Composition of PAHs in Biochar and Implications for Biochar Production

The content of polycyclic
aromatic hydrocarbons (PAHs) in biochar
has been studied extensively; however, the links between biomass feedstock,
production process parameters, and the speciation of PAHs in biochar
are understudied. Such an understanding is crucial, as the health
effects of individual PAHs vary greatly. Naphthalene (NAP) is the
least toxic of the 16 US EPA PAHs but comprises the highest proportion
of PAHs in biochar. Therefore, we investigate which parameters favor
high levels of non-NAP PAHs (∑16 US EPA PAHs without NAP) in
a set of 73 biochars. On average, the content of non-NAP PAHs was
9 ± 29 mg kg–1 (median 0.9 mg kg–1). Importantly, during the production of the biochars with the highest
non-NAP PAH contents, the conditions in the post-pyrolysis area, where
pyrolysis vapors and biochar are separated, favored condensation and
deposition of PAHs on biochar. Under these conditions, NAP condensed
to a lower degree because of its high vapor pressure. In biochars
not contaminated through this process, the average non-NAP content
was only 2 ± 3 mg kg–1 (median 0.5 mg kg–1). Uneven heat distribution and vapor trapping during
pyrolysis and cool zones in the post-pyrolysis area need to be avoided.
This demonstrates that the most important factor yielding high contents
of toxic PAHs in biochar was neither a specific pyrolysis parameter
nor the feedstock but the pyrolysis unit design, which can be modified
to produce clean and safe biochar.