Separation and Purification Technology 80 (2011) 246–261

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Separation and Purification Technology

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A systematic study on triazine retention by fouled with humic substancesNF/ULPRO membranes

Konstantinos V. Plakas, Anastasios J. Karabelas ⇑Chemical Process Engineering Research Institute, Centre for Research and Technology – Hellas, P.O. 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 15 February 2011Received in revised form 3 May 2011Accepted 4 May 2011Available online 17 May 2011

Keywords:Water membrane nanofiltrationTriazine herbicides retentionOrganic foulingCake-layer resistance

1383-5866/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.seppur.2011.05.003

⇑ Corresponding author.E-mail address: karabaj@cperi.certh.gr (A.J. Karabe

a b s t r a c t

Naturally occurring organic compounds tend to form complexes with divalent cations and micro-pollutants, and to foul membrane surfaces; both phenomena have a significant effect on pollutant rejection.Previous study results show the significant influence of triazine herbicides complexation, with dissolvedhumic substances, on their rejection by NF/ULPRO membranes. The net effect of fouled membranes, on tri-azine retention, is systematically investigated herewith by comparing the performance of three types ofclean and fouled membranes and relating them to changes observed in membrane surface characteristics.Two typical triazines (atrazine and prometryn) and three well characterized humic substances (HS) areemployed. The results show that humic substances deposited on the membrane surfaces cause consider-able changes of their characteristics, including the contact angle and salt retention, which affect water per-meability and triazine retention. Specifically, a strong correlation is identified between the hydrophobicity/hydrophilicity of dissolved HS and the resistance to flow of the fouling layer, which affects the retention ofthe smaller-size triazines. This trend is related to the condition of organic layers on the membrane. Gener-ally, relatively loose fouling layers on the membranes are associated with reduced triazine retention. How-ever, rather dense fouling layers formed by complexes of HS with calcium exhibit significant flux declineand an improved sieving effect on triazines. Moreover, tight and hydrophobic membranes display signifi-cant changes of triazine retention when fouled by HS of increased hydrophobicity. On the contrary, porousand hydrophilic membranes display significant changes of triazine retention only when fouled by HS ofreduced hydrophobicity. The new results highlight the need for good knowledge of the properties charac-terizing the organic matter present in natural waters, before their treatment process is designed.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

The increasingly complex cocktail of anthropogenic organicchemicals detected at very small concentrations (i.e. at the levelof ppb) in water bodies, and consequently in drinking water sup-plies, constitutes a major threat to humans, since many of thesechemicals and their residues (e.g. hormones, polycyclic aromatichydrocarbons and organochlorine pesticides) are toxic with knownadverse health outcomes. The need for essentially complete re-moval of these compounds from water has increased the interestin membrane processes, such as nanofiltration (NF) and ultra-lowpressure reverse osmosis (ULPRO), which are considered efficientand cost effective treatment methods for potable water productionof high and stable quality.

A significant number of studies have been carried out on the reten-tion characteristics of pesticide active ingredients, pharmaceuticallyactive compounds (PhACs), polycyclic aromatic hydrocarbons (PAHs),polychlorinated biphenyls (PCBs), endocrine disrupting compounds

ll rights reserved.

las).

(EDCs) and for many other toxic organic substances, which have im-proved our understanding of the prevailing mechanisms and factorsaffecting membrane retention efficiency [1–10]. The majority of therelated experimental research so far is focused on the removal of or-ganic residues of crop protection products (pesticides) and theirmetabolites due to their widespread application in agriculture andtheir significant toxicity which has led authorities to take measuresto protect human health [11].

Laboratory research, pilot and industrial-scale activity show thatthe retention of pesticides residues varies from very good, by somemembranes (mainly tight NF and RO membranes), to moderate orlow removal by others (loose NF membranes). Recent reviewsindicate the importance of solute–solute and solute–membraneinteractions in membrane performance, which in turn is influencedby solute parameters (molecular size, charge and polarity),membrane properties (pore size, surface charge and hydrophobic-ity/hydrophilicity), feed water composition and process conditions[12,13]. The significant number of parameters affecting pesticideretention is characteristic of the complex interactions taking place,which can be further influenced by the changes occurring in mem-brane surface properties as a result of fouling. Indeed, the retention

Table 1Characteristics of membranes used in this study; manufacturer Dow (Filmtec).

NF270 NF90 XLE

MWCOa (Da) 200 200 <100Membrane pore sizeb (nm) 0.71 ± 0.14 0.55 ± 0.13 (0.82) (0.67)Contact anglec (�) 28 ± 2 62 ± 2 65 ± 2Zeta potentiald (mV) �21.6 �24.9 �3.2NaCl retentione (%) 66.4 99.5 95.9CaCl2 retentione (%) 77.1 98.8 97Na2SO4 retentione (%) 96.5 99.9 98.1Specific fluxf (L m�2 h�1 bar�1) 11.5 ± 0.3 5.8 ± 0.3 5.7 ± 0.4

a Molecular weight cut off, as reported by the manufacturer.b Values obtained from Ref. [30]. Values in the brackets obtained from Ref. [31].c Sessile drop contact angle measurements (G10 Krüss).d Measured at pH 7 and 30 lS/cm KCl solution (PAAR EKA-Electro Kinetic Ana-

lyzer RV. 4.0).e Filtration tests with 2 mM salt concentration (5 bar).f Determined at 5 bar (filtration of Milli-Q water at 25 �C).

K.V. Plakas, A.J. Karabelas / Separation and Purification Technology 80 (2011) 246–261 247

of organic micropollutants (EDCs, PhACs, pesticides, etc.) is deter-mined by electrostatic, steric and hydrophobic/hydrophilic solute–membrane interactions, which can be modified due to foulantsdepositing on the membrane surface.

The effect of fouling on organic micropollutant retention hasbeen the subject of extensive research in the past decade [14–23].Investigations on the influence of colloidal and/or organic foulingon retention of various trace organics suggest that two cases maybe distinguished, depending on the relative solute selectivities ofthe fouling layer and the membrane. First, if the membrane rejectssolutes better than the deposited layer, hindered back diffusion ofsolutes (by the fouling layer) would cause solute accumulation nearthe membrane surface. This enhanced concentration polarizationresults in greater concentration gradient across the membraneand, hence, an increase of solute permeation. Second, if solutes arerejected better by the deposited layer than the membrane, the foul-ing layer controls solute retention which tends to improve. Forexample, the severe decline observed [14] in the retention of twosteroid hormones due to accumulation of colloidal silica particlesat the RO membrane surface was attributed to the increasedhormone adsorption onto the membrane polymeric skin layer as aresult of the cake enhanced concentration polarization, which inturn facilitated their diffusion across the skin layer to the permeateside. The effect of cake-enhanced concentration polarization on theretention of 21 PhACs was also recently invoked by Verliefde et al.[22] to explain the observed retention variations depending on typeof fouling. Specifically, filtration of surface water, which was firstpretreated with anionic exchange resins, resulted in the depositionof a mainly colloidal fouling layer which led to increased flux declineand to significant changes in pharmaceuticals retention. On theother hand, retention by membranes fouled by natural organic mat-ter (surface water pretreated with ultrafiltration) or by untreatedsurface water exhibited smaller variation as a result of steric andelectrostatic effects [22].

