Polycyclic aromatic hydrocarbons (PAHs) in biochar – Their formation, occurrence and analysis: A review

Application of biochar for soil development has been increasingly practiced over the last decade. However, during the pyrolytic production of biochar, polycyclic aromatic hydrocarbons (PAHs) can form and are present on the surface of biochar. Consequently, there may be a potential risk for human health as well as the environment. In the present paper, research progress related to PAHs in biochar with respect to their formation mechanisms, factors that influence their formation, and the methods used to determine the total and bioavailable PAHs are discussed. The pyrolysis process is the key factor responsible for the yield of PAHs in biochar. Slow pyrolysis and longer residence time result in lower PAHs yield than the fast pyrolysis and shorter residence time. Temperature is also another significant determinant affecting the formation and yield of PAHs. Low molecular weight PAHs are usually formed at low temperature (<500°C) whereas the high molecular weight PAHs commonly appear under high temperature (>500°C). Original composition, mineral and moisture content of feedstock, appearance of O2 during pyrolysis or after some treatments also have a considerable effect on the yield of PAHs. Analytical methods for extraction of PAHs from biochar mainly include Soxhlet extraction and accelerated solvent extraction. However, the extraction efficiency depends both on the extractant method and the type of biochar. This review of the literature has noted that future studies should focus on (i) development of a standard protocol for quantifying total and bioavailable PAHs in biochar, (ii) revealing the effect of key chemical components of feedstock and high temperature on the formation of PAHs, and (iii) carrying out eco-toxicological studies at long timescale to evaluate the environmental risk of biochar application.