Assessment of the Possibility of Soil Contamination with By-Products of Pyrolysis upon Application of Biochars

In the recent decades, there has been increasing interest in the application of biochars (BCs) to soils to solve a wide range of problems: from sequestration of atmospheric carbon and long-term increase in fertility to their use as effective sorbents in the remediation of contaminated soils. However, uncontrolled use of low-quality BC has environmental risks associated with the danger of soil contamination with toxic by-products of pyrolysis. The aim of the work was to study the content and composition of labile hydrophobic by-products of pyrolysis (lipid fraction) extracted by organic solvents from BC obtained from various plant materials at different pyrolysis temperatures. Six types of BC prepared from corn and millet straw and willow wood by pyrolysis to final temperatures of 400 and 600°C (low- and high-temperature biochars, LT/BCs and HT/BCs, respectively) were used in the experiment. All HT/BСs had a significantly lower content of lipid fraction (CLF) compared to LT/BCs: 0.16–0.46 and 0.54–3.38% of the BC weight, respectively. Also, HT/BCs obtained were characterized by a higher content of total organic carbon (CTOC) with a lower proportion of organic carbon of the lipid fraction (CTOCLF). For a qualitative characterization of the lipid fraction, SUVA254 and Sr indices reflecting the degree of aromaticity and molecular weights of colored soluble organic compounds were calculated on the basis on the UV-VIS absorption spectra of their extracts. Based on the obtained results, it was concluded that, regardless of the feedstock, an increase of the final pyrolysis temperature resulted in the decrease in the degree of aromaticity and molecular weight of organic compounds in the lipid fraction. As pyrolysis temperature increased, the total content and proportion of polynuclear representatives of polycyclic aromatic hydrocarbons (PAHs)—the most dangerous by-products with high toxicity, carcinogenicity, and resistance to biodegradation—decreased. It was also found that the total PAH content in all LT/BCs exceeded the maximum permissible level (20 mg/kg) recommended by the International Biochar Initiative (IBI), while the total PAH content in the HT/BCs was below this value. Thus, a conclusion was made about the need to develop general quality standards for BCs produced for application to soil, taking into account the content of hydrophobic pyrolysis by-products, including PAHs, which are hazardous soil pollutants.