The study by Nghiem and Hawkes [19] highlighted the signifi-cance of NF membrane pore size on fouling and indirectly on theretention of low MW pharmaceuticals, which was mainly attributedto pore restriction and cake enhanced concentration polarizationphenomena. In particular, the retention behavior of a very looseNF membrane was apparently dominated by pore restriction typeof fouling, while there was evidence of the cake enhanced concen-tration polarization effect with the smaller pore size NF membranes[19]. Experiments with NF membranes [18], pre-fouled by activatedsludge and landfill leachate, showed a marked reduction of reten-tion of the larger contaminants, and an increased retention ofsmaller MW contaminants (36 neutral trace plastic additives, EDCsand other organics tested). Fouling experiments using a microfil-tered secondary effluent [15] resulted in an increased adsorptioncapacity and reduced mass transport through partitioning anddiffusion in NF/RO membranes for several organic compounds, rep-resentative of emerging organic contaminants. Interestingly, mem-brane fouling by a mixture of polysaccharides, silicate colloids andorganic sulfonic acids (i.e. the major foulants in the microfilteredsecondary effluent) had little effect on retention by thin film com-posite RO membranes [15]. In a recent work by Yangali-Quintanillaet al. [23], membrane fouling by a hydrophilic anionic polysaccha-ride (sodium alginate) led to different retention behavior, depend-ing on the NF membrane used. Specifically, there was a significantreduction of retention for all 14 organic micropollutants used(including atrazine) when treated with a fouled NF200 membrane(a hydrophilic one); on the contrary, a fouled NF90 membrane (ahydrophobic one) exhibited negligible or increased retentionimprovement, depending on the hydrophobic/hydrophilic charac-ter of the organic micropollutants [23]. Similar observations weremade by Bellona et al. [20]. From the above studies it is evident thatmembrane fouling may significantly affect the retention of low MW

organic compounds depending mainly on foulants characteristics,membrane properties, and chemical composition of feed water.

It is well known that humic substances (HS) make up more than50% of the NOM in surface waters and are considered a major causeof NF fouling [24–27]. Moreover, low MW humic substances maystill be present in UF pretreated feed water [28], which can causeorganic fouling of the NF/ULPRO membranes used for the retentionof micropollutants. In preliminary work [16], the effect of fouling(by humic and fulvic acids) on pesticides retention by a relativelyporous NF membrane (NF270) was evident. The observed signifi-cant change of membrane performance due to organic foulingpointed to the need for a more detailed study of this topic. Thesame conclusion was reached in a recent study [29], where thecapacity of triazines for complexation with dissolved humic sub-stances could not fully account for the observed triazine retentionduring such solution filtration through NF/ULPRO membranes. Toclarify the net effect of organic fouling on both triazine and saltretention, and to obtain an improved understanding of the role ofhumic substances on triazine rejection by NF/ULPRO membranes,this systematic study was undertaken, involving a multi-step filtra-tion protocol, three well characterized humic substances (IHSSstandards), two triazine herbicides (atrazine and prometryn) andthree types of NF/ULPRO membranes. In the following, experimen-tal conditions are described first; experimental results and discus-sion are presented next.

2. Experimental work

2.1. Materials and methods

2.1.1. Membranes and their characterizationThree flat sheet type commercial membranes (Dow, Filmtec)

denoted as NF90, NF270 and XLE were used in this study. NF90and NF270 are nanofiltration membranes while XLE is describedby the manufacturer as extra low energy membrane (ULPRO). Theircharacteristic properties are summarized in Table 1. For the exper-iments, flat sheet membrane specimens with an active membranearea of 14.6 cm2 were employed. Before use, all membrane speci-mens were rinsed with tap water for several minutes and storedat 4 �C in a 0.75% Na2S2O5 aqueous solution (to suppress develop-ment of micro-organisms), which was regularly replaced. Prelimin-ary filtration tests with ultra-pure water and separate solutions ofthree different salts (2 mM NaCl or CaCl2 or Na2SO4, at 5 bar)showed that the rather porous and hydrophilic NF270 membraneexhibits the highest permeability among the three membranestested, as well as fairly high retention of charged ions. On the otherhand, the rather hydrophobic NF90 and XLE membranes exhibit

248 K.V. Plakas, A.J. Karabelas / Separation and Purification Technology 80 (2011) 246–261

similar permeability values and almost complete retention ofcharged ions. The relatively better desalting performance of NF90membrane, in contrast to XLE, is attributed to its greater negativesurface charge (measured at neutral pH), which does not greatlydiffer from the one characterizing the NF270 membrane (Table 1).

Membrane specimens were dried after the experiments at roomtemperature for at least 24 h prior to surface analysis. Membranehydrophobicity was characterized by sessile drop contact anglemeasurements with ultra-pure water, using contact angle measur-ing equipment (Krüss G10, Hamburg). To account for differences ofsurface morphology at least five readings were taken at differentpositions across the samples. A JEOL scanning electron microscope(JSM-6300) with an ISIS energy dispersive X-ray spectrometer(ISIS-EDS, Oxford Instruments) were used to examine the morphol-ogy and the elemental composition of the fouled membranes,respectively.

2.1.2. TriazinesTwo herbicides belonging in the triazine family, i.e. atrazine and

prometryn, were selected for this study. Both herbicides were ofanalytical grade, purchased from Riedel de-Haën (Sigma–Aldrich):atrazine (purity 97.4%), prometryn (purity 99.2%). Information onthe chemical structures and the physicochemical properties ofthe triazines are included in Table 2. Both triazines are consideredas non-ionic, hydrophobic (log Kow >2) compounds that are moder-ately soluble in water (weak polar compounds). Between the two,prometryn is the largest molecule due to its branched structure,whereas its greater pKa value indicates a greater basicity in com-parison to atrazine.

Concentrations of the two triazines in feed, permeate and con-centration samples were determined using an Agilent TechnologiesModel 7890A gas chromatograph system interfaced with an Agi-lent Technologies Model 5975C mass-selective detector (MSD).Pre-concentration of the water samples and transfer to the organicphase was based on off-line solid phase extraction (SPE) whichtook place prior to the chromatographic analysis. Simazine (Riedelde-Haën, purity 99.9%), a triazine compound similar to atrazineand prometryn, was used as internal standard in order to assessthe recovery of the two triazines during the SPE-GC-MSD analyses.Recoveries from pure triazine solutions varied between 82% and112%, while the limit of quantification (LOQ) was 1 lg/L for bothtriazine compounds, i.e. well above the lower concentrationsexpected in the permeate samples in the case of �10 lg/L feed

Table 2Properties of the herbicides used in this work [32].

Herbicide Atrazine

Chemical structure

Molecular formula C8H14ClN5

Chemical class Cl-TriazineMolecular weight (Da) 215.69Molecular sizea (nm) 0.788Log Kow 2.68Aqueous solubility (mg/L) 33Dipole momentb (debye) 2.460pKa (20 �C) 1.7

a Obtained from Ref. [33].b Obtained from Ref. [34].

concentrations and a SPE concentration factor equal to 25 (from50 mL to a final volume of 2 mL). The chromatographic conditionsand the SPE protocol followed are described in detail in previouspublications [16,29].

2.1.3. Humic substancesThree types of water born humic substances (HS), purchased

from the International Humic Substances Society (IHSS, Universityof Minnesota, SWC Dept.), were used in this investigation, whichare denoted as Suwannee River humic acid (HA), fulvic acid (FA)and natural organic matter (NOM). They are well characterized ref-erence materials of known origin, widely used in research. TheIHSS humic and fulvic acids contain mainly hydrophobic organicacids, while the NOM sample contains not only hydrophobic andhydrophilic acids but also other soluble substances that are presentin natural waters [35].

The characteristic chemical parameters of the three humic sub-stances (HS) employed are summarized in Table 3, while the aver-age molecular masses reported in the literature are presented inTable 4. The distribution of carbon among different functionalgroups of the three HS is shown in Table 5. Humic acids (HA) havethe highest MW among the three HS, followed by NOM and FA.However, the carboxyl content of HS seems to be inversely relatedto the MW, since the relatively smaller FA exhibits higher carbox-ylic and total acidity, and therefore greater charge. UV absorbanceat particular wavelengths shows that HA is more aromatic thanNOM, and NOM slightly more aromatic than FA, which is in linewith the 13C NMR estimates of carbon distribution in the threeIHSS samples (Table 5).

Measurement of the HS concentration (more specifically of thearomaticity) was performed in a UV–Vis spectrophotometer (UV-1700, Shimadzu, Japan) at 254 nm in the case of Suwannee RiverHA and NOM, and at 275 nm in the case of FA. Quality control ofthe data was achieved by means of calibration and repeated anal-yses according to the procedure described in a previous publication[29].

