Presented at the First International Workshop between the Center for the Seawater Desalination Plant and the EuropeanDesalination Society, November 15–16, 2007, GIST, Gwangju, Korea.

0011-9164/09/$– See front matter © 2009 Elsevier B.V. All rights reserved

Desalination 238 (2009) 60–69

Fouling of an anion exchange membrane in the electrodialysisdesalination process in the presence of organic foulants

Hong-Joo Lee, Min-Kyoung Hong, Sang-Don Han, Seung-Hee Cho,Seung-Hyeon Moon*

Department of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST),1 Oryong-dong, Buk-gu, Gwangju 500-712, Korea

Tel. +82 (62) 970-2435; Fax: +82 (62) 970-2434; email: shmoon@gist.ac.kr

Received 15 November 2007; Accepted 31 January 2008

Abstract

Interest in desalination using membrane processes has increased with the growth of membrane science andtechnology. Fouling of ion exchange membranes is one of the most significant considerations in the operation of theelectrodialysis desalination process. The influence of the adsorption capacity of foulants on the fouling potential wasinvestigated in an electrodialysis desalination process in the presence of negatively charged organic foulants, humate,bovine serum albumin and sodium dodecylbenzenesulfonate (SDBS). The SDBS showed higher adsorption capacityon the anion exchange membrane surface than the other two foulants in the adsorption equilibrium experiments. Ofthree different organic foulants, the SDBS had a more severe fouling effect on the process performance in theelectrodialysis desalination of synthetic brackish water, with irreversible fouling due to the high adsorption capacity.The study clearly showed that the adsorption capacity affects the fouling potential of the electrodialysis desalinationprocess.

Keywords: Electrodialysis; Fouling; Organic foulant; Anion exchange membrane; Adsorption capacity

1. Introduction

Water shortages ares reported to affect about90% of the developing countries, half of theworld’s population. In addition, the number of

*Corresponding author.

people affected by a severe water shortage isexpected to increase fourfold over the next 25years [1]. Therefore, interest in desalination hasincreased to overcome water shortage problemsworldwide. Desalination of brackish water andseawater is classified with three basic approaches:thermal means; membrane separation processes;

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and chemical methods including ion exchange,liquid–liquid extraction and other precipitationprocesses. Among these approaches, the thermalmeans effects a phase change of the water andseparates water to liquid form using thermalenergy. Distillation has been considered as thefirst process on a large commercial scale. Multi-stage flash (MSF) and multi-effect evaporation(MEE) were mainly considered as one of thedesalination experiences on a large scale for morethan 50 years [2,3].

Nowadays, interest in desalination using mem-brane processes has increased with the growth ofmembrane science and technology. Of the mem-brane processes, two major desalination processesare reverse osmosis and electrodialysis. In reverseosmosis, water passes through a membraneimpermeable to the solute in response to pressuredifference. Reverse osmosis is the most versatiledesalination method for the treatment of water ofany salinity from brackish water to high salinityseawater, which overtook MSF as a leadingdesalination technology [4,5]. Meanwhile, elec-trodialysis is an electrochemical separation pro-cess which employs electrically charged ionexchange membranes with an electrical potentialdifference as a driving force. In electrodialysis,ions in solution migrate through ion selectivemembranes in an electric field.

The advantages of electrodialysis desalinationover reverse osmosis are the following [6,7]:(1) higher water recovery rates for raw water withhigh sulfate content, (2) longer operation life ofmembranes due to higher chemical and mechani-cal stability, (3) less scale or membrane fouling,(4) less raw water pretreatment, and (5) a moreflexible process for various feed water qualities.However, the electrodialysis desalination processhas a limit to apply to the desalination of seawaterwith high salinity. Therefore, a two-stage proce-dure with the combination of reverse osmosis andelectrodialysis was considered for long-term ope-ration and satisfactory product water quality inthe desalination of seawater with salt contents in

the range of 35,000–45,000 mg/L. The salt con-centration of the seawater was reduced to about2,000 to 4,000 mg/L in the first stage usingreverse osmosis, and then to less than 500 mg/Lin the second stage using electrodialysis. The ad-vantages of the two-stage over a one-stage systemare high reliability and possibly lower costs [8,9].

