On the Amphoteric behavior of Desal DK nanofiltration membranes at low salt concentrations

Tangential streaming potential measurements are performed along Desal DK membranes in contact with dilute aqueous solutions containing 1:1 or 2:1 electrolytes at pH values kept at 3 and 6. Salt concentrations in the range from 0.1 to 10 mol/m3 are investigated. Measurements are carried out according to the extrapolation method that consists in determining the zeta potential from measurements of the pressure-induced electrical potential difference across membrane channels of various heights. The zeta potential data are elaborated according to the Gouy-Chapman theory of flat interfaces and the corresponding volumetric membrane charge values are calculated assuming slit-like pore geometry. Experimental charge densities show an unimodal behavior for both electrolytes in the range of concentration under consideration. The behavior can be explained by the Adsorption Amphoteric Model, in which the mechanism of charge formation is related to the acid-base dissociation of hydrophilic sites, to counterion site-binding, as well as to the competitive adsorption onto hydrophobic sites. The model is able to describe the experimental trend obtained at both pH values. At low salt concentrations, the behavior results from the combination of the counterion site-binding on hydrophilic surface sites (compensating the proper charge of the membrane) and of the competitive adsorption of both coions and counterions on hydrophobic surface sites.