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.