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

Abstract

A general simple versatile “adsorption–amphoteric” model is developed to describe the mechanism of membrane charge formation in nanofiltration membranes. The membrane is schematized with a catalyst-type site representation in which two different groups of sites are considered related to hydrophilic and hydrophobic functional groups existing on the membrane material. Each site supports charges derived from different mechanisms. Hydrophilic sites are involved in acid/base dissociation as well as in counter-ions site binding, whereas hydrophobic sites support charges derived from competitive adsorption between anions and cations. The pseudo-chemical reactions approach is used to describe the phenomena, including competitive adsorption. The model is structured to take into account of different chemical and electrochemical interfacial effects; the role of each mechanism can be studied separately and the prevailing contribution can be determined case by case. Amphoteric behaviour is also predicted. 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.