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
E-mail: abramovic@ih.ns.ac.yu

Abstract
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.