Catalysis of advanced oxidation reactions by ultrasound: A case study with phenol

The present study is about the enhancement in ozone-mediated degradation and UV (254 nm) photolysis of phenol in aqueous solutions by 300 kHz acustic cavitation and the selection of operating parameters for optimum phenol removal efficiency. The method was based on monitoring of the concentration of phenol during 90 min exposure to ozonation, sonication, UV photolysis, O-3/ultrasound, UV/ultrasound and O-3/UV/ultrasound operations. It was found that ozonation at alkaline pH was an effective method of phenol destruction, but it was considerably more effective when applied simultaneously with ultrasonic irradiation. The observed synergy particularly at alkaline pH was attributed to combined effects of: (i) increased ozone mass transfer (upon hydrodynamic shear forces created by ultrasonic bubbles) and (ii) excess hydroxyl radical formation (upon thermal decomposition of ozone in the gaseous cavity bubbles). Degradation of phenol by photolysis alone was negligible, while combination of UV-irradiation and ultrasound rendered considerable degrees of decay. The synergy here was explained by excess hydroxyl radicals that are formed by photolysis of ultrasound-generated H2O2. Maximum rate of phenol degradation was observed in case of combined application of ozone, UV and ultrasound at basic pH. (C) 2007 Elsevier B.V. All rights reserved.