or interaction with triazines as evidenced by the GPC tests appar-
ently plays also a significant role in the relative permeability of the
organic species More specifically the retention of the four organic
compounds examined is increased in the presence of the two her-
bicides which verifies also the binding of atrazine and prometryn
with the four humic substances Thus a comparison between the
retention values of the HS in the absence or presence of triazines
gives also a good insight into the HSndashtriazine interactions The slight
increase (lt1) of HA retention in the presence of the two triazines
is in accord with the relatively low HA binding capacity observed
in the GPC tests On the other hand the increase of 2ndash6 in FA
NOM and TA retention is in agreement with the observed increased
interactions between those substances and the two triazines
In the case of calcium addition in the organicndashtriazine solu-
tions the UV absorbing species of the four organics examined are
retained almost completely by all three membranes used This is
again in agreement with the results obtained with gel chromatogra-
phy According to the literature [12] the addition of calcium results
in charge shielding and neutralization of the organic matter charged
functional groups and can shrink the humic matrix The formation
of aggregates of larger apparent molecular weight due to the sub-
stantial increase in the hydrophobicity of the organic molecules
tends to reduce their permeability through membranes and there-
fore to increase their retention
Apart from the increased retention of triazines and humic sub-
stances there is also a significant improvement of calcium rejection
when the latter is present in the solution In comparison to pure
water solutions of elevated calcium ion content [20] the rejection of
calcium in humic solutions is almost complete This is attributed to
size exclusion mechanisms related to the binding tendency of diva-
lent ions like calcium to the negatively charged humic organics
Among the four humic substances used calcium is rejected more in
the presence of the polyphenol tannic acid which is accompanied
by a higher organic adsorption on all three NFULPRO membranes
tested (Table 8)
From the data summarized in Table 8 it can be seen that the
adsorption of humic substances onto the membrane surfaces is
markedly enhanced in the presence of calcium ions due to the
formation of HA-complexed divalent cations This is also clearly
depicted in the SEM images of virgin and fouled membranes in
the absence or presence of calcium ions Characteristic SEM images
in the case of the Suwannee River NOM filtration with the three
NFULPRO membranes are included in Fig 10 these images show
the severe fouling on the three NFULPRO membranes as a result
of the combined filtration of NOM with CaCl2 salt It is possible
that the divalent cations such as Ca2+ act as a bridge between the
membrane surface and the negatively charged humic molecules
that tend to be repelled by the negatively charged membrane As a
result the electrical repulsion between the membrane surface and
the humic molecules is weakened thus rendering the membrane
Fig 10 SEM images of virgin fouled with NOM and fouled with NOM + calcium membranes (a) NF270 (b) NF90 and (c) XLE membrane
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 99
fouling process more severe [45] On the other hand the relatively
smaller adsorption rates of NOM in the absence of calcium result
in less fouled membranes A similar trend is also observed in the
case of the other three humic substances used in this study Among
the three membranes used XLE exhibits in the absence of calcium
the highest adsorption rates for all four humic substances This is
explained by the relatively high hydrophobicity and the low surface
charge that characterize the XLE membrane surface as previously
explained
4 Concluding remarks
The influence of humic substances on retention of two tri-
azines (atrazine and prometryn) by NFULPRO membranes is rather
