Solidification/stabilization treatment of selenium in municipal solid waste incineration fly ash and its mechanisms

Municipal solid waste incineration (MSWI) fly ash is classified as hazardous waste because of its toxic constituents and requires solidification/stabilization (S/S) prior to safe landfill disposal. Selenium (Se), a particularly harmful element, presents significant environmental risks; however, research on its effective treatment remains limited, and no widely accepted remediation technology is currently available. This study evaluated the effectiveness of various S/S agents in stabilizing Se in fly ash, with a particular focus on iron micro-powder (IMP). The results indicate that IMP demonstrates superior solidification efficiency compared to conventional heavy metal chelating agents such as sodium sulfide and sodium dimethyldithiocarbamate, which exhibit negligible effectiveness. Under acidic conditions, the IMP achieved a Se stabilization rate of up to 97.6%, whereas its efficacy decreased in alkaline environments. To clarify the stabilization mechanism, the interactions between IMP and Se were examined in simulated leachates at different pH levels. SEM, EDS, and XPS analyses revealed that the primary S/S mechanism of IMP involves adsorption and reduction reactions. Specifically, the tetravalent selenium ions in solution were reduced to zero-valent selenium, which was subsequently deposited onto IMP surface, thereby achieving stabilization. Furthermore, the reaction process is significantly influenced by the environmental pH.