2.1.4. Feed solutionsAccording to the filtration protocol, three different feed solu-

tions were prepared; i.e. solutions of NaCl in ultra-pure water withconcentration 2 mM, solutions of atrazine–prometryn in ultra-purewater with concentration 10 lg/L for each triazine, and foulantsolutions of one humic substance in the presence or not of calcium

Prometryn

C10H19N5SS-Triazine241.35�3.0833�4.09

Table 3Acidicity and elemental composition of HS used in this study [35].

Humic substance Cat. no Acidity (meq/g C) Elemental composition

Carboxylic Phenolic Total Ca Sa H2Ob

HA IHSS 2S101H 9.13 3.72 12.85 52.63 0.54 20.4FA IHSS 2S101F 11.87 2.84 14.01 52.34 0.46 16.9NOM IHSS 1R101N 9.85 3.94 13.79 48.80 0.02 8.15

a Elemental composition in %(w/w) of a dry, ash-free sample.b %(w/w) of H2O in the air-equilibrated sample (a function of relative humidity).

Table 4Molecular mass data of IHSS organics employed in this study.

Humic substance Technique [ref.] �Mna �Mw

b �Mw�Mn

c

Suwannee River HA FFF [36] Ultracentrifugation [37] 1580�

43904260 ± 280

2.78�

Suwannee River FA FFF [36]Ultracentrifugation [37]

1150�

19101460 ± 80

1.66�

Suwannee River NOM HPSEC [38] 1760 2360 1.34

a Number average molecular weight.b Weight average molecular weight.c Polydispersivity index; Suwannee River HA has a much larger polydispersivity compare to FA.

Table 513C NMR estimates of carbon distribution in IHSS samples used in this study [35].

Humic substance Carbonyl220–190 ppm

Carboxyl190–165 ppm

Aromatic165–110 ppm

Acetal110–90 ppm

Heteroaliphatic90–60 ppm

Aliphatic60–0 ppm

HA, 2S101H 6 15 31 7 13 29FA, 2S101F 5 17 22 6 16 35NOM, 1R101N 8 20 23 7 15 27

K.V. Plakas, A.J. Karabelas / Separation and Purification Technology 80 (2011) 246–261 249

ions. In the latter case, model solutions of the three selected humicsubstances were prepared with ultra-pure water, of concentration10 or 20 mg/L, at neutral pH. The HS were obtained in powder formand used without further purification as the bound iron and ashcontents were very low. The calcium ion content in the corre-sponding foulant solutions was fixed at 40 mg/L (�1 mM) by add-ing calcium chloride (J.T. Baker).

Standard stock solutions of the two triazines (of concentration100 mg/L) were prepared in high-performance liquid chromatogra-phy grade methanol and stored at 4 �C. The feed herbicide solu-tions were prepared from ultra-pure water by dilution of stocksolution at a level of 10 lg/L for each triazine. Finally, all feed solu-tions were neutralized (pH 7.0) prior to their introduction in thestirred cells. The ultra-pure water used (resistivity >18 MXcm)for solution preparation was obtained from a Milli-Q purificationsystem (Millipore, Milford, MA, USA).

2.2. Experimental set-up and filtration protocol

Tests were conducted in the batch mode (dead-end experi-ments) using an experimental set-up, comprised of three highpressure filtration cells (volume of 300 mL each) made of stainlesssteel which does not allow adsorption of the two herbicides [7].The three test cells were connected to a nitrogen cylinder to im-pose/control a constant filtration pressure and were operatedsimultaneously to assess the reproducibility and accuracy of theresults. For mixing and minimization of concentration polarizationphenomena, the test cells were equipped with Teflon-coated mag-netic stirring elements, rotating at a rate of 250 rpm (measuredwith an Extech Instruments Model 461893 digital photo tachome-ter). Electronic balances connected to PCs were used to monitorpermeate fluxes. The temperature of water in the test cells was

kept constant at 25 ± 0.2 �C by a water cooling system. A newmembrane specimen was used in each filtration test.

A filtration protocol was designed to determine the impact ofmembrane fouling by humic substances on triazine and salt reten-tion, as schematically shown in Fig. 1. First, virgin membrane spec-imens are placed in the stirred cells and rinsed with Milli-Q waterfor 1 h at a transmembrane pressure of 10 bar to remove the pre-servatives and to compact the membranes (step a). The initial purewater flux is measured by filtering Milli-Q water through themembrane at 5 bar for 30 min (step b). The next two filtrationsteps (c and d) involve retention experiments of NaCl and triazinesby the clean membranes at 5 bar. The purpose of using separatefeed solutions of NaCl and triazines is twofold; i.e. to avoid the ef-fect of the increased ionic strength on triazine retention, as previ-ously observed [7], and to permit a comparison between theretention values measured for salts and triazines with virgin andfouled membranes. Afterwards, the fouling procedure takes place,in which the foulant solution is filtered in three sequential cycles(e–g). Specifically, a 200 mL foulant solution is filtered at 5 bar un-til 100 mL of permeate are collected (50% recovery). After the firstand second filtration, the permeate is returned to the stirred cell, sothat fluid is filtered three times. Then, the final permeate and con-centrate samples are collected to determine overall retention. Thispermeate recycle procedure was adopted from Schäfer [39] andwas considered appropriate for fouling studies in the case ofbatch-type filtration experiments, to achieve stabilization of theorganic deposits on the membrane surface. Afterwards, the reten-tion efficiency of fouled membranes is tested by filtering triazineand NaCl solutions in two separate cycles (h and i), at 5 bar. Finally(step j), pure water flux was measured at 5 bar for 30 min to deter-mine the final flux decline after the termination of all filtrationcycles, for comparison with the initial pure water flux (step b).

0

20

40

60

80

100

120

0 100 200 300 400 500 600 700 800 900Filtration time (min)

Flux

(L/m

2 h)

a

b d

ce f g h i j

Fig. 1. Flux as a function of filtration time according to the filtration protocolimplemented in the present study; experiment with NF270 membrane: (a)membrane conditioning at 10 bar, (b) initial pure water flux measurement at5 bar, (c) filtration of 2 mM NaCl solution at 5 bar, (d) filtration of atrazine–prometryn solution (10 lg/L each) at 5 bar, (e) first filtration cycle of the foulantsolution (10 mg/L HA + 1 mM Ca2+) at 5 bar, (f) second filtration cycle of the samefoulant solution at 5 bar, (g) third filtration cycle of the same foulant solution at5 bar, (h) filtration of atrazine–prometryn solution (10 lg/L each) at 5 bar, (i)filtration of 2 mM NaCl solution at 5 bar, (j) final pure water flux measurement at5 bar.

250 K.V. Plakas, A.J. Karabelas / Separation and Purification Technology 80 (2011) 246–261

The evolution of flux as a function of filtration time, for allmembranes and materials used, followed a similar pattern withthe one presented in Fig. 1. More specifically, the differences ob-served between the initial pure water flux (straight line in Fig. 1)and the fluxes in each filtration step can be interpreted as follows:

Step c: NaCl-solution filtration; some flux reduction due to theincreased osmotic pressure of the rejected ions (concentrationpolarization) and/or due to ion adsorption to membranes.

Step d: Herbicide-solution filtration; relatively stable water fluxattributed to the negligible osmotic pressure of the herbicidesolution.

Step e: Flux decline mainly due to fouling in this first filtrationwith foulants.

Steps f, g: The similar fluxes measured during the second and thethird filtration cycles indicate that fouling has reached a pseudosteady-state condition.

Step h: The flux reduction (compared to step d) during filtrationof the herbicide solution is attributed to the humic layer existingon the membrane surface.

Step i: Similar flux with that during herbicide filtration (step h).The limited flux decline is attributed to the small solution osmoticpressure.

