Comb-shaped sulfonated poly(ether ether ketone) as a cation exchange membrane for electrodialysis in acid recovery

Abstract

To produce a cation exchange membrane (CEM) with high permselectivity and proton flux for electrodialysis (ED), a series of comb-shaped sulfonated poly(ether ether ketone) (SPEEK) membranes with long alkyl (butyl and octyl) side chains were synthesized using 1,1′-carbonyldiimidazole (CDI) as an activating reagent. The degree of substitution can be easily controlled by controlling the added amounts of CDI and reagents. The chemical structures were confirmed using a ¹H NMR technique. Transparent and tough membranes were prepared by solution casting. A well-defined hydrophilic–hydrophobic separation was achieved, as confirmed by small-angle X-ray scattering (SAXS). The comb-shaped membranes with the well-defined hydrophilic–hydrophobic separation effectively allow the transport of H⁺, so higher H⁺ flux is achieved in the ED process with the comb-shaped SPEEK membranes than that obtained with the SPEEK membrane despite the higher membrane resistance resulting from the lower ion-exchange capacity and lower water uptake. Interestingly, the lower swelling ratio of the comb-shaped membranes can block Fe²⁺ transport through the membranes. Thus, the comb-shaped membranes show much higher H⁺/Fe²⁺ permselectivity than the pristine SPEEK membrane. The highest H⁺ flux of 2.41 × 10⁻⁷ mol cm⁻² s⁻¹ and H⁺/Fe²⁺ permselectivity of 32.13 are achieved for the SPEEK membrane with 30% substitution by octyl side chains. The performance is comparable with and even better than that of a commercial monovalent CEM from ASTOM. Considering the excellent thermal stability and mechanical properties, the comb-shaped SPEEK membranes are promising CEM materials for ED in acid recovery.