Anionic covalent organic framework engineered high-performance polyamide membrane for divalent anions removal

Porous framework materials hold great promise in tuning the chemical and physical structure of polyamide membranes for efficient nanofiltration. In this work, anionic covalent organic framework (aCOF) with abundant sulfonic acid groups was embedded into polyamide matrix via interfacial polymerization to modulate both membrane charge property and thickness for the removal of divalent ions. Benefiting from the high negative charged density, aCOF not only enhanced the membrane electronegativity from −25.6 mV to −71.5 mV but also slowed down the diffusion rate of piperazine by electrostatic interaction to decrease the membrane thickness from 93 nm to 18 nm. The enhanced electronegativity can intensify the charge exclusion to divalent anions (Na₂SO₄ rejection above 97%), while the ultrathin structure endows membrane with high water permeance of up to 39 L m⁻² h⁻¹ bar⁻¹, about 2.4 times higher than that of pristine PA membrane. Our membranes provide a new path to rational design and controllable construction of high-performance nanofiltration membranes.