Environmentally persistent free radicals (EPFRs) are emerging contaminants occurring in combustion-borne particulates and atmospheric particulate matter, but information on their formation and behavior on fly ash from municipal solid waste (MSW) incinerators is scarce. Here, we have found that MSW-associated fly ash samples contain an EPFR concentration of 3–10 × 1015 spins g–1, a line width (ΔH p‑p) of ∼8.6 G, and a g-factor of 2.0032–2.0038. These EPFRs are proposed to be mixtures of carbon-centered and oxygen-centered free radicals. Fractionation of the fly ash-associated EPFRs into solvent-extracted and nonextractable radicals suggests that the solvent-extracted part accounts for ∼45–73% of the total amount of EPFRs. Spin densities of solvent-extracted EPFRs correlate positively with the concentrations of Fe, Cu, Mn, Ti, and Zn, whereas similar correlations are comparatively insignificant for nonextractable EPFRs. Under natural conditions, these two types of EPFRs exhibit different stabilization that solvent-extracted EPFRs are relatively unstable, whereas the nonextractable fraction possesses a long life span. Significant correlations between concentrations of solvent-extracted EPFRs and generation of hydroxyl and superoxide radicals are found. Overall, our results suggest that the fractionated solvent-extracted and nonextractable EPFRs may experience different formation and stabilization processes and health effects.

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Sulfate radical (SO4−•), formed by persulfate (PS) activation during advanced oxidation process, can be used for the remediation of organic contaminated soil. However, the role of biochar and microwave (MW) in the activation of PS are not fully understood, especially the corresponding mechanism. Herein, biochar combined with MW were used to activate PS for the remediation of ethyl-parathion (PTH)-polluted soil. The dynamic evolutions of PTH under different conditions, such as biochar content, types, and reaction temperature, and the degradation mechanisms of PTH were also systematically investigated. Significant enhancement performance on PTH removal was observed after adding BC, which was increased from 75.35–88.78% within 80 min. Meanwhile, activating temperature and the PFRs in biochar exhibited remarkable abilities to activate PS for PTH removal. Furthermore, chemical probe tests coupled with quenching experiments confirmed that biochar combined with MW system contributed to the decomposition of PS into active species, such as SO4−•, •OH and O2−•, which could oxidize PTH into oxidative products, including paraoxon, p-nitrophenol, and hydroquinone. The results of this study provided valuable insights into the synergistic effects of biochar and MW in the PS activation, which is helpful for the potential application of biochar materials combined with MW-activated PS in the remediation of pesticide-polluted soils.