Studies on the role of site-binding and competitive adsorption in determining the charge of nanofiltration membranes

The “adsorption Á amphoteric” model is used to investigate the role of the main phenomena determining the mechanism of the membrane charge formation in nanofiltration membranes. The prevailing mechanisms considered are: acid/base dissociation of the hydrophilic groups of the membrane, counter-ion site-binding on dissociated hydrophilic sites and competitive adsorption of ions on the hydrophobic groups of the membrane. 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, and by the contribution of an adsorption charge located on the hydrophobic sites. Aqueous solutions containing single symmetric (1:1, 2:2) as well as non-symmetric (1:2, 2:1) electrolytes are studied, by a simulation of the performances of a polyamide Desal-type membrane, taken as a reference, as a function of pH and salt concentration. The role of counter-ion site-binding on the total membrane charge is investigated in comparison with competitive adsorption. A sensitivity analysis is performed in which the overall effect of competitive adsorption is studied. With symmetric salts, the membrane charge is determined by the competitive adsorption on hydrophobic sites. With non-symmetric salts, on the contrary, the behaviour of the total volume membrane charge as a function of salt concentration is given by the relative effects of counter-ion site-binding vs. competitive adsorption.

In this paper, the “adsorption Á amphoteric” model has been used to describe the fundamental phenomena determining the mechanism of charge formation in NF membranes and to investigate the relative role of each of them. The effect of counter-ion site-binding has been investigated in comparison with the competitive adsorption; in addition, a sensitivity analysis has been performed in which the overall effect of competitive adsorption has been studied on the total membrane charge. Aqueous solutions containing single symmetric (1:1, 2:2) as well as non-symmetric (1:2, 2:1) electrolytes have been considered, simulating the performances of a polyamide Desal-type membrane, taken as a reference, as a function of pH and salt concentration. In the cases in which symmetric salts are processed, as the salt concentration increases, the total membrane charge approaches the adsorption charge located on the hydrophobic sites. Since the proper charge of the membrane is screened by site-binding effects at the same extent, the absolute value of the total membrane charge is directly related to the feed ionic strength. The amphoteric behaviour is always the result of the competitive effect of anion versus cation adsorption on hydrophobic sites and it depends only on the feed pH. In the case in which non-symmetric salts are considered, we can observe that the behaviour of the total volume membrane charge as a function of the salt concentration is given by the relation between the multivalent ion valence and the proper charge sign. In the case in which the divalent ion is the co-ion, the total membrane charge shows a monotone behaviour with the salt concentration; on the contrary, in the case in which the divalent ion is the counter-ion, the total membrane charge is obtained as a unimodal function. In addition, the total membrane charge can assume a monotone behaviour or alternatively a unimodal behaviour, depending on the relative values used for the competitive adsorption parameters. In the case in which site-binding and competitive adsorption are co-current in determining the membrane charge, the total volume membrane charge is directly related to the feed ionic strength; on the other hand, in the case in which competitive adsorption effects are opposite to site-binding phenomena, a unimodal trend is obtained and the amphoteric behaviour of the membrane may depend on salt concentration.