Persistent free radicals in humin under redox conditions and their impact in transforming polycyclic aromatic hydrocarbons

Highlights

• Regulation of redox conditions promotes the generation of free radicals on HM.

• HM-PFRs can be fractionated into active and inactive types depending on stability.

• The newly produced PFRs readily release electrons to oxygen and generate ROS.

• PFR-induced ROS mediate the transformation of organic contaminants adsorbed on HM.

Abstract

The role of humic substance-associated persistent free radicals (PFRs) in the fate of organic contaminants under various redox conditions remains unknown. This study examined the characterization of original metal-free peat humin (HM), and HM treated with varying concentrations of H₂O₂ and L-ascorbic acid (VC) (assigned as H₂O₂-HM and VC-HM). The concentration of PFRs in HM increased with the addition of VC/H₂O₂ at concentrations less than 0.08 M. The evolution of PFRs in HM under different environmental conditions (e.g., oxic/anoxic and humidity) was investigated. Two types of PFRs were detected in HM: a relatively stable radical existed in the original sample, and the other type, which was generated by redox treatments, was relatively unstable. The spin densities of VC/H₂O₂-HM readily returned to the original value under relatively high humidity and oxic conditions. During this process, the HM-associated “unstable” free radicals released an electron to O₂, inducing the formation of reactive oxygen species (ROS, i.e., ^•OH and ^•O₂⁻). The generated ROS promoted the degradation of polycyclic aromatic hydrocarbons based on the radical quenching measurements. The transformation rates followed the order naphthalene>phenanthrene>anthracene>benzo[a]pyrene. Our results provide valuable insight into the HM-induced transformation of organic contaminants under natural conditions.

Graphical abstract

Keywords

Humic substance / Polycyclic aromatic hydrocarbons (PAHs) / Persistent free radicals (PFRs) / Redox / Reactive oxygen species (ROS)

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Acknowledgements

This work is supported by the National Natural Science Foundation of China (Grant Nos. 41571446 and 41877126), Shaanxi Science Fund for Distinguished Young Scholars (Grant No. 2019JC-18), National Key Technologies R&D Program (Grant No. 2018YFC1802004). Authors thank Drs. Kyriakos Manoli and Mingbao Feng for their comments, which improved the manuscript.

Electronic Supplementary Material

ƒSupplementary material is available in the online version of this article at https://doi.org/10.1007/s11783-020-1252-y and is accessible for authorized users.

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