The kinetics and mechanism of H₂O₂decomposition at the U₃O₈surface 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 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.