Insights into closed-wet ultrasonic carbonation of MSWI fly ash: Carbon sequestration, heavy metals stabilization, and PCDD/Fs migration and degradation

This study proposed a novel closed-wet accelerated carbonation method based on ultrasonic chemistry for treating municipal solid waste incineration fly ash (MSWI FA), assessing various reaction parameters (time, temperature, CO₂ pressure, ultrasonic power, and alkaline additives) on the detoxification and synergistic carbon fixation. The mechanisms of carbon sequestration, heavy metal migration-stabilization, and PCDD/Fs migration-degradation were elucidated. Key findings include the reaction kinetic of MSWI FA ultrasonic carbonation aligning with the surface coverage model (R² > 0.999), achieving a carbonation efficiency of 29.4% within 30 min. Optimal conditions-room temperature, 1-2 MPa CO₂ pressure, and 500 W ultrasonic power-enhanced carbon sequestration and heavy metal stabilization. The formation of fine calcium carbonate resulted in the co-precipitation of heavy metals, such as Ba, Pb, and Zn, avoiding liquid phase heavy metal pollution and reducing their leaching concentrations in solid samples well below the HJ 1134-2020 standard. The particle size reduction rate was 69.9% (18.6 μm) for non-ultrasonic carbonated FA compared to 89.2% (6.7 μm) for ultrasonic carbonated FA (UFA). NaOH and ammonia addition effectively decreased calcium concentration in the carbonation reaction solution (<200 mg/L) and calcium sulfate content in UFA. The ultrasonic carbonation process prevented PCDD/Fs from migrating into the reaction solution and achieved 35.0% PCDD/Fs degradation. This comprehensive evaluation supports the viability of ultrasonic carbonation for treating MSWI FA.