Solidification/stabilization of heavy metals in tannery sludge char with various binders

The leaching potential of heavy metals (Cr, Pb, Ni, Zn, and Cu) from the tannery sludge charred at above 350°C under oxygen-depleted conditions was evaluated for re-use as landfill cover materials. Solidification/stabilization (S/S) was employed for the immobilization of heavy metals in the tannery sludge chars (TSCs) to further reduce the leaching potential. The effects of charring temperatures, different types of binders (FeSO₄, lime, cement, HAP, and ladle slag), and aging on the immobilization of heavy metals (Pb, Ni, Zn, Cu, and Cr) in the TSCs were investigated. The immobilization efficiencies were estimated using the toxicity characterization leaching procedure (TCLP). The results show that the extractable concentrations of heavy metals decreased with the charring temperature. The single binder (FeSO₄ alone) was highly effective for Cr immobilization, but not for other heavy metals such as Ni and Zn. The binary binders, FeSO₄ combined with lime, cement, or ladle slag, were highly effective in the simultaneous immobilization of Cr, Ni, and Zn. Among the binary binders, the FeSO₄ + lime was the best combination in the heavy metal immobilization. TCLP-extractable concentrations decreased after aging for 30 days. The metal immobilization efficiency increased with CaO and SiO₂ contents in the binders (lime > cement > ladle slag) and with aging.