Identification and toxicity prediction of metribuzin transformation products in activated sludge using suspect and nontarget screening

Pesticides entering wastewater treatment plants (WWTPs) are often only partially degraded and can be transformed into various transformation products (TPs) through processes such as microbial degradation. This study focused on metribuzin, a widely used selective herbicide that is particularly toxic to freshwater algae and frequently reaches aquatic environments through rural WWTP effluent. To better understand its fate, we investigated metribuzin's transformation behavior in an activated-sludge system, aiming to identify its TPs and evaluate their potential toxicity to aquatic organisms. Biotransformation experiments were performed in activated-sludge-seeded batch reactors over 16 days. Suspect and nontarget screening using liquid chromatography-high-resolution mass spectrometry enabled TP identification, while the ECOSAR model was used to estimate aquatic toxicity. A total of 10 TPs were detected, and a transformation pathway was proposed. Among them, metribuzin-desamino and metribuzin-diketo were the major products. Several TPs were predicted to be more toxic than the parent compound, and all retained metribuzin's toxicophore, indicating a similar mode of action and potentially greater toxicological risk. Given that metribuzin and its TPs can coexist with other organic pollutants in WWTP effluents and may exert combined toxic effects on aquatic life, these findings underscore the importance of managing not only the parent compound but also its TPs in wastewater treatment systems.