Preparation of glycine mediated graphene oxide/g-C3N4 lamellar membranes for nanofiltration

In this work, we fabricated a series of composite membranes for nanofiltration by filtration-assisted assembly of graphene oxide (GO, with a thickness of ca. 2 nm) and graphitic carbon nitride (g-C3N4, with a thickness of ca. 5.2 nm) nanosheets with the assistance of glycine as a molecular linker to enhance their interactions. The effects of g-C3N4 and glycine concentrations on the lamellar spacing of GO and membrane performance were subsequently investigated. Interestingly, experimental and characterization results showed that both g-C3N4 and glycine could increase the interlayer spacing of pristine GO membranes, but the former reduced the dimensions of nanochannels while the latter led to enlarged channels. As compared with pristine GO membranes, the composite membranes resulted in faster water transportation without sacrificing solute retention. With further functionalization by introducing hyperbranched polyethyleneimine (HPEI) coating, we demonstrated that an integrated Gly-GO/g-C3N4 membrane (ca. 116 nm in thickness) exhibited excellent separation performance for various organic dye solutions under different operational conditions (feed concentration, pH, etc.). Long-term stability experiments showed that this membrane yielded 90%-93% dye rejection with only a slight decline in permeance over a 40 h testing period, indicating acceptable stability. The pure water permeance of the Gly-GO/g-C3N4 membrane reached as high as 207 L h(-1) m(-2) bar(-1). These results indicate that the covalent modification of small molecules can improve the packing of GO nanosheets and lead to outstanding nanofiltration performance which has great potential applications in the field of water purification and separation.