Redox Oscillation-Driven Production of Reactive Oxygen Species from Black Carbon

Wildfire and stubble burning release substantial amounts of black carbon (BC) into natural environments that experience frequent redox oscillations, such as wetlands and farmlands. Here, we report that redox oscillations can effectively drive ROS production from BC. Following sequential microbial reduction and air exposure, 6.5 ± 0.2 μM/g_C hydrogen peroxide (H₂O₂) and 285.3 ± 9.5 nM/g_C hydroxyl radical (•OH) were produced from BC. Moreover, BC derived from various biomass sources, temperatures, and particle sizes exhibits 111.5-fold variations in ROS production. Electrochemical analyses revealed that both the electron transfer capacity and the ROS production selectivity are critical determinants of ROS generation under redox oscillations. The variation in electron transfer capacity (0.3-5.7 mmol e^-/g_C) is primarily governed by the abundance of electron-storing moieties such as quinones, while the ROS generation selectivity (26.2-72.0%) is influenced by the presence of competitive sites for oxygen reduction reactions, such as carbon defects. These findings provide insights into ROS production from BC under fluctuating redox conditions, with potential implications for elemental cycles and pollutant dynamics in regions prone to wildfire and stubble burning events and substantial BC deposition (e.g., wetlands and rice paddies).