Use of selected scavengers for the determination of NF-TiO2 reactive oxygen species during the degradation of microcystin-LR under visible light irradiation

Abstract Although UV-induced TiO 2 photocatalysis involves the generation of several reactive oxygen species (ROS), the formation of hydroxyl radicals is generally associated with the degradation of persistent organic contaminants in water. In this study, a variety of radical scavengers were employed to discriminate the roles of different ROS during visible light-activated (VLA) photocatalysis using nitrogen and fluorine doped TiO 2 (NF-TiO 2 ) in the degradation of the hepatotoxin, microcystin-LR (MC-LR) in water. The addition of hydroxyl radical scavengers, methanol and tert -butyl alcohol to the reaction mixture resulted in negligible inhibition of NF-TiO 2 photocatalytic degradation of MC-LR at pH 3.0 and only partial inhibition at pH 5.7 under visible light. While hydroxyl radicals ( OH) generally play the primary role in UV-TiO 2 photocatalysis, the minimal influence of MeOH and t-BuOH on the degradation process under these experimental conditions indicates that OH are not crucial in VLA NF-TiO 2 photocatalysis. However, strong inhibition was observed in VLA NF-TiO 2 photocatalytic degradation of MC-LR in the presence of superoxide dismutase, benzoquinone and catalase at pH 3.0 and 5.7 indicating that O 2 − and H 2 O 2 play critical roles in the degradation process. Similar degradation rates were observed in the presence of deuterium oxide, which enhances singlet oxygen mediated processes further suggesting singlet oxygen is not a key species in the degradation of MC-LR. Formic acid and cupric nitrate were added to probe the roles of the valence band holes and conduction band electrons, respectively. Under UV–vis light irradiation, almost complete inhibition of MC-LR removal is observed with NF-TiO 2 in the presence of OH scavengers at pH 5.7. These results demonstrate that the solution pH plays a major role in the formation and reactivity of ROS during VLA NF-TiO 2 photocatalysis. The adsorption strength of scavengers and MC-LR onto NF-TiO 2 as well as the speciation of ROS as a function of pH needs to be carefully considered since they also play a major role in the efficiency of the process. These results indicate that the reduction of molecular oxygen by photo-generated electrons rather than hydroxyl radicals produced by oxidative reactions of photo-generated holes is the key factor in the VLA NF-TiO 2 photocatalytic degradation of MC-LR.