Micro-Raman spectroscopy and Petrography for unraveling the complex heterogeneous physicochemical structures of biochar from the scale of bulk to micro: A comparison and discussion

Abstract

This study produced biochar from coconut shells and corncobs through pyrolysis under a nitrogen atmosphere with temperatures ranging from 350℃ to 1400℃. The surface regions of the biochar were characterized at the micro-scale using a combination of micro-Raman spectroscopy and petrography. Extensive data from both methods were compared and correlated from the scale of bulk to micro- levels. The results indicate that for bulk structures, the average random reflectance (R_f) increases and Raman parameter α decreases with temperature, indicating a higher thermal maturity and lower C-H, C-O etc. structures. However, A_D/A_G, A_(VR+VL+GR)/A_D and A_(VR+VL+GR)/A_G exhibit significant inflection points at specific temperatures. These inflection points are linked to key structural transformations: aromatization at 600 ℃ and graphitization at 1000 ℃. At microscale, biochar contains pores of varying shapes and sizes (approximately 5–100μm) with ash deposits embedded within them. These features significantly influence the heterogeneity in R_f measurements, resulting in a broaden distribution of R_f as the pyrolysis process progressed. Besides, micro-Raman spectroscopy shows that biochar particles with higher substituent and side-chain abundances have a tendency towards preferential reactivity. Furthermore, the chemical structural distribution of biochar became more concentrated and focused below 1000 ℃. However, biochar undergoes heterogeneous graphitization at 1000 ℃, and the distribution of aromatic rings and graphite structure becomes even more dispersed between 1000 ℃-1400 ℃. The correlations between the results of petrographic method and micro-Raman at the bulk and micro-scale have been set up and discussed, and it can provide guidance for the comprehensive characterization of the heterogeneous structure of biochar.