Simultaneous realization of heavy metal Cd solidification and chloride separation in municipal solid waste incineration fly ash: Mechanism and DFT analysis

The solidification of heavy metals and the separation-recycling of chloride salts are effective approaches for the safe disposal of municipal solid waste incineration (MSWI) fly ash. This study achieved the solidification of the excessive heavy metal Cd by transforming MSWI fly ash into a solidified body mainly composed of ettringite, while also purifying the calcium and sulfate ions in the chloride solution. The research results demonstrate that the introduction of aluminum ions rapidly decreases the pH value of the MSWI fly ash reaction system. By adjusting the reaction system to a pH value of 11.50, the conversion rate of calcium ions reaches 99.68%, the separation rate of chloride reaches 95.99%, and the solidification rate of heavy metal Cd reaches 98.92%. Density functional theory (DFT) calculations indicate that the vacancy formation at the Ca-2 position of ettringite and Cd entering the vacancy has a higher probability. The combination of DFT calculations and experiments validates that heavy metal Cd leads to charge redistribution, increased interplanar spacing, and decreased thermal stability of ettringite. The concentration of calcium ions in the solution decreases to 22.64 mg/L, achieving efficient recovery of sodium chloride and potassium chloride in a shorter process. The ettringite-based solidified body contains 0.87% chloride ions, showing potential for resource utilization in cement-based materials.