The role of the electrolyte on the mechanism of charge formation in polyamide nanofiltration membranes

The role of the electrolyte on the mechanism of charge formation in polyamide Desal-5 DK Nanofiltration membranes is studied by the application of the “adsorption-amphoteric” (ADS-AMF) model. Membrane performances with NaCl as well as with CaCl2-water solutions are compared. Basic equations of the ADS-AMF model are completely revised and a general structure of the model is introduced. The membrane is modelled through a site-representation as the sum of hydrophobic as well as of hydrophilic functional groups which can support charges derived from various chemical and physical mechanisms. The prevailing mechanisms in determining the volume membrane charge are considered as: the acid/base dissociation of the hydrophilic functional groups, the counter-ion site binding on the dissociated hydrophilic groups and the ionic competitive adsorption on the hydrophobic functional groups. A simple thermodynamic modelling of the phenomena is performed which allows to describe the effect of each single mechanism specifically. Typically, the total membrane charge is given by the contribution of a proper charge, which is screened by the dissolved ions giving rise to site-binding phenomena vs. the dissociated hydrophilic sites, and by the contribution of an adsorption charge located on the hydrophobic sites of the membrane. For each electrolyte solution, the membrane charges calculated through the elaboration of rejection data by the Donnan steric pore and dielectric exclusion model are used as reference values. In the case of NaCl-water solutions, the ionic competitive adsorption on hydrophobic sites according to a Langmuir-type mechanism is dominant with respect to the effects of counter-ion site binding. The competitive effect of chloride versus sodium adsorption on hydrophobic sites determines the amphoteric behaviour of the membrane; the points of zero charge are independent of salt concentration. In the case of CaCl2-water solutions, at low salt concentrations, the mechanism of charge formation is controlled by calcium site-binding on the dissociated hydrophilic sites, whereas, as the salt concentration increases, chloride adsorption on hydrophobic sites prevails. The competition between calcium site-binding and chloride adsorption determines the amphoteric behaviour of the membrane. The remarkable dependence of the points of zero charge on salt concentration is also well predicted.