Crosslinked terpolymer anion exchange membranes for selective ion separation and acid recovery

Abstract

Anion exchange membrane (AEM) is useful for the removal of salts via electrodialysis (ED) process. AEM is also used for the recovery of acid from an aqueous feed containing acid and metal ion through diffusion dialysis (DD). However, for the selective separation of monovalent and divalent ions, special type of AEM is required. Herein, we report the preparation of poly(acrylonitrile-co-n-butyl acrylate-co-polydimethylamino ethyl methacrylate) (PAN-co-PnBA-co-PDMA) terpolymer-based crosslinked AEMs for the separation of monovalent ion from bivalent ion via ED process and recovery of hydrochloric acid (HCl) from aqueous feed of HCl and FeCl2 through DD. The slowing down the movement of bivalent anion under electrical potential, and hindering the diffusion of metal ion are the key factors for the optimization of the membranes. The tuning the polarity of the microenvironment of quaternized nitrogen (from DMA moieties) of the terpolymer by changing the length of alkyl chain gave membranes with different monovalent to bivalent anions selectivity, and separation factor for acid recovery. AEM-TP-C1, AEM-TP-C4 and AEM-TP-C10 membranes were prepared by quaternizing the DMA moieties (C1 to C10 denote length of alkyl) of the terpolymer followed by crosslinking. The AEM-TP-C10 membrane exhibited best selectivity (4.76) during desalination of sodium chloride and sodium sulfate mixture (0.01 M, 0.01 M) via ED process. Best performance in terms of acid recovery was obtained with AEM-TP-C4 membrane with dialysis coefficient of 0.031 m/h and separation factor of 38 during acid recovery. The length of alkyl and the hydrophobicity of the environment of the quaternized nitrogen is a governing factor for deciding the performance of the AEMs. This work provides an insight to design crosslinked AEMs for selective separation and acid recovery. The tertiary amine presence in the terpolymer may be quaternized with varieties of halide compounds to further correlate the structure of quaternized amine and property of the AEMs.