Atomic force microscopy as a suitable technique for surface characterization of activated composite membranes for metal ion facilitated transport

Abstract

Activated composite membranes (ACM) have been made based on a polysulfone layer over a non-woven support with a dense layer of aromatic polyamide, and deposited on the polysulfone using interfacial polymerisation by a carrier agent – di(2-ethylhexyl) dithiophosphoric acid – added to the membranes at several concentrations. Two polysulfone supports have been used, one of them commercial (PSf-Com) an other made ad hoc by us (PSf-GTS). The membrane structure has been characterised by both atomic force microscopy (AFM) and scanning electron microscopy (SEM). Firstly, the two polysulfonic bases were studied. The results show that structurally both are quite similar. PSf-Com presents a more regular sponge like structure in the bulk with some macrovoids randomly distributed, with an average size of 25 µm. The results obtained demonstrate that after interfacial polymerisation, some structural defects appear with relatively big valleys and peaks. The roughness has been studied versus the AFM scan size, which allowed the evaluation of the corresponding fractal dimensions. This permitted a detailed comparison and detection of the differences and similarities of the surface structure of all the membranes studied. The membranes made on PSf-GTS, whose surface structure seems to yield a better coverage of polyamide, gave good performances in the extraction of certain heavy metals. After extraction the membranes were analysed by EDS to confirm the permanence of the extractant along with some deposits of salt and extracted ions.