Covalent organic framework membranes for acid recovery: The effect of charges

Acid recovery has drawn an intensive interest due to the tremendous consumption of acid chemicals in various industry applications, which produce large amounts of acidic wastewater. For conventional anion exchange membranes (AEMs) via diffusion dialysis, the low selectivity between H+ and monovalent cations restricts the precise recovery of acid. Herein, we develop a self-standing COF (TpTAPA) membrane with high stability both in the acid and base solutions. The as-prepared membrane exhibits excellent separation performance for H+ over other monovalent and multivalent cations, where the selectivities of H+/K+, H+/Na+, H+/Li+, H+/Mg2+, and H+/Fe2+ are 136, 143, 145, 608, and 1499, respectively. Whereafter, the membrane nanochannels are decorated with different charges, where the positively charged Q-TpTAPA nanochannels are beneficial for H+ transport and the negatively charged S-TpTAPA nanochannels are adverse for the transport of H+ compared with the pristine TpTAPA membrane. However, both negatively and positively charged nanochannels can increase the leakage of multivalent cations, resulting in the decline of H+ selectivity. These significant findings shed light on the design of high-efficiency nanoporous membranes for exquisite acid recovery from complex feed system.