Importance of Pyrolysis Temperature and Pressure in the Concentration of Polycyclic Aromatic Hydrocarbons in Wood Waste-Derived Biochars

Biochar addition to soil can lead to potential environmental risks due to its content of polycyclic aromatic hy-drocarbons (PAHs). Until now, previous research focused on assessing the influence of pyrolysis peak temper-ature and feedstock on the formation and evolution of PAHs. Nevertheless, the effects of other important process parameters -such as pressure, gas residence time, and type of carrier gas- have not been comprehensively explored. To fill this gap, a 2-level full factorial design of experiments was conducted to assess the influence of the above-mentioned parameters on the pyrolysis behavior of an untreated wood waste as well as the properties of resulting biochars, including their PAHs contents. Results showed that the highest production of PAHs was reached at lower peak temperatures, whereas an increase in temperature led to a substantial reduction of the final PAHs content. An increased pressure also resulted in a marked decrease in PAHs, probably as a consequence of the higher carrier gas flow rates used under pressurized conditions, which could inhibit the generation of PAHs by condensation and polymerization. The outstanding results obtained from the phytotoxicity assessment for three plant species (barley, watercress, and basil) suggest that PAHs were not the major responsible for the observed short-term phytotoxic effects of biochars, since a considerable part of the phytotoxic compounds in biochar can be removed by a simple water washing step.