Dielectric analysis of nanofiltration membrane in electrolyte solutions: Influences of permittivity of wet membrane and volume charge density on ion permeability

Dielectric measurements were carried out for the systems composed of nanofiltration membranes and electrolyte solutions over a frequency range of 40 Hz to 10 MHz, and double dielectric relaxations were observed at about 102–106 Hz. Based on the interfacial polarization theory, the dielectric relaxation at high frequency was analyzed theoretically, and the permittivity and conductivity of the membrane and electrolyte solutions, were obtained. By using the permittivity of wet membrane, the ion solvation energy barrier in eight electrolyte solutions was calculated. The results showed that the ion solvation energy barrier of 2-2 type of electrolyte solution was the biggest among all types of electrolyte. The influence of solvation energy barrier on separation performance was also evaluated. Because the permittivity of wet membrane takes the place of that of dry membrane, the calculations can ensure more accurate results close to practical separation process. Furthermore, the variation of the ratio of membrane/solution conductivity, κm/κw, with the electrolyte concentration was explained by Donnan equilibrium theory, and the ratio together with the TMS model was used to calculate the volume charge density of membrane. On the basis of this, by using the volume charge density of membrane and the existing transport models, such as TMS, TMS-DE, DSPM model, four transport parameters were calculated. It was concluded that in the process of ion permeation through the membrane, Donnan exclusion is the most important influencing factor and steric effect is secondary in low electrolyte concentration, whereas dielectric exclusion plays a dominant role in high electrolyte concentration.