Tuning side group structures of series-connected di-cations to achieve improved electrodialysis acid recovery performances

Abstract

The electrodialysis (ED) technology for waste acids recovery has become a hotspot due to the advantages of low cost, high efficiency and no-pollution. The ED acid recovery efficiency is strongly controlled by the ionomer aggregation structure of the applied anion exchange membrane (AEM). To investigate the effects of different side group structures on the ED acid recovery performance of AEMs, a series of series-connected di-cation ionomers based AEMs named QPPO-DABCO-R with high ion exchange capacities were tailor-made from the commercial polyphenylene oxide with series-connected 1,4-diazabicyclo[2.2.2]octane di-cation groups as the transport cores, and hydroxyethyl, methyl and hexyl groups of various hydrophilic-hydrophobic balances as the auxiliary side groups. The aggregation structure, water uptake, mechanical properties, thermal stability, ion selectivity and ED acid recovery performance of the AEMs were systematically investigated. The results confirm that the modified microphase separation structures with the strong hydrophobic side groups show the enhanced abilities to increase the fixed charge density, to block proton leakage and promote the transport of counterions through the membrane. Interestingly, QPPO-DABCO-C6 with the most hydrophobic side group exhibits the acid recovery performances much better than those of the commercial acid block ACM. The results demonstrate convincingly that the strategy of combining appropriately sized hydrophobic weak polar alkyl groups with high-density series-connected di-ions is an effective method to develop electrodialysis AEM with excellent acid recovery performance.