The Chemical Effects of Collapsing Cavitation Bubbles

Abstract

A comprehensive mechanism is developed for aqueous-phase oxidation of S(-II), where [S(-II)] = [H_2S] + [HS^-] + [S^(2-)], by ·OH radical in the presence of oxygen. The oxidation of S(-II) is initiated by reaction with ·OH, but it is further propagated by a free-radical chain sequence involving O_2. This mechanism can adequately model the observed oxidation of S(-II) in air-saturated aqueous solutions sonicated at 20 kHz and 75 W/cm^2 at pH ≥ 10, assuming a continuous and uniform ·OH input into solution from the imploding cavitation bubbles. At this pH range, practically all S(-II) is present in the form of HS^- and cannot undergo thermal decomposition. Our work suggests that the use of simplified approaches for modeling the liquid-phase sonochemistry of a well-mixed solution may be justified when ·OH radical reactions predominate. For the immersion probe at 20 kHz and 75 W/cm^2, the effective ·OH uniform release into the bulk solution was estimated to be 3.5 μM/min with a corresponding steady-state ·OH concentration of ≤0.1 μM.

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