| Summary: | Emission of hazardous trace elements, especially arsenic from fossil fuel combustion, have become a major concern. Under an oxidizing atmosphere, most of the arsenic converts to gaseous As<sub>2</sub>O<sub>3</sub>. CaO has been proven effective in capturing As<sub>2</sub>O<sub>3</sub>. In this study, the mechanisms of As<sub>2</sub>O<sub>3</sub> adsorption on CaO surface under O<sub>2</sub> atmosphere were investigated by density functional theory (DFT) calculation. Stable physisorption and chemisorption structures and related reaction paths are determined; arsenite (AsO<sub>3</sub><sup>3−</sup>) is proven to be the form of adsorption products. Under the O<sub>2</sub> atmosphere, the adsorption product is arsenate (AsO<sub>4</sub><sup>3−</sup>), while tricalcium orthoarsenate (Ca<sub>3</sub>As<sub>2</sub>O<sub>8</sub>) and dicalcium pyroarsenate (Ca<sub>2</sub>As<sub>2</sub>O<sub>7</sub>) are formed according to different adsorption structures.
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