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Journal of Membrane Science 351 (2010) 205–213

Contents lists available at ScienceDirect

Journal of Membrane Science

journa l homepage: www.e lsev ier .com/ locate /memsci

ffect of solution chemistry on organic fouling of reverse osmosis membranes ineawater desalination

oungbeom Yu, Sangyoup Lee, Seungkwan Hong ∗

epartment of Civil, Environmental & Architectural Engineering, Korea University, 1–5 ga, Anam-dong, Sungbuk-gu, Seoul, 136-713, Republic of Korea

r t i c l e i n f o

rticle history:eceived 22 October 2009eceived in revised form 15 January 2010ccepted 25 January 2010vailable online 1 February 2010

eywords:eawater desalinationrganic fouling

a b s t r a c t

The influence of pH and calcium ion concentrations on the organic fouling of reverse osmosis (RO) mem-branes was investigated under two distinguished ionic strengths representing surface water (i.e., 10 mM)and seawater conditions (i.e., 600 mM). Variations in flux decline with respect to the feed water pH andcalcium concentrations under these ionic environments were compared. Organic foulants deposited onthe membrane surface were collected and characterized in terms of specific UVA (SUVA) and fluorescenceexcitation and emission matrix (FEEM) in conjunction with XAD-8/4 resin fractionation. Flux-declinecurves obtained by various feed water pH and calcium concentrations under the low ionic strength con-dition were quite different with each other, while flux-decline curves obtained at the seawater-level ionicstrength was almost identical regardless of variations in the feed water pH and calcium concentration.

O membranes

H and calcium effectseawater-level ionic strengthUVA and FEEM

SUVA and FEEM results showed that the characteristics of organic matters attached on the membranesurface at high ionic strengths were significantly different from those at low ionic strengths. SUVA val-ues and FEEM images indicated that organic foulants at the seawater-level ionic strength were mostlyhydrophobic. Consequently, enhanced hydrophobic interaction dominantly controlled the rate and theextent of organic fouling at seawater-level ionic strength where the impacts of feed water pH and calcium

d.

were significantly maske

. Introduction

One of the best solutions to overcome water shortage problemsn the coming decade is the desalination of sea and brackish waters,specially using membrane-based technology [1]. Reverse osmosisRO) membranes have been widely used in brackish and seawa-er desalination over the past few years. It has been now widelyccepted that RO membrane processes exhibit better economicalfficiency than conventional thermal processes such as multistageash (MSF) and multieffect distillation (MED) [2,3]. However, foul-

ng of RO membranes has been a recalcitrant obstacle to a successfulpplication in RO desalination [4–8]. Membrane fouling results inhe deterioration of both the quality and quantity of drinking waterroduction. For most membrane processes, a major constraint isrganic fouling associated with bulk organic matters [9,10].

Various studies have been performed to find factors affectingrganic fouling of RO membranes [6,7,9,10]. It has been shown that

eed water solution chemistry, membrane property, and hydro-ynamic operating conditions are the major factors affecting therganic fouling of RO membranes. Among these factors, feedater pH and divalent cation concentration greatly influences the

∗ Corresponding author. Tel.: +82 2 3290 3322; fax: +82 2 928 7656.E-mail address: skhong21@korea.ac.kr (S. Hong).

376-7388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2010.01.051

© 2010 Elsevier B.V. All rights reserved.

flux-decline behaviors during organic fouling [7,9]. Organic mat-ters usually have functional groups such as carboxyl (−COOH)and phenolic groups (−OH) which are directly related to thefeed water pH and divalent cation concentration. It has beenknown that organic fouling is usually accelerated with decreasingpH and increasing divalent cation (i.e., calcium ions) concentra-tion [11–14]. In these conditions (i.e., low pH and high divalentcation concentration), charge property of organic matters dimin-ishes through the neutralization of functional groups as well asorganic-calcium complexation. These results in the acceleratedaccumulation of organic matters on the membrane surface enhancethe foulant–membrane interaction as well as foulant–foulant inter-actions [7,9,11,13]. Based on these observations, it has beensuggested that some pretreatment steps such as pH adjustmentand hardness removal are necessary for reducing organic fouling ofRO membranes [15–17]. Increasing pH results in the deprotonationof functional groups and hardness removal prevents organic-calcium complexation and, thus, the increase in electrostaticrepulsion between foulant–membrane as well as foulant–foulantinteractions.

