Effects of dissolved gas species on ultrasonic degradation of (4-chloro-2-methylphenoxy) acetic acid (MCPA) in aqueous solution

Sonochemical degradation of MCPA ((4-chloro-2-methylphenoxy) acetic acid) in dilute aqueous solutions was studied using ultrasound with a frequency of 500 kHz. The effect of gas atmosphere on MCPA degradation was investigated in nitrogen (N2), air (O2/N2), oxygen (O2), argon (Ar) and Ar/O2 (60/40% v/v) atmospheres. For sonochemical degradation of MCPA in N2, air (O2/N2), O2 and Ar atmospheres, the rate enhancement of MCPA decomposition by sonolysis was found to be more effective in an O2-enriched atmosphere compared to Ar atmosphere. It was considered that a higher amount of oxidants was formed in a higher O2 partial pressure, which accelerated MCPA decomposition in a radical reaction system. On the other hand, both dechlorination and total organic carbon (TOC) removal rates were higher in Ar atmosphere, compared to those in O2/N2 atmosphere. It was found that, MCPA was most effectively decomposed by sonication in Ar/O2 (60/40% v/v) atmosphere, with higher rates of decomposition, dechlorination and TOC removal.