Defect-enabled covalent hydrophilization reconciles high permeability and antifouling in covalent organic framework membranes

Covalent organic framework (COF) nanofiltration membranes exhibit intrinsically high permeability and considerable potential for dye/salt separation in printing and dyeing wastewater treatment. However, severe membrane fouling under high-flux operation remains a major obstacle to their practical application. Herein, a defect engineering–enabled surface hydrophilization strategy is proposed to substantially enhance antifouling performance while preserving high permeation selectivity. Tunable free amino groups are incorporated into the COF framework through controlled defect engineering and subsequently serve as anchoring sites for the covalent grafting of hydroxyl-rich gluconic acid chains via EDC/NHS-mediated coupling. Precise regulation of defect content allows accurate control over the grafting density of hydrophilic chains, facilitating the formation of a stable hydration layer on the membrane surface through hydrogen-bonding interactions with water. This hydrated interface effectively suppresses foulant adhesion without sacrificing the intrinsic permeability of the COF membrane. The optimized dCOF-40-OH membrane delivers a high water permeance of 123.8 L m⁻² h⁻¹·bar⁻¹. It exhibits markedly enhanced antifouling performance against pump oil emulsions, achieving a flux recovery ratio of 96.4%, a total flux decline ratio of 11.8%, and an irreversible flux decline ratio of 8.2%. Meanwhile, the membranes exhibit excellent dye/salt separation performance for various dyeing saline solutions, among them the separation factors for Eriochrome Black T (EBT)/NaCl and EBT/Na₂SO₄ reach up to186 and 168, respectively. This work provides a rational and effective strategy for reconciling high permeability, robust antifouling capability, and efficient dye/salt separation in COF-based nanofiltration membranes.