Use of ArsenXnp, a hybrid anion exchanger, for arsenic removal in remote villages in the Indian subcontinent

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Reactive & Functional Polymers 67 (2007) 1599–1611

REACTIVE & FUNCTIONAL POLYMERS www.elsevier.com/locate/react

Use of ArsenXnp, a hybrid anion exchanger, for arsenic removal in remote villages in the Indian subcontinent Sudipta Sarkar a, Lee M. Blaney a, Anirban Gupta b, Debabrata Ghosh b, Arup K. SenGupta a,* b

a Department of Civil and Environmental Engineering, Lehigh University, Bethlehem, PA 18015, USA Environmental Engineering Cell, Civil Engineering Department, Bengal Engineering and Science University, Howrah, India

Available online 1 August 2007

Abstract Many of the arsenic removal units operating in remote villages of West Bengal, India now use a hybrid anion exchanger (HAIX) which are essentially spherical anion exchange resin beads containing dispersed nanoparticles of hydrated ferric oxide (HFO). HAIX, now commercially available as ArsenXnp, offers a very high selectivity for sorption of oxyanions of arsenic due to the Donnan membrane effect. The sorption columns used in the field for removal of arsenic are either single column or split-column design. The sorption columns allow flow of atmospheric oxygen, thereby promoting oxidation of dissolved Fe(II) species of arsenic-contaminated raw water to insoluble Fe(III) oxides or HFO particulates. Apart from the usual role played by the sorbents like ArsenXnp or activated alumina towards arsenic removal, HFO particulates also aid in the treatment process. Each unit is attached to a hand-pump driven well and capable of providing arsenic-safe water to three hundred (300) households or approximately one thousand villagers. No chemical addition, pH adjustment or electricity is required to run these units. On average, every unit runs for more than 20,000 bed volumes before a breakthrough of 50 lg/L of arsenic, the maximum contaminant level in drinking water in India, is reached. In addition to arsenic removal, significant iron removal is also achieved throughout the run. Upon exhaustion, the media is withdrawn and taken to a central regeneration facility where 2% NaCl and 2% NaOH solution are used for regeneration. Subsequently, the regenerated resin is reloaded into the well-head sorption column. Following regeneration, the spent solutions, containing high arsenic concentration, are transformed into solids residuals and contained in a way to avoid any significant arsenic leaching. Laboratory investigations confirmed that the regenerated ArsenXnp is amenable to reuse for multiple cycles without any significant loss in capacity. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Arsenic; Hybrid ion exchanger; Ion exchange; ArsenXnp; Groundwater

1. Introduction *

Corresponding author. Present address: Fritz Laboratory, 13 E Packer Avenue, Bethlehem, PA 18015, USA; Tel.: +1 610 758 3534; fax: +1 610 758 6405. E-mail address: [email protected] (A.K. SenGupta).

Arsenic present in drinking water drawn from underground sources is the cause of wide-spread arsenic poisoning affecting nearly 100 million people living in Bangladesh and West Bengal, a neighboring

1381-5148/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.reactfunctpolym.2007.07.047

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Indian state [1–4]. While the maximum contaminant level (MCL) of arsenic in drinking water is 50 lg/L [5,6] in India, arsenic concentrations in this region well exceed the MCL. Health effects related to arsenic ingestion through drinking water take a long time before becoming fatal and life-threatening [7]. Average annual precipitation in this geographic location is significantly high, often exceeding 1500 mm/year. But poor sanitation practices which prevail in this area have contaminated surface waters leading to a potential risk of water borne diseases if used as drinking water without appropriate treatment. On the other hand, relative abundance and ease of finding bacteriologically safe groundwater sources promoted the wide-spread use of wells with hand pumps as drinking water sources. There remain thousands of villages where arsenic-laced ground water is the only viable source of drinking water. Several treatment technologies and equipment have been developed for removal of arsenic from water. It is well known that hydrated oxides of polyvalent metals like Fe(III), Al(III), Ti(IV) and Zr(IV) exhibit ligand sorption properties by forming innersphere complexes [8–13]. A non-regenerable adsorption media, granulated ferric hydroxide (GFH) has been widely used in many places including West Bengal, India [14]. It has also been reported that the above-mentioned metal oxides, when dispersed within a polymeric host material, offer tunable behaviors for sorption of a wide variety of anionic ligands and transition metal cations [15–18]. One such hybrid sorbent, produced by dispersing hydrated ferric oxide (HFO) nanoparticles inside a polymeric anion exchanger host material, exhibits high affinity for removal of arsenic from natural waters due to the Donnan membrane effect exerted by the host material [18–20]. The hybrid anion exchanger (HAIX) is now commercially available as ArsenXnp from SolmeteX Co. in Northborough, MA and Purolite Co. in Philadelphia, PA; however, no endorsement is implied. Earlier investigations showed that the chelating polymers with nitrogen donor atoms, when loaded with copper(II), are very selective to inorganic arsenic species and also are reusable [21–23]. However, high price of the parent chelating polymer was a major obstacle toward wider applications related to water and wastewater treatment. Since 1997, Bengal Engineering and Science University, Howrah, India and Lehigh University, USA have collaborated to develop a sustainable solution

