Principles of colour loss. Part 2: Degradation of azo dyes by electron transfer, catalysis and radical routes

摘要

This article is a distillation of recent progress made in the last few years on the degradation of azo dyes via electron transfer, catalysis and radical routes. Exciting progress has been made possible by the application of the pulse radiolysis technique to dyes and by rigorous kinetic analysis. It extends earlier reviews on photochemistry and oxygen atom transfer routes to complete a comprehensive review of mechanisms of dye degradation. Extraction of an electron from the dye by one-electron oxidants represents a novel and generic route for dye bleaching. Most common one-electron oxidants preferentially oxidise the dye common anion and exhibit this property in parallel with oxygen atom transfer oxidants. However, electron transfer oxidation is both more efficient and shows less specificity due to the presence of other dye reducing groups. The azo tautomeric form is much more resistant to oxidative electron transfer than the hydrazone form. Hydroxyl radicals are most efficient at destroying dyes and can bleach both the common anion and the azo or hydrazone tautomeric forms with equal facility. They insert into aromatic rings and - unlike dye radicals formed from electron transfer, which disproportionate - the resulting radicals are bleached bimolecularly. Both the dye tautomeric forms can also be bleached by reductive methods. In this case the hydrazyl radicals initially formed react via disproportionation and the hydrazide formed is subsequently degraded. In contrast to oxidation, the common ion is less susceptible to reduction than the azo or hydrazone tautomeric forms. The mechanism of dye oxidation by peroxidase enzymes, porphyrins and other metal catalysts has also been reviewed. Most of these function via electron transfer routes initiated by high oxidation state species. However, porphyrins appear to be much more versatile and can operate via several mechanisms simultaneously.