The sonochemical decolorization and decomposition of azo dyes, such as C. I. Reactive Red 22 and methyl orange, were performed from the viewpoints of wastewater treatment and to determine the reaction kinetics. A low concentration of the azo dye solution was irradiated with a 200 kHz and 1.25 W/cm2 ultrasound in a homogeneous aqueous solution. The azo dye solutions were readily decolorized by the irradiation. The sonochemical decolorization was also depressed by the addition of the t-butyl alcohol radical scavenger. These results indicated that azo dye molecules were mainly decomposed by OH radicals formed from the water sonolysis. In this paper, we propose a new kinetics model taking into account the heterogeneous reaction kinetics similar to a Langmuir-Hinshelwood mechanism or an Eley-Rideal mechanism. The proposed kinetics model is based on the local reaction site at the interface region of the cavitation bubbles, where azo dye molecules are quickly decomposed because an extremely high concentration of OH radicals exists in this region. To confirm the proposed kinetics model, the effects of the initial concentration of azo dyes, irradiated atmosphere and pH on the decomposition rates were investigated. The obtained results were in good agreement with the proposed kinetics model.
The sonochemical reduction of gold(III) [tetrachloroaurate(III)] was carried out under various irradiation conditions in an aqueous solution containing only a small amount of 1-propanol. Identical starting materials and the same initial concentrations were used in each preparation. The rates of gold(III) reduction were strongly dependent on the atmosphere, the temperature of the bulk solution, the intensity of the ultrasound, and the distance of the reaction vessel from the oscillator. For example, the rates of reduction under several atmospheres were in the order of CH4 = CO2 < N2 < Ne < He < Ar < Kr, where no reduction proceeded under the CH4 and CO2 atmospheres. It was clearly found that the rates of reduction were influenced by the cavitation phenomenon. Upon irradiation, colloidal gold particles having a surface plasmon absorption were formed, although in the absence of any stabilizers for the gold particles. It was found by TEM observations that the average size of the formed gold particles changed from 30 to 120 nm by selecting the irradiation parameters. The size of the gold particles was closely correlated to the initial rate of gold(III) reduction; the higher the rate of reduction, the smaller the particles. This relationship suggested that the rate of reduction would affect the initial gold nucleation processes. In a 200 kHz standing wave system used in the present study, the strength of the generated shock waves as well as the mechanical effects could be regarded as relatively weak.
Gold(III) (tetrachloroaurate(III)) was reduced in an aqueous solution containing only a small amount of 2-propanol to form colloidal gold nanoparticles in a standing wave system generated by a 200 kHz ultrasonic generator. The rates of gold(III) reduction and the sizes of the formed gold particles could be sonochemically controlled by controlling the irradiation parameters such as the temperature of the solution, the intensity of the ultrasound, and the positioning of the reactor. The size of gold particles strongly depended on the rate of gold(III) reduction, suggesting that the rate of gold(III) reduction affects the initial nucleation of the gold particles.
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