Facile preparation of COF-LZU1 modified membrane via electrostatic spraying for emerging contaminants treatment in direct contact membrane distillation

Emerging contaminants (ECs) in aqueous ecosystem are becoming a global threat to human beings, which are mainly discharged from industrial wastewater. Direct contact membrane distillation (DCMD) can utilize waste heat to treat industrial wastewater containing ECs, achieving a better balance between removal efficiency and operation cost than other technologies. However, membrane fouling is the major obstacle that impedes DCMD application. Therefore, anti-fouling covalent organic framework COF-LZU1/PVDF composite membranes were prepared via electrostatic spraying and used in DCMD for the first time. Four typical EC compounds (ibuprofen, acesulfame, sulfamethoxazole, and N,N-diethyl-3-methylbenzamide) were selected as targets to investigate the effect of water affinity and charge on membrane fouling. Results showed that COF-1.0 membrane with relatively hydrophilic COF-LZU1 structure exhibited higher permeate flux and salt rejection than commercial PVDF substrate when treating sulfamethoxazole and ibuprofen within pH range of 4-7 and temperature of 40-70 degrees C. This could be attributed to the weaker hydrophobicity and closer-to-neutral zeta potential of COF-1.0, which weakened the electrostatic and hydrophobic-hydrophobic interactions between membrane and ECs. In addition, the membrane performance comparison in temperature changing DCMD process further showed that COF-1.0 performed better in terms of permeate flux, especially in the gradually decreased temperature condition. Overall, these findings would advance the fundamental understanding and practical application of COF-based membranes in DCMD for ECs treatment.