Ind. Eng. Chem. Res. 2004, 43, 6035-6039
6035
Kinetics of the Photocatalytic Decolorization of an Azo Reactive Dye in Aqueous ZnO Suspensions H. C. Yatmaz,*,† A. Akyol,† and M. Bayramoglu‡ Gebze Institute of Technology, Environmental Engineering Department, 41400 Gebze, Turkey and Istanbul University, Chemical Engineering Department, 34850, Avcilar, Istanbul, Turkey
The photocatalytic decolorization of Remazol Red RR, a commercial azo reactive textile dye, in ZnO suspension, has been investigated in a quartz batch reactor with the use of artificial light source (UV-C). The reaction kinetics are modeled by pseudo-first-order rate law. Regression analysis related pseudo-first-order rate constant, k, to the catalyst loading, empirically as k ∝ [ZnO]0.6. The decolorization rate increases with increasing pH, attaining maximum value at pH 10. The rate constant is inversely related to the initial dye concentration, empirically according to the relation k ∝ [C0]-1.5. Finally, the rate constant is found proportional to the square root of the light intensity as k ∝ [I] 0.5. These empirical models are therefore combined as k ∝ [ZnO]0.6 [C0]-1.5 [I]0.5 1. Introduction The destruction of persistent organic pollutants has acquired novel technologies because of inadequate conventional biological, physical, and chemical treatment methods. Therefore, new studies have been carried out to promote new technologies for at least two decades; since then, recent developments have evolved advanced oxidation processes (AOPs). Andreozzi and co-workers1 have reviewed all technologies related to oxidation processes and summarized the fundamentals with comparative assessment. One of these processes, heterogeneous photocatalysis, has emerged as an efficient process not only converting the wide range of chemicals to less harmful products but also destroying them to complete mineralization products. Some studies2-10 reviewed and highlighted the area of photocatalysis providing the reaction mechanism and various factors affecting the process. Because of the complex processes involved and the variety of wide range of chemicals investigated, the semiconductor photocatalytic oxidation still poses several questions with respect to the phases, that is, water, catalyst, oxygen, UV light, and their interactions. Finding the solution to these questions poses a great challenge but the major step is to understand a systematic way of the process variables affecting the photocatalytic degradation rate. In the study of kinetic evaluation of photocatalytic degradation in a TiO2 slurry system, Mehrotra et al.11 discussed the rate-determining steps to distinguish kinetic and transport limited regimes. They have reviewed and evaluated various macrokinetic factors on the basis of semiconductor photocatalytic oxidation. In the field of photocatalysis, kinetic studies have either concentrated on heterogeneous kinetic model of Langmuir-Hinshelwood type or rather on a basic model as pseudo first order for low substrate concentrations. Many studies evaluated these models for several organic contaminants in water12,13 or in air.14 Some only used a single organic pollutant for detailed investigation of kinetic models.15,16 * To whom correspondence should be addressed. E-mail:
[email protected]. † Gebze Institute of Technology. ‡ Istanbul University.
Since commercial reactive dyes are one of the major pollutants released by textile industry effluents, kinetic studies using different types of reactive dyes were carried out for UV-H2O2 oxidation17 and for photocatalytic oxidation.18-22 Tang and An23 investigated the pH effect of the photocatalyzed oxidation of acid blue40 dye and found that at low pH values the kinetic model followed Langmuir-Hinshelwood mechanism, while at higher pH values no adequate kinetic model fitted the kinetic data. In the comparative study of photosensitized and photocatalytic degradation of azo dyes,24 kinetic and mechanistic evaluations have been done using TiO2 and ZnO. In another detailed investigation of reactive dyes,25 a Langmuir-Hinshelwood type kinetic model was presented and kinetic factors such as catalyst and initial dye concentration, effect of pH, and light intensity were evaluated using a slurry reactor. In this study, photocatalyst ZnO powders were used in a slurry batch reactor and kinetic evaluation was performed for the decolorization rate of a synthetic aqueous solution of a commercial textile dye (Remazol Red RR). The pseudo-first-order rate constant was expressed as function of the relevant process variables such as catalyst loading, light intensity, and initial dye concentration. 2. Experimental Section 2.1. Materials. Azo-reactive dye, Remazol red RR, was obtained from DyStar (Germany). The characteristics of the dye were provided as monoazo type and reactive groups were given as vinylsulfonyl (VS) and monohalogentriazine (MHT) by DyStar.26 The photocatalyst ZnO (BET,
