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 degrees C (T-CHAR) 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 T-CHAR > 500 degrees C (wood) or > 700 degrees 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 T-CHAR 250 and 500 degrees C. With increasing Tow, 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 crosslinkages 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 C-13 benzene chemical shift NMR (Nuclear Magnetic Resonance) approach. Pyrolysis-GC/MS provides more information on molecular structures in the low temperature range (T-CHAR <= 500 degrees C) while the BPCA and NMR ring current methods provide more reliable estimations of charring intensity, especially at higher temperatures (T-CHAR >= 500 degrees C). 2012 Elsevier Ltd. All rights reserved.