SOLUBILITY AND SPECIATION OF (UO2)(2)SIO4-CENTER-DOT-2H(2)O IN AQUEOUS SYSTEMS

The solubility-product constant log K-sp(o)([UO22+](2)[SiO2]/[H+](4)) of uranyl orthosilicate ([UO2]2SiO4 . 2H(2)O) has been determined at p H 3 and 25 degrees C in air and in N-2-atmosphere beginning from under saturation conditions. The value derived under N-2-atmosphere was 6.03 +/-0.45 and in air 6.15+/-0.53 at infinite dilution. The experiments w ere carried out at 25 degrees C in 0.1 M NaClO4 solution and at a pH r anging from 3 to 9. Most solutions reached steady-state concentrations after about 40 days. X-ray powder diffraction (XRD) measurements of t he solid residues at steady-state showed that (UO2)(2)SiO4 . 2H(2)O al one was the solubility-controlling solid. The results indicate that th e solubility of uranyl orthosilicate depends on pH. It exhibits a mini mum at a pH from 6 to 7. The observed concentration minimum is more pr onounced in the air experiments than in the ones under N-2-atmosphere. The results allow for a source term evaluation for uranyl orthosilica te in natural systems.