Contamination Characteristics Of Neonicotinoid Insecticide In Agricultural Water Environmental Systems And Its Degradation Mechanism

Neonicotinoid insecticides,due to their high efficiency,broad-spectrum,and low toxicity,have gradually replaced traditional insecticides and have played an extremely important role in increasing agricultural productivity,preventing and controlling pests and diseases.However,due to its extensive use has led to the detection of these substances in water environments around the world,exacerbating the prominent contradiction between agricultural production and regional ecological security.This has become an important bottleneck problem restricting the green and sustainable development of agriculture.As a ballast stone of China’s food security,it is necessary for Heilongjiang Province to carry out forward-looking research on the monitoring,risk assessment,environmental behavior identification,and pollution removal of neonicotinoid insecticides in typical agricultural water environments.This is not only an important supplement to the basic environmental data of emerging contaminants in agricultural water environments but also an important embodiment of implementing the strategic deployment of the 20th National Congress of the Communist Party of China on"comprehensively consolidating the foundation of food security".Based on monitoring of the presence of neonicotinoid insecticides in typical agricultural water environments in Heilongjiang Province,this study clarifies the residual characteristics and spatial distribution patterns of neonicotinoid insecticides in different agricultural water environment systems.Evaluated the ecological and/or human health risks caused by the contamination of neonicotinoid insecticides in the target study area.The degradation of typical neonicotinoid insecticides in UV/H₂O₂system and biochar/PDS system was investigated,and the effects of different interfering factors on the removal efficiency of neonicotinoid insecticides were clarified.The main research results are as follows:（1）Seven（imidacloprid,thiamethoxam,clothianidin,dinotefuran,thiacloprid,acetamiprid,and imidaclothiz）and four neonicotinoid insecticides（imidacloprid,thiamethoxam,clothianidin,and acetamiprid）were detected in the water and sediment samples collected from the Harbin section of the Songhua River,with total concentrations ranging from 30.8 to 135 ng/L and from 0.61 to 14.7 ng/g（dry weight,similarly hereinafter）,respectively.Larger spatial variations in water neonicotinoid insecticides concentrations were observed in tributary than mainstream（p<0.05）.There was a significant positive correlation（p<0.05）between the concentrations of residual imidacloprid,clothianidin,andΣ₄NNIs in the sediment and total organic carbon（TOC）,and sediment can act as a secondary emission source,releasing the neonicotinoid insecticides stored in it back into the water body.Human exposure risk was assessed using the relative potency factor（RPF）,which showed that infants have the highest exposure level(estimated daily intake(ΣIMI_eq EDI):31.9 ng/kg bw/d).The concentration thresholds of neonicotinoid insecticides for aquatic organisms in the Harbin section of the Songhua River were determined using the species sensitivity distribution（SSD）approach,resulting in a value of 355 ng/L for acute hazardous concentration for 5%of species（HC₅）and 165 ng/L for chronic HC₅.Moreover,Aquatic organisms at low trophic levels were more vulnerable to potential harm from neonicotinoid insecticides.（2）In a typical farmland-river-marsh continuum in the Sanjiang Plain（Qixing River Basin）,six neonicotinoid insecticides（imidacloprid,thiamethoxam,clothianidin,acetamiprid,nitenpyram,and imidaclothiz）were detected.Imidacloprid,thiamethoxam,and clothianidin were commonly found in different environmental media.Concentrations of target neonicotinoid insecticides in soil,water,and sediment samples were 2.23-136 ng/g,3.20-51.7 ng/L,and 1.53-8.40 ng/g,respectively.The total concentration of neonicotinoid insecticides in Qixing River was lower than that in the marsh wetland,but the buffering effect of plants in the marsh wetland resulted in a spatial distribution of neonicotinoid insecticides in sediment that was opposite to that in the water body.In different cultivation types of soil,the concentration of neonicotinoid insecticides in soybean soil（71.5 ng/g）was significantly higher than that in rice soil（18.5 ng/g）（p<0.05）,but there was no significant difference between the concentrations in corn soil（33.8 ng/g）and both of them.Driven by precipitation,the total mass of imidacloprid transported through surface runoff from application to sample collection in the study area ranged from2636-3402 kg.Neonicotinoid insecticides entering rivers through surface runoff can accumulate in sediment,with residue levels ranging from 45.9-252 ng/cm².In addition,the calculation of risk quotients in the water body indicates that the residual concentration of neonicotinoid insecticides has a low risk value to aquatic organisms（RQs<0.1）.