Hydrothermal Co-Treatment of Municipal Solid Waste Incineration Fly Ash with Sewage Sludge Ash: Phosphorus-Induced Mineral Phase Transformation for Stabilizing Heavy Metals

Municipal solid waste incineration fly ash (MSWI-FA), typically classified as hazardous waste, contains highly toxic heavy metals. Hydroxyapatite (HAP) can effectively stabilize heavy metals in MSWI-FA under hydrothermal conditions, although it requires an external phosphorus (P) source for its formation. Sewage sludge ash (SSA) is a P-enriched solid waste that requires urgent treatment. Therefore, this study investigated whether heavy metal stabilization in MSWI-FA can be effectively achieved via hydrothermal cotreatment (HCT) with SSA. The results showed that mineral phase in MSWI-FA was effectively transformed during hydrothermal treatment with the addition of SSA. HAP became predominant mineral (∼24.83%) in MSWI-FA after hydrothermal treatment with 30.0 wt % SSA under 1.2 mol/L NaOH at 400 °C for 20 min. Heavy metal leaching concentrations were significantly reduced compared with raw MSWI-FA, and their hydrothermal migration rates were below 5.50%. Partial least-squares regression analysis showed that heavy metals were transformed into residual fractions by promoting HAP formation. Specifically, heavy metals adsorbed or complexed onto the HAP surface progressively occupied or substituted Ca 2+ sites within HAP crystals, ultimately encapsulating them within the HAP structure, as confirmed by BET, SEM-EDS, and XPS analyses. This study offers a practical approach for the cotreatment of MSWI-FA with SSA.