Roles of Reactive Oxygen Species and Holes in the Photodegradation of Cationic and Anionic Dyes by TiO2 under UV Irradiation

This study compared the roles of different reactive oxygen species (ROS) in the photodegradation of cationic dye methylene blue (MB) and anionic dye orange II (OII) by the TiO2/ultraviolet (UV) system using different radical quenchers to elucidate the oxidation mechanisms for cationic and anionic dyes. In this study, the holes and surficial HO center dot rather than HO center dot in the bulk solution were found to play the dominant role in the photodegradation of the two types of dyes, accounting for 60% of the total degradation (the remaining 40% degradation could be contributed by O-2(center dot-), O-1(2) and the UV photolysis). Interestingly, potassium iodide (KI) as the scavenger for surficial radicals could inhibit the OII degradation but enhanced the MB degradation, probably due to the formation of oxidized products such as I2 or IO3- that could react with MB. O-2(center dot-) played a more contributing role in the photodegradation of OII than MB, whereas O-1(2) exhibited the same contribution to the photodegradation of OII and MB. The results suggested that the photocatalytic reaction mechanisms for cationic and anionic dyes may vary with the types of ROS generated in the TiO2/UV system. (C) 2015 American Society of Civil Engineers.