Saturday, 15 July 2006

Impact of Phosphate on Iron Oxide Bioreducibility and Mineralization.

Thomas Borch1, Yoko Masue2, and Scott Fendorf2. (1) Colorado State Univ, Dept of Soil and Crop Sciences, Fort Collins, CO 80523, (2) Stanford Univ, Geological and Environmental Sciences, Braun Hall, Building 320, Room 118, Stanford, CA 94305

Iron oxides account for the majority of metallic oxides in soils and are found in most climate zones. These oxides are particularly important because they are often contributing, if not controlling, factors in the fate and transport of nutrients and contaminants. Numerous microorganisms have been identified to catalyze Fe(III) reduction although Dissimilatory Iron-Reducing Bacteria (DIRB) such as Shewanella and Geobacter species have been found to be responsible for most of the Fe(III) reduction occurring under non-sulfidogenic anoxic conditions. Dissimilatory iron reduction has been extensively studied over the past two decades with pristine synthetic iron-(hydr)oxides serving as the electron acceptor. However, natural environments often include a complex interaction of numerous organic and inorganic constituents that may control the bioreducibility of iron-(hydr)oxides and the mineralization products formed. For instance, oxyanions such as phosphate and arsenate are know to interact strongly with iron-(hydr)oxides via surface complexation and may therefore effect their reducibility and mineralization pathway. Accordingly, the objectives of this study are to determine the impact of surface-associated oxyanions, using phosphate as a model compound, on the extent of iron-(hydr)oxide bioreduction and nature of the resulting mineralization products. Batch and column studies show that alterations in surface composition have a profound impact on the bioreducibility and mineralization pathway of ferrihydrite. Phosphate induced a linear inhibition on the extent of biomineralization and resulted in a fivefold decrease in iron reduction at high surface-coverage. Magnetite was the most significant mineralization product formed while lepidocrocite and especially goethite formation was inhibited in the presence of phosphate. Minor amounts of vivianite and green rust like phases were formed in systems favoring high concentrations of phosphate, ferrous iron and microbial metabolites (i.e. bicarbonate). Advective flow conditions also influenced the biomineralization pathway of ferrihydrite and the microbial respiration. Furthermore, decreased mobility of phosphate was observed under iron-reducing conditions when compared to column systems in the absence of the iron reducing bacterium Shewanella putrefaciens. Our results reveal the importance of considering hydrodynamics and the molecular-scale heterogeneity of iron (hydr)oxides, inclusive of adsorbates, when evaluating dissimilatory iron reduction and biomineralization within soils and sediments.

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