Effect of alkyl chain length of tri-n-alkyl phosphate extractants on the electrochemical behaviour of U(VI) in ionic liquid medium

• Trialkyl phosphates for extraction of uranium
• Electrochemical behaviour of U(VI) in ionic liquid medium
• Effect of trialkyl phosphats extractants on U(VI) electrochemistry
• Determination of diffusion coefficient and electron transfer rate
• Stability constant and stoichiometry of U(VI)–ligand complexes

Higher homologs of trialkyl phosphates have been proposed for the separation of U(VI) from fast reactor dissolver solution by liquid – liquid extraction. The electrochemical behaviour of U(VI) in tri-n-octyl phosphate (TOP) in the ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (C4mimNTf2) was studied by cyclic voltammetry. The results were compared with those obtained with reference extractant, tri-n-butyl phosphate in C4mimNTf2. The cyclic voltammogram of U(VI) in C4mimNTf2 exhibited a prominent quasi-reversible reduction wave at − 0.24 V(Vs. Fc/Fc+) was due to the reduction of U(VI) to U(V) and a couple of oxidation waves at + 0.5 V(Vs. Fc/Fc+). The cathodic peak current was lowered and peak potential shifted cathodically in the presence of TBP and TOP due to the formation of co-ordinate complex of these ligands of with U(VI). The diffusion coefficient (D, cm2/s) of U(VI) and the heterogeneous charge transfer rate constant (ks, cm/s) were determined for various systems and they increased with increase of temperature and decreased in the presence of ligands. Both these quantities decreased in the order U(VI) in C4mimNTf2 > U(VI) – TBP in C4mimNTf2 > U(VI) – TOP in C4mimNTf2. The stability constant and the stoichiometry of U(VI)-TBP and U(VI)-TOP in ionic liquid medium was determined by cyclic voltammetry.