Design and screening of suitable ligand/diluents systems for removal of Sr2+ ion from nuclear waste: Density functional theoretical modelling

Abstract

Density functional theoretical (DFT) calculation has been used for the design and screening of ligand/solvent systems for the selective and efficient extraction of Sr2+ ion from aqueous solution using free energy analysis. Present DFT study has shown that the metal ion selectivity cannot be predicted using intrinsic gas phase interaction energy alone. Even implicit solvation employing conductor like screening model (COSMO) approach unable to predict the correct trend of experimental selectivity. The COSMO solvation model when used explicitly by considering first solvation sphere around the metal ion it captures the correct trend of experimental selectivity. Organic solvent of high dielectric constant was found to be suitable for the extraction of the metal ion. Further, COSMO for real solvent (COSMO-RS) was successfully used to predict the solubility and partition coefficients of the ligands for screening. Presence of nitrate anion in the complexation reaction was shown to increase the gas as well as solvent phase complexion energy. The calculated theoretical extraction energy was found to be well correlated to the experimentally observed distribution constant of Sr2+ ion in the biphasic systems. The preferential selectivity of Sr2+ ion over other competing ions towards macrocyclic ligands was demonstrated by calculating free energy of extraction in the biphasic systems. Natural bond order (NBO) analysis was also performed to investigate the nature of bonding in Sr2+ ion-crown ether interaction.