Biochar is a carbon‑rich material widely recognized for improving the physicochemical properties of soils that can be produced via pyrolysis of agri‑food residues [1]. However, during biomass pyrolysis, polycyclic aromatic hydrocarbons (PAHs) are commonly formed, mainly due to incomplete combustion of organic material or through transformation reactions [2]. PAHs are toxic environmental pollutants with carcinogenic, teratogenic, and mutagenic properties. This study aimed to assess the occurrence of 16 priority PAHs listed by the US EPA in biochar derived from basil (Ocimum basilicum L.) waste, evaluating its suitability as a soil conditioner and its hidden risks to food safety and the environment [3]. Slow pyrolysis was conducted at 500 °C for 14 hours under oxygen-limited conditions. A Central Composite Design (CCD) experiment was performed to investigate four factors—solvent type, solvent‑to‑biochar ratio, extraction time, and temperature—and identify the most influential variables on PAHs extraction. Extractions employed greener solvents (acetone and acetonitrile) assisted by ultrasound. Optimal conditions predicted by the fitted model were validated through two replicate trials. Finally, a comparative Soxhlet extraction was conducted. The resulting extracts were analyzed by HPLC with fluorescence detection to quantify the target 16 PAHs.
Basil biochar showed concentrations of PAHs such as phenanthrene, fluoranthene, and pyrene below the standard regulatory limits, according to the criteria defined by the European Biochar Certificate (Σ16 PAHs ≤ 6.0 ± 2.4 mg/kg) [4]. Strict quality control of biochar is essential, especially in agricultural food applications, due to the toxicity of these contaminants.

References
[1] Z. Ashkanani, et al. (2024), J. Hazard. Mater., 468, 133813.
[2] G. Greco, et al. (2021). J. Anal. Appl. Pyrolysis, 159, 105337.
[3] E.V. Smirnova, K.G. Giniyatullin, & R.V. Okunev (2025). Eurasian Soil Sci., 58(3), 40.
[4] EBC and WBC guidelines & documents. Accessed: June 20

Maria Eduarda Marques*, Tanara Motta, Cristina Soares, Antón Puga, Ana Rita Alves, Henri Nouws, Cristina Delerue-Matos ; REQUIMTE/LAQV, Instituto Superior de Engenharia do Porto, Instituto Politécnico do Porto, Porto, Portugal