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 con-tained in wastewater. Nevertheless, positively charged NF membranes possessing both high permeance and re-jections 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 char-acteristics 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 m2 h1 bar1 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