Phototransformation of metobromuron in the presence of TiO2

The photocatalytic degradation of the herbicide metobromuron was studied, using TiO2 photocatalysts (Degussa P25 and Millennium PC50, PC100, PC105 and PC500). Intermediate products were identified by means of liquid chromatography-mass spectrometry (LC-MS-MS) using electrospray (ES) interfacing technique. In order of importance, the main reaction pathways were demethoxylation of the urea chain, ring oxidation leading to ortho hydroxylation with respect to the N atom, substitution of Br by OH, ring oxidation with formation of the meta hydroxylated compound. Demethylation of the demethoxylated product occurred as a secondary reaction. Formation of bromide ions was observed too. In the presence of 2-propanol used as a hydroxyl radical scavenger, demethoxylation was drastically inhibited while the other reactions were only partly inhibited. It suggests that demethoxylation was exclusively due to the oxidation by hydroxyl radicals, whereas the other reactions involved positive holes too. The degradation rate of metobromuron varied in the order: PC50 < PC100 < PC105 < PC500 < Degussa P25. TiO2 Degussa P25 was therefore the most efficient photocatalyst although its surface area was lower than that of most of Millennium TiO2.