The solubility of ErPO4 and Er speciation in hydrothermal fluids at varying pH and salinity between 350 and 450 °C

Here, we measured the solubility of endmember ErPO4 in NaCl-HCl and NaCl-NaOH bearing aqueous solutions at temperatures from 350 to 450 °C and water vapor saturation pressure to 500 bar using batch-type Inconel reactors. Erbium speciation was investigated as a function of pH from 2.8 to 8, where Er chloride species predominant at acidic conditions and Er hydroxyl complexes predominant at near-neutral to alkaline conditions. The measured ErPO4 solubility (-9.8 to -7.5 log mEr) is up to 2.5 orders of magnitude lower than thermodynamic predictions (-9.4 to -6.7 log mEr) using existing thermodynamic databases. At elevated temperature, large deviations are observed at alkaline conditions (pH 7–9), indicating a need to revise the thermodynamic properties of Er hydroxyl species. The partial molal Gibbs energy of formation (ΔfG0T,P) for Er aqueous hydroxyl and chloride species are optimized in this study and the logarithmic formation constants (logβn(Cl,OH)) are fitted as a function of temperature.