Dissolved black carbon's role in electron transport

Dissolved black carbon (DBC) plays a crucial role in electron transport due to its redox-active properties. Formed during biomass combustion and pyrolysis, DBC's electron-donating and accepting capacities are well-documented, but its electron-transfer rate has been less understood. Researchers at Kunming University of Science & Technology, led by Yufei Wu, Peng Zhang, Zhaofeng Chang, and Bo Pan, have quantified the apparent heterogeneous electron-transfer rate constant, k₀, to measure DBC reactivity.

The study compared DBC samples from rice straw and sawdust, produced at pyrolysis temperatures ranging from 200 to 600 °C. Using cyclic voltammetry and differential pulse voltammetry, the team assessed electron-exchange properties, while particle-size analysis and mediated electrochemical measurements characterized diffusion. Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) provided molecular composition insights. Microbial electrochemical systems with Shewanella oneidensis MR-1 evaluated DBC's impact on extracellular electron transfer.

Results indicated that k₀ values increased 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 showed a similar trend, increasing from (1.05 ± 0.02) × 10⁻⁴ to (6.18 ± 3.08) × 10⁻² cm s⁻¹. DBC at 400–600 °C demonstrated superior electron-shuttle capabilities compared to natural dissolved organic matter from various environments.

Sawdust-derived DBC generally exhibited higher k₀ values than rice straw-derived DBC at the same temperatures. Smaller particle sizes correlated with faster diffusion, and k₀ was strongly linked to diffusion coefficients and electron-exchange capacity. In microbial systems, higher-temperature DBC increased both peak and steady currents, correlating with log₁₀(k₀) and indicating enhanced microbial extracellular electron transfer.

FT-ICR MS analysis showed that higher-temperature DBC contained more CHNOS, condensed aromatic, and tannin-like compounds, while lignin-like compounds decreased. Condensed aromatic and tannin-like molecules were positively associated with k₀, whereas lignin-like molecules were negatively associated. Tests confirmed these findings: tannic acid increased k₀ in low-temperature DBC, while lignin decreased it in medium-temperature DBC. Redox-active quinone and hydroquinone moieties in aromatic structures are likely electron exchange sites.

DBC's environmental electron-shuttle ability is influenced by both redox activity and diffusivity. Once in soils or aquatic systems, DBC with high k₀ could affect microbial metabolism, contaminant transformation, metal cycling, and greenhouse-gas production. However, environmental factors like ionic composition may impact particle aggregation and diffusion.

The study acknowledges limitations in the tannic-acid and lignin-addition experiments, as these compounds may have acted independently rather than altering DBC's structure. The reported k₀ and diffusion coefficients are apparent parameters from standardized conditions. Further research under realistic environmental conditions is needed to understand how laboratory findings apply to natural settings.

This research framework integrates electron-transfer kinetics and molecular composition into pyrogenic carbon assessments. Future studies could explore how feedstock, pyrolysis conditions, and environmental factors influence DBC reactivity and its impact on contaminant fate and carbon cycling at the field scale.