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.