Gradient-charged cellulose acetate membranes enabled by ionic COF nanosheets for enhanced nanofiltration-based desalination

Cellulose acetate (CA), a biomass-derived material for desalination membranes, faces challenges in nanofiltration-based desalination due to its weakly charged hydroxyl groups. Here we report gradient-charged CA membranes (GC-CAMs) engineered through the surface segregation of ionic covalent organic framework nanosheets (iCOFNs). Pre-evaporation drives sulfonated iCOFNs to migrate from the bulk to the surface due to the density difference between the nanosheets and CA solution, which creates a gradient distribution of iCOFNs and forms GC-CAMs via phase inversion. The charged nanosheets enriched on the front surface increase the membrane charge density from −0.89 to −1.13 mC m −2 while decreasing membrane pore size from 0.74 to 0.68 nm, synergistically enhancing ion exclusion via electrostatic repulsion and size sieving. By positioning the front membrane surface downstream, salt rejection is further improved by mitigating the charge shielding effect. By regulating the pre-evaporation time and iCOFN content, the optimized GC-CAM achieves a Na 2 SO 4 rejection of ∼95 %, which is maintained at ∼90 % over two months. When applied to natural water purification, it reduces total dissolved solids by ∼83 % while moderately removing heavy metal ions. The proposed gradient structure offers a novel approach for the charge engineering of nanofiltration membranes.