Elucidation of the 1,4-dioxane decomposition pathway at discrete ultrasonic frequencies

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). To maintain fully saturated solutions, argon and oxygen were used as sparge gases. 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% O-2 (k = 4.32 +/- 0.31 x 10(-4) s(-1)) and lowest in the presence of pure argon (k = 8.67 +/- 0.47 x 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. A reaction mechanism involving these byproducts was proposed concerning primarily reactions with oxidizing species ((OH)-O-., (OOH)-O-., O-.) in and near the interfacial region of the cavitation bubble. The highest observed first-order 1,4-dioxane decomposition rate occurred at 358 followed by 618, 1071, and 205 kHz. At each frequency, approximately 85% of the initial carbon is accounted for as the parent compound, as an intermediate, or as CO2. The major byproducts formation was investigated at all four-frequencies, and the results indicate that free radical mechanisms are significant over the entire range of frequencies studied.