Environmental contextualisation of potential toxic elements and polycyclic aromatic hydrocarbons in biochar

Nine dissimilar biochars, produced from varying feedstock at different pyrolysis temperatures, are appraised with respect to concentrations of potentially toxic elements, specifically, metals, metalloids and polycyclic aromatic hydrocarbons (PAHs). Concentrations of the metals and metalloids varied with the following ranges (mg kg−1): 0.02–0.94, Cd; 0.12–6.48, Cr; 0.04–13.2, Cu; 0.1–1.37, Ni; 0.06–3.87, Pb; 0.94–207, Zn and 0.03–0.27, As. Σ16PAH concentrations (16 Environmental Protection Agency (EPA) PAHs) range between 0.08 mg kg−1 to 8.7 mg kg−1. Subsequent comparison with background soil concentrations, concentration applied to the regulation of composted materials (Publicly Available Specification (PAS 100)) and European Union (EU) regulations relating to the application of sewage sludge to agricultural land suggest low risk associated with the concentrations of PTEs observed in biochar. Collectively, results suggest that environmental impacts attributable to metals, metalloids and PAHs associated with biochar following its application to soil are likely to be minimal.

► Concentrations of PTEs varied with feedstock and temperature of production.
► Of the PTEs Zn (0.94–207 mg kg−1) was of most priority.
► PTE levels did not infringe guidance values for compost or sewage sludge.
► Biochar (<100 t ha−1) is unlikely to make any real difference to PTE concentrations in soil.