Low-fouling membrane surfaces by using thin coating technologies

Among others hydrophilicity and low surface roughness are the key factors in the development of low-fouling surfaces for thin-film-composite reverse osmosis (TFC RO) membranes. The main focus of the recent study was to develop and evaluate different thin coating technologies which could have potentiality to introduce low-fouling performance on commercial TFC RO membrane surface. Furthermore, it was aimed to improve the mechanical and chemical resistance of the TFC RO membrane surface. The selected thin coating technologies, including inorganic-organic sol-gel and inorganic ALD (atomic layer deposition) thin films and also antibacterial polymer films, were studied. Typical sol-gel coating consisted of organically modified siloxanes whereas aluminium oxide (Al2O3) thin film was applied by the ALD method. In addition, different active agents, such as zinc oxide (ZnO) and carbon nanotube (CNT), were used to functionalise the polyvinyl alcohol (PVA) thin coatings. Commercial DOW LE membrane was used as a substrate for the coating applications. The coating thickness varied from 5 nm to 200 nm depending on the applied thin coating method. The surface chemistry and structure of the coated surfaces were characterised by using ATR-FTIR, AFM and SEM-EDS. Based on the surface analysis all the applied coatings decreased the roughness of the membrane surface. Contact angle measurement was used to determine the hydrophilicity and hydrophobicity as well as to calculate the surface energy of the coated surfaces. Some of the applied coatings were clearly more hydrophilic compared to the uncoated membrane surface. The bacteria attachment test with a flow was carried out to evaluate the anti-fouling performance of the surfaces. In the particular method the coated samples were contacted with Pseudomonas aeruginosa cells for 24 hours in modified standard seawater (ASTM D1141-98 (2008)) in room temperature (130 rpm orbital shaker) and after 24 h bacterial exposure the number of adhered cells on the surface was analysed. In general it was seen that less bacteria was attached to hydrophilic surface, but on the other hand it was noticed that the low surface roughness decreased the bacteria attachment even the surface was relatively hydrophobic. Furthermore it is suggest that some of the functional chemical groups also decrease the bacteria attachment. The flux and salt rejection test was performed for the selected samples to evaluate the membrane's filtration performance. As expected all the tested coatings showed lower flux compared to unmodified TFC RO membrane.