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

Abstract

The reactions of OH, H 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: ∼18%, C2/C4/C5: total ∼28% and C3/C6: total ∼41%. The overall rate constants with OH-radicals were k( OH+2,4,5-T)=6.4 (±0.5)×10 9 mol dm −3 s −1 and k( OH+MCPA)=8.5 (±0.8)×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 μmol dm −3 herbicides a dose of ∼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.