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

Positively charged nanofiltration (NF) membranes with high perm-selectivity are highly desired to remove divalent cations such as Ca 2+ and Mg 2+ 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 MgCl 2 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.