Biological and chemical degradation of azo dyes under aerobic conditions.

摘要

Azo dyes represent {dollar}>{dollar}50% of synthetic industrial dyes. Azo dyes are recalcitrant to aerobic bacterial degradation. Reductive cleavage of the azo linkage under anaerobic conditions yields potentially carcinogenic aromatic amines. This thesis examines aerobic azo dye degradation by the white-rot fungus Phanerochaete chrysosporium, by peroxidases, and by hydroxyl radicals.; P. chrysosporium, a lignin-degrading basidiomycete, extensively mineralized several hydrophobic azo dyes over a 12-day period. All dyes were degraded most extensively in ligninolytic cultures. Hydroxyl, acetamido, nitro, and N-alkylamino substituents enhanced dye degradation.; Disperse Yellow 3 (DY3, 2-(4{dollar}spprime{dollar}-acetamidophenylazo) -4-methylphenol), a dye mineralized by P. chrysosporium, yielded acetanilide as a major metabolite during fungal degradation in cultures that produce lignin and manganese peroxidases (LiP and MnP). Degradation of DY3 by LiP, Mn(III)-malonate (a MnP mimic), and horseradish peroxidase (HRP) was studied. The major products were acetanilide, 4-methyl-1,2-benzoquinone, and dimerized DY3. A mechanism for DY3 degradation is suggested. Either Mn(III) or the H{dollar}sb2{dollar}O{dollar}sb2{dollar}-oxidized forms of the peroxidases oxidize the phenolic ring of the dye by two electrons, producing an azo-bearing carbonium ion. Hydrolytic azo cleavage forms the quinone product and an acetamidophenyldiazene intermediate. The acetamidophenyldiazene is oxidized by metal or oxygen to produce an acetamidophenyldiazenyl radical, which cleaves homolytically to acetamidophenyl radical and molecular nitrogen. The acetamidophenyl radical abstracts a hydrogen from the surroundings, yielding acetanilide. Consistent with this mechanism, dyes containing phenylazo substitutions were degraded to quinones and benzenes by HRP. Further support for the mechanism was obtained through deuterium labeling studies.; Hydroxyl radicals, produced for 24 h by reaction of ferric nitrate and hydrogen peroxide at pH 2.8, degraded large amounts of hydrophobic azo dyes to CO{dollar}sp2{dollar} and water-soluble compounds. Products included benzene, formed during degradation of phenylazo-substituted dyes, and aliphatic acids. A mechanism resembling that for the peroxidase-catalyzed degradation of azo dyes is proposed for dye degradation by hydroxyl radical.