| Pymetrozine, or6-methyl-4-[(E)-(pyridine-3-ylmethylene)amino]-4,5-dihydro-2H-[1,2,4]-triazin-3one (IUPAC), is a selective insecticide against Hemipteran and Homopteran insects such as aphids, whiteflies and planthoppers. It represents a new chemical class of insecticides-pyridine azomethine group and acts in a unique way. Recently, pymetrozine is in widespread use in rice and wheat fields, and the prohibition of highly toxic organophosphate pesticides in China even promotes its extensive application. However, it has been classified recently as a "likely" human carcinogen by the United States Environmental Protection Agency (EPA). As a result, the research on the environmental behaviors of pymetrozine is of significant importance. Adsorption on soil affects the mobility of the pesticide and therefore determines its potential to reach ground and surface water; besides that, the binding to soil reduces the bioavailability of the pesticide and thus its risk to environment. However, our current understanding of pymetrozine sorption on soil systems is lacking. Therefore, the aim of our work was to evaluate the adsorption behavior of pymetrozine on different soils in China.For this purpose, the adsorption and desorption of pymetrozine on six Chinese soils were studied by batch equilibrium method. The adsorption and desorption isotherms fitted well with Freundlich model (0.981<r<1.000). The Freundlich adsorption coefficient KF (mL g-1) varied from3.37to58.32, and the adsorption isotherms were observed to be nonlinear with1/n ranged between0.57-0.91. The adsorption of pymetrozine on soil could be devided as a fast-adsorption phase and a slow-adsorption phase. The kinetics of pymetrozine adsorption on soil A could be regressed by Elovich equation as:y=2.007Ln(x)+8.6651with R2=0.9601. The order of Kf at desorption was the same as Kf at adsorption between the six soils. For all the tested six soils, the Freundlich coefficients were higher at desorption step than adsorption step, indicating that once pymetrozine was adsorbed on the soils, it was not easy to be desorbed. A regression equation was proposed via Multi-ANOVA analysis for predicting the adsorption of pymetrozine on different soils:log KF=3.9021-3.5339x Log (pH in0.01M CaCl2)+0.7106×log OC%, with R2=0.9661. Generally, the organic carbon content of soil influenced the adsorption of pymetrozine positively and to the contrary, soil pH had a negative influence on the adsorption.In addition, the effects of CaCl2concentration, soil to solution ratio and pesticide form were investigated. A factorial experiment with3variables at2levels was performed in advance to screen the significant effect on adsorption of pymetrozine in soils. The result showed that all the three parameters significantly influenced the adsorption (p<0.05), following the order:soil to solution ratio>> CaCl2concentration> pesticide form. After that, three individual experiments to study the effect of each parameter were performed separately. When the soil to solution ratio was adjusted to0.05,0.1,0.2and0.5, the results showed exponential growth of adsorption coefficient along with the increase of soil to solution ratio, the distribution coefficient Kd (mL g-1) equaled to0.75,3.17,5.60and8.17respectively. The adsorption decreased significantly with CaCl2concentration particularly up to0.1M CaCl2. The value of Kd decreased at a rate of7.5%,24.0%,36.2%respectively comparing to blank (no CaCl2was added) at the CaCl2concentration of0.01,0.1and1M. That was, the decrease in adsorption did not vary linearly with ionic strength. Five formulated products with different dosage form or strength were selected to assess the influence of formulation types to the adsorption of pymetrozine on soil. The adsorption of F1, F2, and F3were significantly higher than technical material (p<0.05); whereas the adsorption of F4was weaker than technical material (p<0.05) and no significant difference was observed for F5(p=0.35). As a consequence, the adsorption of formulated products in field might be under or over estimated as per the results from laboratory batch technique with technical materials. |
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