These observations, however, were mostly based on the resultsfrom fouling experiments performed with feed waters containinglow to moderate ion concentrations (i.e., 10–100 mM in terms ofionic strength), which is significantly lower than ionic strengthsunder seawater conditions (i.e., 600 mM). This implies that the

206 Y. Yu et al. / Journal of Membrane Science 351 (2010) 205–213

Table 1Surface characteristics of the RO membrane used in this study.

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Membrane code RMS roughness(nm)

Zeta potential(mV) at pH 7.0

Contact angle(�)

RE8040-SHN400 82.11 −28.99 73.6

mpacts of feed water pH and calcium concentration on organicouling of RO membranes during seawater desalination might beuite different from those during surface and wastewater treat-ents due to the distinguished difference in feed water quality with

espect to ionic strength. Therefore, investigating the influence ofhese solution chemistries on organic fouling of RO membranes ateawater-level ionic strength is of paramount and practical impor-ance for efficient applications of RO processes as well as properelection of pretreatments during seawater desalination.

The objective of this study is to investigate the impacts ofolution chemistry (i.e., feed water pH and calcium concentra-ion) on organic fouling of RO membranes at seawater-level ionictrength. Fouling experiments were performed under two distin-uished ionic strength levels (i.e., 10 and 600 mM). At the endf each fouling run, foulants on the membrane surface were col-ected for further organic characterizations in terms of specific UVASUVA) and fluorescence excitation and emission matrix (FEEM)n conjunction with XAD-8/4 resin fractionation. Based on theseharacterizations, dominant organic constituents fouled on theembrane surface under low and high ionic strength conditionsere compared. Emphasis is placed on showing the differences inux-decline behaviors as well as foulant characteristics betweenhe low and high ionic strength conditions to see whether or nothese solution chemistry impacts on organic fouling in seawateresalination are similar to those in surface and wastewater treat-ents.

. Materials and methods

.1. RO membrane and surface characterizations

A commercial RO membrane was used in this study. The ROembrane was Woongjin RE8040-SHN400 (Seoul, Korea). Theembrane was a polyamide thin-film-composite (TFC) RO mem-

rane with an average salt rejection over 99.8%. Membranes weretored in deionized (DI) water at 4 ◦C with water replaced regu-arly prior to the experiments. The membrane was characterized inerms of surface roughness, zeta potential, and contact angle. Theesults are summarized in Table 1.

Membrane surface roughness was determined by AFM analysisPUCOStation AFM, Surface Imaging Systems, Herzogenrath, Ger-

any). Liquid phase AFM analysis was performed in contact modeith silicon probes with 30-nm thick aluminum reflex coating for

etter resolution and stability in liquid phase applications. Mem-rane surface roughness was quantified by root mean square (RMS)oughness, which is the RMS deviation of the peaks and valleys fromhe mean plane [18].

Contact angle measurements were performed with a goniome-er (DM 500, Kyowa interface Science, Japan). Equilibrium contactngle measurements as described by Marmur were adopted [19].he equilibrium contact angle was the average of the left and rightontact angles. Ten measurements for each membrane were carriedut. The reported values are the average of ten equilibrium contactngles.

Membrane zeta potential was determined by a streaming cur-ent electro kinetic analyzer (SurPass, Anton Paar GmbH, Graz,ustria) following the procedure described by Luxbacher [20]. Theeta potential was calculated based on the Fairbrother and Mastinubstitution [21]. For zeta potential measurements, 10 mM KCl was

Fig. 1. Surface zeta potential of the RO membrane plotted as a function of solutionpH at a background electrolyte concentration of 10 mM KCl.

used as a background electrolyte solution and solution pH was var-ied from 3 to 11. The zeta potential determined with respect tosolution pH was presented in Fig. 1. As shown in Fig. 1, the mem-brane surface was most negatively charged at the solution pH andapproached to −30 mV above pH 6.0.

