Photolysis of herbicide butafenacil and toxicity of its products on marine microalgae

Photolysis plays a critical role in the elimination of herbicides in the environment. Butafenacil (BFA) is a protoporphyrinogen oxidase-inhibiting herbicide regarded as eco-friendly, but the toxicity of BFA photolysis products remains unclear. Natural light was found to enhance the degradation of BFA across different aqueous matrices. In particular, 10 mg/L BFA in ultrapure water could be completely degraded within 1 h under ultraviolet (UV) light. Natural organic matter (fulvic acid) and nitrate could inhibit, while alkaline conditions (pH 9) could accelerate the photolysis process. After UV photolysis of BFA, a significant reduction in acute toxicity towards two marine microalgae (Chlorella pyrenoidosa and Phaeodactylum tricornutum) was observed. Using liquid chromatography-high-resolution tandem mass spectrometry, nine previously unreported photolysis products were identified, which were distinct from those generated by hydrolysis. Toxicity predictions from a computational model indicated that the major photolysis products were remarkably less toxic to algae than their precursor BFA, suggesting that photolysis was key to BFA detoxification. Notably, one major photolysis product with high stability was predicted to bear a high potential risk to daphnia and fish. Given the complexity of environmental aqueous conditions, the persistent toxicity of BFA photolysis products is likely more complex, calling for further in-depth investigation.