Cationic COF nanosheets engineered positively charged polyamide membranes for efficient divalent cations removal

Ionic covalent organic frameworks (COFs) hold great promise in regulating the structure of polyamide nanofiltration membranes for divalent ions removal. In this work, we demonstrate cationic COF (cCOF) nanosheets engineered polyamide membrane with excellent divalent cations rejection. cCOF nanosheets as versatile regulator and an interlayer accelerate the self-sealing and self-termination of interfacial polymerization between polyethyleneimine and trimesoyl chloride, endowing membranes with dense network and small thickness. Meanwhile, the quaternary ammonium groups on cCOF nanosheets increase positive charges on membranes. Through synergistic intensification of electrostatic repulsion and size sieving, the resulting cCOF-PA membranes display excellent rejection for divalent cations (MgCl2, 99.3%), superior to most reported polyethyleneimine-based polyamide membranes. This work highlights the significance of ionic COF materials in developing positively charged membranes for efficient ionic and molecular separations.