Rapid molecular screening of black carbon (biochar) thermosequences obtained from chestnut wood and rice straw: A pyrolysis-GC/MS study

Rice straw and chestnut wood were heated between 200 and 1000 °C (TCHAR) to produce Black C ‘thermosequences’. The molecular properties of the charred residues were assessed by pyrolysis-GC/MS to investigate the relation between charring intensity and pyrolysis fingerprint. Samples obtained at TCHAR > 500 °C (wood) or >700 °C (straw) gave low quality pyrograms and poor reproducibility because of high thermal stability, but pyrolysis-GC/MS allowed to track the thermal degradation of the main biocomponents (polysaccharides, lignin, methylene chain-based aliphatics, triterpenoids, chlorophyll and proteins) in the lower temperature range, mostly occurring between TCHAR 250 and 500 °C. With increasing TCHAR, the charred residues of these biocomponents lose characteristic functional groups, aromatise and finally condense into non-pyrolysable biomass. The proportions of the pyrolysis products of unspecific origin (benzene, toluene, PAHs, etc.), increase with charring intensity, while the ratios that reflect the abundance of alkyl cross-linkages between aromatic moieties (e.g. benzene/toluene, naphthalene/alkylnaphthalene) decrease. These results provide the guidelines to using pyrolysis-GC/MS for the molecular characterisation of different components in Black C and biochar, which is an important parameter for predicting Black C/biochar behaviour in soil. Results are consistent with earlier studies of these samples using the BPCA (benzenepolycarboxylic acid) method and the ring current-induced 13C benzene chemical shift NMR (Nuclear Magnetic Resonance) approach. Pyrolysis-GC/MS provides more information on molecular structures in the low temperature range (TCHAR ≤ 500 °C) while the BPCA and NMR ring current methods provide more reliable estimations of charring intensity, especially at higher temperatures (TCHAR ≥ 500 °C).