Quantification of the Mercury Adsorption Mechanism on Brominated Activated Carbon

Emissions from coal-fired power plants are a major source of atmospheric mercury. To remove toxic mercury from exhausts, special filters that bind mercury are used in the duct systems of power plants. Researchers from Stanford University have approximated the conditions of an industrial exhaust in their lab by placing filter materials in a gas stream doped with trace amounts of mercury. Using SSRL’s Beam Line 7-3, the scientists determined the binding mode of mercury in their samples. Understanding the precise mercury-binding mechanism is of great interest for improving existing filters and designing new materials.

In the study, published in the journal Fuel, the researchers used brominated activated carbon as sorbent, which they studied as powder and fiber samples. The team reacted the sorbents with the mercury-containing gas stream (at 30°C and 140°C) and determined the local mercury environments in the reaction products using x-ray absorption spectroscopy (XAS). The data showed that mercury (Hg) is present in the form of Hg²⁺ – evidence of mercury being taken up through an oxidative process rather than merely through electrostatic adhesion of elemental Hg⁰. Moreover, the scientists found that the reaction temperature alters the mercury-uptake capacity but not the structural mercury-binding mode, indicating that the oxidation of mercury takes place on the sorbent surface and not in the gas stream.

From the extended x-ray absorption fine structure (EXAFS) region of the XAS signal, the team determined that each mercury ion is, on average, surrounded by approximately two bromide ions, suggesting either (1) monodentate binding of mercury to a surface carbon with an additional bond to bromide and a non-binding interaction with a surface-adsorbed bromide, or (2) binding of mercury to two bromide ions with a non-binding interaction with the carbon surface. However, density functional theory (DFT) calculations show that neither of these binding modes corresponds to the most stable configuration – mercury bound to a surface carbon with a single bromide bound atop of mercury. It therefore appears that the reactions of mercury on the carbon surface are kinetically controlled and favor HgBr₂-type adsorption over more stable configurations.

 

Primary Citation

E. Sasmaz, A. Kirchhofer, A. D. Jew, A. Saha, D. Abram, T. F. Jaramillo, and J. Wilcox, “Mercury Chemistry on Brominated Activated Carbon”, Fuel 99, 188 (2012), doi: 10.1016/j.fuel.2012.04.036.

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Contact

Jennifer Wilcox, Stanford University