2.2. Lab-scale crossflow fouling experiments

A laboratory-scale crossflow RO membrane test unit was usedin the fouling experiments. The membrane test unit consists of aflat-sheet type membrane cell, high-pressure pump, mixer, feedreservoir, temperature control system, and data acquisition sys-tem. In this unit, the test solution is held in a 20-l reservoir andfed to the membrane cell by the high-pressure pump. The mem-brane cell contains a flat-sheet membrane, placed in a rectangularchannel with cell geometry of 213 mm length, 165 mm width, and50 mm height. Feed water temperature was controlled by circulat-ing chilled water through a stainless-steel coil immersed in the feedwater at approximately 20 ◦C. The crossflow velocity and trans-membrane pressure in the membrane cell was controlled by usinga back-pressure regulator as well as bypass valve.

Fouling experiments were carried out at two distinguished ionicstrengths of 10 and 600 mM. The higher ionic strength of 600 mMis approximately equivalent to that of seawater. At each ionicstrength, feed solution pH as well as calcium concentration werevaried (i.e., pH 4, 7, and 10; Ca2+ = 0, 0.5, and 3.0 mM). Ionic strength,pH, and calcium concentration of each feed water were adjustedby using NaCl, HCl (or NaOH), and CaCl2 stock solutions, respec-tively. When investigating the impacts of feed water pH on organicfouling behaviors at 10 and 600 mM ionic strengths, calcium wasnot added in the feed waters. Similarly, the feed water pH wasfixed to 7.0 when investigating the impacts of feed water calciumconcentration on organic fouling behaviors at 10 and 600 mM.

In this study, Suwannee River humic acids (HA) obtained fromInternational Humic Substances Society (IHSS) was used as modelorganic foulants. The bulk (i.e., in the feed water) and fouled (i.e., onthe membrane surface) HA were further characterized in terms ofSUVA and FEEM in conjunction with XAD-8/4 resin fractionations.These characterization results are discussed later in this study alongwith the corresponding flux-decline data. Hydrodynamic operatingcondition, solution chemistry, and organic foulant concentrationemployed during each fouling experiment is listed in Table 2. Thereason for the significant difference in the trans-membrane pres-

sure (�P) between the low and high ionic strengths is due to thedifference in osmotic pressure produced at each condition (notethat the initial fluxes at each ionic strength were adjusted to beidentical).

Y. Yu et al. / Journal of Membrane S

Table 2Experimental conditions employed during fouling runs.

IS (mM)a 10 600

Solution chemistry

pH 4.0 4.07.0 7.0

10.0 10.0Ca2+ (mM) 0 0

0.5 0.53.0 3.0

Hydrodynamic operating conditionJ0 (�m/s)b 10 10UXF (cm/s)c 8.5 8.5�P (kPa)d 1000 4000

Organic foulant HA (mg/l) 50 50

a Ionic strength.b Initial flux.c Crossflow velocity.

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of thick and dense fouling layers on the membrane surface due tothe favourable multi-layer accumulation of organic foulants. Thelatter results in the reduction of electrostatic repulsion betweenorganic foulants and membrane surfaces, leading to an acceler-

d Trans-membrane pressure

.3. Characterization of fouled organic matters on RO membrane

In order to understand the fouling phenomena precisely, it isecessary to thoroughly characterize the organic foulants. Humicubstances (HS) are generated from the degradation of organic mat-er and represent a significant fraction of the total organic mattern aquatic environments. The HS are mostly composed of humiccids (HA) and fulvic acids (FA) in natural water [22–24]. Humicnd fulvic acids possess a significant negative charge density and aulky macromolecular shape. Subsequently, humic and fulvic acidsre not easily adsorbed onto such a negatively charged membrane,ven if it is intrinsically hydrophobic [25]. Carbon, hydrogen, anditrogen analyses of humic acids derived from either surface waterr seawater reveal no substantial differences in the ratio of theselements between the two environments [26].

Humic acids (HA) exhibit relatively high specific UV absorbancealues and contain relatively large amounts of aromatic carbon27,28]. Major organic foulants have been identified by variousechniques such as the XAD-8/4 resin fractionation followed byD fluorescence excitation and emission matrix (FEEM) analysis.EEM is increasingly used in the characterization of organic mat-ers as it can provide the multiple number of information abouthese substances [29–31].

At the end of each fouling run, organic foulants deposited on theembrane were collected with extreme care and re-dissolved in DIater for further analyses of specific UVA (SUVA) and fluorescence

xcitation and emission matrix (FEEM). SUVA and FEEM analysesllow the identification of preferentially deposited organic mat-ers (i.e., hydrophobic (HPHO), transphilic (TPHI), and hydrophilicHPHI) fractions isolated by XAD-8/4 resin fractionations [32] onhe membrane surface with respect to the solution chemistriespplied during the fouling experiments. FEEM analysis was also car-ied out with the bulk HA (i.e., in the feed water) to obtain indicativeEEM spectra which needs to be compared with those of isolatedA fractions (i.e., HPHO, TPHI, and HPHI).

