Desalination 270 (2011) 57–63

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Desalination

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Arsenate retention from aqueous solution by hydrophilic polymers throughultrafiltration membranes

Julio Sánchez, Bernabé L. Rivas ⁎Polymer Department, Faculty of Chemistry, University of Concepción, Casilla 160-C, Concepción, Chile

⁎ Corresponding author. Tel.: +56 41 220 4190.E-mail address: brivas@udec.cl (B.L. Rivas).

0011-9164/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.desal.2010.11.021

a b s t r a c t

a r t i c l e i n f o

Article history:Received 25 July 2010Received in revised form 2 November 2010Accepted 4 November 2010Available online 3 December 2010

Keywords:Liquid-phase polymer-based retentionWater-soluble polyelectrolyteArsenic removal

Combining the liquid-phase polymer-based retention, LPR technique with an ultrafiltration membranefacilitates the separation of arsenic ionic species that are retained by the functional groups of hydrophilicpolyelectrolytes.Arsenate retention by P(ClAETA) at a high arsenate concentration (47.6 mg L−1) was 58% and this removalcapacity increases gradually, reaching 100% retention when the arsenate concentration in the cell was atminimum (5.5 mg L−1) using molar ratio (20:1) polymer:As(V).Arsenic removal was also determined at low concentrations (in μg L−1). The results show that P(ClAETA)removes 65% of arsenate at lower concentration and that the arsenate concentration in each 20 mL of filtrateabove Z=3 is below the maximum permissible level of the World Health Organization (WHO).The charge–discharge process shows that the discharge process of the arsenate ions from polymers can beperformed when the polymer–arsenate was in contact with the acid solution from the reservoir.Removal of arsenic from the Camarones River water was also performed by using P(ClAETA). The water-soluble polymer showed a high performance (100%) for the first Z values and then decreased up to 16% forZ=10.

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1. Introduction

Arsenic is a very toxic element that occurs in a variety of forms andoxidation states. The main arsenic species present in natural waters arearsenate (oxidation stateV) and arsenite (oxidation state III) oxy-anions[1–4]. Arsenate is less toxic than arsenite, but more abundant and moremobile in natural surface waters, whereas arsenite is found mostly inanaerobic environment such as groundwater [5]. The maximumpermissible concentration in Europe and accepted by theWorld HealthOrganization (WHO) for arsenic in drinking water is 10 μg L−1. On theother hand, developing countries are struggling to find and implementsystems to reach the standard of 50 μg L−1 in areas affected by thepresence of arsenic [6,7].

In order to remove traces of arsenic fromwater, variousmethods likeion-exchange, adsorption (especially with reagents impregnated resinsandmetal-loaded chelating resins), chemical precipitation–coagulation,membrane processes like reverse osmosis, and complexation have beenused to remove toxic species, such as arsenic [8]. Still, the completeextraction of arsenic from drinking water, wastewaters, and industrialeffluents in order to reach acceptable levels still represents a truechallenge.

Among the recently developed materials for arsenic removal are:the water-soluble cationic polyelectrolytes that are combined withmembrane filtration to remove arsenates from aqueous solutions[9–12]. This method is known as liquid-phase polymer-basedretention (LPR), and it involves the use of an ultrafiltrationmembranethat separates the ionic species interacting with the functional groupsof water-soluble polymers with high molecular weights, thuspreventing them from passing through the membrane [13]. Thegreat advantage of the LPR method is that it is carried out inhomogeneous media, and thus largely avoids the phenomenon ofmass transfer or diffusion that occurs in heterogeneous media [14].

Cationic polyelectrolytes with quaternary ammonium salts havegreat capacity to link to arsenate oxy-anions. This interaction occursbetween the nitrogen of the ammonium group (positively charged)and the oxygen of anion arsenate forming a dipole [9]. Theinteractions are produced mainly by the anion exchange betweenthe counter ion of the quaternary ammonium salt and the arsenateanions at basic pH, as can be corroborated by the polymers' higherretention capacity at basic pH where divalent As(V) species arepredominant [10,11]. On the other hand, the retention capacity islimited by the polymer concentration. Previous results indicate anoptimal polymer:arsenate molar ratio for complete separation is 20:1[12]. These cationic polyelectrolytes remove arsenate ions moreefficiently that arsenite ions in a wide pH range [15].

