The reaction of acetamiprid with OH radicals in the environment: a theoretical study

The chemical fate of acetamiprid (AMP), a neonicotinoid pesticide, is determined by photo-oxidation: a combination of radical degradation pathways driven by the action of hydroxyl radicals. This study utilizes quantum chemical calculations to investigate the reaction of AMP with hydroxyl radicals in atmospheric, lipidic, and aqueous media. It was shown that the degradation process has a steep temperature dependence with the overall rate constant decreasing from 9.04 × 10⁹ to 5.01 × 10⁹ M^-1 s^-1 in the temperature range of 253-323 K thus AMP lifetime in the gas phase varies from 17.26 to 41.37 hours. In lipid media, the AMP + HO˙ reaction exhibited an overall rate constant k _overall of 1.63 × 10⁸ M^-1 s^-1, while in water, it was 2.95 × 10⁸ M^-1 s^-1, closely matching the experimentally measured rate constant (k _exp = 7.59 × 10⁸ M^-1 s^-1). In natural water, where hydroxyl radical concentrations range from 10^-18 to 10^-15 M, AMP degradation is predicted to occur over approximately 6.47 × 10² to 1.06 × 10⁶ hours at 273-373 K, corresponding to a range of ∼27 days to ∼121 years. Across all examined media and temperature conditions, the AMP + HO˙ reaction followed primarily the hydrogen transfer mechanism, with a minor role also played by the radical adduct formation pathway.