Positively Charged Poly(Piperazinamide) Nanofiltration Membranes for the Fast Removal of Metal Ions

Abstract Positively charged nanofiltration (NF) membranes with high perm‐selectivity are highly desired to remove divalent cations such as Ca2+ and Mg2+ ions to ensure the safety of domestic and industrial water supply. In this work, a poly(piperazinamide) NF membrane with a positively charged surface is fabricated via the surfactant‐assembly regulated interfacial polymerization (SARIP) of piperazine (PIP) and 1,3,5‐trimesoyl chloride (TMC). The surfactant assembly can accelerate PIP monomer into the hexane to react with TMC monomer, thus reducing the difference in the trans‐interface diffusion of PIP and the diffusion of TMC in hexane, forming an interfacial polymerization with a stoichiometric characteristic. By tuning the concentration ratio of PIP and TMC, a positively charged poly(piperazinamide) NF membrane will be achieved. At the same time, utilizing the superior hydrophilicity of bacterial cellulose nanofibers nanofilm, a crumpled poly(piperazinamide) active layer is successfully constructed, resulting in a permeating flux up to 22 L m−2 h−1 bar−1 and simultaneously with a MgCl2 rejection >96%. The performance of this positively charged poly(piperazinamide) NF membrane is superior to most other positive NF membranes for the removal of divalent cations so far.