Removal of arsenic from contaminated groundwater by membrane-integrated hybrid treatment system

Journal of Membrane Science 354 (2010) 108–113

Contents lists available at ScienceDirect

Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci

Removal of arsenic from contaminated groundwater by membrane-integrated hybrid treatment system Mou Sen, Ajoy Manna, Parimal Pal ∗ Environment and Membrane Technology Laboratory, Department of Chemical Engineering, National Institute of Technology Durgapur, Mahatma Gandhi Avenue, Durgapur, West Bengal 713209, India

a r t i c l e

i n f o

Article history: Received 27 November 2009 Received in revised form 13 February 2010 Accepted 25 February 2010 Available online 3 March 2010 Keywords: Membrane separation Nanofiltration Pre-oxidation Arsenic removal

a b s t r a c t Experimental investigations were carried out to separate arsenic from contaminated groundwater by using three different types of nanofiltration membranes in a flat sheet cross flow membrane module with a pre-oxidation step for conversion of trivalent arsenic to pentavalent form. KMnO4 was used as an oxidising agent in the pre-oxidation step. Effects of pressure, iron concentration in groundwater, pH, and pre-oxidation on the performance of the membranes in terms of flux as well as percentage rejection of arsenic were studied. While transmembrane pressure was found to have strong impact on flux and retention of arsenic, pH and pre-oxidation exhibited strong influence on percentage removal of arsenic. Introduction of pre-oxidation step remarkably enhanced arsenic separation in a largely foulingfree membrane module. Overall arsenic removal increased from 50–63% to 97–100% for all the three types of membranes over a transmembrane pressure range of 5–12 kgf/cm2 on pre-oxidation of trivalent arsenic. With increase in pH from 3 to 10, arsenic rejection increased by 23% for NF-1, 33% for NF-2 and 26% for NF-20 membranes. Polyamide composite nanofiltration membrane fitted in a cross flow membrane module could successfully remove arsenic from contaminated groundwater with a pre-oxidation step. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Arsenic is a toxic semi-metallic element that is carcinogenic for human health. WHO had recommended a maximum permissible contaminant level (MCL) of 10 ␮g/L of arsenic in drinking water standard considering the epidemiological evidence of arsenic carcinogenicity [1–4]. Subsequently, the European Union and the EPA have accepted this permissible limit in drinking water. About 21 countries are at present facing the crisis of groundwater contamination by arsenic. Arsenic contamination of groundwater in the Bengal Delta Basin in India and Bangladesh has been well documented as the largest arsenic-affected population live in this region. For example, groundwater in 59 districts, out of total 64 districts in Bangladesh is arsenic-contaminated affecting some 53 million people. Several studies on arsenic removal from groundwater have been taken up to meet the WHO-prescribed limit. From such studies [5–12] carried out over the last few decades, adsorption, chemical coagulation–precipitation, ion-exchange and membrane separation have been established as the broad technology options of purification [13]. Arsenic removal efficiencies of these techniques have also been broadly established where reverse osmosis, nanofil-

∗ Corresponding author. Tel.: +91 343 2755380/2547378x5381; fax: +91 343 2547375. E-mail address: [email protected] (P. Pal). 0376-7388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2010.02.063

tration and membrane distillation have been identified as the most efficient technologies capable of removal of more than 95% of arsenic from contaminated groundwater [14]. Though by virtue of highly selective membranes, reverse osmosis can ensure the highest degree of purification, it involves the highest cost also that arises from the cost of membrane as well as the requirement of pumping at high pressure. While Pal et al. [15] have shown that for largescale treatment, chemical precipitation could be an ideal option for low cost treatment in the widely affected South-East Asian countries, Manna et al. [14] have shown that solar-driven membrane distillation could be very effective for small scale treatment where almost 99% pure water can be produced from arsenic-contaminated water. Nanofiltration considered to be one of the best performers with respect to arsenic separation capability can be a potential technology option if operated in an appropriate module avoiding fouling. Water with very high arsenic content can also be treated by nanofiltration membrane [16] without necessitating high transmembrane pressure. When a solution containing ions like arsenate is brought in contact with a membrane possessing a fixed like surface charge, the passage of ions through the membrane is inhibited due to Donnan effect [17]. So, the rejection of arsenate will be higher if the selected membranes are negatively charged. And the rejection of anions will be higher compared to neutral arsenites. During separation of solutes by nanofiltration membranes, both sieving mechanism (size exclusion) and Donnan mechanism (electrostatic charge repulsion) may play their role. However, at higher