Dissolved organic carbon (DOC) and ultraviolet absorbanceUVA) at a wavelength of 254 nm were measured by a total organicarbon analyzer (TOC-VCPH, Shimadzu, Japan) and a UV visiblepectrophotometer (DR5000, HACH, USA) with a 1 cm quartz cell,espectively. FEEM spectra (Safire2TM, Tecan, Germany) of organicoulants were measured with a xenon lamp as an excitation source.he FEEM spectra are the collection of a series of emission spectra’sver a range of excitation wavelengths. In these determinations,

he FEEM spectra were collected with subsequent scanning emis-ion spectra incremented at wavelengths by 10 nm and varied thexcitation wavelength at the same rate.

cience 351 (2010) 205–213 207

3. Results and discussion

3.1. Flux-decline behaviors

3.1.1. pH effect under seawater-level ionic strengthTo see how organic fouling behaves at different feed water

pH with respect to the surface water and seawater-level ionicstrengths, fouling experiments using feed waters with different pHwere carried out at the low (i.e., 10 mM) and high (i.e., 600 mM)ionic strengths. Flux-decline curves obtained from different feedwater pH at ionic strengths of 10 mM (Fig. 2(a)) and 600 mM(Fig. 2(b)) were compared. It is clearly shown in Fig. 2 that therate and extent of organic fouling at low ionic strength is sig-nificantly varied respect to the feed water pH employed, whilethe impact of feed water pH on flux-decline behaviors at thehigh ionic strength was almost negligible. At low ionic strength,the most severe flux decline was observed at pH 4 and the fluxdecline decreased with the increasing pH. The negligible differencebetween pH 7 and 10 is attributed to the fact that the dominantfunctional group (i.e., –COOH) is almost deprotonated above pH6.0 [10]. Feed water pH affects both the charge properties of bulkorganic foulants and the interfacial interaction between organicfoulants and membrane surfaces. The former leads to the formation

Fig. 2. Influence of feed water pH on flux-decline behaviors: (a) IS = 10 mM and (b)IS = 600 mM. Experimental conditions for each fouling run are described in Table 2.

208 Y. Yu et al. / Journal of Membrane Science 351 (2010) 205–213

FId

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ig. 3. Influence of feed water calcium concentration on flux-decline behaviors: (a)S = 10 mM and (b) IS = 600 mM. Experimental conditions for each fouling run areescribed in Table 2.

ted accumulation of foulants on the membrane surfaces. Theseesults were quite well accorded with previous studies showing thenfluence of solution pH on the rate and extent of organic fouling7,9].

The feed water pH, however, showed almost a negligible

mpact on organic fouling under high ionic strength conditionss shown in Fig. 2(b). Flux-decline curves obtained at differentH were almost identical. Even though the organic functionalroups could be deprotonated with increasing pH at such high

ig. 4. Percent flux reduction with respect to feed water pH and calcium concen-ration. Data were calculated based on the results shown in Figs. 2 and 3.

Fig. 5. SUVA values for foulant samples collected after fouling runs at the low andhigh ionic strengths: (a) pH effect and (b) calcium effect.

ionic strengths as that of seawater, the overall charge density oforganic macromolecules is completely masked. This leads to sim-ilar characteristics in organic foulants in terms of charge densityregardless to the pH variation. Instead, charge screening as well aselectrostatic double layer compression induced by huge ion contentof seawater-level ionic strength makes organic foulants as well asmembranes more hydrophobic and, thus, enhances the hydropho-bic interactions between foulants and membranes as well as amongfoulants. Through this it can be proposed that the feed water pHmay not play such a significant role in organic fouling of RO mem-branes during seawater desalination as it does in case of surfaceand wastewater treatments. Previous studies suggested that chargescreening induced by increased indifferent salt concentration doesnot significantly contribute to humic acid fouling [7,9]. The for-mer experiments, however, were carried out at moderate ionicstrength (i.e., 10–100 mM); thereof the results could not be appliedto seawater conditions. By comparing Fig. 2(a) with Fig. 2(b), it wasverified that the overall flux decline was more substantial on thecondition of seawater-level ionic strength. Consequently, it can besuggested that the feed water pH is not a dominant factor affectingorganic fouling during seawater desalination. One practical impli-cation from these results is that pH adjustment as a pretreatment tocontrol organic fouling may not be necessary especially in seawaterdesalination.

