Increased oxygen content in biochar lowered bioavailability of polycyclic aromatic hydrocarbons–related toxicity to various organisms

[Display omitted] •Present in BC HM and PAHs are crucial considering biochar toxicity.•Predicting the bioavailability of PAHs using the total content is misleading.•Toxicity was influenced by BC’s bioavailable PAHs > ash > total PAHs content.•Oxygen-containing functional groups were crucial in revealing the toxicity.•Indicating a safe material for soil and environmental applications is difficult. Agricultural or environmental application of biochar (BC) is connected with the introduction of biochar-derived components among which polycyclic aromatic hydrocarbons (PAHs) and heavy metals are the most toxic. Their presence and bioavailability are crucial considering biochar toxicity. The effect of feedstock and pyrolysis temperature on the physicochemical properties of produced biochar and contaminant content was established and combined with toxicity to a broad range of living organisms. The obtained data revealed that predicting the bioavailability of PAHs using the total content is misleading. The toxicity was influenced by factors in the following way: the bioavailable PAHs > ash > total PAHs content in BC stressing the role of BC physicochemical characteristics. Among tested BC properties, surface functionalization, e.g. presence of oxygen-containing functional groups was crucial in revealing the toxicity. The data clearly indicate that additional research is required to determine BC’s impact on various organisms and performing one ecotoxicity test is not sufficient.