Selective separation of Ni2+/Co2+/Mn2+ enabled by tunable intrapore sulfonic acid group content in covalent organic framework membranes via EDTA-chelation-assisted process

Covalent organic frameworks (COFs) membranes have demonstrated exceptional adaptability and promising potential for precision ion-selective separation in aqueous solutions due to the fact that their pore size and charge intensity can be tailored through coordinated structural modulation. Chelation-assisted membrane separation technologies represent a highly effective strategy for separating and recovering similar divalent transition-metal ions (e.g., Ni 2+ , Co 2+ , and Mn 2+ ). In this approach, conventional chelating agents, such as ethylenediaminetetraacetic acid (EDTA), selectively bind to specific divalent metal (M 2+ ) ionic species in aqueous mixtures, forming negatively charged M–(EDTA) 2− complexes. By regulating the COFs membranes with appropriate pore size and intrapore charge density, the significant physicochemical differences between the M 2+ ions and the M–(EDTA) 2– complexes formed through the selective chelation can be leveraged to achieve their efficient separation. This enables the effective separation of Ni 2+ , Co 2+ , and Mn 2+ ions via membrane-based techniques. In this study, a two-step solvothermal–casting approach was employed to synthesize highly crystalline COF membranes with tunable content of the intrapore sulfonic acid (–SO 3 H) group. By modulating the –SO 3 H group content, the issue associated with the inherent trade-off between the M 2+ permeation flux and the M–(EDTA) 2− complex ion rejection efficiency could be overcome, and an optimal membrane separation performance was obtained. The synthesized membranes can selectively separate Ni 2+ , Co 2+ , and Mn 2+ in both binary and ternary systems through an EDTA-chelation-assisted dialysis process, which is remarkably stable under operational conditions. Furthermore, the membranes demonstrate superior M 2+ permeability compared with commercial cation-exchange membranes in EDTA-chelation-assisted electrodialysis. These findings provide valuable insights into the design of membrane structures for the effective separation and recovery of similar divalent transition metal ions (e.g., Ni 2+ , Co 2+ , and Mn 2+ ) in aqueous solutions.