to combat the arsenic problem in West Bengal, India. Under this initiative, about 160 well-head arsenic removal systems have been installed. These units are community based and serve about 250– 300 families; additionally, the units require no electricity, chemical addition or pH adjustments. The adsorbent media used commonly is activated alumina. Characteristics and performance of these units have been previously reported [24]. Since 2004, ArsenXnp media, along with activated alumina has been utilized in the units. The primary objective of this article is to present the performance of ArsenXnp for arsenic removal over a long period of run in the field, regenerability of the media, and elucidation of arsenic removal mechanism and containment strategies of arsenic removed. 2. Experimental 2.1. Well-head treatment units The main component of the well-head treatment unit is an adsorption column (diameter 35 cm, height 2 m), which is a gravity-fed system operating in downflow mode. Apart from the adsorption column, there is a coarse-sand filter to contain the backwash waste water from the column, which contains arsenic-laden precipitates of ferric hydroxide or hydrated ferric oxide (HFO). The adsorption column mounted on top of a hand-pump driven well is a stainless steel (SS304) cylindrical tank with two distinct functional regions. At the top of the tank, there are atmospheric vent connections to allow passage of atmospheric oxygen. The inlet water is sprayed in and is further divided in fine droplets by means of a splash distributor installed at the top of the tank. Sufficient volume is kept at the top of the tank to facilitate longer residence time to allow oxidation of dissolved Fe(II) species to insoluble Fe(III) species before the well water enters the second region, which contains fixed-bed of sorbent media supported by graded gravels. Ultimately, arsenic-safe treated water is collected at the bottom of the unit. The sorbent used is ArsenXnp. The necessary constructional and operational features of the adsorption column are schematically indicated in Fig. 1a. The amount of sorbent employed in each column is approximately 100 kg. The design flow rate through the column is 8–10 L/minute. The column is routinely backwashed every morning for about 10–15 min in order to drive out precipitated HFO particles and to

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Fig. 1. (a) Schematic detail and operational mechanism of the well-head unit and (b) details of the coarse-sand filter for entrapment of waste backwash containing HFO particles (all dimensions in mm).

maintain a desirable flow rate. Fig. 1b provides pertinent details of the coarse-sand filter used to trap HFO particles from the waste backwash water. Another type of design later evolved where the single adsorption column is split in two separate columns in order to optimize on the use of sorbent like

HAIX and to take better advantage of HFO precipitates. Fig. 2 provides schematic details of the splitcolumn design. The uppermost column contains components necessary for oxidation of dissolved iron similar to the single-column unit; however, sorbent media is only 50 kg of activated alumina. The

Fig. 2. Schematic detail of construction and operation of a split-column unit used in the field.

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partially treated water then enters the second column containing 50 kg ArsenXnp, which basically acts as a polishing unit. Finally, treated water is collected at the bottom of the second column. Both columns have provisions for backwashing. However, the second column does not require backwash because iron oxidation and precipitation only occurs in the first column. At the effluent port of each unit, a flow totalizer (mechanical type) is provided to record total volume of water treated. 2.2. Preparation and characteristics of HAIX SolmeteX, Inc. manufactures ArsenXnp using the procedure developed at Lehigh University [25]. Dispersing HFO nanoparticles within an anion exchanger is scientifically challenging because the ferric ion (Fe3+) and quaternary ammonium group (R4N+) in anion exchange are both positively charged. Fig. 3 illustrates the multi-step process protocol for ArsenXnp synthesis. The parent anion exchanger is macroporous strong base type and has polystyrene matrix with quaternary ammonium functional groups; the capacity is 0.8 meq/g of resin. Fig. 4 shows a photograph of the beads and TEM image of the interior of the bead. HFO loading on the anion exchanger was found to be 150 mg Fe/g. The hybrid sorbent with size range of 0.5–0.7 mm

