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Response to Comment on “Effect of Groundwater Iron and Phosphate on the Efficacy of Arsenic Removal by Iron-Amended BioSand Filters” Noubactep (1) raises several interesting points regarding the treatment of arsenic-contaminated groundwater using ironamended BioSand filters. The author begins with the premise that a successful filter design will remove arsenic and biological contaminants and that well designed filters are capable of doing so. The author argues that there is sufficient iron in modified BioSand filters to effectively remove arsenic from a large volume of groundwater. Furthermore, he indicates that similar filter designs (Kanchan filters) have been successfully implemented in Nepal (2). In contrast, we find in our study of filter performance that bacterial removal was moderate and that arsenic removal was incomplete (3). The author attributes this apparent discrepancy to methodological factors that influence arsenic removal, including differences in filter design and operation, and the quantity of iron available for arsenic (and phosphate) removal. While some design differences exist between Kanchan filters and those used in our study, we maintain that the partial removal of arsenic primarily results from differences in groundwater composition rather than differences in filter design. Noubactep asserts correctly that iron oxidation will generate soluble Fe and Fe(III) oxides at neutral pH and that the precipitation of these iron oxides, if complete will provide adequate iron oxides to remove arsenic from a large volume of groundwater. However, the residence time of water in the upper basin, which contains the iron nails, is short and under the neutral pH conditions common for arsenic-bearing groundwater is insufficient to oxidize sufficient iron metal. Thus, only a small fraction of the iron from the nails is oxidized and thereby available to scavenge arsenic or phosphate, and arsenic is not removed in the upper basin. The short residence time in the upper basin also limits the retention of arsenic to the corrosion products that encrust the nails in the diffuser (3). This result suggests several future design improvements to iron-amended BioSand filters that should be thoroughly investigated, many of which are also suggested by Noubactep. A more reactive form of iron would help considerably. These nails are mild steel, which corrodes relatively quickly; increasing the surface area will facilitate increased iron release, but also may decrease porosity. Ideally, the residence time should be extended in the upper basin while maintaining sufficient overall flow rates for the filter to be useful. Keeping the nails submerged may also influence their passivation, which can prevent oxidation. Future studies should investigate how these and other potential factors affect filter performance and arsenic removal. Given the limited production of iron in the filter, we find groundwater represents the major source of Fe(II) for iron oxide production and arsenic removal. Dissolved iron concentrations in arsenic-impacted groundwater are often significant because iron reduction is the dominant process by which arsenic is released into groundwater (4, 5). However, iron concentrations are variable and are not well-correlated to arsenic levels (6). Surveys of groundwater composition in Cambodia and elsewhere indicate that frequently groundwater compositions have insufficient iron to achieve sufficient arsenic removal (7-9). The presence of phosphate in the water further increases the iron requirement for effective 8696
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arsenic removal (9, 10). Groundwater phosphate levels can be quite high due to reductive dissolution and long-term contamination by sewage and agricultural runoff. In our study, we find that groundwater in Cambodia contains insufficient iron and excess phosphate for water treatment without additional iron sources. However, iron-amended BioSand filters may remove considerably more arsenic in regions where groundwater typically contains high dissolved Fe (or lower phosphate). Last, Noubactep suggests that well flushing could introduce oxygen into the water that would influence iron (and arsenic) levels in the influent. In fact, well flushing does not introduce oxygen; it is done to remove partially oxidized groundwater from the pump so that the study is completed with unaltered (anoxic) groundwater. Influent concentrations are measured in acidified samples and are not subject to precipitation. The iron and arsenic levels in this water are stable during extraction and do not change in the influent prior to filtration. Regardless, the oxidation of groundwater following extraction from the well would lead to additional iron precipitation and arsenic removal and thus would bias the results toward more effective arsenic removal, not less, as implied by the author. Point-of-use water treatment options are needed in areas affected by groundwater arsenic contamination in Cambodia and elsewhere. We agree wholeheartedly with Noubactep that more research is needed to ensure that filters effectively treat water and to establish the lifetime of such treatments under field conditions prior to their widespread deployment in affected areas. This includes continued basic research into the chemistry of water treatment, applied research that examines treatment under variable field conditions, and engineering design improvements that incorporate this research into filters that function under a wide variety of conditions. While we have a considerable knowledge base on which to design effective technologies, many of those technologies are not well tested in the field. Our findings and those of others (3, 10, 11) indicate that groundwater composition (dissolved iron and phosphate) plays a very important role in regulating the efficacy of arsenic removal. Engineered systems that will be widely implemented must remove arsenic from groundwater containing moderate dissolved iron and phosphate.
