Degradation of phenol under combined irradiation of microwaves and ultrasound.

A novel combined system of irradiation by microwaves and ultrasound is used to efficiently destroy phenol in aqueous solutions via sono-generated hydroxyl radicals and H2O2, in conjunction with the rapid thermal effect of microwaves on polar chemicals. Microwave irradiation enhances the sono-degradation of phenol without any additional catalyst although the effect is more marked when H2O2 is added. The degradation of phenol by MW or US alone or by combined irradiation of MW-US follows zero order kinetics. Degradation rate constants and yields of the main intermediates, catechol and hydroquinone, follow the order of MW-US > MW > US. The degradation rate also increases with higher reaction temperature in the MW reactor. The synergistic effects of MW and US were observed at 93 degrees C without addition of H2O2 and at 60 degrees C with addition of H2O2.