The sonochemical degradation of azobenzene and related azo dyes: Rate enhancements via Fenton's reactions

The sonochemical degradation of aqueous solutions of azobenzene and related azo dyes (methyl orange, o-methyl red, and p-methyl red) was performed at 500 kHz and 50 W, under air, O-2, Or Ar saturation at 288 K. Reaction produ cts and intermediates were identified by HPLC-ES-MS. Total organic carbon ( TOC) was also determined as a function of reaction time. We propose a react ion mechanism based on the observed species and the extent and rate of TOC depletion. The addition of OH radicals to the azo double bond is considered to be the first step of the sequence of oxidative bond cleavages leading t o the production of carboxylic acids, quinones, carbon dioxide, and nitrate ions as the main degradation products. The effects of the dye structures a nd of the background gas on the sonochemical bleaching rates were also inve stigated. The reaction rates for o-methyl red were approximately 30-40% fas ter than those for the other compounds. This appears to be a strong influen ce by a carboxylic group ortho to the azo group. Saturating with Ar instead of air or O-2 increased the pseudo first-order rate constants for the degr adation by 10%. The acceleration of the sonochemical bleaching and the mine ralization process upon addition of Fe(II) was also investigated in Ar-satu rated methyl orange solutions. A 3-fold increase in the reaction rate was o bserved at optimal Fe(II) concentrations. This kinetic effect is quantitati vely accounted for by a simple kinetic model based on the reaction of Fe(II ) with sonochemically produced H2O2 (Fenton's reaction). This latter effect illustrates a simple way of achieving a substantial improvement in the eff iciency of sonochemical degradation reactions.