Two approaches are suggested for the acceleration of the photocatalytic oxidation of organic contaminants of water: acceleration by oxidants and photo-enhancement by dyes. These processes were examined with several substances: two widely applied herbicides, bromacil (a uracil) and metribuzin (a triazine), and three proteins, studied as models of biocontaminated waters. The effects of oxygen and hydrogen peroxide indicated two different reaction patterns of photo-oxidation of the herbicides. With metribuzin, oxygen had a pronounced effect on the rate of photo-oxidation, while the influence of hydrogen peroxide was quite moderate; with bromacil, oxygen had a limited effect on the rate of photo-oxidation, which however was considerably enhanced by hydrogen peroxide. Acceleration of the photo-catalytic oxidation of colourless refractory contaminants by photo-excited dye was observed. Both UV and visible light were required for the enhanced decomposition. The mechanism of the reaction seems to involve a combination of oxidation by hydroxyl radicals, via the hole-electron semiconductor route, with subsequent oxidation of photo-intermediates by singlet oxygen formed by dye sensitization. The TiO2-photocatalyzed oxidation of proteins (albumin, ovalbumin and gamma-globulin) showed the susceptibility of proteins to photocleavage and of the amino acids to photocatalytic degradation. Tyrosine was the most sensitive, while the degradation of the aliphatic amino acids Gly and Asp was slow.
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.
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