Correspondence Comment on “Arsenic Removal from Groundwater by Household Sand Filters: Comparative Field Study, Model Calculations, and Health Benefits” The article “Arsenic removal from groundwater by household sand filters: Comparative field study, model calculations, and health benefits,” by Berg et al. appearing in a recent issue of Environmental Science and Technology, discusses the favorability of iron oxidation within sand filters and employment of that conversion toward arsenic removal (1). At near-neutral pH, oxidation of dissolved Fe(II) into ferric hydroxide or hydrated Fe(III) oxide (HFO) is thermodynamically favorable (2):
4Fe2+ + O2 + 10H2O f 4Fe(OH)3(s) + 8H+ (∆GR° ) -18 kJ/mole) The rapid oxidation of dissolved iron followed by hydroxide precipitation in contact with solid surfaces (sand) provides arsenic removal through ligand sorption mechanism (3-4). Percent of arsenic removal is governed primarily by the concentration of dissolved Fe(II), distribution of As(V) and As(III), and the concentration of silica and phosphate in groundwater. Comments on three specific issues presented in the paper follow. First, the authors claim that ion exchange and activated alumina arsenic removal technologies have not been “applied on a broad scale in developing countries because they require
sophisticated technical systems and are therefore unpractical in low income regions.” The statement is factually incorrect. Currently, over 150 gravity-fed well-head community based arsenic removal units using activated alumina are in operation in the bordering areas of West Bengal, India, and Bangladesh (5). Each unit provides safe drinking water to nearly 1000 villagers. No chemical addition, pH adjustment, or electricity are required for unit operation and villagers are responsible for day-to-day maintenance. Figure 1 demonstrates arsenic reduction from approximately 500 µg/L in groundwater to less than 50 µg/L in the treated water in Debnagar Village; additionally, over 90% iron removal was obtained by oxidation of Fe(II) into HFO precipitates, which enhanced arsenic removal capacity of the bed by nearly an order of magnitude. Activated alumina filters were specifically designed to achieve near-complete oxidation of Fe(II) within the column. The subject study (1) observed greater arsenic removal with sand filters than with coprecipitation. The authors speculated that oxidation of As(III) to As(V) by birnessite might be responsible. Thermodynamically, the concurrent presence of Mn(IV) along with Fe(II) is very unfavorable; it is Mn(II) which accompanies Fe(II) under groundwater conditions as recorded in the open literature. In the filtration column, arsenic removal by HFO precipitates approximates plug-flow reactor configuration while continuous stirred tank reactor behavior is operative in coprecipitation. Under otherwise identical conditions, PFR conditions always lead to higher sorption capacity; furthermore, oxidation of As(III) to As(V) by HFO and subsequent arsenic removal enhancement is more likely to occur in a plug-flow type arrangement as demonstrated in a previous study (6).
FIGURE 1. Arsenic and iron influent and effluent histories of a well-head unit using activated alumina filter. 10.1021/es062403q CCC: $37.00 Published on Web 12/30/2006
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Our third and last point is to emphasize that the collection, handling, and long-term containment of arsenic waste in developing countries is as important as arsenic removal from groundwater. In this regard, community based well-head units serving one hundred or more families offer specific advantages because the arsenic-laden sludge is not handled by individual households at all. Any quality control effort, such as arsenic analysis, is also reduced by 2 orders of magnitude.
Literature Cited (1) Berg, M.; Luzi, S.; Trang, P. T. K.; Giger, W.; Stu ¨ ben, D. Arsenic removal from groundwater by household sand filters: Comparative field study, model calculations, and health benefits. Environ. Sci. Technol. 2006, 40, 5567-5573. (2) Morel, F. M. M., Hering, J. G. Principles and Applications of Aquatic Chemistry; John Wiley & Sons: New York, 1993. (3) Raven, K. P.; Jain, A.; Loeppert, R. H. Arsenite and arsenate adsorption on ferrihydrite: Kinetics, equilibrium, and adsorption envelopes. Environ. Sci. Technol. 1998, 32, 344-349.
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(4) Roberts, L. C.; Hug, S. J.; Ruettimann, T.; Billah, M.; Khan, A. W.; Rahman, M. T. Arsenic removal with iron(II) and iron(III) waters with high silicate and phosphate concentrations. Environ. Sci. Technol. 2004, 38, 307-315. (5) Sarkar, S.; Gupta, A.; Biswas, R. K.; Deb, A. K.; Greenleaf, J. E.; SenGupta, A. K. Well-head arsenic removal units in remote villages of Indian subcontinent: Field results and performance evaluation. Water Res. 2005, 39 (10), 2196-2206. (6) Greenleaf, J. E.; Cumbal, L.; Staina, I.; SenGupta, A. K. Abiotic As(III) oxidation by hydrated Fe(III) oxide (HFO) microparticles in a plug flow columnar configuration. Trans. Inst. Chem. Eng., B 2003, 81, 87-98.
Lee Blaney, Arup K. SenGupta*
Sudipta
Sarkar,
Department of Civil & Environmental Engineering Lehigh University Bethlehem, Pennsylvania 18015 ES062403Q
and