is mechanically strong, durable and suitable for use in fixed-bed columns. 2.3. Sample preparation and analysis For analysis of As(V) and As(III), samples collected at site are adjusted to pH 4.0 and are immediately separated using a strong-base anion exchange resin mini-column in accordance with a technique developed earlier [26,27]. Total arsenic is determined from the original sample, As(III) from the sample collected at the exit of the anion exchange column and As(V) by difference. The technique is validated using samples of known As(III) and As(V) concentrations. Bengal Engineering and Science University analyzes arsenic using a automatic flow injection atomic absorption spectrophotometer (Chemito, India) with hydride vapor generation accessory. At Lehigh University, arsenic is analyzed using an atomic absorption spectrophotometer with graphite furnace accessory (Perkin–Elmer model SIMAA 6000). Samples for analysis of iron are preserved at pH < 2 through addition of 8 M HNO3. Dissolved oxygen, hardness, alkalinity, silica and phosphate analyses are carried out in accordance with the procedures available in Standard Methods [28].

Fig. 3. Synthesis of hybrid anion exchanger or ArsenXnp from parent anion exchange resin.

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Fig. 4. (a) Photograph of ArsenXnp and (b) TEM image of the interior of the bead.

All laboratory isotherms and column tests were conducted with background concentrations of commonly occurring electrolytes. Fixed bed column runs utilized ELDEX fraction collectors. Superficial liquid velocity and empty bed contact times (EBCT) were recorded for every column run. Isotherm tests were carried out in plastic bottles in a gyratory shaker. An extended toxicity characteristic leaching procedure (ETCLP) was also performed with the dried sludge (treatment residual) at different pH towards determination of leaching potential [29].

The general procedure for regeneration of exhausted activated alumina is similar except that NaCl is not used in the regenerant. The exhausted media is air-dried and kept in a safe place for reuse. At the end of the regeneration, spent acid, alkali and rinse water are mixed together and pH is adjusted to 6.5 by adding 10% hydrochloric acid. Thick brown slurry immediately forms and settles overnight before being disposed of on top of a coarse-sand filter. Arsenic-laden solids and HFO particles are intercepted and retained at the top of the filter. The entire regeneration, including the spent regenerant treatment, is completed in 5 h.

2.4. Regeneration 3. Results Upon exhaustion of the adsorption column, media from each unit is replaced by fresh or regenerated media. The exhausted ArsenXnp and activated alumina is taken to a central regeneration facility where the media is regenerated inside a manually operated stainless steel batch reactor that can be manually rotated about its horizontal axis. Fig. 5 is a sketch of the batch reactor used for regeneration. For regeneration of ArsenXnp, a solution containing 2% NaOH and 2% NaCl is utilized; the exhausted ArsenXnp is reacted with two bed volumes of regenerant solution in the batch reactor for 45 min. The process is repeated once with a fresh regenerant solution. During regeneration, pH remains near 12.0; spent alkali is collected. After a thorough rinse with well water, the media is subjected to two bed volumes of dilute HCl solution to neutralize the media so that resultant solution pH is 5.5; subsequently, the spent acid is collected. The media is then rinsed with well water.

3.1. Isotherms for activated alumina and ArsenXnp Figs. 6 and 7 show the As(III) and As(V) adsorption isotherms onto ArsenXnp and activated alumina, respectively. It may be noted that As(III) and As(V) isotherms for ArsenXnp are comparable, but activated alumina prefers As(V) well over As(III). 3.2. Performance of the single and split-column unit Fig. 8 shows performance of a single-column unit containing ArsenXnp located at Nabarun Sangha, Kankpul, Ashoknagar in North 24 Parganas district of West Bengal. For an average inlet concentration of 85 lg/L, the unit ran for almost 29,000 bed volumes before breakthrough of 50 lg/L. The breakthrough curve is gradual in nature. Arsenic speciation in the raw water demonstrated an arsenate, or As(V), to arsenite, or As(III), ratio of

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Fig. 5. Sketch of the batch reactor used at central regeneration facility.

Fig. 6. Isotherm of ArsenXnp and activated alumina (AA) for adsorption of As(III) species.

