Fe(III) Oxide Reactivity Toward Biological versus Chemical Reduction

Initial rates of biological (Shewanella putrefaciens strain CN32, pH 6.8) and chemical (ascorbate, pH 3.0) reduction of synthetic Fe(III) oxides with a broad range of crystallinity and specific surface area were examined to assess how variations in these properties are likely to influence the kinetics of bacterial Fe(III) oxide reduction in heterogeneous natural Fe(III) oxide assemblages. The results indicate that bacterial Fe(III) oxide reduction does not respond strongly to oxide crystal thermodynamic properties (∆Gf) which exert a significant impact on the kinetics of abiotic reductive dissolution. These findings suggest that oxide mineral heterogeneity in natural soils and sediments is likely to affect initial rates of bacterial reduction (e.g. during the early stages of anaerobic metabolism following the onset of anoxic conditions) mainly via an influence on reactive surface site density and that inferences regarding the competitiveness of bacterial Fe(III) oxide reduction as a pathway for organic matter oxidation in anoxic environments cannot be based on assumed thermodynamic properties of the dominant oxide phase(s) in the soil or sediment.