Dominant Mechanism of Nanofiltration for Chloride/Sulfate Ion Separation in High Salinity Solutions: The Quantification of Pore Size-Influenced Dielectric Exclusion

Nanofiltration membranes attract extensive attention in solute selective separation, especially in resource extraction and recovery. A prevalent strategy to enhance the monovalent and multivalent ion selective separation performance involves modifying the membrane surface charge properties to influence the Donnan exclusion. However, the counterion adsorption and shielding effects are aggravated with increasing ionic strength, which severely weaken the Donnan exclusion. This study revealed that the contribution of Donnan exclusion was fairly moderate to SO₄^2- rejection in high salinity solutions, while it was dielectric exclusion that exerted the most important influence on Cl^-/SO₄^2- selective separation with a pore radius at 0.35-0.44 nm (molecular weight cutoff at 180-300 Da). Consequently, we proposed that tailored design of nanofiltration membranes with a precise pore radius to fully utilize the steric and dielectric exclusion instead of increasing membrane charge density is more crucial for monovalent/multivalent ion selective separation in high salinity solutions. Overall, our study reveals the importance of dielectric exclusion and provides new insights into nanofiltration membrane customization and application for ion selective separation in high salinity solutions.