Degradation of thiacloprid via unactivated peroxymonosulfate: The overlooked singlet oxygen oxidation

Abstract

Unactivated peroxymonosulfate (PMS) degrading organic contaminants has been reported. Previous studies have focused on direct PMS oxidation, while the role of singlet oxygen (1O2) is often overlooked. Here, we studied the oxidation of thiacloprid (THIA) of the neonicotinoid insecticides by PMS without explicit activation. According to electron spin resonance spectroscopy and quenching experiments, the nonradical mechanism (i.e., direct PMS oxidation and 1O2 oxidation) was responsible for the THIA degradation. There are two pathways to generate 1O2. The pyridine nitrogen of THIA structure can combine with PMS to produce epoxide, which is beneficial to generation of 1O2. However, the contribution of 1O2 from PMS self-decomposition can be ignored. According to kinetic solvent isotropic effect analysis, the overall THIA degradation was attributed to 36.5% 1O2 oxidation and 63.5% direct PMS oxidation. A structure-activity assessment and density functional theory suggested that thioether sulfur, amidine nitrogen and the cyanoimino groups were the main reaction sites. Product analysis further confirmed the involvement of 1O2 and PMS. Moreover, three major THIA degradation pathways were proposed, including (i) heterolytic cleavage of peroxo bonds and transfer of oxygen atoms, (ii) electron transfer and (iii) oxidation of cyanoimino groups to generate nitroso/nitro-THIA. In addition, identification byproducts (Except for TP141) had lower ecotoxicity toxicity (using ECOSAR) in fish, daphnid and green algae. Effects of pH and natural water matrices on THIA oxidation via PMS were further evaluated. This study investigated the overlooked 1O2 oxidation reaction to elucidate the mechanism. The new mechanistic knowledge has important implications for other contaminants for their interactions with PMS.