In the event of nuclear waste canister failure in a deep geological repository, groundwater interaction with spent fuel will lead to dissolution of uranium (U) into the environment. The rate of U dissolution is affected by bicarbonate (HCO3−) concentrations in the groundwater, as well as H2O2produced by water radiolysis. To understand the dissolution of U3O8by H2O2in bicarbonate solution (0.1-50 mM), dissolved U concentrations were measured upon H2O2addition (300 μM) to U3O8/bicarbonate mixtures. As the H2O2decomposition mechanism is integral to the dissolution of U3O8, the kinetics and mechanism of H2O2decomposition at the U3O8surface was investigated. The dissolution of U3O8increased with bicarbonate concentration which was attributed to a change in the H2O2decomposition mechanism from catalytic at low bicarbonate (≤5 mM HCO3−) to oxidative at high bicarbonate (≥10 mM HCO3−). Catalytic decomposition of H2O2at low bicarbonate was attributed to the formation of an oxidised surface layer. Second-order rate constants for the catalytic and oxidative decomposition of H2O2at the U3O8surface were 4.24 × 10−8m s−1and 7.66 × 10−9m s−1respectively. A pathway to explain both the observed U3O8dissolution behaviour and H2O2decomposition as a function of bicarbonate concentration was proposed.
ASJC Scopus subject areas
- Chemical Engineering(all)