Visible-light-induced photocatalytic degradation of herbicide mecoprop in aqueous suspension of TiO2

The visible-light-induced degradation reaction of RS-2-(4-chloro-o-tolyloxy) propionic acid (mecoprop) was investigated in aqueous suspension of TiO2 Degussa P25. Diffuse reflectance spectra showed that mecoprop adsorbed on TiO2 powder induced visible-light absorption (lambda > 400 nm). Formation of charge- transfer complexes was confirmed by recording FTIR spectra. The efficiency of TiO 2 as a photocatalyst with artificial visible light was compared to sunlight and UV light, as well as direct photolysis with visible- light, sunlight and UV light. The rate of mecoprop decomposition, in the presence of visible light, is 0.86 mu mol dm(-3) min(-1), which is about four times higher in comparison to direct photolysis. The effect of catalyst loading was investigated as well, and it was found that even at 8 mg cm(-3) TiO2 the efficiency of photocatalytic degradation increases, which is significantly higher than when UV radiation is used. This difference in the effect of catalyst loading is probably a consequence of a different photodegradation mechanism under visible and UV illumination, i. e. that surface complexation between mecoprop and TiO2 is a reasonable explanation for the visible light reactivity. Besides, the addition of 2- methyl-2-propanol, a common (OH)-O-center dot radical scavenger, did not considerably affect the photocatalytic degradation of mecoprop under visible irradiation, which indicates that (OH)-O-center dot radicals are not involved.