Dissolution behavior of calcium uranate under oxidizing and reducing conditions

Abstract

Calcium uranate solid phase is a secondary mineral found in geological environments. It may form in the residues of high-level radioactive liquid waste and in the fuel debris of the TEPCO's Fukushima Daiichi Nuclear Power Plant under high-temperature conditions. To assess the chemical stability of CaUO₄ in different aqueous environments, static immersion tests were performed under various redox conditions and carbonate ion concentrations. pH and Eh values, as well as concentrations of uranium, calcium, and total carbonate in the solutions, were measured after the supernatants were filtered through a membrane with a 10 kDa molecular weight cutoff. The components of the solid phase were also evaluated using X-ray diffraction and X-ray absorption fine structure analyses. The dissolution mechanism of CaUO₄ was examined using data from solid and liquid analyses, along with chemical thermodynamic calculations. Under reducing conditions and without carbonate, U(VI) in CaUO₄ was reduced to U(V) and the mineral was converted into non-stoichiometric CaUO_4−x. The dissolved uranium was then further reduced to U(IV) in the aqueous media, forming UO₂(am), which controlled the U solubility. Under oxidizing conditions and in the absence of carbonate, dissolved uranium formed metaschoepite ((UO₃)·2H₂O(cr)) at pH ≤ 7 and sodium diuranate (Na₂U₂O₇·H₂O(cr)) at pH > 7, which controlled uranium solubility. In oxidizing conditions with carbonate present, the apparent solubility of U was lower than predicted by the solid-phase solubility calculations. The concentration of U was constrained to levels similar to that of calcium when the calcium concentration reached saturation with CaCO₃. Additionally, the dissolution of calcium from CaUO₄ influenced uranium dissolution.