Decomposition of hydroxybenzoic and humic acids in water by ultrasonic irradiation

Sonochemical decomposition of a series of hydroxybenzoic acids such as monohydroxy-, 3,4-dihydroxy-, 3,4,5-trihydroxybenzoic acids,tannic acid, and reagent and prepared humic acids in water under argon or air atmosphere was investigated. The decomposition followed first-order kinetics at initial stage, and initial rates were in the range of 1.9-5.1 mu M min(-1) under argon and 1.9-16.4 mu M min(-1) under air. The rates of OH radical formation in the sonolysis of water were estimated to be 20 mu M min(-1) under argon and 15 mu M min(-1) under air from the yield of Fe(III) formed by the sonication of Fe(ll) solution. The decomposition of 3-hydroxybenzoic acid was almost completely inhibited by the addition of 0.1 mM t-BuOH, which is an effective scavenger of OH radicals. It is suggested that, in sonolysis under argon, the main sonochemical decomposition of the substances employed in this study proceeds via reactions with OH radicals in the bulk solution and that the contribution of thermal decomposition in cavitation bubbles or the interfacial region (between the bubbles and bulk solution) is small. In the sonolysis under air atmosphere, the role of oxygen was small in monohydroxybenzoic acids but increased with increasing numbers of OH groups substituted on the aromatic ring, suggesting the occurrence of decomposition of polyhydroxybenzoic acids induced by oxygen molecules at the interface. The chloroform formation potentials of 3-hydroxybenzoic acid and humic acids decreased due to the sonication, but the reduction in the potential was less than the corresponding amounts of decomposition of the starting substances.