Understanding roles of different radicals (·OH, SO4·- and ClO·) in the thiacloprid degradation

Thiacloprid (THD), a widely used neonicotinoid insecticide, has become a pervasive contaminant in global aquatic systems. While advanced oxidation processes (AOPs) show promise for THD remediation, the degradation mechanisms mediated by AOP-generated radicals remain unclear. This study employs density functional theory (DFT) to systematically investigate THD degradation pathways initiated by three key radicals (·OH, SO₄^·- and ClO·), while assessing the ecotoxicity of resulting transformation products. Mechanistic analysis reveals distinct reaction patterns: ·OH predominantly undergoes hydrogen atom abstraction, whereas SO₄^·- and ClO· exhibit dual pathways combining hydrogen abstraction and radical adduct formation. Thermodynamic and kinetic analysis identifies preferential reaction sites: hydrogen abstraction primarily occurs at the side chain and 2-thiazoline ring positions, while radical addition favors C11, C13, and C14 positions. Kinetic calculations establish radical reactivity as SO₄^·- (5.46 × 10⁸ M^-1 s^-1) > ·OH (4.30 × 10⁸ M^-1 s^-1) > ClO· (7.62 × 10⁶ M^-1 s^-1). Subsequent transformations primarily proceed through oxygen addition and radical addition pathways, yielding hydroxylated, carbonylated, and ring/chain-cleavage products. The ecotoxicity assessment reveals that THD exhibits aquatic toxicity and mutagenicity, with most transformation products showing reduced toxicity but retaining mutagenic potential. This study provides valuable insights for optimizing radical selection strategies and advancing the development of more efficient, sustainable AOPs in the remediation of aquatic environments contaminated with neonicotinoid insecticides.