Interactions Of Two Neonicotinoid Insecticides With Dissolved Organic Matter Derived From Soil

Neonicotinoid insecticides have been widely used in agriculture because of their high insecticidal activity,broad spectrum,and resistance since their emergence.However,long-term applications of neonicotinoid insecticides result in their accumulation in soil and water after entering soil or water.The large-scale application of neonicotinoid insecticides caused the environmental contamination,which has been attracted attention from researchers.Dissolved organic matter(DOM)plays an important role in regulating degradation,removal,transformation of organic pollutants in the environment.Therefore,investigation of the interaction between neonicotinoid insecticides and dissolved organic matter in soil is beneficial from environmental risk assessment.In this study,the interactions between two neonicotinoid insecticides(imidacloprid and thiamethoxam)and DOM are studied by spectroscopic techniques,molecular docking and density functional theory calculations,which provides useful information of their environmental behavior in soil.The results are as follows:1.The quenching mechanism of imidacloprid and thiamethoxam binding to DOM was explored.The analysis of synchronous fluorescence shows that imidacloprid/thiamethoxam binds to DOM whether protein-like substance or humic acid-like substance through the static quenching mechanism.These results are good agreement with fluorescence lifetime.2.The binding ability and order of imidacloprid/thiamethoxam with different components of DOM was different.The binding constant K_a were obtained from the modified Stern-Volmer equation.Imidacloprid and thiamethoxam bind protein-like substance more strongly than humic-like substance,which is verified by DFT calculations.In addition,the synchronous fluorescence spectra are analyzed by two-dimensional correlation analysis.The binding of the protein-like fraction of DOM to imidacloprid and thiamethoxam was later than humic-like fraction of DOM.3.The binding model of thiamethoxam/imidacloprid with protein-like and humic-like substance was determined by van't Hoff equation.Based on the Ross'theory,the values of?H,?S and?G obtained by van't Hoff equation suggested that DOM was interacted with thiamethoxam/imidacloprid by hydrophobic force and electrostatic force in spontaneous process.The value of?G calculated by molecular docking is close to?G obtained by van't Hoff equation,which demonstrates that the fluorescence experiments was reliable.4.The binding process of imidacloprid/thiamethoxam with DOM results in the conformational changes and functional groups of DOM.Peak I and peak II in synchronous fluorescence spectra shows a red shift,and peak B(protein-like substance)in the three-dimensional fluorescence spectra also shows a red shift,which explains that the conformation of DOM has some changes during the binding process with imidacloprid and thiamethoxam.The functional groups such as aliphatic C-H,amide,carboxyl,and hydroxyl were involved in the binding process of DOM with imidacloprid/thiamethoxam as important active sites from Fourier infrared spectra.