Size reduction of biochar to nanoscale decrease polycyclic aromatic hydrocarbons (PAHs) and metals content and bioavailability in nanobiochar

Objective of this research was to investigate ball milling of biochar from various feedstocks (willow, rice husk, oilseed rape, sewage sludge) under different pyrolysis conditions (500 and 750 °C, N₂ and CO₂ atmosphere) on polycyclic aromatic hydrocarbons (PAHs) and metals content. A significant reduction in both total (C_total Σ16) (1.1 to 2.4-fold decrease) and freely available (C_free Σ16) (1.1 to 2.9-fold decrease) PAHs in nanobiochar (n-BC), particularly in sewage sludge (SSL)-derived n-BC was observed. The changes in biochar surface characteristics induced by ball milling, such as surface area, pore volume, and hydrophilicity, contribute to the decreased PAHs content. Mechanisms involving mechanochemical degradation, free-radical reactions, and defects on the biochar surface may explain the decrease of PAHs. The n-BC showed lower log K_d values for PAHs (reduced by 14 to 41 %) compared to bulk biochar (b-BC), indicating reduced effectiveness in immobilizing PAHs. The ball milling significantly reduced also metal contents by 37 to 55 %, especially in willow pellets and agricultural residue (rice husk and oilseed rape straw) with optimal reduction (by 55 %) observed for WL-derived n-BC produced at 650 °C. Importantly, no metals were detected in the aqueous phase after ball milling, ensuring environmental safety of these materials. Reduction mechanisms of metals during ball milling include stable aggregate formation, enhanced defects in crystalline structures, and irreversible adsorption onto new surfaces. The results demonstrate that the application of an eco-friendly ball milling method allows for the production of materials characterized by reduced contamination levels, which is crucial from an environmental protection viewpoint.