Residue Dissipation And Dietary Risk Assessment Of Oxadiargyl,Diflufenican And Pendimethalin In Rice

As one of China’s major food crops,rice accounts for one-third of China’s total grain output.However,the annual output loss due to problems caused by weeds is 20 billion kilograms and the economic loss reaches 30 billion yuan.Currently,chemical weeding,is considered to be a fast and effective method to control weeds.The emergence of herbicides has ensured rice yield and quality.However,due to improper use,a large number of herbicides are applied,which damages the environment,has negative impact on the agricultural economy,and threatens human health.In this paper,a multi-residue detection method for oxadiargyl,diflufenican and pendimethalin has been established with HPLC/MS/MS in order to examine the digestion laws of this three pesticides in plants,soil,and water,to detect the residues of three pesticides in plants,brown rice,rice husks and soil,and to assess their dietary risk in rice.The results of the experiments are as follows:1.A multi-residue detection method for oxadiargyl,diflufenican and pendimethalin in rice has been established with high performance liquid chromatography tandem mass spectrometry（HPLC/MS/MS）.A mixture of acetonitrile（0.6 m L formic acid）and water was employed to extract the oxadiargyl,diflufenican and pendimethalin in plant,brown rice,rice husk and soil samples while the oxadiargyl,diflufenican and pendimethalin in field water samples were extracted with acetonitrile（0.6 m L formic acid）only.Then,both were purified by 60 mg PSA adsorbent.Gradient elution was conducted with methanol and water（5 mmol ammonium acetate+0.1%formic acid）as mobile phases.Detection of analytes was performed by high performance liquid chromatography tandem mass spectrometry（HPLC-MS/MS）.The average added recoveries of oxadiargyl,diflufenican and pendimethalin were 74.1-102.4%,78.1-102.7%and88.0-102.8%,respectively.Besides,the relative standard deviations（RSDs）were 1.4-5.8%,0.8-6.6%and 0.8-5.6%.The minimum detectable concentration of the three pesticides in brown rice and rice husk was 0.005 mg/kg,and the minimum detectable concentration in plants,soil and field water was 0.01 mg/kg.The method is simple,fast and accurate,saves reagents and meets the requirements for pesticide residue detection.2.The digestion laws of oxadiargyl,diflufenicanand pendimethalin in plants,soil,and water have been examined.In 2007,the half-lives of oxadiargyl in plant,soil and water samples were 0.3-0.7 d,2.7 d and 0.1-3.6 d respectively;the half-lives of diflufenican were only calculated in plants and soil samples,which were 0.6-0.8 d and 5.9-6.0 d,respectively,as the degradation rate of diflufenican in the field water not detected in the residue after 3 days of application,a first-order kinetic equation could not be used to establish a digestion curve.The initial residues of the pendimethal in plant,soil and field water in Changchun,Changsha and Guangzhou were all less than the limit of quantitation,which is 0.01 mg/kg after 2 hours of application.so the digestion curves of the three places could not be fitted with first-order kinetic equations,and thus were used for monitoring purposes only.Oxadiargyl,diflufenican and pendimethalin are easily degradable pesticides because the half-lives t_1/2 of the three pesticides are all less than 30 d.In this test,the half-lives of oxadiargyl and pendimethalin in plants,soil,and field water were slightly different from those of previous research,The degradation rate of diflufenican in the field water was so fast that its half-life could not be calculated.The factors which affect the degradation of pesticides may be the relatively low proportion of pesticides in the mixture,leaching of rainwater,microbial degradation,and photolysis.3.The residues of oxadiargyl,diflufenican and pendimethalin in plants,brown rice,rice husks,soil and field water were measured.In 2017,final residue tests were carried out in three places in one year with the low dose of 1500 g/hm²（540 g a.i./hm²）and the high dose of 3125g/hm²(810 g a.i./hm²).In 2018,the final residue test was conducted in nine places in one year,and the application dose was 1500 g/hm²（540 g a.i./hm²）,all of which were applied once with the toxic soil method.From 2017 to 2018,the residues of oxadiargyl,diflufenican and pendimethalin in plants,soil,and water were all less than 0.01 mg/kg,and the residues in brown rice and rice hust are both less than 0.005 mg/kg.Based on the results of the residual digestion test,a 36%oxadiargyl·diflufenican·pendimethalin dispersible oil suspending agent is recommended to prevent annual weeds in rice fields.The poisonous soil method should be applied 3-7 d before rice transplanting,and before the leaf stage of barnyard grass.The highest dose is 1500 g/hm²（540 g a.i./hm²）.At present,China,Japan and the United States have not formulated the maximum residue limit value of diflufenican on brown rice.The EU has established a maximum residue limit of diflufenican on brown rice of 0.01 mg/kg.According to the maximum residual limit value of diflufenican of EU and the results of this research,I recommend that the maximum residue limit value of diflufenican on brown rice in China should be 0.01 mg/kg.4.Dietary risk assessment of oxadiargyl,diflufenican and pendimethalin in rice was conducted.Based on the final residue test results from 2017 to 2018,taking the lowest detected concentration of the three pesticides as the median residue,and combining with the calculation formula of dietary risk assessment,the results showed that general population’s the national estimated daily intakes of oxadiargyl,diflufenican and pendimethalin were 0.0058,0.0265 and0.0630 mg,accounting for 1.2%,0.2%and 3.3%of the acceptable daily intake.The RQ values of oxadiargyl,diflufenican,and pendimethalin on rice in different groups were 2.74×10^-3～6.79×10^-3,1.10×10^-4～2.72×10^-4and 7.30×10^-4～1.81×10^-3,all of which are far less than 1,and thus the dietary risk is low.