Enhanced photocatalytic degradation of polycyclic aromatic hydrocarbons (PAHs) in biochar using nano-TiO2 under ultraviolet light irradiation: Effect and mechanisms

Numerous studies have focused on the removal of polycyclic aromatic hydrocarbons (PAHs) in polluted soil using biochar (BC) as amendments, little attention was paid to the degradation of PAHs associated with BC. In this study, photocatalytic degradation experiments were conducted using different BC to nano-TiO2 catalyst ratios. With the optimal BC to nano-TiO2 ratio at 5:1, the degradation rates of total PAHs and high-molecular-weight PAHs in BC were 89.30-94.51 % and 94.28-95.14 %, respectively. The PAHs degradation rate reached 93.38 % at 60 min, meanwhile, the final PAHs concentration decreased to 6.32 mg center dot kg- 1 , which was below the maximum limit value of 16 PAHs contents permitted by the European Biochar Certificate. Therefore, the photocatalytic time of 60 min was suggested comprehensively considering the efficiency and effect in PAHs removal. The degradation mechanism of PAHs with a self-catalytic system comprised of nano-TiO2 and BC was investigated by degradation products analysis and radical capture experiment and the degradation pathways were proposed. center dot OH and center dot O2- were the main active substances participating in PAHs degradation reaction. Nano-TiO2 produced e--h+ pairs under the UV light and transferred electrons to O2 and OH-, generating center dot O2- and center dot OH. Finally, the ecological risk assessment results demonstrated that the TEQ value and phytotoxicity significantly decreased, and the germination index of BC after photolysis under the suggested parameters reached 86.78 %, indicating no toxicity toward plants. This study provided a photocatalytic degradation method with costeffectiveness, high efficiency and environmental friendliness for PAHs degradation in the field of BC production and application.