The role of the electrolyte on the mechanism of charge formation in polyamide NF membranes: NaCl and CaCl2 solutions in comparison

In the case of electrolyte solutions charge formation on the Nanofiltration membrane surface is relevant in determining rejection properties. Most of NF membranes show amphoteric behaviour. In this work the “adsorption-amphoteric” model (ADS-AMF model) is considered, in which the membrane is modelled through a site-representation as the sum of hydrophobic as well as of hydrophilic groups which can support charges derived from acid/base ionization, ions adsorption and counter-ions site-binding. The pseudo-chemical reactions approach is used to describe both chemical and physical phenomena. The model is applied to the case of polyamide/polysulfone Desal DK membranes, in order to understand which phenomenon is dominant in determining its amphoteric behaviour. Parameters calculation is performed on volume membrane charge data obtained in previous papers, from the elaboration of experimental rejections data through the Donnan Steric Pore Model and Dielectric Exclusion developed by the same authors.