EXPERIMENTAL STUDY OF THE SOLUBILITY OF REE PHOSPHATES IN AQUEOUS CHLORIDE SOLUTIONS FROM 23 TO 150ºC

Solubilities of end member REE (rare earth element) phosphates (monazite and xenotime) have been experimentally determined in aqueous chloride solutions in the temperature range 23 to 150ºC, at saturated water vapor pressure. The goal of the experiments was to determine solubility products that can be used to model REE mobility in geological environments. Data on the solubility and stabilities of REE complexes are of critical importance to those concerned with safe nuclear waste disposal, geochemical exploration for REE deposits, and the use of REE as tracers in seawater and fresh water.

The solubilities of REE phosphate end members containing La, Sm, Nd or Y were measured in solutions of fixed HCl+NaCl or HClO4+NaClO4 concentrations (0.01 to 5.0 m). The solubility experiments indicate a greater solubility of LREE (La, Nd, Sm) phosphates relative to the HREE (Y) phosphate under the same physico-chemical conditions. We found that dissolution of (La,Nd,Sm)PO4 is nearly stoichiometric. However, up to 50% deviation from stoichiometry occurs in favor of phosphorous for the dissolution of YPO4.

It has been reported previously that the solubility of REE phosphates increases with decreasing pH. The experimental data confirm that REE phosphate solubilities are higher at very low pH and high ionic strength. The results of high-temperature experiments suggests that REE phosphates have retrograde solubility (i.e., solubility decreases with increasing temperature). Measurements and model calculations showed that Ln-chloride complexation is weak and Ln3+ is the dominant form of REE in aqueous solutions at ambient temperature but the degree of complexation increases with temperature and the speciation changes to the LnCl2+ complex, consistent with literature data. Furthermore, speciation calculations suggested that pH and the complexation with organic ligands are by far the most important factors that can significantly increase the mass transfer of rare earth elements in low-temperature geological environments.