Hydrothermal solution calorimetry in acidic aqueous solutions and revisiting the standard partial molal thermodynamic properties of Nd3+ from 25 to 300 °C

The mobility of rare earth elements (REE) can be predicted in aqueous fluids using geochemical modeling but the accuracy of these models strongly depends on the availability of robust thermodynamic properties for the REE aqueous species. The REE³⁺ aqua ions are important in the derivation of the formation constants of all the major REE complexes including the chloride, sulfate, and fluoride species which predominate in many hydrothermal-magmatic systems. However, the thermodynamic properties of the REE³⁺ aqua ions are still commonly derived from the Helgeson-Kirkham-Flowers (HKF) equation of state parameters tabulated several decades ago. The standard state thermodynamic properties at reference conditions (25 °C and 1 bar) and their extrapolations to high temperature need to be verified, if not revised, based on hydrothermal experiments. In this study, the enthalpy of solution was measured for synthetic Nd hydroxide from 25 to 150 °C to retrieve the standard partial molal thermodynamic properties of Nd³⁺ as a function of temperature. The experiments were conducted in aqueous perchloric acid based solutions with starting pH of 2 and varying ionic strength (0.01–0.09 mol/kg NaClO₄). The standard partial molal enthalpy of formation (Δ_fH°) of Nd³⁺ derived from the experimental study displays differences of up to 10 kJ/mol compared to the enthalpy values derived from the HKF equation of state in the studied temperature range. These inaccuracies are resolved by adjusting the standard partial molal Gibbs energy of formation (Δ_fG°) of Nd³⁺ at 25 °C and 1 bar from −672.0 to −679.7 kJ/mol. The heat capacity function (C_p°) derived between 25 and 150 °C can be described by: C_p° = a₀ + a₁⋅T + a₂⋅T⁻², with a₀ = 1256, a₁ = −2.68, a₂ = −55.56⋅10⁶ and T in Kelvin. A set of recommended thermodynamic properties is provided for the Nd³⁺ aqua ions and corrections are provided for the chloride and fluoride species to remain internally consistent with the experimentally derived properties. These results allow predicting accurately the solubility of monazite between 25 and 300 °C. Before these corrections, the properties for the Nd³⁺ aqua ions derived from the HKF parameters resulted in up to ∼1.5 orders of magnitude lower monazite solubility than determined experimentally. Therefore, a revision of the REE⁺³ aqua ions properties is necessary to accurately predict the mobility of REE in hydrothermal acidic solutions.