complicated and involves combinations of solutendashmembrane and
solutendashsolute interactions In summary the retention of triazines
depends on the type of the humic material present in the feed
water In most cases the combined nanofiltration of triazines and
naturally occurring humic substances facilitates the formation of
complexes with triazines which in turn enhance their removal by
means of membrane filtration This complexation appears to be
related not to the characteristic acidity (phenolic carboxylic) of
the HS used but rather to their molecular conformation More
specifically a preferential binding is observed between triazines
and low molecular weight fractions of humic compounds (espe-
cially of FA and TA) which results in higher retention values for the
two triazines Under all conditions tannic acid exhibits the greatest
effect on triazine retention among the four standard HS compounds
used leading to an almost complete removal of the two triazines
(95ndash100) for all three membranes tested
The removal of triazines is improved in the presence of calcium
which tends to enhance the interaction between HS and triazines
Moreover triazine retention is increased with increasing HS con-
centration to a degree depending on the type of HS Additionally
triazine retention is apparently also affected by the interactions
between the organics and the membrane surface which in turn
may alter triazine retention These results illustrate the complex-
ity of the triazine adsorption and retention mechanisms in the
presence of humic substances that needs further investigation for
improved understanding Moreover regarding triazine removal by
nanofiltration it is of particular interest to distinguish the effect
of the HSndashtriazine interactions from that of membrane HS fouling
Research in this direction is carried out in this laboratory
References
[1] DP Birardar AL Rayburn Chromosomal damage induced by herbicide con-tamination at concentrations observed in public water supplies J Environ Qual24 (1995) 1222ndash1225
[2] IK Konstantinou DG Hela TA Albanis The status of pesticide pollution insurface waters (rivers and lakes) of Greece Part I Review on occurrence andlevels Environ Pollut 141 (3) (2006) 555ndash570
[3] European Environment Agency (EEA) Groundwater quality and quantity inEurope Report Copenhagen June 1999 available in httpreportseeaeuropaeugroundwater07012000enenviassrp199903
[4] EM Thurman DA Goolsby MT Meyer MS Mills ML Pomes DW Kolpin AReconnaissance study of herbicides and their metabolites in surface water of theMidwestern United States using immunoassay and gas chromatographymassspectrometry Environ Sci Technol 26 (1992) 2440ndash2447
[5] C Bellona JE Drewes P Xu G Amy Factors affecting the rejection of organicsolutes during NFRO treatmentmdasha literature review Water Res 38 (2004)2795ndash2809
[6] A Bhattacharya Remediation of pesticide-polluted waters through mem-branes Sep Purif Rev 35 (2006) 1ndash38
[7] KM Agbekodo B Legube S Dard Atrazine and simazine removal mechanismsby nanofiltration influence of natural organic matter concentration Water Res30 (1996) 2535ndash2542
[8] P Berg G Hagmeyer R Gimbel Removal of pesticides and other micropollu-tants by nanofiltration Desalination 113 (1997) 205ndash208
[9] EC Devitt F Ducellier P Coumlteacute MR Wiesner Effects of natural organic mat-ter and the raw water matrix on the rejection of atrazine by pressure-drivenmembranes Water Res 32 (1998) 2563ndash2568
[10] R Boussahel S Bouland KM Moussaoui A Montiel Removal of pesticideresidues in water using the nanofiltration process Desalination 132 (2000)205ndash209
[11] Y Zhang B Van der Bruggen GX Chen L Braeken C Vandecasteele Removalof pesticides by nanofiltration effect of the water matrix Sep Purif Technol38 (2004) 163ndash172
[12] EC Devitt MR Wiesner Dialysis investigations of atrazinendashorganic matterinteractions and the role of a divalent metal Environ Sci Technol 32 (1998)232ndash237
