Investigating characteristics of increasing molecular weight cutoff polyamide nanofiltration membranes using solutes rejection and atomic force microscopy

In this article, three nanofiltration (NF) membranes of increasing molecular weight cutoff ranging from 200 to 2000 were characterized by using solute rejection method and atomic force microscope (AFM) imaging. The membranes were all made of polyamide; thus, by using membranes made from the same polymer, it is hoped that the uncertainty imposed by the polymeric materials is eliminated and a better understanding of the relationship between the membrane characteristics and its separation performance is gained. Three approaches were used to fit the solute rejection data: characterization using information provided by the manufacturer, characterization using limiting rejection data, and characterization using the full rejection data. The characterization based on the information by the manufacturer was found to be quite misleading especially for larger pore size membranes. The method based on solute rejection data shows that the characteristics of the membranes were within that expected for NF membranes. It was possible to explain some of the membrane rejection performance based on the characteristics obtained. However, the result from AFM study shows the existence of wide pore size distribution of the membranes. This may be the reason why there have been some inconsistencies and non-uniformity in the membranes performance.