Experimental investigation of the solubility of the REE phosphate minerals monazite/xenotime and chloride complexation in hydrothermal solutions at 23°C, 50°C, 150°C and saturated water vapor pressure.

摘要

Solubilities of pure REE (rare earth element) phosphate minerals (monazite and xenotime) have been experimentally determined in aqueous chloride solutions in the temperature range 23 to 150°C, at saturated water vapor pressure. The goal of the experiments is to determine thermodynamic data (i.e., monazite/xenotime solubility products and REE-chloride complex stability constants) 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 endmembers (La, Nd, SM, Y)PO₄ were measured in solutions of fixed HCl +NaCl or HClO₄+NaClO ₄ 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. It has been recognized that dissolution of (La, Nd, SM)PO₄ is nearly stoichiometric (ΣREE = ΣPO₄). 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 monazite increases with decreasing pH. The experimental data confirm that monazite/xenotime solubilities are higher at very low pH and high ionic strength. The results of high-temperature experiments suggests that monazite has 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 the 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 geological environments.