Removal of arsenic from drinking water by anion exchangers

Removal of arsenic from drinking water by anion exchangers

DE!%LINATION ELSEVIER Desalination 141 (2001) 81-84 www.elsevier.comhcate/desal Removal of arsenic from drinking water by anion exchangers E. Korngo...

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DE!%LINATION ELSEVIER

Desalination 141 (2001) 81-84 www.elsevier.comhcate/desal

Removal of arsenic from drinking water by anion exchangers E. Korngold*, N. Belayev, L. Aronov The Institute for Applied Research, Ben-Gurion University of the Negev, PO Box 653, Beer-Sheva 84105, Israel Tel. +972 (8) 646-1940; Far i-972 (8) 647-2960

Received 24 April 2001; accepted 14 May 2001

Abstract Selective removal of arsenic(V) f?om drinkmg water was carried out with strong-base anion-exchange resins. The influence on the effkiency of the process of parameters such as type of anion-exchange resin and water composition was investigated. Recycling of the sodium chloride regenerant solution was carried out by precipitating the As with FeCl,. Keywords:

Arsenic removal; Anion-exchange resin; Tap water treatment

1. Introduction

In many parts of the world, drinking water is contaminated by arsenic percolating from mining and refinery wastes or traceable to arseniccontaining pesticides. Arsenic is known to be toxic and to cause skin, liver, lung and kidney cancer. For the last 59 years the permitted level of arsenic concentration in drinking water has been set at 50 ppb. Recently the EPA has been considering lowering the maximum level of contamination (MCL) to 10 ppb in the face of evidence showing higher toxicity than previously thought. There are several technologies [ 1,2] for removing arsenic: coagulation with iron or aluminum salts [3], lime softening, electrodialy*Corresponding author.

sis, reverse osmosis [4], nanofiltration [5], and ion-exchange processes. Inorganic arsenic occurs in water in trivalent and pentavalent form. In neutral solutions (pH 6.5-8.5), the trivalent form is found as H,AsO, (Pk, = 9.22, Pk, = 12.3, Pk, = 13.4) and the pentavalent form as H,AsO; and HAsO; (Pk, = 2.2, Pk,= 7.1, Pk,= 11.5). The ionic form ofthe pentavalent arsenic is easier to remove, and oxidation of the trivalent arsenic to pentavalent by chlorination or ozonation is necessary for all the processes mentioned above. Strong-base anion-exchanger resin absorbs arsenic ions from water according to the following reactions: R-Cl + H,AsO; + R- H,As 0, + Cl-

00 1l-9 164/Ol/$- See front matter 0 200 1 Elsevier Science B.V. All rights reserved PII: SOOll-9164(01)00391-5

(1)

82

E. Korngold et al. /Desalination

2R-C1+ HAsO;* -, R2-HAsO, + 2Cl-

(2)

As the selectivity of such resin to divalent anions is higher than its selectivity to monovalent anions, the efficiency of the process at high pH, under which the proportion of divalent arsenic anions in the water is greater, may be expected to be higher too. Regeneration is carried out with excess of chloride ions in the following reaction: RH,As04 + Cl- -+ RCl + H,AsO,Both HCl and NaCl solutions can be used for regeneration. When HCl solution is used, the arsenic anions are transformed to arsenic acid (H,AsO,), which has no influence on the ionexchange equilibrium, and the regeneration is more efficient. When water contaminated with arsenic also contains high concentrations of sulfate, nitrate, and chloride, competition from reactions between these anions and the resin is significant and the efficiency of the process decreases accordingly. Selective removal of arsenic(V) from drinking water with strong-base anion-exchange resins was examined in the study reported below.

141 (2001) 81-84

3. Results and discussion 3.1. Type of resin

The functional group of the quaternary ammonium ion has a significant influence on selectivity with respect to As. Purolite A-505 has three methyl groups connected to the nitrogen, whereas Relite-A-490, which is tailored for nitrate removal, has longer chain groups (ethyl, propyl or others) connected to the nitrogen. The latter resin has a higher selectivity for the H,AsOi and HA,O; ions and a better performance (see Fig. I).

