Photocatalytic degradation with immobilised TiO2 of three selected neonicotinoid insecticides: Imidacloprid, thiamethoxam and clothianidin

This research focused on photocatalytic degradation of imidacloprid, thiamethoxam and clothianidin employing a tailor-made photoreactor with six polychromatic fluorescent UVA (broad maximum at 355 nm) lamps and immobilised titanium dioxide (TiO2) on glass slides. The disappearance was followed by high pressure liquid chromatography (HPLC–DAD) analyses, wherein the efficiency of mineralization was monitored by measurements of total organic carbon (TOC). Within 2 h of photocatalysis, all three neonicotinoids were degraded following first order kinetics with rate constants k = 0.035 ± 0.001 min−1 for imidacloprid, k = 0.019 ± 0.001 min−1 for thiamethoxam and k = 0.021 ± 0.000 min−1 for clothianidin. However, the rate of mineralization was low, i.e. 19.1 ± 0.2% for imidacloprid, 14.4 ± 2.9% for thiamethoxam and 14.1 ± 0.4% for clothianidin. This indicates that several transformation products were formed instead. Some of them were observed within HPLC–DAD analyses and structures were proposed according to the liquid chromatography–electro spray ionization tandem mass spectrometry analyses (LC–ESI–MS/MS). The formation of clothianidin, as thiamethoxam transformation product, was reported for the first time.

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► Neonicotinoids are linked with bee deaths, thus their transformation products were studied.
► TiO2 photocatalysis of imidacloprid, clothianidin and thiamethoxam are described.
► No publications are available on clothianidin and thiamethoxam photocatalysis.
► The formation of clothianidin from thiamethoxam was proved for the first time.