Iron (Hydr)oxide Transformation and Release of Arsenic From Ferrihydrite and Tropical Soils During Sulfate Reduction.
Benjamin Kocar, Yoko Masue, Katharine Tufano, Samantha Ying, Matthew Polizzotto, Thomas Borch, and Scott Fendorf. Dept. of Geological and Environmental Sciences, Stanford University, Building 320, Room 118, Stanford, CA 94305-2115
Sulfidogenesis and dissimilatory iron reduction are ubiquitous processes that occur in a variety of anaerobic environments and 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. Reduced aqueous constituents produced during sulfate reduction, such as S(-II) and subsequently Fe(II), are partly responsible for the transformation and dissolution of ferrihydrite, as well as the desorption of surface species such as arsenic. Here, we examine the effect of arsenic loading on the solid phase transformation of ferrihydrite and arsenic desorption during sulfidogenesis. Columns initially packed with arsenic loaded ferrihydrite-coated sand were inoculated with either a sulfur or iron reducing bacterial culture. Arsenic is mobilized during biologically mediated sulfidogenesis when initially loaded with low arsenic concentrations; the dominant solid phases are amorphous iron sulfide, magnetite, and ferrihydrite. However, at high initial loadings of arsenic, mobilization is not enhanced. Rather, sulfidogenesis appears to occur at a much slower rate relative to low arsenic loadings, and remaining reaction products predominately consist of ferrihydrite and green rust. Thus, sulfidogenesis may induce arsenic desorption but mobilization is a function of arsenic loading. Additionally, mineralogical transformation pathways are drastically influenced by the type of bacterial respiration (sulfate versus iron reduction) and the extent of arsenic surface coverage. Soils and surface sediments are enriched with arsenic bearing iron (hydr)oxides throughout sedimentary basins of southeast Asia– arsenic concentrations typically range from 10 to 40 mg kg-1 (DW). Seasonal monsoons result in drastically fluctuating water levels, which drive wide swings in redox processes within soils and near-surface sediments. Extensive flooding results in anaerobic soil/sediment conditions, with ensuing reduction of iron (hydr)oxides and sulfate. Aqueous concentrations of arsenic vary with redox conditions and processes and when elevated, result in migration through the soil profile to the deeper subsurface. Using ceramic-cup lysimeters and passive samplers (peepers), we measured a suite of reduced aqueous constituents, including arsenic, within soil profiles in the lower Mekong delta, Cambodia. Within a zone of seasonal wetting/drying, we observed increased aqueous concentrations of arsenic in surface soils during periods of flooding. Arsenic release is coincident with the production of Fe(II) and, to a lesser degree, S(-II) in the porewater. These results indicate that arsenic mobilization occurs in near-surface soil environments upon soil wetting, and that iron reduction and sulfidogenesis are likely dominant processes responsible for arsenic release.