Formation of polycyclic aromatic hydrocarbons and their derivatives in biochars: The effect of feedstock and pyrolysis conditions

Abstract

During pyrolysis and several environmental processes, polycyclic aromatic hydrocarbons (PAHs) are altered and O/N/S-derivatives may be formed. The thermal utilization of sewage sludge (SSL) and bio-wastes (e.g. plant materials) towards biochars (BC) may be connected with the formation of both PAHs and their derivatives, known toxic compounds. Their toxicity depends on bioavailability. In the presented studies the effect of pyrolysis temperature on PAHs and their derivatives formation was investigated. The results clearly indicated that temperature (500, 600, and 700 °C) and feedstock (sewage sludges, wheat straw, willow) affected both the amount and type of bioavailable PAHs in biochars. The total content of bioavailable PAHs in SSL-derived BC was higher than in plant-derived BC and ranged from 8.58 to 16.07 ng L−1 for BC obtained from SSL, 1.67–3.53 ng L−1 for straw-derived biochars, and 3.17–3.53 ng L−1 for willow-derived biochars. The percentage of quantified PAHs derivatives in SSL-derived BC was up to 10.3 %. Plant-derived BC were enriched in PAHs derivatives (1-methyl-5-nitronaphthalene, 1-methyl-6-nitronaphthalene, 4H-cyclopenta(def)phenanthrene, and nitropyrene). The increase of the pyrolysis temperature caused an increase in the percentage of PAHs derivatives from 5 % to 37.4 % (500−700 °C) for straw-derived BC whereas decrease to 15 % for willow-derived BC. The presence of toxic compounds such: 1-methyl-5-nitronaphthalene, 1-methyl-6-nitronaphthalene, nitropyrene in plant-derived BC, and B[a]P, nitronaphthalene, 9,10-anthracenedione in SSL-derived BC may significantly limit their agricultural applications.