Standard Thermodynamic Properties and Heat Capacity Equations for Rare Earth Element Hydroxides: I. La(OH)₃(s) and Nd(OH)₃(s). Comparison of Thermochemical and Solubility Data

Standard thermodynamic properties at 298.15 K, 1 bar and heat capacity equations for La(OH)₃(s) and Nd(OH)₃(s) were generated from thermochemical data available in the literature. Calculated solubility products of these crystalline hydroxides at 298.15 K were compared to those derived from critically assessed solubility and potentiometric measurements. Good agreement between the calculated and measured solubility was found for La(OH)₃(s). In contrast, the Nd(OH)₃(s) solubility at 298.15 K, calculated from thermochemical data, was more than one and half orders of magnitude higher than that derived from experimental solubility data. This result shows that Nd³⁺ is much less mobile in natural environments than previously thought and suggests that accepted standard thermodynamic properties for the Nd³⁺ aqueous ion at 298.15 K, 1 bar [2] need to be re-examined. By calculating equilibria between La- and Nd-crystalline hydroxides, hydroxycarbonates and carbonates we show in this study that Ln-hydroxides are the stable phases at low CO₂ partial pressures typical for concrete backfill environments in proposed radioactive waste storage sites.