The aim of the present work is to study the arsenate retentionproperties of polyelectrolytes P(SAETA) and P(ClAETA) using the LPR

58 J. Sánchez, B.L. Rivas / Desalination 270 (2011) 57–63

technique and considering the interference of other ions, such as NaCland Na2SO4 in solution, using both the washing method at constantionic strength and variable ionic strength. The influence of arsenicconcentration was another parameter studied and even at very lowconcentrations (μg L−1 levels).

Arsenic removal from the Camarones River water was also tested.

2. Material and methods

2.1. Hydrophilic polymers

In this study, the hydrophilic polymers with different counter ionswere prepared by free-radical polymerization in the same conditions.The following monomers were used for the free-radical polymeriza-tion: [2-(acryloyloxy)ethyl]trimethylammonium chloride solution(ClAETA) (80 wt.% in water; Aldrich) and [2-(acryloyloxy)ethyl]trimethylammoniummethyl sulfate (SAETA) (80 wt.% inwater;Aldrich).

The products were dissolved in water, purified with ultrafiltrationmembranes of poly(ethersulfone), and fractionated by ultrafiltrationmembranes with different molar mass cut-offs (MMCO) range(10,000, 30,000, 50,000, and 100,000 Da). The maximum yieldobtained in mass over a fraction above than 100,000 g mol−1. Thestructures of both polymers are shown in the Fig. 1.

2.2. LPR procedure

LPR equipment and procedure were previously described [13]. Inthis study, two different modes of LPR separation were used toremove arsenate ions. The first one is the washing method (seeFig. 2a), which is a batch-like procedure wherein washing isperformed with water at constant pH. Before carrying out ultrafiltra-tion, the pH of the solution was adjusted to 8. The resulting mixturepolymer/arsenate was stirred for 1 h at room temperature, and thenplaced in the ultrafiltration cell. The solution was submitted toultrafiltration and washed with reservoir water at the same pH.Ultrafiltration was performed under a total pressure of 3.5 bar usingan ultrafiltration membrane of polyethersulfone with molecular masscut-off, MMCO, 10,000 Da. Total cell volume was kept constant duringthe filtration process. Fractions of 20 mL were collected up to a totalvolume of 200 mL. All experiments were performed with a solution ofpolymer and As(V) (20:1 polymer:As(V) mole ratio). Results of the As(V) uptake are systematically presented as the percentage of retentionR(%) versus the filtration factor Z (volume of filtrate/volume of the cell).

The second mode is the enrichment method (see Fig. 2b), whichdetermines the maximum retention capacity of polymer and it isanalogous to a columnmethod. A solution containing the arsenate ionsto be separated is passed from the reservoir through the ultrafiltrationcell containing a polymer solution. Both cell and reservoir solutionsmay

Fig. 1. Structures of the water-soluble polyelectrolytes: poly[2-(acryloyloxy) ethyl]trimethylammonium chloride, P(ClAETA), poly[2-(acryloyloxy) ethyl] trimethylammoniummethyl sufate, P(SAETA).

be adjusted to the samevalues of pHand ionic strength. The enrichmentmethod was used in aqueous solution, using 4 mM de As(V) solutionand 0.8 mmol of water-soluble polymers at 300 mL of total filtratevolume. In the charge–discharge process, the enrichment method andwashing method were alternately used.

In both cases, a blank experiment (in the absence of thewater-solublepolymer) is included inorder toevaluate the interactionof themembranewith arsenate ions. Arsenic concentrationwasmeasured in the filtrate byatomic absorption spectrometry (AAS) using a Perkin Elmer 3100spectrometer and Perkin Elmer AAnalyst T200; a HGA 900 graphitefurnace was used for measurements at low arsenic concentration. Thequantity of arsenic species retainedwas calculated as the difference withthe initial concentration. The pHwasmeasured by a pHmeter (H. Jürgenand Co). A solution of 1000 mg L−1 of Na2HAsO4·7H2O (Merck) wasused. The pH was adjusted by adding 0.1 M NaOH or HNO3 (by Merck)and the ionic strength was adjusted using different concentrations from0.01 to 0.1 M of NaCl and Na2SO4 (by Sigma Aldrich).

3. Results and discussion

In order to systematize the interactions of polymer with ions insolution using the ultrafiltration technique, two factors should bedefined: 1) filtering factor and 2) retention (Rz), which is the fractionof ions remaining in the cell.

