Sorption and Degradation of Imidacloprid in Soil and Water

Imidacloprid, the major component of many widely used insecticide formulations, is highly persistent in soils. In this study, the sorption of imidacloprid by six soils as well as its photodegradation and hydrolysis in water were studied. The soils differed significantly in organic matter content and other physical and chemical properties. Sorption increased with increasing soil organic matter content but was not significantly correlated with other soil properties. Removal of organic matter via H₂O₂ oxidation decreased the sorption. By normalizing the Freundlich coefficients (K _f ) to organic matter contents, the variability in obtained sorption coefficient (K _om ) was substantially reduced. These results indicate that soil organic matter was the primary sorptive medium for imidacloprid. The low heat of sorption calculated from K _om suggests that partition into soil organic matter was most likely the mechanism. The photodegradation and hydrolysis of imidacloprid in water followed pseudo-first-order kinetics; however, the latter process needed a six-time-higher activation energy. While both processes produced the same main intermediate, they occurred via different pathways. The hydrolysis of imidacloprid was not catalyzed by the high interlayer pH in the presence of metal-saturated clays, which appeared to result from the lack of the pesticide adsorption in the interlayers of clays.

Key Words:

• Imidacloprid
• Sorption
• Soil organic matter
• Photodegradation
• Hydrolysis
• Pathway

ACKNOWLEDGMENTS

This work was supported by the National Basic Research Program of China (2002CB410800), the National Natural Science Foundation of China (No. 30270767), and the National Natural Science Foundation of China for Distinguished Young Scholars Program to Weiping Liu (No. 20225721).

Notes

* Oven-dry basis.

** Measured in 1:1 (w:w) soil:water mixture.

* T _1/2 is half-life.

* T _1/2 is half-life.