Modelling the mechanism of charge formation in NF membranes: Theory and application

Model validation is performed for the case of Desal DK membranes, with reference to NaCl-water solutions. In this case the general model is simplified and a relationship, containing five adjustable parameters, is obtained in which the volume membrane charge is a function of the feed bulk electrolyte concentration and pH. Electrolyte adsorption on hydrophobic sites according to a Langmuir-type mechanism is highly dominant with respect to the effect of counter-ion site-binding on the charged hydrophilic groups. The pH effect on the adsorption charge is also remarkable; it decreases the net negative charge located on the hydrophobic sites with respect to the values existing at higher pH's. The model is able to predict with good accuracy both the membrane charge behaviour versus electrolyte concentration and the sign change; prediction of points of zero charge is remarkably satisfactory.