Regulating post-treatment of polyamide nanofiltration membranes: The role of solvent polarity and miscibility on membrane performance

Solvent post-treatment is an effective strategy for regulating polyamide nanofiltration membranes. However, the regulatory mechanism behind this process requires further supplementation and improvement. In this study, different polar solvents (polarity order: acetonitrile > methanol > ethanol > acetone > n-butanol > n-hexane) were used for post-treatment of PEI nano-filtration membranes to compare and analyze the influence of solvent post-treatment on the performance of PEI nano-filtration membranes (flux, salt and heavy metal retention). Combined with the comprehensive physicochemical characterization (SEM, FTIR, XPS, contact angle), the regulatory mechanisms of different solvent post-treatments on PEI nanofiltration membranes were analyzed and summarized. The results revealed that the effect of solvent post-treatment on PEI-TMC polyamide is different from that of the MPD and PIP systems, possibly due to the larger molecular weight and acyclic molecular chain structure of PEI. Moreover, the solvent post-treatment mainly modulates the polyamide structure through two mechanisms: the swelling effect and the miscibility-promoted further reaction or hydrolysis. The former is the primary mechanism that determines the changes in separation properties. And the chemical structure of the polyamide surface is primarily influenced by the mutual dissolution promoted reaction or hydrolysis. This study improves the understanding of solvent post-treatment of interfacial polymerization and provides inspiration for performance modulation of positively charged PEI NF membranes.