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 (AKash ), 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 AKash  = 2.0 to AKash  = 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 (STIM ) calculation model was introduced to investigate the potential ecological risk (PEI) of heavy metals in the recycling and utilization of molten slag. Based on STIM calculation model, PEI of heavy metal in hazardous materials was classified into five ranks, such as serious pollution (STIM  > 462), heavy pollution (330 < STIM  < 462), moderate pollution (132 < STIM  < 330), mild pollution (0 < STIM  < 132), and no pollution (STIM  = 0). The molten slags produced from fly ash can be directly reused as building materials like freestone and ceramics due to the mild PEI.