The kinetics and mechanism of H 2 O 2 decomposition at the U 3 O 8 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 (HCO 3 - ) concentrations in the groundwater, as well as H 2 O 2 produced by water radiolysis. To understand the dissolution of U 3 O 8 by H 2 O 2 in bicarbonate solution (0.1-50 mM), dissolved U concentrations were measured upon H 2 O 2 addition (300 μM) to U 3 O 8 /bicarbonate mixtures. As the H 2 O 2 decomposition mechanism is integral to the dissolution of U 3 O 8 , the kinetics and mechanism of H 2 O 2 decomposition at the U 3 O 8 surface was investigated. The dissolution of U 3 O 8 increased with bicarbonate concentration which was attributed to a change in the H 2 O 2 decomposition mechanism from catalytic at low bicarbonate (≤5 mM HCO 3 - ) to oxidative at high bicarbonate (≥10 mM HCO 3 - ). Catalytic decomposition of H 2 O 2 at low bicarbonate was attributed to the formation of an oxidised surface layer. Second-order rate constants for the catalytic and oxidative decomposition of H 2 O 2 at the U 3 O 8 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 U 3 O 8 dissolution behaviour and H 2 O 2 decomposition as a function of bicarbonate concentration was proposed.