The role of the electrolyte on polyamide NF membranes performances: experimental analysis with NaCl and CaCl2 solutions

Nanofiltration separation efficiency depends on the membrane material as well as on the chemical nature of the process solutions. In the case of electrolyte solutions the membrane performance is greatly affected by the type of the electrolyte used. In the case of single symmetric salts, such as NaCl, rejection generally decreases as the concentration increases at constant pH values, whereas rejection goes through a minimum value as feed pH increases. In the case of non-symmetric electrolytes, on the contrary, chemical interactions with the membrane can be relevant and different opposite trends are often observed. In this work, the role of pH and concentration is studied on the ion rejection of Desal-DK polyamide membranes, in the case of aqueous binary mixtures containing NaCl or CaCl2, in order to put in evidence the different role of the electrolyte on the membrane performance. The behaviours observed in the membrane rejection put in evidence that the chemical nature of the electrolyte can make the difference; in this case calcium interactions with the membrane material are different from those existing with sodium, and might not be only related to the different ionic strengths in the mixtures. In other words, specific adsorption of ions can occur on the membrane surface and it can greatly affect the corresponding membrane charge values.