Balancing benefits and risks in food waste biochar: Pyrolysis severity effects on contaminant fate, nutrient leaching, and biological safety

Thermochemical conversion of food waste into biochar offers a compelling circular economy pathway for soil amendment, yet comprehensive safety assessments remain limited. This study assessed the stability, contaminant fate, nutrient leaching, and microbial compatibility of bone and mixed vegetable biochars pyrolyzed at 300-600 °C. Feedstock identity dominated variance in yield, ash, and carbon (η² = 0.73-0.91), whereas temperature governed the volatile heteroatoms H, O and N (η² = 0.44-0.74; all p < 0.001). At 600 °C, bone retained higher yield (60.3 ± 0.7 % vs 34.4 ± 0.7 %) and appreciable porosity (118.0 m²/g), whereas vegetable biochar showed negligible surface area (≤1.0 m²/g) yet preserved a carbon-rich matrix (59.5 ± 0.5 % vs 11.5 ± 1.3 % C). Vegetable biochar attained thermal stability (H/C < 0.7) by 400 °C, while bone did not cross this threshold. Bone was nutrient-dense (Ca 145,285 ± 25,963; P 73,321 ± 18,435 mg/kg), but its ions remained matrix-bound, raising leachate pH/EC only to 8.5 ± 0.18 and 235.7 ± 7.9 µS/cm versus 10.3 ± 0.08 and 1,202.5 ± 6.4 µS/cm for K-rich (25,325 ± 5,060 mg/kg) vegetable biochar, releasing far more labile ions (Na 96.4 % vs 44.8 %; p < 0.001). Mild pyrolysis (300 °C) eliminated key agrochemical and pharmaceutical contaminants (carbamazepine, pyrimethanil, o-hydroxybiphenyl) without generating EPA-priority polycyclic aromatic hydrocarbons; industrial plasticizers persisted at 600 °C, highlighting the need for upstream feedstock screening. Pyrolysis reduced dissolved organic carbon to < 10 mg/L, lowering colony counts to control levels, unlike raw bone (59-fold higher, p = 0.001). Bone biochar thus functions as a slow-release mineral scaffold, while vegetable biochar confers liming potential for acidic soils, manageable by pre-washing. Pyrolysis severity and feedstock selection are jointly critical to biochar safety.