Dielectric analysis of multi-layer structure of nanofiltration membrane in electrolyte solutions: Ion penetrability, selectivity, and influence of pH

Dielectric measurements were carried out on the systems composed of NF membrane and dilute electrolyte with different concentration and 0.1 mol/m3 solutions with different pH. Double dielectric relaxations were observed in frequency range of 40 Hz–10 MHz. The two relaxations are caused by the interfacial polarization between the membrane and solution and the multi-layer structure of the membrane. A triple-layer-plane model was adopted to analyze the dielectric spectra. It was found that the electric properties change with the concentration and pH of electrolyte solution. The fixed charge density was estimated, and the ion permeations in both sub-layers were compared. The results were interpreted based on Donnan equilibrium and dielectric exclusion principle. The two sub-layers were then confirmed as a dense active layer and a porous support layer, respectively. Special attention was paid on the permeability and selectivity of the active layer. The results indicated that the active layer has the different solvation energy barrier for divalent and monovalent co-ion and the selectivity for co-ion penetrating into it. The permeability of the porous support layer, however, has no selectivity for different electrolytes, and electrolyte easily passes through this sub-layer. Moreover, the pH dependence of the dielectric spectra was also analyzed.

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► This research developed a skill to probe inner information of multilayer membrane.
► This information includes structural and electrical parameters of sub-layer membrane.
► The ion permeability and selectivity of each layer in membrane were discussed.
► The distribution of the co-ion in effective separation layer was calculated.