[13] SK Dalton JA Brant MR Wiesner Chemical interactions between dissolvedorganic matter and low-molecular weight organic compounds impacts onmembrane separation J Membr Sci 266 (2005) 30ndash39
[14] NA Kulikova IV Perminova Binding of atrazine to humic substances fromsoil peat and coal related to their structure Environ Sci Technol 36 (2002)3720ndash3724
[15] RL Wershaw The importance of humic substancendashmineral particle complexesin the modeling of contaminant transport in sedimentndashwater systems in RABaker (Ed) Organic Substances and Sediments in Water Humics and SoilsLewis Publishers Chelsea MI 1991 pp 23ndash34
[16] SM Schrap A Opperhuizen Relationship between bioavailability andhydrophobicity reduction of the uptake of organic chemicals by fish due tothe sorption on particles Environ Toxicol Chem 9 (1990) 715ndash724
[17] E Barriuso M Schiavon F Andreux JM Portal Localization of atrazine non-extractable (bound) residues in soil size fractions Chemosphere 12 (1991)1131ndash1140
[18] L Loiseau E Barriuso Characterization of the atrazinersquos bound (nonextractable)residues using fractionation techniques for soil organic matter Environ SciTechnol 36 (2002) 683ndash689
[19] G Sposito L Martin-Neto A Yang Atrazine complexation by soil humic acidsJ Environ Qual 25 (1996) 1203ndash1209
[20] KV Plakas AJ Karabelas Membrane retention of herbicides from single andmulti-solute media the effect of ionic environment J Membr Sci 320 (2008)325ndash334
[21] V Freger J Gilron S Belfer TFC polyamide membranes modified by graftingof hydrophilic polymers an FT-IRAFMTEM study J Membr Sci 209 (2002)283ndash292
[22] P Xu JE Drewes Viability of nanofiltration and ultra-low pressure reverseosmosis membranes for multi-beneficial use of methane produced water SepPurif Technol 52 (2006) 67ndash76
[23] Z Knez A Rizner-Hras K Kokot D Bauman Solubility of some solid triazineherbicides in supercritical carbon dioxide Fluid Phase Equilibria 152 (1998)95ndash108
[24] Weed Science Society of America (WSSA) Herbicide Handbook 7th ed 1994[25] B Van der Bruggen J Schaep W Maes D Wilms C Vandecasteele Nanofiltra-
tion as a treatment method for the removal of pesticides from ground watersDesalination 117 (1998) 139ndash147
[26] KN Reddy MA Locke Molecular properties as descriptors of octanol-waterpartition coefficients of herbicides Water Air Soil Pollut 86 (1996) 389ndash405
[27] IHSS website httpwwwihssgatechedu[28] F Flores-Ceacutespedes M Fernaacutendez-Peacuterez M Villafranca-Saacutenchez E Gonzaacutelez-
Pradas Cosorption study of organic pollutants and dissolved organic matter ina soil Environ Pollut 142 (2006) 449ndash456
[29] R Beckett Z Jue JC Giddings Determination of molecular weight distribu-tions of fulvic and humic acids using flow field-flow fractionation Environ SciTechnol 21 (1987) 289ndash295
[30] PM Reid AE Wilkinson E Tipping MN Jones Determination of molecularweights of humic substances by analytical (UV scanning) ultracentrifugationGeochim Cosmochim Acta 54 (1990) 131ndash138
[31] S Lee B Kwon M Sun J Cho Characterizations of NOM included in NF and UFmembrane permeates Desalination 173 (2005) 131ndash142
[32] MN Jones ND Bryan Colloidal properties of humic substances Adv ColloidInterf Sci 78 (1998) 1ndash48
[33] KV Plakas AJ Karabelas T Wintgens T Melin A study of selected herbicidesretention by nanofiltration membranesmdashthe role of organic fouling J MembrSci 284 (2006) 291ndash300
[34] APHA-AWWA-WPCF Standard Methods for the Examination of Water andWastewater 17th ed Port City Press Baltimore MD 1989 pp 567ndash569
[35] R Artinger G Buckau JI Kim S Geyer Characterization of groundwater humicand fulvic acids of different origin by GPC with UVVis and fluorescence detec-tion Fresenius J Anal Chem 364 (1999) 737ndash745
[36] AI Schaumlfer N Andritsos AJ Karabelas EMV Hoek R Schneider M Nys-troumlm Chapter 8 Fouling in nanofiltration in AI Schaumlfer AG Fane TDWaite (Eds) Nanofiltration Principles and Applications Elsevier Oxford UK2005 pp 173ndash174
