Deciphering the overlooked role of dielectric exclusion in the ion selectivity of nanofiltration membranes

In response to emerging environmental challenges and the growing demand for resource recycling, nanofiltration (NF) technology has evolved from primarily targeting solute/solvent separation to enabling solute/solute separation. This includes the effective recovery of phosphates while simultaneously rejecting emerging micropollutants (EMPs), ensuring the safe reuse of nutrients. Among the key mechanisms influencing solute rejection, steric exclusion and Donnan effects have garnered considerable attention, driving the rapid development of electroneutral or low-charge membranes. In contrast, dielectric effects have received limited scrutiny. This study investigates the interplay between ion partitioning mechanisms and the influence of feedwater pH on the rejection of solute species, including phosphate ions and EMPs, by preparing electroneutral membranes with varying pore sizes and isoelectric points spanning different pH ranges. Experimental data were fitted using the Donnan-Steric Pore Model with Dielectric Exclusion (DSPM-DE) and the Steric, Electric and Dielectric Exclusion (SEDE) models, enabling a comprehensive examination of how membrane pore size, charge characteristics, and ion properties influence ion partitioning and transmembrane processes. The SEDE model, which integrates both the Born effect and image forces within the dielectric effects framework, outperformed the DSPM-DE model in accurately predicting ion rejection, making it well-suited for designing and optimizing NF membranes in scenarios that require selective separation of multivalent ions from other solutes. These findings underscore the previously overlooked importance of dielectric exclusion, offering valuable insights for the future development of customized NF membranes with enhanced selectivity for nutrient recovery and broader applications in environmental and industrial water treatment.