Oxidative UO2 dissolution induced by soluble Mn(III).

Area of Science:

• Environmental Geochemistry
• Biogeochemistry
• Nuclear Waste Management

Background:

• Uranium dioxide (UO2) stability is crucial for uranium bioremediation.
• Manganese (Mn) redox cycling can remobilize uranium by oxidizing UO2.
• Ligand-stabilized soluble Mn(III) is a key intermediate in Mn biogeochemical cycling.

Purpose of the Study:

• To evaluate the kinetics of UO2 oxidative dissolution by soluble Mn(III) stabilized by pyrophosphate (PP) and desferrioxamine B (DFOB).
• To understand the role of ligand coordination in Mn(III)-mediated UO2 oxidation.
• To investigate the direct oxidation of UO2 by Mn(III) without particulate Mn species.

Main Methods:

• Kinetic experiments measuring UO2 dissolution rates in the presence of Mn(III)-PP and Mn(III)-DFOB complexes.
• Varying ligand-to-metal ratios, pH, and concentrations to study reaction kinetics.
• Development of kinetic models to describe UO2 oxidative dissolution.

Main Results:

• Mn(III)-PP complex efficiently oxidized and dissolved UO2 at rates exceeding dissolved O2.
• Mn(III)-DFOB complex did not induce significant UO2 oxidative dissolution.
• Reactivity was higher at lower PP:Mn(III) ratios and lower pH, suggesting ligand and protonation state influence.
• Observed 2:1 reaction stoichiometry between Mn(III) and UO2.
• At pH 9.0, Mn(III)-PP disproportionated, with both MnO2 and soluble Mn(III) driving UO2 oxidation.

Conclusions:

• Soluble Mn(III), particularly when stabilized by pyrophosphate, is a potent oxidant of UO2.
• Ligand coordination significantly influences the ability of Mn(III) to oxidize UO2, likely by affecting surface attachment.
• Mn(III) can directly oxidize UO2, a critical consideration for uranium bioremediation and contaminant transport.