Equilibrium and kinetic aspects of soddyite dissolution and secondary phase precipitation in aqueous suspension

The dissolution and transformation of soddyite ([UO2]2SiO4·2H2O) have been examined in aqueous suspension at pH 6 and 0.01 M NaNO3. Soddyite is an important component of the paragenetic sequence of secondary minerals that arises from the weathering of uraninite ore deposits and corrosion of spent nuclear fuel. A soddyite of high purity and crystallinity was synthesized in the laboratory for use in dissolution experiments. In batch experiments, rapid dissolution occurred over an initial period of several hours followed by continuing steady-state dissolution for up to 700 h. Up to 200 h. U and Si were released into solution at their stoichiometric 2:1 ratio in soddyite. A decrease in the dissolved U concentration was observed at longer times, indicating the precipitation of a new phase. Even after precipitation of the secondary phase, the continuing dissolution of soddyite could be inferred from increasing dissolved Si concentrations. Through the use of X-ray diffraction, Raman spectroscopy, and scanning electron microscopy, the precipitated phase was identified as a clarkeite-like sodium uranyl oxide hydrate. The sodium uranyl oxide hydrate was ultimately the solubility-controlling solid, despite being only a minor component. Soddyite dissolution rates were quantified in flow-through experiments, in which reaction products were flushed from the reactors, thereby avoiding reprecipitation of U. The measured dissolution rate at pH 6 was 0.71 μmol U m-2 h-1. A slower dissolution rate of 0.44 μmol U m-2 h-1 was observed when 100 μM dissolved Si was added to the reactor influent. Copyright © 2002 Elsevier Science Ltd.