Impact of HCl and O2 on removal of elemental mercury by heat-treated activated carbon: Integrated X-ray analysis

- Heat-treated activated carbons (TAC) play an important role in Hg0 adsorption.
- HCl promote Hg0 capture on TAC through Eley-Rideal reactions.
- When Hg0 adsorbed, one electron bond with Cl while the other covalently bond with C.
- O2 has synergistic effect on Hg0 capture through the formation of OFGs.
- Hg0 can be adsorbed as Hg2Cl2 or HgCl but not as HgCl2 on TAC.

Treated activated carbons, prepared by heat treatment of activated carbon sorbents under the protection of argon (Ar), were subjected to synthetic flue gases to investigate its elemental mercury (Hg0) capture and the specific role of HCl and O2. To elucidate the adsorption mechanisms & binding properties, subsequent sorbents were characterized by X-ray photoelectron spectroscopy and X-ray absorption fine structure spectroscopy. The results suggest that Hg0 capture on the heat treated activated carbon (TAC) under HCl atmosphere mainly follows Eley-Rideal reaction mechanism, where, HCl reacts with the basic active carbon sites on the TAC leading to the formation of Cl-containing active sites, subsequently gaseous Hg0 heterogeneously reacts with these Cl-containing active sites through electron-transfer and forms HgClx compounds. Both the 2.42 Å and 2.39 Å bonding distance of Hg-Cl shows that only one of the two valence electrons on mercury bond with Cl, while the other electron bonds with the basic active carbon site covalently. In presence of HCl and O2, the synergistic effect of O2 on Hg0 adsorption might attribute to the O2 adsorption and the subsequent formation of oxygen-containing functional groups (OFGs) on TAC, which increase the amount of effective activated sites for Hg0 adsorption and at the same time enhance the adsorbing kinetics of adjacent Cl-containing sites.

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