Physico-chemical characterization of NF/RO membrane active layers by Rutherford backscattering spectrometry

The physico-chemical properties of the active layer of nanofiltration/reverse osmosis (NF/RO) membranes were characterized by Rutherford backscattering spectrometry (RBS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). NF/RO membranes with active layer materials including polyamide (PA), polyvinyl alcohol derivative (PVA), PA–PVA, and sulfonated-polyethersulfone (SPES) were investigated. SEM and TEM images of membrane cross-sections confirmed the presence of a denser active layer supported by an asymmetric porous polysulfone structure but could only provide a rough estimate of membrane active layer average thickness. RBS provided an accurate tool to determine the elemental composition of NF/RO membrane active and support layers, and the thickness and roughness of the membrane active layer. The oxygen-to-nitrogen molecular ratio obtained for PA membranes was in the range of 1.1–3.0, which is consistent with the reported presence of unreacted carboxyl groups in the active layer surface of this type of membranes.