Highly permeable positively charged nanofiltration membranes with multilayer structures for multiple heavy metal removals

Positively charged nanofiltration (NF) membranes have been vitally employed to treat heavy metal ions contained in wastewater. Nevertheless, positively charged NF membranes possessing both high permeance and rejections continue to be a bottleneck in engineering applications owing to the trade-off between rejection and permeance. Herein, a highly permeable and positively charged NF membrane with multilayer structures was prepared using a chitosan interlayer-assisted low-temperature interfacial polymerization and surface grafting polyethyleneimine (CIL-IP-P) strategy. The chemical components, surface morphologies, and separation characteristics of the prepared NF membranes were investigated systematically. The construction of a chitosan interlayer increased hydrophilicity and influenced the IP reaction by modulating the monomer diffusion. Grafting polyethyleneimine (PEI) further resulted in NF membranes with higher electro-positivity, larger specific surface area, and narrower average pore diameter. Hence, the trade-off between rejection and permeance in conventional positively charged NF membranes was surmounted. An excellent water permeance of 10.9 ± 0.6 L m−2 h−1 bar−1 and a satisfactory rejection for heavy metals (93.3, 92.8, 91.0, 90.4, and 89.2 % for Ni2+, Mn2+, Cu2+, Cd2+, and Pb2+) were observed. The CIL-IP-P strategy provided in this study is promising to tailor a high-performance positively charged NF membrane for fast heavy metal removals. [Display omitted] •Synergistic CS interlayer and PEI grafting for fabricating highly permeable positively charged NF membranes•NF membranes exhibited a higher positive charge and greater hydrophilicity.•Narrower pore sizes and higher filtration areas were achieved with the crumpled NF membranes.•Highly efficient NF membranes for multiple heavy metal removals were constructed.•The multilayer-structured NF membranes are promising for ultrafast heavy metal removals.