Removal mechanisms of polycyclic aromatic hydrocarbons in biochar and its effects on plant growth

Polycyclic aromatic hydrocarbons (PAHs) could be produced during the preparation of biochar, which may pose potential risks to the environment and human health. Existing research mainly focuses on the removal efficiency of PAHs in biochar using different methods. Still, there are few reports on the removal mechanism of PAHs and the impact of treated biochar on plant growth. This study prepared biochar by pyrolysis of sewage sludge, pig manure, and distillers grains at 300, 500, and 600 °C, respectively. The prepared biochar was subjected to thermal treatment, water washing treatment, and hydrothermal treatment to investigate the removal mechanism of PAHs in biochar and evaluate its effect on plant growth. The results showed that the removal rates of PAHs in biochar by hydrothermal treatment, thermal treatment, and water washing were 36.79 %-86.09 %, 80.00 %-89.90 %, and 19.15 %-72.40 %, respectively. Compared with thermal treatment and water washing treatments, the hydrothermal treatment is more effective. The removal rate of PAHs in sludge biochar with a pyrolysis temperature of 300 °C by hydrothermal treatment reached 86.09 % under 80 °C for 2 h. The removal mechanisms of PAHs include weakening hydrophobic interaction and thermal desorption. The germination results indicate that treated biochar reduces malondialdehyde content in mung bean seedlings and increases superoxide dismutase, peroxidase, and catalase activities. Sludge biochar after hydrothermal treatment is more conducive to the growth of mung beans than pristine biochar. This study provides an effective pathway for the safe utilization of biochar.