previously he was at Sandia National Laboratories. His research concerns the energetics and kinetics of the reactions between minerals and aqueous solutions, and in recent work, he has determined rate coefficients for dissociation of Al−O bonds in various dissolved aluminum complexes.

Dissimilatory iron-reducing bacteria (DIRB) couple the oxidation of organic matter or H 2 to the reduction of iron oxides. The factors controlling the rate and extent of these reduction reactions and the resulting solid phases are complex and poorly understood. Batch experiments were conducted with amorphous hydrous ferric oxide (HFO) and the DIRB Shewanella putrefaciens, strain CN32, in well-defined aqueous solutions to investigate the reduction of HFO and formation of biogenic Fe(II) minerals. Lactate-HFO solutions buffered with either bicarbonate or 1,4-piperazinediethanesulfonic acid (PIPES) containing various combinations of phosphate and anthraquinone-2,6-disulfonate (AQDS), were inoculated with S. putrefaciens CN32. AQDS, a humic acid analog that can be reduced to dihydroanthraquinone by CN32, was included because of its ability to function as an electron shuttle during microbial iron reduction and as an indicator of pe. Iron reduction was measured with time, and the resulting solids were analyzed by X-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) with energy-dispersive X-ray spectroscopy (EDS) and selected area electron diffraction (SAED). In HCO 3 Ϫ buffered medium with AQDS, HFO was rapidly and extensively reduced, and the resulting solids were dominated by ferrous carbonate (siderite). Ferrous phosphate (vivianite) was also present in HCO 3 Ϫ medium containing P, and fine-grained magnetite was present as a minor phase in HCO 3 Ϫ medium with or without P. In the PIPESbuffered medium, the rate and extent of reduction was strongly influenced by AQDS and P. With AQDS, HFO was rapidly converted to highly crystalline magnetite whereas in its absence, magnetite mineralization was slower and the final material less crystalline. In PIPES with both P and AQDS, a green rust type compound [Fe (6-x) II Fe x III (OH) 12 ] xϩ [(A 2Ϫ) x/2 ⅐ yH 2 O] xϪ was the dominant solid phase formed; in the absence of AQDS a poorly crystalline product was observed. The measured pe and nature of the solids identified were consistent with thermodynamic considerations. The composition of aqueous media in which microbial iron reduction occurred strongly impacted the rate and extent of iron reduction and the nature of the reduced solids. This, in turn, can provide a feedback control mechanism on microbial metabolism. Hence, in sediments where geochemical conditions promote magnetite formation, two-thirds of the Fe(III) will be sequestered in a form that may not be available for anaerobic bacterial respiration.

Quantitative aspects of microbial crystalline iron(III) oxide reduction were examined using a dissimilatory iron(III) oxide-reducing bacterium (Shewanella alga strain BrY). The initial rate and long-term extent of reduction of a range of synthetic iron(III) oxides were linearly correlated with oxide surface area. Oxide reduction rates reached an asymptote at cell concentrations in excess of ≈1 × 109/m2 of oxide surface. Experiments with microbially reduced goethite that had been washed with pH 5 sodium acetate to remove adsorbed Fe(II) suggested that formation of a Fe(II) surface phase (adsorbed or precipitated) limited the extent of iron(III) oxide reduction. These results demonstrated explicitly that the rate and extent of microbial iron(III) oxide reduction is controlled by the surface area and site concentration of the solid phase. Strain BrY grew in media with synthetic goethite as the sole electron acceptor. The quantity of cells produced per micromole of goethite reduced (2.5 × 106) was comparable to that determined previously for growth of BrY and other dissimilatory Fe(III)-reducing bacteria coupled to amorphous iron(III) oxide reduction. BrY reduced a substantial fraction (8−18%) of the crystalline iron(III) oxide content of a variety of soil and subsurface materials, and several cultures containing these materials were transferred repeatedly with continued active Fe(III) reduction. These findings indicate that Fe(III)-reducing bacteria may be able to survive and produce significant quantities of Fe(II) in anaerobic soil and subsurface environments where crystalline iron(III) oxides (e.g., goethite) are the dominant forms of Fe(III) available for microbial reduction. Results suggest that the potential for cell growth and Fe(II) generation will be determined by the iron(III) oxide surface site concentration in the soil or sediment matrix.