Development of loose thin film nanofibrous composite nanofiltration membrane with modified g-C3N4 nanosheets barrier layer for efficient separation of salt/dye mixtures

To effectively separate salt/dye mixtures from textile wastewater, the development of nanofiltration (NF) membranes with stable and loose structure for high permeability is getting an urgent task. In this study, a novel loose thin film nanofibrous composite NF membrane was fabricated by filtration-assisted interfacial polymerization for dye/salt separation, in which two-dimensional g-C3N4 nanosheets (GNs) modified with polyethylenimine (PEI) were sturdily integrated together via reaction with organic phase trimesoyl chloride (TMC) to form a polyamide (PA) soldered GNs barrier layer onto polyacrylonitrile (PAN) nanofibrous substrate. Benefiting from the inimitable photocatalytic activity of GNs, the organic dyes adhered and accumulated on the membrane surface could be easily cleared up under visible light irradiation for the great recyclability of the composite membrane. The optimized robust GNs composite membrane possessed high water permeability (40.9 L m−2 h−1 bar−1), superior dyes rejections (>94%) for Congo Red, Methyl Blue and Reactive Black 5 and low salt rejections (<7%) for NaCl and Na2SO4 for excellent salt/dye separation performance (the dye/NaCl separation factor was up to 236). The resultant GNs barrier layer endowed the composite membrane with long-term stability and sustainability, the dye permeability could still recover to 93% of the initial membrane during three cycles of visible light irradiation. Therefore, this work provided a new solution for the design and development of loose NF membrane for the sustainable separation of salt/dye wastewater.