The kinetics and mechanism of H2O2 decomposition at the U3O8 surface in bicarbonate solution.

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 H2O2 produced by water radiolysis. To understand the dissolution of U3O8 by H2O2 in bicarbonate solution (0.1-50 mM), dissolved U concentrations were measured upon H2O2 addition (300 µM) to U3O8/bicarbonate mixtures. As the H2O2 decomposition mechanism is integral to the dissolution of U3O8, the kinetics and mechanism of H2O2 decomposition at the U3O8 surface was investigated. The dissolution of U3O8 increased with bicarbonate concentration which was attributed to a change in the H2O2 decomposition mechanism from catalytic at low bicarbonate (≤5 mM HCO3 -) to oxidative at high bicarbonate (≥10 mM HCO3 -). Catalytic decomposition of H2O2 at low bicarbonate was attributed to the formation of an oxidised surface layer. Second-order rate constants for the catalytic and oxidative decomposition of H2O2 at the U3O8 surface were 4.24 × 10-8 m s-1 and 7.66 × 10-9 m s-1 respectively. A pathway to explain both the observed U3O8 dissolution behaviour and H2O2 decomposition as a function of bicarbonate concentration was proposed.