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 OH, H and eaq− 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)×109 mol dm−3 s−1 and k(OH+MCPA)=8.5 (±0.8)×109 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.

► Degradation of phenoxyacetic acid herbicides by ionizing radiation.
► Primary pattern of OH-radical addition to the aromatic ring of 2,4,5-T.
► Strong influence of oxygen concentration on chloride elimination, formation of phenolic intermediates and aliphatic acids.
► Detoxification of 500 μmol herbicide solutions is obtained with 10 kGy.