Dye photo-enhancement of TiO2-photocatalyzed degradation of organic pollutants : the organobromine herbicide bromacil

Dye chemicals from the textile industry are a principal source of environmental pollution. The industrial effluents must therefore be treated before their release into the aqueous environment. However, application of conventional biological methods for textile wastewater treatment is limited by unsatisfactory degradation of the refractory dyes and associated chemicals. In the present study a complete degradation of selected azo- and thiazine-dyes has been achieved by TiO2-solar photocatalytic oxidation (SPO). Here the SPO is accompanied by a reaction of dye sensitization. Enhanced degradation of colourless refractory pollutants has been observed in the combined process. The main advantage of the combined approach is the utilization of visible light for assisting the degradation of refractory organic pollutants, not only coloured, but also colourless, which cannot be photosensitized in the visible region. Although very little is known on the subject, the present results demonstrate that the process is promising for the removal of undesirable toxic chemicals from textile industry wastewater. To simulate such conditions we have recently studied the dye photosensitized solar photocatalysis of an uracil derivative. Both UV and visible light are required for enhanced substrate and dye decomposition. Irradiation by UV alone decreased the degradation rate of dye, while visible light alone did not lead to any decomposition at all. It seems that the mechanism of the studied reaction involves a combination of oxidation by hydroxyl radicals, via the hole-electron semiconductor route, with oxidation by singlet oxygen.