Development of nucleobase functionalized sulfonated poly(ether ether ketone) electrodialysis membranes with enhanced proton flux and permselectivity

Electrodialysis has the advantages of simple operation, low energy consumption and environmental friendliness, making it an efficient and sustainable waste acid recovery technology. In this work, two series of nucleobase functionalized sulfonated poly(ether ether ketone)s (SPEEK) with different degrees of adenine (SPEEK-xA) or cytosine (SPEEK-xC) were successfully synthesized. The introduction of nucleobase groups can form acid-base pairs with SO3H groups in SPEEK, thereby improving the thermal and mechanical properties of the membranes. Compared to SPEEK, the tensile strength of the nucleobased modified membranes increased by 1.2–1.6 times (56.9–75.3 MPa), and the elongation at break decreased by more than 70 % (22.3–34.7 %). Due to the proton binding ability of the multiple N atoms in the nucleobase groups, the H+ fluxes of the SPEEK-10A and SPEEK-10C membranes during electrodialysis increased by about 10 % compared to the SPEEK membrane, reaching 1.06 and 1.03 mmol·m−2·s−1, respectively. Remarkably, all modified membranes showed much higher H+/Fe2+ permselectivity than SPEEK because of the strong repulsive effect of protonated planar nucleobase cations. The permselectivities of the SPEEK-10A and SPEEK-10C membranes were as high as 91.6 and 94.8, respectively, which were 6.0–6.2 times higher than that of SPEEK. Therefore, the introduction of nucleobase groups into SPEEK not only increases proton flux but also enhances the permselectivity between H+ and divalent metal ions.