Tracing the Speciation Origin of Acidity-Dependent Extractability from Nitrilotriacetamide-Mediated Am(III)/Eu(III) Selectivity Separation

The performance of nitrilotriacetamide (denoted as NTAamide or L) in extracting and separating Am(III)/Eu(III) is strongly governed by the nitric acid concentration; however, the interaction mechanism connecting acidity to extractability remains unclear. Herein, five types of representative NTAamide-extracted Am(III)/Eu(III) species were comprehensively examined by density functional theory calculations and ab initio molecular dynamics simulations. The extracted complexes were classified as ligand-neutral M-L (M:L = 1:1, 1:2, and 2:2) and ligand-protonated M-HL (M:HL = 1:1 and 1:2), with one to three HNO3 molecules explicitly included to simulate the nitric acid media. With increasing acidity, most extracted complexes prefer forming thermodynamically less stable species, corroborated by electronic structure analyses. Specifically, structural optimizations and dynamic simulations find acidity-weakened metal–oxygen coordination in the 1:1 M-L complex, decreasing the inner-sphere coordination number from nine to eight; in contrast, the rest of the species mainly reorganize and weaken outer-sphere noncovalent interactions. Meanwhile, high acidity promotes NTAamide protonation, and the produced M-HL complexes are less stable on average than the M-L species. It is evident that the former ones have more positive total bond interaction energies (e.g., weaker bonding strength) and less thermodynamic stability. Thus, the established acidity–extractability correlation rationalizes experimental findings and guides amide extractant design for selective lanthanide/actinide separation.