The degradation of phenol in aqueous solution by means of ultrasound was performed at two frequencies: 20 and 487 kHz. Using the same acoustical power (30 W) determined by the calorimetric method, the treatment appears more efficient for the higher frequency. The initial rates were found to be dependent on the initial phenol concentration, reaching limit values k20wz = 1.84 x M min-'. Identification of the first intermediates of the reaction (hydroquinone, catechol, benzoquinone) indicates that 'OH is involved in the degradation pathways. Correlation with hydrogen peroxide formation in water saturated with air has shown that the rate of H202 formation is more elevated at 487 kHz (k = 4.9 x M min-') than at 20 kHz (k = 0.75 x M min-I). It has been shown that the rate of sonochemical degradation is directly linked to the 'OH availability in the solution. Using luminol as a probe to visualize the region where 'OH radicals are produced, it was shown that there is a great difference between the ultrasonic field at the two frequencies.

Organic compounds in aqueous solution submitted to an ultrasonic irradiation behave differently according to their physical and chemical properties. In this work, hydrogen peroxide formation and the degradation rate of phenol and carbon tetrachloride have been studied at different frequencies: 20, 200, 500 and 800 kHz. Whatever the frequency, it is easier to decompose CCl4 than phenol by means of ultrasonic wave. It is shown that the rates of reactions involving hydroxyl radicals (hydrogen peroxide formation and phenol degradation) have a maximum value at 200 kHz. The best yield observed at 200 kHz for the phenol degradation may be the result of better HO radicals availability outside of the bubble of cavitation. The degradation rate for carbon tetrachloride which decomposes into the bubble of cavitation increases with frequency. Calculating the reaction rate for one ultrasonic period shows that the efficiency of one ultrasonic cycle decreases as frequency increases.

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