Step j: Flux decline after final pure water filtration.All filtration steps were performed with a feed solution of

200 mL; the tests performed with the triazine or the sodium chlo-ride solutions were carried out until 100 mL of permeate were col-lected (50% recovery). In the case of triazines, a feed sample, twopermeate samples (50 mL each) and the final concentrate were re-tained for chromatographic analysis. As in previous studies [7,29],the triazine feed solutions were initially stirred in the cells for 1 hwithout pressure. In this way, triazine adsorption on the mem-brane surface was considered to reach equilibrium, as shown bypreliminary adsorption experiments (data not presented here).[After termination of the filtration experiments, the membranespecimens were dried for subsequent analysis, while the test cellswere thoroughly washed with acetone and repeatedly rinsed withMilli-Q water.]

2.3. Calculation of main parameters

Experimental results are expressed in terms of the retention bythe membrane of the organic and inorganic compounds, which wascalculated according to the method described in a previous study[7]. In the present work, the retention of the two triazines is deter-mined from the so-called ‘‘stable’’ permeate sample (j = 2) whichcorresponds to 50% water recovery. The amount of the two tria-zines and of the humic substances adsorbed on the membrane sur-face (MHS) is determined from mass balances as follows:

Adsorbed organics ¼ 1�

Pj

i¼1Vpi

Cpiþ CrVr

Cf Vfð1Þ

where Cpi, Cr, Cf are the concentrations of specific organic species in

the permeate sample i, retentate and feed, respectively, whereas Vpi,

Vr , Vf are the respective volumes of permeate sample i, retentateand feed. In the case of the humic substances, Cp, Cr, Vp and Vr cor-respond to the final permeate and retentate samples, collected afterthe termination of the fouling phase (third filtration cycle; step g,Fig. 1).

The extent of membrane fouling is described by three parame-ters: percent flux reduction due to fouling (FR), mass of depositedhumic substances on the membrane surface (MHS) and the averagespecific resistance (a) of the fouling layer [40]. FR is determined asa percentage reduction of water flux right before (JWb) and after(JWa) the fouling phase (i.e. between steps d and h in Fig. 1):

FR ¼ JWb � JWa

JWb� 100 ð2Þ

The average specific resistance, a, of the fouling layer was deter-mined as follows. The fouling behavior during membrane filtrationcan be described by the common resistance-in-series model:

J ¼ 1A

dVdt¼ DP � Dpm

gðRm þ Rf Þð3Þ

where J is the permeate flux, A the active membrane area,ðDP � DpmÞ the effective transmembrane pressure, g the permeateviscosity, Rm the clean membrane resistance and Rf the total foulantresistance. In the case of calcium free solutions, the osmotic pres-sure difference, Dpm, is practically zero due to the negligible osmo-tic pressure caused by the selected humic substances. In the case ofNF/ULPRO filtration of mixed HS/Ca solutions, the effective trans-membrane pressure (DH-Dpm) is determined by estimating the os-motic pressure difference, including concentration polarizationeffects (see Appendix); the estimated osmotic pressure differenceis also negligible (of order 10�5 bar). Therefore, Eq. (3) can be sim-plified for the present experiments as:

J ¼ DPgðRm þ Rf Þ

ð4Þ

Under constant pressure, the fouling layer resistance, Rf, is re-lated to an average specific resistance a [40] through theexpression

Rf ¼aCb;HSV

Að5Þ

where Cb,HS is the bulk humic substance concentration. The value ofspecific resistance a is calculated by interpreting the fouling data int/V vs V plots, where the slope of the linear region of the filtrationcurve, k, is characteristic of a foulant layer formed on the membranesurface, under constant pressure (DH) [40], i.e.

k ¼ gaCb;HS

2DPA2 ð6Þ

It should be pointed out that the parameters FR, MHS and a areused in the present work to interpret triazine retention in relationto membrane fouling.

0

10

20

30

40

0 40 80 120 160 200

Filtration time (min)

Perm

eate

flux

(L/m

2 h)

1mM Ca2+

10mg/L HA

10mg/L HA+1mM Ca2+

Fig. 3. Permeate flux temporal variation for filtration of various feed waters withNF90 membrane.

K.V. Plakas, A.J. Karabelas / Separation and Purification Technology 80 (2011) 246–261 251

3. Results and discussion

3.1. Changes in membrane permeability due to fouling

Fig. 2 shows typical filtration data (plotted in terms of t/V vs V),obtained in repetitive filtration of a HA/Ca2+ solution through theNF90 membrane. Similar plots were obtained for all three mem-branes and dissolved humic substances, in the presence or not ofcalcium ions, from which the fouling layer specific resistances, a,were calculated. Specifically, resistance a was calculated in allcases from the slope of the linear region of the first filtration curve,which is typical of the cake formation process. The increased levelof the quantity (t/V) in the 2nd and 3rd filtration cycle (Fig. 2) isattributed to a greater amount of deposited HA and perhaps togreater compactness of the fouling layer. Furthermore, as is well-known (e.g. [41]), fouling layers in the presence of Ca2+ exhibitsmaller permeability compared to those forming in the absenceof calcium. This is clearly shown in data obtained with the NF90membrane (Fig. 3), as well as with the other two membranes, forsolutions with and without Ca2+.

Data on percent flux reduction (FR), adsorption of humic sub-stances (MHS) and of specific cake resistance (a) for the fouledmembranes are summarized in Table 6. These data suggest thatfouling is related to the type of foulant, the concentration of thehumic substance, and the presence of calcium ions in the feed solu-tion. In the absence of calcium ions, humic substances did notcause much fouling, as also observed in previous studies with UFand RO membranes; e.g. [40,41]. This is reflected in the slight fluxdecline and the rather small HS adsorption on the membrane sur-faces; the latter varies between 0 and 0.171 g/m2, depending onthe membrane and the humic substance type. Specifically, NF270and NF90 membranes appear to absorb HA to a larger extent thanNOM and FA, while XLE membranes seem to be more susceptibleto fulvic acids fouling. These differences in organic adsorption arelikely related to the characteristic acidity and aromaticity of thethree Suwannee River substances (Tables 3 and 5), since the morearomatic and, therefore, hydrophobic humic acids are more sus-ceptible to hydrophobic interactions with the membranes, espe-cially with the hydrophobic ones (NF90). Moreover, the loweracidity of the HA and NOM compounds is likely leading to reducedelectrostatic repulsion by the negatively charged membranes, thuspromoting adsorption. On the other hand, very small interactionslikely occur between FA and the two NF membranes due to thesmaller aromaticity and the greater acidity of the fulvic acid mole-

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 20 40 6

Cumulative

t/V (m

in/m

L)

Fig. 2. Filtration data of HA plus calcium solutions plotted as t/V versus V for three sequPressure 5 bar, pH �7, stirring rate 250 rpm.

cules. Specifically, the higher negative charge of NF90 and NF270membranes renders them less susceptible to adsorption of nega-tively charged organics; this is not observed in the case of neutrallycharged XLE membrane.

The very significant effect of calcium ions on organic fouling(reflected in the data of Table 6) has been well documented inthe literature [24,40–45], where the increased flux decline isattributed to two different mechanisms. The first is related to thecharge neutralization capacity of the calcium ions, which bind spe-cifically through complex formation with the acidic functionalgroups (predominantly carboxylic) of the humic materials. Theconcomitant charge reduction of the humic compounds is consid-ered [24] to result in the formation of small, coiled humic macro-molecules which are readily deposited on the membrane surface,forming quite compact organic layers. The second mechanism isrelated to the unique intermolecular bridging capability of calciumions which can induce the arrangement of humic acid moleculesinto a ‘‘cross-linked’’ structure in the fouling layer, rendering itvery compact [44]. Moreover, calcium ions may directly link thehumic molecules with the membrane surface, leading to a strongand highly resistant (to mechanical and hydrodynamic forces)fouling layer. It is interesting to note that the specific cake resis-tances obtained in dead-end experiments [40] with UF membranesand humic acids (from Aldrich) are in substantial agreement withthe results of the present NF experiments.

0 80 100 120

volume, V (mL)

1st cycle

2nd cycle

3rd cycle

o Experimental data--- Linear curve fitting

ential cycles (NF90 membrane). Feed solution composition 10 mg/L HA, 1 mM Ca2+.

