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

In this study, hydrothermal solution calorimetry was used to measure the standard partial molal enthalpy of solution (ΔsolH°) of Nd chloride solid (NdCl3⋅6H2O) from 25 to 150 ºC and 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 ΔsolH° 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 (ΔfH°) of NdCl3·6H2O(s) and aqueous NdCl2+ species were optimized to reproduce the experimentally derived ΔsolH° 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.