Migration and transformation of lead species over CaO surface in municipal solid waste incineration fly Ash: A DFT...

Heavy metal pollutants generated from municipal solid waste (MSW) incineration are mainly concentrated in the fly ash, among which lead species have received considerable attention due to their high content and biotoxicity. CaO is an active component in fly ash to adsorb heavy metal species. In this study, based on density functional theory (DFT) calculations, the migration and transformation mechanisms of lead species over the CaO (100) surface were investigated by calculating the adsorption configurations, energies, and electronic structures, etc. The results indicate that the adsorption of lead species over the CaO (100) surface is dominated by chemisorption, and PbCl₂ molecule exhibits a stronger affinity to the CaO surface than Pb⁰. The dissociation of HCl molecule on the CaO (100) surface facilitates the adsorption and chemical reactivity of lead species. The chlorination of Pb⁰ to PbCl₂ is a two-stage route. In the first stage, two HCl molecules are exothermically adsorbed on the surface without an energy barrier, and Pb⁰ is directly bonded to the active Cl atom, which is controlled by the Eley-Rideal mechanism. In the second stage, PbCl intermediate bonds with another Cl atom over the surface to form the PbCl₂ molecule, following the Langmuir-Hinshelwood mechanism, which is also the rate-determining step. Compared with the homogeneous chlorination, CaO catalyzes the heterogeneous process to greatly reduce the oxidation energy barrier and promotes the formation of PbCl₂. Consequently, CaO is able to accelerate the lead enrichment in fly ash, which is favorable for lead species purification.