60:40. Silica and phosphate concentration in the influent water were 19 and 3 mg/L, respectively. The dissolved iron concentration was 3.5 mg/L. After passage of 5800 bed volumes of water, the unit was connected to an adjacent well due to problems in the existing well. With lower dissolved iron and phosphate concentration of 2.5 mg/L and 0.5 mg/ L, respectively, however, no significant impact was

observed in the breakthrough curve. There was an early breakthrough of silica at less than 1500 bed volumes; while phosphate breakthrough occurred at 3000 bed volumes. Approximately 164.9 mg of total arsenic was removed by the ArsenXnp column. Fig. 9 represents the arsenic history for a splitcolumn unit located at Binimoypara, Ashoknagar in the North 24 Parganas district of West Bengal,

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Fig. 7. Isotherm of ArsenXnp and activated alumina (AA) for adsorption of As(V) species.

Fig. 8. Arsenic removal performance of a single-column unit at Nabarun Sangha, Ashoknagar, West Bengal. The closed squares represent arsenic concentration in raw water whereas the closed triangles represent arsenic concentration in treated water.

India. For an average inlet arsenic concentration of 160 lg/L, the column ran for approximately 22,000 bed volumes. The ratio of As(V) to As(III) in raw water was 60:40. Analysis of the breakthrough curves demonstrates that approximately 228.8 mg

of total arsenic was removed in the first column loaded with activated alumina; the second column which is loaded with ArsenXnp, removed about 54.4 mg of total arsenic. It may be noted that although activated alumina showed a poorer

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Fig. 9. Arsenic removal performance of the split-column unit at Binimoypara, Ashoknagar, West Bengal. The closed squares, closed triangles and open triangles denote raw water, effluent from first column and final treated water, respectively.

adsorption capacity compared to ArsenXnp for removal of both As(V) as well as As(III), (Figs. 6 and 7), activated alumina column removed much more arsenic compared to ArsenXnp column.

of the split-column unit at Binimoypara removed about 4600 g of total iron. 3.4. Regeneration of HAIX and safe transformation of treatment residues

3.3. Iron removal Figs. 10 and 11 represent the iron histories of the single column as well as the split-column unit, respectively. It is observed that in both cases, there is a significant removal of dissolved iron that was originally present in the raw water. Also, the concentration of iron in the treated effluent does not have any dependence on the number of bed volumes passed through the column. This evidence points out that the removal mechanism is not adsorptive but typically an oxidative precipitation followed by filtration. Iron removal in the arsenic removal units is possible due to its unique construction features that allow oxidation of Fe(II) species to an insoluble Fe(III) species. It may also be noticed that for the split-column unit, the iron removal took place in the first column only. This observation also signifies the advantage of the split-column design; users need to backwash only the first column in order to wash out the HFO particles. The total amount of iron removed by the unit at Nabarun Sangha was approximately 5820 g. The first column

Regeneration of the exhausted media was carried out at a central regeneration facility following the procedure indicated earlier. Table 1 shows arsenic and iron concentrations in the spent solutions (treatment residual) from the regeneration of the exhausted ArsenXnp of the single-column unit at Nabarun Sangha. It may be noted that the spent regenerant along with the acid and water rinses contain high concentrations of arsenic. In an effort to detoxify the spent solutions, all the spent regenerants are mixed together, the pH is lowered to 6.5– 7.0 producing thick brown precipitates of ferric hydroxide. The particulates of ferric hydroxide adsorb arsenic from the bulk solution leaving the bulk phase with a fairly low concentration of arsenic (Table 1). A mass balance on the regeneration data demonstrates that the dry weight of the precipitated mass (sludge) should not be more than 1 kg. The bulk water along with the sludge is disposed on top of a coarse-sand filter which is similar to the one described in Fig. 1b but larger in size. The top of the coarse-sand filter is open to atmosphere.

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Fig. 10. Iron removal by single-column unit at Nabarun Sangha, Ashoknagar, West Bengal. Closed squares denote iron concentration in inlet water whereas closed triangles denote the same in the treated water.

Fig. 11. Iron removal by the split-column unit at Binimoypara, Ashoknagar, West Bengal. Closed squares, closed triangles and open triangles denote raw water, effluent from first column and final treated water, respectively.

Fig. 12 shows the leaching potential of a similar sludge (obtained from regeneration of activated alumina) at different pH. The results demonstrate that the arsenic concentration in the leachate tends to be minimum at a pH 5.5; arsenic

was present as As(V). Earlier laboratory studies suggest that ArsenXnp can adsorb As(V) species over many cycles of sorption and desorption without significant capacity loss. Fig. 13 provides such data.

