Photo-chemical Transformation And Photo-induced Toxicity Of Neonicotinoid Insecticides To Vibrio Fischeri ——Pathway And Mechanism

Neonicotinoids has posed severe pollution of aqutic environment all over the world and this kind of compounds has been widely investigated,but few attentions have been paid to the environmental risks of their photolysis products.The analysis on photolysis of neonicotinoids and their photo-induced toxicity to Vibrio Fischeri is of great significance to the corresponding environmental researches.At the same time,the results will benifit the assessment of environmental risk,application and regulation of neonicotinoids and other agricultural chemicals.In this study,the solutions of eleven neonicotinoids were illuminated under simulated sunlight radiation at room temperature and samples were collected at certain time intervals during the radiation period.The photolysis kinetics of test neonicotinoids were determined and the photolytic products were identified for each neonicotinoid,respectively.The dissolved organic matter(DOM)and scavengers of reactive oxygen species(ROSs)were added into the test solutions and the influence of adding DOM and scavenging ROSs over photolysis rates were investigated for the eleven neonicotinoids.At the same time,the bioluminescence inhibition rate to Vibrio Fischeri was employed to indicate the variation in toxicity of these neonicotinoids after simulated sunlight radiation in different solutions.The objectives of this dissertation were to fill the blank of photo-induced toxicity for neonicotinoids via determining the photolysis rates and photo-induced toxicity of neonicotinoids under simulated sunlight radiation and deducing photolysis pathways;To investigate the mechanisms of photo-degradation and photo-induced toxicity of neonicotinoids via structure-activity relationship analysis based on statistics of the photolysis rates,photo-induced toxicity values and molecular descriptors.The results are listed as follows:(1)The photolysis results show that all test neonicotinoids can be photo-degraded under simulated sunlight radiation.The photolysis rates of neonicotinoids can be well described by(pseudo-)first-order kinetics.Above all eleven neonicotinoids,the structures with nitro-enamine(nitenpyram and cycloxaprid)exhibite fast photolysis rates(half lives of the two compounds are less than 30 min).The structrues with nitroguanidine(imidacloprid,thiamethoxam,clothianidin,dinotefuran,imidaclothiz and flupyradifurone)exhibite moderate photolysis rates(half lives are within the range of 100 to 240 min).The structures with cyano or cyano-amidine(thiacloprid,acetamiprid and sulfoxaflor)exhibite slow photolysis rates(half lives are greater than 720 min).(2)Based on the photolysis experiments of neonicotinoids in the presence of DOM and ROSs scavengers,it shows that direct photolysis dominates photolysis of imidacloprid,nitenpyram,thiamethoxam,clothianidin,dinotefuran,imidaclothiz and sulfoxaflor.Addition of DOM declines the photolysis rates and addition of ROSs scavengers has no effect to the photolysis rates for these neonicotinoids.The photolysis processes of thiacloprid and acetamiprid are the reaction via hydroxyl(·OH).The photolysis rates are increased due to the increased amount of·OH generated by DOM.Scavenging singlet oxygen(~1O₂)will not influence the photolysis of the two compounds,but scavenging·OH will inhibit the photolysis processes.For the photolysis of cycloxaprid and flupyradifurone,both direct photolysis and photo-sensitization reactions constitute the photo-chemical transformation process of parent compounds.Both DOM and ROSs scavenger can decrease the photolysis rates for the two compounds.(3)Based on LC-MS/MS results,the photolytic products of neonicotinoids were identified.The corresponding highly toxic photolytic products of neonicotinoids are identified via combining relative peak area of detected structures against the photo-radiation time with toxicity determination of radiated solution and standard substances of detected photolytic products.The ROSs participate the photo-chemical transformation of intermediates and play important roles to the generation and transformation of highly toxic products.The photolysis pathways of neonicotinoids are deduced and the corresponding energy barriers and molecular descriptors are computed via Gaussian 09.(4)All test neonicotinoids exhibit photo-induced toxicity to Vibrio Fischeri but with different light-driven responses due to the difference of photolysis reactions,ability of generating ROSs and toxicity of photolytic products.Imidacloprid,thiamethoxam,clothianidin and nitenpyram exhibit low photo-induced toxicity due to the small amounts of highly toxic products,which is posed by the insufficient ROSs that generated by neonicotinoids parent compounds.Addition of DOM increases the photo-induced toxicity and addition of ROSs scavengers decreases the photo-induced toxicity.The enhanced toxicity of acetamiprid and thiacloprid are posed by the photolytic products generated by·OH reactions.Addition of DOM will increase the photo-induced toxicity of these two compounds due to the increased amounts of ROSs,but scavenging·OH will significantly inhibit the photo-induced toxicity.Dinotefuran,flupyradifurone,cycloxaprid,imidaclothiz and sulfoxaflor exhibit significant photo-induced toxicity.Adding DOM into test solutions decreases the photo-induced toxicity because the DOM decreases the accumulation of highly toxic products.Adding histidine into these solutions decreases the photo-induced toxicity of dinotefuran,imidaclothiz,flupyradifurone and cycloxaprid solutions because scavenging ~1O₂ will inhibit the formation of corresponding highly toxic products,but increases the photo-induced toxicity of sulfoxaflor because it will inhibit the further photo-chemical transforamtion of corresponding highly toxic products.Adding tert-Butanol decreases the photo-induced toxicity of dinotefuran and imidaclothiz,but increases the toxicity of flupyradifurone and sulfoxaflor.Scavenging·OH will increase the photo-induced toxicity of cycloxaprid at the first part of radiation and decreases that of the second part of radiation as compared with the pure water condition.This is posed by that cycloxaprid can generate at least two highly toxic products and the latter product is formed via the reaction between hydroxyl and the former highly toxic product.(5)The ability to generate ROSs is the key point to the photo-induced toxicity variation of neonicotinoids.The corresponding photo-chemical properties can be calculated by Gaussian 09and compared via total photon with high energy to excite neonicotinoids.The neonicotinoids with high photo-induced toxicity have relatively higher total photons to excite compounds.A theoretical model is developed for photo-induced toxicity variation trend based on simplified formula and same mode of action.This model can well describe and explain the variation of toxicity curves.