Heavy-metal speciation redistribution in solid phase and potential environmental risk assessment during the conversion of MSW incineration fly ash into molten slag

The alkalinity (AK_ash), BCR sequential extraction method, and principle component analysis (PCA) were adopted to investigate the heavy-metal partitioning and their speciation redistribution in solid phase in ash-melting process. The results indicated that the conversion of Zn-OXI (oxidisable fraction) into Zn-RES (residual fraction) and the decomposition of Cu-OXI fraction were prevailing in solid-phase reaction. Moreover, the alkalinity reduction from AK_ash = 2.0 to AK_ash = 1.2 had positive implications for the concentration reduction of As-RED (reducible fraction), Zn-RED, and Pb-RES fractions in slags. The modified synthesis toxicity index (STI_M) calculation model was introduced to investigate the potential ecological risk (PEI) of heavy metals in the recycling and utilization of molten slag. Based on STI_M calculation model, PEI of heavy metal in hazardous materials was classified into five ranks, such as serious pollution (STI_M > 462), heavy pollution (330 < STI_M < 462), moderate pollution (132 < STI_M < 330), mild pollution (0 < STI_M < 132), and no pollution (STI_M = 0). The molten slags produced from fly ash can be directly reused as building materials like freestone and ceramics due to the mild PEI.