3.1.2. Calcium effect under seawater-level ionic strengthVarious studies have demonstrated that among the solution

chemistry factors (i.e., solution pH, ionic strength, and calciumconcentration) affecting organic fouling of RO membranes, feedwater calcium concentration plays a key role in governing the

Y. Yu et al. / Journal of Membrane Science 351 (2010) 205–213 209

droph

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Fig. 6. FEEM spectra for the model organic matters: (a) unisolated, (b) hy

ate and the extent of organic fouling through organic-calciumomplexation [6,7,9,11–13]. Active organic-calcium complexationeads to thicker, denser, and more compact fouling layer onhe membrane surface through intermolecular bridging amongrganic macromolecules. These findings, however, are also basedn the flux-decline experiments performed at low to moderateonic strength conditions significantly lower than that of sea-

ater. Therefore, to confirm whether or not these findings arepplicable to interpreting organic fouling behavior during seawa-er desalination, it is essential to perform these experiments ateawater-level ionic strengths. To see how organic fouling behav-or at different feed water calcium concentrations with respecto the surface water and seawater-level ionic strengths, foulingxperiments using feed waters with different calcium concen-rations were carried out at the low (i.e., 10 mM) and high (i.e.,00 mM) ionic strengths. Flux-decline curves obtained from dif-erent feed water calcium concentrations. At 10 mM (Fig. 3(a))nd 600 mM (Fig. 3(b)) of ionic strengths were compared. Ashown in Fig. 3(a), calcium effect on organic fouling is quite sim-lar to the previous studies conducted at low ionic strengths. Inhe presence of calcium ions, the flux declined more rapidly. Thisesulted from the formation of cross-linked organic fouling layersn the membrane surface though active organic-calcium com-lexation. The slight difference in flux-decline curves between 0.5nd 3.0 mM calcium concentrations implies that 0.5 mM calcium

s nearly the saturation point for the complexation. Further-

ore, by comparing Figs. 2(a) and 3(a), it can also be knownhat calcium effect on organic fouling is more influential thanhe pH effect, which is also well accorded with previous studies9,11–13].

obic (HPHO), (c) hydrophilic (HPHI), and (d) transphilic (TPHI) fractions.

Astonishingly, this noticeable impact of calcium on flux-declinebehaviors at low ionic strengths was almost disappeared at theseawater-level ionic strength as shown in Fig. 3(b). Similar flux-decline curves were obtained regardless of the feed water calciumconcentrations. Similar to the result of pH effects on organic foulingat high ionic strengths, the effect of divalent cation concentration,therefore, can be less significant under seawater conditions con-taining high electrolyte concentrations. It is interesting to notethat flux-decline behaviors at the seawater-level ionic strength arealmost identical regardless to the variations in either the pH orcalcium concentration (see Figs. 2(b) and 3(b)). This implies thatseawater ionic strength itself is the key factor controlling the rateand the extent of organic fouling during seawater desalination.Furthermore, these results suggest that hardness removal prior toRO application to lessen organic fouling may not be necessary inseawater desalination.

3.1.3. Summary of flux-decline behaviors under seawaterconditions

The feed water pH and calcium concentration impacts onorganic fouling at low and high ionic strengths were summarizedin Fig. 4. The percent flux reduction was calculated by compar-ing the initial flux to the flux at the end of fouling runs (i.e., 20 h).At low ionic strength, flux reduction varies from 28 to 56%, whilethe variation in flux reduction is almost negligible at high ionic

strengths. Flux reduction increased substantially with decreasingfeed water pH as well as increasing calcium concentration at lowionic strengths, while negligible difference in flux reduction wasobserved at high ionic strengths, regardless of variation in both feedwater pH and calcium concentration. The impacts of these solution

210 Y. Yu et al. / Journal of Membrane Science 351 (2010) 205–213

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ig. 7. FEEM spectra for foulant samples collected after fouling runs at: (a) IS = 10 me) IS = 600 mM and pH 7, and (f) IS = 600 mM and pH 10.

actors are significantly masked at the seawater-level ionic strengthue to the rigorous charge screening and electrostatic double layerompression for both the organic foulants and membrane surfaces.hus, the enhanced hydrophobic interaction mainly controls theate and the extent of organic fouling rather than the electrostaticnteraction.

