Studies on diffusional mobility and selectivity of I− ion in plasticized anion-exchange membrane using radiotracer

Abstract

Summary Plasticized anion-exchange polymer inclusion membranes (PIM) were prepared by physical immobilization of a liquid anion-exchanger trioctylmethylammonium chloride (Aliquat-336) into the polymer matrix of cellulose triacetate (CTA) plasticized with 2-nitrophenyl octyl ether (NPOE). The self-diffusion coefficients (D m) of I− ion in these PIMs were obtained by analysing the experimental 131I radiotracer exchange rates with an analytical solution of Fick's second law. The D m was found to increase exponentially as a function of volume fraction of liquid (plasticizer) in the membrane, which can be attributed to reduction of physical obstruction in diffusion path of the ions in CTA polymer matrix. The selectivity of PIMs towards different anions, with respect to I− ion, was obtained from a determination of mole fraction of I− ion (ratio of moles of I− ion to total ion-exchange sites) in the membrane as a function of mole fraction of I− ion in the equilibrating aqueous solution containing a competing X− ion (X− = F−/Cl−/Br−/NO3 −/ClO4 −). This study indicated that the PIM has preference for I− ion in the presence of Cl−, Br− and NO3 − ions in equilibrating solution, while ClO4 − ion are preferred over I− ion. The selectivity trend (ClO4 −>I−>NO3 −>Br−>Cl−>F−) in PIM was found to correspond with Hofmeister series.