Impact of the Donnan electrolytes on selectivity of cation exchange membranes evaluated via the ionic membrane conductivity

The selectivity of an ion exchange membrane (IEM) is an essential property in related electro-chemical separation and clean production processes. The impact of the Donnan electrolytes on the counter-ion selectivity of three representative cation exchange membranes (CEMs) evaluated via the ionic membrane conductivity is systematically studied in the mixed MgSO4/Na2SO4 and H2SO4/Na2SO4 electrolytes, over an external total sulfate concentration range of 0.1 ∼ 1.0 M. The counter-ions selectivity is decoupled into the partitioning selectivity coefficient (Kji) and the mobility ratio (uim/ujm), both of which are related to the CEM conductivity. If the Donnan electrolytes conductivity is excluded, for the heterogeneous CEM (3361BW), the selectivity calculated from the ion-exchanged conductivity (κE) can be perfectly matched with the electrodialysis (ED) results under a lower external sulfate concentration (0.1 M). The Mg2+/Na+ selectivity values of two homogeneous CEMs (Nafion117 and CSE) obtained from the nonlinear correlation method derived from the Kohlrausch's law (σ) are closer to the ED values, while the linear correlation approaches are more suitable at the presence of H2SO4. By analyzing the variation of Kji and uim/ujm with the κE separately, the main reason for the variation of the Mg2+/Na+ selectivity for these three CEMs, when the external electrolytes concentration increases, is the different equilibrium partitioning selectivity coefficients (KNa+Mg2+) between the membrane and the solution. On the contrary, the H+ ions transport much faster than the Na+ ions within the CEMs, and it is the change of this mobility ratio that causes the different H+/Na+ selectivity results. This work not only expands the application of the proposed conductivity method, but also provides a new perspective for studying the sorption and transport mechanism of ions in electro-chemical processes based on IEMs.