Thermodynamic Model for ThO2(am) Solubility in Alkaline Silica Solutions

摘要

No thermodynamic data for Th complexes with aqueous Si are available. To obtain such data, extensive studies on ThO2(am) solubility were carried out as functions of: ( 1) a wide range of aqueous silica concentrations ( 0.0004 to 0.14 mol.L-1) at fixed pH values of about 10, 11, 12, and 13; and ( 2) and variable pH ( ranging from 10 to 13.3) at fixed aqueous Si concentrations of about 0.006 mol.L-1 or 0.018 mol.L-1. The samples were equilibrated over long periods ( ranging up to 487 days), and the data showed that steady-state concentrations were reached in < 29 days. X-ray diffraction, FTIR, and Raman analyses of the equilibrated solid phases showed that the Th solids were amorphous ThO2(am) containing some adsorbed Si. The solubility of ThO2(am) at pH values ranging from 10 to 13.3 at fixed 0.018 mol.L-1 aqueous Si concentrations decreases rapidly with an increase in pH, and increases dramatically with an increase in Si concentrations beyond about 0.003 mol.L-1 at fixed pH values > 10. The data were interpreted using both the Pitzer and SIT models, and required only the inclusion of one mixed-hydroxy-silica complex of Th [Th( OH)(3)(H3SiO4)(3)(2-)]. Both models provided similar complexation constant values for the formation of this species. Density functional theory calculations predict complexes of this stoichiometry, having six-fold coordination of the Th cation, to be structurally stable. Predictions based on the fitted value of log(10)K(0) = -18.5 +/- 0.7 for the ThO2(am) solubility reaction involving Th(OH)(3)(H3SiO4)(3)(2-)[ThO2(am) + 3H(4)SiO(4) + H2O reversible arrow Th(OH)(3)(H3SiO4)(3)(2-) + 2H(+)], along with the thermodynamic data for aqueous Si species reported in the literature, agreed closely with the extensive experimental data and showed that under alkaline conditions aqueous Si makes very strong complexes with Th.