On the mechanism of charge formation in nanofiltration membranes

The role of the fundamental phenomena determining the mechanism of charge formation in Nanofiltration membranes is presented. The study is performed through the development of a theoretical model, called “adsorption-amphoteric model” in which 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 phenomena considered are acid/base dissociation of the hydrophilic functional groups, counter-ions site binding and competitive adsorption of ions on the hydrophobic functional groups. The model is quite general and it is able to put in evidence which are the relevant phenomena occurring in determining the membrane charge as a function of the operative conditions. The model is applied to the case of Desal DK polyamide membranes for which a wide series of data is available, with binary aqueous solutions containing NaCl or CaCl2. In the pH range investigated, the membrane charge is the result of the opposite effects caused by counter-ion site-binding on dissociated hydrophilic sites and by ionic competitive adsorption on hydrophobic sites, which occurs according to a Langmuir-type mechanism. In the case of NaCl-water solutions, the membrane amphoteric behaviour, observed as a function of pH, is controlled by the competitive effect of chloride versus sodium adsorption on hydrophobic sites. In the case of CaCl2-water solutions, at high pH values, the competition between calcium site-binding on dissociated hydrophilic sites and chloride adsorption on hydrophobic sites determines the membrane amphoteric behaviour depending on the salt concentration. The most innovative aspect of the model is represented by the possibility to describe each phenomenon individually, which in turn allows to study the effect of each mechanism on the membrane charge formation. The effect of counter-ion site-binding is investigated in comparison with the competitive adsorption, at various operative conditions, for specific cases of aqueous solutions containing single symmetric (1:1, 2:2) and non-symmetric (1:2, 2:1) electrolytes; reference is made to a polyamide Desal-type membrane. With non-symmetric electrolytes, the behaviour of the total volume membrane charge vs. salt concentration is given by the relation existing between the multivalent ion valence and the proper charge sign. In the case in which the divalent ion is the co-ion, the membrane charge is directly related to the feed ionic strength. In the case in which the divalent ion is the counter-ion, on the contrary, the total membrane charge appears as an unimodal function of the salt composition and can change its sign: the amphoteric behaviour is greatly affected by the salt concentration.