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.本研究采用热水溶液量热法，在25至150 ℃范围内测定了氯化钕固体（NdCl3·6H2O）的标准偏摩尔溶解焓（ΔsolH°），并确定了氯化钕水溶液物种的标准热力学性质。实验在初始pH为2、离子强度（0.08~0.42 mol/kg）可变的盐酸基水溶液中开展，在此条件下，Nd³+与氯化钕水溶液物种均保持稳定。在研究的温度区间内，实测得到的ΔsolH°值与基于现有文献数据的热力学预测值之间存在35 kJ/mol的偏差。此类显著偏差对地球化学模拟具有重要意义，因为稀土元素（Rare Earth Element, REE）水溶液物种的热力学性质均由其氯化物固体的性质推导而来。为解决该问题，本研究对NdCl3·6H2O(s)的标准生成焓（ΔfH°）以及水溶液中的NdCl₂+物种进行了优化，使其能够复现实验得到的ΔsolH°值，并与近期更新的Nd³+热力学性质保持内在一致性。本研究的结果可推导出一套推荐的、内在一致的钕水溶液物种热力学性质，该套性质可提升含酸性氯化钠的水溶液流体中稀土元素迁移的热力学模拟精度。