Interaction mechanism between cadmium species and SiO2 of municipal solid waste incineration fly ash: Effect of HCl

Abstract

Cadmium (Cd) pollutants from the incineration of municipal solid waste (MSW) has drawn considerable attention due to their high volatility and biological toxicity. Current experimental results have revealed the strong interaction between SiO2 and Cd species, which provides an insight for the removal of Cd species, but the detailed mechanism is still unclear. Thus, with the density functional theory (DFT) calculations, the interaction mechanisms of Cd species over the SiO2(0 0 1) surface were investigated, considering the effect of HCl in MSW incineration flue gas. It is found that the adsorption manners of Cd0 and CdO are chemisorption over the SiO2 surface and the active sites are O sites, while the interaction between CdCl2 and the surface is dominated by weak chemisorption. HCl plays a dual role in the interaction of Cd0 and CdO with the surface. The introduction of HCl can promote the fixation of CdO and Cd0, but also promote their chlorination into CdCl2 whose adsorption on SiO2 is too weak to be captured. The chlorination of Cd0 can be achieved by two-step and one-step routes, both following the Eley-Rideal mechanism. Whereas, CdO can only be chloridized through a two-step pathway, in which the first step follows the Eley-Rideal mechanism and the second step follows the Langmuir-Hinshelwood mechanism. Compared with Cd0, the chlorination of CdO has a higher reaction energy barrier and is more difficult to proceed. Therefore, CdO is the most effortless cadmium pollutant for SiO2 to control.