Production of hydrogen peroxide in an intra-meander hyporheic zone at East River, Colorado

Abstract

The traditionally held assumption that photo-dependent processes are the predominant source of H2O2 in natural waters has been recently questioned by an increrasing body of evidence showing the ubiquitiousness of H2O2 in dark water bodies and in groundwater. In this study, we conducted field measurement of H2O2 in an intra-meander hyporheic zone and in surface water at East River, CO. On-site detection using a sensitive chemiluminescence method suggests H2O2 concentrations in groundwater ranging from 6 nM (at the most reduced region) to ~ 80 nM (in a locally oxygen-rich area) along the intra-meander transect with a maxima of 186 nM detected in the surface water in an early afternoon, lagging the maximum solar irradiance by ∼ 1.5 h. Our results suggest that the dark profile of H2O2 in the hyporheic zone is closely correlated to local redox gradients, indicating that interactions between various redox sensitive elements could play an essential role. Due to its transient nature, the widespread presence of H2O2 in the hyporheic zone indicates the existence of a sustained balance between H2O2 production and consumption, which potentially involves a relatively rapid succession of various biogeochemically important processes (such as organic matter turnover, metal cycling and contaminant mobilization). More importantly, this study confirmed the occurrence of reactive oxygen species at a subsurface redox transition zone and further support our understanding of redox boundaries on reactive oxygen species generation and as key locations of biogeochemical activity.

Reactive Iron, Not Fungal Community, Drives Organic Carbon Oxidation Potential in Floodplain Soils

“…In Arctic surface waters, Fenton chemistry is calculated to produce CO 2 at rates similar to microbial oxidation (Page et al, 2013). Moreover, Yuan et al (2022) found 6-80 nM H 2 O 2 in a more reducing nearby East River meander and cutoff oxbow sediments, confirming the likelihood of H 2 O 2 being consistently available at our field site. Floodplains experience dynamic oxygen and Fe(II) supply to high-carbon surface soils, making them prime locations for Fenton-mediated SOC oxidation.…”

“…It is not likely coincidence that abiotic Fe(II)-mediated organic carbon oxidation has been observed in environments experiencing frequent water saturation -e.g. rain forests, floodplains, and bogs and fenswith active Fe(II)/Fe(III) redox cycling (Inubushi et al, 1984;Liptzin and Silver, 2009;Dubinsky et al, 2010;Lipson et al, 2010) and often significant H 2 O 2 production (Hall and Silver, 2013;Yuan et al, 2022). Floodplains encompass a range of redox conditions over space and time from relatively oxic in meander surface soils to periodically reducing in subsurface soils, and completely reducing in river and cutoff channels.…”

Significant Contribution of Solid Organic Matter for Hydroxyl Radical Production during Oxygenation

“…Dark formation of reactive oxygen species (ROS) including superoxide (O 2 •– ), hydrogen peroxide (H 2 O 2 ), − and hydroxyl radicals (•OH) − has been increasingly recognized at oxic–anoxic interfaces in subsurface environments. Particularly, •OH (standard reduction potential: 2.8 V) is the most powerful ROS in natural systems which plays an important role in biogeochemical element cycles, such as greenhouse gas emission (e.g., CO 2 and CH 4 ) ,, and natural attenuation of contaminants. ,− Generally, ferrous iron (Fe­(II)) and reduced natural organic matter (NOM) are considered to be the main contributors to O 2 reduction and •OH formation in the subsurface. − ,− Many studies have reported •OH production from oxygenation of different Fe­(II) species, including dissolved Fe­(II), ligand-complexed Fe­(II), − adsorbed Fe­(II), ,, and mineral structure Fe­(II) (e.g., Fe-bearing clay minerals, ,, mackinawite, pyrite, , and siderite).…”