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Frontiers of Environmental Science & Engineering

Front. Environ. Sci. Eng.    2020, Vol. 14 Issue (4) : 73
Persistent free radicals in humin under redox conditions and their impact in transforming polycyclic aromatic hydrocarbons
Hanzhong Jia1,2, Yafang Shi1, Xiaofeng Nie1, Song Zhao1, Tiecheng Wang1(), Virender K. Sharma3()
1. College of Resources and Environment, Northwest A&F University, Yangling 712100, China
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
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
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• 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.

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.

Keywords Humic substance      Polycyclic aromatic hydrocarbons (PAHs)      Persistent free radicals (PFRs)      Redox      Reactive oxygen species (ROS)     
Corresponding Author(s): Tiecheng Wang,Virender K. Sharma   
Issue Date: 19 June 2020
 Cite this article:   
Hanzhong Jia,Yafang Shi,Xiaofeng Nie, et al. Persistent free radicals in humin under redox conditions and their impact in transforming polycyclic aromatic hydrocarbons[J]. Front. Environ. Sci. Eng., 2020, 14(4): 73.
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Hanzhong Jia
Yafang Shi
Xiaofeng Nie
Song Zhao
Tiecheng Wang
Virender K. Sharma
Fig.1  (a) EPR spectra of the isolated peat humins (HM) (or original HM), and H2O2 and L-ascorbic acid (VC) treated HM samples, assigned as H2O2-HM and VC-HM samples, respectively; (b) Variation of peak area and g-Factor for the treated HM by H2O2 and L-ascorbic acid (VC) at varied concentrations ranging from 0.0 to 0.10 mol/L.
Fig.2  X-ray photoelectron spectra (XPS) of the original and treated peat HM, including (a) original HM (or untreated samples), (b) HM treated with 0.04 mol/L H2O2, (c) HM treated with 0.10 mol/L H2O2, (d) HM treated with 0.04 mol/L L-ascorbic acid, and (e) HM treated with 0.10 mol/L L-ascorbic acid.
Fig.3  Evolution of spin densities as a function of aging time for the originally isolated HM, H2O2- and L-ascorbic acid-treated HM samples under different conditions, such as (a) Air, RH= 60%, (b) air, RH= 7%, (c) Air, ~100%, and (d) no Air, RH= 0%.
Fig.4  Variation of concentrations of hydroxyl radials and ROS with concentration of EPFRs of HM samples treated by H2O2 and L-ascorbic acid.
Fig.5  (a) Evolution of PAHs, including benzo[a]pyrene (B[a]P), anthracene (ANT), phenanthrene (PHE), and naphthalene (NAP), as a function of reaction time during the transformation process by 0.10 mol/L L-ascorbic acid-HM sample; Evolution of ANT as a function of reaction time in the reaction systems involving in HM samples treated by different amounts of (b) H2O2 and (c) L-ascorbic acid. (d) Transformation of ANT by 0.10 mol/L L-ascorbic acid-HM in the presence of coumarin and benzoquinone.
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