Kinetic study on photocatalytic degradation of C.I. Acid Yellow 23 by ZnO photocatalyst
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 + hν → e− + h+e− + h+ → heath+ + H2Oads → OHads + H+h+ + OHads− → OHadse− + O2 → O2−O2− + HO2 + H+ → H2O2 + O2O2− + AY23 → AY23–OOOHads + 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):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.
References (15)
Toxicology
(2002)- et al.
Appl. Catal. B
(2004) - et al.
J. Hazard. Mater.
(2004) - et al.
Appl. Catal. B
(2004) Chemosphere
(2004)- et al.
Dyes Pigments
(2005) - et al.
J. Photochem. Photobiol. A
(2004)
Cited by (867)
Preparation of ZnO/TiO<inf>2</inf> NTs-loaded materials and their photocatalytic performance
2024, Chemical Physics LettersFabrication of BiVO<inf>4</inf> photoanode loaded with Zn-doped Co<inf>9</inf>S<inf>8</inf> for enhanced photoelectrochemical performance
2024, Journal of Photochemistry and Photobiology A: ChemistryAu nanoparticles dispersed chitosan/ZnO ternary nanocomposite as a highly efficient and reusable visible light photocatalyst
2023, Materials Science in Semiconductor ProcessingTreatment of landfill leachate using photocatalytic based advanced oxidation process – a critical review
2023, Journal of Environmental ManagementSolar light-driven photocatalytic decontamination of MB using Co and Cu doped ZnO with excellent antibacterial activity
2023, Inorganic Chemistry Communications