The fouling of ion exchange membranes is oneof the most significant considerations in theoperation of the electrodialysis desalination pro-cess in spite of the wide application areasincluding the environmental and biotechnologicalindustries as well as desalination. It is known thatmost organic foulants in many effluent streamsare negatively charged, thus fouling anionexchange membranes due to deposition and/oradsorption [10–12]. Among organic foulants,natural organic matter with a large molecularweight foul anion exchange membranes due toaccumulation on membrane surfaces [13]. Also,a protein foulant is known to have fouling effectson the membrane by the formation of a gel layeron the membrane surface [14,15]. Protein depo-sition results in the formation of a compressibleprotein deposit on the surface of the membrane,leading to an additional hydraulic resistance. Oforganic foulants, a surfactant has fouling effectson anion exchange membranes by the accumu-lation on the membrane surface at a high concen-tration. Even at a low concentration, the foulingincreases the resistance considerably due to theslow transport rate through the inner structure ofan anion exchange membrane [16].

It is known that fouling potential and rever-sible/irreversible fouling depend on the propertiesof a foulant and a membrane [17]. The foulingpotential can be predicted by characterization oftheir properties [13]. In addition, the interactionbetween a foulant and a membrane is one of theimportant considerations for the fouling potential,which is estimated by the adsorption capacity ofthe foulant on the membrane surface.

The objectives of this study are (1) to inves-tigate influence of the adsorption capacity on the

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fouling potential between a foulant and a mem-brane and (2) to analyze fouling potential in thefouling experiments in the presence of organicfoulants by comparing electrodialysis desali-nation process performances.

2. Experimentals

2.1. Characterization of organic foulants

In this study, three synthetic organic foulants,sodium humate (Aldrich, USA), a heat-shockprecipitated bovine serum albumin (BSA)(Sigma, USA), and sodium dodecylbenzenesul-fonate (SDBS) (Aldrich, USA), were chosen asrepresentative of natural organic matter, proteinsand anionic surfactants, respectively. The surfacecharges of organic foulants were estimated withzeta potential values using the Smolouchowskiequation based on the electrophoretic mobilitymeasurement in an electrophoretic apparatus,ELS-8000 (Ostuka Electronics, Japan) [18].

Adsorption equilibrium experiments for threeorganic foulants were carried out in batch experi-ments in order to observe adsorption capacity onthe anion exchange membrane. The batch experi-ments were performed in a wide variety of con-centrations at a constant pH value of 6.5±0.3 atthe room temperature after the AMX membraneswere soaked in the solutions containing organicfoulants for 2 days. The concentration of organicfoulants was analyzed by the measurement of UVabsorbance using the UV-1601PC (Shimadzu,Japan) at a wavelength of 224 nm for SDBS,254 nm for humate, and 280 nm for BSA,respectively.

2.2. Characterization of anion exchangemembranes

The fouling potential of the membranes wasanalyzed by comparing the properties for thefresh and fouled anion exchange membranes. Thephysical and chemical properties of an anionexchange membrane were analyzed in terms of

transport number, contact angle of the membranesurface, electrical resistance, water content, andexchange capacity. The transport number wasestimated by measurement of the membranepotential of a counter-ion (Cl!) in the anionexchange membrane using Ag/AgCl electrodes ina 0.01 M and 0.05 M KCl solution. Contact angleof the membrane surface, an indicator of hydro-phobicity, was measured using a sessile dropmethod. In addition, the water content of mem-branes was estimated after measuring driedweight, found elsewhere [10]. Electrical resis-tance was measured at the frequency of 100 KHzusing a clip cell connected to a LCZ meter, NF2321 (NF Electronic Instruments, Japan) in 0.5 MNaCl. Exchange capacity was estimated by thetitration method with 0.05 N HCl after examinedmem-branes were soaked in 0.1 N NaOH for 24h. For the functional groups of anion exchangemem-branes, attenuated total reflection–Fouriertrans-form infrared (ATR–FTIR) spectroscopy(FT/IR-460 Plus, Jasco, Japan) was used at ameasuring range between 700 and 4000 1/cm.

