Constructing proton selective pathways using MOFs to enhance acid recovery efficiency of anion exchange membranes

Abstract

Efficient recovery of acids from industrial wastewater containing metal ions is crucial for resource recycling and environmental safety. Dialysis based on anion exchange membranes (AEMs) is a promising method for acid recovery but typically suffers from low selectivity. Herein, we develop highly stable AEMs based on metal-organic frameworks (MOFs) to improve the acid recovery performance. For membranes based on quaternary ammonium polysulfone (QAPSF) and quaternary ammonium poly (2,6-dimethyl-1,4-phenylene oxide) (QPPO), embedded MOFs can provide selective proton transport paths because of a precise size-sieving effect and abundant hydrogen-bonding networks, thus improving both the acid dialysis selectivity and flux. Remarkably, the QPPO membrane incorporated with 20 wt% UiO-66 exhibits a high dialysis coefficient of 16 mm/h and a separation factor of 683. The MOF-hybrid AEMs are sufficiently stable and retain their original structure and morphology after dialysis tests. In addition, molecular dynamics simulations suggest that the competitive Fe2+ ions are immobile and present a high energy barrier to diffuse in UiO-66, whereas water molecules can hop between the cavities of MOFs, thereby facilitating fast proton conduction and thus improving proton selectivity. Therefore, Zr-MOFs can be incorporated as porous sieving fillers into AEMs to develop advanced hybrid membranes for acid recovery.