Rz = Mzc =Mzinit ð1Þ

where Mzc is the absolute amount of ions that are in the feed phaseand Mzinit is the absolute amount of ions at the start of theexperiment. The subscript z refers to ion charge.

The filtration factor (Z) is the ratio between the total permeatevolume and the retentate volume:

Z = Vf = Vo ð2Þ

Depending on the experimental data, a graph (retention profile) inwhich Rz is represented as a function of Z, can be drawn.

3.1. Competitive effect of other monovalent and divalent anions onarsenate retention at constant ionic strength

The cationic polyelectrolytes present the highest retention ofarsenate species by the LPR technique when no other anions arepresent in the solution.

In previous reports, the retention experiments were studied atdifferent polymer:arsenic mole ratio by the washing method and thebest conditions was 20:1 [12]. Both polymers were capable ofinteracting and removing arsenate species at pH 8. This resultdemonstrates that polyelectrolytes with chloride exchanger groups,such as P(ClAETA), show a higher ability to remove arsenate than thepolymer that contains methyl sulfate as anion exchanger group,P(SAETA), at the same conditions. Polymers with chloride exchangergroups have the highest capacity to remove arsenate oxy-anions (97%)at basic pH (see Fig. 3). These results can be attributed to the easierrelease of the chloride anion in comparison with the methyl sulfateanion, which are associated with the quaternary ammonium groups.

In order to determine the influence of other monovalent anddivalent anions, different experiments in presence of divalent andmonovalent anions, such as a sulfate and chloride, were performedusing different concentrations of these salts at pH 8. In this study, weused the washing method at constant ionic strength adding to boththe reservoir and the ultrafiltration cell concentrations in the range of1×10−3 M to 1×10−1 M NaCl and Na2SO4 in separate experimentswith a P(ClAETA):As(V) mole ratio of 20:1 inside of ultrafiltration cell.

The arsenate retention is found to decreasewith the increasing saltconcentration and the increased charge of the added anion. The

Fig. 2. General procedure of arsenate extraction using LPR technique at pH 8. The different experiments, a) washingmethod, b) enrichment method and c) interference of external ions.

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decrease in the retention was due to the presence of the added saltsdeclining in the following order Na2SO4NNaCl.

According to the literature [16], the order of interference in thearsenic retention is: trivalent ionsNdivalent ionsNmonovalent ions.The effect of added electrolytes on arsenic binding to the polyelec-trolyte can be understood as due to the competition between arsenateand other anions for binding sites on the polymer. The affinity ofanions to bind onto the polymer is similar to the behavior observed inthe ion-exchange resin containing ammonium groups when remov-ing arsenic by ion-exchange process [16]. Another way of explainingthe effect is that the electrical double layer is compressed around thepolymer as the ionic strength increases, thus reducing the polymer'selectrical potential. The divalent anions produce a greater reduction inarsenic retention than the monovalent anions because the divalentanions bind more strongly to the polymer's charged sites and alsocompress the electrical double layer around the polymer moreeffectively than the monovalent anions [17].

It is reasonable that sulfate or chloride anions present differentinterference toward arsenate retention. The results prove the adsorp-

Fig. 3. Retention profile of As(V) by (♦) P(ClAETA) and (▲) P(SAETA) at pH 8, using30 mg L−1 of As(V) and mole ratio of 20:1 polymer:As(V) (1.6×10−4 mol:8×10−6 mol).

tion of the interfering ions at the same active sites on the polymer,especially in the case of sulfate, which like arsenate has a tetrahedralstructure and divalent charge at basic pH. The results showed thatarsenic retention decreased from 96% to 20% at Z=10 when just1×10−3 M of sodium sulfate was added. Moreover, arsenate retentiondropped to zerowhen sulfate ion concentration increased to 5×10−3 M(see Fig. 4b). The competition between arsenate and monovalentchloride was lower than that between sulfate and arsenate. In anotherseparate experiment, when the minimum chloride concentration wasadded, corresponding to 1×10−3 M, the arsenate retention capacity ofarsenate decreased from96% to55%at Z=10 (see Fig. 4a). This behaviorshows that when the concentration of chloride was increased, it wasblocking the polymer active sites and the retention of arsenate wasdecreasing gradually. These results proved that when the ionic strengthincreases the retention capacity of the polymer decreases due to thecompetition between ions in solution. This behavior depends directly ofthe type and charge of ion interfering. Even at a low concentration,interfering ions block and diminish the extracting capability of thewater-soluble polymer.

