Oxidative dissolution of uraninite nanoparticles in the presence of manganite

Abstract

One of the possible uranium pollution remediation methods is the immobilization of uranium by reduction, transforming from soluble hexavalent uranium to barely soluble uraninite (UO2). However, the uraninite is susceptible to be re-oxidized and releases the uranium into the subsurface environments. Understanding the process of uraninite oxidative dissolution is necessary for evaluating uranium migration behavior and improvements of the uranium remediation strategies. Manganese (hydr)oxides are among the strong inorganic oxidants in natural environments. In this work, we investigated the interaction between manganite, a stable Mn(III) oxyhydroxide, and uraninite nanoparticles in subalkaline solutions under both oxic and anoxic conditions by batch experiments. Under oxic conditions, a small amount of manganite effectively catalyzed the oxidative dissolution of uraninite by oxygen, and the catalysis was suppressed with the increase of the manganite concentration or the decrease of the bicarbonate concentration. Under anoxic conditions, the oxidative dissolution of uraninite happened at a much slower rate with manganite acting as the oxidant. The dissolution process involved two stages of reaction as illustrated by the correlation of uranium concentration and time. Our results suggest under both oxic and anoxic conditions, the oxidative dissolution of uraninite could be strongly affected by the presence of Mn(III) oxyhydroxide minerals.