Substituent effects on azo dye oxidation by the Fe^III-EDTA-H₂O₂ system

The effect of substituents on the oxidation of azo dyes in the Fe^III-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 Fe^III-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.