Standard Gibbs free energies of formation at the temperature 303.15 K of four uranyl silicates: soddyite, uranophane, sodium boltwoodite, and sodium weeksite

Abstract

The solubility products and standard molar Gibbs free energies of formation were obtained from solubility measurements at 303.15 K for the following synthetic uranium-silicate phases: soddyite, (UO2)2SiO4·2H2O; uranophane, Ca(H3O)2(UO2)2(SiO4)2·3H2O; sodium boltwoodite, Na(H3O)(UO2)SiO4·H2O; and sodium weeksite, Na2(UO2)2(Si2O5)3·7H2O. These phases were identified as uranium precipitates likely to form near a high-level nuclear waste repository. The experimental standard molar Gibbs free energies of formation for soddyite, uranophane, sodium boltwoodite, and sodium weeksite are: −(3658.0±4.8) kJ·mol−1, −(6210.6±7.6)kJ·mol−1, −(2966.0±3.6)kJ·mol−1, and −(9088.5±18.4) kJ·mol−1, respectively. The value for sodium boltwoodite is a lower bound and therefore should be used with caution.