Mechanism of Arsenic Sorption by Hydrated Lime

Abstract

Arsenic, along with mercury and selenium, represents one of the trace metallic emissions from fossil fuel-fired power plants that exhibit a tendency to remain in the gas phase. In this study, the effectiveness of some commonly used mineral sorbents is tested for the removal of arsenic from flue gas. Investigations are conducted with hydrated lime sorbent to identify the mechanism of As/Ca interaction and the chemical state and characteristics of captured species. Arsenic oxide (As2O3) is used as the representative arsenic species, and investigations are conducted in a differential fixed bed reactor at medium (400−600 °C) and high (1000−800 °C) temperature conditions. Comparison of Ca(OH)2 with three other mineral sorbents (kaolinite, alumina, and silica) reveals that calcium hydroxide is the most effective in capturing arsenic. The capture mechanism of Ca(OH)2 does not involve a simple physical adsorption but proceeds by means of an irreversible chemical reaction leading to a solid product. X-ray diffraction and ion chromatography analyses of the post-sorption sample confirm that calcium arsenate is the dominant reaction product. Sorption of arsenic by fly ash is found to be reversible and physical in nature.