Manganese oxides are widespread in the environment and their surface reactivity has the potential to modify the geochemical behavior of uranium. We have investigated the effect of different concentrations of U and Mn on the coupled biogeochemical oxidation-reduction reactions of U and Mn. Experiments conducted in the presence of Mn(II)-oxidizing spores from Bacillus sp. strain SG-1 and 5% headspace oxygen show that the Mn oxides produced by these spores can rapidly oxidize UO2. Thirty to fifty times more UO2 is oxidized in the presence of Mn oxides compared to Mn oxide free controls. As a consequence of this UO2 oxidation, Mn oxides are reduced to soluble Mn(II) that can be reoxidized by SG-1 spores. SG-1 spores cannot directly oxidize UO2, but UO2 oxidation proceeds rapidly with Mn(II) concentrations of <5 μM. The rate of UO2 oxidation is equal to the rate of MnO2 reduction with UO2 oxidation controlled by the initial concentrations of UO2, dissolved Mn(II) (in systems with spores), or Mn(IV) oxides (in systems containing preformed MnO2). U(VI) and UO2 decrease the Mn(II) oxidation rate in different ways by inhibiting the Mn(II)-oxidizing enzyme or decreasing the available Mn(II). These results emphasize the need to consider the impact of Mn(II)-oxidizing bacteria when predicting the potential for UO2 oxidation in the subsurface.
ASJC Scopus subject areas
- Environmental Chemistry