Decolourisation of Reactive Orange 4 by Fenton and photo-Fenton oxidation technology
Introduction
Wastewaters from textile and dye industries are highly coloured with significant amount of auxiliary chemicals. The discharge of these wastewater introduced intensive colour and toxicity to aquatic environment causing serious environmental problem [1]. Reactive dyes are widely used in the textile industries because of its simple dyeing procedure and good stability during washing process [2]. Due to the complex aromatic structure and stability of these dyes, conventional biological treatment methods are ineffective for degradation [3]. Hence, the concentration remains constant in the environment. A number of physical and chemical techniques had been reported for the treatment of dye effluents. Among these, the advanced oxidation processes are more efficient as they are capable of mineralizing a wide range of organic pollutants. In our laboratory, we had reported the photocatalytic degradation of Reactive Orange 4 (RO4) using TiO2P25 as catalyst [4], [5] and H2O2–UV process [6].
In recent years attention have been focused on photochemical advanced oxidation processes using Fenton reagent with UV light for the treatment of wastewater. Fenton reagent had been found to be effective in degrading the refractory organic contaminants such as chlorophenols [7], [8], chlorobenzene [9], nitrophenols [10] and dye pollutants [11], [12], [13], [14]. The oxidation power of Fenton reagent is due to the generation of hydroxyl radical (OH) during the iron catalysed decomposition of hydrogen peroxide in acid medium. The hydroxyl radical with a high oxidation potential (2.8 eV) attacks and completely destroys the pollutants in Fenton process. The degradation of pollutants can be considerably improved by using UV-radiation. This is due to the generation of additional hydroxyl radicals. This photo-Fenton process had been effectively used to degrade the pollutants [15], [16], [17], [18].
For economic colour removal of dye wastewater by Fenton and photo-Fenton processes, there is a need to determine the optimal conditions of experimental parameters. We have undertaken a reactive class mono azo dye Reactive Orange 4 and investigated the influence of various experimental parameters on the photooxidation.
The Reactive Orange 4 dye (C.I. 18260, M.wt. 769.21) is extensively used in dyeing the textile fabrics. The chemical structure and absorption maxima of RO4 are shown in Fig. 1.
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Material
Reactive Orange 4 dye obtained from Colour Chem, Pondicherry, was used as received. AnalaR H2O2 (30 w/w), FeSO4·7H2O (Merck) were used as received. The experimental solutions were prepared using double distilled water. The pH of the solution was adjusted using H2SO4 and NaOH.
Photoreactor
Heber multilamp photoreactor model HML-MP 88 (Fig. 2) was used for photoreaction. This model consists of eight medium pressure Mercury vapour lamps (8 W) set in parallel and emitting 365 nm wavelength. It has a reaction
Photochemical degradability
Initially, the experiments were carried out under the following conditions: (i) dye + UV, (ii) dye + H2O2 + dark, (iii) dye + H2O2 + UV, (iv) dye + H2O2 +Fe2+ in dark (Fenton process), (v) dye + H2O2 + Fe2+ + UV (photo-Fenton process) and (vi) dye + Fe2+ + UV. The decolourisation results are shown in Fig. 3. From the results it is clear that the dye is resistant to (i) direct photolysis of UV light and (ii) irradiation in the presence of Fe2+ alone. For H2O2 with dye in dark 8.55% decolourisation was observed. But
Conclusions
Based on the results the following conclusions have been drawn. Though Fenton and photo-Fenton processes can be used for the decolourisation of RO4, the photo-Fenton process is more efficient. The photochemical oxidation is maximum at pH 3 in both processes. H2O2 concentrations of 15 mmol and 20 mmol appears as optimum dosages for Fenton and photo-Fenton processes, respectively. The decolourisation of dye increases by increasing the Fe2+ dosage from 0.01 mmol to 0.1 mmol. Increase in dye
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