Hydroxyl radical-induced oxidation of azo dyes: a pulse radiolysis study

The hydroxyl radical ((OH)-O-.)-mediated oxidation of two simple textile azo dyes, methyl orange and calmagite, was studied by the pulse radiolysis technique. The oxidation of methyl orange and calmagite by hydroxyl radicals was extremely fast, with second-order rate constants of (2.0 +/- 0.3) x 10(10) and (1.1 +/- 0.2) x 10(10) 1 mol(-1) s(-1), respectively, at 25 degrees C and pH 9.2. The transient intermediates formed by the interaction of hydroxyl radical with the dyes displayed absorption bands at 300, 480 and 720 nm for calmagite and 360 and 580 nm for methyl orange. These transients decompose by first-order kinetics with half-lives of 150-200 mu s. In the case of methyl orange, hydroxyl radical reacts by one-electron oxidation at the nitrogen center, forming the anilino cation radical. This was confirmed by comparing the absorption spectrum of the intermediate formed by the interaction of (OH)-O-. with methyl orange and that obtained by the reaction between a one-electron oxidant such as carbonate radical (CO3-.) and methyl orange. Hydroxyl radicals react with calmagite by addition to the benzene ring, producing hydroxycyclohexadienyl radicals, which rapidly decompose to phenoxyl-type radicals by water elimination. Copyright (C) 1999 John Wiley & Sons, Ltd.