Cationic covalent organic framework membranes for efficient dye/salt separation

Wastewater containing organic dyes and high content of salts has posed a serious challenge on global water security. Herein, a cationic nanoporous covalent organic framework (COF) membrane is constructed via a facile interfacial growth strategy. The prepared membranes show excellent dye/salt separation performance with a pure water flux above 24 L m−2 h−1 bar−1. The rejection rate of methylene blue of positive charges is above 99%, while the rejection rates of NaCl, Na2SO4, MgSO4 are only 3.2%, 2.6%, and 9.4%, respectively. The high rejection of methylene blue is mainly governed by the electrostatic repulsion between cationic COF channels and the positively charged dye. Moreover, the size sieving effect dominates the rejection of dyes with larger molecular sizes than that of the COF channel. These findings provide strategies for developing various nanoporous COF membranes for efficient dye/salt separation.