.2. Characteristics of organic foulants

.2.1. SUVA analysisAt the end of each fouling run shown in Figs. 2 and 3, organic

oulants accumulated on the membrane surface were collectedy rinsing the membranes with DI water. Each foulant sampleas re-dissolved in DI water to make the foulant volume of 0.2 l.

hese samples were subjected to the measurements of DOC as

ell as UVA254 to calculate SUVA. The calculated SUVA values of

ach foulant sample for different pH and calcium concentrationsere depicted in Fig. 5 (i.e., pH and calcium effects for Fig. 5(a)

nd (b), respectively). As shown, SUVA values at the low ionictrengths under various pH and calcium ion concentrations were

pH 4, (b) IS = 10 mM and pH 7, (c) IS = 10 mM and pH 10, (d) IS = 600 mM and pH 4,

much smaller than those at the high ionic strengths. This impliesthat organic foulants collected at the high ionic strength are muchmore hydrophobic compared to those collected at the low ionicstrength. At low ionic strengths, SUVA values varied from 0.35 to1.37 where the value increased with increasing feed water pH anddecreasing calcium concentration. On the other hand, at high ionicstrengths, all SUVA values were much higher than 4.0 regardless ofvariation in feed water pH and calcium concentration. This is wellaccorded with the discussion made in the previous section. At theseawater-level ionic strength, the enhanced hydrophobic interac-tion mainly controls the rate and the extent of organic fouling asthe high salt content in seawater is quite a favorable condition forenhancing the hydrophobicity of both organic macromolecules andmembrane surfaces through charge screening as well as electro-static double layer compression.

Humic substances generally contain relatively large amountsof aromatic carbon and exhibit relatively high SUVA values rep-resenting the hydrophobicity of organic matters. Alternately, thisstrong hydrophobicity compensated by the charge density ofhumic substances through deprotonation of functional groups

Y. Yu et al. / Journal of Membrane Science 351 (2010) 205–213 211

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ig. 8. FEEM spectra for foulant samples collected after fouling runs at: (a) IS = 10S = 600 mM and No Ca2+, (e) IS = 600 mM and Ca2+ = 0.5 mM, and (f) IS = 600 mM and

epends on the solution chemistry. As discussed previously, how-ver, this charge density does not play a role in lessening thetrong hydrophobic characters of organic matters when the chargeensity is severely masked by seawater-level ionic strength and,hus, the enhanced hydrophobic interaction among foulants asell as between foulants and membrane. Therefore, the enhancedydrophobic interaction at the seawater-level ionic strength is theajor reason for the similar severe flux-decline curves shown in

igs. 2(b) and 3(b) regardless of variations in feed water pH andalcium concentration.

.2.2. FEEM analysis

FEEM fluorescence spectra were obtained by collecting exci-

ation (Ex) and emission (Em) spectra over a range from 230 to00 nm. Prior to FEEM analysis for each foulant sample, the modelrganic foulants were isolated by XAD-8/4 resin fractionation.hen, FEEM spectra for hydrophobic (HPHO), transphilic (TPHI),

d No Ca2+, (b) IS = 10 mM and Ca2+ = 0.5 mM, (c) IS = 10 mM and Ca2+ = 3.0 mM, (d)= 3.0 mM.

and hydrophilic (HPHI) fractions were obtained and used as anindicative FEEM map. These indicative FEEM spectra are shownin Fig. 6 along with the spectra for the bulk organic matter (i.e.,unisolated). The indicative FEEM spectra shown in Fig. 6 indicatesthat the hydrophobic fraction was located at Ex = 230–420 nm andEm = 290–600 nm, the hydrophilic fraction at Ex = 230–350 nm andEm = 290–510 nm, and the transphilic fractions at Ex = 230–300 nmand Em = 290–540 nm. These values are quite similar to thoseshown in other researches even though the model organic mat-ters used were slightly different (i.e, [29] for NOM, [30] for Aldrichhumic acids, and [31] for IHSS fulvic acids).

