Dissipation and Risk Assessment of Polycyclic Aromatic Hydrocarbons in Industrial-Scale Biochar Composting

Biochar is considered beneficial to composting and is a common composting additive. However, biochar may contain polycyclic aromatic hydrocarbons (PAHs) residues due to its production by oxygen-limited pyrolysis. This study investigated PAHs dissipation in biochar compost, the influence of biochar on composting characteristics and the health risk of compost. Biochar was mixed into compost at 10%, 5%, 2% and 0% (v/v). The compost temperature, time, pH, germination index (GI) and PAHs concentration were determined to evaluate the effect of biochar application rate on PAHs dissipation and compost chemical properties. The added biochar content, which should not be less than 2% (v/v) in compost, positively affects PAHs dissipation. PAHs dissipation follows first-order kinetics. Low molecular weight (LMW) PAHs primarily dissipate early in the thermophilic phase of composting, while high molecular weight (HMW) PAHs primarily degrade later in the thermophilic phase and the cooling phase. Multivariable interactions analysis shows that the compost GI has the greatest effect on PAHs content (λ = -0.936, p < 0.001). The lifetime incremental cancer risks (ILCRs) for adults and children in four composts with different biochar contents are negligible.