[37] I Franco L Catalano M Contin M De Nobili Interaction between organicmodel compounds and pesticides with water-soluble soil humic substancesActa Hydrochim Hydrobiol 29 (2001) 88ndash99
[38] AA Helal DM Imam SM Khalifa HF Aly Interaction of pesticides with humiccompounds and their metal complexes Radiochemistry 48 (4) (2006) 419ndash425
[39] MG Browman G Chester Fate of Pollutants in the Air and Water EnvironmentWiley New York 1977 p 50
[40] LD Nghiem AI Schaefer M Elimelech Nanofiltration of hormone mimickingtrace organic contaminants Sep Sci Technol 40 (2005) 2633ndash2649
[41] R Celis J Cornejo MC Hermosin WC Koskinen Sorptionndashdesorption ofatrazine and simazine by model soil colloidal components Soil Sci Soc AmJ 61 (1997) 436ndash443
100 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
[42] SW Runge KR Shelton SA Melton WM Moran Maintaining the ionic per-meability of a cellulose ester membrane J Biochem Biophys Methods 64(2005) 200ndash206
[43] R Boussahel A Montiel M Baudu Effects of organic and inorganic matter onpesticide rejection by nanofiltration Desalination 145 (2002) 109ndash114
[44] Z Wang DS Gamble CH Langford Interaction of atrazine with Laurentianhumic acid Anal Chim Acta 244 (1991) 135ndash143
[45] C Jucker MM Clark Adsorption of aquatic humic substances on hydrophobicultrafiltration membranes J Membr Sci 97 (1994) 37ndash52
KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100 99
fouling process more severe [45] On the other hand the relatively
smaller adsorption rates of NOM in the absence of calcium result
in less fouled membranes A similar trend is also observed in the
case of the other three humic substances used in this study Among
the three membranes used XLE exhibits in the absence of calcium
the highest adsorption rates for all four humic substances This is
explained by the relatively high hydrophobicity and the low surface
charge that characterize the XLE membrane surface as previously
explained
4 Concluding remarks
The influence of humic substances on retention of two tri-
azines (atrazine and prometryn) by NFULPRO membranes is rather
complicated and involves combinations of solutendashmembrane and
solutendashsolute interactions In summary the retention of triazines
depends on the type of the humic material present in the feed
water In most cases the combined nanofiltration of triazines and
naturally occurring humic substances facilitates the formation of
complexes with triazines which in turn enhance their removal by
means of membrane filtration This complexation appears to be
related not to the characteristic acidity (phenolic carboxylic) of
the HS used but rather to their molecular conformation More
specifically a preferential binding is observed between triazines
and low molecular weight fractions of humic compounds (espe-
cially of FA and TA) which results in higher retention values for the
two triazines Under all conditions tannic acid exhibits the greatest
effect on triazine retention among the four standard HS compounds
used leading to an almost complete removal of the two triazines
(95ndash100) for all three membranes tested
The removal of triazines is improved in the presence of calcium
which tends to enhance the interaction between HS and triazines
Moreover triazine retention is increased with increasing HS con-
centration to a degree depending on the type of HS Additionally
triazine retention is apparently also affected by the interactions
between the organics and the membrane surface which in turn
may alter triazine retention These results illustrate the complex-
ity of the triazine adsorption and retention mechanisms in the
presence of humic substances that needs further investigation for
improved understanding Moreover regarding triazine removal by
nanofiltration it is of particular interest to distinguish the effect
of the HSndashtriazine interactions from that of membrane HS fouling
Research in this direction is carried out in this laboratory
References
[1] DP Birardar AL Rayburn Chromosomal damage induced by herbicide con-tamination at concentrations observed in public water supplies J Environ Qual24 (1995) 1222ndash1225
