Journal of Membrane Science 240 (2004) 105–111

Effect of operating variables on rejection ofindium using nanofiltration membranes

Ming Wua, Darren Delai Sunb,∗, Joo Hwa Tayb

a Intel Products (Shanghai) Ltd, No. 999 Ying Lun Road, Waigaoqiao Free Trade Zone, Pudong, Shanghai, PR Chinab School of Civil and Environmental Engineering, Nanyang Technological University, Singapore 639798, Singapore

Received 3 December 2003; accepted 30 April 2004

Available online 24 June 2004

Abstract

Indium and its compounds exhibit excellent semiconductor properties however they are suspected carcinogenic to human beings. For thefirst time, we applied nanofiltration (NF) technology to the separation of indium from a synthetic wastewater as a literature review revealedlittle information on the treatment of such a waste. In this research, three types of nanofiltration membranes, NTR7450, ES10 and ES10C, wereemployed to compare their performances under various operating conditions. With increasing indium concentration in the feed solution, therejection rates decreased in all the membranes, which could be ascribed to concentration polarization and ion-shielding effects. The changes ofindium concentration in the permeate (Cp) were then correlated to the concentration factor (CF) during nanofiltration of the feed solution. Theexperimental results were well predicted by the theoretical analysis. Increase of operating pressure enhanced their rejection rates of indium,which might be attributed to the “dilute effect”. The real rejection (fr) of indium by nanofiltration was found permeate flux dependent. Basedon the results obtained, the nanofiltration mechanisms of multivalent cations such as In3+ were delineated and discussed. It was found thatmost of the models developed from nanofiltration of univalent and divalent cations were still valid for the nanofiltration process of trivalentcations. However, the strong chemical potential of trivalent cations to form complexes in the solution around neutral pH exerted a significantimpact on indium rejection rates of the NF membranes. The experimental results suggest a stable performance of nanofiltration when appliedto the semiconductor wastewater, however, acidic conditions should be avoided.© 2004 Elsevier B.V. All rights reserved.

Keywords:Electrochemistry; Indium phosphide; Nanofiltration; Semiconductor; Wastewater

1. Introduction

Indium is an extremely rare element. Indium is estimatedto have abundance in the earth’s crust of 0.05 mg l−1, whichis similar to that of silver, but it is widely dispersed in con-centrations of 0.001% or less[1]. Indium does not formany minerals of its own. Instead, it is distributed in minuteamounts in many minerals, usually but not exclusively be-ing concentrated in sulfide deposits. Zinc, which it also re-sembles in size and other properties, is an important carrier,and zinc blendes afford the principal commercial source ofthe metal. Although indium was discovered in 1863, no usewas made of this metal for many years and the world supplywas measured in grams until well into the twentieth century.

∗ Corresponding author. Tel.:+65 6790 6273; fax:+65 6791 0676.E-mail address:ddsun@ntu.edu.sg (D. Delai Sun).

The first reported commercial application was as a minoraddition to gold-based dental alloys in which indium servedas a scavenger for oxygen[2]. In the last two decades, ithas been found that indium combines with elements of othergroups of elements such as antimony and phosphorus to pro-duce compounds that exhibit semiconductor characteristics.It has been reported recently that indium phosphide (InP)has a high refractive index allows a smaller radius of cur-vature leading to components that are at least 10–100 timessmaller than current silicon technology[3]. Another charac-teristic of InP is its direct bandgap, leading to very easy andfast quantum transitions when photons are either absorbedor emitted[4]. With the rapid evolution of semiconductorfabrication and assembly technologies in the last few years,it is anticipated higher and higher production volumes ofInP wafers in the near future.

A potential issue associated with increasing applicationsof indium compounds in the semiconductor industry is their

0376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.memsci.2004.04.017

106 M. Wu et al. / Journal of Membrane Science 240 (2004) 105–111

impacts on the environment. Before 1990s, it was gener-ally recognized that indium appeared to pose no threat tohuman health mainly because that environmental exposureof the general population to indium should be negligible[5]. However the latest research findings indicated thatInP might cause lung and breast cancers[6,7]. It is there-fore essential to develop corresponding pollution controland abatement processes to catch up with the comingup semiconductor technologies although these technolo-gies have not so far been widely applied in the industry.With this practical motivation, the main objective of thiswork was to study the performance of indium (In3+) re-jection by a few types of nanofiltration (NF) membraneunder various operating conditions. The rejection mech-anisms of multivalent cations by nanofiltration were alsodiscussed.

