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

Abstract

Dark formation of hydroxyl radicals (•OH) from soil/sediment oxygenation has been increasingly reported, and solid Fe­(II) is considered as the main electron donor for O2 activation. However, the role of solid organic matter (SOM) in •OH production is not clear, although it represents an important electron pool in the subsurface. In this study, •OH production from oxygenation of reduced solid humic acid (HAred) was investigated at pH 7.0. •OH production is linearly correlated with the electrons released from HAred suspension. Solid HAred transferred electrons rapidly to O2 via the surface-reduced moieties (hydroquinone groups), which was fueled by the slow electron transfer from the reduced moieties inside solid HA. Cycling of dissolved HA between oxidized and reduced states could mediate the electron transfer from solid HAred to O2 for •OH production enhancement. Modeling results predicted that reduced SOM played an important or even dominant role in •OH production for the soils and sediments possessing high molar ratios of SOC/Fe­(II) (e.g., >39). The significant contribution of SOM was further validated by the modeling results for oxygenation of 88 soils/sediments in the literature. Therefore, reduced SOM should be considered carefully to comprehensively understand •OH production in SOM-rich subsurface environments.

Effect of C/Fe Molar Ratio on H2O2 and •OH Production during Oxygenation of Fe(II)-Humic Acid Coexisting Systems

“…Compared with the initial stage, the positive influence shifted to pathway (c) at C/Fe values of 0.5 and 1.6 when the oxygenation progressed to 20 min (Figure S20a), which is in agreement with the production of surface-adsorbed Fe­(II) (Figure b). • OH. Both Fe­(II) and HA red species can decompose H 2 O 2 to • OH through Fenton or Fenton-like reactions. ,,,, NSC analysis revealed that the important reactions for H 2 O 2 production were also important for • OH production in Fe­(II)–HA red coexisting systems (Figure ). Besides, the reactions for • OH production include the decomposition of H 2 O 2 by different reductants (Table ).…”

“…• OH. Both Fe­(II) and HA red species can decompose H 2 O 2 to • OH through Fenton or Fenton-like reactions. ,,,, NSC analysis revealed that the important reactions for H 2 O 2 production were also important for • OH production in Fe­(II)–HA red coexisting systems (Figure ). Besides, the reactions for • OH production include the decomposition of H 2 O 2 by different reductants (Table ).…”

“…Both Fe(II) and HA red species can decompose H 2 O 2 to • OH through Fenton or Fenton-like reactions. 24,32,35,62,63 NSC analysis revealed that the important reactions for H 2 O 2 production were also important for • OH production in Fe(II)−HA red coexisting systems (Figure 5). Besides, the reactions for The previously established mechanisms of ROS production from oxygenation of Fe(II)−NOM associations in oxic surface waters (i.e., seawater and freshwater) only include the electron transfer pathways from NOM-complexed Fe(II) to O 2 because of the extremely high level of C/Fe(II) ratios (>200) (Table S5).…”

NOM-Induced Dissolution of CrxFe1–x(OH)3 Precipitates and Formation of Cr(III)-NOM-Fe Colloids under Oxic and Anoxic Conditions

“…With the AHA concentration elevated to 170 mg C/L, the aqueous Cr­(III) concentrations were ∼61 μM for AHA ox under oxic concentration and ∼90 μM for AHA red under anoxic conditions. While the reactive oxygen species (ROS) can be produced from oxygenation of reduced NOM by molecular oxygen, ,− the ROS production was unlikely to occur in this study because the reduced AHA was reacted in the absence of oxygen. Therefore, the redox reaction between Cr­(OH) 3 and AHA is lacking.…”

Microscale Spatiotemporal Variation and Generation Mechanisms of Reactive Oxygen Species in the Rhizosphere of Ryegrass: Coupled Biotic–Abiotic Processes

“…Previously, individual soil components wre considered to be the main contributors to ROS formation in the subsurface. ,, Only limited studies have been performed on whole soil to evaluate the integrated effects between multiple inorganic minerals and organic matter mixtures for ROS production. The present study not only elucidated the important role of biotic–abiotic coupling processes in ROS production but also demonstrated that the rhizosphere was a pervasive but previously unrecognized hotspot for ROS production.…”