Sonochemistry and sonoluminescence at discrete ultrasonic frequencies

Michael Andrew Beckett, Purdue University

Abstract

The use of ultrasound in environmental applications is a novel advanced oxidation process that is currently being investigated as a method to degrade recalcitrant organic wastes. Upon exposure of an aqueous solution to ultrasound, large pressure gradients occur with the liquid causing the transient formation and collapse of microscopic bubbles. This process is known as acoustic cavitation and leads to localized spots of high temperatures and pressures. Hydroxyl radicals that are formed during the homolytic cleavage of water molecules upon bubble collapse can be used to degrade many environmental pollutants. Currently, the influence of frequency, an important factor in determining optimal reactions conditions, on ultrasonic activity is not well understood. This research has investigated ultrasonic frequency and its role in sonochemical activity and sonoluminescence (SL). The sonolytic decomposition chemistry of the refractory compound 1,4-dioxane in aqueous solution has been investigated at four ultrasonic frequencies (205, 358, 618, and 1071 kHz). Argon and Oxygen were used as sparge gases to maintain fully saturated solutions. Using a frequency of 358 kHz, the observed first-order kinetic rate constants for 1,4-dioxane destruction were highest with a sparge gas ratio of 75% Ar/25% O2 (k = 4.32 ± 0.31 × 10 −4 s−1) and lowest in the presence of pure Argon (k = 8.67 ± 0.47 × 10−5 s−1 ). Ethylene glycol diformate, methoxyacetic acid, formaldehyde, glycolic acid, and formic acid were found to be the major intermediates of 1,4-dioxane degradation. The highest observed first-order 1,4-dioxane decomposition rate occurred at 358 followed by 618, 1071, and 205 kHz. The augmentation of the ultrasonic decomposition of 1,4-dioxane by the Fenton process was also investigated at 205, 358, 618, and 1071 kHz. The Fenton process improved the 1,4-dioxane decomposition rate and mineralization efficiency at all frequencies studied by increasing the concentration of available •OH radicals. SL spectra and intensity were examined at 205, 358, 618, and 1071 kHz in the presence of varying Argon and Oxygen saturation ratios. Chemical reactivity via H2O2 generation and the impact of a hydroxyl radical scavenger, bicarbonate ion, on SL intensity and H2O2 formation was also measured at all four frequencies. 358 kHz was the optimal frequency for maximum SL intensity and chemical reaction rates. The luminescent spectra of sodium emission in aqueous solutions and the effect of ultrasonic irradiation on iron surfaces were also determined to ascertain the impact of sonication at different frequencies on microcavity interfaces. This research provides experimental evidence that is consistent with fewer cavitation events of greater collapse intensity at low frequencies and more frequent but less violent events at high frequencies. Mass transfer of chemical species to the microbubble gas-liquid interface occurs more rapidly at higher frequencies. Furthermore, results from this investigation indicate that non-linear (asymmetrical) bubble implosions play a more significant role at lower frequencies and cause more pronounced mechanical abrasion effects near solid surfaces than that observed at higher frequencies.

Degree

Ph.D.

Advisors

Hua, Purdue University.

Subject Area

Environmental engineering|Chemical engineering|Environmental science

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