[2] IK Konstantinou DG Hela TA Albanis The status of pesticide pollution insurface waters (rivers and lakes) of Greece Part I Review on occurrence andlevels Environ Pollut 141 (3) (2006) 555ndash570
[3] European Environment Agency (EEA) Groundwater quality and quantity inEurope Report Copenhagen June 1999 available in httpreportseeaeuropaeugroundwater07012000enenviassrp199903
[4] EM Thurman DA Goolsby MT Meyer MS Mills ML Pomes DW Kolpin AReconnaissance study of herbicides and their metabolites in surface water of theMidwestern United States using immunoassay and gas chromatographymassspectrometry Environ Sci Technol 26 (1992) 2440ndash2447
[5] C Bellona JE Drewes P Xu G Amy Factors affecting the rejection of organicsolutes during NFRO treatmentmdasha literature review Water Res 38 (2004)2795ndash2809
[6] A Bhattacharya Remediation of pesticide-polluted waters through mem-branes Sep Purif Rev 35 (2006) 1ndash38
[7] KM Agbekodo B Legube S Dard Atrazine and simazine removal mechanismsby nanofiltration influence of natural organic matter concentration Water Res30 (1996) 2535ndash2542
[8] P Berg G Hagmeyer R Gimbel Removal of pesticides and other micropollu-tants by nanofiltration Desalination 113 (1997) 205ndash208
[9] EC Devitt F Ducellier P Coumlteacute MR Wiesner Effects of natural organic mat-ter and the raw water matrix on the rejection of atrazine by pressure-drivenmembranes Water Res 32 (1998) 2563ndash2568
[10] R Boussahel S Bouland KM Moussaoui A Montiel Removal of pesticideresidues in water using the nanofiltration process Desalination 132 (2000)205ndash209
[11] Y Zhang B Van der Bruggen GX Chen L Braeken C Vandecasteele Removalof pesticides by nanofiltration effect of the water matrix Sep Purif Technol38 (2004) 163ndash172
[12] EC Devitt MR Wiesner Dialysis investigations of atrazinendashorganic matterinteractions and the role of a divalent metal Environ Sci Technol 32 (1998)232ndash237
[13] SK Dalton JA Brant MR Wiesner Chemical interactions between dissolvedorganic matter and low-molecular weight organic compounds impacts onmembrane separation J Membr Sci 266 (2005) 30ndash39
[14] NA Kulikova IV Perminova Binding of atrazine to humic substances fromsoil peat and coal related to their structure Environ Sci Technol 36 (2002)3720ndash3724
[15] RL Wershaw The importance of humic substancendashmineral particle complexesin the modeling of contaminant transport in sedimentndashwater systems in RABaker (Ed) Organic Substances and Sediments in Water Humics and SoilsLewis Publishers Chelsea MI 1991 pp 23ndash34
[16] SM Schrap A Opperhuizen Relationship between bioavailability andhydrophobicity reduction of the uptake of organic chemicals by fish due tothe sorption on particles Environ Toxicol Chem 9 (1990) 715ndash724
[17] E Barriuso M Schiavon F Andreux JM Portal Localization of atrazine non-extractable (bound) residues in soil size fractions Chemosphere 12 (1991)1131ndash1140
[18] L Loiseau E Barriuso Characterization of the atrazinersquos bound (nonextractable)residues using fractionation techniques for soil organic matter Environ SciTechnol 36 (2002) 683ndash689
[19] G Sposito L Martin-Neto A Yang Atrazine complexation by soil humic acidsJ Environ Qual 25 (1996) 1203ndash1209
[20] KV Plakas AJ Karabelas Membrane retention of herbicides from single andmulti-solute media the effect of ionic environment J Membr Sci 320 (2008)325ndash334
[21] V Freger J Gilron S Belfer TFC polyamide membranes modified by graftingof hydrophilic polymers an FT-IRAFMTEM study J Membr Sci 209 (2002)283ndash292
[22] P Xu JE Drewes Viability of nanofiltration and ultra-low pressure reverseosmosis membranes for multi-beneficial use of methane produced water SepPurif Technol 52 (2006) 67ndash76
[23] Z Knez A Rizner-Hras K Kokot D Bauman Solubility of some solid triazineherbicides in supercritical carbon dioxide Fluid Phase Equilibria 152 (1998)95ndash108
[24] Weed Science Society of America (WSSA) Herbicide Handbook 7th ed 1994[25] B Van der Bruggen J Schaep W Maes D Wilms C Vandecasteele Nanofiltra-
