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

Abstract

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.