Reactivity and reaction pathways of peroxymonosulfate and peroxydisulfate with neonicotinoid insecticides

Neonicotinoid insecticides (NNIs), which have been detected across diverse aquatic environments, have sparked substantial concerns regarding their potential adverse ecological and health risks. In this study, the removal of NNIs by unactivated peroxymonosulfate (PMS) and peroxydisulfate (PDS) was systematically investigated. Results showed that PMS/PDS direct oxidation is mainly responsible for the degradation of imidacloprid (IMD), and the degradation kinetics can be well described by a second-order kinetics model, first-order in both IMD and PMS/PDS concentration. The species-specific reaction rate constants of HSO₅^- and SO₅^2- with IMD were calculated to be 429.36 ± 15.41 M^-1h^-1 and 9.72 ± 35.48 M^-1h^-1, while the corresponding rate constant between S₂O₈^2- and IMD is 25.04 ± 3.04 M^-1h^-1. Over 100 transformation products in the degradation of IMD by PMS/PDS were identified by HPLC/Q-Orbitrap HRMS, and five major reaction pathways were proposed thereafter: hydroxylation on imidazolidine ring, olefin reaction on imidazolidine ring, desnitro reaction on nitroguanidine moiety, and two chain-breaking reactions between imidazolidine ring and chloro-pyridyl moiety. Toxicity evaluation on the transformation products found that their ecotoxicity is various at a wide range with an overall indeterminacy, while their bioconcentration factors show a definite decrease. The reactivity of six NNIs with PMS/PDS was found varied by structures but generally low, indicating that in-situ oxidation with unactivated PMS/PDS is safe but inefficiency for the mitigation of NNIs. It is thus suggested that further investigations into activated PMS/PDS systems involving radicals promise enhanced remediation of NNIs, and fundamental data in this study has laid the groundwork.