Degradation mechanism of alachlor during direct ozonation and O 3/H 2O 2 advanced oxidation process

Abstract

The degradation of alachlor by direct ozonation and advanced oxidation process O 3/H 2O 2 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 O 3/H 2O 2, 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 O 3 and O 3/H 2O 2 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 O 3/H 2O 2). Nitrite and nitrate formation was negligible. Alachlor degradation occurred via oxidation of the arylethyl group, N-dealkylation, cyclization and cleavage of benzene ring. After O 3 or O 3/H 2O 2 treatment, the toxicity of alachlor solution examined by the Daphnia magna bioassay was slightly reduced.