Mechanically Strengthened Graphene Oxide: Covalent Organic Framework Membranes for Monovalent/Divalent Cation Selectivity via Electrodialysis

Electrodialysis is an energy efficient method for water desalination and resource recovery; however, the lack of ion selectivity in conventional ion-exchange membranes limits its performance, particularly in separating monovalent and divalent cations. In this study, we present the design and fabrication of mechanically reinforced graphene oxide (GO)-covalent organic framework (COF) composite membranes with variable selectivity for monovalent over divalent cations, enhancing the performance of electrodialysis processes. The composite membranes, created by stacking GO and sulfonate-functionalized COF nanosheets, exhibit a nacre-inspired "brick-and-mortar" structure that imparts mechanical robustness and high ion selectivity. By adjusting the GO-to- COF ratio, we achieved varying Na⁺/Ca²⁺ and Li⁺/Ca²⁺ selectivity ratios, reaching up to 15.34 and 6.99, respectively, without compromising the charge efficiency (greater than 75%). The membranes have also shown good stability in synthetic hypersaline brine. These results demonstrate the potential of GO/COF membranes for high-performance, energy-efficient ion separation in electrodialysis, offering a promising solution for desalination, wastewater treatment, and resource recovery.