Coupling Activation and Curing to Tune the Permselectivity of Poly(Piperazine Amide) Nanofiltration Membranes

To tune both water transport and solute rejection (i.e., permselectivity) of nanofiltration (NF) membranes, polyamide (PA) films were synthesized under the same interfacial polymerization (IP) conditions following distinct posttreatments, and the underlying relationship between the PA structure and membrane performance was systematically investigated. The combination of hexane activation and water curing was the optimal posttreatment process, which significantly increased membrane permeability from 6.7 ± 0.4 to 19.8 ± 0.8 L·m−2·h−1·bar−1 and enhanced the selectivity of NaCl/Na2SO4 from 9.6 ± 1.2 to 45.8 ± 0.8. For this combined posttreatment process, the outstanding permeability was due to the enlarged PA film pore size, the enhanced hydrophilicity and the reduced PA thickness and crosslinking degree, and the high NaCl/Na2SO4 selectivity was attributed to the increased electrostatic repulsion between the hydrolyzed acyl chloride groups (i.e., carboxyl groups) and divalent anions. Furthermore, a continuous filtration of 10 days was implemented, and the permeation flux and salt rejection trends for the fabricated PA membranes were maintained at high levels, which indicated excellent long-term stability for the combined posttreatment process. The results of the current study are of paramount practical significance for the reconciliation of permeability and selectivity in water treatment and resource recovery from wastewater.