TiO2/UV Photodegradation of Azo Dyes in Aqueous Solutions

Photocatalytic degradation kinetics of eleven azo dyes by TiO2/UV was studied using a Rayonet photoreactor. The degradation kinetics of azo dyes seems to be significantly influenced by their electrical nature, the number of azo bonds present in a dye molecule, and type of auxiliary groups attached to the azo bond. For example, trisazo dyes were the most difficult to degrade, and monoazo dyes were the most readily degradable, while the disazo dyes were in the middle range. The experimental results indicate that positive hole oxidation is the dominant mechanism contributing to the degradation of neutral or negatively charged dyes at pH 3. The degradation kinetics of positively charged basic dyes may be through reduction by electrons in the conduction bands of the excited TiO2 particles. On the other hand, hydroxyl radicals should be the major oxidation species at pH greater than 7. The pH effects on the degradation kinetics of different dyes may be due to the intrinsic reactivities of the neutral and ionized dye species at various pH levels towards positive hole oxidation or hydroxyl radical attack.