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Frontiers of Environmental Science & Engineering

ISSN 2095-2201

ISSN 2095-221X(Online)

CN 10-1013/X

Postal Subscription Code 80-973

2018 Impact Factor: 3.883

Front. Environ. Sci. Eng.    2020, Vol. 14 Issue (6) : 106    https://doi.org/10.1007/s11783-020-1285-2
RESEARCH ARTICLE
Light-induced variation in environmentally persistent free radicals and the generation of reactive radical species in humic substances
Yafang Shi1, Yunchao Dai1, Ziwen Liu1, Xiaofeng Nie1, Song Zhao1, Chi Zhang1, Hanzhong Jia1,2()
1. College of Natural Resources and Environment, Northwest A&F University, Yangling 712100, China
2. Key Laboratory of Plant Nutrition and the Agri-environment in Northwest China, Ministry of Agriculture, Yangling 712100, China
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Abstract

• Light irradiation increased the concentration of free radicals on HS.

• The increased spin densities on HS readily returned back to the original value.

• The “unstable” free radicals induced the formation of reactive radical species.

• Reactive radicals’ concentration correlated strongly with EPFRs’ concentration.

Environmentally persistent free radicals (EPFRs) in humic substances play an essential role in soil geochemical processes. Light is known to induce EPFRs formation for dissolved organic matter in aquatic environments; however, the impacts of light irradiation on the variation of EPFRs in soil humic substances remain unclear. In this study, humic acid, fulvic acid, and humin were extracted from peat soil and then in situ irradiated using simulated sunlight. Electron paramagnetic resonance spectroscopy results showed that with the increasing irradiation time, the spin densities and g-factors of humic substances rapidly increased during the initial 20 min and then gradually reached a plateau. After irradiation for 2h, the maximum spin density levels were up to 1.63 × 1017, 2.06 × 1017, and 1.77 × 1017 spins/g for the humic acid, fulvic acid, and humin, respectively. And the superoxide radicals increased to 1.05 × 1014–1.46 × 1014 spins/g while the alkyl radicals increased to 0.47 × 1014–1.76 × 1014 spins/g. The light-induced EPFRs were relatively unstable and readily returned back to their original state under dark and oxic conditions. Significant positive correlations were observed between the concentrations of EPFRs and reactive radical species (R2 = 0.65–0.98, p<0.05), which suggested that the newly produced EPFRs contributed to the formation of reactive radical species. Our findings indicate that under the irradiation humic substances are likely to be more toxic and reactive in soil due to the formation of EPFRs.

Keywords Peat      Humic substances      Environmentally persistent free radicals      Light irradiation      Reactive radical species     
Corresponding Author(s): Hanzhong Jia   
Issue Date: 19 June 2020
 Cite this article:   
Yafang Shi,Yunchao Dai,Ziwen Liu, et al. Light-induced variation in environmentally persistent free radicals and the generation of reactive radical species in humic substances[J]. Front. Environ. Sci. Eng., 2020, 14(6): 106.
 URL:  
https://academic.hep.com.cn/fese/EN/10.1007/s11783-020-1285-2
https://academic.hep.com.cn/fese/EN/Y2020/V14/I6/106
Fig.1  Evolution of g-factor and concentration of free radicals in humic acid (a and d), fulvic acid (b and e), and humin (c and f) during in situ photo irradiation and decay in dark conditions.
Fig.2  Electron paramagnetic resonance (EPR) spectra obtained from humic substances (HS) after irradiation for 0h (black) and 2h (red) extracted by a mixture of H2O with the spin-trapping agent of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). (HA-CK: original humic acid; HA-2 h: humic acid samples after irradiation for 2 h; FA-CK: original fulvic acid; FA-2 h: fulvic acid samples after irradiation for 2 h; HM-CK: original humin; HM-2 h: humin samples after irradiation for 2 h).
Fig.3  Electron paramagnetic resonance (EPR) spectra obtained from humic substances (HS) after irradiation for 0 h (black) and 2h (red) extracted by a mixture of dimethyl sulfoxide (DMSO) with the spin-trapping agent of 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). (HA-CK: original humic acid; HA-2 h: humic acid samples after irradiation for 2 h; FA-CK: original fulvic acid; FA-2 h: fulvic acid samples after irradiation for 2 h; HM-CK: original humin; HM-2 h: humin samples after irradiation for 2 h).
Fig.4  Dynamic changes in the concentrations of alkyl radicals (DMPO-R) and O2 (DMPO-OOH) in humic acid (a and d), fulvic acid (b and e), and humin (c and f) under the irradiation.
Fig.5  Correlativity between the environmentally persistent free radicals (EPFRs) and alkyl radicals (DMPO-R) and O2 (DMPO-OOH) in humic acid (a and d), fulvic acid (b and e), and humin (c and f) after different irradiation time.
Fig.6  X-ray photoelectron spectroscopy (XPS) characterization of humic substances (HS) after irradiation for 0 h and 2 h. (HA-CK: original humic acid; HA-2 h: humic acid samples after irradiation for 2 h; FA-CK: original fulvic acid; FA-2 h: fulvic acid samples after irradiation for 2 h; HM-CK: original humin; HM-2 h: humin samples after irradiation for 2 h).
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