Unique Selectivity Reversal in the Complexation of Uranium(VI) and Plutonium(IV) with Two Novel C-Pivoted Tripodal Amide Ligands: Extraction and Liquid Membrane Transport Studies

Two new carbon pivoted tripodal amide-containing ligands with different spacers (L_I: -CH₂- spacer; L_II: -CH₂-CH₂- spacer) have been investigated for the liquid–liquid extraction of tetravalent plutonium and hexavalent uranium from nitric acid feed solutions. The studies were carried out using the ligand solutions prepared in 95% n-dodecane + 5% isodecanol. Pu(IV) extraction was higher than that of U(VI) at 3 M HNO₃ for ligand L_I which was just the opposite for ligand L_II. Increase of the nitric acid concentration resulted in increasing extraction of both metal ions. Isodecanol variation studies exhibited decrease in the extraction of Pu(IV) and U(VI) with increasing isodecanol content in the organic phase. Greater that 98% stripping of Pu(IV) and U(VI) was obtained from the complexes of L_I and L_II using 0.5 M HNO₃ + 0.1 M HEDTA and 1 M Na₂CO₃, respectively. The slope analysis study suggested the formation of 1:2 (ML₂) species with Pu(IV) as well as U(VI) with both ligands at 3 M HNO₃. Temperature variation studies showed that the extraction of U(VI) and Pu(IV) with both ligands is exothermic in nature. Supported liquid membrane transport experiments using 0.11 M ligands (L_I and L_II) in 95% n-dodecane + 5% isodecanol indicated poor transport (<10% after 2 h) of U(VI) and Pu(IV).