Visible-light-induced Photocatalytic Degradation of Herbicide Mecoprop in Aqueous Suspension of TiO2

Biljana Abramović*, Daniela Šojić and Vesna Anderluh
Department of Chemistry, Faculty of Sciences, University of Novi Sad, Trg D. Obradovi}a 3, 21000 Novi Sad, Republic of Serbia

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 (λ > 400 nm). Formation of charge-transfer complexes was confirmed by recording FTIR spectra. The efficiency of TiO2 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 µmol 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 radical scavenger, did not considerably affect the photocatalytic degradation of mecoprop under visible irradiation, which indicates that ·OH radicals are not involved.

Keywords: Visible light irradiation, charge-transfer complexes, photocatalyst, photodegradation, TiO2, mecoprop.