Photocatalytic degradation of a model contaminant in the presence of nanocatalysts ZnO and SnO2 powders under UV irradiation

Wastewater from textile, paper, and some other industries contain residual dyes, which are not readily biodegradable. One of them is tartrazine. C.I.Acid Yellow 23 is an azo dye present in thousands of foods and drugs and has been reported as a possible cause of asthma, urticaria, and angioedema [1]. It also has phototoxic potentials. Adsorption and chemical coagulation are two common techniques of treatment of such wastewater. However, these methods merely transfer dyes from the liquid to the solid phase causing secondary pollution and requiring further treatment [2].Semiconductor photocatalysis is a newly developed AOP, which can be conveniently applied for the degradiation of dye pollutants. Semiconductors (such as TiO2, ZnO, Fe2O3, CdS, and ZnS) are important due to the electronic structure of the metal atoms in chemical combination, which is characterized by a filled valence band, and an empty conduction band [3]. Photodecomposition of organic pollutants employing semiconductor photocatalyst like ZnO has been investigated [4]. Under irradiation of semiconductor, conduction band electrons and valence band holes would be formed, these electrons and holes along with hydroxyl ions and dissolved oxygen will be responsible for redox reactions on the surface of semiconductor, resulting in oxidation of organic pollutants. It is obvious that combination of electrons and holes in semiconductor would affect the degradation efficiency [5]. In order to avoid recombination process an interparticle electron transfer (IPET) process has been suggested, where two semiconductor with different redox energy levels are coupled. This process would enhance charge separation and consequently increases photocatalytic activity[6]. The aim of the present study is to monitor the photocatalytic activity of coupled semiconductors ZnO/SnO2 and the degradation a AY23 as a model compound and comparing it with ZnO alone under different operational conditions.