Solubility and speciation of (UO2)2SiO4.2H2O in aqueous systems

The solubility-product constant log K0sp([UO2+2]2[SiO2]/[H+]4) of uranyl orthosilicate ([UO2]2SiO4.2H2O) has been determined at pH 3 and 25 C in air and in N2-atmosphere beginning from undersaturation conditions. The value derived under N2-atmosphere was 6.03±0.45 and in air 6.15±0.53 at infinite dilution. The experiments were carried out at 25 C in 0.1 M NaClO4 solution and at a pH ranging from 3 to 9. Most solutions reached steady-state concentrations after about 40 days. X-ray powder diffraction (XRD) measurements of the solid residues at steady-state showed that (UO2)2SiO4.2H2O alone was the solubility-controlling solid. The results indicate that the solubility of uranyl orthosilicate depends on pH. It exhibits a minimum at a pH from 6 to 7. The observed concentration minimum is more pronounced in the air experiments than in the ones under N2-atmosphere. The results allow for a source term evaluation for uranyl orthosilicate in natural systems. (orig.)