2. Materials and methods

2.1. Membranes

Three flat-sheet NF membranes, supplied by Nitto DenkoCo. (Japan), were used in this study. Their main propertiesare summarized inTable 1. NTR7450 is a NF membranewith loose structure, which has been well characterizedby other researchers. However, the reported molecularweight cut-off (MWCO) is found contradictory in the lit-erature, from 600–800 Da[8] to 10,000 Da[9]. NTR7450is negatively charged over the pH range, mainly due tothat the sulfonic acid groups are strongly acidic and com-pletely dissociated over nearly the entire pH range[10].ES10 and ES10C are oppositely charged NF membranes,with MWCO around 100 Da[11]. They are so-called“low pressure RO” membranes, which are relatively newmembers in the NF family. Characterization of thesetwo membranes has been reported in our previous study[12].

2.2. Crossflow membrane test unit

A typical laboratory scale crossflow membrane test unitwas employed for the experimental runs. The nanofiltration

Table 1Main properties of NF membranes

Nanofiltration membranes

NTR7450 ES10 ES10C

Material Sulfonated polysulfone Aromatic polyamides Aromatic polyamidesOperating pressure (kPa) 490–1470 490–980 490–1470Operating temperature (◦C) 5–60 <40 <40Feed pH range 2–11 1–11 2–10Membrane surface charge Negative Negative PositiveNaCl rejection (%) 40.00 99.30 99.00

Fig. 1. Setup of nanofiltration experimental system.

system is presented inFig. 1. The flat-sheet membranes wereplaced in the C10-T unit (Nitto Denko Co., Japan). The cell,designed in rectangular form (46 mm× 180 mm), could beoperated at up to 1000 kPa pressure (40◦C). The feed streamwas pumped from the solution storage tank to the test unit,tangentially crossing the membrane surface. Temperatureof the feed solution was maintained at 25± 2 ◦C using aheat-exchanger in the storage tank. The retentate was recir-culated into the 10 l feed solution vessel while the permeatewas collected in a tank for indium analysis. The system wasequipped with both pressure and flowrate gauges for onlinemonitoring purpose.

2.3. Synthetic wastewater and analytical methods

Reagent grade indium chloride (InCl3, 98% purity)supplied by Sigma–Aldrich was dissolved in de-ionized(DI) water (resistivity greater than 17.5 M� cm) to pre-pare the synthetic wastewater. The weight of solid InCl3and volume of DI water were pre-determined accordingto the desired indium concentration and quantity. Afteradding InCl3 into DI water, the solution was fully mixedwith a stirrer at 30 rpm for 2 min. The solution was thenused for the experimental runs. Indium was analyzed us-ing ICP-MS (Agilent 7500A) with a detecting limit of3.0�g l−1. The observed rejection rate (f0) was defined asthe comparison of indium concentrations in the feed waterand permeate while the real rejection rate (fr) was definedas the comparison of indium concentrations across themembrane.

