Kinetic study on photocatalytic degradation of C.I. Acid Yellow 23 by ZnO photocatalyst

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Abstract

The potential of a common semiconductor, ZnO, has been explored as an effective catalyst for the photodegradation of C.I. Acid Yellow 23 (AY23). The effects of process parameters such as, catalyst loading, initial dye concentration, light intensity, and pH on the extent of photodegradation have been investigated. Substantial reduction of COD, besides removal of color, was also achieved. A rate equation for the degradation based on Langmuir–Hinshelwood (L–H) model has been proposed. The results show that the adsorption constant (Kads) and rate constant (kL–H) in L–H model are dependent to the light intensity, and increase with increasing the light intensity. With inserting the light intensity parameter to L–H equation, this model can be used for predicting the removal rate at different light intensities and initial concentrations of AY23. A comparison between experimental and calculated apparent reaction rate constants shows that the results obtained from the L–H modified model are in good agreement with experimental data.

Introduction

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 degradation 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]. The biggest advantage of ZnO in comparison with TiO2 is that it absorbs over a larger fraction of UV spectrum and the corresponding threshold of ZnO is 425 nm. Upon irradiation, valence band electrons are promoted to the conduction band leaving a hole behind (Eq. (1)). These electron–hole pairs can either recombine (Eq. (2)) or interact separately with other molecules. The holes at the ZnO valence band can oxidize adsorbed water or hydroxide ions to produce hydroxyl radicals (Eqs. (3) and (4)). Electron in the conduction band on the catalyst surface can reduce molecular oxygen to superoxide anion (Eq. (5)). This radical may form organic peroxides or hydrogen peroxide in the presence of organic scavengers (Eqs. (6) and (7)). The hydroxyl radical is a powerful oxidizing agent and attacks to organic compounds and intermediates (Int.) are formed. These intermediates react with hydroxyl radicals to produce final products (P) (Eq. (8)). The mechanism of heterogeneous photocatalysis presented in Fig. 1 [4].ZnO +   e + h+e + h+  heath+ + H2Oads  radical dotOHads + H+h+ + OHads  radical dotOHadse + O2  O2radical dotO2radical dot + HO2radical dot + H+  H2O2 + O2O2radical dot + AY23  AY23–OOradical dotradical dotOHads + AY23  Int.  P

The aim of the present work is to investigate the influence of operational parameters on the decolorization kinetics of AY23 in UV/ZnO process and also relation between L–H model parameters and light intensity.

Section snippets

Materials

C.I. Acid Yellow 23 (AY23), a mono azo anionic dye was obtained from ACROS organics (USA). Its chemical structure and other characteristics are listed in Table 1. ZnO, NaOH, and HCl were purchased from Merck (Germany). Solutions were prepared by dissolving appropriate amount of the dye in double distilled water before each experiment.

Photoreactor

All the experiments were carried out in a batch photoreactor. The radiation source was a UV lamp (30 W, UV-C, λmax = 254 nm, manufactured by Philips, Holland), which

Effect of the photocatalyst concentration

Some dyes are degraded by direct UV radiation. Therefore, it should be examined to what extent the AY23 are ‘photolyzed’ if no catalyst was used. Blank experiments were carried out for the dye without catalyst for this purpose. It is also interesting to determine, the minimum amount of catalyst required to decolorize the maximum amount of dye at a particular experimental condition. With an increased catalyst loading from 150 to 750 mg l−1 the percent of degradation increased from 49.1 to 92.98 at

Kinetic modeling

The photocatalytic oxidation kinetics of many organic compounds have often been modeled with the Langmuir–Hinshelwood equation, which also covers the adsorption properties of the substrate on the photocatalyst surface. This model was developed by Turchi and Ollis [12] and expressed as Eq. (16):R=d[AY23]dt=kLHKads[AY23]1+Kads[AY23]where R is the reaction rate (mg l−1 min−1), kL–H the reaction rate constant (mg l−1 min−1), Kads the adsorption coefficient of dye on the ZnO particles (mg−1 l), and

Conclusions

Effective destruction of AY23, a mono azo dye, is possible by photocatalysis in the presence of ZnO suspension and UV light. The kinetic of the photocatalytic decolorization follows a Langmuir–Hinshelwood model and depends on several factors such as, dye concentration, catalyst loading, light intensity, and pH. The results show that the adsorption constant Kads and rate constant kL–H in L–H model increases with increasing light intensity. The modified L–H model can be used for predicting

Acknowledgement

The authors thank the Islamic Azad University of Tabriz branch for financial and other supports.

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