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

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 (HA(red)) was investigated at pH 7.0. .OH production is linearly correlated with the electrons released from HA(red) suspension. Solid HA(red) transferred electrons rapidly to O(2 )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 HA(red) 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.

Automated summary

This study investigated the production of hydroxyl radicals (.OH) from solid humic acid during oxidation. The results demonstrated that solid humic acid transferred electrons to oxygen, leading to the generation of .OH. The modeling results also confirmed that reduced solid organic matter played a significant role in .OH production.