<p>Since municipal solid waste incineration fly ash (MSWI FA) contains a large amount of potentially toxic heavy metals that need to be safely treated before landfill, solidification/stabilization chemical stabilization method is one of the most commonly used methods for MSWI FA treatment. To investigate the effects of different inorganic and organic chelating agents on the solidification of heavy metals (such as Pb, Cd, etc.), immobilization, here we selected three inorganic chelating agents (Na₂S, NaH₂PO₄ and Na₃PO₄) and four organic chelating agents (thiourea, diethyl-mercaptan, DDTC and EDTA Na) for a comprehensive comparison of the heavy metal stabilization and immobilization performance. The leaching and BCR results indicate that under the same low addition conditions, diethyl-mercaptan containing double bond groups exhibits a better chelating effect on most heavy metals (such as Pb, Cd, Cr, Cu and etc.), and the addition of Na₂S can better increase the residual state ratio of Cd. Moreover, we further selected inorganic Na₂S and organic diethyl mercaptan for compounding and it was found that the content of cadmium and lead in the leaching product after the composite chelating agent NS-2 solidified the heavy metals was low, and the lead content was far below the safety threshold of the landfill standard of China (GB16889-2008), which can significantly save the cost of chemicals. This work can provide a reference for the selection and design of effective chelators for stabilizing toxic heavy metals.</p> Graphical Abstract <p></p>