tion as a treatment method for the removal of pesticides from ground watersDesalination 117 (1998) 139ndash147
[26] KN Reddy MA Locke Molecular properties as descriptors of octanol-waterpartition coefficients of herbicides Water Air Soil Pollut 86 (1996) 389ndash405
[27] IHSS website httpwwwihssgatechedu[28] F Flores-Ceacutespedes M Fernaacutendez-Peacuterez M Villafranca-Saacutenchez E Gonzaacutelez-
Pradas Cosorption study of organic pollutants and dissolved organic matter ina soil Environ Pollut 142 (2006) 449ndash456
[29] R Beckett Z Jue JC Giddings Determination of molecular weight distribu-tions of fulvic and humic acids using flow field-flow fractionation Environ SciTechnol 21 (1987) 289ndash295
[30] PM Reid AE Wilkinson E Tipping MN Jones Determination of molecularweights of humic substances by analytical (UV scanning) ultracentrifugationGeochim Cosmochim Acta 54 (1990) 131ndash138
[31] S Lee B Kwon M Sun J Cho Characterizations of NOM included in NF and UFmembrane permeates Desalination 173 (2005) 131ndash142
[32] MN Jones ND Bryan Colloidal properties of humic substances Adv ColloidInterf Sci 78 (1998) 1ndash48
[33] KV Plakas AJ Karabelas T Wintgens T Melin A study of selected herbicidesretention by nanofiltration membranesmdashthe role of organic fouling J MembrSci 284 (2006) 291ndash300
[34] APHA-AWWA-WPCF Standard Methods for the Examination of Water andWastewater 17th ed Port City Press Baltimore MD 1989 pp 567ndash569
[35] R Artinger G Buckau JI Kim S Geyer Characterization of groundwater humicand fulvic acids of different origin by GPC with UVVis and fluorescence detec-tion Fresenius J Anal Chem 364 (1999) 737ndash745
[36] AI Schaumlfer N Andritsos AJ Karabelas EMV Hoek R Schneider M Nys-troumlm Chapter 8 Fouling in nanofiltration in AI Schaumlfer AG Fane TDWaite (Eds) Nanofiltration Principles and Applications Elsevier Oxford UK2005 pp 173ndash174
[37] I Franco L Catalano M Contin M De Nobili Interaction between organicmodel compounds and pesticides with water-soluble soil humic substancesActa Hydrochim Hydrobiol 29 (2001) 88ndash99
[38] AA Helal DM Imam SM Khalifa HF Aly Interaction of pesticides with humiccompounds and their metal complexes Radiochemistry 48 (4) (2006) 419ndash425
[39] MG Browman G Chester Fate of Pollutants in the Air and Water EnvironmentWiley New York 1977 p 50
[40] LD Nghiem AI Schaefer M Elimelech Nanofiltration of hormone mimickingtrace organic contaminants Sep Sci Technol 40 (2005) 2633ndash2649
[41] R Celis J Cornejo MC Hermosin WC Koskinen Sorptionndashdesorption ofatrazine and simazine by model soil colloidal components Soil Sci Soc AmJ 61 (1997) 436ndash443
100 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
[42] SW Runge KR Shelton SA Melton WM Moran Maintaining the ionic per-meability of a cellulose ester membrane J Biochem Biophys Methods 64(2005) 200ndash206
[43] R Boussahel A Montiel M Baudu Effects of organic and inorganic matter onpesticide rejection by nanofiltration Desalination 145 (2002) 109ndash114
[44] Z Wang DS Gamble CH Langford Interaction of atrazine with Laurentianhumic acid Anal Chim Acta 244 (1991) 135ndash143
[45] C Jucker MM Clark Adsorption of aquatic humic substances on hydrophobicultrafiltration membranes J Membr Sci 97 (1994) 37ndash52
100 KV Plakas AJ Karabelas Journal of Membrane Science 336 (2009) 86ndash100
[42] SW Runge KR Shelton SA Melton WM Moran Maintaining the ionic per-meability of a cellulose ester membrane J Biochem Biophys Methods 64(2005) 200ndash206
[43] R Boussahel A Montiel M Baudu Effects of organic and inorganic matter onpesticide rejection by nanofiltration Desalination 145 (2002) 109ndash114
[44] Z Wang DS Gamble CH Langford Interaction of atrazine with Laurentianhumic acid Anal Chim Acta 244 (1991) 135ndash143
[45] C Jucker MM Clark Adsorption of aquatic humic substances on hydrophobicultrafiltration membranes J Membr Sci 97 (1994) 37ndash52
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