Reconciling competing objectives in carbonated MSWI fly ash washing: Dechlorination, calcium retention, and heavy...

The resource utilization of MSW incineration fly ash is constrained by its high chloride content, calcium loss during washing, and the environmental risk of residual heavy metals. This study proposes a multistage pretreatment integrating carbonation, ball milling, ultrasonication, and CO₂-coupled washing to balance dechlorination, calcium retention, and heavy metal stabilization. Citric acid (CA), ammonium bisulfate (AB), and ethylenediaminetetraacetic acid (EDTA) were selected as washing agents and evaluated on carbonated fly ash. For the investigated fly ash, results demonstrated that carbonation provided the foundational calcium fixation, while ball milling physically disrupted particle agglomerates to expose encapsulated chlorides, with 60 min selected as the milling duration. Ultrasonication for 10 min further reduced soluble chloride contents in heavily carbonated fly ash residues to 0.72%, 0.79%, and 0.81%, respectively, with calcium loss rates of 14.30%, 10.81%, and 12.08%. However, EDTA washing caused severe remobilization of Pb, Zn, Cu, and Cd. Further coupling ball milling and ultrasonication with in-situ CO₂ sparging (20 min for CA and 10 min for AB) improved the overall treatment performance. The lowest soluble chloride content (0.71%) was obtained with CA, whereas AB achieved the lowest calcium loss (8.87%). All heavy metal leaching concentrations in CA and AB residues met the regulatory limits. Risk assessment code analysis indicated that AB generally resulted in lower environmental risks than CA, although Cd remained the primary residual concern. Overall, the coupled treatment alleviated the trade-offs among chloride removal, calcium retention, and heavy metal stabilization, thereby improving the resource recovery potential of treated fly ash.