Raman Spectroscopy and Reflectance as Probes for Structural Evolution of Biochar

The production of biochar and its incorporation into soils have recently gained attention as a carbon dioxide sequestration pathway within negative emissions strategies. The long-term persistence of biochar in soil is governed by its intrinsic material properties, which are controlled by pyrolysis conditions and feedstock type, as well as by environmental factors. This study investigates the evolution of biochar structural properties derived from three biomass feedstocks—peanut shells, sycamore wood, and wheat straw—across a range of pyrolysis temperatures (300–700 °C) using a combined reflectance and Raman spectroscopic approach. Raman spectra were analyzed using two deconvolution approaches: a 2-band model (D and G bands) and a 4-band model (D1, D3, D4, and G bands). The resulting Raman parameters effectively capture structural transformations in biochar as functions of pyrolysis temperature and biochar reflectance. Across all samples, consistent trends were observed, including systematic shifts in band positions, increasing band separation, and progressive band narrowing with increasing temperature. Several Raman parameters—specifically the D and D1 band positions, D-band full width at half maximum (FWHM), and Raman band separation—exhibit near-linear relationships with biochar reflectance, indicating their potential as spectroscopic proxies for reflectance. Notably, several Raman parameters display inflection points near 500 °C and at reflectance values of approximately 2.0–2.5%, marking a transition from biomass decomposition–dominated reactions to the growth and coalescence of polyaromatic clusters. This convergence of Raman-derived inflection points with reflectance thresholds closely corresponds to the stage at which biochar acquires substantially enhanced permanence in soil. Overall, the strong agreement between Raman and reflectance trends demonstrates that Raman spectroscopy provides a valuable complementary tool for biochar characterization, offering molecular-scale insights that enhance assessments of biochar stability and carbon sequestration potential.