Heavy metal speciation in bottom slags from the co-combustion of municipal sludge and organic medical solid waste

Heavy metal toxicity in bottom slags constitutes a major challenge limiting their reutilization. This study collected bottom slag from co-combustion experiments of municipal sludge (MS) and organic medical solid waste (OMSW) at various ratios in a tubular furnace, followed by analysis of Cr, Ni, Cu, and Zn concentrations. The European Community Bureau of Reference (BCR) sequential extraction method was used to analyze the speciation characteristics of the heavy metals, and the chemical speciation of Cr, Ni, and Cu in the bottom slag at 700 °C and 850 °C was dominated by the residual state, which accounted for 57.20–77.25 % of the heavy metals. When the reaction temperature reached 1000 °C, the proportion of the reducible state of Cr was most obviously elevated, accounting for 35.86–59.07 %, and the proportion of the acid soluble state of Zn was the highest, accounting for more than 46.65 %. Thermodynamic analysis via HSC Chemistry 6.0 demonstrated that increasing the temperature increased the reactions of the four heavy metals with iron to form the corresponding ferrate salts, explaining the increased proportion of the reducible state at 1000 °C. The Hakanson potential ecological hazard index method was used to evaluate the ecological risk of heavy metals. The highest value of the individual pollution factor of Zn reached a high risk, while the pollution levels of Cr, Ni, and Cu were all under elevated risk. The comprehensive potential ecological risk value of the heavy metals in the bottom slag at 1000 °C was the highest, and the pollution levels of the four heavy metals were Cu, Ni, Cr, and Zn in descending order.