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

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 (Fe3+/H2O2/u.v.). At 10−2 M H2O2 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 NH3 and HNO3, > 2 h), and CO2 (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 HNO3 and H2SO4 (5 min) and H3PO4 (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 HNO3 concomitant with disappearance of starting material, and evolved 14CO2 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.