The kinetics and mechanisms of the reaction of azo dye methyl orange with ·OH, e_aq^- and ·H in dilute aqueous solutions have been studied using pulse radiolysis. It is shown that the three radicals can all destroy the conjugative system made up of azo group and aryl rings, resulting in the decoloration of methyl orange. ·OH radicals mainly attack the azo group and the aryl ring bearing methylamino group, forming corresponding OH-adducts. Under the attack of e_aq^-, methyl orange undergoes H+ catalyzed reduction, leading to the formation of hydrazyl radicals. ·H radicals mainly induce the reductive addition to methyl orange, producing hydrazo type radicals. The rate constants for the reaction of methyl orange with ·OH, e_aq^- and ·H are 5.7×10⁹, 7.2×10⁹ and 1.2×10¹⁰ dm³·mol^-1·s^-1 respectively. These results shed light on the degradation mechanism of hydrosoluble azo-dyes in aqueous solution and hence provide the theoretical foundation for the treatment of the water contaminated with this class of compounds.

Key words: methyl orange, pulse radiolysis, transient absorption spectrum, free radical, degradation mechanism