M. Sen et al. / Journal of Membrane Science 354 (2010) 108–113

pH (greater than 7.0) most of the nanofiltration membranes of polyamide composite types possess negative zeta potential [18]. When membrane surface potential measured as zeta potential changes from positive to negative ones with high medium pH, electrostatic interaction, charge repulsion and hence separation of ionic species based on Donnan effect become dominant in nanofiltration. Arsenic may occur in various oxidation states (3−, 0, 3+ and 5+) and speciation may change from neutral to anionic forms and vice versa depending on the pH of the medium. Thus polyamide type nanofiltration membranes which largely assume negative surface potential (at pH value greater than 7.0) can be effectively exploited in removal of arsenic from water provided arsenic speciation is favourably changed making Donnan mechanism the dominant separation mechanism. However, efficient removal of arsenic by nanofiltration depends on a host of factors like types of membranes, modules of membranes, operating pressure, pH, oxidation states of arsenic, concentration of arsenic and presence of other impurities like iron etc. in water. A very few studies [19,20] have been reported on nanomembrane separation of arsenic using some specific membrane modules and simulated arsenic solution. Varieties of other modules with different types of membranes have the potential of arsenic separation from groundwater offering longer fouling-free operation of the polymeric membranes and thus adding economy to the process. Such modules need to be tested using actually contaminated groundwater for judging the potential of nanofiltration process in effective separation of arsenic in presence of other possible contaminants. In the present study, thus investigations were carried out using a few commercially available polyamide composite nanofiltration membranes in a cross flow module while using contaminated groundwater from some affected areas of Bengal Delta basin in India. 2. Materials and methods 2.1. Standards, reagents and membranes All chemical reagents used were of reagent grade and were used without further purification. Arsenic standards and supra pure HCl were procured from E. Merck, Germany. Potassium permanganate was procured from Ranbaxy, India. The NF membranes were thin film composite polyamide membranes (NF-1, NF-2 and NF-20) manufactured by Sepromembranes Inc. Membrane surface area of each module used was 100 cm2 . Some major characteristics of the used membranes have been presented in Table 2. 2.2. Experimental set up Schematic diagram of the NF experimental set up has been shown in Fig. 1. The set up consisted of a stirred feed tank (that also served as a pre-oxidation reactor) and three parallel cross flow membrane modules. A reciprocating pump (Milton-Roy, India) connected to the reactor tank circulated feed water through the

109

Fig. 1. Experimental set up of cross flow nanofiltration module.

module where up and downstream pressure gauges and bypass control valves were used to monitor and control transmembrane pressure. Flow through the system was monitored and controlled using attached rotameters and control valves. Every new set of membrane was initially operated for 30 min under a pressure of 15 kgf/cm2 using deionised water before actual separation study employing the membrane. 2.3. Experimental procedure The set up was run in a continuous flow mode. Contaminated groundwater collected from arsenic-affected areas of West Bengal, India was used for investigation. Characteristics of these water samples have been shown in Table 1. Flux for arsenic-contaminated water was determined over a pressure range of 5–12 kgf/cm2 for all membranes used. To determine the effect of pre-oxidation on arsenic removal by NF-1, NF-2 and NF-20, contaminated water with arsenic was oxidised by addition of KMnO4 solution prepared in deionised water following the procedure of our earlier study [15]. Analysis for As(III) and As(V) was done before and after oxidation and nanofiltration. The tank was provided with a mechanical stirrer. The stirring was done for 40 min (pre-oxidation time) at 180 rpm. After charging each type of feed water into the tank the system was kept in running condition for 45 min continuously to allow the system to attain steady state. Both permeate and the retentate streams were recycled to the feed tank to keep arsenic concentration in the feed water at constant level during filtration. 2.4. Analytics Analysis for arsenic in feed water and membrane permeate was done at 193.7 nm wavelength in a PerkinElmer atomic absorption spectrophotometer (AAS-100) using flame-fias technique [15] in which arsenic was analysed after its conversion to volatile hydride. Samples to be analysed for arsenic was pre-reduced [As(V) to As(III)] using a reducing solution 5% (w/v) potassium iodide (KI) and 5% (w/v) ascorbic acid. In this flame-fias technique, oxy-acetylene flame was used to atomise the sample element and FIAS (flow injection with atomic spectroscopy) was used to inject an exactly

