ADVANCES IN THE AQUEOUS SOLUTION CHEMISTRY OF TRIVALENT RARE EARTH ELEMENTS AT TEMPERATURES UP TO 250°C: RELEVANCE TO NEAR-FIELD RADIOACTIVE WASTE DISPOSAL ENVIRONMENTS

The trivalent rare earth elements (REE) are considered good analogues of trivalent actinides such as Am(III) and Pu(III). Radioactive REE also occur as fission products in nuclear waste and REE minerals such as monazite may sequester actinides. The near-field environment in radioactive waste repositories may be subjected to temperatures as high as 200ºC for extended periods of time. Therefore, the aqueous solution chemistry of trivalent REE at elevated temperatures is relevant to radioactive waste disposal. Significant advances in our knowledge of the thermodynamics of aqueous REE complexation, REE solid-phase solubility, and REE adsorption have been made via a variety of experimental techniques. These advances are reviewed and discussed in this paper. Experimentally measured stability constants now exist at elevated temperatures for REE complexes with acetate, chloride, fluoride, hydroxide, and sulfate. Solubility products at elevated temperatures have also been determined for selected solid REE fluoride, hydroxide and phosphate phases. Finally, adsorption of trivalent Nd onto rutile has been measured. The experimental data show that acetate complexes are more stable and fluoride and hydroxide complexes are less stable than predicted based on extrapolation of stability constants determined at standard conditions. The measured stability constants for chloride and sulfate complexes are in reasonable agreement with predicted values. At elevated temperatures, the stabilities of both chloride and fluoride complexes decrease with increasing atomic number across the REE series. For fluoride complexes this trend is contrary to that observed at standard conditions and predicted theoretically. The experimental data suggest that chloride complexation is likely to be more important relative to hydroxide and fluoride complexation than predicted from extrapolations. Phosphate mineral solubility and adsorption onto various oxides, carbonates and other minerals should maintain trivalent REE (and by extension trivalent actinide) concentrations in solutions relatively low in most nuclear waste environments.