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 therm ochemical data available in the literature. Calculated solubility prod ucts of these crystalline hydroxides at 298.15 K were compared to thos e derived from critically assessed solubility and potentiometric measu rements. 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 an d half orders of magnitude higher than that derived from experimental solubility data. This result shows that Nd3+ is much less mobile in na tural 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 CO 2 partial pressures typical for concrete backfill environments in prop osed radioactive waste storage sites.