Oxidative UO₂ dissolution induced by soluble Mn(III)

The stability of UO₂ is critical to the success of reductive bioremediation of uranium. When reducing conditions are no longer maintained, Mn redox cycling may catalytically mediate the oxidation of UO₂ and remobilization of uranium. Ligand-stabilized soluble Mn(III) was recently recognized as an important redox-active intermediate in Mn biogeochemical cycling. This study evaluated the kinetics of oxidative UO₂ dissolution by soluble Mn(III) stabilized by pyrophosphate (PP) and desferrioxamine B (DFOB). The Mn(III)-PP complex was a potent oxidant that induced rapid UO₂ dissolution at a rate higher than that by a comparable concentration of dissolved O₂. However, the Mn(III)-DFOB complex was not able to induce oxidative dissolution of UO₂. The ability of Mn(III) complexes to oxidize UO₂ was probably determined by whether the coordination of Mn(III) with ligands allowed the attachment of the complexes to the UO₂ surface to facilitate electron transfer. Systematic investigation into the kinetics of UO₂ oxidative dissolution by the Mn(III)-PP complex suggested that Mn(III) could directly oxidize UO₂ without involving particulate Mn species (e.g., MnO ₂). The expected 2:1 reaction stoichiometry between Mn(III) and UO₂ was observed. The reactivity of soluble Mn(III) in oxidizing UO₂ was higher at lower ratios of pyrophosphate to Mn(III) and lower pH, which is probably related to differences in the ligand-to-metal ratio and/or protonation states of the Mn(III)-pyrophosphate complexes. Disproportionation of Mn(III)-PP occurred at pH 9.0, and the oxidation of UO₂ was then driven by both MnO₂ and soluble Mn(III). Kinetic models were derived that provided excellent fits of the experimental results.