Table 6Humic substance deposition (MHS), flux reduction (FR%) and specific cake resistance (a) of fouled NF270, NF90 and XLE membranes (pressure 5 bar, pH �7).

Foulant solution(mg/LHS/mg/LCa2+)

NF270 NF90 XLE

a(�1015 m/kg)

MHS

(g/m2)FR (%)a a

(�1015 m/kg)MHS

(g/m2)FR (%)a a

(�1015 m/kg)MHS

(g/m2)FR (%)a

HA/Ca2+ 10/0 2.9 0.075 4.1 (1.1) 4.3 0.027 3.6 (1.2) 4.3 0.021 3.5 (11.1)20/0 4.1 0.158 7.9 (7.7) 16.0 0.082 10.7 (9.2) 8.6 0.075 9.6 (16.1)

10/40 10.0 0.301 17.3 (14.5) 39.0 0.158 30.1 (27.4) 36.0 0.219 36.1 (25.8)20/40 16.0 0.781 22.4 (18.3) 32.0 0.144 36.6 (19.7) 27.0 0.151 32.3 (22.0)

FA/Ca2+ 10/0 �b 0.0 1.1 (1.9) 8.6 0.068 9.7 (12.5) 5.8 0.082 5.7 (9.6)20/0 0.7 0.034 3.6 (3.1) 14.0 0.082 7.3 (9.3) 20.0 0.171 6.8 (10.3)

10/40 4.3 0.062 8.2 (8.9) 16.0 0.096 17.0 (19.7) 10.0 0.116 20.1 (17.8)20/40 10.0 0.151 20.3 (16.0) 17.0 0.096 18.8 (14.5) 26.0 0.212 29.5 (20.3)

NOM/Ca2+ 10/0 �b 0.0 1.4 (4.4) 2.9 0.014 6.1 (8.7) 13.0 0.068 11.7 (15.8)20/0 1.4 0.010 6.8 (5.5) 12.0 0.096 11.9 (14.6) 12.0 0.055 14.0 (17.5)

10/40 4.3 0.048 9.2 (5.4) 12.0 0.082 10.9 (17.0) 15.0 0.197 24.4 (21.3)20/40 7.2 0.068 16.6 (15.9) 29.0 0.253 25.3 (19.7) 13.2 0.103 19.6 (16.6)

a Values in the brackets describe the pure water flux reduction; comparison between the stabilized permeate fluxes measured at the beginning and the end of theexperiment (steps a and j, Fig. 2).

b No positive slope in the respective t/V vs V curves (linear curve fittings).

252 K.V. Plakas, A.J. Karabelas / Separation and Purification Technology 80 (2011) 246–261

It is occasionally reported in the literature (e.g. [19]) that organ-ic fouling on NF membranes is well correlated with the membranepore size, meaning that flux decline is more severe on loose NFmembranes compared to the tighter ones. Such correlation was

0

10

20

30

40

50

60

70

80

NF270

Con

tact

Ang

le (o )

Virgin 10mg/L HA20mg/L FA 10mg/L NOM

0

10

20

30

40

50

60

70

80

072FN

Con

tact

Ang

le (o )

Virgin 10mg/L HA +10mg/L FA + 1mM Ca2+ 20mg/L FA +20mg/L NOM + 1mM Ca2+

a

b

Fig. 4. Membrane surface contact angle values; (a) after the filtration of humic substancions.

not observed in this study (Table 6). The present findings ratherunderpin the marked effect of the type of the organic species pres-ent in the raw water on NF fouling, which appears to be directly re-lated to the surface characteristics of the membranes.

NF90 XLE20mg/L HA 10mg/L FA20mg/L NOM

ELX09FN 1mM Ca2+ 20mg/L HA + 1mM Ca2+

1mM Ca2+ 10mg/L NOM + 1mM Ca2+

es alone, and (b) after the filtration of humic substances in the presence of calcium

0

10

20

30

40

50

60

70

80

MHA (g/m2)

NF270

NF90

XLE

0

10

20

30

40

50

60

70

80

0.1 0.15 0.2 0.25

NF270

NF90

XLE

0

10

20

30

40

50

60

70

80

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

0 0.05

0 0.05 0.1 0.15 0.2 0.25 0.3

NF270

NF90

XLE

Con

tact

Ang

le (o )

Con

tact

Ang

le (o )

Con

tact

Ang

le (o )

MFA (g/m2)

MNOM (g/m2)

a

b

c

Fig. 5. Membrane contact angle vs deposited mass surface density for (a) humicacids (MHA), (b) fulvic acids (MFA), and (c) NOM (MNOM).

K.V. Plakas, A.J. Karabelas / Separation and Purification Technology 80 (2011) 246–261 253

3.2. Characterization of fouled membranes

3.2.1. Contact angle measurementsThe mean contact angle values of the virgin and the fouled with

humic substances membranes are depicted in Fig. 4, in which the

Table 7Elemental composition (%) of fouled by humic substances (10 mg/L) and calcium ions (1 m

% Relative composition NF270 NF90

HA/Ca2+ FA/Ca2+ NOM/Ca2+ HA/Ca2

Al 0.96 � 1.46 0.41Si 2.06 0.70 1.96 12.46S 42.85 83.33 57.94 35.02Ca 15.79 3.71 6.71 5.18Fe 8.35 1.81 15.04 2.23Ni 4.46 � 0.71 1.07Cu 25.54 6.83 17.04 43.62Cr � 4.54 � �

bars correspond to the minimum and maximum values measuredfor each membrane specimen (from at least five readings). Thesedata suggest that the effect of fouling on membrane hydrophobic-ity is not the same for the three membranes. In the absence of cal-cium, humic substance filtration through the denser NF90 and XLEmembranes resulted in small changes of the membrane surfacecontact angle. However, the NF270 membrane became less hydro-philic due to fouling. On the other hand, the increased adsorptionof HS-calcium complexes on the membrane surfaces enhanced sig-nificantly the hydrophobicity of the loose NF270 membrane, butaltered slightly the hydrophobic character of the NF90 and XLEmembranes. An exception is observed in the case of the FA-calciumcomplexes which rendered the XLE membrane considerably morehydrophilic, and the NF270 membrane slightly less hydrophilicthan the virgin one.

The results shown in Fig. 4 are indicative of the adsorption ofhumic substances onto the membranes and consequently of the ef-fect of foulant physicochemical properties on membrane surfacecharacteristics. This is evident in Fig. 5, in which the membranecontact angle appears to be linearly related with adsorbed foulantmass; this effect is either positive or negative, possibly dependingon the number of the hydrophilic and/or hydrophobic functionalgroups characterizing the deposited molecules. For instance, thehydrophobic humic acids are related with increased values ofmembrane contact angles, while fulvic acids tend to reduce hydro-phobicity as a result of their limited number of hydrophobicgroups. A similar trend is also observed in the work performedby Xu et al. [46], where the slight decrease observed in contact an-gle values of the NF90 and XLE membranes could be attributed tothe hydrophilic character of the foulants used in their study.Although it is difficult to clearly isolate the effect of organic foulingon membrane hydrophobicity/hydrophilicity, the results of thepresent study support the commonly held opinion that the greaterthe extent of fouling, the greater the contribution of foulant prop-erties on membrane surface characteristics.

3.2.2. Energy dispersive X-ray spectroscopy measurements (X-ray EDS)The elemental composition of the fouled membranes is given in

Table 7. The major inorganic constituents of the foulants quantifiedby energy dispersive spectroscopy (EDS) are Cu, Fe, Si, and Ca,while the large values of %S are most likely due to the active layersupport (made of polysulfone). In support of this interpretation isthe low content of sulfur characterizing the three IHSS substances(Table 3), as well as the small thickness of the membrane activelayers (<100 nm) which may allow probing deep into the mem-brane. Therefore, one may suggest that the sulfur content (%S) ofHS-fouled membranes (determined through X-ray EDS and X-rayphotoelectron spectroscopy (XPS) measurements performed inprevious studies [22]), could serve as a qualitative indicator ofthe deposition of organic substances (especially of the hydrophobicones) on the membrane surface. For instance, the high %S in mem-branes fouled by fulvic acids and calcium ions may be indicative of

M) membranes, as determined by X-ray EDS.