M. Wu et al. / Journal of Membrane Science 240 (2004) 105–111 107

2.4. Experimental procedures

A series of experiments were designed to measure indiumrejection at various feed concentrations, operating pressureand pH. Fresh NF membranes with an effective area of60 cm2 were stabilized for at least 30 min prior to any samplecollection. In experiment I to evaluate the concentration ef-fect, the initial indium concentration of the synthetic wastew-ater was 40 mg l−1. Both feed and permeate samples werecollected to measure indium rejection rate during concentra-tion of the feed water from 10 to 2 l at 0.5 l interval, whichwas equivalent to a concentration factor (CF) of 1.0, 1.5, 2.0,2.5, 3.0, 3.5, 4.0, 4.5 and 5.0, respectively. In this study, theCF was defined as the ratio between the initial feed to re-tentate volume. The operating pressure was kept at 650 kPawith a recirculation flowrate of about 1.6–2.4 l min−1 (cir-culation Reynolds number is less than 1000). In experimentII to evaluate the trans-membrane pressure effect, indiumremoval efficiencies were measured under different operat-ing pressures ranging from 150 to 750 kPa with a recircula-tion flowrate of about 0.4–2.8 l min−1 (circulation Reynoldsnumber is less than 1200). The indium concentration of thefeed water was kept at 40 mg l−1. In experiment III to eval-uate the pH effect, pH was adjusted to 6 and 8, respectively,using sodium hydroxide. The indium concentration of thefeed water was 40 mg l−1 and the testing unit was operatedat 650 kPa.

3. Results and discussion

The change of operating conditions is a useful tool bywhich to determine the ion transfer and segregation proper-ties of nanofiltration membranes. The results of indium re-jection by three NF membranes are presented and discussedin this section.

3.1. Effects of feed water concentration

In order to simulate the real process, the selectedrange of indium concentration in the synthetic wastewater(40–200 mg l−1) matches the measured indium concentra-tion in the semiconductor wastewater[13]. Variations ofindium concentration in the discrete permeate time-seriessamples during nanofiltration are shown inFig. 2. It canbe seen from the figure that feed concentration exerts amarked effect on indium rejection. With increasing feedconcentrations (Cf ), indium concentrations in the permeates(Cp) increase in all the three NF membranes as shown inFig. 2a–cfor NTR 7450, ES10, and ES10C, respectively.This phenomenon has been ascribed to the concentrationpolarization and cation shielding effects. Concentrationpolarization refers to the build up of solute species at themembrane surface that adversely affects the membrane per-formance[14]. Meanwhile, the cation shielding effects onthe membrane negatively charged groups become progres-

sively stronger with increasing electrolyte concentration,leading to the decrease of the membrane repulsion forceson the anions. The decrease of co-ion exclusion from themembrane often results in a deteriorated rejection rate. Thiseffect is more evident for the ions with a low charge densitysuch as Cl− [15].

ES10 and ES10C are NF membranes with similar MWCOof 100 Da, however indium rejection rate of ES10C is muchhigher than that of ES10 over the concentration range tested.The rejection rate of ES10 is about 96% only at 40 mg l−1

indium feed concentration and it drops to about 87% withincreasing indium concentration up to about 160 mg l−1.ES10C shows relatively high and stable rejection ratesof indium throughout the experiments, around 99%. Thismight be due to their different charge properties. Chargeexclusion is an important feature of NF membranes[16,17].A charged membrane always tends to attract counter-ionsonto its surface in order to attain equilibrium of electricalpotentials at its surface and across the membrane. Indiumions (3+) are positively charged and therefore easily at-tracted onto the negatively charged membrane surface(ES10), which promotes the transport of cations through themembrane.

Another observation is that the two negatively changedmembranes, NTR7450 and ES10, present different rejec-tion capacities of indium and sodium (Fig. 2 andTable 1).NTR7450 shows a higher indium rejection than that ofsodium whilst ES10 shows a reverse trend. A plausibleexplanation is the hydrolysis of indium as a multiva-lent cation. Almost all cations of charge 3+ or highergive rise to mononuclear/polynuclear species, known as‘structure-making’ ions, in aqueous solution. The hy-drolyzed products thus generated are held together byhydroxo- or oxo-bridges. Increasing the size of the poly-meric unit eventually leads to the formation of insolublehydroxides or hydrated oxides as the limiting form. Thestability of a polynuclear cation can be expressed by the hy-pothetical reaction (simplified form) as shown below[18].

qMn+ + pH2O = Mq(OH)p(nq–p) + pH+ (1)

At the simplest level, this behavior can be ascribed to the in-fluence of the positive charge on the metal ion facilitating theloss of a water proton. However it should be noted that thereal reactions in the aqueous solution are much more com-plicated including ion–ion, ion–solvent and solvent–solventinteractions. In chloride solution, an analogous di-indiumspecies In2(OH)Cl4+ is claimed[19].

