ABSTRACT
Presented in this paper is a study on the photodegradation of two widely used neonicotinoid insecticides clothianidin and thiamethoxam in three soils and in solid phase. The effects of light with differing wavelengths were examined using the natural sunlight and single ultraviolet A (UVA) and ultraviolet B (UVB) light sources. The results indicated that UVB played a key role in the photodegradation of clothianidin and thiamethoxam while the effects of visible and UVA lights were negligible. The degradations of clothianidin and thiamethoxam under all the light sources followed the first-order kinetics, and the half-lives of clothianidin and thiamethoxam in the three soils under the sunlight ranged from 97 to 112 h and 88 to 103 h, respectively. When clothianidin and thiamethoxam were directly exposed to the sunlight without soil, the degradation rates were remarkably higher with half-lives being 13 and 10 h, respectively. Therefore, the insecticides fallen on the surface of soils would be degraded under sunlight much faster than those that enter the soils. The examination of the degradation products revealed four compounds from the photodegradation of clothianidin and three from thiamethoxam, and clothianidin was one of the photodegradation products of thiamethoxam.