Zwitterionic cyclodextrin membrane with uniform subnanometre pores for high-efficient heavy metal ions removal

Polymer nanofiltration (NF) membranes have been widely applied in the desalination, resource substances purification and separation, and wastewater treatment. Nevertheless, direct construction of subnanometre channel polymer membranes for achieving high-efficient metal ions separation is still challenging because of the complicated membrane preparation and difficultly manipulating the membrane pore-size in sub-nanometer. Herein, we exploited a novel subnanometre channel membrane with zwitterionic β-cyclodextrins (ZCDs) by interfacial polymerization. Wherein ZCDs act as building blocks with hydrophobic inner subnanometre cavities and hydrophilic outer rims, allowing for simultaneously improved water permeability and metal ions selectivity. By modulating the zwitterionic monomer grafting ratio of ZCDs and their loading content in the membrane, the ZCDM membrane exhibits high water permeance (J = 21.7 L∙m−2 h−1∙bar−1) and heavy metal ions rejection, e.g., RCr3+ = 99.8%, RFe3+ = 98.9%, respectively, these values outperform the state-of-the-art NF membranes. Moreover, the ZCDM membrane has good stability and anti-fouling/-bacterial properties, e.g., the water flux recovery ratio and anti-bacterial ratio is >97% and >99%, in the long-term running process. This work offers a feasible strategy for constructing polymer membranes with uniform subnanometre pore, and demonstrates their potential for the heavy metal separation and wastewater treatment.