Environmental safety assessment of cocoa pod husk–derived biochar: Atmospheric control of PAH formation and zebrafish-based ecotoxicological evaluation

Polycyclic aromatic hydrocarbons (PAHs) are recalcitrant organic contaminants whose multi-ring aromatic structures contribute to their persistence and ecological toxicity. In this work, cocoa pod husk (CPH), a major agricultural residue, was converted into biochar through thermochemical treatment to investigate how pyrolysis atmosphere regulates the generation and retention of PAHs. Biochar samples were produced across 300–900 °C under CO 2 or N 2 atmospheres, yielding a set of cocoa pod husk biochar (CPHBC) with distinct chemical characteristics. High-ring PAHs, particularly dibenzo[ a , h ]anthracene (DA) and indeno[1,2,3-cd]pyrene (IP), dominated the aromatic profiles. The CO 2 -pyrolyzed material at 700 °C exhibited the greatest PAH burden (2478 ± 228 ng g⁻ 1 ), whereas high-temperature treatment under N 2 (900 °C) resulted in substantial suppression of PAH accumulation (104 ± 22 ng g⁻ 1 ). Ecotoxicological testing with zebrafish ( Danio rerio ) embryos from 0 to 4 dpf revealed no detectable alterations in assessed developmental endpoints, locomotor behavior, or oxidative stress biomarkers under short-term laboratory exposure, indicating that the CPHBC did not induce observable acute toxicity within the tested conditions. Collectively, the results demonstrate that judicious control of pyrolysis atmosphere can effectively limit PAH formation in CPH-derived biochar, reducing potential ecological and food-chain hazards while supporting the sustainable reuse of biomass within a low-carbon circular bioeconomy. • Cocoa pod husk was converted into biochar under controlled CO 2 and N 2 pyrolysis atmospheres. • Pyrolysis atmosphere and temperature jointly governed PAH formation and molecular distributions in biochar. • N 2 -assisted pyrolysis markedly reduced total PAH contents and BaP-equivalent toxicity compared with CO 2 conditions. • Zebrafish ( Danio rerio ) embryo assays revealed no acute developmental, behavioral, or oxidative stress effects. • Atmospheric control emerged as a critical parameter for enhancing the environmental safety of biochar.