QPPO-Surface-Functionalized SPPSU/MXene Composite Membranes for Efficient Separation of Mono/Multivalent Cations through Selectrodialysis

Monovalent cation exchange membranes (MCEMs) are central to selectrodialysis (SED), which enables precise ion fractionation. In this study, a novel type of MCEMs were designed for the separation of mixed acid/salt systems via SED. The membranes were fabricated by first doping Ti3C2TX nanosheets into a sulfonated polysulfone (SPPSU) matrix, followed by spray-coating quaternized poly(2,6-dimethyl-1,4-phenylene oxide) (QPPO) layers synthesized from brominated polyphenylene oxide and tertiary amines with varying alkyl chain lengths. The incorporation of 0.5 wt % Ti3C2TX into SPPSU (yielding the SPPSU-T membrane) enhanced the ion exchange capacity by 1.10-fold (to 1.53 mmol·g–1) and water uptake by 1.01-fold (to 16.10%) relative to the pristine SPPSU membrane, resulting in a reduced area resistance (2.08 vs 2.14 Ω·cm–2) and a 1.30-fold increase in H+ flux (7.31 × 10–8 mol·cm–2·s–1). Although the optimized SPPSU-T-QPPO18-1.02 (coated with 1.02 g·m–2 of QPPO quaternized with N,N-dimethyldodecylamine) exhibited a 3.11-fold increase in area resistance compared to the pristine SPPSU-T membrane, it had a 12.32-fold higher H+/Mg2+ selectivity (PMgH = 87.59). Moreover, the SPPSU-T-QPPO18-1.02 also achieved 14.23-fold and 5.13-fold higher selectivity for H+ over Zn2+ (PZnH = 29.32) and Fe2+ (PFeH = 9.149), respectively, compared to SPPSU (PZnH = 2.06, PFeH = 1.78). In the three investigated acid/salt systems, the SPPSU-T-QPPO18-1.02 delivered competitive H+ flux (6.97∼8.78 × 10–8 mol·cm–2·s–1) and selectivity (9.14∼87.59) compared to the commercial CIMS membrane (flux: 6.00∼8.68 × 10–8 mol·cm–2·s–1; selectivity: 1.98∼4.11). Furthermore, it demonstrated excellent long-term operational stability in SED processing of MgCl2/HCl mixtures. This work demonstrates that incorporating Ti3C2TX nanosheets into the membrane matrix and coating with a tailored QPPO layer can synergistically overcome the trade-off between ion flux and selectivity in MCEMs, providing an effective strategy for advanced membrane design.