Degradation of the insecticides thiamethoxam and imidacloprid by zero-valent metals exposed to ultrasonic irradiation in water medium: electrospray ionization mass spectrometry monitoring

The degradation of thiamethoxam (1) and imidacloprid (2), prototype neonicotinoid insecticides bearing a characteristic N-NO2 moiety in their structures, promoted by a number of zero-valent metals (Fe, Sn, Zn) upon ultrasonic irradiation in acidic aqueous solution (pH 2) was investigated. It was verified that thiamethoxam (1) and imidacloprid (2) are quickly and almost completely consumed under these experimental conditions (degradation >90% after a reaction time of 30 min) and that ultrasonic irradiation strongly enhances the degradation rate for both insecticides, especially when zinc and tin are employed. Based on the results from electrospray ionization mass (and tandem mass) spectrometry in the positive ion mode, degradation routes for both insecticides, comprising an initial NO2 --> NH2 reduction, were proposed. In addition, products from the dehydrochlorination of imidacloprid were also found to be formed under these conditions.