Migration pathway and solidification mechanism of heavy metal Pb during the conversion of municipal solid waste incineration fly ash into ettringite and simultaneously purification of chloride salts solution process

Abstract

The solidification/stabilization of heavy metals and valuable component recovery from municipal solid waste incineration (MSWI) fly ash are of great significance for its safe disposal. In this study, MSWI fly ash was transformed into a new solid phase mainly composed of ettringite, achieving the solidification of excessive heavy metal Pb while obtaining a mixed solution of sodium chloride and potassium chloride with extremely low impurity content, which can be recovered by evaporation-crystallization respectively. The solidification mechanism of heavy metal Pb by ettringite was investigated through a combination of DFT calculations and experiments. The results indicate that a high conversion rate of calcium ions (99.68%), separation rate of chloride (95.99%), and conversion rate of heavy metal Pb (99.42%) can be achieved by controlling the ions ratio of the MSWI fly ash reaction system to n(Ca²⁺):n(Al³⁺):n(SO₄^2-) = 6:2:3. DFT calculations show that the reaction pathway of the formation of a vacancy-Pb entering the vacancy at the Ca-2 site of ettringite is more likely to occur. The substitution of heavy metal Pb at the Ca-2 site leads to an increase in the unit cell volume, redistribution of charges, and a decrease in the thermal stability of the ettringite. The solidified body of ettringite presents a promising potential for application in cement-based materials due to its negligible risk of heavy metals leaching and low chloride content.

Keywords: Density functional theory; Ettringite; Heavy metal Pb; MSWI fly ash; Solidification/stabilization.