Indirect UO₂ oxidation by Mn(II)-oxidizing spores of Bacillus sp. strain SG-1 and the effect of U and Mn concentrations

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.