Recent advances in covalent organic framework-based membranes for water purification: Insights into separation mechanisms and applications

With the accelerating steps of urbanization, the freshwater scarcity has been emerging as a serious threat to sustainable economic and social development. Under this circumstance, increasing attention is shifting to alternative water sources, such as wastewater and seawater, to harvest the embedded freshwater for supplement. Membrane technology has been predominately applied in this scenario due to its huge advantages in separation efficiency and economic investment. However, the application, especially with polyamide (PA) composite membranes, is significantly limited by the unsatisfied water flux when reclaiming high-quality freshwater. Recently, covalent organic framework (COF)-based membranes with pure COF or COF-incorporated PA as the selective layer exhibited great promise in alleviating this current dilemma benefiting from its tunable aperture, regular channels and desirable hydrophilicity. Unfortunately, relative research works were mainly focusing on the structure–function relationship between COF and the resultant membrane. In the meantime, the accompanied reviews were derived mainly from summarizing membrane preparation processes and practical applications while paid insufficient attention in the separation mechanisms of the COF-based membranes, which are highly desired for directionally optimizing membrane performance from the theoretical perspective. Herein, the mechanisms involved in water purification through COF-based membranes are systematically investigated, especially in a novel view of adsorption. In addition, the up-to-date synthesis strategies of high-performance COF-based membranes are highlighted. Besides, the latest advances in structure-performance relationship of emerging COF-based membranes for wastewater purification and saltwater desalination are also described in this paper. This review aims to provide theoretical guidance and potential methods for design and preparation of next-generation COF-based membranes with higher-performance for high-quality recovery of freshwater.