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The combination of sonolysis and ozonolysis as an advanced oxidation process was investigated to gain insight into factors affecting enhancement of the combined system. Sonolysis, ozonolysis, and a combination of the two were used to facilitate the degradation of three known organic contaminants, nitrobenzene (NB), 4-nitrophenol (4-NP), and 4-chlorophenol (4-CP), in water. Experiments were performed at frequencies of 20 and 500 kHz. At 20 kHz, there appeared to be an enhancement due to sonolytic ozonation, while at 500 kHz, an apparent retardation was seen. The catalytic effects of NB, 4-NP, and 4-CP degradation at 20 kHz increased with decreasing k O3 of the compounds, whereas retardation at 500 kHz was correlated with increasing k O3 . The correlation of apparent rate enhancement at 20 kHz and retardation at 500 kHz with k O3 is consistent with a pathway involving the thermolytic destruction of ozone to form atomic oxygen. Atomic oxygen then reacts with water vapor in cavitation bubbles, yielding gasphase hydroxyl radical. Enhancement in loss of total organic carbon (TOC) by sonolytic ozonation was considerable at both 20 and 500 kHz with all three compounds. In addition, intermediate product formation was observed.

We model the collapse of a bubble by taking into account all the energy forms involved (i.e., mechanical, thermal, chemical, and radiative) and compare the calculated radical yields with sonochemical data in H2O. Water decomposition plays a critical role in the energy balance, but trails equilibrium even in bubbles collapsing at subsonic speeds. Integration of the equation of bubble motion coupled with a full chemical mechanism reveals that (1) terminal gas temperatures and Mach numbers M L increase in cooler water, (2) ΓOH, the number of OH-radicals produced per unit applied work at maximum M Lwhen bubbles become unstable and disperse into the liquiddecreases at small and very large sound intensities. We show that available data on the sonochemical decomposition of volatile solutessuch as CCl4, which is pyrolyzed within collapsing bubblesare compatible with the efficient conversion of ultrasonic energy into transient cavitation. On this basis we calculate ΓOH = (1 ± 0.5) × 1017 molecules/J for R 0 = 2 μm bubbles optimally sonicated at 300 kHz and 2.3 W/cm2 by assuming mass and energy accommodation coefficients of α ≤ 7 × 10-3 and ε ≤ 0.04, respectively, in gas−liquid collisions, and values about 3-fold smaller after averaging over the nuclei size distribution. Since there is negligible radical recombination during dispersal, these ΓOH values represent available oxidant yields, that agree with experimental data on iodide sonochemical oxidation. Bubbles emit little radiation, suggesting that only radial shock waves may heat small regions to the 104−105 K range required by some sonoluminescence experiments. The contribution of this sonoluminescent core to sonochemical action is, however, insignificant. We show that much larger accommodation coefficients would lead to higher temperatures, but also to O atoms rather than OH radicals and ultimately to excess O2, at variance with experimental evidence.

Ultrasound (US) was shown to activate persulfate (PS) providing an alternative activation method to base or heat as an in situ chemical oxidation (ISCO) method. The kinetics and mechanism of ultrasonic activation of PS were examined in aqueous solution using an in situ electron paramagnetic resonance (EPR) spin trapping technique and radical trapping with probe compounds. Using the spin trap, 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), hydroxyl radical (OH) and sulfate radical anion (SO) were measured from ultrasonic activation of persulfate (US-PS). The yield of OH was up to 1 order of magnitude greater than that of SO. The comparatively high OH yield was attributed to the hydrolysis of SO in the warm interfacial region of cavitation bubbles formed from US. Using steady-state approximations, the dissociation rate of PS in cavitating bubble systems was determined to be 3 orders of magnitude greater than control experiments without sonication at ambient temperature. From calculations of the interfacial volume surrounding cavitation bubbles and using the Arrhenius equation, an effective mean temperature of 340 K at the bubble-water interface was estimated. Comparative studies using the probe compounds tert-butyl alcohol and nitrobenzene verified the bubble-water interface as the location for PS activation by high temperature with OH contributing a minor role in activating PS to SO. The mechanisms unveiled in this study provide a basis for optimizing US-PS as an ISCO technology.

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