Degradation Of Neonicotinoids In Soil By Activated Persulfate Process

Neonicotinoids are globally important chemical insecticides that are used effectively against pests such as Homoptera,Coleoptera,Diptera and Lepidoptera.However,the long-term use of neonicotinoids has caused serious environmental pollution and human health risks.In this thesis,an ultra high performance liquid chromatography tandem mass spectrometry（UPLC-MS/MS）method was developed for the detection of the neonicotinoids in soil,and the neonicotinoids in farmland soils of six provinces in China was monitored.Degradation experiments of typical neonicotinoids was studied based on persulfate advanced oxidation processes to elucidate the degradation process,kinetics and degradation mechanism,providing scientific basis and technical support for the treatment of neonicotinoid pesticide contaminated soil.Specific results of the study are as follows.（1）Detection method for 10 neonicotinoids residue in soil was developed based on QuEChERS combined with UPLC-MS/MS.The results showed that the ten neonicotinoids had a good linearity in the set concentration range with R² able to reach 0.999.The spiked recoveries in soil ranged from 61.8-124.2%with the relative standard deviations between 0.5-12.2%,meeting the requirements for the determination of pesticide residues.The results showed that imidacloprid,thiamethoxam,thiamethoxam and acetamiprid occurred with high residue frequency in rice,maize and wheat farmlands soil,with detection rates ranging from 38.9-100%and mean concentrations between 0.3-24.8μg/kg.（2）Effective degradation of imidacloprid in soil by a thermally activated persulfate technology was investigated.The degradation efficiencies of imidacloprid were in the range of 82-97%in 60 mins with the PS dosage of 10 m M,when the initial concentrations of imidacloprid were 5-50 mg/kg in the soil.Degradation of the imidacloprid was fitted with a pseudo-first-order kinetic model under different reaction temperatures.Inhibition effects of the common inorganic anions on the imidacloprid degradation in the system followed the order Cl^->HCO₃^->H₂PO₄^->NO₃^-.Soil pH and soil organic matter were also main factors affecting the degradation of imidacloprid.Three other typical neonicotinoids（acetamiprid,thiamethoxam and dinotefuran）were degraded using this process with 64-97%degradation.Quenching experiments indicated that the major reactive species in imidacloprid degradation were SO4^·-,O2^·-,and·OH.Six degradation intermediates of imidacloprid were inferred in the soil,and degradation pathways of IMI included hydroxylation,denitrification,C-N bond break and further oxidation.（3）The simultaneous degradation of three representative neonicotinoids（i.e.,imidacloprid,thiamethoxam and dinotefuran）in soil by a nanoscale zero-valent iron（nZVI）activated PS process was investigated.Under the optimum conditions with 1 mg g^-1 nZVI,10 m M PS at an initial pH value of 6.9 and a water-soil ratio of 1.5,the degradation efficiency of imidacloprid,thiamethoxam and dinotefuran reached 89%,86%and 69%in 15 mins,respectively.Degradation of the three neonicotinoids was fitted with a pseudo first-order kinetic model.High nZVI dosage and PS concentration were found to enhance the neonicotinoids degradation.In contrast,the presence of Cl^-,HCO₃^-and humic acid（HA）reduced the degradation rates of the three neonicotinoids.Soil physicochemical properties showed a significant effect on the degradation efficiencies of the three neonicotinoids.In addition,quenching experiments combined with electron paramagnetic resonance showed that free radicals SO₄^·-and·OH contributed to the degradation of three neonicotinoids in the nZVI activated PS system.Degradation pathways were proposed based on identified intermediates of imidacloprid,thiamethoxam and dinotefuran.In summary,this paper investigates the degradation of typical neonicotinoids widely detected in agricultural soils based on thermal activation and zero-valent iron-activated persulfate advanced oxidation techniques.The focus is on elucidating the degradation kinetics and mechanisms,and providing scientific basis and technical guidance for the treatment of neonicotinoids contaminated soil.