Determination and theoretical evaluation of selectivity coefficients of monovalent anions in anion-exchange polymer inclusion membrane

The selectivity of an anion-exchange polymer inclusion membrane (PIM) towards a number of anions (F−/OH−/HCO3−/CH3COO−/IO3−/Cl−/BrO3−/NO2−/Br−/NO3−/SCN−/MnO4−/ClO4−) was studied with respect to I− ions using 131I radiotracer. The PIM was prepared by physical immobilization of a liquid anion-exchanger trioctylmethylammonium chloride (Aliquat-336) into polymer matrix of cellulose triacetate (CTA) plasticized with 2-nitrophenyl octyl ether (NPOE). The highly hydrated ions (F−/OH−/HCO3−/CH3COO−/IO3−/Cl−) did not exchange appreciably with I− counterion present in the membrane sample, while the less hydrated ions like ClO4− were strongly exchanged. The averaged selectivity coefficients (KI−X−) for those anions, which were appreciably exchanged with I− ions (Br−, NO2−, BrO3−, NO3−, SCN−, and ClO4−) in the membrane, were determined from ion exchange isotherms. Attempt was made to theoretically evaluate the observed selectivity trends based on the Marcus formalism, which is commonly used for the calculation of selectivity coefficient in solvent extraction involving liquid ion exchangers. The model reproduced well the experimentally observed selectivity coefficients using the adjusted dielectric constant of the membrane medium. The dielectric constant of the plasticizer (NPOE) was not found to be appropriate for prediction of experimentally measured selectivity coefficients.