Co-Mn-Fe spinel-carbon composite catalysts enhanced persulfate activation for degradation of neonicotinoid insecticides: (Non) radical path identification, degradation pathway and toxicity analysis

The extensive utilization of neonicotinoid insecticides (NNIs) in agricultural practices ultimately poses a significant threat to both the environment and human health. This work focuses on the efficient degradation and detoxification of the representative NNI, thiamethoxam (THX), and explores the underlying mechanism using a Co-Fe-Mn mixed spinel doped carbon composite catalyst activated persulfate. The findings demonstrate that the composite effectively degrades THX, achieving a degradation rate of 95% in 30 mins, while requiring only a fraction (one-sixteenth) of the oxidant dosage compared to pure carbon. The study aimed to examine the negative impact of reactive halogens on reactive oxygen species within a saline environment. The degradation byproducts were linked to the presence of two common electron-withdrawing groups, namely halogens and nitro in the THX molecule. It was hypothesized that the degradation process was primarily influenced by C-N bond breaking and hydroxylation occurring between the diazine oxide and 2-chlorothiazole rings. Consequently, dehalogenation and carbonylation processes facilitated the elimination of halogenated components and pharmacophores from the THX, leading to detoxification. In addition to the identified free radical pathway including SO4•-, •OH and O2•- contributed to THX degradation, the participation of non-radical pathways (1O2 and electron transfer) were also confirmed. The efficacy of detoxification was further validated through toxicity assessment, employing quantitative conformation relationship prediction and microbial culture utilizing Bacillus subtilis.