Black carbon alleviates electron transfer bottlenecks for remote ROS generations at centimeter scales

Electron transfer coupled to molecular O₂ activation drives ROS formation and underpins pollutant removal processes. Yet in heterogeneous matrices electron transfer is typically confined to nano-micrometer distances, which limits ROS efficacy. Here, we show that black carbon (BC) can bridge spatial gaps between electron source and O₂, enabling centimeter-scale ROS generation. Using a solidified agarose reactor that mimics restricted transport, BC mediated electron transfer from zero-valent iron (ZVI) to distal zones, producing hydroxyl radicals (^•OH) and hydrogen peroxide (H₂O₂) up to 18 mm from the ZVI source within 30-day experimental period; H₂O₂ reached 28.6 ± 3.6 µM and ^•OH reached 103.3 ± 3.2 nM with rice-straw BC (300 °C) at 3 mm and then plateaued beyond 12 mm. Mechanistic analyses revealed that surface redox-active functionalities, rather than the polyaromatic conductive backbone, control BC's electron-shuttling performance. Consistent with this mechanism, low-temperature BCs (≤500 °C), which retain more oxygenated surface moieties, gave up to 61.0-fold higher ROS yields than high-temperature (900 °C) samples. Furthermore, chemical oxidation of high-temperature BC reinstated these oxygenated groups and increased its electron-accepting and -donating capacities (to as high as 2.81 mmol e⁻ g^-1 and 1.59 mmol e⁻ g^-1), producing corresponding rises in ROS output. Because ROS were generated over an expanded spatial region, BC/ZVI composites accelerated degradation rates of phenol, bisphenol A, and diuron by 5.4-9.8-fold. Overall, BC functions as an effective electron shuttle that extends interfacial redox reactivity across centimeter scales, offering a sustainable approach to overcome electron-transport limitations in environmental remediation.