A Study of the Mechanism for the Extraction of Uranium, Water, and Nitric Acid with Trioctylphosphine Oxide Using a Binary Molten Mixture of Biphenyl and Naphthalene as the Solvent

Abstract Liquid-liquid distribution with trioctylphosphine oxide (TOPO) and a binary molten mixture of biphenyl and naphthalene (weight ratio=3:1) has been examined for the extraction of uranium from a 0.1 mol kg−1 nitric acid solution. The stoichiometry of the TOPO complex of uranyl nitrate in the molten biphenyl–naphthalene phase was established to be UO2(NO3)2(TOPO)2. The extracted species of water and nitric acid are H2·TOPO and H3O+·TOPO·NO3− at concentrations of TOPO of less than 0.1 mol kg−1. The thermodynamic quantities (ΔH°, ΔG°, and ΔS°) were obtained for the extraction of uranyl nitrate, nitric acid, and water into TOPO–bi-phenyl–naphthalene and TOPO–cyclohexane solutions from the temperature dependence of the extraction constants. More exothermic enthalpy and smaller entropy changes were observed for the binary molten mixture; these differences were attributed to the aromatic and viscous properties of the solvent.