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

Hydroxyl radical (•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 •OH (40.3–1061.4 μmol kg−1) was produced during oxygenation of anoxic paddy slurries under circumstance conditions. Wet chemical sequential extractions, Mössbauer 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 •OH production than aqueous Fe2+. The produced •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 •OH-attacking mechanisms. Quenching experiments showed that hydrogen peroxide (H2O2) was the important intermediate for •OH formation via Haber-Weiss mechanisms during oxygenation processes. These findings indicate that abundant •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.