3.2. Effect of arsenate concentration on retention capacity

The effect of arsenate concentration on arsenate removal inpresence of NaCl was also studied. All the experiments were carriedout at polymer:As(V), 20:1 M ratio and pH 8. The arsenateconcentration in the feed ranged from 2.46×10−6 M (5.5 mg L−1)to 1.27×10−5 M (47.6 mg L−1), and all were at constant ionicstrength in presence of 1.54×10−3 M NaCl.

In comparison with P(SAETA), P(ClAETA) shows higher retentionarsenate capacity in all the cases. At higher arsenate concentration(47.6 mg L−1), arsenate retention by P(ClAETA) was 58% and thisremoval capacity increased gradually reaching 100% retention whenthe arsenate concentration in the cell wasminimum (5.5 mg L−1) (seeTable 1).

The effect of the conformational changes of polyelectrolyte and theinfluence of ionic strength cannot be discarded. Indeed, it may be dueto a conformational change on the polymer chains [18], the filtrationof arsenate ions and their subsequent release from the polymerinduces an increase of the net charge on the polymer surface and thenin an expansion of the chains in order to increase the total surface,minimizing the electrostatic repulsions at low arsenic concentration.

Fig. 4. Retention profile of As(V) by P(ClAETA) in presence of different concentrations of(a) NaCl and (b) Na2SO4 in both the reservoir and ultrafiltration cell at pH 8, usingmolarratio 20:1 polymer:As(V) (3.2×10−4 mol:1.6×10−5 mol).

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Related with this, at high arsenic concentrations, the decrease on thesurface charge density of the polymer induces a decrease in thestrength of the interactions with the arsenic ions, and in consequence,their easier release into the solution from the polymer domain duringfiltration.

3.3. Arsenate retention capacity at low concentration

Arsenic removal was also determined at low concentrations (inμg L−1) in order to study the capability of the cationic water-solublepolymers by the LPR technique to extract traces of arsenic untilreaching permissible levels.

Table 1Effect of As(V) concentration on the removal. Retention percentages of P(ClAETA) and P(SAETA) polymer:As(V) ratio 20:1 at pH 8 and Z=10 in the presence of 1.54×10−3 MNaCl (90 mg L−1).

Mol ofpolymer

Mol of As(V) As(V) in feed(mg L−1)

R(%) ofP(ClAETA)

R(%) ofP(SAETA)

2.54×10−4 1.27×10−5 47.6 58 198.05×10−5 4.02×10−6 15.1 65 205.33×10−5 2.66×10−6 10.0 80 422.93×10−5 2.46×10−6 5.5 100 56

The experiments were carried out by the washing methodusing an arsenate standard solution of 4.47×10−6 M (200 μg L−1)and a polymer:arsenate 20:1 mol ratio (1.78×10−6 mol of poly-mer:8.94×10−6 mol of arsenate) at pH 8. The arsenic concentrationwasmeasured in the filtrate by atomic absorption spectrometry (AAS)Perkin Elmer AAnalyst T200; HGA 900 graphite furnace.

In these utrafiltration experiments, P(ClAETA) and P(SAETA) wereused as extracting reagents and the results show almost the samebehavior previously observed. The polymer with the chloride counterion showed greater removal ability than the onewithmethyl sulfate. P(ClAETA) reached 65% of retention at Z=10, while P(SAETA) onlyreached 20% at the same condition. This behavior of P(ClAETA) atlower concentrations shows that the arsenate concentration, in each20 mL of filtrate above Z=3 is below the maximum permissible levelof the WHO. In the case of P(SAETA), arsenic concentrations in each20 mL of filtrate below than maximum permissible limits wasobtained above Z=5 (see Fig. 5).

