Chemical Effects of Acoustic Cavitation

Citation

Lesko, Timothy Michael (2004) Chemical Effects of Acoustic Cavitation. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/TY3J-F564. https://resolver.caltech.edu/CaltechETD:etd-04262004-184449

Abstract

A novel high-frequency, high-power, pilot-plant scale sonochemical reactor was developed and used to study the degradation of organic pollutants in aqueous solutions. The degradation rates of trichloroethylene, dichloromethane, and phenol were found to exceed those of similar frequency, small-scale bench reactors by factors ranging from 2.5 to 7. Experiments with 10 µM methyl orange in the large reactor operating at different total volumes exhibited a linear dependence between the observed sonolytic rate constants and the applied power density. Likewise, steady-state •OH-radical (aq) in each reactor were calculated and shown to correlate with the applied power density in the vessel.

The sonochemical decomposition of phenol was further studied in a bench-scale ultrasound reactor combination with ozonolysis. The addition of ozone during sonication did not affect the first-order degradation rate constants of phenol compared to the linear combination of separate sonication and ozonation experiments. However, enhancement of the degradation rates of the total organic carbon (TOC) by 43% was observed for sonolytic ozonation compared to the separate sonication and ozonolysis experiments. The synergistic action of O₃ (aq) and ultrasound enhanced oxalate degradation rates 16-fold compared to the simple linear addition of the two independent systems. Several degradation pathways are considered which may account for the rate enhancements observed when ultrasonic irradiation is applied concurrently with ozonolysis.

In addition, the decomposition of aqueous ozone in the presence of hydrogen peroxide was investigated. H₂O₂ enhances the reactivity of O₃ (aq) by reactions that remain obscure. Several free-radical degradation mechanisms for O₃ decomposition correctly predict the ozone-decay kinetics in pure water but vastly overestimate reaction rates in the presence of H₂O₂. Results from solvent deuteration experiments in neat water are compatible with a chain-process driven by electron transfer and/or O-transfer processes. However, the large kinetic isotope effect (KIE) found in the O₃/H₂O₂ system provides compelling evidence for an elementary reaction (O₃ + HO₂⁻) involving H-O₂⁻ bond cleavage and does not support appreciable radical production from the O₃ + HO₂⁻ reaction. The magnitude of the observed KIE is consistent with a hydride transfer process yielding a closed-shell trioxide HO₃⁻, the conjugate anion of H₂O₃.

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