Reactions of neonicotinoids with peroxydisulfate: The generation of neonicotinoid anion radicals and activation pathway to form sulfate radicals.

To activate persulfate to generate reactive species such as sulfate radical (SO4•-) for micropollutants abatement, external energy or chemicals are often needed. In this study, a novel SO4•- formation pathway was reported during the oxidation of neonicotinoids by peroxydisulfate (S2O82-, PDS) without any other chemical additions. Thiamethoxam (TMX) was used as a representative neonicotinoid and SO4•- was the dominant specie contributing to its degradation during PDS oxidation at neutral pH. TMX anion radical (TMX•-) was found to activate PDS to generate SO4•- with the second-order reaction rate constant determined to be (1.44 ± 0.47)× 106 M-1s-1 at pH 7.0 by using laser flash photolysis. TMX•- was generated from the TMX reactions with superoxide radical (O2•-), which was formed from the hydrolysis of PDS. This indirect PDS activation pathway via anion radicals was also applicable to other neonicotinoids. The formation rates of SO4•- were found to negatively linearly correlated with Egap (LUMO-HOMO). The DFT calculations indicated the energy barrier of anion radicals to activate PDS was greatly reduced compared to the parent neonicotinoids. The pathway of anion radicals' activation of PDS to form SO4•- improved the understanding of PDS oxidation chemistry and provided some guidance to enhance oxidation efficiency in field applications.