Complete oxidation of metolachlor and methyl parathion in water by the photoassisted Fenton reaction

Abstract

Metolachlor [2-chloro- N-(2-methyl-6-ethylphenyl)- N-(2-methoxy-1-methylethyl)acetamide] or methyl parathion [ O,O-dimethyl-4-nitrophenyl phosphorothioate] at (1–2) × 10 −4 M was rapidly decomposed using a photoassisted Fenton reaction (Fe 3+/H 2O 2/u.v.). At 10 −2 M H 2O 2 and blacklight u.v. (300–400 nm) comparable in intensity to midday summer sunlight, metolachlor reacted in 8 min and was completely mineralized to HCl (40 min), inorganic N (7:1 ratio of NH 3 and HNO 3, > 2 h), and CO 2 (6 h). The aromatic ring was mineralized in 2.5 h. The transient organic intermediates identified—chloroacetate, oxalate, formate, serine, and several derivatives with the aromatic ring intact—indicate non-selective attack on the molecule. Under the same conditions, methyl parathion reacted in 5 min giving quantitative yields of HNO 3 and H 2SO 4 (5 min) and H 3PO 4 (30 min). Oxalic acid, 4-nitrophenol, dimethyl phosphoric acid, and traces of O,O-dimethyl-4-nitrophenyl phosphoric acid were identified as intermediates and shown to be oxidized further. Solutions of 14C-4-nitrophenol evolved HNO 3 concomitant with disappearance of starting material, and evolved 14CO 2 within 45 min. Based on the intermediates and their yields the initial oxidant attack on methyl parathion appears to be on the PS group leading to elimination of 4-nitrophenol and dimethyl phosphate. The results suggest that photoassisted Fenton oxidation can be a mild and effective remedy for dilute pesticide wastes.