Thermal characteristics and fate of heavy metals during thermal treatment of Sedum plumbizincicola, a zinc and cadmium hyperaccumulator

• Most of the original mass of S. plumbizincicola was reduced during thermal processing.
• Most of the heavy metals (> 99%) were enriched in fly ash and/or bottom ash fractions.
• During the combustion process heavy metals were found as pure metals and their oxides.
• During the reduction process oxides, sulphides or complex compounds were found.

A horizontal tube furnace was designed to test the disposal of ground hyperaccumulator biomass. X-ray diffraction (XRD), thermodynamic analysis and chemical extraction were used to characterise the heavy metals. The recoveries of Zn, Pb, and Cd in bottom ash decreased with increasing incineration temperature but increased in fly ash. The recoveries of Zn, Pb, and Cd fluctuated with increasing air flow rate and most of the metals were found in the fly ash. Cd, Pb, and Zn occurred during the combustion process in the form of elementary substances and their oxides. During reduction in the thermal process both Pb and Cd formed metal complexes such as Ca0.75Cd0.25O and Pb2O3.333 or their mixtures and more of the bottom ash Zn under the reducing conditions was present as pure metal, crystalline oxides, sulphides or complex compounds combined with other metals.