Fabrication of advanced positively charged polyamide nanofiltration membrane for effective removal of heavy metal ions via surfactant-plasticizer-synergy assisted interfacial polymerization

Positively charged nanofiltration (NF) membrane has shown great potential for the removal of heavy metal ions from contaminated water. In order to fabricate positively charged membranes, polyethyleneimine (PEI) is widely used considering the existence of abundant amine groups in this branched macromolecule. However, the PEI-based NF membranes often suffer from low permeance due to the highly dense polyamide film. In this study, a surfactant-plasticizer-synergy (SPS) assisted fabrication strategy is proposed to fabricate PEI-based NF membrane with high performance. The surfactant sodium dodecyl sulfate (SDS) and nontoxic plasticizer acetyl tributyl citrate (ATBC) are added into the aqueous phase and organic phase respectively to regulate the interfacial polymerization (IP) reaction between PEI and terephthaloyl chloride (TPC). As a result of the enhanced miscibility of the two phases at the presence of SDS, the diffusion of PEI monomer is promoted and this contributes to a positively charged membrane surface, which effectively improves the rejections of NF membrane towards positive ions. On the other hand, the plasticizer ATBC softens the polyamide film as confirmed by the reduced Young's modulus. This could enhance the flexibility of polymer chains and promote the transportation of water molecule. Compared with the nascent PEI-TPC membrane, the permeance of PEI/SDS + TPC/ATBC experiences a dramatic promotion from 2.5 L∙m−2∙h−1∙bar−1 to 9.7 L∙m−2∙h−1∙bar−1 and the MgCl2 rejection also increases from 95.9% to 97.8%. More importantly, the rejections of PEI/SDS + TPC/ATBC towards various heavy metal ions are all above 90.0% and they also follow the order: Mn2+(97.6%) > Ni2+(96.8%) > Cu2+(95.7%) > Cd2+(91.0%). This SPS assisted IP reaction may provide a simple and low-cost strategy for the fabrication of high-performance TFC membranes for diverse applications in the future.