Long-term physical and chemical aging of biochar affected the amount and bioavailability of PAHs and their derivatives.

Biochar applied into the soil is recommended as an effective tool for increasing its properties and crop productivity. However, biochar can contain some potentially toxic compounds such as polycyclic aromatic hydrocarbons (PAHs). Moreover, during biochar production or environmental application (e.g. as soil fertilizer), more toxic PAHs derivatives containing nitrogen, oxygen or sulfur can be formed. There is a lack of information on how the environmental factors affect the bioavailability of such compounds during the long-term application of BC into the soil. In the presented studies the effects of physical (freeze-thaw cycles) and chemical aging (temperatures 60 °C and 90 °C) on the total and bioavailable content of PAHs and their derivatives were estimated. The results indicate that long-term (6 months) aging affected the physicochemical characteristic of biochars promoting the formation of new C and O-containing species on the BC surface increasing their polarity and hydrophilicity. Physical and chemical aging promoted the formation of compounds with higher molecular weight and a significant (up to 550 %) increase in the bioavailability of PAHs and their derivatives. The results of this study highlight the importance of the bioavailable fraction of PAHs and their derivatives for evaluation of the toxicity of aged biochar.