3.2. pH of water The second dissociation constant of H,AsO, is 7.1, and therefore tap water with a higher pH will have more divalent arsenic ions. As noted above, anion-exchange resins show a higher selectivity for divalent ions and therefore a more effective process results when the pH of the tap water is high (Fig. 2).

3.3. Type of regenerate 2. Experimental The experiments were carried out in a transparent column 70 cm high loaded with 100 ml of resin. Two types of strong-base anion-exchange resins were used: Purolite A-505 (Purolite Co.) and Relite-A-490 (Mitzubishi Co.). The former belongs to type 1, trimethyl ammonium, while the latter is designed for selective nitrate removal and has other strong-base groups. The arsenic compound KH,AsO, (Sigma Co.) was introduced into tap water containing 180 ppm Cl, 75 ppm SO.,, and 190 ppm HCO;. The concentration of As was determined by atomic absorption measurement (VGA method). Regeneration was carried out with 200 ml 2N NaCl or 200 ml 2N HCl.

Arsenic acid (V) is a weak acid with Pk, = 2.2. Therefore, when regeneration of the spent column is carried out with strong acid, the arsenic anions in the resin, H,AsO; and HAsO,‘, are transformed into the acid H,AsO,, which is a non-charged molecule and does not influence the equilibrium. The regeneration is thus more efficient, and less leakage occurs during column operation (Fig. 3).

3.4. Flow rate According to Fig. 4, the flow rate of water through the resin can be held at 20 BV/h. When a higher flow rate is used, significant As leakage occurs.

E. Korngold et al. /Desalination 601

141 (2001) 81-84

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Fig. 6. Concentration of As and chloride in sodium chloride (2N, 2 BV) regenerant solution vs. resin bed volumes.

E. Korngold et al. /Desalination

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141 (2001) 81-84

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Fig. 8. Concentration of As vs. bed volumes for recycled and non-recycled NaCl regenerant solution.

3.5. Water composition

2. When sulfate, chloride and other anions are present in the water in high concentrations, the process is not efficient. 3. The regenerant solution contains a high concentration of As and could therefore pose a serious environmental problem. Recyling of the solution by precipitation of the As with FeCl, is recommended.

When the sulfate and/or chloride concentration in the tap water is high, the efficiency of As removal decreases sharply (Fig. 5). The process is therefore suitable only for ordinary drinking water with a salinity below 1000 TDS. 3.6. Recycling with NaCl solution When regeneration is carried out with sodium chloride solution, the As can be precipitated from the regenerant solution to enable reuse. The composition of the regenerant solution is given in Fig. 6. Addition of FeCl, at 10-15 times excess concentration on a molar basis and subsequent adjustment of the pH to 7-8 results in precipitation of more than 99% of the As (Fig. 7). The regenerant solution can then be filtered and used again after adjustment of the sodium chloride concentration (Fig. 8).

Acknowledgments This research was made possible by a generous bequest by the late Lily and Leon Zaslavsky. The paper was edited by Ms. Alice Sen.

References

PI E.O. Kartinen and C.J. Martin, Desalination, 103 4. Conclusions 1. By using a strong-base anion-exchange resin, it is possible to eliminate over 99% of the As in drinking water. The As concentration can be reduced to below 10 ppb, which is likely to be set as the highest permitted level in the near future.

(1995) 79. PI SD. Chang, in: Critical Issues in Water and Wastewater Treatment, AXE, Boulder, CO, USA, 1994. 131 J.G. Hering, B.-Y. Chen, J.A. Wilkie and M. Elimelech, J. Environ. Engineering, Aug (1997) 800. [4 J.J. Waypa, J.G. Hering and M. Elimelech, J. AWWA, 89(10) (1997) 102. PI E.M. Vrijenhoek and J. Waypa, Desalination 130 (2000) 265.