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

Description

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 of 4.7 to 5.5), from -2.4 to -5.6 at 200 °C (pH of 3.9 and 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 DyPO4 solubility data in the literature. A comparison of the optimized ∆fG°T values 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.

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