Tracking a Missing Kinetic Dimension
Dissolved black carbon (DBC) is a mobile, redox-active fraction of carbon formed during biomass combustion and pyrolysis. Although electron-donating and electron-accepting capacities describe how much charge a material can exchange, they do not indicate how quickly that exchange occurs. Work led by Yufei Wu, Peng Zhang, Zhaofeng Chang, and Bo Pan at Kunming University of Science & Technology quantified the apparent heterogeneous electron-transfer rate constant, k₀, as a kinetic measure of DBC reactivity.
Comparing Carbon Sources and Temperatures
The team examined 10 DBC samples produced from rice straw and sawdust at pyrolysis temperatures of 200–600 °C. Cyclic voltammetry and differential pulse voltammetry were conducted with a three-electrode system, while particle-size analysis and mediated electrochemical measurements characterized diffusion and electron-exchange properties. Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) was used to resolve molecular formulas and assign compound classes. Microbial electrochemical systems containing Shewanella oneidensis MR-1 were used to assess how DBC affected extracellular electron transfer.
Temperature, Diffusion, and Molecular Composition
The k₀ values increased markedly with pyrolysis temperature. For rice straw-derived DBC, k₀ rose from (0.54 ± 0.27) × 10⁻⁷ cm s⁻¹ at 200 °C to (1.13 ± 0.73) × 10⁻² cm s⁻¹ at 600 °C. Sawdust-derived DBC increased from (1.05 ± 0.02) × 10⁻⁴ to (6.18 ± 3.08) × 10⁻² cm s⁻¹ across the same temperature range. At 400–600 °C, DBC showed greater electron-shuttle ability than natural dissolved organic matter from wetland, soil, river, and lake samples.
DBC derived from sawdust generally exhibited higher k₀ values than DBC derived from rice straw at corresponding temperatures. Smaller average particles were associated with faster apparent diffusion, and k₀ correlated strongly with both diffusion coefficients and electron-exchange capacity. In microbial electrochemical systems, higher-temperature DBC increased peak and steady currents, with current responses showing significant correlations with log₁₀(k₀), linking the kinetic parameter to microbial extracellular electron transfer.
Molecular Drivers and Environmental Context
FT-ICR MS profiles indicated that higher-temperature DBC contained greater relative intensities of CHNOS, condensed aromatic, and tannin-like compounds, whereas lignin-like compounds declined. Condensed aromatic and tannin-like molecules were positively associated with k₀, while lignin-like molecules were negatively associated with it. Additional tests supported these relationships: adding tannic acid increased k₀ in low-temperature DBC, whereas adding lignin decreased k₀ in medium-temperature DBC. Redox-active quinone and hydroquinone moieties within aromatic structures provide plausible sites for electron donation and acceptance.
The findings indicate that DBC's environmental electron-shuttle ability depends on both redox activity and diffusivity, rather than on electron-exchange capacity alone. Once transported into soils or aquatic systems, DBC with high k₀ could alter the rates of microbial metabolism, contaminant transformation, metal cycling, and greenhouse-gas production. These implications remain context-dependent because environmental ionic composition, including Ca²⁺ and Mg²⁺, may promote particle aggregation and reduce effective diffusion.
Scope and Next Steps
A stated limitation concerns the tannic-acid and lignin-addition experiments: the added compounds were physical mixtures and may have acted as independent electron shuttles in parallel with DBC, rather than changing DBC's intrinsic structure. The reported k₀ and diffusion coefficients are also apparent parameters obtained under standardized electrochemical conditions. Further evaluation under environmentally realistic pH, ionic strength, mineral associations, microbial communities, and transport conditions would help determine how laboratory kinetic rankings translate to soils, sediments, and natural waters.
The work provides a framework for incorporating electron-transfer kinetics and molecular composition into assessments of pyrogenic carbon. Future investigations can test how feedstock, fire or pyrolysis conditions, particle aggregation, and environmental aging jointly regulate DBC reactivity, and can establish whether the observed kinetic differences produce measurable changes in contaminant fate, denitrification, methanogenesis, or carbon cycling at field scale.
Corresponding Author: Peng Zhang, Zhaofeng Chang or Bo Pan
Original Source: https://doi.org/10.1007/s44246-026-00299-9
Contributions: All authors contributed to the study conception and design. Material preparation, methodology, supervision, validation, visualization, writing-original draft, and writing-review & editing were performed by Yufei Wu; Project administration, supervision, validation, funding acquisition, and writing-review & editing were performed by Peng Zhang; Material preparation, methodology and data collection were performed by Hanxue Li; Methodology was performed by Ting He; Data analysis was performed by Wenmei Tao; Validation and visualization were performed by Zhaofeng Chang; Project administration, supervision, and writing-reviewing & editing were performed by Bo Pan. All authors read and approved the final manuscript.