Table 1 Groundwater characteristics for samples taken from three different districts. Parameters

Sample without arsenic

Sample I Chakdah, Nadia

Sample II Debnagar, Murshidabad

Sample III Sangrampur, N-24 Parganas

Total hardness (mg/L) (AsCaCO3 ) Total alkalinity (mg/L) (AsCaCO3 ) Chloride (mg/L) (AsCl− ) Sulfate (mg/L) (AsSO4 2− ) pH Total Fe (mg/L) Total As (␮g/L) As(III) As(V)

403 367 320 3 7.10 2.5 Nil Nil Nil

538 390 310 3 7.2 4.2 150 62 88

400 320 270 5 7.6 10.5 376 152 224

650 520 310 3 7.15 7.5 252 98 154

110

M. Sen et al. / Journal of Membrane Science 354 (2010) 108–113

Fig. 3. Effect of applied pressure on arsenic rejection percentage for all the three membranes. Operating conditions: As concentration in initial water 150 ␮g/L, flow rate of water 700 L/h, cross flow velocity 1.16 m/s, temperature 35 ◦ C.

Fig. 2. Effect of transmembrane pressure on flux through NF-1, NF-2 and NF-20 membrane. Operating conditions: pH 7, pressure range 5–12 kgf/cm2 , temperature 35 ◦ C, flow rate 700 L/h, cross flow velocity 1.16 m/s, pre-oxidised by KMnO4 .

highest flux followed by NF-20 and NF-1 respectively indicating that NF-2 was the loosest type whereas NF-1 was the tightest in nature. Water flux was observed to be independent of concentration of arsenic over the investigated range. 3.2. Effect of transmembrane pressure on removal of arsenic

reproducible volume of the sample into a continuously flowing carrier system. After measurement of total arsenic, analysis for As(V) was suppressed by adjusting pH of the mixed solution to 3 using sodium hydroxide and citric acid buffer enabling measurement of As(III) only [21]. The measured As(III) was then deducted from total arsenic giving measurement for As(V). Percentage removal of arsenic was calculated using the initial value (CAs0 ) and the residual value (CAs ) of arsenic concentration in feed water and treated water (permeate) respectively using Eq. (1). % removal of arsenic =

1 − CAs × 100 CAs0

(1)

2.5. Membrane morphology Membrane morphology was studied through SEM analysis (scanning electron microscopy, Hitachi S-3000 N, Japan). Prior to SEM analysis, the membranes were freeze-fractured in liquid nitrogen. After thin gold coating in ion sputter (Hitachi, E-1010) the pieces of membranes were transferred to the scanning electron microscope. SEM analysis at 20 kV was done for the top surfaces of the membranes. 3. Results and discussion 3.1. Effect of transmembrane pressure on permeate flux Effect of transmembrane pressure on the permeate flux was determined for three membranes with water of three varieties: (a) groundwater with 150 ␮g/L arsenic (As(III) 62 ␮g/L:As(V) 88 ␮g/L); (b) groundwater without arsenic and (c) groundwater with 150 ␮g/L arsenic pre-oxidised with 5 mg/L of KMnO4 . Water flux data as presented in Fig. 2 shows that permeates flux increased with increase of transmembrane pressure for all the NF membranes. For NF-2, water flux increased from about 150 L/m2 h to 350 L/m2 h as transmembrane increased from 5 kgf/cm2 to 12 kgf/cm2 and almost similar trend was followed by the other two membranes. Water flux was found to vary linearly with transmembrane pressure with all feed water used in the present study indicating cleanliness of groundwater as such flux behaviour is observed while studying pure water permeation through nanofiltration membrane. The nanofiltration membrane NF-2 yielded the

