Polyesteramide loose nanofiltration membranes from p-hydroxyphenylpiperazine: tunable structure for dye desalination

The microstructure of polymer membranes critically governs two key performance parameters, namely permeance and selectivity, and thus directly influencing overall separation efficiency. Novel loose nanofiltration (LNF) membranes were prepared by interfacial polymerization using p -hydroxyphenylpiperazine (HPZ), a nonplanar aqueous phase monomer containing phenolic hydroxyl and secondary amine groups. By modulating NaOH additive concentrations, hierarchical tuning of the separation layer's microstructure was achieved by altering ester-amide condensation-hydrolysis equilibria. The optimized HPZ@0.33 membrane (0.33 wt% NaOH) demonstrated exceptional selectivity with a Congo red/NaCl separation factor of 210 and a permeance of 40 L m −2 h −1 ·bar −1 . Increasing NaOH concentration to 0.50 wt% induced ester bond hydrolysis, expanding pore radii and reducing layer thickness. This yielded the HPZ@0.50 membrane with ultrahigh permeance (220 L m −2 h −1 ·bar −1 ) while maintaining 98.65% Congo red rejection. Due to the twisted conformation and rigid benzene ring of HPZ, the as-prepared separation layer had a high porosity and improved the pore interconnection, enabling it to operate stably at low pressure (0.5-2 bar). Furthermore, dual-membrane coupling technology effectively desalinated high-salinity dye wastewater. This study provides insights into mitigating the permeability-selectivity trade-off in membrane design, offering a potentially scalable and cost-effective strategy to advance resource recovery in sustainable water management systems.