Journal of Membrane Science 336 (2009) 86ndash100

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Journal of Membrane Science

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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

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

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

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

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

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

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

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

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

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

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