Mercury chemistry on brominated activated carbon

Abstract

Activated carbon-based sorbents are the most widely tested sorbents for mercury removal in coal-fired power plants. A major problem in mercury removal is the limited understanding of the mechanism associated with elemental mercury (Hg0) oxidation and its subsequent adsorption. This work investigates the possible binding mechanism of Hg0 onto brominated fiber and powder activated carbon sorbents through packed-bed experiments in a stream of air. To better understand the mechanisms involved, a combination of spectroscopy and quantum mechanical modeling were used to characterize the sorption process. X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS) spectroscopy were used to analyze the surface and bulk chemical compositions of brominated activated carbon sorbents reacted with Hg0. It was found that Hg0 is oxidized at the brominated carbon surfaces at both 30°C and 140°C. The oxidation state of adsorbed Hg is found to be Hg2+, and coordinated to two Br atoms with no detectable bonding between Hg and O. Though plane-wave density functional theory (DFT) and density of states (DOSs) calculations indicate that Hg is more stable when it is bound to the edge C atom interacting with a single Br bound atop of Hg, a model that includes an interaction between the Hg and an additional Br atom matches best with experimental data obtained from EXAFS spectroscopy. Because the most stable structures optimized in the DFT simulations were not found on the samples analyzed using EXAFS spectroscopy, Hg surface reactions on the carbon surface are thought to be kinetically controlled.