Occurrence, distribution, and toxicity assessment of polycyclic aromatic hydrocarbons in biochar, biocrude, and biogas obtained from pyrolysis of agricultural residues

Occurrence, distribution, and toxicity assessment of polycyclic aromatic hydrocarbons (PAHs) in pyrolysis steams (biochar, biocrude, and biogas) of three agricultural residues was investigated at pyrolysis temperatures of 400-800 degrees C. Increasing PAHs formation was observed in the narrow temperature range (500-600 degrees C) in all feedstocks due to temperature-induced dehydration, decarboxylation, and dehydrogenation reactions. Low molecular weight PAHs (naphthalene, phenanthrene) were dominant in all product streams while high molecular weight PAHs were found in negligible concentrations. Leaching studies showed that pyrolyzed biochars produced at lower temperatures are more prone to leaching due to the presence of hydrophilic amorphous uncarbonized structures, while the presence of hydrophobic carbonized matrix with denser and stronger polymetallic complex prevents the leaching of PAHs in the high temperature pyrolyzed biochar. Low leaching potential, low toxic equivalency, and permissible total PAHs values in biochar derived from all three feedstocks warrant the broader application and ensure ecological safety.

Automated summary

The occurrence, distribution, and toxicity of polycyclic aromatic hydrocarbons (PAHs) were investigated in pyrolysis streams (biochar, biocrude, and biogas) of three agricultural residues at pyrolysis temperatures of 400-800 degrees C. The formation of PAHs increased in a narrow temperature range (500-600 degrees C) due to temperature-induced reactions. Low molecular weight PAHs dominated in all product streams, while high molecular weight PAHs were found in very low concentrations. Leaching studies revealed that biochars produced at lower temperatures were more prone to leaching, while high temperature pyrolyzed biochar prevented the leaching of PAHs due to the presence of a hydrophobic carbonized matrix. The low leaching potential, low toxic equivalency, and permissible total PAHs values in biochar derived from all three feedstocks ensure ecological safety and warrant broader application.