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Table 1 Volumes and compositions of individual regenerant streams for regeneration of exhausted media used at Nabarun Sangha, Ashoknagar, N 24 Parganas, West Bengal, India Description

Volume (L)

pH

Arsenic (lg/L)

Total iron (mg/L)

Spent caustic 1st batch Spent caustic 2nd batch Acid rinse Treated wastewater

140 140 180 460

12.5 12.5 5.5 7.1

39600 11200 320 30

2500 245 2.86 0.96

Fig. 12. Leaching potential of the treatment residue as determined by ETCLP test.

4. Discussion 4.1. Arsenic removal: design of well-head units and role of dissolved iron The uppermost part of the unit ensures nearcomplete oxidation of dissolved iron to hydrated ferric hydroxide or HFO by oxygen as shown below: 4Fe2þ þ O2 þ 10H2 O ! 4FeðOHÞ3ðsÞ þ 8Hþ  ðDG0Reac ¼ 18kJ=moleÞ ð3Þ The standard state free energy change for the above reaction is highly negative implying that the forward reaction is thermodynamically favorable. Hydrogen ions generated by the precipitation reaction are neutralized instantaneously by alkalinity (HCO 3 Þ present in groundwater. As a result, no significant pH change has been observed at any site regardless of

dissolved iron content. X-ray diffraction (XRD) analysis confirmed that precipitated HFO particles are present in the amorphous state and no crystalline iron oxide (e.g., goethite, hematite) was formed even after several weeks. In an activated alumina column of similar design, precipitates of HFO particles played a vital role for removal of arsenic, especially As(III) [24,30]. As(III) oxidation is thermodynamically possible but previous field observation has indicated that there is minimal conversion of As(III) to As(V) within the bed [24]. So, from a mechanistic view point, the role of freshly precipitated HFO is significant. Fig. 14 depicts arsenic removal mechanisms that are operative within the adsorption column. The exceptionally high removal capacity shown by the activated alumina column (Fig. 9) as compared to ArsenXnp is thus attributable to adsorption effort provided by freshly precipitated HFO in coordination of activated alumina. It has been reported

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Fig. 13. Performance of ArsenXnp for multiple cycles of sorption–desorption in the laboratory.

Fig. 14. An illustration depicting interplay of different variables for simultaneous removal of As(III) and As(V) in the adsorption column.

that high concentration of dissolved iron in water allow formation of HFO particulates towards successful household arsenic treatment in Vietnam [31,32]. 4.2. Residue management Managing and containing arsenic-laden waste products is almost as important as removing arsenic from drinking water. Local environmental laws/

guidelines with regard to the safe disposal of arsenic-containing treatment residuals do not exist or are not enforceable. However, to avoid future hazard, proper management of treatment residuals is considered an important part of the overall treatment scheme. Regeneration of the media is the first step towards volume reduction of the treatment residuals. If there was no regeneration, every well-head unit would produce 100 kg of disposable sorbent

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media at the end of each cycle, posing a sizeable obstacle to safe waste management. Following regeneration and containment of arsenic into about a kilogram of solid sludge, the amount of treatment residue is reduced by approximately 100 times. The sludge is contained and stored at the top of a coarsesand filter which is deliberately designed to maintain atmospheric conditions. The oxidizing environment inside of coarse-sand filters helps to keep iron in the form of Fe(III) and arsenic in As(V) form. Fig. 15 presents a pe–pH or predominance diagram for arsenic. During the regeneration procedure, all the solutions are nearly saturated with atmospheric oxygen. The following half reaction and the resulting pe value tend to determine the redox environment for them [33]: 1=4O2 þ Hþ þ e ¼ 1=2H2 O 0

pe ¼ pe þ 1=4 log PO2  pH

pe0 ¼ 20:79

ð4Þ ð5Þ

where, PO2 = partial pressure of oxygen = 0.21 atm for atmospheric oxygen The estimated boundary of pe–pH conditions for both spent alkali regenerant and the precipitated sludge are marked on the pe–pH diagram. It may be noted that As(V) is the dominating species in both situations. Experimental observations support this fact. Since any reduction of HFO particles,

