Molecular characterization of biochar and the relation to carbon permanence

Molecular compounds present in biochar carbon structure are studied from biochar produced from forest, food, and agricultural wastes and sewage sludge using pyrolysis gas chromatography mass spectrometry (Py-GC/MS). The results show that with increasing biochar production temperature (PT), the total pyrolysis yield decreases, and the macromolecular structure becomes more condensed with the aromatic linkages becoming less alkylated, hence indicating a stable carbon structure. These highly stable biochar samples consist predominantly of inertinite and have the entire random reflectance (R_o) distributions above the inertinite benchmark (IBR_o2%). The results are aligned with high carbon stability of high-temperature biochar. In contrast, biochar samples that were insufficiently carbonized and comprised of mainly semi-inertinitic biochar contain alkane traces, volatile compounds, and higher degrees of alkylation with aromatic linkages in their molecular structure. This indicates the more proneness to oxidative and microbial breakdown, and therefore a less condensed and less stable carbon structure. Additionally, occurrence of these compounds in inertinitic biochar indicate retention of free hydrocarbons within the biochar carbon structure. Complimenting microscopic and bulk geochemical data, Py-GC/MS data is additionally advantageous to assess the stability conditions of the biochar samples.