Profiles of Volatile Organic Compounds in Biochar: Insights into Process Conditions and Quality Assessment

Sustainable application of biochar to soil requires careful consideration of sorbed mobile compounds that can be released into the environment and their relationship with bulk chemical properties. Seven biochars were produced by pyrolysis of corn stalk from 350 to 650 °C, and residual volatile organic compounds (VOCs) were investigated by head space solid-phase microextraction and gas chromatography-mass spectrometry. Over 80 compounds were detected in poorly carbonized biochars, including thermal degradation products of lignin (2-methoxyphenols), cellulose and hemicellulose (1,4:3,6-dianhydro-β-d-glucopyranose, C1-C3 furans and furanones, C1-C2 cyclopentenones), lipids (aromatic and C2-C8 aliphatic acids), and proteins (aromatic nitriles, amides). The presence of potentially harmful compounds, such as benzene, toluene, ethylbenzene, xylenes, phenols, volatile fatty acids, and polycyclic aromatic hydrocarbons, highlights the importance of controlling the biochar production process. Statistically significant decreasing trends emerged between the quantity of all VOC classes and the increasing carbonization degree estimated by the hydrogen to carbon (H/C) ratios and volatile matter content. Biochars with H/C < 0.70 (pyrolysis temperature >400 °C) did not release VOCs at ambient temperatures (25, 50 °C). VOCs fingerprinting can contribute to the evaluation of biochar quality. The samples with the highest levels of VOCs did not inhibit the germination of cress seeds (40 g L-1 water suspensions) indicating that biochar contamination is not necessarily associated with adverse effects.