Degradation mechanism of alachlor during direct ozonation and O₃/H₂O₂ advanced oxidation process

The degradation of alachlor by direct ozonation and advanced oxidation process O₃/H₂O₂ was investigated in this study with focus on identification of degradation byproducts. The second-order reaction rate constant between ozone and alachlor was determined to be 2.5 ± 0.1 M^-1 s^-1 at pH 7.0 and 20 °C. Twelve and eight high-molecular-weight byproducts (with the benzene ring intact) from alachlor degradation were identified during direct ozonation and O₃/H₂O₂, respectively. The common degradation byproducts included N-(2,6-diethylphenyl)-methyleneamine, 8-ethyl-3,4-dihydro-quinoline, 8-ethyl-quinoline, 1-chloroacetyl-2-hydro-3-ketone-7-acetyl-indole, 2-chloro-2′,6′-diacetyl-N-(methoxymethyl)acetanilide, 2-chloro-2′-acetyl-6′-ethyl-N-(methoxymethyl)-acetanilide, and two hydroxylated alachlor isomers. In direct ozonation, four more byproducts were also identified including 1-chloroacetyl-2,3-dihydro-7-ethyl-indole, 2-chloro-2′,6′-ethyl-acetanilide, 2-chloro-2′,6′-acetyl-acetanilide and 2-chloro-2′-ethyl-6′-acetyl-N-(methoxymethyl)-acetanilide. Degradation of alachlor by O₃ and O₃/H₂O₂ also led to the formation of low-molecular-weight byproducts including formic, acetic, propionic, monochloroacetic and oxalic acids as well as chloride ion (only detected in O₃/H₂O₂). Nitrite and nitrate formation was negligible. Alachlor degradation occurred via oxidation of the arylethyl group, N-dealkylation, cyclization and cleavage of benzene ring. After O₃ or O₃/H₂O₂ treatment, the toxicity of alachlor solution examined by the Daphnia magna bioassay was slightly reduced.