Effects of pyrolysis temperature on biochar’s characteristics and speciation and environmental risks of heavy metals in sewage sludge biochars

Sewage sludge pyrolysis into biochar is an effective way to realize sludge harmless and resourceful utilization. The study aimed to investigate the relationship between pyrolysis temperatures (300 °C, 500 °C and 700 °C) and biochar properties and environmental risks associated with heavy metals (Cu, Zn, Pb, Cd, Ni and Cr) and to discuss possible mechanisms of heavy metal immobilization. The results showed that the biochar pH value increased from 5.87 to 10.50 and the specific area grew from 5.26 m2/g to a maximum of 15.23 m2/g with pyrolysis temperature rose. Pyrolysis enhanced aromatization and increased stability of biochar, and part of oxygen-containing functional groups decomposed. Although heavy metals enriched in pyrolysis, high temperatures promoted the bioavailable fractions converted to more stable ones, and the heavy metals leaching rate was reduced by 5.5% on average. This may be related to the functional group decomposition, pore collapse and wrapping, and the new crystalline phases formation. The environmental risk was significantly reduced (RI value below 51.00), indicating the relatively safe and reliable performance of sludge biochar. The study can provide a theoretical basis for sludge management and the biochar safe use in soil.