Environmental Fate and Ecological Risk Assessment: Revealing the Abiotic Degradation of HPPD-Inhibiting Herbicide Fenpyrazone in Aquatic Environments

Fenpyrazone is a hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicide registered in China in 2020, while its aquatic environmental fate remains unclear. This study provides the first investigation of its degradation characteristics, products, and ecological risks in water. Results indicate that the degradation of fenpyrazone in aquatic environments is influenced by temperature, pH, and ionic conditions. Photolysis under neutral conditions was 193 times faster than hydrolysis, indicating photosensitivity. Five new metabolites, M440, M156, M140, M96 and M82 were identified, primarily formed via hydrolysis, decarboxylation, demethylation, and hydroxylation. In the photolysis process, ·OH, ·O 2 –, and 1 O 2 play key roles. Two key reactive sites are identified, the carboxylic ester bond for hydrolysis and the 1,3-dimethylpyrazole ring for photooxidation. Toxicity assessment revealed that metabolite M440 exhibited higher toxicity than the parent compound. Furthermore, most products were predicted to pose significantly higher chronic toxicity risks, highlighting long-term environmental hazards and providing critical data for ecological risk assessment.