Behavior Study of Migration and Transformation of Heavy Metals during Oily Sludge Pyrolysis

This paper studied the migration, transformation, leaching toxicity, and risk assessment of heavy metals when oily sludge was pyrolyzed at multiple pyrolysis temperatures and residence times. Results indicated that heavy metals were enriched in char, while less than 3% of heavy metals were distributed to gas and liquid products. By the modified European Community Bureau of Reference sequential extraction method, it was found that the most immobilized Cr, Ni, and Pb were obtained under conditions of 600 °C for 30 min, 400 °C for 60 min, and 500 °C for 60 min, respectively. However, pyrolysis reduced the percentage of stable fractions in Cu and Zn. Similarly, the leaching concentrations of Cr, Ni, and Pb decreased through pyrolysis, while that of Zn increased. Risk assessment code results suggested that pyrolysis reduced the environmental risk of Pb but increased that of Zn. Meanwhile, the mechanism of heavy metal migration and transformation was explored by characterizing with proximate analysis, ultimate analysis, X-ray fluorescence, Fourier transform infrared spectroscopy, and X-ray diffraction. The results indicated that aromatization and mineral crystals in chars influenced the immobilization of heavy metals.