（3）Dynamic degradation experiments of neonicotinoid insecticides in paddy field systems showed that the dissipation of imidacloprid,thiamethoxam,and clothianidin in paddy water,plants,and soil almost followed a first-order kinetic equation.They belong to the category of easily degradable pesticides,and their dissipation rate in paddy water was faster than in rice plants and roots.The initial deposition of imidacloprid,thiamethoxam,and clothianidin in paddy water was 0.97,2.74,and 3.08μg/L,respectively;and the digestion half-lives were 6.11,10.5,and 6.00 d,respectively.In the paddy field soil,the residual concentrations of imidacloprid,thiamethoxam,and clothianidin were the highest at 2 h after application,with concentrations of 68.3,6.80,and 27.1 ng/g,respectively.In this case,the K_d of three neonicotinoid insecticides had the maximum value.In the roots of rice plants,the initial deposits of imidacloprid,thiamethoxam,and clothianidin were 5.19,3.02,and 5.24μg/g,respectively,with half-lives of 1.07,0.51,and 3.52 d.In the rice plants themselves,the initial deposits of imidacloprid,thiamethoxam,and clothianidin were 19.3,9.36,and 52.6μg/g,respectively,with half-lives of 1.32,0.99,and 1.96 d.The results of the dietary risk assessment showed that the risk of neonicotinoid intake through rice consumption was low and at an acceptable safety level for all age groups.（4）The degradation of neonicotinoid insecticides by the UV/H₂O₂ system showed that the degradation efficiency of the UV/H₂O₂ system was significantly higher than that of UV and H₂O₂ system,which was attributed to the synergistic effect of both UV-induced direct degradation and indirect degradation by reactive radicals.The degradation rates of neonicotinoid insecticides increased with the addition of H₂O₂ in the system,however,the higher concentration of H₂O₂ reduced the steady-state hydroxyl radical concentration in the reaction system,which inhibited the degradation rates.The effect of p H on the degradation of imidacloprid and clothianidin showed that the degradation rate was relatively fast under neutral and acidic conditions and slow under alkaline conditions,while the degradation rate of thiamethoxam was relatively slow under acidic conditions and fast under neutral and alkaline conditions.Due to the reaction of anions with radicals,Cl^-and HCO₃^-showed an inhibitory effect on the degradation of imidacloprid and clothianidin.The inhibitory effect of HA on imidacloprid and clothianidin was attributed to light-shielding and radical scavenging.The degradation of neonicotinoid insecticides was all inhibited in Songhua River water bodies.The complex composition of the real water body is the trapping agent of the active radicals,which is the main reason for the decrease of the degradation rate,and the influence of many factors needs to be considered in the practical application.Neonicotinoid insecticides failed to be completely mineralized in the UV/H₂O₂ system,but were degraded to compounds with smaller molecular weights.（5）A magnetic biochar Fe/Zn-WBC was obtained by using white melon peel biochar as a precursor and modified with ferric chloride and zinc acetate.The results of characterization analysis showed that the modified Fe/Zn-WBC had increased defect structure and pore size,rich oxygen-containing functional groups,and could be separated from aqueous by an external magnetic field.The experimental results showed that the original biochar WBC and Fe/Zn-WBC were weak in removing neonicotinoid insecticides,while Fe/Zn-WBC/PDS could effectively achieve the removal of neonicotinoid insecticides.When the initial p H of Fe/Zn-WBC/PDS system was 3-9,the removal rate of neonicotinoid insecticides was almost unchanged.The removal rate of neonicotinoid insecticides kept increasing as the amount of PDS dosing increased and the number of free radicals involved in the reaction in the system increased.The free radical quenching experiments and electron paramagnetic resonance showed that the degradation of neonicotinoid insecticides by Fe/Zn-WBC/PDS system is a process in which multiple active substances of·SO₄^-,·OH,·O₂^-,and ¹O₂ participate together.Due to the complexity of the composition of the Songhua River,the removal of neonicotinoid insecticides is inhibited.However,in the tap water system,the removal of neonicotinoid insecticides is promoted due to the generation of chlorine free radicals.This study systematically conducted the identification of characteristics and degradation mechanisms of neonicotinoid insecticides pollution through the"environmental monitoring,spatial distribution,environmental behavior,risk identification,pollution removal"approach.It initially generalized the technologies and methods for identifying the neonicotinoid insecticides existence characteristics analysis,environmental behavior process identification,ecological/health risk quantification,and pollution obstacle removal suitable for agricultural water environments（watersheds,farmlands,wetlands）.The research results provide important data and scientific basis for enriching and improving China’s neonicotinoid insecticides pollution database,formulating corresponding control policies,realizing pollution control and risk avoidance.To effectively reduce the risk of neonicotinoid insecticides,source management should be strengthened,correct doses and scientific methods should be followed,and the promotion of alternative insecticides and biological control strategies should be encouraged.It is also important to establish buffer zones between planting areas and receiving waters to minimize their negative impacts.