XLE

+ FA/Ca2+ NOM/Ca2+ HA/Ca2+ FA/Ca2+ NOM/Ca2+

0.88 0.41 5.84 0.65 3.730.72 7.35 15.52 0.44 29.87

86.93 78.75 5.41 89.18 57.120.66 5.07 8.08 0.81 1.764.85 2.07 21.24 0.49 0.79� � 0.77 0.89 �8.75 7.71 35.48 7.95 8.83� 0.33 7.66 � �

254 K.V. Plakas, A.J. Karabelas / Separation and Purification Technology 80 (2011) 246–261

reduced fouling, which is in accord with the smaller flux decline(Table 6) and limited changes on the membrane surface morphol-ogy (as evidenced by SEM images not shown here).

3.3. Effect of fouling on salt retention

The influence of organic fouling on salt retention (expressed asconductivity ratio), for all humic substances and membranestested, is shown in Fig. 6. The retention values are plotted as thedifference in NaCl retention observed between the virgin and thefouled membranes (steps c and i in Fig. 1, respectively); positivevalues correspond to increased retention by the fouled mem-branes, compared to the virgin ones. The values in Fig. 6 corre-spond to the final measurements made after the recovery of 50%water as permeate (mean values). The error bars designate theminimum and maximum values measured for each membranespecimen; i.e. values obtained from triplicate experiments withdifferent membrane specimen of the same type.

The differences in salt retention vary between the three mem-branes, being more pronounced for the relatively loose NF270membrane. In general, significant improvement in membranedesalting efficiency was observed in the case of membranes fouledby calcium complexes of all foulants. Salt retention was greatly im-proved in the case of membranes fouled by HA-Ca2+, being greaterfor higher HA concentrations (20 mg/L). In the absence of calciumions, different behavior was observed in the case of the relativelyloose NF270 membrane; i.e. fouling by all HS used led to signifi-cantly reduced salt retention compared to the virgin membrane.Although it is difficult to isolate the contribution of organic foulingand membrane zeta potential on salt retention, the above resultssuggest that foulant deposits may significantly change the mem-brane desalting capacity as a result of modified charge, steric andhydrophobic effects.

Interestingly, the differences observed in the retention valuesappear to be related with the acidity properties of the adsorbedHS and specifically with their carboxylic acidity [23]. As shownin Fig. 7a, an increased carboxylic content of the adsorbed HS (cal-culations based on carbon composition, humic concentration andcarboxylic acidity of HS used) is related to increased salt retention.This trend is possibly related to both steric and charge effects, due

-6

-4

-2

0

2

4

6

8

10

12

10mg/L HA 20mg/L HA 10mg/L HA+ 1mMCa2+

20mg/L HA+ 1mMCa2+

10mg/L FA 20mg/L F

Diff

eren

ce in

NaC

l rej

ectio

n w

ith v

irgin

mem

bran

e (%

)

HA

Fig. 6. Difference in salt retention between fouled and virgin NF270, NF90 and XLE mretention, respectively. Rejection values for clean NF270, NF90 and XLE membranes: 66

to pore restriction and increased negative charge on the fouledmembrane surfaces, respectively. Specifically, the adsorbed car-boxylic acidity inside the membrane pores can lead to increased in-tra membrane repulsion (within the membrane pores) which maycause a reduction of the effective pore size, leading to reduced per-meability of the membranes (Fig 7b), and consequently to a betterretention of hydrated ionic species. Fig 7b indicates that this mech-anism may be involved in the case of rather porous NF270 mem-brane, for which the adsorbed carboxylic acidity appears to bepositively correlated with the pure water flux reduction, some-thing that is not clearly observed for the tight NF90 and XLE mem-branes. Moreover, as shown in Fig. 7a, the filtration of low acidityHS through NF270 tends to increase the ionic permeability, andconsequently reduce rejection. This is in accord with the expectedreduced steric effects of the loose NF270 membranes after the pre-liminary treatment with HS of low carboxylic acidity. On the otherhand, increased charge effects may be attributed to the adsorbedhumic acidity on the membrane surfaces, which possibly accountfor the increased NaCl rejection by the tight NF90 and XLE mem-branes; this is especially true for the neutrally charged XLEmembrane.

3.4. Effect of membrane fouling on triazine retention

Figs. 8–10 show the mean values of triazine retention by fouledwith humic substances, in the absence or presence of calcium ions,for NF270, NF90 and XLE membranes, respectively. The error barscorrespond to the minimum and maximum values obtained fromtriplicate experiments with different membrane specimen of thesame type. In the same figures, the adsorption capacity of thetwo triazines on the virgin and fouled membranes is also included(as a percentage of the initial feed concentration).

Triazine retention between fouled and virgin membranes variedfrom �34.8% to +17.1% in the case of NF270 membranes, from�28.0% to +19.5% in the case of NF90 membranes, and from�17.7% to +14.3% for the XLE membranes, depending on the typeof the humic substances and the presence or not of calcium ionsin the foulant solution. The corresponding range of the differencesobserved in triazine retention (51.9%, 47.5% and 32% for the NF270,NF90 and XLE membranes, respectively) indicates that the

A 10mg/L FA+ 1mMCa2+

20mg/L FA+1mMCa2+

10mg/LNOM

20mg/LNOM

10mg/LNOM +

1mM Ca2+

20mg/LNOM +

1mM Ca2+

NF270 NF90 XLE

FA

NOM

embranes. Negative and positive values correspond to a decreased and increased.4% ± 1.3%, 99.5% ± 1.8% and 95.9% ± 0.5%, respectively.

-6

-4

-2

0

2

4

6

8

0 1 2 3 4 5 6

Carboxylic acidity of adsorbed HS (meqx10-3)Diffe

renc

e in

NaC

l rej

ectio

n w

ith v

irgin

mem

bran

e (%

)

NF270

NF90

XLE

0

5

10

15

20

25

30

0 1 2 3 4 5 6

Carboxylic acidity of adsorbed HS (meqx10-3)

Pure

wat

er fl

ux re

duct

ion

(%)

NF270

NF90

XLE

a

b

Fig. 7. (a) Difference in salt retention between fouled and virgin membranes versus the estimated? total carboxylic acidity characterizing the adsorbed humic substances(rejection values for clean NF270, NF90 and XLE membranes: 66.4% ± 1.3%, 99.5% ± 1.8% and 95.9% ± 0.5%, respectively). (b) Pure water flux reduction versus the adsorbedhumic carboxylic acidity.

K.V. Plakas, A.J. Karabelas / Separation and Purification Technology 80 (2011) 246–261 255

influence of organic fouling on triazine retention is more signifi-cant for the more porous NF270 membrane. This is consistent withresults from previous studies [15,19] which suggest that the mag-nitude of the influence of membrane fouling on retention, eitherpositive or negative, decreases as the membrane pore size de-creases. It is interesting to note that the influence of organic foulingon triazine retention by the NF270 and NF90 membranes differsfrom that observed in a previous study [19], where these mem-branes exhibited small changes in the retention of three pharma-ceutically active compounds when fouled by a cocktail consistingof soil-born humic acids (Sigma–Aldrich) and background electro-lytes. This trend may be attributed to the different solute and fou-lant characteristics, since the smaller solubility of the larger MWSigma–Aldrich HA used may have resulted in greater fouling andas a consequence in improved rejection for all solutes tested in thatstudy [19].

