Abstract
Along with rapid social development, the use of insecticides and caffeine-containing products increases, a trend that is also reflected in the composition of surface waters. This study is focused on the phototreatment of a surface water containing three neonicotinoids (imidacloprid, thiamethoxam, and clothianidin) and caffeine. Firstly, the radiation absorption of the target pollutants and the effect of the water matrix components were evaluated. It was observed that the maximum absorption peaks appear at wavelengths ranging from 246 to 274 nm, and that the water matrix did not affect the efficiency of the removal of the target pollutants. It was found that the insecticides were efficiently removed after a very short exposure to UV irradiation, while the addition of hydrogen peroxide was needed for an efficient caffeine depletion. The electrical energy per order was estimated, being the lowest energy required (9.5 kWh m−3 order−1) for the depletion of thiamethoxan by indirect photolysis, and a concentration of hydrogen peroxide of 5 mg dm−3. Finally, a preliminary evaluation on the formation of by-products reveals that these compounds play a key role in the evolution of the ecotoxicity of the samples, and that the application of direct photolysis reduces the concentration of these intermediates.
Highlights
Neonicotinoids are a class of insecticides that includes imidacloprid, acetamiprid, thiacloprid, dinotefuran, nitenpyram, thiamethoxam, and clothianidin
According to the kinetic constants obtained, it can be concluded that the optimal concentration of hydrogen peroxide was 15 mg dm−3, as further additions of this chemical did not significantly improve the degradation of caffeine
It was observed that caffeine reaches its maximum absorption at 274 nm, imidacloprid at 272 nm, clothianidin at 246 nm, and thiamethoxam at 251 nm
Summary
Neonicotinoids are a class of insecticides that includes imidacloprid, acetamiprid, thiacloprid, dinotefuran, nitenpyram, thiamethoxam, and clothianidin. This class represents about 20% of the actual global insecticide market. According to Bonmatin et al [5], neonicotinoids are expected to accumulate in soils, since their half-lives can exceed 1000 days. Even though these compounds were reported in laboratory studies to be susceptible to rapid degradation through photolysis (e.g., aqueous photolysis DT50 < 1 day for example for clothianidin), slow rates of dissipation have been described under field conditions. The authors state that the high-to-moderate solubility, leaching potential, and persistence of most neonicotinoids cause a continuing and increasing risk to the aqueous environment [5]
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