Low bioavailability of derivatives of polycyclic aromatic hydrocarbons in biochar obtained from different feedstock

Abstract

In the last years, there is great progress in the field of studies on the thermal transformation of wastes into valuable materials such as biochar. High-temperature processes, however, are connected with the formation of polycyclic aromatic hydrocarbons (PAHs) with confirmed toxicity. However, during pyrolysis, some derivatives containing oxygen, nitrogen, or sulfur can also be formed. Their toxicity is expected to be higher than parent PAHs. However, the key parameter in the agricultural application of carbonaceous materials is PAHs’ bioavailability. The aim of the presented studies was the determination of the effect of various feedstock (wheat straw (Triticum L.), willow (Salix viminalis), sunflower, residues from softwood and hardwood, sewage sludges, and residues from biogas production) on the formation of PAHs and their derivatives (O-, N-PAHs) in biochar and their bioavailability. The results indicated that the content of total and bioavailable PAHs in obtained biochar was rather low. The concentration of total PAHs in plant-derived biochar reached 57 ± 3 ng g−1 - 181 ± 8 ng g−1, whereas sewage sludge-derived biochar contained from 121 ± 6 ng g−1 to 188 ± 9 ng g−1 of PAHs. The highest concentration of PAHs was noted in biochar obtained from residues from biochar production – up to 202 ± 9 ng g−1. The total concentration of bioavailable PAHs was lower and reached 2–4.45 ng L−1 for plant-derived biochar, 3–40 ng L−1 for sewage sludge-derived biochar. The highest content of bioavailable PAHs was noted in biochar obtained from residues from biogas production: 9–42 ng L−1 indicating that increased attention should be paid to using this type of biochar. Among PAHs derivatives, nitronaphthalene, 1-methyl-5-nitronaphthalene, 1-methyl-6-nitronaphthalene, 9,10-anthracenedione, 4H-cyclopenta(def)phenanthrene, nitropyrene were determined at various levels and their concentrations were from below the limit of detection (LOD) to 28 ng L−1 for plant-derived biochar, 3–16 ng L−1 for biochar obtained from residues from biogas production, and 5–45 ng L−1 for sewage sludge-derived biochar. The content of bioavailable PAHs derivatives was, generally, one order of magnitude lower than parent PAHs derivatives, and reached from below LOD up to almost 1 ng L−1 for plant-derived biochar, from 0.5 to 2 ng L−1 for biochar obtained from residues from biogas production, and from 0.2 to almost 5 ng L−1 for sewage sludge-derived biochar confirming the safety of agricultural usage of biochar.