Influence mechanism of chlorine on arsenic release and transformation during municipal solid waste incineration

Arsenic release presents a critical environmental pollution issue during municipal solid waste (MSW) incineration. The increase of chlorine-containing waste for incineration processes has heightened interest in the volatilization of heavy metals induced by chlorine. However, the migration and transformation of arsenic, which has a strong affinity for oxygen at high temperatures, are less understood in the context of chlorine influence, leading to the absence of corresponding regulatory strategies. Therefore, this study first investigated the mass distribution of arsenic during co-incineration of MSW and chlorine-rich solid waste (plastics) in a grate incinerator. In addition, Ca3(AsO4)2 was selected as the predominant form of arsenate in the bottom ash to perform decomposition experiments under the influence of HCl, supported by thermodynamic calculations and density functional theory (DFT) calculations. The results demonstrated that chlorine enhances the migration of arsenic from the bottom ash to the fly ash during the co-incineration process. For the first time, it has been discovered that Ca3(AsO4)2 transforms into Ca5(AsO4)3Cl when exposed to HCl at high temperatures, releasing As2O3 (g) into the flue gas. Importantly, the presence of HCl significantly lowers the initial temperature for the release of As2O3 (g). This effect is attributed to the adsorption of chlorine from HCl onto the calcium atoms in Ca3(AsO4)2, which affects the binding energy of adjacent Ca-O-As structure, thereby facilitating arsenic release at lower energies. Furthermore, the initial transformation temperature of Ca3(AsO4)2 into Ca5(AsO4)3Cl is reduced from 1300 degrees C to 600 degrees C in a high-chlorine atmosphere, which was obtained by incorporating the thermal properties of Ca5(AsO4)3Cl. These insights are crucial for the understanding of arsenic transformation in high-chlorine environments and offer guidance for arsenic emissions control during the incineration of chlorine-rich solid waste.