Effects of pyrolysis conditions on sewage sludge-biochar properties and potential risks based on PAH contents

Pyrolysis of sewage sludge can significantly reduce industrial waste while producing high-value biochar for soil improvement. This study aimed to evaluate the quality and safety of biochar from sewage sludge under different pyrolysis conditions. Optimal carbonization conditions (700 °C, 60 min, 5 °C/min) were identified by analyzing the physicochemical properties, elemental composition, structural characteristics, and the specific surface area of biochar. Results show that pyrolysis of waste sludge reduces the total content of priority polycyclic aromatic hydrocarbons (PAHs) by 48%, from 6367 ng/g to 3317 ng/g, mainly due to a reduction in low-molecular-weight compounds. The composition of polyarenes in biochars is represented primarily by low-molecular compounds, among which naphthalene and phenanthrene predominate. At the same time, among high-molecular compounds, fluoranthene, pyrene, and chrysene stand out, significantly dominating the overall picture. According to the Incremental Lifetime Cancer Risk model, the carcinogenic risks associated with biochar usage are primarily driven by hazardous compounds such as chrysene, benzo(a)pyrene, and dibenz(a,h)anthracene, evaluated through toxic equivalent concentrations. It was found that with oral or dermal exposure to these pollutants, the likelihood of cancer in children is 1.1-1.4 times higher than in adults. At the same time, with inhalation, this threat increases by 1.5 times for adults compared to children. However, with increased pyrolysis temperature, heating rate, and holding time of sewage sludge, the carcinogenic risks of biochar decrease. Biochar produced under optimal conditions contains PAH levels below toxic threshold standards set by the International Biochar Initiative. The safe application rate for biochar in Haplic Chernozem soils at 0-20 cm depth is up to 26 t/ha.