Effect of transmembrane pressure on removal of arsenic on three different membranes was determined for the water sample with 150 ␮g/L arsenic. Arsenic rejection was found to be the highest by NF-1 followed by NF-20 and NF-2 when the contaminated water contained 150 ␮g/L arsenic. Fig. 3 shows that arsenic rejection increased slightly with increase of applied pressure for a cross flow velocity of 1.16 m/s. This was found to be similar for all the NF membranes. This may be attributed to the solution diffusion mechanism that applies to nanofiltration. In solution diffusion mechanism, solute flux and the solvent flux are uncoupled as a result increase of solvent flux following an increase in transmembrane pressure does not result in increase of solute flux. Rather increase of solvent flux stands in the way of transport of solute. Transmembrane pressure leads to the increase in solvent flux. Presence of As(V) as monovalent or divalent anionic forms in water results in a charge repulsion when it comes in contact with negatively charged NF membranes and this results in rejection of As(V). Due to the neutral character of the As(III) molecule within the pH range of 3–10, increase of pressure increases the flux. This in turn, decreases the As(III) retention as in this case, it is the convective transport that becomes dominant due to the neutral character of As(III) that passes through the membrane following size exclusion principle. As tested groundwater contained both As(V) and As(III), total arsenic removal varied from 48% to 63% at 12 kgf/cm2 transmembrane pressures for the three tested membranes when there was no pre-oxidation step (Table 2). 3.3. Effect of pre-oxidation by KMnO4 Effect of oxidant dose was determined for the groundwater with arsenic. The arsenic concentration of the groundwater was 376 ␮g/L (sample-II). Fig. 4 shows the effect of oxidant dose on arsenic rejection by the nanofiltration membrane. Pre-oxidation of As(III) by KMnO4 resulted in substantial improvement in removal efficiency of the membranes. Rejection of arsenic by all the membranes was found to increase with increase in doses of oxidant. KMnO4 dose was varied from 1 mg/L to 9 mg/L. The optimum oxidant dose was obtained as 5 mg/L. Pre-oxidation of As(III) resulted in increase in removal of total arsenic by NF-1, NF-2 and NF-20 membranes from 63% to 98%, 57% to 96% and 60% to 96% respec-

M. Sen et al. / Journal of Membrane Science 354 (2010) 108–113

111

Table 2 Membrane characteristics and its performance at 12 kgf/cm2 pressure, treating As water (pH 7) and without pre-oxidation. Membrane codes

Membrane thickness (cm)

Maximum process temperature

pH resistance

Flux (L/m2 h) at 12 kgf/cm2 for water with arsenic

NF-1 NF-2 NF-20

0.0165 0.0165 0.0165

50 ◦ C 50 ◦ C 50 ◦ C

2–11 2–11 2–11

40 350 96

tively. Pre-oxidation with KMnO4 results in conversion of neutral As(III) into anionic As(V) and thus changes the transport regime where charge repulsion and solution diffusion mechanism dominate the overall separation process as explained in the previous section. Thus as high as 98% removal was achieved by NF-1 membrane. This is certainly improvement over the separation efficiency achieved by Saitua et al. [22] and Vrijenhoek and Waypa [23] in their nanofiltration studies. In both these studies [22,23] separation of only As(V) has been attempted after spiking deionised water with As(V). Saitua et al. [22] achieved 95% separation of As(V) in a hollow fibre module which is prone to fouling whereas in the other study [23] a plate and frame module was used. In terms of potential of offering fouling-free operation, the cross flow membrane module used in the present study was much better compared to most of the modules reported in earlier studies. 3.4. Effect of pH on the removal of arsenic Fig. 5 shows that with increase of pH of groundwater (with 150 ␮g/L arsenic) from 3.0 to 10.0 arsenic rejections increased from 50% to 76% for NF-1, 33% to 69% for NF-2 and 43% to 71% for NF-20

