Tailoring the surface charge density of polyamide nanofiltration membrane for efficient emerging micropollutant separation

Active layered modification of thin-film composite (TFC) nanofiltration·(NF) membranes has enhanced the rejection performance of emerging micropollutants (EMPs) in water treatment applications. However, the influence of surface charge density on the NF membrane has been overlooked when evaluating the efficacy of tailoring membrane performance. In this study, five distinct membranes of NF-PEI₁, NF-PEI₂, pristine NF, NF-SA, and NF-PSS with continuously tuned charge intensity from positive, neutral to negative were fabricated by introducing polyelectrolytes into the active layer. The addition of polyelectrolytes slightly decreased the pore size of membranes while increasing the water flux. In general, the modified membranes with positive and negative charges exhibited an increase of rejection from 66–85 % to 75–97 % for a variety of EMPs and 5–17 % to 7–44 % for salts, with the rejection improving as the surface charge intensity of NF-X membranes increased. Notably, the selectivity of charged EMPs and salts improved significantly from 5 to 11 of NF membrane to 12–32 of NF-PEI₁ and NF-PSS membranes, while the selectivity of neutral EMP and salts remained unchanged. That demonstrated the significant role of Donnan interaction in enhancing the rejection of EMPs and the selectivity of EMPs/salts. The NF-X membranes were promising for separating EMPs and salts in applications such as textile wastewater treatment, antibiotic purification, and aquiculture seawater treatment. This study provided valuable insights into the separation mechanism of NF membranes with charged active layers for removing emerging pollutants, highlighting the synergistic interplay of the Donnan effect and size exclusion for efficient treatment of emerging pollutants.