which is the predominant constituent of the sludge, to Fe(II) will result in an enhanced leaching of arsenic, the top of the coarse-sand filter is deliberately kept open to atmosphere through provision of vents. In this region, the presence of Fe(III) and As(V) is thermodynamically favorable. Hence, there will not be any significant leaching of arsenic as long as an oxidizing condition prevails, and the pH of the water passing through the storage chamber is in the range 5 –10. Experimental observations of the leaching study carried out in such conditions indicated a very minimal leaching of arsenic, in the tune of 30–100 lg/L. On the other hand, if the sludge is kept under reducing environment, such as in sanitary landfills, it is thermodynamically possible that arsenic and iron will be reduced to As(III) and Fe(II), respectively, causing enhanced leaching. TCLP (Toxicity Characteristic Leaching Procedure) may indicate minimal leaching of arsenic from the sludge, as it is performed under oxidizing environment [34]. However, under reducing condition, such as that inside landfills there will be enhanced leaching of arsenic [35,36]. Also, there is evidence of research findings that TCLP underestimates leaching of arsenic as the protocol essentially proposes to carry out tests with a headspace of air in the bottles [37]. 5. Conclusions

Fig. 15. pe–pH diagram of arsenic confirming the predominance of As(V) in the precipitated sludge after the spent regenerant treatment.

ArsenXnp is the first polymer-based commercially available arsenic-selective sorbent. The field performances of well-head arsenic removal units using ArsenXnp demonstrate that such units can effectively produce arsenic-safe water for more than 20,000 bed volumes. The units do not require any chemical addition or electricity. These community based units are run and maintained by the villagers. At the end of each run, the exhausted sorbent is regenerated at a central regeneration facility; after regeneration, the treatment residue is contained as a solid sludge. Furthermore, water treatment residuals amount to approximately 1 kg, about 100 times lower than the exhausted sorbent which if disposed of in a landfill would have leached significant concentration of arsenic back to the environment. The solid sludge is disposed on top of a sand chamber, which under atmospheric conditions, prevents arsenic leaching. Therefore the treatment technology and the residue management offer a sustainable solution for the problem of arsenic in drinking water. The technology can be replicated in other

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developing nations including Mexico and Vietnam where arsenic in groundwater poses severe threat to human health. Acknowledgements Partial financial assistance from private donors like Hilton Foundation and Rotary International through Water For People is acknowledged. Also, the authors would like to thank SolmeteX, Inc. for providing ArsenXnp for the units installed in the villages. The authors like to thank Mr. Alok Pal, Mr. Dilip Ghosh and Mr. Morshed Alam for their assistance in field and laboratory work. References [1] A.H. Smith, E.O. Lingas, M. Rahman, Bull. World Health Organ. 78 (2000) 1093–1103. [2] P. Bagla, J. Kaiser, Science 274 (1996) 174–175. [3] W. Lepkowski, C&EN News 16 (November) (1998) 27–28. [4] A. Mukherjee, M.K. Sengupta, M.A. Hossain, S. Ahamed, B. Das, B. Nayek, D. Lodh, M.M. Rahman, D. Chakraborti, J. Health Popul. Nutr. 24 (2006) 142–163. [5] World Health Organization, Guidelines for drinking water quality, vol. 1, WHO, Geneva, 2004. [6] Indian Standard, Drinking water specification (IS: 10500), Bureau of Indian Standards, New Delhi, 1993. [7] National Research Council, Arsenic in Drinking Water: 2001 Update, National Academy Press, Washington, DC, 2001. [8] Y. Gao, A.K. SenGupta, D. Simpson, Water Res. 29 (1995) 2195–2205. [9] T.M. Suzuki, J.O. Bomani, H. Matsunaga, Y. Yokoyama, React. Funct. Polym. 43 (2000) 165–172. [10] P.K. Dutta, A.K. Ray, V.K. Sharma, F.J. Millero, J. Colloid Interf. Sci. 278 (2004) 270–275. [11] M.L. Pierce, C.B. Moore, Water Res. 6 (1982) 1247. [12] M.M. Ghosh, J.R. Yuan, Environ. Prog. 3 (1987) 150–157. [13] J.H. Jang, Ph.D. dissertation, the Pennsylvania State University, 2004. [14] W. Driehuas, M. Jekel, U. Hildebrandt, J. Water SRT Aqua 47 (1) (1998) 30–35. [15] P. Puttamraju, A.K. SenGupta, Ind. Eng. Chem. Res. 45 (2006) 7737–7742.

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