HR: 17:00h
AN: B24D-05    [Abstracts]
TI: Sulfidogenesis Controls on Ferrihydrite Transformation and Repartitioning of Sorbed Arsenic
AU: * Kocar, B D
EM: kocar@stanford.edu
AF: Stanford University, Department of GES, Building 450 Serra Mall, Braun Hall, Building 320, Stanford, CA 94305,
AU: Fendorf, S
EM: fendorf@stanford.edu
AF: Stanford University, Department of GES, Building 450 Serra Mall, Braun Hall, Building 320, Stanford, CA 94305,
AB: Iron (hydr)oxides are ubiquitous sorbents of arsenic (As) that undergo reductive dissolution and transformation upon reaction with dissolved sulfide. Here, we examine diverging pathways of solid phase iron (Fe) transformation during sulfate reduction in the presence of varying As loadings. Columns initially containing As(V)- ferrihydrite coated sand, inoculated with the sulfate reducing bacteria Desulfovibrio vulgaris (Hildenborough), were eluted with artificial groundwater containing sulfate and lactate. Additionally, abiotic batch reaction experiments were conducted to examine Fe secondary products rapidly formed during sulfidization of As-loaded ferrihydrite. Rapid and consistent sulfate reduction coupled with lactate oxidation is observed within column solids possessing low As(V) surface coverage (10% of the adsorption maximum). Column experiments illustrated that at high As(V) surface coverage (50% of the adsorption maximum), sulfate reduction and lactate oxidation are initially slow but gradually increase over time, and all As(V) is reduced to As(III) by the end of experimentation. The dominant Fe solid-phase transformation products at low As coverage include amorphous FeS within the zone of sulfate reduction (near the inlet of the column) and magnetite downstream where Fe(II)_aq concentrations exceed 1 mM. Arsenic(V) is reduced to As(III) and displaced from the zone of sulfidogenesis and Fe(III)_s depletion. At higher As coverage, green rust carbonate, as opposed to magnetite, is a dominant Fe solid phase product. Independent of loading, As is strongly associated with magnetite and residual ferrihydrite, while being excluded from green rust and iron sulfide. Abiotic batch reactor experiments illustrate that As is readily released from ferrihydrite during sulfidization, and that low As loadings yield initial Fe secondary products of lepidocrocite and FeS, while high loadings inhibit rapid secondary Fe mineral formation. Our observations illustrate that sulfidogenesis occurring in proximity with Fe (hydr)oxides may govern pathways of Fe solid phase transformation and As partitioning; formation of As sulfide minerals, in particular, is inhibited by reactive Fe either through sulfide oxidation or complexation.
DE: 0461 Metals
DE: 0463 Microbe/mineral interactions
DE: 0471 Oxidation/reduction reactions (4851)
DE: 0488 Sulfur cycling
DE: 0489 Trace element cycling (4875)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting