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.