The immobilization mechanisms of Pb in borates during low-temperature vitrification process of municipal solid waste incineration (MSWI) fly ash

B₂O₃ was used as a fluxing agent can reduce the melting temperature of municipal solid waste incineration (MSWI) fly ash which related to the formation of the minerals Ca₃(BO₃)₂, Ca₂B₂O₅ and CaB₂O₄ during vitrification process. However, the relationship between immobilization of heavy metals and borates transformation during melting process remains unclear. In this study, the immobilization of heavy metal Pb inside Ca₃(BO₃)₂, Ca₂B₂O₅ and CaB₂O₄ based on density functional theory (DFT) were comparatively studied in this paper. The results showed that the defect formation energies for substitutional doping model are higher than interstitial model which is the more favorable the immobilization model for Pb. Pb cause a volume expansion in all borates. In the substitution doping model, Pb can replace Ca to balance the electronegativity of O atom and form chemical bonds with the surrounding O atoms, and the covalency of Pb–O is stronger than that of Ca–O. In addition, the order of covalency between Pb and surrounding O atoms is as follows: Ca₃(BO₃)₂ > Ca₂B₂O₅ > CaB₂O₄. In the interstitial doping model, bonds of O–Pb and B-Pb exhibited negative population values, revealing that filling of electrons into the antibonding states was the major orbital contribution. In addition, B 2s and 2p orbital and Pb 6p orbital form hybridization near the Fermi level in the CaB₂O₄ (I) doping model.