UC Merced

This study investigates possible geochemical interactions between microbial reduced U(IV) species (Bio-U(IV)O2(s)) and abiotic and biotic factors (Chemolithoautotrophic bacteria) affecting the stability of U(IV)-oxide minerals in anaerobic aquifers. Microbially mediated precipitation of Bio-U(IV)O2(s) produced a mixture of a U(IV) solid, mineral uraninite (UO2(s)) and trace amounts of U(VI). Flow-through column experiments were performed to quantify biotic or abiotic oxidation of Bio-U(IV)O2(s), along with spectroscopic and microscopic techniques to characterize reactants and products. Results from flow through column experiments showed that release of U under anaerobic conditions is higher in presence of oxidants (NO3-, NO2-, Fe(III)) than with no oxidants. The presence of Thiobacillus denitrificans enhanced U release in the presence of 1 mM NO3-. However, in the system in which 5 mM soluble Fe2+ and 5 mM NO3- were present the release was significantly suppressed. One possible explanation is that Thiobacillus denitrificans favors oxidation of soluble Fe2+ over Bio-U(IV)O2(s) . Soluble Fe2+ might form insoluble ferric oxide solids that sorb oxidized products of Bio-U(IV)O2(s). Spectroscopic data provided evidence of U(VI) in reacted columns in which Fe(II) was passed. These findings suggest that the presence or addition of another electron donor such as soluble Fe2+ in the field might suppress re-oxidation of uranium into soluble products that are released in aquifers.