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Solubility and Speciation of (UO2)2SiO4 · 2H2O in Aqueous Systems

The solubility-product constant log O [ U 0 r ] 2 [ S i 0 2 ] / [ H + ] 4 ) of uranyl orthosilicate ([U02]2Si04 · 2H20) has been determined at pH 3 and 25 °C in air and in N2-atmosphere beginning from undersaturation conditions. The value derived under ^-a tmosphere 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 NaC104 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 steadystate showed that (U02)2Si04 · 2H 2 0 alone was the solubilitycontrolling 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.