Mixed-charge poly(2,6-dimethyl-phenylene oxide)anion exchange membrane for diffusion dialysis in acid recovery

In order to achieve high H+ dialysis coefficients and highly selective of anion exchange membranes (AEMs) for diffusion dialysis (DD) in acid recovery applications, a series of mixed-charge PPO AEMs with quaternary ammonium(QA) and carboxylic acid groups were synthesized quantitatively via Cu(I)-catalysed “click chemistry”. In diffusion dialysis, using an acidic solution (HCl, 1 mol/L; FeCl2, 0.2 mol/L) as a simulated waste solution indicated that the as-obtained mixed-charge PPO AEMs displayed higher H+ dialysis coefficients and a higher H+/Fe2+ selectivity over that of typical AEMs despite their increased water uptake and lower volumetric IECv values. The highest H+ dialysis coefficients, 0.021 m/h, and H+/Fe2+ selectivity 34.52, membranes were achieved with PPO-X35Y20 at 30 °C. These values were much higher than that of the PPO-X40 membrane without carboxylic acid groups. Importantly, unlike previously reported AEMs for DD in which the H+/Fe2+ selectivity decreased as the IECw increased, (i.e., a trade-off effect between the UH+ and selectivity), the high IECw of the mixed-charge PPO AEMs tended to result in not only high H+ dialysis coefficients but also high H+/Fe2+ selectivities. It is assumed that the carboxylic acid groups in AEMs likely enhance the dialysis of H+ by the ion changing and hydrogen-bonding more with H+ than Fe2+, thus, cancelling out the 'trade-off' effect in DD for acid recovery.