| Neonicotinoid pesticides are currently the most widely used insecticides and have become a hot research topic in the environmental field because of their high toxicity to bees and invertebrates.However,because only 1.6%to 10%will be absorbed by crops,most of the pesticide molecules collect in the soil environment.Meanwhile,with rainfall and surface runoff,these excess pesticide molecules will gradually enter the deep soil layer or even groundwater,causing soil and groundwater pollution,and these excess pesticide molecules will also pose a potential threat to humans who are exposed to this environment for a long time.In this paper,the solubility of thiamethoxam in soil leachate was measured by laser dynamic method,and the Apelblat equation was used to correlate the solubility data to obtain the solubility pattern of thiamethoxam in soil leachate.The leaching behavior of different initial concentrations of thiamethoxam in different soil profiles was studied by column leaching,and the relationship between thiamethoxam solubility and leaching migration capacity was investigated.（1）A combined solid-phase extraction-high performance liquid chromatography（SPE-HPLC）method was used to detect thiamethoxam in leachate and soil,and the extraction solution was selected for the pretreatment process.Under this method,the linearity of the standard solution of thiamethoxam in the concentration range of 0～100 mg/L was good,and the R~2reached above 0.999.（2）The solubility of thiamethoxam in soil leachate under different conditions was measured by dynamic laser monitoring technique.The solubility of thiamethoxam was the best in the leachate obtained after drenching in neutral drench solution compared to acidic and alkaline drench solution;the solubility value of thiamethoxam was the highest on the fifth day with increasing number of days of drenching at the same temperature,while it was the highest on the third day under acidic and alkaline conditions.（3）The GUS value of thiamethoxam was calculated to be 2.1,which has a moderate to high mobility in soil.The soil leaching experiments also showed that the leaching and migration behavior of thiamethoxam in soil was related to the initial concentration of pollutants and leaching time.The leaching solubility of thiamethoxam was positively correlated with the initial concentration of pollutants,and the larger the pollution concentration,the more obvious the downward migration ability;with the increase of days,the concentration of thiamethoxam in the leachate of different pollution concentrations showed a trend of rising to the highest value and then slowly decreasing.（4）Fitting the solubility with the residual concentration of thiamethoxam in the leachate,the results showed that:during the leaching process,thiamethoxam could reach the solid-liquid partitioning equilibrium on the fourth day;at low concentrations,thiamethoxam mainly carried out the solid-liquid partitioning process in the soil column,and the residual concentration in the leachate was not related to its solubility in the leachate;while at high concentrations,the residual concentration in the leachate and its solubility was positively correlated（R~2=0.99）.（5）Three parallel feasible degradation pathways were proposed by LC-MS analysis and determination of soil samples in soil columns.Based on the identified intermediates obtained,it can be concluded that nitroreduction is the main pathway for microbial transformation of thiamethoxam;while·OH radicals play a very important role in the hydrolysis process. |
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