The solubility of uranium, a radionuclide contaminant at many sites, is highly dependent on its valence state and speciation. Transformation of U(VI) to U(IV) under reducing conditions can decrease U solubility due to nanoparticulate uraninite precipitation (UO2); however, recent evidence suggests that U(IV) often occurs in environmental solids as complexed species, e.g. labile precipitates or surface-adsorbed U(IV) atoms. We will present examples of U(VI) reduction by bacteria (Geobacter, Anaeromyxobacter), by minerals (magnetite, green rust), and by natural sediments where we identified complexed U(IV) species using synchrotron x-ray spectroscopy (XANES, EXAFS). Detailed studies of U(VI) adsorption to carboxyl-functionalized colloids (surrogates for organic surfaces) and subsequent reduction by Fe(II) oligomers or AH2DS indicate that the presence of trace phosphate or titanium prevent uraninite formation by inner-sphere complexation to U(IV). We will also show how observed U(IV) speciation suggests  differences between gram-positive and gram-negative bacteria in their cell-wall electron transfer mechanisms to U(VI).

Reduction of U(VI) at biological and mineral surfaces: Mechanisms and factors controlling the speciation of U(IV)

245th National Meeting (2013)

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