High-flux nanofiltration membranes tailored by bio-inspired co-deposition of hydrophilic g-C3N4 nanosheets for enhanced selectivity towards organics and salts
Surface modification with advanced nanomaterials (i.e., 2D nanosheets) can be used to strategically tailor membrane properties, providing improved solute permselectivity to targeted molecules. In particular, 2D graphite-like carbon nitride (g-C3N4) nanosheets are a promising alternative for membrane modification, due to their exceptional physicochemical properties and facile synthesis. Herein, high-flux nanofiltration (NF) membranes were designed using bio-inspired co-deposition of hydrophilic g-C3N4 nanosheets with a polydopamine (PDA)/polyethylenimine (PEI) layer onto porous ultrafiltration (UF) substrates. The g-C3N4 nanosheets created additional nanochannels in the PDA/PEI layer to facilitate water molecule transport, resulting in high permeability (28.4 ± 1.2 L m−2 h−1 bar−1). Particularly, the bio-inspired layer structure was tailored from the UF to the NF (592 Da) scale by incorporating g-C3N4 nanosheets, thereby breaking through the permeability–selectivity trade-off effect. The tailored NF membrane enabled ultrahigh retention of three reactive dyes (610–630 Da, >99.3%) and low salt rejection (2.9% for NaCl; 7.6% for Na2SO4), significantly promoting the fractionation of dyes and salts for dye desalination. Additionally, the hydrophilic g-C3N4 nanosheets with oxygen plasma treatment further enhanced the wettability of the membrane surfaces, resulting in a superior antifouling performance. This study indicates the promise of g-C3N4 nanosheets to engineer high-flux NF membranes with desirable fractionation performance for sustainable treatment of highly saline wastewater.