Post-grafting has been commonly used to modify NF membranes and reduce fouling tendency by adjusting their hydrophilicity and charged properties. However, surface grafting often results in significant pore narrowing or blocking, leading to a loss in permeability. In addition, monocharged surfaces struggle to resist mixed foulants with different charges.
"In our strategy, we used an ionic liquid (IL)-ethanol (EtOH) to induce the deep grafting of polyethyleneimine (PEI) and rearrange the polyamide layer, improving the surface and aperture properties of the NF membrane," Prof. WAN explained.
The proposed system utilized the strong affinity of the IL to the polyamide layer and the low diffusion steric hindrance of EtOH to induce polyamide swelling and achieve deep grafting of PEI. This also allowed for targeted filling of larger pores, resulting in a narrower pore size distribution. The deep grafting of amino groups made the polyamide layer highly hydrophilic and nearly charge-neutral, thereby maintaining excellent antifouling ability against hydrophobic macromolecules and mixed charged molecules.
Furthermore, the NF membrane demonstrated remarkable long-term antifouling and separation selectivity stability when tested with coking wastewater and molasses, respectively. Its performance was comparable to NF270.
"This study highlights the possibility of using IL-induced deep grafting of polyelectrolytes to improve the antifouling performance of NF membranes," said Prof. LUO Jianquan, the corresponding author of the study. "It underscores the significance of solvent properties in grafting behavior, and provides a versatile platform for the fabrication of advanced NF membranes for wastewater treatment and resource recovery."