Effects of HCO3- on the kinetics of UO2 oxidation by H2O2

The effect of HCO3- on the kinetics of UO2 oxidation by H2O2 in aqueous solution has been studied using powder suspensions where the concentration of H2O2 was monitored as a function of time. By varying the UO2 surface to solution volume ratio second order rate constants were obtained for HCO3- concentrations ranging from 0 to 100 mM. The second order rate constant increases linearly with HCO3- concentration from 0 to approximately 1 mM. Above 1 mM HCO3- the rate constant is 4.4 × 10−6 m min−1 independent of [HCO3-]. This indicates that the kinetics of the reaction depends on both oxidation and dissolution below 1 mM HCO3- while at higher concentrations it is solely governed by oxidation. Hence, the rate constant obtained at HCO3- concentrations above 1 mM is the true rate constant for oxidation of UO2 by H2O2. The results also imply that the reaction between HCO3- and oxidized UO2 on the UO2 surface (i.e. HCO3- facilitated dissolution) is limited by diffusion (ca 10−3 m min−1 in the present system). Furthermore, the experimental results were used to estimate the oxidation site density of the powder used (126 sites nm−1) and the rate constant for dissolution of UO22+ from the UO2 surface (7 × 10−8 mol m−2  s−1).