Total rejection of arsenic without pre-oxidation of As(III) 63% 57% 60%

membranes respectively without pre-oxidation. But increase of pH along with pre-oxidation of arsenic species resulted in much higher arsenic rejection. Rejections of arsenic at pH 7 (normal groundwater pH), for NF-1, NF-2 and NF-20 membranes were 57%, 60% and 63% respectively when operated at a pressure of 12 kgf/cm2 . Arsenic rejection was 96–98% for all the three types of membranes at pH 7.0. At pH 10.0, arsenic rejection increased up to 99% for pre-oxidised water. Such effect of pH on arsenic removal by nanofiltration has also been observed in some earlier studies [24,25] that show that pH has a significant role on removal for arsenic as speciation of arsenic changes with pH of the medium. Up to a pH value of 8.0, As(III) remains largely as a neutral molecule while As(V) remains as an anion. As(V) speciation even changes from monovalent (H2 AsO4 − ) to divalent form (HAsO4 2− ) enhancing further retention of arsenic due to Donnan exclusion. As the NF membranes are mostly negatively charged, rejection of arsenic seems to be affected by the charge valency of arsenate in the solution (Donnan exclusion). Apparent pore size of polyamide NF membranes can also vary with solution pH. At the pore surface point of zero charge (isoelectric point), the membrane functional groups are minimal in charge and hence opens up, as the absence of repulsion forces contribute to the widening of the membrane pores. At high or low pH values, functional groups of membrane polymer can dissociate and take on positive or negative charge functions. Repulsion between these functions in the membrane polymer reduces or closes up membrane pores. Braghetta [26] has shown effect of solution pH and ionic state on apparent pore size of membranes. At high ionic strength and high pH, apparent pore size reduces remarkably.

3.5. Effect of iron concentration on arsenic separation

Fig. 4. Effect of oxidant dose on percentage arsenic removal for () NF-1, () NF-2 and () NF-20. Pressure maintained at 12 kgf/cm2 , As concentration in initial water 150 ␮g/L, flow rate of water 700 L/h, cross flow velocity 1.16 m/s, temperature 35 ◦ C.

Fig. 5. Effect of pH on removal of arsenic over NF-1, NF-2 and NF-20 membranes with and without pre-oxidation. Operating conditions: As concentration 150 ␮g/L, transmembrane pressure 12 kgf/cm2 , flow rate 700 L/h, cross flow velocity 1.16 m/s, raw water As concentration 150 ␮g/L, temperature 35 ◦ C, pre-oxidation done by KMnO4 .

Feed water for the present investigation was collected from three different arsenic-affected areas of West Bengal in India. Characteristics of such feed waters in terms of impurities like salts, iron, and arsenic were found to vary from source to source as shown in Table 1. Arsenic(III) in feed water was oxidised by addition of KMnO4 following the procedure of Pal et al. [15]. Nanofiltration of the pre-oxidised water was then performed using NF-1 membrane which was already found to be most effective in arsenic removal. Fig. 6 shows that with the increase in iron concentration in the groundwater, the arsenic separation efficiency of the membrane increased. Nguyen et al. [10] observed similar positive effect of nanoscale zero valent iron on removal efficiency during microfiltration and nanofiltration of arsenic-contaminated water. But their approach was different as they attempted to find out whether addition of zero valent iron to simulated water could improve separation whereas the present study attempted to investigate the effect of iron already present in arsenic-contaminated groundwater on separation performance of nanofiltration membranes. Presence of iron in arsenic-contaminated water as observed in the water samples of Table 1 poses real problem in treatment of contaminated water by adsorption [6]. Higher rejection of arsenic during nanofiltration in the present study may be attributed to enmeshed co-precipitation of arsenic with ferric hydroxide. Pentavalent arsenic precipitates out from water through adsorption, enmeshment and formation of loose As(V)–Fe(OH)3 complexes

112

M. Sen et al. / Journal of Membrane Science 354 (2010) 108–113

Fig. 6. Effect of Fe concentration in groundwater over arsenic removal efficiency of NF-1. Operating conditions: transmembrane pressure 12 kgf/cm2 , flow rate 700 L/h, cross flow velocity 1.16 m/s, raw water as per Table 1, temperature 35 ◦ C, preoxidation done with KMnO4 .

[15]. At the same time it was also observed that the flux decreased with increase in iron concentration as exhibited by Fig. 7. Formation of such precipitates leads to flux reduction of water. About 10% reductions in flux were observed when iron concentration increased from 4.2 mg/L to 10.5 mg/L. Such flux reduction can be attributed to possible clogging of the membrane by the precipitates. 3.6. Membrane fouling Figs. 8 and 9 show the SEM images of the best found membrane NF-1 before and after use. The surface conditions show that membranes largely remained fouling-free even after long 120 h of operation. This is due to the very cross flow pattern of the fluid that

Fig. 9. SEM image of the upper surface of NF-1 membrane after 120 h of operation.

