Sonochemical oxidation of phenol and three of its intermediate products in aqueous media: Catechol,hydroquinone, and benzoquinone. Kinetic and mechanistic aspects

The sonochemical oxidation of phenol has been examined in airequilibrated aqueous media at various pH’s and at various insonation powers. Its disappearance follows zero-order kinetics at phenol]_initial ~ 30 to 70 μM Three principal intermediate species formed at pH 3: catechol (CC), hydroquinone (HQ), and p-benzoquinone (BQ); at natural pH (5.4–5.7) only catechol and hydroquinone formed. No intermediate species were detected at pH 12 under the conditions used. The sonochemical fate of CC, HQ, and BQ was also examined at pH 3 and at natural pH’s. At pH 3, BQ is the major species formed during insonation of HQ, while HQ is produced during insonation of BQ. In both cases, an additional intermediate formed in trace quantities that is identified as hydroxy-p-benzoquinone. These same intermediate species have been identified in the heterogeneous photocatalyzed oxidation of phenol in irradiated titania suspensions. The present results confirm the important role of ’OH radicals in degradation processes. Although CO₂ is the ultimate product in heterogenous photocatalysis, irradiation of a phenolic aqueous solution by ultrasounds showed no loss of total organic carbon (TOC) after several hours, even though the aromatic substrate and the intermediates had degraded. A simple kinetic model/scheme is described to account for the events in the conversion of the substrates to products. It is concluded that the hydrophobic benzoquinone reacts with ¹OH and H¹ radicals at the hydrophobic gas bubble/liquid interface, while the hydrophilic species (phenol, CC, and HQ) react, to a large extent, with the ¹ OH radicals in the solution bulk.