Hydrothermal solubility of Dy hydroxide as a function of pH and stability of Dy hydroxyl aqueous complexes from 25 to 250 °C

The rare earth elements (REE) have important applications in green energy technologies. The formation of mineral deposits in geologic systems commonly involves hydrothermal fluids which can mobilize the REE. However, the REE speciation is not well known as a function of pH. The thermodynamic properties of REE hydroxyl complexes used in geochemical models are based on the Helgeson-Kirkham-Flowers (HKF) equation of state parameters which were derived by extrapolation of low temperature experimental and estimated data. In this study, Dy hydroxide solubility experiments are combined with available literature data to improve these models from 25 to 250 °C and optimize the thermodynamic properties of Dy3+ and Dy hydroxyl complexes using GEMSFITS. Batch-type solubility experiments were conducted from 150 to 250 °C and at saturated water vapor pressure in perchloric acid solutions with initial pH values of 2 to 5 in 0.5 pH unit increments. The measured solubility of Dy hydroxide is retrograde with temperature and decreases with pH. The logarithm of total dissolved Dy molality ranges from −2.3 to −5.3 at 150 °C (pH 4.7–5.5), from −2.4 to −5.6 at 200 °C (pH 3.9–5.1), and from −3.7 to −6.9 at 250 °C (pH of 3.4 and 5.0). The optimized standard partial molal Gibbs energies of formation (∆fG°T) derived for Dy3+ and DyOH2+ display a close to linear relationship with temperature, fitting with previous optimizations based on DyPO4 solubility data in the literature. A comparison of the optimized ∆fG°T values for aqueous Dy species with predictions from available HKF parameters indicates significant differences ranging from +11 to −26 kJ/mol between 25 and 250 °C. The experimental fits are used to derive the Dy hydroxide solubility products (Ks0) and formation constants for the hydrolysis of Dy (βn with n = 1 to 3; Dy3+ + nOH− = DyOHn3-n) as a function of temperature. The optimization method presented yields accurate thermodynamic properties for the Dy3+ aqua ions and the DyOH2+ species at the acidic to mildly acidic pH studied whereas more experimental work is needed at near-neutral and alkaline conditions to better constrain the other hydroxyl complexes. The optimized thermodynamic data have a significant impact on geochemical modeling of the mobility and solubility of REE minerals in acidic hydrothermal fluids.