Effect of blast furnace slag on heavy metal speciation in municipal sludge pyrolysis: experimental and theoretical study

Abstract

The presence of unstable heavy metals in sludge restricts its potential for resource utilization, making it crucial to minimize the secondary risk associated with heavy metals during sludge pyrolysis. This study explored the influence of the addition of blast furnace slag (slag), pyrolysis temperature, and residence time on the morphological distributions of Zn, Pb, and Cr in biochar co-pyrolyzed with slag. The slag dosage was consistent at 7.5% for all samples to immobilize different heavy metals. High-temperature pyrolysis (600–700 °C) promoted the immobilization of Cr and Pb, while lower pyrolysis temperatures (400 °C) favored the stabilization of Zn. The appropriate residence time was determined to be 15 min for Zn and Cr, and 60 min for Pb, across all samples. Brunauer–Emmett–Teller (BET) characterization revealed that all the samples exhibited a mesoporous structure dominated by a type IV isothermal profile. Additionally, the biochar demonstrated the largest specific surface area (69.48 m²/g) after pyrolysis at 500 °C. XRD (X-ray diffractometer) characterization indicated that the addition of slag facilitated the formation of crystals composed of Ca–Si-based substances bound to heavy metals. FTIR spectroscopy tests revealed that the addition of slag resulted in a reduction in the intensity of the O–H telescoping vibration peaks, which in turn promoted the formation of aromatic and epoxide structures and enhanced heavy metal stabilization. Density functional theory (DFT) calculations revealed that slag can form covalent bonds with Zn, reducing the volatilization risk of Zn species during pyrolysis. The experimental and simulation findings presented in this paper provide new insights into the management of heavy metals in the sludge pyrolysis process.

Graphical abstract