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.