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'-sulf ophenylazo)phenol, with methyl, methoxy, and halo substituents on the pheno lic 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 decolorize d in the Fe-III-EDTA-H2O2 system, but the degree of decolorization varied o ver 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 m ake the phenolic moieties more acidic, which favors the phenolate anion, wh ich 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 Oran ge II, a probable mechanism for decolorization of phenolic azo dyes by . OH is proposed. In addition, the optimal levels of H2O2 needed for the proces s have been examined. It appears that high levels of H2O2 could reduce deco lorization by scavenging the . OH. (C) 2001 Elsevier Science Ltd. All right s reserved.