Enzyme-Labile Functional Groups Override Fluorination to Govern the Bioaccumulation and Metabolism of Fluorinated Pesticides in Earthworms ( Eisenia fetida )

The increasing application of fluorinated pesticides raises environmental persistence and bioaccumulation risks. This study investigated how fluorination and overall molecular structure govern pesticide fate in soil-earthworm systems. Using a matched pair of compounds, we first confirmed that fluorination of a simplified model (2-hydroxy-5-trifluoromethylpyridine) enhanced its soil persistence (half-lives: 21.95 d vs 6.13 d for its nonfluorinated analogue) and bioaccumulation in earthworms ( Eisenia fetida ) (bioaccumulation factors (BAFs): 1.12–2.15 vs 0–0.67). However, for seven commercial fluorinated pesticides, BAFs varied dramatically (0 to 13.96) and did not correlate with fluorine number. Suspect and nontarget screening identified 51 and 22 metabolites in earthworms after filter paper and soil exposure, respectively. The proposed metabolic pathways typically involved hydroxylation, hydrolysis, and subsequent conjugation (glutathione, sulfate, and glucose conjugation, as well as novel fatty acid conjugation). A key inverse relationship was found: pesticides with high BAFs (e.g., trifluralin) had low metabolic transformation, while those with low BAFs (e.g., flufenacet) were metabolized at enzyme-labile sites (e.g., ether bonds). Critically, no defluorinated metabolites were detected, confirming C–F bond stability. This study demonstrates that enzyme-labile functional groups, rather than the presence of fluorination per se, govern the overall bioaccumulation of fluorinated pesticides, while simultaneously generating persistent fluorinated metabolites that pose distinct ecological hazards.