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.