HR: 0800h
AN: H21C-1025    [Abstracts]
TI: Arsenic Mobilization Influenced By Iron Reduction And Sulfidogenesis Under Dynamic Flow
AU: * Kocar, B D
EM: kocar@stanford.edu
AF: Department of Geological and Environmental Sciences, Stanford University, Braun Hall, #118, 450 Serra Mall, Building 320, Stanford, CA 94305 United States
AU: Stewart, B D
EM: stewartb@stanford.edu
AF: Department of Civil and Environmental Engineering, Stanford University, M42 Terman Engineering Center, Stanford, CA 94305 United States
AU: Herbel, M
EM: mherbel@ussl.ars.usda.gov
AF: USDA-ARS-USSL Salinity Laboratory, 450 Big Springs Road, Riverside, CA 92507 United States
AU: Fendorf, S
EM: fendorf@stanford.edu
AF: Department of Geological and Environmental Sciences, Stanford University, Braun Hall, #118, 450 Serra Mall, Building 320, Stanford, CA 94305 United States
AB: Sulfidogenesis and iron reduction are ubiquitous processes that occur in a variety of anoxic subsurface and surface environments, which profoundly impact the cycling of arsenic. Of the iron (hydr)oxides, ferrihydrite possesses one of the highest capacities to retain arsenic, and is globally distributed within soils and sediments. Upon dissimilatory iron reduction, ferrihydrite may transform to lower surface area minerals, such as goethite and magnetite, which decreases arsenic retention, thus enhancing its transport. Here we examine how arsenic retained on ferrihydrite is mobilized under dynamic flow in the presence of {\it Sulfurosprillum barnesii} strain SES-3, a bacteria capable of reducing both As(V) and Fe(III). Ferrihydrite coated sands, loaded with 150 mg kg$^{-1}$ As(V), were inoculated with {\it S. barnesii}, packed into a column and reacted with a synthetic groundwater solution. Within several days after initiation of flow, the concentration of arsenic in the column effluent increased dramatically coincident with the mineralogical transformation of ferrihydrite and As(V) reduction to As(III). Following the initial pulse of arsenic, effluent concentration then declined to less than 10 $\mu$M. Thus, arsenic release into the aqueous phase is contingent upon the incongruent reduction of As(V) and Fe(III) as mediated by biological activity. Reaction of abiotically or biotically generated dissolved sulfide with iron (hydr)oxides may have a dramatic influence on the fate of arsenic within surface and subsurface environments. Accordingly, we examined the reaction of dissolved bisulfide and iron (hydr)oxide complexed with arsenic in both batch and column systems. Low ratios of sulfide to iron in batch reaction systems result in the formation of elemental sulfur and concomitant arsenic release from the iron (hydr)oxide surface. High sulfide to iron ratios, in contrast, appear to favor the formation of iron and arsenic sulfides. Our findings demonstrate that iron (hydr)oxides may quench reactions between sulfide and constituents sorbed to iron (hydr)oxide surfaces, forming elemental sulfur as opposed to sulfide-arsenic complexes. In addition, reductive transformation of iron (hydr)oxide by dissolved sulfide may release sorbed constituents. Hence, moderate to low concentrations of dissolved sulfide in association with iron (hydr)oxides may inhibit sequestration of important contaminants that are attenuated by Fe(III) and/or S(-II) bearing phases.
DE: 1831 Groundwater quality
DE: 1832 Groundwater transport
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting