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Free radical property, redox capacity, and the relevant influencing factors of soil humin
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This study characterized the free radical properties and electron paramagnetic resonance signals of soil humin, finding that humin contains persistent free radicals with distinct redox properties. The research improves understanding of the role of humic substance radicals in soil biogeochemistry.
Humic substances (HS) contain considerable quantities of persistent free radicals (PFRs), which have been detected by the electron paramagnetic resonance (EPR) technique. Among three HS fractions (humin, fulvic acid, and humic acid), humin is recognized as the predominant fraction possessing PFRs. Most of the previous studies of HS-PFRs involved their environmental effect; however, very few investigations have been conducted to explore the relationship among structural characteristics, PFRs, and redox capacity of humin. To address the above-mentioned scientific questions, the types and concentrations of PFRs in 10 representative types of soils were investigated by EPR technique. We found that humin from different soils contained PFRs concentration of 0.11×1016–5.79×1016 spins/g and-factors of less than 2.0030. These PFRs were assigned as carbon-centered “aromatic-type” radicals. Fourier transform infrared (FTIR) spectroscopy, ultraviolet-visible, and the contents of acid groups indicated that the aromatic structure of humin was beneficial for the formation of PFRs. The concentration of PFRs was negatively correlated with phenolic hydroxyl contents (<0.05) and positively correlated with carboxyl contents (<0.01). The potential generation of reactive oxygen species (ROS) in humin was determined by spin-trapping EPR technique. Superoxide radicals (average concentration of up to 5.25×1013 spins/g) were detected in humin from all soils while hydroxyl radicals were detected only in the samples extracted from red soil, black soil, chestnut soil and brown soil. The redox capacity of humin was evaluated by dithiothreitol (DTT) and 1,1-diphenyl-2-picrylhydrazyl (DPPH). The oxidation capacity of humin was in the range of 24−376 nmol/and the IC50 was in the range ofand the It was shown by correlation analysis that the oxidation capacity of humin was directly related to its ROS concentration, and phenolic hydroxyl contributed to the reduction capacity. The removal of bisphenol A (BPA) by humins derived from 10 representative soils indicated that the removal rate of BPA was positively correlated with the oxidation capacity of humin (<0.05), and the oxidation ability was positively correlated with ROS concentration (<0.05), confirming that the contribution of ROS to BPA removal efficiency. These results provided a theoretical basis for evaluating the ecological and environmental effects of humin in various soil systems.
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