Investigation of the structural and charge properties of a polyamide nanofiltration membrane

Abstract

Nanofiltration (NF) membrane had gained interest worldwide due to the excellent removal of contaminant(s) with lower energy consumption compared to reverse osmosis. NF is used for several environmental applications; for example, pharmaceutical and biotechnology, or desalination. The fundamental principle behind the separation mechanism of nanofiltration can be fully understood by defining the structural and charge properties of the membrane. In the present work, a polyamide thin-film composite NF membrane (AFC 30) was used for characterization of the structural and charge surface properties. The structural properties are the pore radius (rp) and the membrane thickness-to-porosity ratio ()x/Ak), respectively. The interpretation of values from the different uncharged solutes rejection allows one to determine the structural properties of AFC 30. This was done by using modelling of rejection of the uncharged solutes with the aid of the Donnan steric partitioning model (DSPM). In addition, the membrane was considered to be modelled as a bundle of cylindrical pores. It has been observed that the pore radius and thickness-to-porosity ratio from AFC 30 membrane are similar irrespective of the different uncharged solutes. The fixed charge density on the membrane surface was determined by experiments with sodium chloride at different concentrations. The data from such a sodium chloride experiment(s) were used to calculate the effective charge density (KX) by using the Spiegler–Kedem model together with a charge model called Teorell–Meyer–Sievers (TMS). Then, it has been revealed that the membrane charge depends solely on the salt concentration in the solution because of the ion adsorption onto the membrane surface. The reliance of the charge density of the NaCl concentration follows the well-known equation — Freundlich isotherm.