Environmental Fate and Stability of Iron-Bound Organic Matter during Redox Reactions

Soil organic matter (SOM) accounts for a significant fraction of the global carbon pool. Stabilization or destabilization of SOM greatly influences the carbon reservoir the in soil environment, greenhouse gas emission from soil and the consequent climate change process. The stability of SOM is strongly regulated by the interactions between SOM and iron oxide minerals. However, the reduction of iron can break down the association between SOM and iron oxide and lead to the release and potential degradation of SOM. To date, limited information is available for the stability and fate of the iron-bound organic matter during the redox reactions. Herein, we investigated the reductive release of hematite-bound organic matter and the impact of physicochemical properties of SOM on its stability during the redox reactions. Our major findings include: 1) hematite prefer to sorb more aromatic organic matter, while the aromatic organic matter was relatively easier to be released during the reduction; 2) release of organic carbon and iron was asynchronous during the abiotic reduction of hematite, with organic carbon releasing rapidly at the beginning and then maintaining steady but iron release obeying first-order kinetics; 3) aromatic carbon was released more rapidly compared to other compartments of organic matter; and 4) the more rapid release of aromatic carbon was resulted from its potential distribution on the outer layer of hematite-organic matter complexes and possible involvement of quinone functional groups in the reduction. We demonstrate that iron-bound aromatic organic carbon was more mobile during the reduction of iron oxide, although iron minerals prefer to sorb more aromatic organic matter. Such findings provide partial explanation for long-lasting puzzle about the stabilization of aliphatic organic matter in soil and sediment environment. Our results are valuable for evaluating the biogeochemical stability or organic carbon and coupling the redox cycles of iron to the turnover of organic matter.