Wurster, Christopher M., Saiz, Gustavo, Schneider, Maximilian P.W., Schmidt, Michael W.I., and Bird, Michael I. (2013) Quantifying pyrogenic carbon from thermosequences of wood and grass using hydrogen pyrolysis. Organic Geochemistry, 62. pp. 28-32.

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Abstract

Previously studied thermosequences of wood (chestnut) and grass (rice straw) biochar were subjected to hydrogen pyrolysis (hypy) to evaluate the efficacy of the technique for determining pyrogenic carbon (C-P) abundance. As expected, biochar from both wood and grass produced at higher temperature had higher C-P amount. However, the trend was not linear, but more sigmoidal. C-P/C-T ratio values (C-T = total organic carbon) for the wood thermosequence were <= 0.03 at biochar production temperature (T-CHAR) <= 300 degrees C. They increased dramatically until 600 degrees C and remained relatively constant and near unity at higher biochar production temperature. Grass biochar was similar in profile, but C-P/C-T values rose dramatically after 400 degrees C. The findings are consistent with the hypothesis that hypy residues contain polycyclic aromatic hydrocarbons (PAHs) with a degree of condensation above at least 7-14 fused rings, with labile organic matter and pyrogenic PAHs below this degree of condensation removed by hypy. Both wood and grass thermosequences displayed delta C-13(P) values that decreased with increased T-CHAR, indicating that recalcitrant carbon compounds (pyrogenic aromatic PAHs with a relatively high degree of condensation) were first formed from structural components with relatively high delta C-13 values (e. g. cellulose). Relatively constant delta C-13 values at T-CHAR >= 500 degrees C suggested the dominant pyrolysis reaction was condensation of PAHs with no additional fractionation. Comparison of hypy with benzene polycarboxylic acid (BPCA), 'ring current' NMR and pyrolysis gas chromatography-mass spectrometry (GC-MS) results from the same suite of samples indicated a consistent overview of the structure of C-P, but provided unique and complimentary information. (C) 2013 Elsevier Ltd. All rights reserved.

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