Black carbon electron-shuttle speed increases with higher pyrolysis temperature

A recent peer-reviewed study led by researchers Yufei Wu, Peng Zhang, Zhaofeng Chang, and Bo Pan at Kunming University of Science & Technology has revealed that dissolved black carbon transfers electrons more rapidly when produced at higher pyrolysis temperatures. The study focused on biomass-derived carbon's role in microbial electron exchange, quantifying the apparent heterogeneous electron-transfer rate constant, k₀, for ten samples derived from rice straw and sawdust pyrolyzed at temperatures ranging from 200 to 600 °C. This kinetic measurement addresses a gap in conventional redox testing, which typically measures the amount of charge a material can exchange but not the speed of that exchange.

The researchers employed a combination of cyclic voltammetry and differential pulse voltammetry within a three-electrode system, alongside particle-size measurements, mediated electrochemical experiments, Fourier transform-ion cyclotron resonance mass spectrometry, and microbial electrochemical systems using Shewanella oneidensis MR-1. For dissolved black carbon derived from rice straw, the k₀ increased 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 materials showed an increase from (1.05 ± 0.02) × 10⁻⁴ to (6.18 ± 3.08) × 10⁻² cm s⁻¹ over the same temperature range. Generally, sawdust-derived samples exhibited higher k₀ values than rice-straw samples at equivalent temperatures, and smaller average particle sizes were linked to faster apparent diffusion. The rate constant showed a strong correlation with both diffusion coefficients and electron-exchange capacity.

At pyrolysis temperatures between 400 and 600 °C, the dissolved black carbon demonstrated superior electron-shuttle capabilities compared to natural dissolved organic matter from wetland, soil, river, and lake environments. In microbial electrochemical systems, materials produced at higher temperatures generated larger peak and steady currents, with these responses significantly correlated with log₁₀(k₀), thus connecting the electrochemical kinetic parameter to microbial extracellular electron transfer. Mass-spectrometric analysis revealed that higher-temperature samples contained greater relative intensities of CHNOS, condensed aromatic, and tannin-like compounds, while lignin-like compounds were reduced. Condensed aromatic and tannin-like molecules were positively associated with k₀, whereas lignin-like molecules were negatively associated. Additional tests supported these findings: tannic acid increased k₀ in low-temperature materials, while lignin decreased it in medium-temperature materials. The researchers identified redox-active quinone and hydroquinone moieties within aromatic structures as potential sites for electron donation and acceptance.

The study suggests that the ability of dissolved black carbon to shuttle electrons in soils and waters is influenced by a combination of redox activity and diffusivity, rather than solely by total electron-exchange capacity. If these behaviors are applicable under natural conditions, rapidly transferring dissolved carbon could impact microbial metabolism, contaminant transformation, metal cycling, and greenhouse-gas production. However, these implications remain uncertain due to potential environmental alterations of laboratory results. Specifically, Ca²⁺ and Mg²⁺ may promote particle aggregation and reduce effective diffusion. The reported k₀ and diffusion coefficients are apparent parameters measured under standardized electrochemical conditions, and their behavior under realistic pH, ionic strength, mineral association, microbial-community composition, and transport regimes requires further evaluation. The tannic-acid and lignin additions resulted in physical mixtures, suggesting that the added compounds may have functioned as independent electron shuttles rather than altering the intrinsic structure of the black carbon. Field-scale tests are necessary to determine whether the observed kinetic differences have measurable effects on contaminant fate, denitrification, methanogenesis, or carbon cycling.