Degradation of the Novel Heterocyclic Insecticide Pyraquinil in Water: Kinetics, Degradation Pathways, Transformation Products Identification, and Toxicity Assessment

As new pesticides are continuously introduced into agriculturalsystems, it is essential to investigate their environmental behaviorand toxicity effects to better evaluate their potential risks. Inthis study, the degradation kinetics, pathways, and aquatic toxicityof the new fused heterocyclic insecticide pyraquinil in water underdifferent conditions were investigated for the first time. Pyraquinilwas classified as an easily degradable pesticide in natural water,and hydrolyzes faster in alkaline conditions and at higher temperatures.The formation trends of the main transformation products (TPs) ofpyraquinil were also quantified. Fifteen TPs were identified in waterusing ultrahigh-performance liquid chromatography coupled to quadrupoleOrbitrap high-resolution mass spectrometry (UHPLC-Orbitrap-HRMS) andCompound Discoverer software, which adopted suspect and nontargetscreening strategies. Among them, twelve TPs were reported for thefirst time and 11 TPs were confirmed by synthesis of their standards.The proposed degradation pathways have demonstrated that the 4,5-dihydropyrazolo[1,5-a]quinazoline skeleton of pyraquinil is stable enough toretain in its TPs. ECOSAR prediction and laboratory tests showed thatpyraquinil was "very toxic" or "toxic"to aquatic organisms, while the toxicities of all of the TPs are substantiallylower than that of pyraquinil except for TP484, which was predictedto pose a higher toxicity. The results are important for elucidatingthe fate and assessing the environmental risks of pyraquinil, andprovide guidance for scientific and reasonable use.