For NTR7450, as a loose nanofiltration membrane witha reported average pore diameter of 0.81 ± 0.2 nm [10],steric exclusion still plays an important role in solute rejec-tion [20]. As hydrolysis of multivalent cations increases thesize of ions, resulting in an increased steric hindrance factor(friction force), NTR7450 presents a higher rejection rate ofhydrated indium ions than that of dissociated sodium ions inthe solution. For ES10 with a dense structure, which is morelike a RO membrane, diffusion is the predominant mecha-

108 M. Wu et al. / Journal of Membrane Science 240 (2004) 105–111

Fig. 2. Effect of feed concentration on indium rejection at constant operating pressure of 650 kPa. Experimental results of NTR7450 as (�), ES10 as(�), ES10C as (�), and theoretical plot as (—).

nism for ion transport in the membrane. With an increasedStoke radius of the ions, a reduced effective diffusion co-efficient is expected, thus leads to a lower rejection rate ofindium than that of sodium in the ES10 membrane. The dif-ference in mass transfer mechanisms may qualitatively ex-plain the difference in indium rejection rates observed in theexperiments.

For engineering applications, it is of practicable interestto predict the rejection rate with the variation of indiumconcentration in the feed solution. As most of the recycledwastewater is commonly used as the feed water to produceultrapure water (UPW), it is essential to keep all the impu-rities at a low level. According to Cheryan[21], the soluteconcentration in the retentate at any time or stage of mem-brane processing is a function of both the volume reductionand observed solute rejection. Based on this theory, the fol-lowing equation has been developed by other researchersto predict rejection rates of univalent and divalent ions[22–24]:

ln Cp = ln

(Cp × C0

f

Cf

)+(

1 − Cp

Cf

)ln(CF) (2)

where C0f is the initial concentration of indium in the

upstream feed solution.Eq. (2) is used in this study toexamine the correlation between indium in the permeateand the concentration factor. The results are plotted inFig. 2d for the three NF membranes, respectively. A closeagreement between the experimental and theoretical re-sults is observed for the NTR7450 and ES10 membranes.As for the ES10C membrane, the rejection rates are vir-tually high over the test range of feed concentrations,which also reasonably fit the predicted trend as shown inFig. 2d.

3.2. Effects of operating pressure

Indium rejection rates at different operating pressures areshown inFig. 3. An increase in operating pressure resultsin increased removal efficiencies of all the three NF mem-branes although the increase is not significant in the ES10Cmembrane (Fig. 3a). Two competing phenomena dictate theseparation behavior of the ions under increasing permeateflux conditions. With increasing operating pressure, the fluxwill also increase. The indium concentration in the perme-ate decreases as water flux increases while the ion flux iselectrically and sterically hindered, resulting in lower in-dium concentrations in the permeate (the so-called ‘diluteeffect’). On the other hand, more ions are transported fromthe bulk solution toward the membrane surface as perme-ate flux increases, which enhance concentration polarizationand subsequently reduce indium rejection[25]. It seems thatthe latter effect plays an insignificant role in this experiment.

Quite a lot of researches have been carried out to link per-meate flux with ion rejection rate in NF membranes[26–28],among which Garba et al.[29] developed a specific modelfor divalent cations (cadmium), based on the combinationof the well known extended Nernst–Planck equation and thefilm theory:

ln(1 − fr) = − 1

KeJv + ln Φi (3)

The model can be applied to a nanofiltration process usingthree transport parameters: the water permeability, the iontransmittanceΦi and the effective transport coefficientKe ofthe ion. The water permeability can be determined by sta-tistical regression of permeate flux against applied pressure.In the case of dilute feed (In3+ < 200 mg l−1), the perme-ate flux is proportional to the trans-membrane pressure. Ac-

M. Wu et al. / Journal of Membrane Science 240 (2004) 105–111 109

Fig. 3. Effect of operating pressure on indium rejection rate at constant feed indium concentration of 40 mg l−1. Experimental results of NTR7450 as(�), ES10 as (�), ES10C as (�), and theoretical plot as (—).

