Elucidation of complexation of tetra and hexavalent actinides towards an amide ligand in polar and non-polar diluents: Combined experimental and theoretical approach

Abstract

A combined experimental and theoretical study has been put forward to investigate the complexation behaviour of tetravalent Pu4+ and hexavalent UO22+ ions with the N,N-dihexyloctanamide (DHOA) ligand in different diluents. Solvent extraction experiments were conducted with Pu4+ and UO22+ ions using DHOA in different diluents, namely dodecane, toluene, chloroform, octanol, nitrophenyl-octyl ether (NPOE) and nitrobenzene (NB). The experimentally measured distribution coefficients (D) of both the Pu4+ and UO22+ ions followed the order: nitrobenzene>NPOE>octanol>chloroform>toluene>dodecane. In order to complement the experimental distribution data, the structure, energetic and thermodynamic parameters of the Pu4+ and UO22+ complexes with DHOA have been calculated using density functional theory. The free energy of extraction, ΔGext, for the Pu4+ and UO22+ ions with DHOA was calculated using the Born–Haber thermodynamic cycle in the different experimentally studied diluents. The value of ΔGext for the Pu4+ ion in dodecane was found to be higher than that of the UO22+ ion, as measured in the experiments. The value of ΔGext was shown to increase with the increase in the dielectric constant of the diluent for both Pu4+ and UO22+ ions, similar to the experimental studies. Thus, the combined experimental and theoretical studies help to understand the underlying complexation mechanism of the UO22+ and Pu4+ ions with DHOA, which explains its selectivity towards the Pu4+ ion.