Bacterial reduction of crystalline Fe3+ oxides in single phase suspensionsand subsurface materials

Microbiologic reduction of synthetic and geologic Fe3+ oxides associated wi th four Pleistocene-age, Atlantic coastal plain sediments was investigated using a dissimilatory Fe reducing bacterium (Shewanella purrefaciens, strai n CN32) in bicarbonate buffer. Experiments investigated whether phosphate a nd anthraquinone-2, 6-disulfonate, (AQDS, a humic acid analogue) influenced the extent of crystalline Fe3+ oxide bioreduction and whether crystalline Fe3+ oxides in geologic materials are more or less reducible than comparabl e synthetic phases. Anaerobic incubations (10(8) organisms/mL) were perform ed both with and without PO, and AQDS that functions as an electron reposit ory and shuttle. The production of Fe2+ (solid and aqueous) was followed wi th time, as was mineralogy by Xray diffraction. The synthetic oxides were r educed in a qualitative trend consistent with their surface area and free e nergy: hydrous ferric oxide (HFO)>goethite>hematite. Bacterial reduction of the crystalline oxides was incomplete in spite of excess electron donor. B iogenic formation of vivianite [Fe-3(PO4)(2). 8H(2)O] and siderite (FeCO3) was observed; the conditions of their formation was consistent with their s olubility. The geologic Fe3+ oxides showed a large range in reducibility, a pproaching 100% in some materials. The natural oxides were equally or more reducible than their synthetic counterparts, in spite of association with n on-reducible mineral phases (e.g., kaolinite). The reducibility of the synt hetic and geologic oxides was weakly effected by PO4, but was accelerated b y AQDS. CN32 produced the hydroquinone form of AQDS (AHDS), that, in turn, had thermodynamic power to reduce the Fe3+ oxides. As a chemical reductant, it could reach physical regions of the oxide not accessible by the organis m. Electron microscopy showed that crystallite size was not the primary fac tor that caused differences in reducibility between natural and synthetic c rystalline Fe3+ oxide phases. Crystalline disorder and microheterogeneities may be more important.