HR: 16:45h
AN: NS44A-06 INVITED     [Abstracts]
TI: Bacterial-Induced Minerlization of Fe (Hydr)oxides and Subsequent Modification of Surface Reactivity
AU: * Hansel, C M
EM: hansel@stanford.edu
AF: Stanford University, Geological and Environ. Sci. Braun Hall, Bld. 320, Room 1118, Stanford, CA 94305 United States
AU: Benner, S G
EM: sbenner@boisestate.edu
AF: Boise State University, Department of Geosciences Math/Geosciences Bldg, Rm 217, Boise, ID 83725 United States
AU: Fendorf, S
EM: fendorf@stanford.edu
AF: Stanford University, Geological and Environ. Sci. Braun Hall, Bld. 320, Room 1118, Stanford, CA 94305 United States
AB: Due to their ubiquity and intrinsic reactivity, Fe(III) (hydr)oxides serve as an important sink for various metals and nutrients. Upon (a)biotic reductive dissolution, Fe(II) may repartition in the solid-phase and/or serve as a catalyst for Ostwald ripening of the (hydr)oxide substrate. Considering the variable reactivity of Fe(II, III, and II-III) phases coupled with the reducing capacity of Fe(II) species, secondary mineralization of Fe(III) (hydr)oxides will have a direct bearing on the fate and transport of numerous (in)organic constituents within the environment. Here we compare the operating secondary mineralization pathways of various Fe(III) (hydr)oxides (2-line ferrihydrite, goethite, hematite) following dissimilatory iron reduction within a minimal groundwater medium under advective flow. The conversion of Fe(III) (hydr)oxides occurs via a coupled biotic-abiotic reaction pathway such that bacterial-generated Fe(II) reacts abiotically with the residual (hydr)oxide surface. Upon Fe(II) reaction, the ferrihydrite surface is converted to goethite and/or magnetite being a function, for the most part, of bacterial- and flow-regulated Fe(II) concentrations. The operating mineralization pathway is a function of competitive reactions of Fe(II) with the ferrihydrite surface and stability of a lepidocrocite precursor. Conversely, the surfaces of goethite and hematite undergo atomic relaxation followed by electron delocalization within the bulk solid and/or minor organization to spinel-like domains. Surprisingly, while the surfaces of goethite and hematite become more disordered upon reductive dissolution and Fe(II) reaction, the reducing capacity and bioavailability decreases. Similarly, the extensive mineralization of ferrihydrite occludes the residual ferrihydrite leading to slower sustained bacterial reduction rates consistent with those of the more crystalline Fe(III) (hydr)oxides. Dissimilatory reduction of natural Fe(III) (hydr)oxides, however, results in the initial, preferential consumption of more recalcitrant Fe(III) phases (e.g. goethite) relative to ferrihydrite. Iron(II) reaction with natural Fe (hydr)oxide surfaces results solely in the internal ordering of ferrihydrite to hematite, most likely, a consequence of compromised surface reactivity by sorbed and/or coprecipitated ions. Thus, the operating mineralization pathways within soils and sediments undergoing Fe reduction will be a function of Fe(III) (hydr)oxide structural order and surface reactivity, which ultimately controls Fe(II) generation and ensuing surface conversion.
DE: 0400 Biogeosciences
DE: 1030 Geochemical cycles (0330)
DE: 1045 Low-temperature geochemistry
SC: Near-Surface Geophysics [NS]
MN: 2005 Joint Assembly