Environ. Sci. Technol. 2006, 40, 4039
Response to Comment on “Landfill-Stimulated Iron Reduction and Arsenic Release at the Coakley Superfund Site (NH)” Blaney and SenGupta (1) raise several interesting points regarding the release of arsenic from landfills through reductive processes. They argue that solid-phase arsenic in landfills is inherently unstable despite the strong adsorption of arsenic to iron(III) oxides (e.g., 2). They base these observations on our recent findings for the Coakley Landfill Superfund Site, a landfill exhibiting arsenic contamination (3). Our study, along with other recent studies (4-5), finds that reduction of arsenic-bearing iron oxides results in the release of arsenic from either the landfill (resulting in elevated arsenic concentrations in the landfill leachate) or from the surrounding rock formations. This biologically mediated reduction is facilitated by the decomposition of organic matter in the landfill and leachate. Blaney and SenGupta also suggest that arsenic reduction alone may mobilize arsenic. While we have not examined arsenic speciation in our study, we have identified both As(III) and As(V) in groundwater near the landfill, and our study (3) finds negligible As(III) in the solid phase. Thus, we cannot rule out the reductive release of arsenic from other phases (e.g., aluminum oxides) that may be associated with arsenic reduction. In fact, direct arsenic reduction has been implicated in arsenic release in other systems (6). However, the central conclusion of our work, and that of Blaney and SenGupta, that arsenic release results from geochemical reduction, remains intact and consistent with the growing body of literature on arsenic contamination in Bangladesh and elsewhere (7-9). As pointed out by Blaney and SenGupta (1), our research findings have important implications to the long-term fate of arsenic-containing materials in landfills. Current regulations use the toxicity characteristic leaching procedure (TCLP) standard to determine if materials may be disposed in landfills. The TCLP is based on the extraction of arsenic (and other trace metals) under oxic, mildly acidic (pH 5) conditions for 18 h. Under these conditions, arsenic is strongly adsorbed to most oxides, thus, most materials meet TCLP guidelines for landfill disposal (10). However, the landfill conditions may differ considerably from those simulated by the TCLP procedure. Under reducing conditions, TCLP-compliant waste may result in dissolved arsenic concentrations that may be elevated considerably above TCLP guidelines (11). As a result, we agree that considerable care must be used when disposing of arsenic-bearing wastes in landfills. Several types of arsenic-bearing wastes may be affected by geochemical reduction. In particular, water treatment residuals often contain high levels of arsenic bound to iron oxides (1, 11) and may be destabilized when stored in landfills that undergo geochemical reduction. Given the increased need for arsenic water treatment to meet the new 10 µg/L water standard, the use and disposal of these wastes is likely to increase in the future. Depending on distribution of these wastes to landfills, these residuals may contribute a measurable fraction of arsenic to landfill leachate in some landfills. Arsenic in chromated copper arsenate (CCA)-treated wood also is susceptible to reductive dissolution; however, arsenic release from landfills containing these wastes appears to be limited by slow degradation (12). In fact, arsenic-bearing overburden alone may be sufficient to cause high arsenic concentration in leachate. 10.1021/es068004c CCC: $33.50 Published on Web 05/16/2006
2006 American Chemical Society
Given the potential hazards of disposing arsenic-bearing wastes in landfills, we feel it prudent to take several steps to minimize the extent of arsenic release to the environment. First and foremost, it is prudent to minimize the disposal of arsenic in unlined landfills. Fortunately, modern landfills are usually lined to prevent the release of arsenic-bearing leachates to the environment. Furthermore, arsenic-bearing wastes should be isolated and kept in oxidizing environments to keep arsenic in the stable, solid form. This may be achieved by disposing of arsenic in well-aerated systems such as those employed by Blaney and SenGupta (1), but also may involve the disposal in specialized systems that do not contain sufficient organic matter to induce reducing conditions. Other strategies, including the use of alternative adsorbents such as alumina, or stabilization of adsorbed arsenic by ferric mineral recrystallization, may also be useful to minimize rapid arsenic release and the subsequent contamination of landfill leachate and, potentially, groundwater.
Literature Cited (1) Blaney, L. M.; SenGupta, A. K. Comment on “LandfillStimulated Iron Reduction and Arsenic Release at the Coakley Superfund Site (NH)”. Environ. Sci. Technol. 2006, 40, 40374038. (2) 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. (3) deLemos, J. L.; Bostick, B. C.; Renshaw, C. E.; Sturup, S.; Feng, X. H. Landfill-stimulated iron reduction and arsenic release at the Coakley Superfund Site (NH). Environ. Sci. Technol. 2006, 40, 67-73. (4) Mayo, M. J.; Hon, R.; Brandon, W. C.; Ford, R. Arsenic in groundwater at landfill sites in Northern Central Massachusetts. Abstr. Pap. Am. Chem. Soc. 2003, 226, U594-U594. (5) Keimowitz, A. R.; Simpson, H. J.; Stute, M.; Datta, S.; Chillrud, S. N.; Ross, J.; Tsang, M. Naturally occurring arsenic: Mobilization at a landfill in Maine and implications for remediation. Appl. Geochem. 2005, 20, 1985-2002. (6) Zobrist, J.; Dowdle, P. R.; Davis, J. A.; Oremland, R. S. Mobilization of arsenite by dissimilatory reduction of adsorbed arsenate. Environ. Sci. Technol. 2000, 34, 4747-4753. (7) Nickson, R. T.; McArthur, J. M.; Ravenscroft, P.; Burgess, W. G.; Ahmed, K. M. Mechanism of arsenic release to groundwater, Bangladesh and West Bengal. Appl. Geochem. 2000, 15, 403413. (8) Harvey, C. F.; Swartz, C. H.; Badruzzaman, A. B. M.; KeonBlute, N.; Yu, W.; Ali, M. A.; Jay, J.; Beckie, R.; Niedan, V.; Brabander, D.; Oates, P. M.; Ashfaque, K. N.; Islam, S.; Hemond, H. F.; Ahmed, M. F. Arsenic mobility and groundwater extraction in Bangladesh. Science 2002, 298, 1602-1606. (9) Polizzotto, M. L.; Harvey, C. F.; Sutton, S. R.; Fendorf, S. Processes conducive to the release and transport of arsenic into aquifers of Bangladesh. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 18819-18823. (10) Jing, C. Y.; Liu, S. Q.; Patel, M.; Meng, X. G. Arsenic leachability in water treatment adsorbents. Environ. Sci. Technol. 2005, 39, 5481-5487. (11) Ghosh, A.; Mukiibi, M.; Ela, W. TCLP underestimates leaching of arsenic from solid residuals under landfill conditions. Environ. Sci. Technol. 2004, 38, 4677-4682. (12) Khan, B. I.; Jambeck, J.; Solo-Gabriele, H. M.; Townsend, T. G.; Cai, Y. Release of arsenic to the environment from CCA-treated wood. 2. Leaching and speciation during disposal. Environ. Sci. Technol. 2006, 40, 994-999.
Benjamin C. Bostick,* Carl Jamie L. deLemos, Stefan Xiahong Feng
E. Renshaw, Stu1 rup, and
Department of Earth Sciences Dartmouth College 6105 Fairchild Hall Hanover, New Hampshire 03755 ES068004C VOL. 40, NO. 12, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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