OH radical initiated oxidative degradation of imidacloprid in the environment: An assessment of breakdown kinetics and environmental hazards

Imidacloprid (IMI) is a common neonicotinoid pesticide that acts via a similar mechanism of action to nicotine, a naturally occurring insecticide. It is therefore important to understand its chemical fate in the environment. Hydroxyl radicals (HO) are significant oxidizing species in natural aquifers due to their strong reactivity towards organic substrates. Therefore, principal photo-oxidation products are expected to form with the involvement of HO in the self-cleaning process of water in nature. Here quantum chemical calculations are used to examine the reaction of IMI with HO in the atmosphere and aqueous environments. It was found that the principal mechanism of the HO + IMI reaction is the hydrogen transfer that, in a two-step process, produces stable cations in the gas phase. Within the atmospheric temperature range of 253–323 K, the overall rate constants for the HO + IMI reaction decreased from 4.35 × 10¹⁰ to 2.13 × 10¹⁰ M⁻¹ s⁻¹. Consequently, IMI can undergo rapid gaseous degradation within a comparatively brief period of 2.81 × 10⁻⁴ – 5.75 × 10⁻⁴ years. However, differences in environmental temperature and pH in aqueous environments influence the processes, rate constants, and products of IMI breakdown by the HO radical. The data indicate that IMI breakdown by OH radicals is strongly temperature and pH-dependent, resulting in the generation of different reaction products. The results imply that at all pH levels, the interaction between IMI and the ambient HO radical in water produces toxic chemical species. According to the computed data, it appears that IMI and the vast majority of degradation products present potentially carcinogenic and/or mutagenic hazards, and they also lack downstream biodegradability.