Study On The Residual Dynamics And Environmental Behavior Of Hexaconazole In Paddy Field Envionment

Hexaconazole is a triazole fungicide, mainly for the prevention of the Ascomycetes, Basidiomycetes and imperfect bacteria caused diseases such as powdery mildew, gray mold, scab disease. In particular, such as rice sheath blight has a very high biological activity. In order to evaluate the residues in environmental media, the residues of hexaconazole in paddy field and hydrolysis, photodegradation, egradation and absorption in aqueous solutions and soils have been studied. The main results were as follows:1. An analytical method for determining hexaconazole residues was established. Paddy samples were determined with gas chromatography-nitrogen phosphorus detector after extracted with acetonitrile and chloroform. The result showed that the average recoveries of hexaconazole in different paddy samples ranged from81.28%to103.98%, spiked at0.005～0.5mg·L-1and0.05·1.0mg·kg-1. The limit of detection(LOD) in water、soil、paddy straw、brown rice and rice husk were0.005mg·L-1、0.1、0.05、0.02and0.05mg·kg-1, respectively. And the maximum relative standard deviations (RSD) was12.39%. The method satisfied the requirement of pesticide residue analysis. At the same time, in the environmental behavior of test, an analytical method for determining hexaconazole residues in water and soil by GC-ECD was described. The result showed that the linear relationship was good with the range from0.05～5.0mg·L-1, the correlation coefficient r=0.9995. The average recovery for water and soil was83.85%-102.52%, spiked at0.05～1mg·L-1in water samples and0.5～5.0mg·kg-1in soil samples, while the maximum RSD was5.50%. The LOD was0.05mg·L-1and0.5mg·kg-1for water and soil samples, respectively. It indicated that the developed method could be applied for residue determination and environmental behavior of hexaconazole.2. Residues degradation dynamics of hexaconazole in paddy field for two years in three places. The results indicated that the degradation dynamics in paddy samples were in accordance with the kinetic equation. In2009, the half-lives of paddy water, soil and straw in Nanjing, Nanchang and Tianjin were5.0～7.0d,4.4～6.8d,1.0～1.3d, respectively; and in2010, they were6.3～6.4d,8.7～10.0d,1.3～4.1d, respectively. The half-lives were different conditions of three soils and Nanchang paddy straws for two years. This may because the influence by weather and the style of different soils between trial period. The result of the final residue trial of hexaconazole showed that3.5%validamycin-hexaconazole microemulsion was spraied for2times and3times with the dose of36.75g a.i.·ha-1and55.13g a.i.·ha-1in the tillering late. The interval time was7days. After14days, the residues of hexaconazole in rice plants, brown rice and soil were less than the minimum detectable concentration (soil was0.1mg·kg-1, plant was0.05mg·kg-1, brown rice was0.02mg·kg-1). The highest residues of hexaconazole in rice husk was0.08～0.38mg·kg-1. After21days, all residues of hexaconazole were less than the minimum detectable concentrations.3. The result of hydrolysis test showed that hexaconazole is difficult to hydrolysis. At25℃,the half-lives were greater than180days in pH5-9buffer solutions.The hydrolysis rates were as follows:pH7>pH9>pH5.4. The result of photolysis test showed that the photolytic rate of hexaconazole in water under different light sources was significant different to each other. The half-lives under high pressure mercury lamp and xenon lamp were5.62min and12.83h, respectively. Under high pressure mercury lamp, the photolytic rates of hexaconazole in pH5-9buffer solutions showed the following sequence:pH7>pH9>pH5. This is as same as the result of hydrolysis test. The photolysis degradation of hexaconazole accorded with the first order reaction kinetics in the different concentrations of nitrate or nitrite aqueous solutions. The added concentrations of NO3-could increase the photolysis rates. The NO2-could promote the photolysis rates at the concentrations of0.4and2.0mg·L-1, but inhibit the processes at the concentrations of10.0to20.0mg·L-1.5. The adsorption-desorption characteristics of hexaconazole were studied. The results showed that the adsorption-desorption isotherms of hexaconazole were fitted to Freundlich model well. Notable different adsorption behaviors of hexaconazole were observed in the three tested soils, with the adsorption constants were0.791,2.274and43.800. Moreover, the adsorption rate was in a excellent correlation to the OM、CEC and Clay of the soil. Adsorption isotherm of hexaconazole in clay and loam exhibited L-type isotherm, but it was S-type isotherm in sand. The free energy of sorption ranged from-5.186to-8.164kJ·mol-1, which indicated that the adsorption could be the physical adsorption.6. The result of soil degradation test showed that the half-lives of hexaconazole in sand, loam and clay were123.8d,70.0d and30.5d, respectively, and in those three sterilized soils, the half-lives were495.0d,330.0d and169.0d. respectively, while in flooded soils, they were231.0d,161.2d,79.6d, respectively. It indicated that soil microorganisms, especially the aerobic microorganisms, had significant effect on the degradation of hexaconazole in soils.