always sweeps the membrane surface leaving very little scope for building up of concentration polarization layer and thus membrane fouling. Compared to the long period (more than 3 months) over which the applied membranes are at least expected to do the purification job, 120 h of operation seems to be small. However, whereas operation for months is always done with periodic cleaning of the membranes, the present observation period did not include any cleaning of the membranes. Performance over this investigated period indicates a clear trend and the potential of the selected membrane as well as the module in successful removal of arsenic from groundwater without much problem of fouling which is a major concern in membrane separation. 3.7. Error analysis

Fig. 7. Effect of iron concentration on flux over NF-1 membrane. Transmembrane pressure 12 kgf/cm2 , flow rate 700 L/h, cross flow velocity 1.16 m/s, temperature 35 ◦ C, pre-oxidised by KMnO4 .

During the experimental investigations, random errors were kept at the minimum possible levels by averaging. The mean of three values was always considered as the final value. Systematic errors arising from interference during spectrophotometric analysis could not be avoided altogether though could be significantly reduced by using reagent grade chemicals. Fully automated sample handling feature of FIAS-100 system (flow injection with atomic spectroscopy), use of ‘A’ class glassware and high precision electronic balance significantly contributed to reduction of error in the results. Six standard arsenic solutions each of 10 ␮g/L, 100 ␮g/L and 200 ␮g/L were measured to check the standard deviation. The overall percentage coefficient of variation which is standard deviation expressed as percentage of the mean value was found to be around 4.2. 4. Conclusions

Fig. 8. SEM image of upper surface of the selected best membrane NF-1 before use.

Polyamide composite nanofiltration membranes have high potential of arsenic separation from contaminated groundwater largely by Donnan-steric effect. However, only arsenic in pentavalent form can be so efficiently removed from water producing almost arsenic-free water. But groundwater is likely to contain arsenic both in oxidation states of 3+ as well as 5+ and thus conversion of trivalent arsenic into pentavalent form is a pre-condition for effective arsenic separation by nanofiltration membrane. Moreover, arsenic speciation and hence Donnan effects are very much dependent on medium pH. Thus maintaining proper (high) pH so as to maximise Donnan effects is another very significant operating condition. As fouling of membrane has always stood in the way of successful use of membranes in separation–purification, it

M. Sen et al. / Journal of Membrane Science 354 (2010) 108–113

is essential to explore the potential of various modules in ensuring largely fouling-free operation for very long duration much beyond 120 h. Nanofiltration membranes and the associated modules are very sensitive to operating conditions and characteristics of groundwater. Impurities like iron and other salts may significantly alter the separation characteristics. The present study considered the effect of presence of iron where it was found that beyond an iron concentration of 10 mg/L, water flux may significantly get reduced though removal efficiency may improve. Thus effects of other such impurities (e.g. salts of Ca, Na, S) on various types of nanofiltration membranes and modules need further study. From the present study, it transpires that polyamide composite nanofiltration flat sheet membranes in a cross flow membrane module can very successfully purify arsenic-contaminated groundwater with pre-oxidation of all As(III) into As(V). Acknowledgements The authors are thankful to the Department of Science and Technology (DST), Government of India for the grants under DST-FIST Program (SR/FST/ET1-204/2007) and Green Chemistry/Technology Program (SR/S5/GC-05/2008) with which the infrastructure for the present research was developed and necessary research materials were procured. References [1] NSF International, Environmental Technology Verification Report, and Removal of Arsenic in drinking water: prepared for USEPA, August 2001. [2] F.W. Pontius, Arsenic removal from drinking water by a loose nanofiltration membrane, Journal of the American Water Works Association 90 (3) (1998) 38. [3] World Health Organisation, WHO Guidelines for Drinking Water Quality. Health Criteria and Other Supporting Information, 2nd edn, WHO, Geneva, Switzerland, 1996. [4] E.M. Vrjenhoek, J.J. Waypa, Effect of operating conditions in removal of arsenic from water by nanofiltration membrane, Desalination 130 (2000) 165. [5] P. Pal, Z. Ahamad, P. Bhattacharya, ARSEPPA: A Visual Basic software tool for arsenic separation plant performance analysis, Chemical Engineering 129 (2007) 113–122. [6] M. Sen, P. Pal, Treatment of arsenic-contaminated groundwater by a low cost activated alumina adsorbent prepared by partial thermal dehydration, Desalination and Water Treatment 11 (2009) 275–282. [7] J.G. Hering, M. Elimelech, Arsenic removal by ferric chloride, Journal of the American Water Works Association 88 (4) (1996) 155–167.