cording to the Nernst–Planck theory,f0 = fr = 0 atJv = 0,which is the boundary condition, other two parameters,Φi

andKe, can be calculated by plotting the rejection rate ver-sus flux. The real rejection rate (fr) can be computed fromthe observed rejection rate (f0) using the following equation:

fr = [f0 exp(Jv/Ke)]

{1 − f0[1 − exp(Jv/Ke)]} (4)

The results are plotted inFig. 3b–d for NTR7450, ES10and ES10C, respectively. The experimental data conform tothe linearity between ln(1− fr) andJv that is predicted byEq. (3). The computed transfer coefficients of indium in thethree NF membranes are shown inTable 2. It can be seenthat coefficientKe drops in the order of NTR7450, ES10 andES10C. A lower transfer coefficient normally means higherremoval efficiency[29].

3.3. Effects of pH

When dissolved in water, indium chloride solution showslow pH of 3–4. Increase of pH greatly enhances indium re-jection of all the three NF membranes, among which ES10and ES10C achieve almost 100% removal efficiencies at pH8 (Fig. 4a). This phenomenon could be ascribed to the for-mation of indium hydroxide and aquo-cation complexes inthe presence of Na+ and Cl−, the so-called inorganic com-

Table 2Computed effective transfer coefficients (40 mg l−1 feed concentration)

Nanofiltration membranes

NTR7450 ES10 ES10C

Effective transfercoefficientKe (m s−1)

1.84E−06 7.9E−07 5.0E−07

plex. At high pH values (pH > 7), base-induced hydrolysisof In3+ solution causes precipitation well before the OH/Inratio reaches 3.0[30], which can be illustrated by its phasediagram as shown inFig. 4b [31]. It is noteworthy that in-dium rejection by nanofiltration has greatly improved evenwhen the feed solution is still slightly acidic, say pH 6 inthis case, which might not be well explained by the theoryof base-induced hydrolysis in InCl3 solution. A plausibleexplanation is that there might exist some indium oxidesin the InCl3–NaOH–H2O solution that exhibit amphotericproperties[32]. Although there are still arguments on theamphoteric character of indium oxides and hydroxide[33],the experimental results of this study tend to support its ex-istence. Another possibility is that a portion of the indiumcomplexes is transformed to hydrolysis products beforeprecipitation occurs. The following equilibrium can beassumed:

InCl2 + In3 + H2O ⇔ In2ClOH4+ + H+ (5)

With a measured equilibrium constant of−2.3±0.1 [19], thereaction can take place under the acidic aquo-environment.Hence, with the addition of an alkaline, the catalyzed pro-cess (NaOH as the catalyst) immediately reaches a hydrody-namic balance in the solution where polymerized hydrolysisproducts such as In2(OH)3Cl3, [InO(OH)2]−, [In(OH)4]−and [In(OH)4(OH2)2]− become dominant although the endproducts are not certain, depending on the actual pH of thesolution. This has resulted in a totally changed performanceof nanofiltration. When complexes are formed, the effect ofcharge exclusion becomes insignificant. Therefore, there islittle difference in the performances of ES10 and ES10C,which have similar MWCO. As for the NTR7450 mem-brane, polymerization of the hydrated aquo-cations furtherincreases the hindrance friction inside the nano-pores,

110 M. Wu et al. / Journal of Membrane Science 240 (2004) 105–111

Fig. 4. Effect of pH on indium rejection rate at constant feed concentration (40 mg l−1) and operating pressure (650 kPa); (b) is adopted from[31].Experimental results of NTR7450 as (�), ES10 as (�), and ES10C as (�).

leading to an improved indium rejection capacity of themembrane.

As in natural water environment where pH is always neu-tral, NF membranes are therefore able to effectively removealmost all trivalent cations. This may explain that most ofnanofiltration R&D focuses on the rejection of univalent anddivalent cations only. However, this may not be true when-ever the water environmental is acidic. The results obtainedfrom this study have important implications on a full-scaleNF system treating indium phosphide wastewater producedfrom semiconductor manufacturing processes. The wastew-ater should not be mixed with any acidic waste, which maydeteriorate indium rejection rates and therefore reduce thechance of water recycling, or at least increase the recyclingcost later on.

