Ion adsorption parameters determined from zeta potential and titration data for a gamma-alumina nanofiltration membrane

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 -alumina nanofiltration membrane material in aqueous solutions of NaCl, Na2SO4, and CaCl2, and literature potentiometric titration data on -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 -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.