Degradation of malathion to low-toxicity products via peroxymonosulfate activated by plastic/K2FeO4-derived porous carbon

Malathion is a widely used insecticide with toxic effects on humans and is considered to be genotoxic and carcinogenic. As the common free radicals in advanced oxidation processes, hydroxyl radical (•OH) and sulfate radical (SO₄^•−) can efficiently degrade malathion, but highly toxic product malaoxon is prone to produce in this process. In response to this issue, polyethyleneterephthalate (PET) plastics and K₂FeO₄ were utilized to prepare Fe⁰-doped porous plastic-derived carbon material (FAC) as peroxymonosulfate (PMS) trigger to perform low toxicity degradation of malathion. FAC (100 mg/L) could trigger 0.5 mM PMS to completely degrade 10 mg/L malathion within 15 min. PMS alone, singlet oxygen (¹O₂), •OH, and SO₄^•− contributed to the degradation of malathion, in which ¹O₂ played the most important role with a contribution of 53.5%. Density functional theory (DFT) was employed to elucidate the reaction site of ¹O₂ for malathion, further illustrating ¹O₂ with the product of desmethyl malathion. Based on the DFT program, we calculated the theoretical second-order rate constants, the reactivity of ¹O₂ with malathion to produce desmethyl malathion, was 1.88 × 10¹² M⁻¹ s⁻¹, which was much higher than another reaction pathway with the highly toxic product of malaoxon (1.36 × 10⁷ M⁻¹ s⁻¹). The binding energies of various key proteins of zebrafish and human beings to the degradation products were analyzed by molecular dynamics to characterize their ecological and human toxicity. Surprisingly, in contrast to the highly toxic intermediate malaoxon in the previous studies, desmethyl malathion, the main degradation product in FAC/PMS system, has a significantly low toxicity.