| Chlorinated organic compounds, including chlorophenols, are a group of special interest due to their high toxicity to humans and aquatic life. The development of remediation techniques to remove these contaminants is necessary to attain water quality standards that fulfill government regulations. Photocatalysis based on the use of dye coated titanic was studied for the photocatalytic oxidation of 4-chlorophenol in aqueous solution. Upon visible light illumination, the various developed photocatalysts can participate in charge transfer and/or energy transfer, leading to the generation of reactive oxygen species that are responsible for the photodegradation of the organic pollutant.; The best photocatalyst among a homologous series for the photooxidation of 4-chlorophenol was selected as ZnTSPP coated on TiO2. Upon visible light irradiation at pH = 6.2 and the intensity used, 67.5% of 20.0 ppm 4-chlorophenol in water was photooxidized in 24 hours, with an initial degradation rate of 17.7 muM per hour. Using a spin trapping technique and detection by electron spin resonance, reactive oxygen species such as singlet oxygen and hydroxyl radicals were found to be present in the system during the photodegradation. Scavenging experiments were further used to confirm the role of these species. Singlet oxygen was found to be the major species responsible for the photooxidation of 4-chlorophenol; whereas hydroxyl radicals had a minor effect. Singlet oxygen and hydroxyl radicals were generated by the processes of energy and charge transfer, respectively.; Photooxidation of the model organic pollutant, 4-chlorophenol, in water using the developed ZnTSPP coated TiO2 was found to produce hydroxylated intermediates, which experienced further oxidation. The subsequent addition of hydroxyl groups could lead to the breakdown of the aromatic ring, and eventually the generation of carbon dioxide as the final oxidation product. Dechlorination of 4-chlorophenol was demonstrated by the production of 4-(4-hydroxyphenoxy)phenol, which was formed by the dimerization of hydroquinone radicals.; The results in the present study suggest an effective dye for TiO 2 photocatalysis through the mechanism of dye sensitization producing singlet oxygen under visible light illumination. The charge transfer to TiO 2 also occurs providing powerful oxidants superoxide and/or hydroxyl radicals in smaller amounts. However, the dye was observed to undergo photobleaching, which limits practical applications until a way to stabilize the dye is identified. |