2.3. Analysis of fouling potential in electro-dialysis experiments

A unit cell pair was assembled in a TS-2electrodialysis stack using a commercial cationexchange membrane, Neosepta® CMX (Astom,Japan) and an anion exchange membrane,Neosepta® AMX. The effective area of the ionexchange membrane was 200 cm2 each. As syn-thetic brackish water, the initial diluate solutionin each experiment was 3.0 L of 6,000 mg/LNaCl containing organic foulants. Three liters of3,000 mg/L NaCl was used as an initial concen-trate solution in the electrodialysis experiments.The diluate and concentrate solutions were circu-lated at a flow rate of 200 mL/min. For theelectrode rinse solution, 8 L of 6% Na2SO4 wasused. A constant current of 1.0 A (current den-sity: 5.0 mA/cm2) was supplied in electrodialysisexperiments.

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3. Results and discussion

3.1. Adsorption of foulants on the anion exchnagemembrane surface

The zeta potential of material is an indicator ofthe surface charge, which is estimated based onthe electrophoretic mobility in the electric field[18]. In this study, the electrophoretic mobilitiesof three organic foulants were measured in10 mM KCl at constant pH values of 6.5±0.3 atroom temperature. The zeta potential of humate,BSA, and SDBS was estimated to be !28.2±3.1 mV, !25.3±4.5 mV, and !26.9±5.3 mV,respectively. In the zeta potential measurements,it was observed that organic foulants have highlynegative charges. This implies that they caneasily be adsorbed on the surface of a positivelycharged anion exchange membrane, thus resultingin fouling on the membrane [13].

The quantity of adsorbable foulants on themembrane surface is related to the relationshipbetween the membrane and foulants. The ad-sorbed amount of materials is determined with theadsorption isotherm. In this study, the Freundlichand Langmuir isotherms were applied for theadsorption equilibria for three different kinds offoulants [19,20]. The Freundlich equation iswritten as follows:

(1)ne f eq K C

(2)ln ln lne f eq K n C

where Kf and n are coefficients which can beestimated from the slope of the Freundlich iso-therm plot, and qe the amount of adsorbed mater-ial at equilibrium (mg/g). The experimentalresults were used for the calibration of parametersof the Freundlich adsorption isotherm and arepresented in Table 1(a) with the correlationcoefficients. The SDBS shows the highest valuesof the parameters of the Freundlich isothermamong three different organic foulants.

Table 1Parameters of the Freundlich and Langmuir isotherms foradsorption of organic foulants on the anion membrane

(a) Freundlich isotherm

Foulant K (mg/g dried AMX)

n Correlationcoefficient

Humate 0.66 0.87 0.948SDBS 7.06 0.13 0.794BSA 0.85 0.39 0.903

(b) Langmuir isotherm

Foulant Q0

(mg/g dried AMX)b(L/mg)

Correlationcoefficient

Humate 1.89 0.45 0.992SDBS 12.41 0.86 0.997BSA 3.24 0.27 0.988

The Langmuir isotherm is often used todescribe the sorption of a solute from a liquidsolution containing a finite number of identicalsites as follows:

(3)0

1e

ee

q bCq

bC

where Ce is the equilibrium concentration (mg/L),qe the amount of adsorbed material at equilibrium(mg/g), b the affinity parameter or Langmuir con-stant (L/mg), and Q0 the capacity parameter(mg/g). Fig. 1 shows the Langmuir isotherm fororganic foulants on the AMX membrane surface.As shown in Fig. 1, the non-linear isotherm iswell fitted to the Langmuir isotherm.

The above equation can be written as followsto determine the parameters of the Langmuirisotherm, i.e.:

(4)1e e

e o o

C C

q Q Q b

The linear relationship between Ce/qe and Ce was

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Fig. 1. Adsorption isotherm of organic foulants on theanion exchange membrane.

Fig. 2. Langmuir isotherm plot of organic foulants.

observed for three organic foulants, indicatingthat the adsorption of the organic foulants obeysthe Langmuir isotherm model. Fig. 2 shows thelinear plot of Ce/qe versus Ce for the threedifferent organic foulants. The values of Q0 andb were determined from the slope and intercept ofthe Langmuir plot and the estimated values aregiven in Table 1(b) with the correlation coeffi-cient for the respective organic foulant. As shownin the table, the adsorption of three organic foul-ants shows a high correlation coefficient, imply-ing that the adsorption of foulants on the AMX

membrane surface can be explained by the Lang-muir isotherm. Among three organic foulants, theSDBS showed the highest values of Q0, which isrelated to the adsorption capacity. Especially, thelarger value of b, an indicative of adsorptionenergy, implies stronger adsorption or mostlyprobably chemically adsorption of SDBS on theAMX membrane surface [21,22]. The adsorptionisotherm parameters in Table 1 shows that theLangmuir isotherm is suitable for the adsorptionof organic foulants and that the SDBS has thehighest adsorption capacity on the anon exchangemembrane surface.

