Abstract:
Theoretical methods are used with the available experimental data to provide estimates of parameters for the revised-HKF equations of state for aqueous uranium species. These parameters are used with standard state thermodynamic data at 25°C and 1 bar to calculate equilibrium constants for redox reactions among the four most common oxidation states of uranium (U(III), U(IV), U(V), and U(VI)), and their hydrolysis reactions at temperatures to 1000°C and pressures to 5 kb. A total of nineteen aqueous uranium species are included. The predicted equilibrium constants are used to construct oxidation potential-pH diagrams at elevated temperatures and pressures and to calculate the solubilities of uraninite as functions of temperature and pH, which are compared to experimental data. Oxidation potential-pH diagrams illustrate the relative stabilities of aqueous uranium species and indicate that U(IV) and U(VI) species predominate in aqueous solution in the U-O-H system. Increasing temperature stabilizes U(VI) and U(III) species relative to U(IV) species, but U(IV) species dominate at oxidation states consistent and mineral-buffer assemblages and near-neutral pH. At low pH, U(VI) is stabilized relative to U(IV) suggesting that uranium transport in hydrothermal systems requires either acidic solutions or potent complexes of U(IV).