Understanding Acid Transport in Anion Exchange Membranes

As the need for acid recovery in mining and industrial wastewater treatment applications increases, there is growing interest in the use of ion-exchange membrane (IEMs) technologies, such as electrodialysis (ED), for this application. However, applying ED to acid recovery is hindered by acid leakage across anion-exchange membranes (AEMs). This phenomenon decreases both current efficiency and recovered acid concentration. To design AEMs with better acid-blocking properties, it is important to have a better understanding of acid transport in current advanced anion exchange membranes (AEMs). In this study, the acid transport properties of 24 different AEMs, commercial and lab-made, with varying water and charge contents, are investigated. The transport properties of the AEMs are comprehensively studied based on the Solution-Diffusion and Nernst-Einstein frameworks. Results show that membranes with better counter-ion/co-ion selectivity generally exhibit better acid-blocking performance. A clear trade-off between the membrane counter/co-ion conductivity-selectivity for commercial membranes is also observed. Recently reported ultrahigh-charge-density (UHCD) and phase-separated AEMs break this trade-off, demonstrating high selectivity and exceptional counter-ion conductivity simultaneously. The results of this study highlight shortcomings in the performance of current commercial membranes for acid recovery applications and showcase how our current strategy of designing lab-made membranes with UHCD or phase separation can enhance acid blocking efficiency.