Light-induced variation in environmentally persistent free radicals and the generation of reactive radical species in humic substances

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 x 10(17), 2.06 x 10(17), and 1.77 x 10(17) spins/g for the humic acid, fulvic acid, and humin, respectively. And the superoxide radicals increased to 1.05 x 10(14)-1.46 x 10(14) spins/g while the alkyl radicals increased to 0.47 x 10(14)-1.76 x 10(14) 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 (R-2= 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.