3.2. Fouling characterization of the anionexchange membrane

The fouling properties are related to theproperties of membranes and foulants, which arecharacterized by the physical and chemical pro-perties. The characteristic values were measuredin terms of electrical resistance, water content,exchange capacity, transport number and contactangle. The results of the fouled AMX membranewere compared with those of the fresh AMXmembrane. As shown in Table 2, the fouledmembranes exhibited higher electric resistance(2.3–2.6 Ωcm2) than the fresh membrane(2.15 Ωcm2). The ion exchange capacity showedslightly decreased values compared with thevalue of 2.5 meq/g-dried AMX for the freshmembrane. Also, the water content decreasedslightly due to the organic fouling. The foulingcaused a decrease in the transport number throughthe anion exchange membrane and an increase inhydrophobicity of the membrane due to deposi-tion on the membrane surface. Considering thedifference in characteristic values between thefresh and fouled membranes, the SDBS had agreater fouling effect on the membrane pro-perties, which is related to a high adsorptioncapacity.

The transmittance of the fouled anion

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Table 2Characteristics of the fresh and fouled AMX membranes

Fresh AMX Fouled AMX

Humate SDBS BSA

Electrical resistance (Ωcm2) 2.15 2.32 2.62 2.40Water content (g H2O/g dried AMX) 0.34 0.28 0.30 0.26Exchange capacity (meq/g dried AMX) 2.5 2.3 2.1 2.3Transport number 0.96 0.94 0.93 0.92Contact angle (E) 66–70 72–75 72–76 70–73

Fig. 3. Transmittance of the fresh and humate fouled AMX membranes.

exchange membranes was measured in the wavenumber range between 700 and 4000 1/cm usingthe ATR–FTIR in order to investigate the func-tional groups of the membrane. The functionalgroups were not changed significantly for all theanion exchange membranes even after the mem-branes were fouled. Fig. 3 shows some of the

functional groups including -CH2-, C=O and–NH- bonds for the humate fouled AMX mem-brane. It is thought that the foulant depositionchanged the functional groups in membranestructure in a small degree. It was observed thatorganic fouling had a small effect on the func-tional groups of the membrane structures even

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though the physical and chemical properties ofthe AMX membrane was affected by organicfoulants.

3.3. Fouling potentials of different foulants in thecell of CMX and AMX

The operating current density was set to5.0 mA/cm2 for the fouling experiment in thepresence of three different kinds of foulants usinga unit cell pair with the effective area of 200 cm2.The concentration of two organic foulants(humate and BSA) was fixed at 1.0 g/L in diluatesolution. In the case of SDBS, the initial con-centration for diluate was 52 mg/L since theSDBS was observed to have the highest adsorp-tion capacity in the adsorption equilibriumexperiments.

The fouling effect on the electrodialysisdesalination process was investigated in terms ofmembrane resistance, conductivity change indiluate. Fig. 4 shows the resistance change of theAMX membrane change for three organic foul-ants: humate, BSA and SDBS. The results revealthat SDBS led to the highest increase in the AMXmembrane resistance. The membrane resistanceincreased to 40 Ω at the lapsed time of 180 minwhere the concentration depletion cannot takeplace due to the existance of ions in the boundarylayer of the membrane facing to the diluate. It isconsidered that the SDBS fouled the AMXmembrane most significantly due to adsorptionon the membrane surface, thus retarding themovement of anion through the AMX membraneand increasing the resistance of the membrane.

Fig. 5 shows change in the conductivity ofdiluate in the presence of organic foulants. Thedecreasing rates were similar to the foulingsystems of the humate and BSA. Also, the diluateconductivity decreased to nearly zero at the endof the experiments for both fouling systems,implying that the humate and BSA did not givesignifcant fouling effect even in the presence of

Fig. 4. Change in the anion exchange membrane resis-tance in the electrodialysis desalination in the presence oforganic foulants.

