The role of co-ion transport in selectrodialysis for mono/divalent anion separation

In addition to the characteristics of monovalent anion perm-selective membrane (MAPM), the separation efficiency between monovalent and divalent anions (e.g., Cl − /SO 4 2− ) can be potentially influenced by the co-ion transport especially through cation exchange membranes (CEMs) in selectrodialysis. However, in such a process, the separation mechanism remains unclear and needs to be addressed. Hence, in this study, the role of co-ion transport in selectrodialysis for mono/divalent anion separation was systematically investigated in a new perspective. Results indicated that the CEM (e.g., SYMC membrane) that has a loose structure, a low ion exchange capacity, and a high transport number for SO 4 2− (0.133) exhibited a high co-ion flux of SO 4 2− and thus a high selectivity of Cl − /SO 4 2− (17.6) in selectrodialysis. Apart from the CEM properties, the paired cation (e.g., K + and Na + ) that has a high diffusion coefficient exhibited a high co-ion transport of SO 4 2− and thus a high selectivity (17.6−18.0). In addition, a high current efficiency (77.3 %), and a low operating expense (0.03 $ kg −1 ), capital expenditure (0.35 $ kg −1 ), and total cost (0.38 $ kg −1 ) can be obtained. These findings suggest that co-ion transport of SO 4 2− through CEM is beneficial for the selective separation of mono/divalent anions in selectrodialysis, offering practical guidance for industrial application of MAPMs to achieve a high selectivity.