Assessment of commercial biochar's stability: Insights from organic petrography and micro-Raman Spectroscopy

Biochar, a carbon-rich material produced by biomass pyrolysis or pyro-gasification, is increasingly considered a reliable carbon dioxide removal (CDR) option and a versatile product for environmental applications. Yet, the properties of commercial biochar remain highly variable, and the lack of standardized characterization methods limits the ability to assess their long-term carbon stability and functional performance. This study addresses this gap by analyzing nine commercial biochar through an integrated approach combining organic petrography and random reflectance, and micro-Raman spectroscopy. These techniques, commonly used to evaluate the thermal maturity and carbonization of geological organic matter, are here adopted to quantify the degree of aromatic condensation and identify structural features linked to long-term carbon stability. Bulk chemical indicators, especially the H/C molar ratio, are also included for comparison. While H/C molar ratio remains a useful proxy of overall biochar stability, spectroscopic and petrographic techniques provide the necessary resolution to identify differences in carbonization to assess long-term biochar stability. Random reflectance measurements of biochar (Ro%) reveal a wide range of carbonization behaviors, from a unimodal and narrow distribution, indicative of uniform charring process, to bimodal or asymmetric distributions, which reflect intrinsic heterogeneity produced by variations in residence time, heat transfer, or feedstock reactivity during production process. Using the value of 2% as inertinite benchmark (IBRo2%), reflectance data effectively discriminates incompletely carbonized domains from fully stabilized aromatic structures. Furthermore, Raman spectra showed systematic evolution of the D1 and G bands, with D1-G separation, intensity (iD1/iG), area (aD1/aG), and width (wD1/wG) ratios increasing with Ro%. These parameters defined two carbonization stages across the dataset. Biochar with Ro > 3% show inertinite-like signatures consistent with high carbonization. In contrast, samples dominated by low-reflectance fractions (Ro < 2%) are characterized by Raman spectral features typical of poorly carbonized, labile material. By integrating micro-Raman spectroscopy with reflectance measurements, this study introduces a set of rapid diagnostic parameters derived from spectroscopy for evaluating the carbonization efficiency and long-term stability of commercial biochar. The approach enables rapid discrimination between poorly carbonized and fully inertinite-like materials, offering practical benchmarks for CDR applications and for optimizing conditions in real production scenarios.