Exploring Bonding Properties, Extraction and Separation Performance of Furan-based N,O-Hybrid Extractants for Am(III) and Eu(III)

The Am(III) and Eu(III) in high-level liquid waste exhibit similar physicochemical properties, which makes their extraction and separation rather challenging. Ten symmetric and asymmetric furan-based N,O-hybrid extractants were constructed from the furan skeleton and the side chains of pyridine, pyridazine, pyrimidine and pyrazine. The density functional theory method was employed to evaluate the coordination structures and bonding properties of the ligands with the Am(III) and Eu(III) ions; the thermodynamic parameters of the two-phase extraction process were calculated to explore the extraction ability and separation performance of the ligands for the Am(III) and Eu(III) ions. The natural bond orbitals and QTAIM analyses show that there is a weak closed-shell interaction between Am(III), Eu(III) and the ligand, and the Am-ligand bonds have more covalent character than the Eu-ligand bonds since the Am-5f orbitals are more involved in bonding with the ligand than the Eu-4f orbitals. Thermodynamic analysis shows that the solvents cyclohexanone and n-dodecane of the organic phase have some impact on the extraction and separation, the ligand was more likely to bind with the metal in the cyclohexanone solvent, while some ligands have better separation effects in the n-dodecane solvent. In either solvents, the asymmetric extractant L⁹ has the most excellent separation performance for the two metal ions, and the asymmetric ligands L⁵ and L⁸ also have good separation effect. This work contributes to a deeper understanding of the selectivity differences of similar furan-based flexible ligands and provides more abundant theoretical bases and insights for the design of new extractants required for the separation of Am(III) and Eu(III).