A kinetic model for the decolorization of C.I. Acid Yellow 23 by Fenton process

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Abstract

The decolorization of azo dye C.I. Acid Yellow 23 (AY23) by Fenton process was investigated. The decolorization rate is strongly dependent on the initial concentrations of the Fe2+ and H2O2. The optimum operational conditions were obtained at pH 3.

A kinetic model has been developed to predict the decolorization of AY23 at different operational conditions by Fenton process. The model allows to simulate the system behavior involving the influence of hydrogen peroxide, Fe(II) and dye concentrations.

Introduction

Synthetic dyes are the major industrial pollutants and water contaminants [1], [2], [3]. Wastewater from textile, paper, and some other industries contain residual dyes, which are not readily biodegradable. Azo dyes, the largest class of synthetic dyes used in food industries, are characterized by the presence of one or more azo bonds (–Ndouble bondN–) in association with one or more aromatic systems, which may also carry sulfonic acid groups. One of them is Tartrazine or C.I. Acid Yellow 23 an azo dye present in thousands of foods and drugs. Many studies indicate that these dyes are toxic or carcinogenic. If these colorants come into contact along with certain drugs in the human body they can induce allergic and asthmatic reactions in sensitive people. An additional difficulty is that, when present, these dyes are not normally removed by conventional wastewater treatment systems. Therefore, the employment of these dyes must be controlled and the effluents must be treated before being released into the aquatic and terrestrial environment [4].

There are several methods used to decolorize the textile wastewater, but they cannot be effectively applied for all dyes [5]. Activated carbon adsorption process for the removal of dyes is an accepted practice, but the cost of treatment is high. Ozone and hypochlorite oxidations are effective decolorization methods, but they are not desirable because of the high cost of the equipments, operating costs and the secondary pollution arising from the residual chlorine [6].

Recently, chemical treatment methods, based on the generation of hydroxyl radicals, known as advanced oxidation processes (AOPs) has been developed [7]. It is one of the potential alternatives to decolorize and to reduce recalcitrant wastewater loads from textile dyeing and finishing effluents. This process implies generation and subsequent reaction of hydroxyl radicals, which are the most powerful oxidizing species after fluorine [8]. Among these processes, the oxidation using Fenton's reagent has proven a promising and attractive treatment method for the effective decolorization and degradation of dyes [9].H2O2 + Fe2+  radical dotOH + OH + Fe3+

The Fenton system uses ferrous ions to react with hydrogen peroxide, producing hydroxyl radicals with powerful oxidizing abilities to degrade certain toxic contaminants [10].

Hydroxyl radicals may react with ferrous ions to form ferric ions or react with organics:radical dotOH + Fe2+  OH + Fe3+radical dotOH + organics  products

Hydroxyl radicals can also react with hydrogen peroxide to produce other radicals, and may also combine with each other to produce hydrogen peroxide, which are shown below [10]:radical dotOH + H2O2  H2O + HO2radical dotradical dotOH + radical dotOH  H2O2

Ferrous ions and radicals are produced during the reactions. The reactions are shown in Eqs. (6), (7), (8), (9) [10]:H2O2 + Fe3+  H+ + FeOOH2+FeOOH2+  HO2radical dot + Fe2+HO2radical dot + Fe2+  HO2 + Fe3+HO2radical dot + Fe3+  O2 + Fe2+ + H+

The reaction rate in Eq. (6) is much slower than that of Eq. (1). It can be derived that ferrous ions are consumed quickly, but reproduced slowly [10]. Consequently, the oxidation rate of organic compounds is fast when large amount of ferrous ions are present because large amount of hydroxyl radicals are produced. However, the Fenton reaction may slow down due to the slow ferrous ion production. Gallard and De Laat [11] suggested that the Fenton's oxidation process (FOP) should be a simple first-order reaction, while Guedes et al. [12] believes that it should be a second-order reaction. Because the information regarding the kinetic study of the removal of AY23 by Fenton's oxidation is still limited, the use of hydrogen peroxide with iron salt to degrade AY23 at various concentrations of Fe(II) and H2O2 was investigated in this study. A mathematical model was also derived to predict the reaction kinetics and process performance under various reaction conditions in water.

Section snippets

Materials

AY23 was obtained from Acros (USA) and used without further purification. Fig. 1 shows the chemical structure of this dye. Hydrogen peroxide solution (30%, w/w), NaOH and H2SO4 were products of Merck (Germany). Ferrous sulfate hepthahydrate (FeSO4·7H2O) was used as a source of Fe2+ and purchased from Fluka (Switzerland).

Analytical methods

The pH of the solution is measured by using METTLER TOLEDO (MP 220) digital pH meter. The glassware was covered with aluminum foil during the tests to minimize the exposure of

Effect of pH

The pH value affects the oxidation of organic substances both directly and indirectly. The Fenton reaction is strongly pH dependent. The pH value influences the generation of hydroxyl radicals and thus the oxidation efficiency. Fig. 2 shows the effect of the initial pH value during the use of the Fenton process. A maximum degradation of 97.4% at 20 min was obtained in Fe2+/H2O2 process at pH 3. It can be seen from Fig. 2 that the removal efficiency increases from 6.3% to 97.4% in Fenton process

Conclusions

The results showed that FOP is a powerful method for decolorization of AY23. A mathematical model has been derived successfully to describe the reaction kinetics at various reaction conditions. The corresponding parameters involved in this model have been identified as the initial AY23 decay rate and the final oxidation capacity. The optimum conditions for the decolorization of AY23 in FOP were observed at pH 3. From the results, as high as 98% of AY23 can be decolorized by 13.95 mg l−1 ferrous

Acknowledgement

The authors would like to thank Islamic Azad university of Tabriz branch for financial supports.

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