Photodegradation of organic compounds in water

The application of photocatalytic processes for the decontamination treatment of polluted water has inspired very extensive studies. Titanium dioxide with its large band gap energy and appropriate redox potential was found as one of the most promising semiconductors for the photodegradation of pollutants in the water as well as in gas phase. The titania-silica aerogels obtained by a simple co-hydrolysis method was applied in the photodegradation of the model organic compound. Different ageing times and heat treatment tempera- tures were found to influence both the activity and the textural properties of the photocatalysts. The obtained aerogels are efficient photodegradation catalysts of methylene blue and allow a removal up to 98 and 78% of the model pollutant from 20 and 500 ppm solutions, respectively. Recently the titania-silica systems in the form of aerogels have been suggested to be more active than the pure titania catalysts. In those aerogels silica provides an excellent porous structure with an extremely high surfaced area and porosity, allowing good adsorption of pollutants. In addi- tion, it is transparent for irradiation in the range needed for the photo excitation of the titania species which can be highly dispersed in/on the silica matrix. The prepara- tion procedure of aerogel is based on the sol-gel process and involves hydrolysis of the metal precursors followed by a condensation of the obtained species with the forma- tion of gel. In order to obtain aerogels from wet gels, the applied solvent must be replaced for air. To prevent the collapse of a highly-porous gel structure during the re- moval of the solvent, special drying procedures are ap- plied, i.e. low or high temperature drying. The prepara- tion process allows obtaining the highly-dispersed titania species which can be better utilized in the photocatalytic process. The titania-silica aerogel were effectively applied in the photodegradation of benzene 3 , cyanides 4, 5 , phenol 6 and its para-derivatives 7 and others. Our recent study on the photodegradation of salicylic acid 8 also indicates bet- ter utilization of titania particles in the titania-silica aerogels than in the case of pure TiO2 catalysts. The objective of this study is to correlate the influence of titania content, the heat treatment temperature and the ageing time of the titania-silica aerogels on the efficiency of the photodegradation process of polluted waters. The aerogels were prepared using the co-hydrolysis of titania and silica precursors and methylene blue (MB) was cho- sen as a model organic compound.