Insights into the speciation of heavy metals during pyrolysis of industrial sludge.

The purpose of this study was to determine the mechanisms in heavy metals immobilisation during pyrolysis of industrial sludge (IS). Pyrolysis was conducted in a fixed-bed unit at the temperature range between 400 and 800°C. Conventional and advanced analytical techniques were employed to assess the influencing parameters on the transformation of heavy metals during pyrolysis process. The examined heavy metals (i.e. Mn, Ni, Cu, Zn, Cd, and Pb) were effectively immobilised in the char derived at ≤600°C. In contrast, for the char derived at ≥700°C, Mn, Cu, and Zn leaching efficiencies were significantly increased. According to BCR sequential extraction procedure (SEP) results, pyrolysis of IS conducted at ≤600°C transformed heavy metals into stable fractions (i.e. associated with Fe-Mn nodules, and organics and sulphides), while Mn, Cu, and Zn bound in these fractions decomposed into easily soluble forms at higher temperatures. Advanced analytical characterisation (XRD, EDX, XPS, and FTIR) of the derived char and anion concentrations in the leachate have demonstrated that the increase in metal leaching was probably due to the formation of metal halides.