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 UO(2). Thirty to fifty times more UO(2) is oxidized in the presence of Mn oxides compared to Mn oxide free controls. As a consequence of this UO(2) oxidation, Mn oxides are reduced to soluble MOO that can be reoxidized by SG-1 spores. SG-1 spores cannot directly oxidize UO(2), but UO(2) oxidation proceeds rapidly with Mn(II) concentrations of <5 mu M. The rate Of UO(2) oxidation is equal to the rate of MnO(2) reduction with UO(2) oxidation controlled by the initial concentrations Of UO(2), dissolved Mn(II) (in systems with spores), or Mn(IV) oxides (in systems containing preformed MnO(2)). U(VI) and UO(2) 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 UO(2) oxidation in the subsurface.