Role of ligand spacer length of a tripodal amide on uranium(vi) and plutonium(iv) complexation: synthesis, solvent extraction, liquid membrane transport and theoretical studies

Liquid-liquid extraction of several actinide and lanthanide ions, viz., U(vi), Pu(iv), Am(iii) and Eu(iii) was investigated from nitric acid feed solutions employing N,N,N′,N′,N′′,N′′-hexa-n-octyl nitrilotripropamide (HONTP) in 90% n-dodecane + 10% isodecanol. The observed trend of metal ion extraction at 0.5 M HNO₃ was Pu(iv) > U(vi) > Eu(iii) ∼ Am(iii). A high distribution ratio was observed for U(vi) at pH 2 using 0.08 M HONTP in 90% n-dodecane + 10% isodecanol. The back extraction study of Pu(iv) and U(vi) using 0.5 M HNO₃ + 0.5 M oxalic acid and 1 M Na₂CO₃, respectively, showed highly efficient stripping (>97%) for both the metal ions from the loaded organic phase. Slope analysis suggested formation of a lower stoichiometry complex of Pu(iv) with HONTP at 3 M HNO₃, whereas U(vi) formed 1 : 1 (ML) and 1 : 2 (ML₂) species. Temperature variation studies on the extraction of the metal ions showed that complexation of U(vi) with HONTP was not favourable, based on entropy changes. A supported liquid membrane study showed very poor transport of U(vi) using 0.08 M HONTP in 90% n-dodecane + 10% isodecanol, whereas Pu(iv) exhibited significant transport through the membrane. Density functional theoretical (DFT) studies were carried out to corroborate the experimental observations. Comparison of the results with those of hexa-n-octyl nitrilotriacetamide (HONTA) revealed a considerable effect of the spacer length on the binding efficiency, selectivity, and complex stoichiometry.