Coal combustion as the largest anthropogenic emission source of trace elements (TEs), such as arsenic and selenium, has attracted widespread attention. CaO has been researched for a long time as an effective adsorbent in arsenic and selenium removal from flue gas due to its advantages of low cost and possible adsorption capability. Calcium looping (CaL) based on CaO sorbents has also been identified as one of the most promising technologies for CO₂ capture. This paper aims at explaining the mechanisms of the effect of CO₂ existed in the flue gas on the As₂O₃ and SeO₂ adsorption characteristics by CaO using density functional theory (DFT) calculations. The results show that As₂O₃ and SeO₂ prefer to locate at the top-sites of O atoms, exhibited by the overlapped electron density cloud. The adsorption energies of As₂O₃ and SeO₂ are -2.21 eV and -2.05 eV, both of which are lower than that of CO₂ adsorption. This phenomenon suggests that the surface bindings to As₂O₃ and SeO₂ are stronger than CO₂. The optimized co-adsorption structures of TEs and CO₂ on CaO (100) surface verify the higher preferential adsorption of As₂O₃ and SeO₂ on the CaO (100) surface, compared with the adsorptions of CO₂. Carbonation is a competitive reaction during the TEs removal by CaO and the effect of CO₂ on the interaction between TEs species and CaO cannot be ignored, considering the flue gas always contains a high concentration of CO₂. Pre-adsorbed CO₂ by CaO changes the original electronic field of CaO surface, which weakens the electron transfers from the surface to As and Se atoms. The average adsorption energies of TEs molecules in four orientations on the surface O atoms next to the pre-adsorbed CO₂ are 0.5%-88% higher than those without CO₂. This work provides the adsorption mechanisms of TEs on CaO-based sorbents, which is beneficial for the design of effective capture technologies.

heavy metal,trace element,calcium looping,CO2 capture,density functional theory