HR: 11:20h
AN: B32B-05 INVITED     [Abstracts]
TI: Biotransformation Rates of Iron Governing Chromium and Uranium Transport
AU: * Fendorf, S
EM: fendorf@stanford.edu
AF: Stanford University, Dept. of Geological and Environmental Sciences, Stanford, CA 94305-2115 United States
AU: Hansel, C M
EM: hansel@stanford.edu
AF: Stanford University, Dept. of Geological and Environmental Sciences, Stanford, CA 94305-2115 United States
AU: Benner, S G
EM: sbenner@boisestate.edu
AF: Boise State University, Dept. of Geosciences, Boise, ID 83725 United States
AB: Soils and sediments are complex assemblages of organic and inorganic material which are seldom at sustained equilibrium. Transition from aerobic to anaerobic conditions, for example, has profound impacts on the reactive mineralogical constituents and the operating biochemical pathways. We have recently explored the alteration in reactive phases and contaminant sequestration mechanisms upon reductive dissolution of ferrihydrite by dissimilatory iron reducing bacteria under hydrodynamic conditions. A complex mineral assemblage results that is dominated by the production of goethite and magnetite. The principal bacterial role is in supplying an Fe(II) source, with the resulting ferrous-iron concentration being the dominant factor controlling the biomineralization pathway. On the basis of the resulting particle morphology and lattice fringe images, goethite is produced through a dissolution/reprecipitation process. Accordingly, formation rates of goethite, as elucidated with X-ray absorption spectroscopy, provide conservative estimates for the rate of ferrihydrite dissolution. The rate of Fe(II)-catalyzed ferrihydrite dissolution is surprisingly rapid (7x10-9 mol m-2 s-1 in the presence of 2 mM Fe(II) at pH 7). In fact, these rates are sufficient to support dissimilatory iron reduction even at micromolar levels, challenging the assumption that the low solubility of iron oxide minerals limits ferric-iron accessibility. These observations further suggest that Fe(II)-induced transformations towards more crystalline phases are a major consumption pathway of reactive ferric hydroxides, dramatically altering nutrient and contaminant sequestration. The fate of redox active contaminants such as chromium and uranium will be impacted dramatically as a consequence of reductive biomineralization. Adsorption properties will be modified appreciably with the shift in mineralogy, and the development of reactive ferrous iron bearing phases (solution, surface, and solid) will have important ramifications on reductive stabilization. While the extent of contaminant adsorption on ferric (hydro)oxides will generally decrease upon biomineralization (a consequence of the crystallization and diminished surface area), the potential for reductive stabilization will be enhanced. Considering the phases resulting from our biomineralization experiments, chromate reduction will be controlled nearly equally by green rust, Fe(II) saturated goethite, and aqueous Fe(II). Similarly, the pertechnetate anion will be rapidly reduced by these ferrous bearing phases. Uranyl reduction, on the other hand, remains dominated by biological (enzymatic) pathways, albeit that chemical avenues (principally pathways involving green rust and Fe(II) bearing goethite) are significant.
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0418 Bioremediation
DE: 0419 Biomineralization
SC: Biogeosciences [B]
MN: Fall Meeting 2005