Extensive production of hydroxyl radicals during oxygenation of anoxic paddy soils: Implications to imidacloprid degradation

Hydroxyl radical (center dot OH) plays a critical role in driving organic pollutants degradation during redox fluctuations. Such processes have been frequently investigated in sedimentary environments, but rarely referred to the agricultural fields, such as paddy soils with frequent occurrence of redox fluctuations. Our findings demonstrated that extensive center dot OH (40.3-1061.4 mu mol kg-1) was produced during oxygenation of anoxic paddy slurries under circumstance conditions. Wet chemical sequential extractions, Mo center dot ssbauer spectra, and X-ray photoelectron spectroscopy characterizations collectively corroborated that 0.5 M HCl-extracted Fe(II) (i.e., surface-bound Fe and Fe in low-crystalline minerals) contributed to more center dot OH production than aqueous Fe2+. The produced center dot OH can efficiently induce the oxidative transformation of organic carbon and the degradation of imidacloprid (IMP), which in turn produced the by-products, such as IMP-urea, IMP-olefin, and 6-chloronicontinic acid, via center dot OHattacking mechanisms. Quenching experiments showed that hydrogen peroxide (H2O2) was the important intermediate for center dot OH formation via Haber-Weiss mechanisms during oxygenation processes. These findings indicate that abundant center dot OH can be produced during the redox fluctuations of paddy soil, which might be of great significance to predict the removal of organic contaminants and the mineralization of organic carbon in paddy fields.

Automated summary

Hydroxyl radical plays a crucial role in driving the degradation of organic pollutants during redox fluctuations, with extensive production demonstrated in oxygenated anoxic paddy slurries. The study found that Fe(II) extracted by 0.5M HCl was a key contributor to hydroxyl radical production. These findings suggest that the redox fluctuations in paddy fields have significant implications for the removal of organic contaminants and the mineralization of organic carbon.