Effect of piperazine-based monomer structure on nanofiltration membrane selectivity: A DSPM-DE analysis of nitrate transport

Nitrate contamination in water supplies poses significant environmental and public health risks, necessitating the development of efficient nanofiltration (NF) membranes. We investigate the impact of two structurally distinct piperazine-based monomers (aminoethylpiperazine (AEP) and 1,4-bis(3-aminopropyl)piperazine (DAP)) on the morphology and nitrate separation performance of thin-film composite (TFC) NF membranes. We fabricated ten membranes via interfacial polymerization by systematically varying the concentrations of AEP or DAP to achieve distinct physicochemical characteristics. Their separation performance was evaluated through water permeance, poly(ethylene glycol) (PEG 200) rejection, and salt retention tests. The Donnan Steric Pore Model with Dielectric Exclusion (DSPM–DE) was employed to understand the underlying ion rejection and transport mechanisms. Compared to AEP-based membranes, DAP-based membranes within the tested concentration range yielded thicker and more open structures with lower net charge density, leading to higher flux but lower nitrate selectivity. In contrast, AEP-based membranes often yielded thinner layers and stronger positive charge densities, enhancing anion partitioning. Negative nitrate retention was observed for some membranes when the net membrane charge density was close to zero. These findings underscore the crucial role of monomer structure and concentration in controlling effective pore size, charge density, and ion transport. This study provides mechanistic insights into the flux–selectivity trade-off, guiding the design of NF membranes through structurally distinct piperazine-based monomers for selective nitrate removal.