Degradation of neonicotinoids by UV irradiation: Kinetics and effect of real water constituents

Neonicotinoids are a new class of insecticides whose potential side-effects on the human health and the environment have not yet been well investigated. Recently, the EU approved the Decision 2015/495 establishing a watch list of substances for Union-wide monitoring in the field of water policy, which includes five neonicotinoids: thiamethoxam (TMX), imidacloprid (ICP), clothianidin (CTD), thiacloprid (TCP) and acetamiprid (AMP). The photodegradation of neonicotinoids by monochromatic UV irradiation (254 nm) has been investigated. The photolysis rate followed pseudo-first order kinetics, with rate constant values depending on the nature of the specific compound, pH, and the presence or absence of radical scavengers. A method based on atrazine actinometry provided the following quantum yields at pH = 7: (77 ± 3) × 10−3, (43 ± 2) × 10−3, (48 ± 1) × 10−3, (17 ± 2) × 10−3 and (5 ± 2) × 10−3 mol E−1 for TMX, CTD, ICP, TCP and AMP, respectively. The effects of the presence of bicarbonate, nitrate, nitrite and chloride ions on the simultaneous photodegradation of nicotinoids were not very significant, and only the removal of AMP, the least photosensitive neonicotinoid, was negatively affected. However, the photodegradation of neonicotinoids was clearly inhibited by the addition of humic acids. The results obtained in experiments performed in the presence of hydrogen peroxide or persulfate indicated that while direct UV photolysis was the main degradation pathway for TMX and even for CTD, ICP and TCP, the contribution of indirect photolysis (radical pathway) was important in the case of pollutants with low values of quantum yield such as AMP. Although the presence of natural organic and inorganic matter in real water matrices decreased the degradation rate of the selected neonicotinoids, UV photolysis and the AOPs UV/H2O2 and UV/PS processes constitute efficient technologies for the removal of these pollutants from contaminated waters.