Fig. 5. Change in diluate conductivity in the electro-dialysis desalination in the presence of organic foulants.

foulants. In the SDBS fouling experiment, how-ever, the electrodialysis desalination was operatedonly at the elapsed time of 180 min because ofthe significant fouling on the membrane surface,as observed in the membrane resistance change.

In this study, the electrodialysis performancesof the three fouling systems were compared interms of the NaCl flux, the current efficiency, andthe power consumption to treat 1 mol of NaCl,and the results are shown in Table 3. Compared

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to the desalination performances in the absence offoulant, the NaCl flux and current efficiency werenotably deteriorated and the power consumptionincreased in the three fouling systems. Of theprocess performances, the NaCl flux of each foul-ing system was not significantly different within0.53–0.55 mol/m2h. It is assumed that the ionictransport was not affected because of the lowdense fouling layer on the anion exchange mem-brane surface [13]. It was clearly observed thatthe current efficiency and the power consumptionchanged significantly in the fouling systems.Among the fouling systems, the SDBS showedthe highest fouling effect with the lowest currentefficiency (70.2%) and the highest power con-sumption (0.647 kWh/mol). It is noted thatdecrease in the electrodialysis performances inthe SDBS fouling system is related to the highestadsorption capacity in the adsorption on theAMX membrane surface.

Table 3Electrodialysis performances in the presence of organicfoulants

Foulingsystem

NaCl flux(mol/m2 h)

Currentefficiency(%)

Powerconsumption(kWh/mol)

Humate 0.542 84.3 0.627BSA 0.530 96.4 0.553SDBS 0.548 70.2 0.647Desalinationwithout foulant

0.587 98.1 0.514

3.4. Fouling behaviors of different foulants in thefouling experiments

Generally, fouling phenomena can be clas-sified as reversible and irreversible fouling inmembrane processes, depending on the recoveryof the process performances. In revsersible foul-ing, foulants physically deposited on the mem-brane surface and the process performance arerecovered after the foulants are removed. In irre-versible fouling, however, process performancecannot be easily recovered due to chemicallyadsorbed foulants [23,24].

In order to investigate the fouling behaviorsfor the three organic foulants, the followingexperiments were conduted sequently: (1) a de-salting experiment of NaCl (before foulung), (2) afouling experiment in the presence of foulant, and(3) a desalting experiment of NaCl (after fouling).Table 4 presents the electrodialysis performancesin the desalting experiment for the fouling systemof three organic foulants. The electrodialysisperformances decreased due to fouling fromorganic foulants, showing the irreversible foulingeffect. The SDBS fouling system showed thelargest decrease in the current efficiency and thepower consumption after fouling, giving moreirreversible fouling effects on the system. Theresults imply that the degree of irreversible foul-ing depends on the adsorption capacity of foul-ants. Therefore, the high fouling potential isattributed to the high adsorption capacity ofSDBS in this study.

Table 4Fouling effect on the process performances in the electrodialysis desalination

Humate BSA SDBS

Before fouling After fouling Before fouling After fouling Before fouling After fouling

Current efficiency (%) 98.4 90.7 97.9 91.2 97.9 86.9NaCl flux (mol/m2h) 0.587 0.539 0.573 0.530 0.569 0.525Power consumption (kWh/mol)

0.545 0.609 0.553 0.606 0.534 0.626

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4. Conclusions

In this study the fouling potential of the AMXmembrane was analyzed in the electrodialysisdesalination of synthetic brackish water in thepresence of three organic foulants (humate, BSA,and SDBS). The fouling potential was predictedthrough the characterization of physical andelectrochemical properties and the adsorptionexperiments. Three foulants were highly nega-tively charged according to the zeta potentialmeasurements, implying that they can foul anionexchange membranes. In the adsorption oforganic foulants, the SDBS showed a higheradsorption capacity on the AMX membrane sur-face than the other foulants. SDBS had greaterfouling effects on the membrane in electrodialysisdesalination, which is related to the high adsorp-tion capacity. The SDBS fouling system showedthe largest decrease in the current efficiency andthe largest increase in the power consumptionafter fouling, producing more irreversible foulingon the system.

This study shows that a foulant with a highadsorption capacity signicantly affects the elec-trodialysis desalination process performance withirreversible fouling.

Acknowledgement

This research was supported by a grant (code#C106A1520001-06A085600222) from the PlantTechnology Advancement Program funded by theMinistry of Construction and Transportation ofthe Korean Government.

