Conductive thin-film nanocomposite nanofiltration membrane comprising N-doped graphene quantum dots with relieved concentration polarization for sulfate separation from high-salinity solution

Nanofiltration (NF) has been an environmentally-friendly technique for separating sulfate from concentrated seawater in the Chlor-alkali industry. However, the commercial thin-film composite (TFC) NF membranes suffered more deteriorated concentration polarization, membrane fouling and the perm-selectivity trade-off effect in practical application. Here, a thin-film nanocomposite (TFN) NF membrane with conductivity and low cross-linking degree was constructed by the synergistic manipulation of N-doped graphene quantum dots (N-GQDs), dopamine and piperazine on the nanostructure of active polyamide layer. With the high-salinity solution containing 15 g·L−1 Na2SO4 and 127 g·L−1 NaCl as feed solution, the well-designed TFN-NF membrane exhibited Na2SO4 rejection of 99.70% and achieved synchronous improvement in water permeability (173.02 L·m−2 h−1 MPa−1, 3.62 times of the pristine TFC membrane), water/Na2SO4 selectivity and NaCl/Na2SO4 selectivity under an electric field. After the cyclic humic acid fouling-regeneration process three times, it still possessed a water permeance recovery ratio (FRR) of 98.55%. Moreover, the improved wettability and higher ratio of quaternary amine groups (N+H3-C) derived from the N-GQDs covalently linked to the polyamide layer endowed it with excellent antibacterial activity. The multifunctional TFN-NF membrane demonstrated its potential application in high-salinity solution separation.