Sonolysis of 2,4-Dichlorophenoxyacetic Acid in Aqueous Solutions. Evidence for •OH-Radical-Mediated Degradation

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2,4-dichlorophenoxyacetic acid (2,4-D) undergoes efficient degradation when an O 2 - or Ar-saturated aqueous solution is subjected to high-frequency (640 kHz) sonolysis. 2,4-dichlorophenol, hydroquinone, and catechol are major reaction intermediates common to various experimental conditions. The similarities between the reaction intermediates of sonolytic and radiolytic reactions indicate • OH radical as the primary reactive species responsible for 2,4-D degradation. Very little 2,4-D degradation occurs if the sonolysis is carried out in the presence of the • OH radical scavenger tert -butyl alcohol, also indicating that little or no pyrolysis of the compound occurs. Most of the 2,4-D eventually forms oxalic acid, which, unlike other • OH-mediated oxidation methods, is not easily mineralized with high-frequency ultrasound.