Kinetic Study of the Oxidative Dissolution of UO2 in Aqueous Carbonate Media

The oxidative dissolution of uranium(IV) dioxide powder at room temperature in aqueous carbonate media has been investigated. Kinetic studies evaluating the efficacy of various oxidants, including K 2 S 2 O 8, NaOCl, and H 2 O 2, for dissolving UO 2 in alkaline solution have been performed, with H 2 O 2 exhibiting the most rapid initial dissolution at 0.1 M oxidant concentrations. This result is due in part to the ability of peroxide to act as both an oxidant and a ligand under alkaline conditions. A spectrophotometric titration was used to confirm peroxide coordination to the U(VI) metal center. The disappearance of characteristic absorbance maxima associated with UO 2 (CO 3 ) 3 4- (e.g., 448.5 nm) and a subsequent change in solution coloration upon titration with hydrogen peroxide indicated a change in speciation. Optimization of the hydrogen peroxide concentration indicated that the initial rate of uranium oxidation increased with increasing peroxide concentration, with a maximum reaction rate estimated at about 0.9 M peroxide. In addition, the effects of both the carbonate countercation and the carbonate concentration were also studied. It was determined that for 40 mg UO 2 0.5 M Na 2 CO 3 was the most propitious choice, exhibiting both a high initial dissolution rate and the highest UO 2 dissolution capacity among the systems studied.