Mechanism of clomazone photocatalytic degradation: hydroxyl radical, electron and hole scavengers

Abstract

The role of •OH radicals (adsorbed and free) and valence band holes as primary oxidants in the photodegradation of clomazone in UV-illuminated TiO2 suspension was investigated. Significant inhibition of the photodegradation of clomazone in the presence of NaI (hole and surface •OH scavenger) suggesting that the surface degradation mechanism played a crucial role rather than the bulk degradation pathway. Also, less impact of tert-butanol on the photodegradation indicated that free •OH radicals were not majorly involved in the photodegradation process of clomazone. On the other hand, when the surface is covered by fluoride, it was concluded that the kinetic pathways for reaction with subsurface holes and with free •OH in solution are predominant. The LC–ESI–MS/MS analyses of the irradiated solution of clomazone in the presence of NaF indicated formation of the same intermediates when free •OH played a crucial role in mechanism instead of surface •OH radicals. Besides, molecular oxygen, H2O2, KBrO3, and (NH4)2S2O8 are generally used as electron scavengers in heterogeneous photocatalytic reactions. It was found that the addition of electron scavengers such as H2O2, KBrO3, and (NH4)2S2O8 has resulted in higher pollutant degradation rate compared to molecular oxygen alone.