Theoretical insights into Am/Nd stripping by N,O-donor ligands: covalency and bond uniformity effects on separation performance.

Actinide and lanthanide separation is a central challenge in wet-method nuclear spent fuel reprocessing. Understanding how ligand geometry, softness, and related properties influence stripping and separation performance is essential for optimizing back-extraction agents for trivalent actinides. Herein, we theoretically evaluate the Am-selective back-extraction ability of the N-2-pyridylmethyl-methyldiethylenetriamine-N,N',N″,N″-tetraacetic acid ligand (H4DTTA-Pym, L1) and its analogs H4DTTA-Pzm (L2) and H4DTTA-Trm (L3) using quasi-relativistic density functional theory (DFT). Electrostatic potential and natural atomic charge analyses indicate stronger partial covalent character in Am-N bonds than in Nd-N bonds, attributed to the energy degeneracy between Am 5f and N 2p orbitals. All three ligands exhibit slightly enhanced covalency relative to their counterparts. Gibbs free energy calculations reveal limited differentiation in stripping efficiency between Am3+ and Nd3+ ions, although L2 and L3 show marginally stronger Am-Nring interactions and more stable back-extraction products. Notably, enhanced M-Nring partial covalency does not necessarily improve Am/Nd selectivity, owing to differences in the uniformity of M-Namine bond lengths. We find that more uniform M-Namine bond lengths correlate with better Am/Nd separation. These results may guide the rational design of water-soluble ligands for An3+/Ln3+ separation.