Dual-activation interfacial polymerization based anionic covalent organic framework nanofiltration membrane for high-flux dye separation

Covalent organic frameworks (COFs) hold great promise in constructing a new-generation of molecular separation membranes. Significantly, ionic COF membranes endow novel functions distinct from those of neutral membranes. However, ionic COF membranes generally suffer from extremely slow reaction rates and low yields, limiting their widespread application. In this work, a self-standing anionic COF membrane was prepared by a dual-activation interfacial polymerization strategy. And the obtained anionic COF membrane possessed a unique ordered pore structure, superhydrophilicity, as well as good chemical stability. Meanwhile, the as-prepared membrane provided the high-efficiency water transport pathways with effective rejection for dye molecules. As expected, the anionic COF membrane exhibited an excellent water permeability of about 270 L m−2 h−1 bar−1, which was much higher than those of commercial nanofiltration membranes, and also approximately 2–15 times significantly higher than those reported for polymer- or neutral COF-based nanofiltration membranes with similar rejection. Owing to the electrostatic attraction between positively charged dyes and anionic COF channels, the rejection rates of cationic dyes were above 96 %. In addition, the size-exclusion effect dominated rejection efficiency of dyes with molecular sizes larger than COF channels. These findings provided a new candidate membrane for efficient dye wastewater treatment.