Introduction There are two main goals of the research associated with this project. Firstly, evaluation of landfill leachate, waste water and air from industrial sources, for organic compound composition. The second aim of the project involves development of a new method of treatment of organic contaminants in water and air using titanium dioxide photocatalysis. Results from the determination of organic content of various waste streams in objective one will be used to design a model pollutant system to which the waste treatment photoreactor will be applied and optimised, with the ultimate aim of its application directly to the waste stream.
Destruction of Aqueous Organic Contaminants by TiO2 Photocatalysis Water treatment by conventional methods such as chlorination and ozonation use potentially toxic oxidants and can result in bi-product formation. More advanced treatment methods, such as the use of activated carbon, are often expensive and non-destructive[1]. It has been recently discovered that the photo-oxidising power of titanium dioxide (TiO2), a feature that has plagued the paint pigment industry for decades, can be harnessed to mineralise organic components[2], [3]. Sunlight energy UV (of around 360-380 nm)[4],[5] excites titanium dioxide particles creating a photo-generated electron and a highly oxidising hole in the conduction and valence band of the semiconductor (Figure 1). The electron can be scavenged by oxygen and the hole can react with surface bound OH on the TiO2, creating a highly reactive OH• radical that can completely mineralise organic components[6], [7]. The aim is to harness this oxidising power for the oxidative destruction of volatile organic compounds (VOCs) and organic leachates.
FIGURE 1. Mechanism of TiO2 Photomineralisation
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Results In this research, experiments are being conducted to increase the efficiency of the photo-oxidation process by using different substrates onto which titanium dioxide (TiO2) is adhered [8]. Progress thus far on the research project includes the development of a bench scale falling film reactor for photocatalysis investigations where TiO2 is immobilised upon a substrate over which a model pollutant dye (RB5) is dispersed ( Figure 3). The progress of photo-oxidation is indicated by the disappearance of the colour over time, which is monitored by a UV/Vis spectrophotometer. From the decay curves for the different substrates the relative rates of reaction for each can be determined and compared. Results to date suggest that the glass fibre substrate provides the greatest photocatalytic efficiency (Figure 2). Investigations to further increase the photo-efficiency of the system are ongoing.
FIGURE 2. Reaction rate versus type of substrate |
FIGURE 3. Falling film reactor |
An important feature of any new technology is that it does not create intermediate species that have greater toxicity than the parent pollutant. As such, mechanistic studies are essential to gauge both the kinetics of destruction and their relationship to molecular structure and also the potential for generating stable reaction intermediates. A separate reactor system has been designed and constructed for this purpose (Figure 4). The model pollutant compound currently being investigated is toluene, which may be studied in both aqueous and vapour phases. Preliminary results for aqueous toluene show rapid disappearance of Toluene in this phase. Work is underway to determine the reaction products and pathway.
Figure 4. Intermediate analysis tube reactor schematic
Analysis of leachate samples from the Port Talbot Steel Works site showed no evidence of organic contaminants. Therefore collection and analysis of samples from alternative sources, such as municipal waste landfill facilities, will be conducted to obtain examples of the types of organic contaminants typically present in landfill leachate. Compilation of existing data about general landfill leachate chemical composition, and also for other waste streams which may comprise landfill contents, is ongoing. This information will then be used to design more realistic model pollutant systems that will be used to optimise the photocatalytic reactor, with the ultimate aim to apply the treatment system directly to municipal waste leachate samples.
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References
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Alas, D. F. (1988) in Photocatalysis and Environment: Trends and Applications, Schiavello, M. Ed., Kluwer Academic Publishers, Dordrecht, pp 663 |
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Herrmann, J-M., Catalysis Today, 53 (1999) 115 |
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Cunningham, J., Al-Sayyed, G., Sedlak, P., Caffrey, J. (1999) Cat. Today, 53, 145 |
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Malato, S., Blanco, J., Fernández-Alba, A.R., Agüera, A. (2000) Chemosphere, 40, 403 |
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Malato, S., Blanco, J., Richter, C., Maldonado, M. I. (2000) App. Cat. B: Env., 25, 31 |
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Turchi, C. S. & Ollis, D. F. (1990) J. of Cat., 122, 178 |
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Ranchella, M., Rol, C., Sebastiani, G. V. (2000) J. Chem. Soc. Perkin Trans., 2, 311 |
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Byrne, J. A., Eggins, B. R., Brown, N. M. D., Mckinney, B., Rouse, M. (1998) App. Cat. B: Env., 17, 25 |
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Angela Stride (MSc (Hons.), NZCS) EPSRC Engineering Doctorate Centre in Steel Technology Materials Engineering University of Wales Swansea Singleton Park Swansea SA28PP