Energy recoveries and heavy metal migration behaviors of different oily sludges treated by pyrolysis versus solvent extraction

The pyrolysis and trace element mitigation characteristics are investigated by contrast to solvent extraction for four oily sludges, including storage tank bottom sediment (OS-1), scum from a wastewater separator (OS-2), white-clay-adsorbed waste oil (OS-3), and settlings from wastewater treatment (OS-4). Slow pyrolysis at 700 °C generated a single oil phase for OS-1 and separate oil and aqueous phases for OS-2, OS-3 and OS-4. Up to 73.0–88.3 % of the total energy were recovered from OS-1, OS-2 and OS-3 in the oil phase with 19.9–77.1 % oil yield; however, the oil phase from OS-4 accounted for only 13.3 % of the total energy, while the aqueous product accounted for 68.0 % of the total energy. Quantification of 16 trace elements revealed that OS-2 and OS-4 had much higher contents of Cu/Zn/As/Se/Cd/Pb and Ni/Cu/Zn/Se/Cd contents than the average crustal abundances, respectively. Correlations between evaporation and extraction rates indicated that the mitigation behaviors of trace elements were related to their occurrence modes in different oily sludges. Except for Cd, As and Se, all other trace elements were enriched in the pyrolysis residues of the oily sludges. Ni in the pyrolysis residue of OS-4 posed a moderate potential ecological risk.