Identification And Ecotoxicity Evaluation Of Environmental Transformation Products Of Difenoconazole And Kresoxim-Methyl

Pesticide transformation products may be more toxic and stable than their parent compounds.The frequent detection of pesticide and their transformation products in the environment has attracted more and more attention.Difenoconazole and kresoxim-methyl belong to triazole and strobilurin fungicides respectively,and they are both toxic to aquatic organisms.The abiotic and biotic degradation pathways,the structure and toxicity of potential transformation products of the two pesticides under environmental related conditions are not clear until now.In this paper,the biotic（soil degradation）and abiotic（hydrolysis and photolysis）degradation behaviors of the two fungicides were studied under environmental related conditions in laboratory.High resolution mass spectrometry（HRMS）was used to identify and verify transformation products of the two fungicides under different environmental conditions with two data acquisition modes.Based on the structures of transformation products,the degradation pathways were proposed.In silico prediction and actual toxicity tests were conducted to evaluate the ecotoxicity of their transformation products.This will provide theoretical basis for the analysis of the degradation mechanism and environmental risk assessment.Furthermore,it will provide technical support and theoretical guidance for scientific and rational use of difenoconazole and kresoxim-methyl.The main results are as follows:1.The results showed that the hydrolysis and photolysis of difenoconazole and kresoxim-methyl in solutions with different p Hs（4,7,and 9）can be easily affected by p H,and the degradation rate of the two pesticides in neutral or alkaline aqueous solutions was significantly faster than that in acidic solutions.In different buffers and ultra-pure water,the DT₅₀ of difenoconazole photolysis was in the range of 13.7 h～33.8 h,and the DT₅₀ of hydrolysis was in the range of 5.8 d～31.6 d.The photolysis DT₅₀ of kresoxim-methyl was in the range of 3.4～74.3 h and the hydrolysis DT₅₀ was in the range of0.5～262 d.The degradation of kresoxim-methyl was more prone to be affected by p H.Different concentrations of natural photosensitizers such as HA,Fe³⁺and NO₃^-,all have effects on the indirect photolysis of difenoconazole and kresoxim-methyl.It is speculated that the interaction of various environmental factors in natural water can jointly affect the photodegradation behavior of pesticide.Difenoconazole and kresoxim-methyl showed biphasic degradation behavior in black,red and fluvo-aquic soils.Difenoconazole was stable while the DT₅₀ of kresoxim-methyl in all three soils was less than 3 days,but the degradation rate was faster under anaerobic conditions.The results clearly described the degradation behavior of difenoconazole and kresoxim-methyl in different environments.2.Data acquisition was performed in MS^E mode on UPLC-QTOF/MS.Based on suspect and non-target screening analysis methods,A total of 14 transformation products were identified on UNIFI platform,and 12 of them were reported for the first time.12 transformation products were detected under irradiation,4 transformation products were detected in hydrolysis transformation process,and 4transformation products were observed in soils.The structures of six transformation products CGA205374,CGA205375,TP295,TP295A,TP354A and TP387A were confirmed.The highest conversion rate of TP387A under acidic conditions was 12.1%,which should be considered to be included in the risk assessment.Based on the structures of the transformation products,new degradation pathways involving oxidation,hydrolysis,hydroxylation,dechlorination and cleavage of ether bonds for difenoconazole under environmentally relevant conditions were proposed.It provided a theoretical basis for better understanding of environmental fate and risk assessment of difenoconazole.3.The identification of transformation products of kresoxim-methyl in water and soil was performed via UHPLC-Q-Exactive MS in DDA mode.Based on suspect and non-target screening analysis methods,they were further analyzed with the help of Compound Discoverer.19 transformation products of kresoxim-methyl were identified,and 18 products were reported in environmental matrices for the first time.Among them,18 transformation products were detected under irradiation,4transformation products were detected in soil,and only BF490-1 was observed during hydrolysis.BF490-1 and TP223C were finally confirmed by standards.BF490-1 was identified as the major transformation product with conversion rate of more than 50%in three different types of soil,and it was more persistent than kresoxim-methyl,which may cause accumulation in soil.So,further risk assessment is needed.New degradation pathways of kresoxim-methyl including photoisomerization,hydrolyzation of methyl ester,hydroxylation,cleavage of oxime ether and cleavage of benzyl ether were proposed.4.ECOSAR was used to predict the acute and chronic toxicity of transformation products of difenoconazole and kresoxim-methyl to fish,Daphnia and green algae respectively.The results showed that TP369C1 was more toxic to Daphnia than difenoconazole.The toxicity of other transformation products of difenoconazole was substantially lower than that of parent compound,but they still had high toxicity to aquatic organisms(LC₅₀/EC₅₀<1 mg/L).The toxicity of all transformation products of kresoxim-methyl was reduced,but TP315、TP301、TP221、TP327 and TP329 were still extremely toxic to aquatic organisms,and their influence on aquatic ecosystem could not be ignored.The experimental acute toxicity LC₅₀or EC₅₀ of difenoconazole transformation products TP295,TP295A,TP354A and TP387A on Brachydanio rerio and Daphnia magna were all greater than 10 mg/L,and the EC₅₀ on Selenastrum capricornutum were greater than 3 mg/L,which was almost consistent with the predicted results,suggesting that the toxicity classification predicted by ECOSAR has good reference significance.Difenoconazole and kresoxim-methyl are two fungicides with high risk to aquatic organisms.Their transformation products are likely to retain most of the structure of the parent and still have toxic effects to aquatic organisms even if they are degraded in the environment.While paying attention to the parent compounds,we should also focus on the long-term harm of potential transformation products to the environment.