4. Conclusions and perspectives

Indium can be effectively removed by the three NF mem-branes under a steady state, i.e., when polymerized hy-drolysis products or complexes are formed at neutral pH.However, their performances may vary when the feed solu-tion is acidic. NTR7450 and ES10 are negatively chargedmembranes with different pore size. Under all test condi-tions, ES10 shows a better indium rejection rate than thatof NTR7450 indicating that the latter, as a loose NF mem-brane, should not be considered for the real wastewater if theformer is a choice. ES10C is positively charged NF mem-brane, which shows stable performance under either acidicor neutral conditions. From appearance, ES10C should bethe best candidate for the semiconductor InP waste. How-ever it should be noted that there is little difference betweenES10 and ES10C when pH of the feed solution is aroundneutral. In addition, most of the particles in a natural wa-ter environment are negatively charged. Thus, ES10 shouldbe also recommended for pilot-scale studies of the semicon-ductor wastewater.

Future studies should focus on nanofiltration of multiva-lent cations under non-steady states where the cations arepartially in the dissociated or hydrated form. Under acidicconditions, polymerization and complexation of cations in

the presence of other anions may take longer time to reachthe steady status. This process has not been fully understoodand described.

Nomenclature

Cf concentration of feedwater (mg l−1)C0

f initial concentration of feedwater (mg l−1)Cp concentration of permeate (mg l−1)CF concentration factorf0 observed rejection ofith cation (%)fr real rejection ofith cation (%)Jv permeate flux (m3 m−2 s−1)Ke effective transfer coefficient ofith cation

(m s−1)MWCO molecular weight cut-offUPW ultrapure waterΦi transmittance ofith cation (%)

References

[1] T. Moeller, Inorganic Chemistry: A Modern Introduction, Wiley, NewYork, 1982.

[2] I.J. Polmear, The elements, in: A.J. Downs (Ed.), Chemistry ofAluminum, Gallium, Indium and Thallium, Blackie Academic &Professional, London, 1993, pp. 81–110.

[3] M. Telford, Advances in InP and competing materials, III-Vs Rev.14 (6) (2001) 28.

[4] D.J. Suh, O.O. Park, T. Ahn, H.K. Shim, Large two-beam couplingin the p-PMEH-PPV/DPP/DO3/C60, Jpn. J. Appl. Phys. 141 (2002)L428.

[5] H. Seiler, H.G. Sigel, Handbook on Toxicity of Inorganic Com-pounds, Marcel Dekker, New York, 1988.

[6] A. Tanaka, M. Hirata, M. Omura, N. Inoue, T. Ueno, T. Homma,L. Sekizawa, Pulmonary toxicity of indium-tin oxide and indiumphosphide after intracheal instillations into the lung of hamsters, J.Occup. Health 44 (2002) 99.

[7] S.M. Snedeker, Environmental chemicals and breast cancer risk—why is there concern?, Institute for Comparative and EnvironmentalToxicology, Cornell University, New York, Fact Sheet #45, 2002.

[8] B.V.D. Bruggen, C. Vandecasteele, Flux decline during nanofiltrationof organic components in aqueous solution, Environ. Sci. Tech. 35(2001) 3535.

M. Wu et al. / Journal of Membrane Science 240 (2004) 105–111 111

[9] A.E. Yaroshchuk, Optimal charged membranes for the pressure-driven separations of ions of different mobilities: theoretical analysis,J. Membr. Sci. 167 (2000) 149.

[10] J. Schaep, C. Vandecasteele, Evaluating the charge of nanofiltrationmembranes, J. Membr. Sci. 188 (2001) 129.

[11] M. Thanuttamavong, K. Yamamoto, J.I. Oh, K.H. Choo, S.J. Choi,Rejection characteristics of organic and inorganic pollutants by ul-tra low-pressure nanofiltration of surface water for drinking watertreatment, Desalination 145 (2002) 257.

