Applied Catalysis B: Environmental 54 (2004) 19–24 Photocatalytic decolorization of Remazol Red RR in aqueous ZnO suspensions A. Akyol a , H.C. Yatmaz a,∗ , M. Bayramoglu b a Environmental Engineering Department, Gebze Institute of Technology, 41400 Gebze, Turkey b Chemical Engineering Department, Istanbul University, 34850, Avcilar, Istanbul, Turkey Received 10 August 2003; received in revised form 25 April 2004; accepted 31 May 2004 Available online 22 July 2004 Abstract The photocatalytic decolorization of aqueous solutions of Remazol Red RR, a commercial azo-reactive textile dye, in the presence of various semiconductor powder suspensions has been investigated in a quartz batch reactor with the use of artificial light sources (UV-C). ZnO and TiO 2 have been found the most active photocatalysts; however ZnO indicated slightly higher efficiency. The effects of various process variables on decolorization performance of the process have been investigated. The results showed that the decolorization efficiency increases with increase in pH, attaining maximum value at pH 10 for ZnO. The zero-point charge for ZnO is 9.0 above which ZnO surface is negatively charged by adsorbed OH - ions, favoring the formation of strong oxidant OH • radicals. The efficiency is inversely related to the dye concentration; increasing dye concentration enhances dye adsorption on the active sites of the catalyst surface, and consequently hinders OH - adsorption on the same sites, this results with a decreasing OH • formation rate. © 2004 Elsevier B.V. All rights reserved. Keywords: Photocatalysis; Decolorization; Azo-reactive dye; ZnO; TiO 2 ; Textile wastewater 1. Introduction Large quantities of dyes used in the textile industry are lost to the effluents during manufacturing and processing op- erations [1]. These colored dye effluents create severe envi- ronmental pollution problems by releasing toxic and poten- tial carcinogenic substances into the aquasphere. Since the increased public concern with these pollutants, international environmental standards are becoming more stringent; there- fore new treatment methods are required for the removal of persistent dye organic chemicals or converting them to harmless compounds in water. Several studies have been carried out for biological, physical and chemical treatment of dye containing effluents [2–5]. Among these, biodegra- dation, adsorption, chlorination and ozonation are the most commonly used conventional methods. Dyes are usually re- sistant to aerobic degradation and carcinogenic compounds may be generated during the anaerobic treatment, for ex- ample aromatic amines from azo dyes; in these respects, ∗ Corresponding author. Tel.: +90 262 754 2360; fax: +90 262 653 8490. E-mail address: yatmaz@gyte.edu.tr (H.C. Yatmaz). bio-treatment alone has been found to be ineffective for the treatment of dye effluents [4]. On the other hand, physical methods such as flocculation, reverse osmosis and adsorp- tion are not destructive and mainly create pollutant concen- trated phases. Furthermore, chemical treatment using chlo- rine or ozone has led to more successful results, but since the required high dosages are not found economically fea- sible [2]. Recent developments of advanced oxidation processes (AOPs), have led to new improvements of the oxidative degradation of the organic compounds. UV radiation in the presence of H 2 O 2 has yielded encouraging results of color removal from azo-reactive dye containing waters [5]. Het- erogeneous photocatalysis has emerged an important de- structive technology leading to the total mineralization of most of the organic pollutants including organic reactive dyes [6–13]. The reason photocatalysis attracts increased in- terest is that the process may use atmospheric oxygen as the oxidant and can be carried out under ambient conditions. Moreover, the process utilizes semiconductor catalysts such as TiO 2 , ZnO which are largely available, inexpensive, non- toxic and leads to total mineralization of organic chemicals to CO 2 , water and mineral acids. Electrochemically assisted 0926-3373/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2004.05.021