This paper presents a comprehensive experimental and numerical investigation of the effects of liquid temperature on the sonochemical degradation of three organic dyes, Rhodamine B (RhB), Acid orange 7 (AO7) and Malachite green (MG), largely used in the textile industry. The experiments have been carried out for an ultrasonic frequency of 300 kHz. The obtained experimental results were discussed using a new approach combining the results of single-bubble event and the number of active bubbles. The single-bubble event was predicted using a model that combines the bubble dynamics with chemical kinetics occurring inside a bubble during the strong collapse. The number of active bubbles was predicted using a method developed in our previous work. The experiments showed that the degradation rate of the three dyes increased significantly with increasing liquid temperature in the range 25-55°C. It was predicted that the main pathway of pollutants degradation is the attack by OH radicals. The simulations showed that there exists an optimum liquid temperature of about 35°C for the production of OH inside a bubble whereas the number of active bubbles increased sharply with the rise of the liquid temperature. It was predicted that the overall production rate of OH increased with increasing liquid temperature in the range 25-55°C. Finally, it was concluded that the effect of liquid temperature on the sonochemical degradation of the three dyes in aqueous phase was controlled by the number of active bubbles in the range 35-55°C and by both the number of bubbles and the single bubble yield in the range 25-35°C.

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Sonochemical degradation at 600 kHz of 2-phenoxyethanol (PhE), an endocrine disrupting compound, was performed in the presence of several organic additives, namely: 2-propanol, Triton X-100 and sucrose, of different volatilities to obtain detailed information on the reaction zone and the oxidation pathway of this priority emerging water contaminate. It was found that sonication at 600 kHz and 120 W completely remove PhE (10 mg L) from aerated solutions within 100 min of irradiation. Very little removal of PhE (∼7%) and low accumulation of HO took place in the presence of adequate amount of 2-propanol, indicating that reaction with OH radical outside the bubble is the major degradation pathway of PhE. Addition of the hydrophobic surfactant Triton X-100, as an OH-probe for the interfacial region, at 10 and 100 mM reduced the degradation event by 57% and 72% and resulted in more than 50% decrease in the yield of HO, confirming that PhE degradation occurs mainly at the bubble/solution interface with hydroxyl radical attack. Addition of the hydrophilic substrate glucose at high doses decreased slightly (∼7%) the degradation of PhE and the formation rate of HO, meaning that the bulk of the solution participate marginally in the degradation of the pollutant. Finally, analyzing the degradation rates at various initial PhE concentrations (2-400 mg L) with a heterogeneous Langmuir type mechanism underlined the predominance of interfacial radical reactions during the oxidation of PhE, particularly at high initial pollutant concentrations.

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