3.4. Maximum arsenate retention capacity by the enrichment method

The maximum retention capacity (C) of arsenate by the polyelec-trolyte was determined by the enrichment method. This methodconsists in adding to a polyelectrolyte solution the maximumconcentration of the arsenate anion that the polymer can bind inorder to reach saturation. The maximum retention (enrichmentmethod) is defined as:

C = MVð Þ= Pm ð3Þ

where Pm is the amount of polymer (g), M is initial concentration ofAs(V) (mg L−1), V is the volume of filtrate (volume set) containing As(V) (mL) that passes through the membrane. Themaximum retentioncapacity (C) of arsenate was calculated for the total filtrate volume(300 mL). Assuming a quantitative retention of As(V), the enrichmentfactor (E) is a measurement of the polymer's binding capacity and it isdetermined as follows:

E = PCð Þ=M ð4Þ

where P is the polymer concentration (g L−1). Since the arsenate ion–polymer interactions are processes in equilibrium, a lower slope in therate of increase of the arsenate concentration in the filtrate is normallyobserved. The differences in the slopes can easily be used to calculate

Fig. 5. Retention profile of As(V) by (♦) P(ClAETA) and (▲) P(SAETA) at pH 8, using200 μg L−1 of As(V) and mole ratio of 20:1 polymer:As(V) (1.78×10−6 mol ofpolymer:8.94×10−6 mol of arsenate).

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the amount of arsenate ions bound to the polymer and free in solutionas well as the maximum retention capacity [13].

The behavior was observed in different cationic water-solublepolyelectrolytes using the enrichment method in aqueous solutions atpH 8 using 8×10−4 mol of polymer into the ultrafiltration cell andadding a solution of 4×10−3 mol of As(V) from the reservoir. Insimilar polymeric structures but with different counter ions, such as P(ClAETA) and P(SAETA), the results were different. The values of Cwere 165 mg g−1 for P(ClAETA) and 79 mg g−1 for P(SAETA), and thetotal filtrate volume was 300 mL. Assuming quantitative retention ofAs(V), the enrichment factor was analyzed (E=4 for P(ClAETA) andE=2.5 for P(SAETA)). The type of anion exchanger was an importantfactor in arsenate retention.

3.5. Desorbing of arsenate: the charge–discharge process

In order to study the charge–discharge process, the enrichmentmethod and washing method were alternately used. In theseexperiments P(ClAETA) and P(SAETA), which differ only from theircounter ions, were studied. The first step of the experiment was thesaturation of the polymers through the enrichment method, using theconditions previously described: the enrichment method was per-

Fig. 6. Charge–discharge process of arsenate ions using P(ClAETA) and P(SAETA). (a) first chaof polymers using washing method at pH 3 with 1×10−1 M HCl. (c) recharge of polymerswashing method at pH 3 with 1×10−1 M HCl.

formed at pH 8, using 8×10−4 mol of polymer into the ultrafiltrationcell (20 mL) and adding a solution 4×10−3 M in As(V) from thereservoir. After reaching saturation, the polymer:As(V) solution waswashed in the ultrafiltration cell with reservoir water buffered at pH3, in a similar way to the washing method. It was assumed that thepolymer activity can be recovered in the media's strongly acidconditions media and that this did not significantly affect thepolymer's active sites because acid pH was used in the radicalpolymerization. The same charge–discharge process was repeatedtwice for each polymer in order to determine the capacity of arsenatedelivery and to regenerate the extracting ability of the water-solublepolyelectrolyte.

Fig. 6 shows the charge–discharge behavior for both polymers.Fig. 6(a) presents the enrichment process (charge) reaching the samemaximum retention capacity (C) obtained previously for bothpolymers at pH 8. The values of C were 165 mg g−1 for P(ClAETA)and 79 mg g−1 for P(SAETA), and the total filtrate volume was300 mL.

After the charge process, the discharge process was initiatedchanging the pH from basic to acid using buffered solution of 1×10−1

MHCl. Fig. 6(b) presents the discharge process of the arsenate ions fromboth polymers when the polymer–arsenate is in contact with acid

rge process of polymers through enrichment method at pH 8, (b) first discharge processthrough enrichment method at pH 8, (d) second discharge process of polymers using

62 J. Sánchez, B.L. Rivas / Desalination 270 (2011) 57–63

solution (pH 3) from the reservoir. The first discharge of arsenate waseffective and was carried out almost entirely in the first 100 mL ofsolution when a higher ion arsenate concentration is discharged from P(ClAETA) in comparison with P(SAETA) at the same volume. Bothpolymers discharge all the amount of arsenate at 300 mL of filtrate.

Fig. 6(c) shows that the second charge process did not improve thepolymers' maximum retention capacity when compared with the firstcharge process. P(ClAETA) lost the capacity to remove arsenate, P(SAETA) was only slightly better at the same conditions. The values ofC were 83 mg g−1 for P(ClAETA) and 47 mg g−1 for P(SAETA), and thetotal filtrate volume was 300 mL. This result is probably due to thepresence of more species in the solution when the pH was adjustedfrom basic to acid in the discharge process and from acid to basic inthe second charge process. Finally, the second discharge process(Fig. 6(d)) showed almost the same behavior in both polymers,releasing most of the arsenate ions into the filtrate in the first 100 mLin a similar manner.

