Internally consistent thermodynamic properties of Nd chloride aqueous species and Nd3+ derived from hydrothermal solution calorimetry at 25–150 °C

Abstract

Rare earth elements (REE) are important critical elements used in the high-technology and green energy industries. Hydrothermal fluid-rock interaction and the stability of aqueous chloride complexes play an important role in the mobilization and deposition of REE in geologic systems. Geochemical modeling can be used to interpret the mobility of REE in these hydrothermal fluids. However, the accuracy of modeling predictions strongly depends on the underlying thermodynamic properties for REE aqueous species. In this study, hydrothermal solution calorimetry was used to measure the standard partial molal enthalpy of solution (Δ_solH°) of Nd chloride solid (NdCl₃·6H₂O) from 25 to 150 °C in order to determine the standard thermodynamic properties of Nd chloride aqueous species. The experiments were conducted in aqueous hydrochloric acid-based aqueous solutions with an initial pH of 2 and varying ionic strength (0.08–0.42 mol/kg), at which conditions the Nd³⁺ and Nd chloride species are both stable. In the studied temperature range, the measured Δ_solH° values differ by 35 kJ/mol in comparison to thermodynamic predictions based on available literature data. These large discrepancies have important implications for geochemical modeling because the thermodynamic properties of REE aqueous species are derived from the properties of REE chloride solids. To address this problem, the standard enthalpy of formation (Δ_fH°) of NdCl₃·6H₂O(s) and aqueous NdCl²⁺ species were optimized to reproduce the experimentally derived Δ_solH° values and keeping internal consistency with the recently updated thermodynamic properties of Nd³⁺. The outcome of this study permits deriving a set of recommended internally consistent thermodynamic properties for the Nd aqueous species which improve thermodynamic modeling of REE mobilization in acidic NaCl-bearing aqueous fluids.