Theoretical models for the prediction of nanofiltration separation performance as a function of, e.g., pH and electrolyte composition require knowledge on the ion−surface adsorption chemistry. Adsorption parameters have been extracted from electrophoretic mobility measurements on a ceramic γ-alumina nanofiltration membrane material in aqueous solutions of NaCl, Na2SO4, and CaCl2, and literature potentiometric titration data on γ-alumina. Various adsorption reaction models and descriptions of the electrostatic double layer have been tested. The adsorption parameters are obtained using a 1-pK triple-layer model. The zeta potential data indicate that on this γ-alumina NaCl acts as an indifferent electrolyte, resulting in an isoelectric point of pH = 8.3. The data can be accurately described with the 1-pK triple-layer model. Furthermore, the surface charge model predictions are in good agreement with literature titration data for this 1:1 electrolyte. Strong adsorption of Ca2+ ions leads to positive zeta potentials over the entire concentration and pH range studied. The model is capable of fitting the potential data reasonably well. Strong adsorption of sulfate ions causes a shift of the isoelectric point to lower pH values. For a bulk concentration of 100 mol/m3 Na2SO4 only negative zeta potentials are observed.

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The electrolyte separation behaviour of a supported bi-layered ceramic membrane is investigated experimentally and the measured ion retentions are compared with the predictions of a site-binding transport model with no adjustable parameters. Due to the difference in iso-electric point between its two separating layers, the bi-layered system is expected to perform better over a large pH range compared with a membrane with only one type of selective layer. The separating layers in the membrane are a microporous silica and a mesoporous ␥-alumina (pore sizes of 0.8 and 2 nm, respectively) and their retention is studied for a binary electrolyte solution of NaCl at 1 mol/m 3 for pH values between 4 and 10. Because of its smaller pores and high charge, the silica layer mainly determines the membrane retention at neutral and alkaline pH, while the ␥-alumina layer has a significant impact on the NaCl retention at 4 < pH < 5. The model predictions are in good agreement with the experimental data for Na + at 4 < pH < 9 and for Cl − at the whole pH range. For a pH of 4, the predicted chloride retention is lower than the sodium retention while the experimental data show the opposite effect.

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