OH-radical induced degradation of 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) and 4-chloro-2-methylphenoxyacetic acid (MCPA): A pulse radiolysis and gamma-radiolysis study

The reactions of (center dot)OH, H(center dot) and e(aq)(-) with 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) and 4-chloro-2-methylphenoxyacetic acid (MCPA) were studied by pulse radiolysis. The site of 'OH-radicals addition to the aromatic ring of 2,4,5-T was found to be-C1: similar to 18%, C2/C4/C5: total similar to 28% and C3/C6: total similar to 41%. The overall rate constants with OH-radicals were k((center dot)OH + 2.4.5-T)= 6.4 (+/- 0.5) x 10(9) mol dm(-3) s(-1) and k((center dot)OH+MCPA)=8.5 (+/- 0.8) x 10(9) mol dm(-3) s(-1). The radiation induced decomposition of the pesticides, chloride- and product formation (phenolic compounds, aliphatic acids) was studied by gamma radiolysis as a function of dose. A mechanism for acetate formation is discussed. The presence of oxygen during irradiation affected the decomposition rate only indiscernibly, however, chloride elimination, ring fragmentation (formation of aliphatic acids), TOC- and toxicity reduction were strongly enhanced. For complete removal of 500 mu mol dm(-3) herbicides a dose of similar to 4 kGy was required. Using air saturation during irradiation a reduction of 37-40% of the TOC was observable at 5 kGy, detoxification (luminescence inhibition <20%) was achieved with 10 kGy. (C) 2011 Elsevier Ltd. All rights reserved.