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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 K2S2O8, NaOCl, and H2O2, for dissolving UO2 in alkaline solution have been performed, with H2O2 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 UO2(CO3)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 UO2 0.5 M Na2CO3 was the most propitious choice, exhibiting both a high initial dissolution rate and the highest UO2 dissolution capacity among the systems studied.

The uranyl iodates, UO2(IO3)2 (AU1−8) and UO2(IO3)2(H2O) (AU2−8) have been prepared from the reaction of UO3 with I2O5 under hydrothermal conditions. Equilibrium between AU1−8 and AU2−8 is established after 3 days at 400 °C, resulting in the isolation of an equimolar ratio of these compounds. Lowering of the reaction temperature to 180 °C or shortening of the reaction duration to 1 day allows for the isolation of AU2−8 in pure form. At 327 °C, AU2−8 undergoes dehydration and structural rearrangement to AU1−8, which is a reversible process. Single-crystal X-ray diffraction, bond valence sum calculations, EDX, DSC, TGA, and vibrational and fluorescence spectroscopy have been used to characterize these compounds. Crystallographic data are as follows:  AU1−8, monoclinic, space group P21/n, a = 4.2454(8) Å, b = 16.636(5) Å, c = 5.284(1) Å, β = 107.57(2)°, and Z = 2; AU2−8, orthorhombic, space group Pbcn, a = 8.452(2) Å, b = 7.707(2) Å, c = 12.271(3), and Z = 4. The structure of AU1−8 is pseudo-three-dimensional with one-dimensional uranyl iodate chains forming close I···O interchain contacts. Each one-dimensional chain consists of edge-sharing UO8 hexagonal bipyramids and chelating iodate anions that also bridge the UO2²⁺ units through μ3-O atoms. AU2−8 has a corrugated layered structure containing pentagonal bipyramidal UO7 units that are formed from the binding of four oxygen atoms from four bridging iodates and the coordination of a water molecule to the UO2²⁺ moiety. Both of these compounds fluoresce when irradiated with UV light (365 nm), yielding emission spectra characteristic of the uranyl dication.