3.6. Arsenic removal from the Camarones River water

The town of Camarones is located in the Atacama Desert in thenorthern Chile. The water from the Camarones River is usedprincipally for human consumption and agricultural activities in thearea [19].

The Camarones River water presents natural arsenic contamina-tion with total arsenic concentrations above 1000 μg L−1 that existsmainly in the form of As(V). The water samples were collected,characterized and reported in a study made by Cornejo et al. [19].

In the present study, we also include some preliminary results ofthe arsenic removal from the Camarones River water using LPRtechnique with P(ClAETA) in the already mentioned conditions: pH 9and polymer:As(V) 20:1 mol ratio.

The preliminary results of the As(V) removal from CamaronesRiver water by P(ClAETA) are presented in the Fig. 7. The water-soluble polymer showed a high performance (100%) for the first Zvalues and then decreased up to 16% for Z=10. This means that underthese conditions the interaction between the polymer and arsenate isnot strong enough, probably due to the presence of other ions. TheCamarones River water presents mainly at pH 8.3, 154 mg L−1 ofsulfate, 541 mg L−1 of chloride, 1650 mg L−1 of total dissolved solids,15.68 mg L−1of boron, among others [19].

In a future research we will try to optimize the conditions of theLPR technique in order to improve the arsenic retention.

Fig. 7. Retention profile of As(V) from Camarones River water by P(ClAETA) using moleratio of 20:1 polymer:As(V) and pH 9.

4. Conclusions

The liquid-phase polymer-based retention (LPR) has proved to bea convenient method to significantly retain anions arsenate solutionusing a polymer with quaternary ammonium groups.

The polymer P(SAETA) containing bulky counter ions (CH3OSO3−),

which are more hydrophobic than Cl− ion, showed lower retentioncapacity for arsenate ions. Thus, the nature of the anionic exchangergroups appears to be an important factor in arsenate retention bythese water-soluble polymers because the electrostatic interactionsare predominant in these systems.

The decrease in the retention ability of the cationic polymer isprobably due to an increase in the solution's ionic strength followingthe addition of Na2SO4, higher than that NaCl, which induced a changein polarization.

The effect of arsenate concentration on arsenate removal in presenceof NaCl was also studied. Arsenate retention by P(ClAETA) at a higherarsenate concentration (47.6 ppm) was 58%, and this removal capacityincreases gradually achieving 100% of retention when the arsenateconcentration in the cell was minimum (5.5 mg L−1).

Arsenic removal was also determined at low concentrations (inμg L−1). The result shows that P(ClAETA) removes 65% arsenate atlower concentrations, and when above Z=3, the arsenate concen-tration in each 20 mL of filtrate is below than the WHO maximumpermissible level.

The enrichment method shows the maximum retention capacity(C) for arsenate anions in aqueous solutions at pH 8. The type of anionexchanger was an important factor in the maximum retentioncapacity of arsenate.

The charge–discharge process shows that it is possible to performthe discharge process of the arsenate ions from polymers when thepolymer–arsenate was in contact with acid solution from thereservoir. The second charge process did not improve the maximumretention capacity of the polymers, when compared with the firstcharge process. The second discharge process showed almost thesame behavior for both polymers, releasing most of the arsenates ionsinto the filtrate in a similar manner. In the future, this experimentshould be repeated several times in order to determine until whatpoint it is possible to use the same polymer in the charge–dischargeprocess.

Preliminary results of the As(V) removal from Camarones Riverwater by P(ClAETA) are presented in this study. The water-solublepolymer showed a high performance (100%) for the first Z values andthen decreased up to 16% for Z=10.

Acknowledgments

The authors are grateful for grants from FONDECYT (No 1070542),CIPA, PIA (Grant Anillo ACT 130), and ECOS-CONICYT, and to Dr. J.Yáñez and Dr. L. Cornejo to supply the Camarones River watersamples.

J.A. Sánchez thanks CIPA, CONICYT for the Ph.D. scholarship for theFranco–Chilean Doctoral School 2008, the Department of MolecularChemistry, University Joseph Fourier, Grenoble 1, France.

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