Photodegradation of Neonicotinoid Active Ingredients and Their Commercial Formulations in Water by Different Advanced Oxidation Processes

This contribution is concerned with the comparison of the efficiency of the removal of four pure neonicotinoid active ingredients (AIs) and their commercial formulations (CFs) from aqueous solutions by using different advanced oxidation processes at the pH 2.8. The AIs of thiamethoxam and imidacloprid, and their CFs (Actara and Confidor), having a nitroguanidine functional group, exhibited low persistence to photolysis. In contrast to them, thiacloprid and acetamiprid and their CFs (Calypso and Mospilan), containing a cyanoimine functional group, were stable during the UV irradiation period. As expected, the degradation rate of the studied neonicotinoids increased significantly in the combined action of UV radiation and H2O2. In the case of thiacloprid and acetamiprid and their CFs, the reaction of the OH radicals formed and molecules of these insecticides was the major destruction pathway. The increased photodegradation efficiency of the UV/7.2Fe/TiO2/H2O2 and vis/7.2Fe/TiO2/H2O2 processes was attributed to the surface photoreduction of Fe3+ to Fe2+, which produces new OH radicals in the reaction with H2O2. In the presence of visible light, the efficiency may be partly due to the formation of the H2O2-TiO2 complexes. For the 7.2Fe/TiO2/H2O2 process in the presence of UV or visible radiation, no significant influence on the efficiency of photodegradation was observed in dependence of the structural differences of selected neonicotinoids. These results strongly suggest that highly reactive hydroxyl radicals, generated on the catalyst's surface in the reaction involving H2O2, are responsible for this oxidation. In order to investigate degree of mineralization for all insecticides, TOC measurements were also conducted. Also, it was observed that the removal of pure AIs and their CFs by dark adsorption was almost negligible.