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 products and intermediates were identified by HPLC-ES-MS. Total organic carbon (TOC) was also determined as a function of reaction time. We propose a reaction 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 to the production of carboxylic acids, quinones, carbon dioxide, and nitrate ions as the main degradation products. The effects of the dye structures and of the background gas on the sonochemical bleaching rates were also investigated. The reaction rates for o-methyl red were approximately 30−40% faster than those for the other compounds. This appears to be a strong influence 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 degradation by 10%. The acceleration of the sonochemical bleaching and the mineralization process upon addition of Fe(II) was also investigated in Ar-saturated methyl orange solutions. A 3-fold increase in the reaction rate was observed at optimal Fe(II) concentrations. This kinetic effect is quantitatively accounted for by a simple kinetic model based on the reaction of Fe(II) with sonochemically produced H_2O_2 (Fenton's reaction). This latter effect illustrates a simple way of achieving a substantial improvement in the efficiency of sonochemical degradation reactions