Abatement of Polycyclic Aromatic Hydrocarbon Residues in Biochars by Thermal Oxidation

Abstract

Biochars often contain residues of polycyclic aromatic hydrocarbons (PAHs) that can pose risks to exposed organisms and ultimately to human health. This study shows that thermal oxidation of an anoxically produced (500 °C) softwood biochar using brief post-pyrolysis thermal oxidation in air caused a substantial decline in solvent-extractable PAHs (up to 85% of Tot-PAH) and a sharp decrease (up to 82%) in the EPA Relative Potency Factor. Optimum conditions were 400 °C for 20 min. An alternative approach of including 1% O₂ in the nitrogen flow stream during pyrolysis was less effective. PAH loss is attributed to combustion processes as opposed to outgassing. Oxidation of PAHs occurs in the gas and adsorbed phases and is promoted by pore reaming of biochar micropores and mesopores during thermal oxidation, which facilitates diffusive exchange of O₂, reactive oxygen species, and PAH molecules. Oxidation also may be promoted by self-heating of the biochar body, which increases the local temperature. The results indicate that a short post-pyrolysis thermal oxidation step can reduce the risks associated with PAH residues in biochar.