Comparison of azo dye degradation efficiency using UV/single semiconductor and UV/coupled semiconductor systems

Abstract

This study examined degradation of azo dyes using photocatalytic oxidation (UV/semiconductor). The model substrates employed in this work were Procion Red MX-5B and Amaranth, while the photocatalysts were TiO 2, ZnO, and SnO 2. UV–Vis spectrum analysis demonstrated that the band gap energies of TiO 2, ZnO, and SnO 2 were 3.17, 2.92, and 4.13 eV, respectively. The band gap energy of SnO 2 is insufficient to initiate photocatalytic reaction after UV irradiation (365 nm). The reaction rate constants fit a first-order reaction model and the reaction rate constant of Procion Red MX-5B for TiO 2 + SnO 2 (0.31 h −1) is larger than that of TiO 2 (0.24 h −1) and SnO 2 at pH 10. The difference between the conduction bands of SnO 2 and TiO 2 enables the former to act as a sink for the photogenerated electrons. Most of the reaction rate constants had higher values at pH 10 than pH 7, and thus the OH attack could be assumed to represent the main reaction in this investigation. The quantities of sulfate and chloride ions released are below stoichiometry during the degradation. Owing to the sulfonate groups of Amaranth exceeding Procion Red MX-5B, Amaranth had larger electrostatic attraction than Procion Red MX-5B with the surface of ZnO, and also had higher adsorption percentage than Procion Red MX-5B on the surface of ZnO. The trend of adsorption is consistent with the reaction rate constant at pH 7, namely Amaranth > Procion Red MX-5B. The sulfate dissociation rate constant of Amaranth in UV/ZnO at pH 7 (0.49 h −1) approaches the overall rate constant (0.53 h −1); therefore, the first step involved in Amaranth can be suggested to the cleavage of the bonds of the C–S in Amaranth, causing sulfate ion formation.