Photocatalytic decolorization of congo red over ZnO powder using Box±Behnken design of experiments G. Annadurai, T. Sivakumar, S. Rajesh Babu Abstract Photocatalytic decolorization of congo red under sunlight illumination has been examined using ZnO cata- lyst. Batch experiments were conducted and Box±Behnken design has been employed to study the effect of different variables on the photodecolorization. Four variables such as dye concentration 0.05, 0.075, 0.10 g/l), weight of cat- alyst ZnO 0.025, 0.14, 0.25 g/l), pH 4, 7, 10) and time 1, 2, 3 h)) were used to identify the signi®cant effects and interactions in the batch studies. A second order polyno- mial regression model has been developed using the ex- perimental data. It was found that the photodecolorization potential of ZnO was strongly affected by the variations in dye concentration, weight of catalyst ZnO, pH and time. The experimental values were in good agreement with the predicted values and the correlation coef®cient was found to be 0.9982. Optimum conditions of the variables for the maximum photodecolorization are dye concentration 0.05 g/l) ZnO 0.16 g/l), pH 7.0) and time 2.0) h). The maximum percentage of photodecolorization was observed to be 97%. 1 Introduction The presence of harmful pollutants in the discharge of waste waters is a topic of global concern. Out of the total of 6,40,000 tons of dye production in 1975, 3,60,000 tons were used by textile industries and 10 to 20% of these consumed by the textile industries was let into waste water [1]. Other industries such as pulp and paper, leather, wool, silk, etc. also use dyes in large quantities which result in increased discharge. Discharge of these colored wastes into natural water bodies is undesirable from aesthetic point of view. Textile waste water is notoriously known to contain strong colour, high COD concentration level and sus- pended solids [2]. Dyes are toxic and may be carcinogenic. Environmental contamination by these toxic chemicals is a serious global problem. Based on the chemical structure of the chromophoric group, dyes are classi®ed as anthro- quinone dyes, triacrylomethane dyes, direct dyes, reactive dyes, azodyes and heterocyclic dyes. The decolorization of such dyes have been carried out with different methods by many workers [3±17]. Since colored solution containing dyes may cause skin cancer due to photosensitization and photodynamic damage, techniques like dissolved air-¯oa- tation [18], coagulation [19], ion-exchange, reverse osmosis, adsorption [20] and oxidation with peroxide or) ozone are usually applied for the removal and destruction of dyes in waste water [21]. Currently no single econom- ically feasible method can be relied upon for treating textile ef¯uents. Conventional carbon adsorption and coagulation, ¯occulation and electrochemical methods resulted in poor colour removal and also disposal prob- lems. But the photocatalytic decolorization process is more effective when compared to other processes. Recently the TiO 2 /UV based degradation of various organic pollutants present in waste is recognized to have emerged as an alternative waste water treatment technology [21]. Photo catalytic process using sunlight is much more advanta- geous than any other because of its low operating cost. Increasing attention has been given recently to photocat- alytic reactions occurring on semiconductor particles suspended in aqueous solutions. Researchers have exam- ined photocatalytic reactions using sun light illuminated TiO 2 in a variety of systems as a means to split water in order to produce H 2 [22]. As environmental problems became more series, scientists have begun to use these same ZnO systems to degrade organic contaminants in water. Previous research has proved the ability to degrade organic compounds with TiO 2 , including surfactants, phenols, herbicides chloroform, pesticides and carboxylic acids. Many of these compounds can be totally and ef®- ciently mineralized. The TiO 2 /ZnO semiconductor oxides photocatalytic reaction is now generating commercial in- terest because of its low cost, simplicity, and ability to achieve extremely low residual organic contaminant levels. Several improvements must be made before these systems can become economically attractive. Improvement is to be made to increase the contact area between the semicon- ductor oxides and the solution, which can be done by using porous semiconductor oxide ®lms made by sol gel method [22, 23]. Advantage of the photocatalytic process is its mild operation conditions and the fact that it can be powered by ZnO in sunlight, thus reducing signi®cantly the otherwise required electric power and therefore the operating costs [24±26]. Variety of semiconductor pow- ders metatoxides, sulphides and others) acting as phot- ocatalysts have been used. Most attention was given to Bioprocess Engineering 23 2000) 167±173 Ó Springer-Verlag 2000 167 Received: 17 August 1999 G. Annadurai, T. Sivakumar &), S. Rajesh Babu Department of Chemical Engineering, Alagappa College of Technology, Anna University, Chennai ± 600 025, India We wish to express our sincere thanks to Mr. Mark Anderson, DOE Instructor and State Ease Corporation Principal for his suggestion and to use his design experiments version 5.0.7 1997).