Enhancing chloride/fluoride selectivity with hydrophobic side-chain anion exchange membranes in electrodialysis

The selective separation of chloride ions (Cl−) from fluoride ions (F−) is a critical concern in various industries due to the potential risks of F− on public health and industrial operations. However, Cl−/F− separation is known to be technologically challenging, considering they have the same charge and valency. This study explores the design of anion exchange membranes (AEMs) in electrodialysis to achieve Cl−/F− separation based on the difference in their hydration energy. Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) polymers brominated at varying reaction temperatures were employed as the basic AEM material, enabling fine-tuning of the hydrophobic properties of the resulting membranes through selective bromination at the benzyl or aryl positions and the subsequent quaternization with tertiary amines of different chain lengths, ultimately enhancing their selectivity. The developed AEMs are tested in electrodialysis involving a binary Cl−/F− mixture. The obtained membranes exhibited remarkable selectivity, reaching as high as 12.5 ± 1.0, surpassing the performance of commercial monovalent selective AEMs. The results demonstrate the potential of Cl−/F− separation by electrodialysis and highlight the significance of tailored membrane design in achieving the separation of ions with the same valency.