Electrodriven Transport of Cs⁺ through Polymer Inclusion Membrane as "Solvent Separated Ions"

In our earlier work, we reported [Env. Sci. Technol. 2014, 48, 12994] a novel electrodialysis based selective separation of Cs⁺ from nuclear waste solution using chlorinated cobalt dicarbollide (HCCD) loaded polymer inclusion membrane (PIM). In continuation with that work, the mechanism of electrodriven transport of Cs⁺ through HCCD loaded polymer inclusion membranes has been explored. PIMs containing fixed amount of cellulose triacetate and nitrophenyl octyl ether (NPOE) but different concentrations of the carrier have been prepared. The experimental flux of Cs⁺ across the PIMs, for two different concentrations of the metal ion in the initial feed solution, has been measured using the radiotracer technique. On the basis of the Nernst-Planck equation, an attempt has been made to calculate the time dependence of concentration changes of the metal ion in the feed compartment. The experimental parameters of the membrane., viz., length, self-diffusion coefficient, distribution ratio, electrical resistance, and current, have been used in the calculation. The experimental results indicate that the transport of Cs⁺ by mobile carrier diffusion or fixed site jumping is not possible. It has been proposed that, under applied electric field, Cs⁺ is mostly transported as "solvent separated ions" through the polar lipophilic solvent NPOE. The proposed mechanism has been substantiated by comparing the experimental and the calculated results.