[12] J.H. Tay, J.L. Liu, D. Sun, Effect of solution physico-chemistryon the charge property of nanofiltration membranes, Water Res. 36(2002) 585.

[13] M. Wu, D. Sun, J.H. Tay, Characterization of indium phosphide(InP) wastewater for treatment and recycling, Enviro. Technol., 2004,under review.

[14] K.M. Pastagia, S. Chakraborty, S. Dasgupta, J.K. Basu, S. De, Pre-diction of permeate flux and concentration of two-component dyemixture in batch nanofiltration, J. Membr. Sci. 218 (2003) 195.

[15] J.P. Hsu, S.W. Huang, Y.C. Kuo, S. Tseng, Stability of a dispersionof particles covered by a charge-regulated membrane: effect of thesizes of charged species, J. Coll. Int. Sci. 262 (2003) 73.

[16] J.D. Sherwood, F. Risso, F.C. Paillot, F.E. Levy, M.C. Levy, Rates oftransport through a capsule membrane to attain Donnan equilibrium,J. Coll. Int. Sci. 263 (2003) 202.

[17] A. Martin, F. Martinez, J. Malfeito, L. Palacio, P. Pradanos, A. Her-nandez, Zeta potential of membranes as a function of pH optimiza-tion of isoelectric point evaluation, J. Membr. Sci. 213 (2003) 225.

[18] C.F. Baes Jr., R.E. Mesmer, The hydrolysis of cations, Wiley/Interscience, New York, 1976.

[19] J. Burgess, Metal Ions in Solution, Ellis Horwood Ltd., New York,1978.

[20] J. Palmeri, P. Blanc, A. Larbot, P. David, Theory of pressure-driventransport of neutral solutes and ions in porous ceramic nanofiltrationmembranes, J. Membr. Sci. 160 (1999) 141.

[21] M. Cheryan, Ultrafiltration and Microfiltration Handbook, TechnomicPublication Co., Lancaster, 1998.

[22] J. Kilduff, W.J. Weber Jr., Transport and separation of organicmacromolecules in ultrafiltration processes, Environ. Sci. Tech. 26(1992) 569.

[23] B. Logan, Environmental Transport Processes, Wiley, New York,1999.

[24] L.D. Guo, B.J. Hunt, P.H. Santschi, Ultrafiltration behavior of majorions (Na, Ca, Mg, F, Cl, and SO4) in natural waters, Water Res.35 (6) (2001) 1500.

[25] A. Seidel, J. Waypa, M. Elimelech, Role of charge (Donna) ex-clusion in removal of arsenic from water by a negatively chargedporous nanofiltration membrane, Environ. Eng. Sci. 18 (2) (2002)105.

[26] P.Y. Pontalier, A. Ismail, M. Ghoul, Specific model for nanofiltration,J. Food Eng. 40 (3) (1999) 145.

[27] I. Koyuncu, D. Topacik, Effect of organic ion on the separationof salts by nanofiltration membranes, J. Membr. Sci. 195 (2002)247.

[28] C. Labbez, P. Fievet, A. Szymxzyk, A. Vidonne, A. Foissy, J. Pagetti,Retention of mineral salts by a polyamide nanofiltration membrane,Sep. Puri. Tech. 30 (2003) 47.

[29] Y. Garba, S. Taha, N. Gondrexon, G. Dorange, Ion transport mod-eling through nanofiltration membranes, J. Membr. Sci. 160 (1999)187.

[30] I.R. Grant, III–V compounds, in: A.J. Downs (Ed.), Chemistry ofAluminum, Gallium, Indium and Thallium, Black Academic & Pro-fessional, London, 1993, pp. 292–320.

[31] J.A. Campbell, R.A. Whiteker, A periodic table based on potential-pHdiagrams, J. Chem. Educ. 46 (1969) 92.

[32] L.C.A. Thompson, R. Pacer, The solubility of indium hydroxide inacidic and basic media at 25◦C, J. Inorg. Nucl. Chem. 25 (1963)1041.

[33] J. Kleinberg, Inorganic Chemistry, Heath, Boston, 1960.