Evaluation of structural and charge properties of polyamide nanofiltration membrane

Nanofiltration (NF) is a highly efficient membrane separation process which has various environmental and industrial applications. Nanofiltration membranes are finely porous membranes with specific interesting features, their characterization being important for understanding of the NF processes and for their prediction for practical use. A polyamide thin-film composite NF membrane (AFC 80) was characterized in the present work by two different techniques: modelling of the rejection of uncharged solutes and modelling of the salt rejection. The interpretation of the data from uncharged solutes rejection experiments by using Donnan steric partitioning pore model (DSPM) allows to determine the structural characteristics of the AFC 80 membrane, i.e., effective pore radius (rp), and thickness to porosity ratio ()x/Ak). The experimental data for sodium chloride rejection were used to calculate the effective fixed charge density (MX) by means of an irreversible thermodynamics model (Spiegler–Kedem model) and of a charge model, namely Teorell–Meyer–Sievers (TMS) model. It was found that the membrane charge depends on the salt concentration in solution, this behaviour being attributed to ion adsorption on the membrane. The dependence of the charge density on NaCl concentration obeys a Freundlich isotherm equation