Theoretically unraveling the separation of Am(iii)/Eu(iii): insights from mixed N,O-donor ligands with variations of central heterocyclic moieties

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Abstract

With the fast development of nuclear energy, the issue related to spent nuclear fuel reprocessing has been regarded as an imperative task, especially for the separation of minor actinides. In fact, it still remains a worldwide challenge to separate trivalent An(iii) from Ln(iii) because of their similar chemical properties. Therefore, understanding the origin of extractant selectivity for the separation of An(iii)/Ln(iii) by using theoretical methods is quite necessary. In this work, three ligands with similar structures but different bridging frameworks, Et-Tol-DAPhen (La), Et-Tol-BPyDA (Lb) and Et-Tol-PyDA (Lc), have been investigated and compared using relativistic density functional theory. The electrostatic potential and molecular orbitals of the ligands indicate that ligand La is a better electron donor compared to ligands Lb and Lc. The results of QTAIM, NOCV and NBO suggest that the Am-N bonds in the studied complexes have more covalent character compared to the Eu-N bonds. Based on the thermodynamic analysis, [M(NO₃)(H₂O)₈]2+ + L + 2NO₃- = [ML(NO₃)₃] + 8H₂O should be the most probable reaction in the solvent extraction system. Our results clearly verify that the relatively harder oxygen atoms offer these ligands higher coordination affinities toward both of the An(iii) and Ln(iii) ions compared to the relatively softer nitrogen atoms. However, the latter possess stronger affinities toward An(iii) over Ln(iii), which partly results in the selectivity of these ligands. This work can afford useful information on achieving efficient An(iii)/Ln(iii) separation through tuning the structural rigidity and hardness or softness of the functional moieties of the ligands.

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