References

[1] J.E. Miller, Review of water resources and desali-nation technologies, SAND 2003-0800, Albu-querque, 2003.

[2] GWI, Desalination markets 2005–2015, MediaAnalytics, Oxford, 2004.

[3] K. Wangnick, Desalination plants inventory report,International Desalting Association, 2000.

[4] M. Wilf, The Guidebook to Membrane DesalinationTechnology, Balaban Desalination, Awerbuch, 2007.

[5] B. Van der Bruggen, A. Koninckx and C. Vande-casteele, Separation of monovalent and divalent ionsfrom aqueous solution by electrodialysis and nano-filtration, Water Res., 38 (2004) 1347.

[6] M. Turek, Cost effective electrodialytic seawaterdesalination, Desalination, 153 (2002) 371.

[7] J.M. Veza, B. Peñate and F. Castellano, Electro-dialysis desalination designed for off-grid windenergy, Desalination, 160 (2004) 211.

[8] H. Schmoldt, H. Strathmann and J. Kaschemekat,Desalination of sea water by an electrodialysis-reverse osmosis hybrid system, Desalination, 38(1981) 567.

[9] A.A. Husseiny and H.L. Hamester, Engineeringdesign of a 6000 m3/day seawater hybrid RO-EDhelio-desalting plant, Desalination, 39 (1981) 171.

[10] H. Strathmann, Ion-exchange Membrane SeparationProcesses, Elsevier, Amsterdam, 2004.

[11] T. Xu, Ion exchange membranes: State of theirdevelopment and perspective, J. Membr. Sci., 263(2005) 1.

[12] H.J. Lee, S.H. Moon and S.P. Tsai, Effects of pulsedelectric fields on membrane fouling in electrodialysisof NaCl solution containing humate, Sep. Purif.Technol., 27 (2002) 89.

[13] H.J. Lee, J.H. Choi, J. Cho and S.-H. Moon,Characterization of anion exchange membranesfouled with humate during electrodialysis, J. Membr.Sci., 203 (2002) 115.

[14] E. Korngold, E. De Korosy, R. Rahav and M.F.Taboch, Fouling of anionselective membranes inelectrodialysis, Desalination, 8 (1970) 195.

[15] D. Marshall, P.A. Munro and G. Trägårdh, Theeffect of protein fouling in microfiltration and ultra-filtration on permeate flux, protein retention andselectivity: A literature review, Desalination, 91(1993) 65.

[16] S.T. Kelly and A.L. Zydney, Mechanims for BSAfouling microfiltration, J. Membr. Sci., 107 (1995)115.

[17] V. Linstrand, A.S. Jönsson and G. Sundström,Organic fouling of electrodialysis membranes withand without applied voltage, Desalination, 130(2000) 73.

H.-J. Lee et al. / Desalination 238 (2009) 60–69 69

[18] Y. Shim, H.J. Lee, S. Lee, S.H. Moon J. Cho, Effectsof natural organic matter and ionic species on mem-brane surface charge, Environ. Sci. Technol., 36(2002) 3864.

[19] F. Gode and E. Pehlian, Removal of Cr(IV) fromaqueous solution by two Lewatit-anion exchangeresins, J. Haz. Mat. B, 119 (2005) 175.

[20] M. Chabani, A. Amrane and A. Bensaili, Kineticmodeling of the adsorption of nitrates by ionexchange resin, Chem. Eng. J., 125 (2006) 111.

[21] Y. Chen, S. Liu and G. Wang, Kinetics and adsorp-tion behavior of carboxymethyl starch on α-alumina

in aqueous medium, J. Coll. Interf. Sci., 303 (2006)380.

[22] E. Pehlian and T. Altun, The study of various para-meters affecting the ion exchange of Cu2+, Zn2+, Ni2+,Cd2+, and Pb2+ from aqueous solution on Dowex50W synthetic resin, J. Haz. Mat. B, 134 (2006) 149.

[23] E.J. Watkins and P.H. Pfromm, Capacitance spec-troscopy to characterize organic fouling of electro-dialysis membranes, J. Membr. Sci., 162 (1999) 213.

[24] G. Belfort, Synthetic Membrane Process—Funda-mentals and Water Applications, Academic Press,New York, 1984.