A molecular dynamics investigation of La3+ and Lu3+-ligand speciation in aqueous solution

Abstract

Ion-pair association of rare earth elements (REE) with ligands is important for its mobilization, enrichment, and fractionation during hydrothermal transport process, which is of great significance to study REE ore genesis, weathering process, the origin and evolution of earth and other related geoscience issues. However, reliable experimental data are limited to a specific system or ambient conditions, which hampered our understanding of the geochemical behavior of REE. In the present study, classical molecular dynamics simulations were carried out to calculate the potential of mean force (PMF) between La3+/Lu3+ and ligands of Cl−, F−, OH−, HCO3−, CO32− and SO42− in dilute aqueous solutions from ambient to supercritical thermodynamic conditions. Stability constants were calculated from the PMFs and compared with literature reported and thermodynamic calculated data. The analyses of the PMFs and stability constants suggest smaller or highly charged ions are easier to form stable complexes. For both La3+ and Lu3+ containing systems, the stability of ion pair decreases in the order of CO32− > OH− > SO42− > F− > HCO3− > Cl−. The smaller Lu3+ is to form more stable aqueous complexes at all thermodynamic conditions and these stability difference may lead to significant fractionation between La (light) and Lu (heavy). The trends of PMFs calculated stability constants are consistent with thermodynamic calculated values and provide association constants of ligands for which no experimental data exist.