Sonolytic Decomposition of Aqueous Bioxalate in the Presence of Ozone

Ultrasonic irradiation in the presence of ozone is demonstrated to be effective for the rapid oxidation of oxalic acid, bioxalate, and oxalate (H_(2)C_(2)O_(4)/HC_(2)O_(4)−/C_(2)O_(4)^2−) in aqueous solution to CO_2 and H_(2)O. The degradation rate of bioxalate exposed to "sonozone" (i.e., simultaneous ultrasonication and ozonolysis) was found to be 16-times faster than predicted by the linear addition of ozonolysis and ultrasonic irradiation rates. The hydroxyl radical (•OH) is the only oxy-radical produced that can oxidize oxalate on a relevant time-scale. Thus, plausible •OH production mechanisms are evaluated to explain the observed kinetic synergism of ultrasonication and ozonolysis toward bioxalate decomposition. •OH production via decomposition of O_3 in the cavitating bubble vapor and via the reaction of O_3 and H_(2)O_2 are considered, but kinetic estimations and experimental evidence indicate neither to be a sufficient source of •OH. A free-radical chain mechanism is proposed in which the HC_(2)O_(4)− + •OH reaction functions as a primary propagation step, while the termination occurs through the O_3 + CO_(2)•− reaction via an O-atom transfer mechanism. Kinetic simulations confirm that ozone reacts efficiently with the superoxide (O_(2)•−) ion that is produced by the reaction of O_2 and CO_(2)•− to form •OH radical, and that the reaction of O_3 + CO_(2)•− must be chain terminating. Oxalate is also readily oxidized by "peroxone" treatment (i.e., H_(2)O_2 and O_3). However, the addition of H_(2)O_2 during the course of the sonolytic ozonation of oxalic acid does not appear to increase the observed degradation rate and decreases rates at millimolar levels.