Literature Cited (1) Noubactep, C. Comment on “Effect of groundwater iron and phosphate on the efficacy of arsenic removal by iron-amended BioSand filters. Environ. Sci. Technol. 2009, 10.1021/es902520n. (2) Ngai, T. K. K.; Shrestha, R. R.; Dangol, B.; Maharjan, M.; Murcott, S. E. Design for sustainable development: Household drinking water filter for arsenic and pathogen treatment in Nepal. J. Environ. Sci. Health 2007, A42, 1879–1888. (3) Chiew, H.; Sampson, M. L.; Huch, S.; Ken, S.; Bostick, B. C. Effect of groundwater iron and phosphate on the efficacy of arsenic removal by iron-amended BioSand filters. Environ. Sci. Technol. 2009, 43, 6295–6300. (4) Cummings, D. E.; Caccavo, F.; Fendorf, S.; Rosenzweig, R. F. Arsenic mobilization by the dissimilatory Fe(III)-reducing bacterium Shewanella alga BrY. Environ. Sci. Technol. 1999, 33, 723–729. (5) Islam, F. S.; Pederick, R. L.; Gault, A. G.; Adams, L. K.; Polya, D. A.; Charnock, J. M.; Lloyd, J. R. Interactions between the Fe(III)-reducing bacterium Geobacter sulfurreducens and arsenate, and capture of the metalloid by biogenic Fe(II). Appl. Environ. Microbiol. 2005, 71, 8642–8648. (6) Zheng, Y.; Stute, M.; van Geen, A.; Gavrieli, I.; Dhar, R.; Simpson, H. J.; Schlosser, P.; Ahmed, K. M. Redox control of arsenic mobilization in Bangladesh groundwater. Appl. Geochem. 2004, 19, 201–214. 10.1021/es902940m CCC: $40.75
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(7) Quicksall, A. N.; Bostick, B. C.; Sampson, M. L. Linking organic matter deposition and iron mineral transformations to groundwater arsenic levels in the Mekong delta, Cambodia. Appl. Geochem. 2008, 23, 3088–3098. (8) Horneman, A.; Van Geen, A.; Kent, D. V.; Mathe, P. E.; Zheng, Y.; Dhar, R. K.; O’Connell, S.; Hoque, M. A.; Aziz, Z.; Shamsudduha, M.; Seddique, A. A.; Ahmed, K. M. Decoupling of As and Fe release to Bangladesh groundwater under reducing conditions. Part 1: Evidence from sediment profiles. Geochimica Et Cosmochimica Acta 2004, 68 (17), 3459–3473. (9) Hug, S. J.; Leupin, O. X.; Berg, M. Bangladesh and Vietnam: Different groundwater compositions require different approaches to arsenic mitigation. Environ. Sci. Technol. 2008, 42, 6318–6323. (10) 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. (11) 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.
Benjamin C. Bostick Lamont-Doherty Earth Observatory of Columbia University, Palisades New York 10964
Hannah Chiew Department of Civil Engineering, University of British Columbia, Vancouver, British Columbia, Canada RDI-C (Resource Development International-Cambodia), Royal Brick Road, Kean Svay, Kandal, Cambodia
M. L. Sampson RDI-C (Resource Development International-Cambodia), Royal Brick Road, Kean Svay, Kandal, Cambodia ES902940M
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