Azo dye transformation by enzymatic and chemical systems

Azo Dye Transformation by Enzymatic and Chemical Systems Sangkil Narn Supervising Professor: V. Renganathan Almost all dyes used in industrial applications are synthetic, and of these, approximately 50% are azo dyes. Azo dyes are recalcitrant to biodegradation; yet they are released into the environment with little or no treatment. This dissertation describes an investigation of azo dye decolorization by peroxidases, hydroxyl radicals, zero-valent iron, and NADH. Since azo dyes have widely differing substituent patterns, the effect of substituents on the reaction was examined using quantitative structure-activity relationships (QSARs). Two different groups of azo dyes were used: 4-(4'-sulfopheny1azo)-phenol and 2-(4'-sulfopheny1azo)-phenol. Substituent changes were made only in the phenolic ring of the azo dyes. Horseradish peroxidase (HRP), manganese peroxidase (MnP), and lignin peroxidase (Lip) oxidized a variety of substituted 4-(4'-sulfopheny1azo)-phenol dyes. None of the peroxidases oxidized the 2-(4'-sulfopheny1azo)-phenol dyes. HRP was the most active in dye oxidation, and Lip was the least active. Enzyme reactions can be controlled by electronic factors, steric factors, or both. In the Hammett correlation analysis, HRP and MnP oxidation of azo dyes exhibited a negative correlation with d constants, suggesting that HRP and MnP prefer substrates with electron-donating substituents (-a) in the phenolic ring. MnP showed a stronger correlation compared to HRP, suggesting that MnP reactions are primarily controlled by electronic factors. HRP showed only a weak correlation, suggesting that its reaction could be controlled by electronic and steric factors. Hydroxyl radicals generated using FelI1-EDTA and H,O, at pH 7.0 readily decolorized all dyes tested. Substitution of an electron-withdrawing substituent increased the rate of dye oxidation. A correlation was observed between the rate of decolorization and the charge density of deprotonated azo dyes, suggesting that the initial attack of . OH is on the phenolate species of azo dyes whose formation is favored when the phenolic ring is substituted with electron-withdrawing substituents. All azo dyes tested were reduced by NADH under aerobic conditions to produce two aromatic mines. Reduction was favored at low pH. The substituent effect was dependent upon the location in the phenolic ring. For example, substitution of a chlorine at the Zposition decreased the azo linkage reduction, whereas its introduction into the 3-position (ortho to azo linkage) enhanced reduction. A Zchlorine substitution decreases the pKa of phenol and favors phenoxide formation. The phenoxide donates electron density to the phenolic ring and the azo linkage. The increased electron density of the azo linkage might resist reduction by NADH. Alternatively, a 3-chlorine substitution might not substantially decrease the dye pKa; nevertheless, it can strongly deplete the electron density from azo linkage via inductive mechanisms and this could favor reduction of azo linkage. However, NADH reduction did not exhibit a strong correlation with parameters such as pKa and substituent constants. Zero-valent iron readily reduced azo dyes at neutral pH under anaerobic conditions. Reduction rates were apparently influenced by mass transport of the dye to the iron metal surface. A weak correlation was observed between kb,, the firstorder rate constant, and energy of the lowest unoccupied molecular orbital, suggesting that reduction might also be influenced by the reduction potential of the dyes. CHAPTER