HR: 0800h
AN: H11E-0820    [Abstracts]
TI: Sequestration Mechanisms of Arsenic Mobilized during In Situ Bioremediation of Chlorinated Solvents
AU: * Choi, S
EM: schoi@ucmerced.edu
AF: University of California, Merced, School of Natural Sciences, P.O. Box 2039, Merced, CA 95344, United States
AU: Beals, D A
EM: dbeals@ucmerced.edu
AF: University of California, Merced, School of Natural Sciences, P.O. Box 2039, Merced, CA 95344, United States
AU: Hering, J
EM: Janet.Hering@eawag.ch
AF: Swiss Federal Institute of Aquatic Science & Technology (EAWAG), Ueberlandstrasse 133, Dubendorf, 8600, Switzerland
AU: O'Day, P A
EM: poday@ucmerced.edu
AF: University of California, Merced, School of Natural Sciences, P.O. Box 2039, Merced, CA 95344, United States
AB: The mobilization and down-gradient attenuation of naturally occurring arsenic (As) was studied at a site where anaerobic bioremediation was employed to treat a tetrachloroethene plume at a former military base. Elevated levels of As in groundwater were induced by anthropogenic introduction of organic substrates, which generated an anaerobic reducing zone that extends ~30 m down gradient of the injection zone. However, ~60 m down gradient and outside of the reducing zone, As and Fe groundwater concentrations have remained below their detection limits, indicating natural attenuation. In this study, As sequestration behavior and mechanisms were investigated in laboratory batch and column uptake experiments and characterized by As, Fe, and Mn X-ray absorption spectroscopy (XAS). Batch sorption experiments showed similar As(III) and As(V) sorption on sediments at high total As concentration (10^-3 M), but slightly more As(V) than As(III) sorption at low total As concentration (10^-6 and 10^-5). At the same As concentration, As uptake in sterilized samples was not significantly different from untreated samples, indicating a lack of microbial influence on As sorption. More Fe(II) was released to solution in samples reacted with As compared to a control sediment with no As, and slightly more Fe(II) was released in the presence of As(III) than As(V). XAS of unreacted core sediments collected from 40-42 m depth showed that native As (15- 24 ppm) was present as As(V). Sediments reacted in laboratory experiments with As(V) solutions retained their As(V) speciation after sorption, as indicated by the XANES spectra. Sediments reacted with As(III) showed spectroscopic evidence for oxidation to As(V). The amount of As(III) oxidation was proportional to the total dissolved concentration, with the majority of sorbed As oxidized to As(V) at low As(III) concentrations and most sorbed As remaining as As(III) at high concentrations. EXAFS results are consistent with adsorption of As as the primary mechanism of retention by sediment minerals. Comparison of unreacted and reacted Fe XANES showed no changes in spectral features. The Mn XANES spectra, however, indicated a spectral shift in absorption maxima to lower energy between unreacted sediments and As(III)-reacted sediments. Qualitatively, the absorption shift can be interpreted as a slightly higher proportion of reduced to oxidized Mn in the reacted sediment compared with the unreacted sediment. These results indicate that the sediment has a limited sorption and abiotic oxidation capacity for As, and that reduction of sediment Mn plays a role in the oxidation of sorbed As(III).
DE: 1831 Groundwater quality
DE: 1871 Surface water quality
SC: Hydrology [H]
MN: 2007 Fall Meeting