Effects of different pyrolysis conditions on chemical speciation transformation and immobilization of heavy metals in sludge

With the acceleration of global industrialization, the environmental risks posed by heavy metals in sludge have become increasingly prominent. Traditional treatment methods struggle to effectively reduce the bioavailability of these metals, whereas pyrolysis facilitates the fixation and transformation of heavy metals via high-temperature conversion. This study investigated the migration and transformation of heavy metals during sludge pyrolysis, focusing on the influence of pyrolysis temperature and heating rate on the distribution and morphology of these metals. The results revealed that both the pyrolysis temperature and heating rate significantly affected the fixation efficiency, migration behavior, and chemical form of the heavy metals. Pyrolysis temperature and heating rate significantly affect heavy metal behavior. Optimal stabilization is achieved at 700 and 10 °C min−1, which drastically reduces the potential ecological risk index (RI) from 156.97 to 9.83 and 10.80. These conditions facilitate metal migration into gas/liquid phases and convert active fractions (F1/F2) into the stable residual fraction (F4). The crystal structure and functional group changes of the pyrolysis products were further analyzed by X-ray diffraction and infrared spectroscopy. The results show that the higher pyrolysis temperature will weaken the characteristic peak intensity of SiO2. The faster heating rate will enhance the crystallization of graphite carbon and accelerate the dehydration/alkane decomposition reaction, which can be confirmed by the weakening of -OH and -CH stretching vibration intensity. This study provides a theoretical foundation for optimizing the sludge pyrolysis process and contributes to the realization of resource utilization in sludge treatment.