Dynamic Production of Hydroxyl Radicals during the Flooding–Drainage Process of Paddy Soil: An In Situ Column Study

Abstract

Frequent cycles of flooding and drainage in paddy soils lead to the reductive dissolution of iron (Fe) minerals and the reoxidation of Fe­(II) species, all while generating a robust and consistent output of reactive oxygen species (ROS). In this study, we present a comprehensive assessment of the temporal and spatial variations in Fe species and ROS during the flooding–drainage process in a representative paddy soil. Our laboratory column experiments showed that a decrease in dissolved O2 concentration led to rapid Fe reduction below the water–soil interface, and aqueous Fe­(II) was transformed into solid Fe­(II) phases over an extended flooding time. As a result, the •OH production capacity of liquid phases was reduced while that of solid phases improved. The •OH production capacity of solid phases increased from 227–271 μmol kg–1 (within 1–11 cm depth) to 500–577 to 499–902 μmol kg–1 after 50 day, 3 month, and 1 year incubation, respectively. During drainage, dynamic •OH production was triggered by O2 consumption and Fe­(II) oxidation. ROS-trapping film and in situ capture revealed that the soil surface was the active zone for intense H2O2 and •OH production, while limited ROS production was observed in the deeper soil layers (>5 cm) due to the limited oxygen penetration. These findings provide more insights into the complex interplay between dynamic Fe cycling and ROS production in the redox transition zones of paddy fields.

Natural Attenuation of 2,4-Dichlorophenol in Fe-Rich Soil during Redox Oscillations: Anoxic–Oxic Coupling Mechanism

“…Sequential extraction was applied to analyze Fe­(II) species in soil slurries (Text S4). ,,, Briefly, 1 mL of the homogenized slurry was sequentially extracted using 1 M CaCl 2 , 1 M HCl, 6 M HCl, and 1.3 M HF/1.8 M H 2 SO 4 to obtain (i) ion exchangeable Fe, (ii) surface-bound Fe and Fe in low-crystalline minerals (e.g., ferrihydrite), (iii) structural Fe in highly crystalline minerals (e.g., hematite), and (iv) Fe in silicates and clays, respectively. We defined it as ion exchangeable, surface-bound, mineral structural, and phyllosilicate Fe, respectively, in this study.…”

Critical Role of Mineral Fe(IV) Formation in Low Hydroxyl Radical Yields during Fe(II)-Bearing Clay Mineral Oxygenation

“…In recent years, the dark formation of • OH from reduced soil/sediment oxygenation has gained increasing recognition, serving as a sustainable • OH source in redox-fluctuating subsurface environments. − This novel pathway of • OH generation is via the activation of molecular oxygen (O 2 ) by reduced species [especially mineral Fe­(II)] in the subsurface, ,− distinguishing it from the photochemical generation of • OH in Earth’s surface environments including atmosphere, surface water, and ocean. − The dark formation of • OH from mineral Fe­(II) oxygenation plays an important role in the microbial activities, − the oxidative transformation of recalcitrant contaminants [e.g., As­(III) and trichloroethylene ] and the chemical pathway of the formation of greenhouse gas (e.g., CO 2 , CH 4 , and N 2 O) at oxic–anoxic interfaces − in redox-fluctuating environments . In soils and sediments, Fe­(II)-bearing clay minerals are typically recognized as the important contributors to O 2 activation and • OH production. , Given the global distribution of clay minerals, which host significant amounts of Fe­(II) , and constitute ∼21.5% of the Earth’s crust by mass, understanding the mechanisms of Fe­(II)-bearing clay mineral oxygenation is critical for gaining insights into the • OH-induced global cycling of elements and contaminants in the shallow subsurface …”