Theoretical studies on the synergistic extraction of Am3+ and Eu3+ with CMPO–HDEHP and CMPO–HEH[EHP] systems

In recent years, there has been a trend to use synergistic systems of neutral and acidic extractants into a single-solvent formulation to achieve Ln3+/An3+ extraction and separation via simplified processing schemes. In this study, geometrical structures, bonding nature, and thermodynamic stabilities of a series of possible extraction complexes of Am3+ and Eu3+ with two neutral-acid synergistic systems (CMPO–HDEHP and CMPO–HEH[EHP]) were theoretically studied using scalar-relativistic density functional theory (DFT). It is found that hydrogen bonds between neutral and acid ligands may greatly influence the Eu3+/Am3+ extraction and separation performance of the two synergistic systems. According to natural bond orbital (NBO) and energy decomposition analyses (EDA), the phosphoryl oxygen atoms of HDEHP or HEH[EHP] have higher affinity for Eu3+ and Am3+ than those of the CMPO ligand. Since weak but different extents of covalency exist in Eu- and Am-complexes, Eu3+/Am3+ separation via these two systems may be attributed to the different compositions of interactions between the metal ions and ligands. Thermodynamic analysis shows that the neutral complexes ML(NO3)2 (M = Eu and Am; L = [CMPO-DEHP]− and [CMPO-EH[EHP]]−) appear to be the most probable species in the complexation process.