Assessment of The Possibility of Soil Contamination by Pyrolysis By-Products When Adding Biochars

In the last decade, there has been a growing interest in the possibility of introducing biochars (BC) into 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. The studies used six types of BC prepared from corn and millet straw, as well as willow wood by pyrolysis to final temperatures of 400 and 600°C (low- and high-temperature – LT/BC and HT/BC, respectively). It was shown that all HT/BС have a significantly lower content of lipid fraction (CLF) compared to LT/BC: within 0.16–0.46 and 0.54–3.38% of the BC weight, respectively. Also, BC obtained at higher temperatures were characterized by a higher content of total organic carbon (CTOC) with a lower proportion of lipid fraction organic carbon (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 based on the UV-VIS absorption spectra of their extracts. Based on the obtained results, it was concluded that, regardless of the feedstock, with an increase in the final pyrolysis temperature, decrease the degree of aromaticity and molecular weight of organic compounds included in the lipid fraction. In addition, with increasing pyrolysis temperature, the total content and proportion of polynuclear representatives of polycyclic aromatic hydrocarbons (PAHs), which are the most dangerous by-products due to their high toxicity, carcinogenicity and resistance to biodegradation, decrease. It was also found that in all HT/BC, the total PAH content exceeds the maximum permissible level (20 mg/kg) recommended by the International Biochar Initiative (IBI), while in HT/BC, the total PAH content was below this value. Thus, a conclusion was made about the need to develop general quality standards for BC produced for application to soil, taking into account the content of hydrophobic pyrolysis by-products, including PAHs, which are hazardous soil pollutants.