Solvent-Extractable Polycyclic Aromatic Hydrocarbons in Biochar: Influence of Pyrolysis Temperature and Feedstock

Despite the increasing agricultural use of biochar as a way of combining the utilization of biomass for energy production with the removal of CO2 from the atmosphere, it is not known how variations in pyrolysis temperature and feedstock type affect concentration and composition of polycyclic aromatic hydrocarbons (PAHs) that inevitably form and associate with biochar. To close this knowledge gap, we quantified 11 unsubstituted three- to five-ring PAHs as well as alkylated forms of phenanthrene and anthracene in grass and wood chars produced in 100 degrees C increments across a temperature range (100 to 700 degrees C). Our results show that solvent-extractable PAH concentrations in biochars produced at heat treatment temperatures (HTTs) of 400 and 500 degrees C greatly exceed those observed at higher and lower temperature, supporting a low HTT solid-phase formation mechanism operable at temperatures commonly used for industrial biochar production. The maximum extractable yield of 'pyrolytic' unsubstituted PAHs for grass (22 mu g g(-1) at HT'T = 500 degrees C) greatly exceeds the value for wood (5.9 mu g g(-1)). Moreover, PAH signatures (e.g., total monomethylphenanthrene to phenanthrene ratios, MP/P similar to 2-3) at intermediate temperatures (400 degrees C) resemble those of fossil oils rather than that commonly attributed to pyrolytic products. Further research is needed to characterize the PAR evolution in modern pyrolysis reactors and assess the fate of biochar-bound PAHs in soils and sediments. Various commonly applied PAR ratios and indicator compounds show promise as markers for specific feedstock materials and pyrolysis conditions of biochars in environmental systems.