Sonolytic Decomposition of Ozone in Aqueous Solution: Mass Transfer Effects

Abstract

The sonolytic degradation of ozone (O_3) was investigated in both closed and open continuous-flow systems to examine effects of mass transfer on chemical reactivity in the presence of ultrasound. Degradation of O_3 followed apparent first-order kinetics at frequencies of both 20 and 500 kHz in all the systems. Degassing of O_3 was observed at 20 kHz due to the effects of rectified diffusion and larger resonant radii of the cavitation bubbles than at 500 kHz. Increased mass transfer of O_3 diffusing into solution due to ultrasound as measured by the mass transfer coefficient, k_La_2, was observed at both frequencies. At 20 kHz, an increase in mass transfer rates in the presence of ultrasound may be partially attributed to turbulence induced by acoustic streaming. However, the main process of increased gas−liquid mass transfer in the presence of ultrasonic waves appears to be due to the sonolytic degradation of O_3 creating a larger driving force for gaseous O3 to dissolve into solution. From first-order cyclohexene degradation kinetics obtained by sonolysis, ozonolysis, sonolytic ozonolysis, and comparing the large diameter of an O_3 diffusing gas bubble to the size of an active cavitation bubble, it appears that diffusing gas bubbles containing O_3 are not directly influenced by ultrasonic fields.