Degradation of the pyrethroid insecticide dimefluthrin in a water-sediment system: detailed insight into the non-extractable residues

The environmental fate of dimefluthrin (DIM), 2,3,5,6-tetrafluoro-4-(methoxymethyl)benzyl (1R,3R)-2,2-dimethyl-3-(2-methylprop-1-en-1-yl)cyclopropanecarboxylate, was investigated in a water-sediment system in the USA. The study focused on the formation of degradation products and non-extractable residues (NER) using 14C-DIM labelled either at the benzyl or cyclopropyl carbon. The degradation half-life was calculated to be 15.6 days at 20°C, which is well below the regulatory persistence threshold of 120 days. The primary degradation pathway was hydrolysis of the ester linkage. Other reactions, such as oxidation at various positions, produced several minor metabolites that were subsequently mineralised to CO2 and/or incorporated into NER. Specifically, in the cyclopropyl-labelled samples, NER exceeded 10% of the applied radioactivity (% AR) with 78–86% of the total NER consisting of non-hazardous biogenic residues (type-III NER) as verified by 14C-labelled amino acid analysis. Potentially hazardous type-I NER were negligible, accounting for less than 0.5% AR for both labels. These insights into NER formation, particularly regarding the role of microbial activity in the environmental distribution of DIM and its metabolites, indicate minimal environmental risks from such residues.