Metal/metalloid elements and polycyclic aromatic hydrocarbon in various biochars: The effect of feedstock, temperature, minerals, and properties

Fourteen metal/metalloid elements and sixteen polycyclic aromatic hydrocarbons (PAHs) within biochars were quantified to investigate how heat treatment temperatures (HTTs) and feedstocks affect their concentration and composition. Concentrations and composition of metals/metalloids were strongly dependent upon feedstocks rather than HTTs. HTTs significantly affected concentrations and composition of PAHs. The highest concentration of PAHs was observed for plant residue-derived biochars (PLABs) produced at 450 °C and the opposite result was for animal waste-derived bichars. High mineral content was responsible for depolymerization of organic matter (OM), which facilitated high production of PAHs. High HTTs pyrolysis or combustion PAHs (COMB) of PLABs possibly blocks their micropores derived from other components within OM and leads to a decline of CO₂-surface areas (CO₂-SAs). Concentration of ∑COMB or individual PAH was affected by biochar properties, including composition and contents of functional groups, ash content, and CO₂-SAs. PLABs produced at 600 °C were recommended for low toxicity. - Highlights: • Yield and composition of metals in biochars were strongly dependent on feedstocks. • Heat treatment temperature notably affected concentration and composition of PAHs. • Low-molecular-weight PAHs may serve as precursors for the combustion PAHs (COMB). • Concentration of ∑COMB or individual PAH was affected by biochar properties. • Plant-residue derived biochars produced at 600 °C are recommended for low toxicity. - The contamination of biochars after their addition to soil can be evaluated through learning about the HTTs, feedstocks, minerals and properties of biochars.