Electron transfer kinetics and molecular structural basis of dissolved black carbon as an electron shuttle

Abstract

Dissolved black carbon (DBC) serves as a crucial electron shuttle in various biogeochemical cycles owing to its highly mobile and redox-active nature. Prior work on DBC electron-shuttling has focused on thermodynamic metrics—electron donating capacity (EDC) and electron accepting capacity (EAC)—which quantify total potential for electron exchange. Those metrics, however, do not capture electron-transfer kinetics, which control the rate and extent of environmental reactions. Consequently, the apparent electron transfer rate constant ( k 0 ) and the molecular structural features that govern it remain poorly characterized, limiting a comprehensive evaluation of DBC’s environmental role. To address this knowledge gap, this study quantitatively evaluated the k 0 value of DBC from 10 BC samples, derived from two feedstocks (rice straw and sawdust) and produced at pyrolysis temperatures ranging from 200 to 600 °C. The electron shuttle ability of DBC increases with rising pyrolysis temperature, ultimately reaching levels in high-temperature DBC that are substantially higher than those of natural dissolved organic matter. Furthermore, at a given pyrolysis temperature, DBC derived from herbaceous plants exhibits lower electron shuttle efficiency compared to DBC derived from woody plants. The electron shuttle ability of DBC is governed by its redox activity and diffusivity. Specifically, the CHNOS compounds, condensed aromatic, and tannin-like moieties, positively contribute to DBC’s electron shuttle ability. Conversely, lignin-like molecules tend to decrease the DBC-mediated electron transfer processes. The molecular structures of condensed aromatic and lignin-like substances that strongly modulate DBC-mediated electron transfer have been characterized. The greater electron shuttle ability of DBC produced at medium and high pyrolysis temperatures, compared with environmental DOM, is primarily attributable to its higher content of tannins and condensed aromatic compounds and its lower content of lignin-like constituents. This study clarifies the key factors and molecular structural basis by which DBC mediates electrochemical electron shuttling, thus providing a theoretical foundation for accurately assessing and managing DBC’s environmental impacts.