Tri-<i>n</i>-butyl Phosphate vs Tri-<i>iso</i>-amyl Phosphate Complexation with Th(IV), U(VI), and Nd(III): From Theory to Experiment

The complexation behavior of tri-<i>iso</i>-amyl phosphate (TiAP) and tri-<i>n</i>-butyl phosphate (TBP) ligands with U(VI), Th(IV), and Nd(III) was investigated using density functional theory (DFT). Quantum chemical calculations yielded identical coordination geometries for TBP and TiAP complexes. Calculated complexation energies indicated a preferential extraction of U(VI) followed by Th(IV) over Nd(III), aligning with solvent extraction experiments conducted in the cross-current mode. Notably, during the separation of Th(IV) from RE(III), an increase in Th(IV) loading in the organic phase suppressed RE(III) extraction. Further analysis highlighted the crucial role of structural features (symmetry and dipole moment) in the extraction behavior of complexes. Energy decomposition analysis underscored the essential role of geometric strain and dispersion interaction energies in deciding the stability of the complexes.