Mass transfer mechanism of the carrier-facilitated transport of uranium(VI) across 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester immobilised liquid membranes

Abstract

Transport of uranium(VI) from nitric acid solutions across immobilized liquid membranes (ILM) containing 2-ethylhexyl phosphonic acid mono-2-ethyhexyl ester (H 2A 2) dissolved in dodecane as carrier has been investigated under varied experimental conditions. The typical permeability of the membrane was 3.02 μm/s when H 2A 2 concentration in the ILM was 0.03 M in dodecane, the feed solution and receiving phase (RP) compositions were 0.5 M nitric acid and 0.3 M oxalic acid+0.05 M HNO 3, respectively. The flux of U across ILM decreased significantly when concentration of H 2A 2 in ILM was increased beyond 0.03 M. The transport mechanism consists of a diffusion process through feed aqueous diffusion layer, interfacial chemical reactions and diffusion of U–H 2A 2 complex across the membrane. The mathematical equation describing the rate of transport was evaluated and correlated with the membrane permeability coefficient, diffusional and equilibrium parameters as well as the chemical composition of the system such as carrier concentration in the membrane phase, HNO 3 concentration in the feed phase, and composition of the receiving phase. The influence of stirring speed in both the feed and receiving phase, membrane phase diluents and the effect of introduction of a modifier such as tributylphosphate in the membrane phase on the flux of U are discussed. The analytical as well as process applications of this method were studied by the transport of U across the ILM from solutions containing diverse ionic impurities.