The formation of uranyl-peroxide complexes was studied at alkaline media by using UV-Visible spectrophotometry and the STAR code. Two different complexes were found at a H 2 O 2 /U(VI) ratio lower than 2. A graphical method was used in order to obtain the formation constants of such complexes and the STAR program was used to refine the formation constants values because of its capacity to treat multiwavelength absorbance data and refining equilibrium constants. The values obtained for the two complexes identified were:At hydrogen peroxide concentrations higher than 10 -5 mol dm -3 , and in the absence of carbonate, the UO 2 (O 2 ) 2 (OH) 2 4-complex is predominant in solution, indicating the significant peroxide affinity of peroxide ions for uranium and the strong complexes of uranium(VI) with peroxide.
UO 2 / Hydrogen peroxide / Alkaline pH / X-ray photoelectron spectroscopy Summary. The dissolution of non irradiated UO 2 was studied in the presence of hydrogen peroxide (10 −4 mol dm −3 ) at alkaline pH (11, 11.5, 12, and 13). Both hydrogen peroxide and uranium concentration in solution were determined as a function of time. The H 2 O 2 consumption was modelled considering a pseudo first order reaction, the rate constants obtained were 0.126 ± 0.004 h −1 , 0.126 ± 0.003 h −1 , 0.078 ± 0.002 h −1 , and 0.056 ± 0.002 h −1 at pH 11, 11.5, 12, and 13, respectively. The uranium concentrations measured at the end of the experiments were close to the solubility of sodium uranate (Na 2 U 2 O 7 ). X-ray photoelectron spectroscopy showed that the surface of the solid was more oxidized at lower pH, indicating that at this pH the limiting step of the oxidative dissolution process is the dissolution of the U(VI) formed on the surface. At more alkaline pH values, the rate limiting step would be the oxidation of the UO 2 surface.
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