Post-synthesis of a covalent organic framework nanofiltration membrane for highly efficient water treatment

Nanofiltration (NF) membranes with ultrahigh water flux and high ion rejection are substantially beneficial for desalination and wastewater treatment. However, synthesis of NF membranes with high water permeance while maintaining high ion rejection remains a great challenge. Herein, we report a highly stable covalent organic framework (COF) IISERP-COOH-COF1 membrane with high ion rejection and relatively high water flux. Through post-modification, the pore aperture of the IISERP-COOH-COF1 membrane can be constricted, thus preventing ions from accessing the pores. Further, covalent post-functionalization is helpful to reduce non-selective transport through invisible intercrystalline defects, thus enhancing salt rejection. Demonstrated by both experimental and simulation studies, the IISERP-COOH-COF1 membrane shows superior ion rejection (e.g., 96.3% for Na₂SO₄, 97.2% for MgSO₄, 99.6% for FeCl₃, 90.6% for MgCl₂, and 82.9% for NaCl) based on size exclusion, with a water flux above 0.5