Comparison of long-term stability under natural ageing between cement solidified and chelator-stabilised MSWI fly ash.

Cement-solidification and chelator-stabilisation of municipal solid waste incineration fly ash (MSWI-FA) are two main treatment techniques to immobilise heavy metals. Differences in the long-term stabilities of those two methods of heavy-metal immobilisation were explored to aid in determining the better MSWI-FA treatment. However, few comparative studies have been conducted on 6-year-old cement-solidified FA (Ce-6-FA) and chelator-stabilised FA (Ch-6-FA). In this study, we compared the physicochemical and heavy metal leaching characteristics of Ce-6-FA and Ch-6-FA. The chemical speciation of heavy metals was modelled using geochemical software to assess long-term stability. The results showed weaker long-term stability in Pb immobilisation under the chelating system. The leaching concentrations of target heavy metals, acetic acid leaching tests, acid neutralising capacity, and pH-dependent leaching results indicated that Ce-6-FA had higher long-term stability than Ch-6-FA. A column experiment indicated that the cumulative release rates of Pb in Ce-6-FA and Ch-6-FA were 2.49% and 4.72%, respectively. The phase-controlled leaching of Pb in Ce-6-FA mainly occurred through Pb 2 (OH) 3 Cl and chloropyromorphite (Pb 5 (PO 4) 3 Cl), whereas that in Ch-6-FA mainly occurred through Pb 5 (PO 4) 3 Cl. The decomposition of heavy metal chelates in Ch-6-FA and salt generation in this process led to the release of Pb via the inorganic complex. Image 1 • We compared heavy metal leaching characteristics of 6 years aged cement-solidified FA and chelator-stabilised FA. • The leaching concentration of Pb and Cd in Ch-6-FA exceeds the MSW landfill thresholds. • We compared the difference of chemical speciation of heavy metal between Ce-6-FA and Ch-6-FA. • Poorer long-term stability in Pb immobilisation under the chelating system. [ABSTRACT FROM AUTHOR]