Assessment of anion exchange membrane selectivity with ionic membrane conductivity, revised with Manning's theory or the Kohlrausch's law

Abstract

The evaluation of anion exchange membrane (AEM) selectivity towards different anions requires facile characterization methods. The anion selectivity of membranes could principally, but practically not easy to, be extracted from the measured anion transport selectivity in electrodialysis (ED), which is a parameter closely related to the process conditions. According to the classical solution-diffusion model, the membrane anion selectivity could be decoupled into two parts: the partitioning selectivity factor of anions between the membrane and the solution phase; the mobility ratio of anions in the membrane. Alternatively, based on the relationship between the anion composition in the membrane and the mixed-ionic membrane conductivities, these two parts can be calculated from the volumetric swelling ratios and ionic conductivities when the AEMs are in pure and mixed-anion forms, respectively. This relationship is proved to be linear in two commercial AEMs (ASE and ACS) for the NO3−/Cl− and SO42−/Cl− pairs. Further, it is possible to obtain the anion selectivity directly for the ASE membrane from corresponding membrane ionic conductivities, with Manning's counter-ion condensation theory to account for the anion activity coefficients in the membrane. While for the NO3−/Cl− pair in the monovalent-anion selective ACS membrane, the selectivity thus calculated is very different from that obtained in ED. For the nonlinear anion composition-conductivity relationship observed in the commercial FAA membrane, the Kohlrausch's parameter (σ) is successfully introduced to describe the mutual influence of anions in the membrane. In this case, it is also possible to obtain the anion selectivity with the proposed approach. This work provides a different perspective to study the selectivity and conductivity of AEMs.