Ion adsorption on nanofiltration membrane surface and its effect on rejection of charged solutes: A zeta potential approach

The net charge on nanofiltration (NF) membrane surface, which differs from the inherent fixed charges due to the adsorption of ions from the feed water, has a great effect on the rejection selectivity of the membrane. In this study, by taking advantage of the electric double layer theory, streaming zeta potentials of the membrane varying with solution composition and concentration were utilized to investigate the characteristics and mechanisms of ion adsorption on the typical negatively charged NF membrane surface, which were then utilized to interpret the (selective) rejection behaviors for different solutes. Results showed that the cation adsorption all followed the Langmuir isotherm, indicative of monolayer adsorption mechanism. Both Langmuir adsorption fitting results and Gibbs free energy calculation showed that the cation adsorption is primarily of Coulomb force origin, rather than the chemical complexation. In addition, Ca2+ and Mg2+ could occupy the membrane surface much more facilely than Na+, with the adsorption of divalent cations reaching near saturation at relatively low concentrations in the magnitude of 1 mmol L-1. Contrary to the original presumption, no remarkable difference was observed between the adsorptions of Ca2+ and Mg2+, as was also the case between -COOH and -SO3H, the two common types of acidic functional groups on membrane surface for cation adsorption. These mechanisms of ion adsorption, when incorporated with the existing NF theory, could well interpret the seemingly "strange" rejection behaviors including the counter-ion competition effect. This study evidenced that zeta potential measurements could act as a useful approach for the study of ion adsorption on NF membrane surface and selective rejection mechanisms of NF membranes as well.