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

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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 mol/L. 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.

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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.

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Authors and Affiliations

  1. College of Resources and Environment, Northwest A&F University, Yangling, 712100, China

    Hanzhong Jia, Yafang Shi, Xiaofeng Nie, Song Zhao & Tiecheng Wang

  2. State Key Laboratory of Soil Erosion and Dryland Farming on Loess Plateau, Institute of Soil and Water Conservation, Northwest A&F University, Yangling, 712100, China

    Hanzhong Jia

  3. Program for the Environment and Sustainability, Department of Occupational and Environmental Health, School of Public Health, Texas A&M University, College Station, TX, 77843, USA

    Virender K. Sharma

Authors
  1. Hanzhong Jia
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  2. Yafang Shi
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  3. Xiaofeng Nie
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  4. Song Zhao
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  5. Tiecheng Wang
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  6. Virender K. Sharma
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Corresponding authors

Correspondence to Tiecheng Wang or Virender K. Sharma.

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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.

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11783_2020_1252_MOESM1_ESM.pdf

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

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Jia, H., Shi, Y., Nie, X. et al. Persistent free radicals in humin under redox conditions and their impact in transforming polycyclic aromatic hydrocarbons. Front. Environ. Sci. Eng. 14, 73 (2020). https://doi.org/10.1007/s11783-020-1252-y

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  • DOI: https://doi.org/10.1007/s11783-020-1252-y

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