Charge and separation characteristics of nanofiltration membrane embracing dissociated functional groups

The current work focused on the investigation of charge and separation characteristics of nanofiltration (NF) membrane embracing dissociated functional groups under different electrolyte solutions. The electro-kinetic method was carried out to assess the membrane volume charge density (X) with different salt concentrations ranging from 0.1 to 10 mol·m^-3 and different electrolyte species, such as type 1–1, type 2–1 and type 3–1. The Donnan steric pore model-dielectric exclusion (DSPM-DE) model was employed to evaluate the separation characteristics of the NF membrane for wide range of electrolyte concentration (from 25.7 to 598.9 mol·m^-3). The results indicated that the dissociation of the hydrophilic functional groups and the specific adsorption contributed to charge formation on membrane surface. The former played a dominant role in type 1–1 and type 2–1 electrolytes at dilute aqueous solutions (0.1–0.5 mol·m^-3). However, for type 3–1 electrolyte, specific adsorption should contribute to the charge effect to a large extent. Moreover, the correlation between the volume charge density and feed concentration was in accordance with Freundlich isotherm. Furthermore, it was found that the separation characteristic of NF membrane could be evaluated well by DSPM-DE model coupling with electro-kinetic method in a whole concentration range.