Synergistic effects of sonolysis combined with ozonolysis for the oxidation of azobenzene and methyl orange

Most advanced oxidation processes (AOP) readily decolorize but are unable t o mineralize aqueous azo dye solutions. The extent of mineralization-measur ed as total organic carbon (TOC) losses-during the 500-kHz sonication of az obenzene or methyl orange solutions increases from 20% to more than 80% in the presence of O-3 The abatement of the total organic load by the joint ac tion of ultrasound and O-3 amounts to chemical synergism. Since TOC losses are not enhanced by ozonation followed by sonication and ground-state O ato ms-that are produced by sonochemical O-3 thermolysis-are relatively unreact ive, synergism likely involves the fast oxidation by ozone of free radical or unsaturated species generated by . OH radical attack on otherwise refrac tory products. Some of these products probably are saturated mono- and dica rboxylic acids, known to be resistant to O-3 oxidation. Nitrobenzene and be nzoquinone, two rather persistent byproducts of sonolysis, are rapidly and completely mineralized by the combined oxidation treatment. Thus, direct oz onation of unsaturated sonolytic byproducts also accounts for part of the o bserved enhancement of the extent of mineralization. The anomalous kinetic behavior of the sonochemical degradation of benzoquinone (in the absence of O-3) is accounted for by its particularly high reactivity toward the relat ively inert HO2. and O-2(-). radicals.