Oxygen Limitation Accelerates Regeneration of Active Sites on a MnO 2 Surface: Promoting Transformation of Organic Matter and Carbon Preservation.

Birnessite (δ-MnO 2 ) is a layered manganese oxide widely present in the environment and actively participates in the transformation of natural organic matter (NOM) in biogeochemical processes. However, the effect of oxygen on the dynamic interface processes of NOM and δ-MnO 2 remains unclear. This study systematically investigated the interactions between δ-MnO 2 and fulvic acid (FA) under both aerobic and anaerobic conditions. FA was transformed by δ-MnO 2 via direct electron transfer and the generated reactive oxygen species (ROS). During the 32-day reaction, 79.8% of total organic carbon (TOC) in solution was removed under anaerobic conditions, unexpectedly higher than that under aerobic conditions (69.8%), suggesting that oxygen limitation was more conducive to the oxidative transformation of FA by δ-MnO 2 . The oxygen vacancies (O V ) on the surface of δ-MnO 2 were more exposed under anaerobic conditions, thus promoting the adsorption and transformation of FA as well as regeneration of the active sites. Additionally, the reaction of FA with δ-MnO 2 weakened the strongly bonded lattice oxygen (O latt ), and the released O latt was an important source of ROS. Interestingly, a part of organic carbon (OC) was preserved by forming MnCO 3 , which might be a novel mechanism for carbon preservation. These findings contribute to an improved understanding of the dynamic interface processes between MnO 2 and NOM and provide new insights into the effects of oxygen limitation on the cycling and preservation of OC.