Hydroxyl radicals induced mineralization of organic carbon during oxygenation of ferrous mineral-organic matter associations: Adsorption versus coprecipitation

The iron (Fe) phases have been widely proposed to preserve organic carbon (OC) via adsorption or coprecipitation pathways, however, such role of Fe phases might be largely reversed under redox-fluctuation conditions, especially for Fe(II) minerals-protected OC. In this study, we synthesized the Fe(II)-OC associations via adsorption and coprecipitation using FeCO3 and three types of low-molecular-weight organic compounds (LMWOCs) at different C/Fe molar ratios, and investigated the OC mineralization induced by hydroxyl radicals (center dot OH) during oxygenation processes. Abundant center dot OH can be produced upon oxygenation of FeCO3-LMWOCs associations within 96 h, giving values of 28.49-151.36 mu M in adsorption and 12.63-76.41 mu M in coprecipitation treatments depended on types of LMWOCs and C/Fe molar ratios. Fe(II) species in coprecipitates with hydroquinone (HQ) mainly transformed into Goethite-like phases after oxygenation, while adsorption samples induced more formation of lower-crystalline Fe phase (e.g., ferrihydrite). The surface-Fe(II) was the primary electron donors to O-2, which further induced hydrogen peroxide (H2O2) formation via one-and two-electron transfer pathways. Finally, the produced center dot OH removed 0.55-9.65 and 0.16-85.54 mg L-1 total OC in adsorption and coprecipitation treatments after oxygenation. Collectively, this study highlights that OC associated with Fe(II) minerals might be labile due to the oxidation of formed center dot OH, and the role of Fe phases in OC sequestration may be further reevaluated under redox fluctuation conditions. (C) 2021 Elsevier B.V. All rights reserved.