Study On Environmental Dissipation,Transport And Removal Behaviors Of Thiamethoxam And Fosthiazate

Neonicotinoid insecticides,widely used for seed treatment in many corps,are of increasing environmental concern due to potential risks to non-target organisms and aquatic environment.Taking thiamethoxam?TMX?as the example,environmental fete and transformation of TMX as corn seed coating were investigated for better understanding potential risks of neonicotinoids to the surroundings.TMX coating corn seeds were grown in soil columns under greenhouse conditions;corn plant,soil,and leachate samples were collected and analyzed by using ultra-performance liquid chromatography tandem mass spectrometry?UPLC-MS/MS?at different growing stages.Different soil textures?sand and clay?and treatment methods?with and without viable corn plants?had an effect on fate and transmission of TMX.Results showed that residue levels of TMX and its metabolite clothianidin?CLO?in different matrices?except for root?trended to decrease during the growing season.TMX had a higher leaching potential and faster dissipation in clay soil with lower total organic carbon and higher water content.Plant uptake could reduce TMX fate and plant root had a negetive effect on TMX leaching in soil column system.More than 90%of TMX transferred into plant and soil or leached into water system from seed coatings at V1 stage?day 8?,and most part of TMX was determined in sand column empirically in 0-30 cm of bulk soil.Nearby 66.6%-83.3%of TMX were lost in sand and clay columns system,only 2.4%-6.4%of TMX transferred to CLO and were detected in whole column system at V5 stage?day 36?.TMX with leaching potential index easily leached into groundwater through unstructured clay soil under heavy rainfall conditions.Fosthiazate,a member of nonfumigant organophosphate nematicide with similar water soluble and application with thiamethoxazole,was widely used to control root-knot nematodes on various types.To investigate dissipation dynamics and residue of fosthiazate in tomato and soil is beneficial for understanding environment fate of pesticide.The field trial was conducted in three sites Beijing,Liaoning,Hubei in China.The half-lives of fosthiazate were 0.75-2.6 days in tomato?tomato plants?and 2.5-11.6 days in soil.In the terminal residue experiment,no higher residue than 0.27 mg kg-1 in soil and 0.023 mg kg-1 in tomato were detected.Residues of fosthiazte in tomato were below China maximum residue levels?MRL,0.05 mg kg-1?.Fosthiazate had potential risk of leaching in soil.Montmorillonite colloids are widely distributed in subsurface environments such as soil,which can affect the mobility of contaminants once they are adsorbed on the colloidal surfaces.However,their influence on transport of fosthiazate,a widely used organophosphorus pesticide,has not been investigated to date.The stability of foshiazate standard solution at room temperature and sorption of fosthiazate by montmorillonite colloids by batch experiment were investigated.Foshiazate standard solution had a better stability at lower pH or lower ionic strength?IS?conditions.The sorption of fosthiazate by the colloids can be well described by Weber-Morris kinetic model and Freundlich isotherm model,and hydrophobie bonds between fosthiazate and the colloids was found by calculating adsorption thermodynamic equation.The individual transport of montmorillonite colloids and fosthiazate in saturated quartz sand porous media and their cotransport were investigated.The retention of fosthiazate in quartz sand column was very few.The attachment of montmorillonite colloids in saturated quartz sand column was significant at high IS and low pH,in agreement with the prediction by Derjaguin-Landau-Verwey-Overbeek?DLVO?interaction energy and colloidal attachment efficiency calculations.The transport of low concentration of montmorillonite colloids could be facilitated with the presence of fosthiazate at lower IS in saturated quartz sand column.With concentration of montomorillonite colloids increasing,total of mobile fosthiazate firstly increased and then decreased in quartz sand column effluents,and fosthiazate sorbed onto mobile montmorillonite colloids increased and fosthiazate in aqueous solution decreased in quartz sand column effluents.The similar conclusions were found in Tianjin soil with higher pH and lower organic matter and Heilongjiang soil with lower pH and higher organic matter.Overall,cotransport of montmorillonite colloids and pesticides exists in the subsurface environment.The removal of organophosphorus pesticide fosthiazate in aqueous solution using zero valent iron?ZVI?and the removal reaction mechanism were investigated.The results indicated that the dissipation of fosthiazate fits a pseudo-first order reaction law.The apparent rate constant of fosthiazate removal could be improved by increasing the ZVI dosage,decreasing concentration of fosthiazate and increasing the control temperature and the initial pH.The observed pseudo-first-order degradation rate constants?Kobs?of fosthiazate removal using ZVI were varied in different electrolyte solutions,which were determined as follows:Kobs?MgSO4?<Kobs?KCl?<Kobs?Control?<Kobs?NaCl?<Kobs?CaCl₂?<Kobs?NaN0₃?<Kobs?Na₂S0₄?.The effects of Fe²⁺ and Fe³⁺ ions on the fosthiazate removal were very weak;however the alkaline environment created by chemical corrosion of ZVI is the key to hydrolysis of fosthiazate.The main degradation products of fosthiazate were investigated by Agilent 1100 LC/MSD Ion Trap.Based on the results,ZVI could be suggested for the efficient removal of organophosphate pesticide fosthiazate in wastewater.