Multi-scale insights into nanofiltration ion selectivity: Effects of operating conditions on membrane structure and performance

Nanofiltration (NF) can help address the growing need for sustainable water treatment and wastewater resource recovery, yet its ion-selectivity mechanisms remain poorly understood under varying pH and pressure. Here, we integrate experimental data, DSPM-DE modeling, and ab initio molecular dynamics (AIMD) to investigate Na + /Mg 2+ and Cl − /SO 4 2− separation by three NF membranes: commercial NF270, NF90, and a custom piperazine-based (PIP) membrane. We find that extreme acidic or alkaline feed conditions expand the PIP membrane pore radius by up to 60 ​% while raising volumetric charge density by 4–5 times, enabling divalent-ion retention to improve even at higher flux. When multiple ions coexist, cation or anion selectivity values hinge on these pH- and pressure-driven changes in pore structure and charge. By capturing dynamic ionization within the membrane, we gain a deeper and more accurate perspective on how molecular-level properties evolve under high salinity, multi-ion competition, and extreme pH, those scenarios are common during sustainable water and wastewater treatment. The results indicate that controlled pH or pressure modulation can be harnessed to enhance divalent-ion retention, optimize NF performance, and reduce overall energy demands. By linking macroscopic modeling with molecular-scale insights, this study provides a mechanistic blueprint for refining NF design and operation, offering a pathway to next-generation high-selectivity membranes for water treatment, wastewater reclamation, and resource recovery. • Multiscale modeling clarifies NF selectivity at the molecular level. • Pore size expands by 60 ​% and membrane charge spikes by 5 times under pH extremes. • Water permeability and ion selectivity simultaneously enhance in acid/base conditions. • Pressure elevation aids anion selectivity; cation selectivity is condition-based. • pH-driven morphological shifts allow selective extraction with minimal chemicals.