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.

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The sonolysis of selected monocomponent PAH aqueous solution is studied at 20 and 506 kHz in the microg l(-1) range. The highest activity observed at 506 kHz, compared to 20 kHz, is tentatively explained by examination of the physical characteristics of bubbles (size and life-time) as well as by the calculation of the number of bubble at both frequency (5 x 10(3)bubbles l(-1) at 20 kHz and 4.5 x 10(9)bubbles l(-1) at 506 kHz). It is demonstrated that the main mechanism of sonodegradation is the pyrolysis of PAHs in the heart of the cavitation bubbles, and that a possible PAH oxidation by means of HO degrees appears as a minor way, since gaseous byproducts such as CO, CO2, C2H2 and CH4 have been detected. Correlations have been found by examination of kinetic variations in terms of the physical-chemical properties of PAHs. The rate constants of PAH degradation increase when the water solubility, the vapour pressure and the Henry's law constant increase.