Separation by competitive transport of uranium(VI) and thorium(IV) nitrates across supported renewable liquid membrane containing trioctylphosphine oxide as metal carrier

Abstract

Supported renewable liquid membrane (SRLM) technique is investigated for the selective separation of uranium from thorium solutions. Trioctylphosphine oxide (TOPO) dissolved in dodecane is used as the metal carrier in the liquid membrane (LM) phase. The renewable LM configuration used is highly stable for consistent long run operations. Systematic investigations were carried out to investigate the transport of U(VI) across the SRLM under different experimental conditions such as, concentration of TOPO in the LM phase, concentration of U(VI), Th(IV) and nitric acid in the feed, receiving phase composition and hydrodynamic conditions across the polymeric membrane support. By combining the process of in situ precipitation of thorium carbonate in the receiving phase, an excellent separation of uranium from thorium with decontamination factor of the order of 350 could be achieved. The solid precipitate of thorium carbonate generated in the receiving phase did not create any hindrance to the transport of U(VI) across the SRLM. The parameters defining the transport mechanism of U(VI) across the SRLM are experimentally evaluated as; maximum permeability coefficient, PU=9.66×10−5cm/min, diffusion coefficient for U(VI) in the aqueous feed solution, Da=2.35×10−4cm2/min with length of the concentration depletion layer from membrane interface, da=4.17×10−2cm and membrane diffusion coefficient for UO2(NO3)2·2TOPO in the LM phase, Do=5.36×10−6cm2/min. The numerical model obtained with these parameters gives satisfactory concordance with the experimental results under given constraint of the concentrations of nitric acid, TOPO and U(VI). Selectivity of this separation process is tested by determining permeability of different metal ions across the SRLM under optimized conditions.