113

[8] J.E. Greenleof, J. Lin, A.K. Sengupta, Two novel applications of ion exchange fibres: arsenic removal and chemical-free softening of hard water, Environmental Progress 25 (2006) 300–311. [9] E. Fagarassy, I. Galambos, Bekassy-Molnar, Gy. Vatai, Treatment of high arsenic content wastewater by membrane filtration, Desalination 240 (2009) 270–273. [10] V.T. Nguyen, S. Vigneswaran, H.H. Ngo, H.K. Shorn, J. Kandasamy, Arsenic removal by a membrane hybrid filtration system, Desalination 236 (2009) 363–369. [11] L.C. Hsieh, Y. Weng, C.P. Huang, K.C. Li, Removal of arsenic from groundwater by electro-ultra filtration, Desalination 234 (2008) 402–408. [12] A.E. Pagana, S.D. Sklari, E.S. Kikkinides, V.T. Zaspalis, Microporous ceramic membrane technology for the removal of arsenic and chromium ions from contaminated water, Microporous and Mesoporous Materials 110 (2008) 150–156. [13] P. Pal, M. Sen, A.K. Manna, J. Pal, P. Pal, S.K. Roy, P. Roy, Contamination of groundwater by arsenic: a review of occurrence, causes, impacts, remedies and membrane-based purifications, Journal of Integrative Environmental Sciences 6 (4) (2009) 1–22. [14] A.K Manna, M. Sen, P. Pal, Removal of arsenic from contaminated ground water by solar-driven membrane distillation, Environmental Pollution 158 (2010) 805–811. [15] P. Pal, Z. Ahammad, A. Pattanayek, P. Bhattacharya, Removal of arsenic from drinking water by chemical precipitation—a modelling and simulation study of the physical-chemical processes, Water Environment Research 79 (4) (2007) 357–366. [16] J.J. Waypa, M. Elimelech, J.G. Hering, Journal of the American Water Works Association 89 (10) (1997) 102. [17] H. Strathmann, in: W.S. Ho, K.K. Sirkar (Eds.), Membrane Handbook, Van Nostrand Reinhold, 1992. [18] A.I. Schafer, D.I. Nghiem, T.D. Waite, Removal of natural hormone estrone from aqueous solutions using nanofiltration and reverse osmosis, Environmental Science & Technology 37 (1) (2003) 182–188. [19] S. Xia, B. Dong, Q. Zhang, B. Xu, N. Gao, C. Causseranda, Study of arsenic removal by nanofiltration and its application in China, Journal of Membrane Science, Desalination 204 (2007) 374–379. [20] Y. Sato, M. Kang, T. Kamei, Y. Magara, Performance of nanofiltration for arsenic removal, Water Research 36 (2002) 3371–3377. [21] W. Driehaus, M. Jekel, Determination of As(III) and total inorganic arsenic by on-line pre-treatment in hydride generation atomic absorption spectrophotometry, Fresenius Journal of Analytical Chemistry 343 (4) (1992) 352–356. [22] H. Saitua, M. Campderros, S. Cerutti, A.P. Padilla, Effect of operating conditions in removal of arsenic from water by nanofiltration membrane, Desalination 172 (2005) 173–180. [23] E.M. Vrijenhoek, J.J. Waypa, Arsenic removal from drinking water by a loose nanofiltration membrane, Desalination 130 (2000) 265–277. [24] B. Van der Bruggen, Removal of pollutants from surface water and groundwater by nanofiltration: overview of possible applications in the drinking water industry, Environmental Pollution 122 (2003) 435–445. [25] H. Saitua, M. Campderros, P.A. Perez, Effect of operating conditions in removal of arsenic from water by nanofiltration membrane, Desalination 172 (2005) 173–180. [26] A. Braghetta, The Influence of Solution Chemistry and Operating Conditions on Nanofiltration of Charged and Uncharged Organic Macromolecules, University of North Carolina, Chapel Hill, 1995.