Substituent effects on azo dye oxidation by the FeIII-EDTA-H2O2 system.
The effect of substituents on the oxidation of azo dyes in the FeIII-EDTA-H2O2 system was examined at pH 7. 4-(4'-sulfophenylazo)phenol and 2-(4'-sulfophenylazo)phenol, with methyl, methoxy, and halo substituents on the phenolic ring, were used as model systems. Oxidation of the naphthol dyes Orange I and Orange II were also examined. All of the dyes tested were decolorized in the FeIII-EDTA-H2O2 system, but the degree of decolorization varied over a factor of 10. Dyes substituted with one or two halogens were oxidized to a greater extent than the corresponding methyl- or methoxy-substituted dyes. One explanation for the effect of halogen substituents is that they make the phenolic moieties more acidic, which favors the phenolate anion, which is more readily attacked by *OH. This explanation is supported by the observed correlation between charge density of the phenolate anion and the degree of decolorization. Based on an analysis of products formed from Orange II, a probable mechanism for decolorization of phenolic azo dyes by *OH is proposed. In addition, the optimal levels of H2O2 needed for the process have been examined. It appears that high levels of H2O2 could reduce decolorization by scavenging the *OH.
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Department of Biochemistry and Molecular Biology, Oregon Graduate Institute of Science and Technology, Beaverton 97006-8921, USA.
Renganathan
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Tratnyek
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D001391:Azo Compounds / Q000737:chemistry*; D002614:Chelating Agents / Q000737:chemistry*; D004492:Edetic Acid / Q000737:chemistry*; D016166:Free Radical Scavengers; D006861:Hydrogen Peroxide / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D017665:Hydroxyl Radical / Q000737:chemistry; D007501:Iron / Q000737:chemistry*; D010084:Oxidation-Reduction; D018508:Water Purification / Q000379:methods
D001391:Azo Compounds; D002614:Chelating Agents; C045076:Fenton's reagent; D016166:Free Radical Scavengers; D017665:Hydroxyl Radical; D004492:Edetic Acid; D006861:Hydrogen Peroxide; D007501:Iron