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) x 10(-4) M was rapidly decomposed using a photoassisted Fenton reaction (Fe)(3+)/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 H3SO4 (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 C-14-4-nitrophenol evolved HNO3 concomitant with disappearance of starting material, and evolved (CO2)-C-14 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.