Sonochemical degradation of phenolic pollutants in aqueous solutions

Abstract The sonochemical degradation of phenol, 2‐chlorophenol and 3,4‐dichlorophenol in aqueous solutions as a function of several operating conditions has been investigated. Experiments were performed at initial substrate concentrations of 0.1, 0.5 and 1 g l−1, liquid phase volumes of 0.05, 0.07 and 0.08 1, electric power outputs of 125, 187.5 and 250 W, liquid bulk temperatures of 20, 35, 50 and 70°C and an ultrasound frequency of 20 kHz. Substrate concentration was determined as a function of time by means of high performance liquid chromatography. At the conditions under consideration, the rate of degradation follows first order kinetics with respect to the substrate concentration and increases with increasing electric power and decreasing liquid volume. The relative reactivity appears to decrease in the order: 2‐chlorophenol>3,4‐dichlorophenol>phenol. Measurements of liquid phase total organic carbon content showed that degradation by‐products are more recalcitrant than the original substrate. The rate of 2‐chlorophenol degradation was also found to decrease with decreasing liquid bulk temperature and increasing initial concentration. Addition of t‐butanol as a hydroxyl radical scavenger only partially inhibited degradation, thus implying that degradation is likely to proceed via both radical‐induced and thermal reactions. Addition of Fe2+ ions at concentrations as low as about 0.2×10−3 g l−1 resulted in increased degradation rates; this is attributed to iron being capable of readily decomposing hydrogen peroxide (generated by water sonolysis) in a Fenton‐like process to form hydroxyl radicals as well as being an effective oxidation catalyst. The implications of the use of ultrasound in wastewater treatment are also discussed.