Sonochemical degradation of phenol in dilute aqueous solutions: Comparison of the reaction rates at 20 and 487 kHz

Abstract

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 k20kHz = 1.84 × 10-6 M min-1, and k487kHz = 11.6 × 10-6 M min-1. 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 H2O2 formation is more elevated at 487 kHz (k = 4.9 × 10-6 M min-1) than at 20 kHz (k = 0.75 × 10-6 M min-1). 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. © 1994 American Chemical Society.