Previous work on triazine retention by virgin membranesshowed that there is a pronounced effect of molecular size andhydrophobicity (log Kow) of the solute and of the physical proper-ties of membrane surface (porosity, hydrophobicity) [7]. Therefore,differences in triazine retention between fouled and virgin mem-branes could be explained by the changes taking place in mem-

brane–solute hydrophobic interactions and membrane–solutesize exclusion as a result of the modified membrane surface char-acteristics. Indeed, findings of the present study indicate that thetransport of the relatively hydrophobic triazine compounds isdependent on solute solubility and diffusion through the fouledmembranes. This is highlighted by the adsorption data summa-rized in Figs. 8–10, which strongly suggest that the changes in tri-azine retention are related to the observed differences in triazineadsorption between virgin and fouled membranes. Specifically,from Figs. 8–10, the following observations can be made:

� An increased triazine adsorption on fouled membranes is nor-mally related to lower retention; the opposite is observed forreduced adsorption on the fouled membranes.� The formation of loose organic layer (Table 6) of intermediate

hydrophobicity (Fig. 5) on the membrane surface, results in anincreased diffusion of the rather neutral and hydrophobic tria-zines, thereby facilitating the triazine transport to the permeateside; this is the case of HS fouling in the absence of calcium ions.� The formation of a rather dense fouling layer (Table 6) formed

by HS-calcium complexes may hinder solute diffusion, thusimproving triazine retention. An exception is observed in the

0

10

20

30

40

50

60

70

80

90

100

FA+1mM Ca2+

Fouled-20mg/LFA+1mM Ca2+

Tria

zine

reje

ctio

n (%

)

0

10

20

30

40

50

60

Tria

zine

ads

orpt

ion

(%)

0

10

20

30

40

50

60

70

80

90

100

Virgin Fouled-10mg/L HA Fouled-20mg/L HA Fouled-10mg/LHA+1mM Ca2+

Fouled-20mg/LHA+1mM Ca2+

Tria

zine

reje

ctio

n (%

)

0

5

10

15

20

25

30

Tria

zine

ads

orpt

ion

(%)

Rej. Atrazine Rej. Prometryn Ads. Atrazine Ads. Prometryn

0

10

20

30

40

50

60

70

80

90

100

Virgin Fouled-10mg/L FA Fouled-20mg/L FA Fouled-10mg/L

Virgin Fouled-10mg/L NOM Fouled-20mg/L NOM Fouled-10mg/LNOM+1mM Ca2+

Fouled-20mg/LNOM+1mM Ca2+

Tria

zine

reje

ctio

n (%

)

0

2

4

6

8

10

12

14

16

18

Tria

zine

ads

orpt

ion

(%)

Fig. 8. Atrazine and prometryn rejection (columns) and adsorption (points) by virgin and fouled (with HA, FA and NOM) NF270 membranes; fouling layers formed in thepresence or absence of Ca2+.

256 K.V. Plakas, A.J. Karabelas / Separation and Purification Technology 80 (2011) 246–261

0

10

20

30

40

50

60

70

80

90

100

HA+1mM Ca2+

Fouled-20mg/LHA+1mM Ca2+

Tria

zine

reje

ctio

n (%

)

0

5

10

15

20

25

30

35

Tria

zine

ads

orpt

ion

(%)

Rej. Atrazine Rej. Prometryn Ads. Atrazine Ads. Prometryn

0

10

20

30

40

50

60

70

80

90

100

FA+1mM Ca2+

Fouled-20mg/LFA+1mM Ca2+

Tria

zine

reje

ctio

n (%

)

0

5

10

15

20

25

30

Tria

zine

ads

orpt

ion

(%)

0

10

20

30

40

50

60

70

80

90

100

Virgin Fouled-10mg/L HA Fouled-20mg/L HA Fouled-10mg/L

Virgin Fouled-10mg/L FA Fouled-20mg/L FA Fouled-10mg/L

Virgin Fouled-10mg/L NOM Fouled-20mg/L NOM Fouled-10mg/LNOM+1mM Ca2+

Fouled-20mg/LNOM+1mM Ca2+

Tria

zine

reje

ctio

n (%

)

0

5

10

15

20

25

30

35

Tria

zine

ads

orpt

ion

(%)

Fig. 9. Atrazine and prometryn rejection (columns) and adsorption (points) by virgin and fouled (with HA, FA and NOM) NF90 membranes; fouling layers formed in thepresence or absence of Ca2+.

K.V. Plakas, A.J. Karabelas / Separation and Purification Technology 80 (2011) 246–261 257

0

10

20

30

40

50

60

70

80

90

100

HA+1mM Ca2+

Fouled-20mg/LHA+1mM Ca2+

Tria

zine

reje

ctio

n (%

)

0

5

10

15

20

25

30

35

40

Tria

zine

ads

orpt

ion

(%)

Rej. Atrazine Rej. Prometryn Ads. Atrazine Ads. Prometryn

0

10

20

30

40

50

60

70

80

90

100

FA+1mM Ca2+

Fouled-20mg/LFA+1mM Ca2+

Tria

zine

reje

ctio

n (%

)

0

5

10

15

20

25

Tria

zine

ads

orpt

ion

(%)

0

10

20

30

40

50

60

70

80

90

100

Virgin Fouled-10mg/L HA Fouled-20mg/L HA Fouled-10mg/L

Virgin Fouled-10mg/L FA Fouled-20mg/L FA Fouled-10mg/L

Virgin Fouled-10mg/L NOM Fouled-20mg/L NOM Fouled-10mg/LNOM+1mM Ca2+

Fouled-20mg/LNOM+1mM Ca2+

Tria

zine

reje

ctio

n (%

)

0

5

10

15

20

25

30

35

40

45

Tria

zine

ads

orpt

ion

(%)

Fig. 10. Atrazine and prometryn rejection (columns) and adsorption (points) by virgin and fouled with HA, FA and NOM, XLE membranes; fouling layers formed in thepresence or absence of Ca2+.

258 K.V. Plakas, A.J. Karabelas / Separation and Purification Technology 80 (2011) 246–261

K.V. Plakas, A.J. Karabelas / Separation and Purification Technology 80 (2011) 246–261 259

case of NF270 membranes fouled by NOM–Ca2+; i.e., atrazineand prometryn retention decreased by 35% and 13%, respec-tively, irrespective of NOM concentration in the foulantsolution.� Tight and hydrophobic membranes (NF90 and XLE) display sig-

nificant changes of triazine retention when fouled by humicsubstances of increased hydrophobicity (HA, NOM). On theother hand, membrane fouling by rather hydrophilic humicsubstances (FA) does not greatly affect triazine retention.� On the contrary, porous and hydrophilic membranes (NF270)

display significant changes of triazine retention when fouledby humic substances of decreased hydrophobicity (FA).� Size exclusion apparently plays a significant role in the trans-

port of hydrophobic non-ionic triazines across fouled mem-branes; the retention of prometryn was always higher than

-40

-30

-20

-10

0

10

20

Specific cake resistance, α (x1015 m/kg)

Specific cake resistance, α (x1015 m/kg)

Specific cake resistance, α (x1015 m/kg)

Diff

eren

ce in

atr

azin

e re

ject

ion

with

vi

rgin

NF2

70 m

embr

anes

(%)

HA

FA

NOM

-40

-30

-20

-10

0

10

20

40 50

Diff

eren

ce in

atr

azin

e re

ject

ion

with

vi

rgin

NF9

0 m

embr

anes

(%)

HA

FA

NOM

-30

-20

-10

0

10

20

Diff

eren

ce in

atr

azin

e re

ject

ion

with

vi

rgin

XLE

mem

bran

es (%

)

HA

FA

NOM

0 5 10 15 20

0 10 20 30

0 5 10 15 20 25 30 35 40

a

b

c

Fig. 11. Difference in atrazine (left) and prometryn (right) retention between fouled andNF270, (b) NF90 and (c) XLE membranes.

that of atrazine (especially for the tighter NF90 and XLE mem-branes), likely due to the larger molecular weight and size ofprometryn.

