TiO₂ Nanostructures for Photocatalytic Degradation of Organic Pollutants

Abstract

Wastewater consists of various colloidal particulates, pathogenic microorganisms, and organic pollutants. The discharge of untreated wastewater with a large quantity of organic pollutants does not only lead to the eutrophication and human health risks, but also contributes significantly to Green House Gas (GHG) emissions in the form of nitrous oxide and methane. To date, the most feasible technology in the research of organic wastewater treatment is photocatalysis using semiconductor metal oxide TiO2. By the illumination of UV light, TiO2 generate hydroxyl radicals (•OH) which can further assist the decomposition and mineralization of organic compound. Although the superiority of TiO2 among photoactive materials has been established, there are still some space related to its performance and efficiency that can be improved further such as the process optimization, morphology modification and improvement of their optical properties. In this thesis, the research mainly focuses on the application of TiO2 for organic degradation and the development of TiO2 photocatalyst, which include: (1) investigation on TiO2 photocatalysis for mixed dyes degradation in a photo-reactor (PR) and in a submerged membrane photo reactor (SMPR) and (2) investigation on the morphology modification by employing alkaline hydrothermal process and surface modification through chemical reduction to the improvements of material properties and photocatalytic activity of TiO2. The results of the study on TiO2 photocatalysis for dyes degradation in a binary system indicate that in the binary or even more complex systems, except for the degradation kinetics, the mechanism, and pathway follow those of in a single system. The reactions will eventually end up with total mineralization of organic dyes, showing the effectiveness of TiO2 photocatalysis. As the effectiveness of TiO2 photocatalysis for dyes degradation has been proven, further improvement in the TiO2 catalyst recovery obviously added more value to the process technically and economically. This is developed by integrating the photo-reactor with membrane filtration system (SMPR) for simultaneous dyes degradation and TiO2 catalyst recovery. Compared to the conventional photo-reactor, SMPR offer additional hydrodynamic force in the solution by its catalyst recovery activity that can improve photocatalytic activity up to 20%. In addition, by applying low flux (66 L/m2h), low concentration of catalyst (0.5 g/L), and aeration (1.3 L/min) can give an optimum photocatalytic performance as well as optimum membrane filtration performance. On the development of TiO2 catalyst, morphology modification via hydrothermal methods with operating condition 180oC for 20 h followed by annealing at 500oC has successfully produced TiO2 nanoribbon with dimensions 200-300 nm in width and several microns in length and possessed relatively high surface area and pore volume that improve the photocatalytic activity. This nanostructured can be used to develop multifunction membrane material for effective photocatalysis. Another important work in the TiO2 photocatalyst development is that the surface modification have been successfully introduced to pristine TiO2 via NaBH4 treatment and its concentration can be controlled by appliying different temperatures. The modified TiO2 (X300-450) has a good visible light absorption. Additionally, for the black TiO2 treated via NaBH4 reduction in 400 and 450oC, it shows low charge recombination compared to the pristine TiO2. Furthermore, in the application for dye degradation, it was found that those properties are not linear to the photocatalytic activity, as it has mainly bulk defects, which play the role of swallow traps for the electron and hole recombination, thus reducing the amount of charge migrated to the surface for photocatalytic reaction.