FEEM spectra for foulant samples collected after fouling runs

with different feed water pH at low and high ionic strengths arepresented in Fig. 7. At low ionic strengths, both the hydrophobicand hydrophilic fractions exist, and the FEEM spectra for hydropho-bic fractions decrease with increasing the feed water pH (seeFig. 7(a)–(c)). The FEEM spectra for hydrophilic fractions, on the

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12 Y. Yu et al. / Journal of Memb

ther hand, have completely disappeared at high ionic strengthsor all pH conditions (see Fig. 7(d)–(f)). These results imply thathe dominant organic foulants are hydrophobic constituents at theigh ionic strength. This results in the accelerated organic foulinghrough the enhanced hydrophobic interactions, where the varia-ion of feed water pH has little impacts on lessening the organicouling behavior. The results obtained from FEEM analysis are inood agreement with the SUVA results showing that the majorrganic foulants on the membrane surface at the seawater-levelonic strength is hydrophobic constituent and, thus, the enhancedydrophobic interactions governs the rate and the extent of organic

ouling.Fig. 8 compares the FEEM spectra obtained with different feed

ater calcium concentration at low and high ionic strength. Sim-lar to Fig. 7, the FEEM spectra for hydrophilic fractions (i.e.,x = 230–350 nm and Em = 290–510 nm) have almost disappearedt high ionic strength for all the calcium concentrations inves-igated (see Fig. 8(d)–(f)). At the low ionic strength, both theydrophobic (i.e., Ex = 230–420 nm and Em = 290–600 nm) andydrophilic (i.e., Ex = 230–350 nm and Em = 290–510 nm) frac-ions exist, and the FEEM spectra for hydrophobic fractionsncrease with increasing the feed water calcium concentration (seeig. 8(a)–(c)). This suggests that the impact of feed water calciumn organic fouling is influential at the low ionic strength, whilehis impact diminished at the seawater-level ionic strength wherehe enhanced hydrophobic interaction mainly controls the organicouling behavior. In addition, the fact that the hydrophilic area ofEEM spectra reduced at the high ionic strength is well matched tohe previous SUVA results, that is the great increase in SUVA valueshen feed water ionic strength reached the seawater-level ionic

trength.

. Conclusion

Impacts of solution chemistry variations such as feed water pHnd calcium concentration on organic fouling of RO membranesere investigated under a seawater-level ionic strength condi-

ion (i.e., 600 mM). Under typical surface water ionic strength (i.e.,0 mM), organic fouling was significantly affected by the solutionhemistry (i.e., pH and calcium concentration) showing more rapidux decline with decreasing pH and increasing calcium concentra-ion. However, these visible impacts of feed water pH and calciumoncentration on organic fouling almost disappeared at the highonic strength similar to seawater TDS. All the flux-decline curvesbtained at ionic strength of 600 mM were almost identical regard-ess of variation in feed water pH and calcium concentration. Thesebservations clearly showed that the influence of solution chem-stry such as feed water pH and calcium concentration would have

uch less impacts on organic fouling of RO membranes duringeawater desalination.

SUVA values for the foulants deposited on the membrane surfacet the high ionic strength (i.e., 600 mM) were much greater thanhose at the low ionic strength (i.e., 10 mM). This indicates thathe dominant constituents of organic foulants accumulated at theigher ionic strength contained hydrophobic fractions. From theEEM analyses, it has also been found that the major FEEM areaor the foulants obtained at the high ionic strength clearly matchedhe FEEM indicative area for hydrophobic constituents. Based onUVA and FEEM results, it could be concluded that organic foulinguring seawater desalination is mainly controlled by the enhancedydrophobic interactions between organic matters and membrane

s well as interactions between the organic matters. Consequently,he impacts of feed water pH and calcium concentration on organicouling were much less influential at seawater-level ionic strengthue to the significant masking of electrostatic interactions such ashe charge neutralization of organic functional groups as well as

[

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cience 351 (2010) 205–213

organic-calcium complexation. The results obtained in this studyimply that pretreatment to manipulate solution chemistry such aspH adjustment and/or hardness control might not be an effectiveway to control organic fouling during seawater desalination.

Acknowledgement

The authors would like to thank the Ministry of Land, Trans-port and Maritime Affairs (MLTM) for supporting this study throughthe Seawater Engineering & Architecture of High Efficiency ReverseOsmosis (SEAHERO) program.

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