It should be noted that it is impossible to distinguish betweenadsorption of the triazines on the fouling layers and on the mem-brane itself. Considering that a preliminary membrane ‘‘saturation’’with the two triazines took place prior to membrane fouling, it canbe assumed that the variations in triazine adsorption, and, there-fore, in triazine retention are due to the modified membrane char-acteristics after their treatment with HS solutions, which can eitherincrease or reduce solute diffusion. For instance, the formation of aloose organic layer of intermediate hydrophobicity enhances thehydrophobic interactions on the membrane surfaces which favorthe accumulation of the two triazines near the membranes, thus

Specific cake resistance, α (x1015 m/kg)

Specific cake resistance, α (x1015 m/kg)

Specific cake resistance, α (x1015 m/kg)

-15

-10

-5

0

5

10

15

20

20

Diff

eren

ce in

pro

met

ryn

reje

ctio

n w

ith

virg

in N

F270

mem

bran

es (%

)

HA

FANOM

-30

-20

-10

0

10

20

30

50

Diff

eren

ce in

pro

met

ryn

reje

ctio

n w

ith

virg

in N

F90

mem

bran

es (%

)

HA

FA

NOM

-10

0

10

20

0 5 10 15

0 10 20 30 40

0 5 10 15 20 25 30 35 40

Diff

eren

ce in

pro

met

ryn

reje

ctio

n w

ith

virg

in X

LE m

embr

anes

(%)

HA

FA

NOM

virgin membranes, as a function of the specific cake-layer resistances of fouled (a)

260 K.V. Plakas, A.J. Karabelas / Separation and Purification Technology 80 (2011) 246–261

hindering their diffusion back to the bulk solution. This effect isdescribed in the literature [14,19,21,22] as cake-enhanced concen-tration polarization leading to reduced retention. However,cake-enhanced concentration polarization phenomena cannot beinvoked to explain the reduced rejection of triazines through essen-tially non-fouled membranes (with negligible HS adsorption andflux decline). Indeed, it is interesting to note that the filtration ofFA and NOM through the relatively hydrophilic and porous NF270membrane resulted in an increased membrane hydrophobicityand a reduced triazine rejection, although the amount of the twohumic substances adsorbed on the membrane surfaces wasinsignificant.

On the other hand, a rather dense fouling layer serves as addi-tional barrier, enhancing the sieving effect (increased retention),as previously observed [16]. This phenomenon appears to be inde-pendent of the hydrophobicity of the fouled membranes. Specifi-cally, the greater hydrophobicity measured in the case of thedense HS/Ca layer was not accompanied by a greater adsorptionand diffusion of the triazine compounds, as in the case of the looseHS layers. Probably, the hydrophobic interactions between thehydrophobic humic-calcium complexes and the triazine com-pounds are more important than the respective interactions withthe hydrophobic groups of the membrane surfaces.

The experimental results indicate that an intermediate case (be-tween the above two cases) is dependent on the characteristics ofthe organics accumulated on the membrane surfaces. As can beseen in Fig. 11, there seems to be a rather positive correlation be-tween the specific cake layer resistance (a) of the HS deposits andthe differences observed in triazine retention between fouled andvirgin NF membranes (NF270, NF90). In the case of the non-porousand relatively uncharged ULPRO membranes (XLE, Fig. 11c) suchcorrelation was only observed in the case of the smaller atrazinemolecules. On the other hand, humic substances characterized bya greater hydrophilicity and acidity (like the fulvic acids) resultin fouling resistances which exhibit a positive but rather smallercorrelation with triazine retention, especially in the case of hydro-philic and smooth membrane surfaces (NF270). Such behavior isnot observed in the case of tight and hydrophobic NF/ULPRO mem-branes (NF90 and XLE membranes), which are characterized by asimilar retention performance regardless of the FA deposition ontheir surface.

The above results seem to explain the differences in soluteretention observed in the literature [14–23], since solute transportthrough fouled membranes can be strongly affected by the physi-cochemical properties of both the foulants (hydrophobicity andcharge) and the solutes (size and hydrophobicity), as well as bythe characteristics of the membranes (MWCO, hydrophobicity, sur-face charge). Moreover, the results of the present study are in ac-cord with the differences observed in triazine retention whenhumic substances are filtered together with the two triazines[29]. Indeed, the differences observed [29] in solute rejection couldnot be fully attributed to the interactions taking place in the bulkbetween triazines and humic substances (i.e. formation ofpseudo-complexes), thereby, indicating the important role ofmembrane fouling on process effectiveness. For instance, thesomewhat reduced retention of the two triazines observed in thecase of XLE membrane, when filtered together with humic acids[29], is probably the result of the increased triazine adsorptionon the membrane, which, according to present study results, isfacilitated by the fouling layer formed on the membrane surface.

4. Conclusions

Organic fouling, depending on the nature and the relative con-centration of humic substances as well as on the presence of cal-

cium, results in considerable changes of the membrane surfacecharacteristics, including the contact angle (an indicator of hydro-phobicity) and salt retention (an indicator of surface charge andDonnan effects), which in turn can significantly affect the retentionof triazine herbicides. In general, the permeation or rejection oftested, relatively hydrophobic and non-ionic, triazine compoundsappears to be influenced by their solubility and diffusivity throughthe fouled membranes; this influence depends on the extent ofmembrane fouling. Specifically, limited deposition of organic fou-lants on the membrane surfaces favors the diffusion of the two tri-azines across the membranes, and therefore, their permeation(reduced retention). The magnitude of this permeation is relatedto the physicochemical properties of the foulants (hydrophobicityand charge) and the characteristics of the membranes used(MWCO, hydrophobicity and surface charge), being greater forsmaller organic compounds (e.g. atrazine) and rather loose NFmembranes (e.g. NF270). On the other hand, the formation of adense fouling layer, comprised of calcium complexes with humicsubstances, results in an increased retention for both triazine com-pounds. In particular, dense layers can serve as additional barrierswhich enhance the sieving effect. This phenomenon appears to bemore pronounced in the case of tight and negatively charged NFmembranes (e.g. NF90).

The new results support the view that there is a need for goodunderstanding of the relevant characteristics of the organic matterpresent in natural waters. This would aid the prediction of the or-ganic fouling tendency of the selected membrane, which in turnmay significantly affect the retention of pesticides, and in generalof low MW organic compounds, from potable water by NF/ULPROmembranes.

Appendix A

A.1. Calculation of the osmotic pressure difference (Dpm) in the case of1 mM Ca2+ feed solution

Taking into account the concentration polarization effects, theosmotic pressure difference can be estimated as follows:

Dpm ¼ 3Cb;CaRTRo expJk

� �ðA:1Þ

where Cb,Ca is the bulk calcium concentration, R is the universal gasconstant, T is the absolute temperature, Ro is the observed calciumion rejection, while constant 3 accounts for a 1:2 electrolyte solu-tion (CaCl2) at low to moderate salt concentrations where the van’tHoff law is valid. The broadly accepted concentration polarizationfactor [exp(J/k)] is determined by estimating the bulk mass transfercoefficient k, for stirred-cells, from the correlation presented bySmith et al. [48]:

Sh ¼ krsc

Dw¼ 0:27Re0:567Sc0:33 ðA:2Þ

where rsc is the stirred cell radius, Dw the bulk diffusivity of the sol-ute (CaCl2), Re the Reynolds number ð¼ xr2

sc=vÞ, m the solution kine-matic viscosity (equal to that of water at 25 �C,=0.8926 � 10�6 m2/s), x the stirring speed in rad/s (=2pN/60, where N = 250 rpm) andSc the Schmidt number (=m/Dw).

Using a constant bulk diffusion coefficient of calcium chloride inwater, Dw = 1.45 � 10�9 m2/s [47], the estimated mass transfercoefficient is k � 3.38 � 10�5 m/s. The magnitude of the concentra-tion polarization factor [exp(J/k)] varies between �1.2 and �1.4 forfluxes between 22 L/m2 h (NF90, XLE membranes) and 42 L/m2 h(NF270 membranes). This correction is applied in Eq. (A.1), fromwhich the estimated osmotic pressure difference is found to benegligible (in the order of 10�5 bar).

K.V. Plakas, A.J. Karabelas / Separation and Purification Technology 80 (2011) 246–261 261

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