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.生物炭（biochar）的制备及其施入土壤的应用，近年来已成为负排放战略中二氧化碳封存路径的研究热点。生物炭在土壤中的长期留存性由固有材料特性决定，而这些特性受热解（pyrolysis）工艺条件、原料种类及环境因素共同调控。本研究以花生壳、悬铃木木材、小麦秸秆三种生物质为原料，在300~700℃的热解温度区间内制备生物炭，采用反射光谱与拉曼光谱联用的表征手段，探究其结构性质的演化规律。 拉曼光谱采用两种解卷积方法进行解析：二带模型（D带与G带）以及四带模型（D1、D3、D4与G带）。所得拉曼参数可有效反映生物炭结构随热解温度与生物炭反射率的变化特征。所有样品均呈现出一致的变化趋势：随着热解温度升高，谱带位置发生系统性偏移、谱带间距逐渐增大，且谱带逐步窄化。多项拉曼参数——具体包括D带与D1带的位置、D带半高全宽（FWHM）以及拉曼谱带间距——与生物炭反射率呈现近线性相关关系，表明这些参数可作为反射率的光谱替代表征指标。 值得注意的是，多项拉曼参数在500℃附近以及反射率约为2.0%~2.5%的区间出现拐点，这标志着反应机制从以生物质分解为主的阶段，转向聚芳族团簇的生长与聚合阶段。拉曼光谱得到的拐点与反射率阈值的高度吻合，恰好对应生物炭在土壤中留存稳定性大幅提升的关键阶段。综上，拉曼光谱与反射光谱的变化趋势具有高度一致性，证明拉曼光谱是生物炭表征的重要互补工具，可提供分子尺度的分析视角，从而优化对生物炭稳定性与碳封存潜力的评估。