Decolorization of dark brown colored coffee effluent by solar photo-Fenton reaction: Effect of solar light dose on decolorization kinetics
Introduction
Dark brown colored coffee effluent from production plants includes a lot of persistent pigments, such as melanoidins. Melanoidins are dark brown complex and have hundreds of thousand of COD (Chemical Oxygen Demand) and BOD (Biochemical Oxygen Demand) (Kumar and Chandra, 2006). The harmful effects of colored effluent on the environment are not only its toxicity and non-biodegradability, but also its color (Tokumura et al., 2006a). Even very low concentration of pigments has repercussions for aquatic life (Karim et al., 2006). Because of these negative properties, decolorization and degradation of melanoidins have been investigated by chemical and biochemical treatments (e.g., Kim et al., 1985, Murata et al., 1992, Tokumura et al., 2006a).
As one of AOPs (Advanced Oxidation Processes) being effectively used to detoxify noxious and recalcitrant pollutants in industrial wastewater, there is the photo-Fenton process which consists of a combination of the Fenton reagents (Fe2+/H2O2) and light energy (e.g., Munoz et al., 2005, Rodriguez et al., 2005). The photo-Fenton process consists of two reactions (e.g., Will et al., 2004, Evgenidou et al., 2007).Fe2+ + H2O2 → Fe3+ + OH + OH−Fe + H2O + hν → Fe2+ + OH + H+The first reaction is a reaction of Fe2+ with H2O2, which generates powerful reactive species OH radicals and oxidize Fe2+ to Fe3+. In other words, the hydroxyl radical generation in Fenton processes is due to iron-catalyzed decomposition of H2O2. In addition to the Fenton reaction the formation of hydroxyl radicals also occurs by the photo-Fenton reaction. The second reaction of photo-Fenton process is a reaction of Fe3+ with water which occurs when the light of wavelength from 300 nm to 650 nm is irradiated (Chacon et al., 2006). In this reaction, OH radicals are generated and Fe3+ is reduced to Fe2+. These two oxidation–reduction reactions in the photo-Fenton process are occurred repeatedly and mineralize organic pollutants to CO2 and H2O completely (Wu et al., 2007).Pollutants + OH → IntermediatesIntermediates + OH → CO2 + H2OThe oxidation power of the photo-Fenton process is attributed to the generation of OH radicals.
Under no irradiation condition, the Fenton-like reaction occurred instead of photo-Fenton reaction. The Fenton-like reaction is a reaction of Fe3+ with H2O2, which causes reduction of Fe3+ to Fe2+ (Kavitha and Palanivelu, 2005, Evgenidou et al., 2007).Fe3+ + H2O2 → Fe2+ + OOH + H+Since Reaction (5) occurs instead of Reaction (2), organic pollutants are mineralized even in the no irradiation condition (Arslan-Alaton and Gurses, 2004, Will et al., 2004). It should be noted, however, that Reaction (5) is rather slower than Reaction (2). As described below, therefore, the degradation rate in the dark condition is rather slower than that of the photo-Fenton reaction.
We have previously studied the decolorization of dark brown colored coffee effluent by the photo-Fenton process with UV light and found the effectiveness of the photo-Fenton reaction (Tokumura et al., 2006a). In the present work, we have investigated the photo-Fenton reaction for decolorization of dark brown colored coffee effluent under solar irradiation. Solar light has been expected to be available for photo-Fenton processes instead of costly and hazardous artificial UV light (e.g., Munoz et al., 2005, Lapertot et al., 2006). The use of sunlight being an abundant natural energy source can significantly reduce the costs of the color removal process (e.g., Feitz et al., 2000, Essam et al., 2007, Mendez-Arriaga et al., 2008).
Section snippets
Experimental
Experiments were carried out in a Pyrex glass cylindrical reactor of 0.10 m diameter and 0.20 m height. The working volume was 1 L and all experiments were conducted in a batch mode. Model brown-colored coffee effluents were prepared by dissolving commercial instant coffee powder, Nescafe Goldblend® (Nestle Japan Group, Japan), in aqueous solution. The concentration of the model coffee effluent in this work was 300 mg L−1 and it was dark brown in color.
Reagent grade hydrogen peroxide (30% solution),
Results and discussion
The color of coffee effluent is attributed to the melanoidins having conjugated carbon–carbon double bonds in their structure (Dahiya et al., 2001, Pena et al., 2003, Morales et al., 2005). During the decolorization and degradation process, cleavage of the conjugated carbon–carbon double bonds occurs. This leads to the decolorization and subsequently the degradation to CO2 and H2O (Tokumura et al., 2006a). Decolorization of coffee effluent by photo-Fenton reaction might be mainly due to
Conclusions
The color removal efficiency increased with an increase in the Fe dosage up to 10 mg L−1. When the amount of Fe loading exceeded 10 mg L−1, the decolorization rate did not changed. Addition of H2O2 up to about 1000 mg L−1 increased the decolorization rate. Further addition of H2O2 could not improve the decolorization rate. At excess dosages of Fenton reagents, the color removal rate was not enhanced due to their scavenging of hydroxyl radicals.
It was found from the plots of decolorization data as a
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