Heterogeneous Mercury Reaction Chemistry on Activated Carbon

Abstract

Experimental and theory-based investigations have been carried out on the oxidation and adsorption mechanism of mercury (Hg) on brominated activated carbon (AC). Air containing parts per billion concentrations of Hg was passed over a packed-bed reactor with varying sorbent materials at 140 and 30°C. Through X-ray photoelectron spectroscopy surface characterization studies it was found that Hg adsorption is primarily associated with bromine (Br) on the surface, but that it may be possible for surfacebound oxygen (O) to play a role in determining the stability of adsorbed Hg. In addition to surface characterization experiments, the interaction of Hg with brominated AC was studied using plane-wave density functional theory. Various configurations of hydrogen, O, Br, and Hg on the zigzag edge sites of graphene were investigated, and although Hg-Br complexes were found to be stable on the surface, the most stable configurations found were those with Hg adjacent to O. The Hg-carbon (C) bond length ranged from 2.26 to 2.34 Å and is approximately 0.1 Å shorter when O is a nearest-neighbor atom rather than a next-nearest neighbor, resulting in increased stability of the given configuration and overall tighter Hg-C binding. Through a density of states analysis, Hg was found to gain electron density in the six p-states after adsorption and was found to donate electron density from the five s-states, thereby leading to an oxidized surface-bound Hg complex.

Sulfurized activated carbons and their mercury adsorption/desorption behavior in aqueous phase

“…The binding energy of *101 eV for 4f 7/2 in the spectra of Hg-saturated AC (Fig. 3a) confirmed the presence of Hg-O and Hg-Cl bonds (Nelson et al 2000;Wilcox et al 2011). In the case of mercury-saturated AC-DMDS (Fig.…”

Mercury chemistry on brominated activated carbon

“…Also, the Si2p spectral lines of Si-C, silicon dioxide (SiO 2 ) and Si compounds are expected to be in the region between 98.5 eV and 104 eV. The Si2p core-level spectra can be separated from Hg4f spectra by using the area of the Si2s core-level spectra and the intensity ratios of the two Si spectral lines [10]. Table 2 provides a summary of the XPS of Hg and Br binding energies for Hg-Br reference compounds [29,30] and for the Hg-containing brominated AC sorbent investigated in the current work compared against the study carried out by Hutson et al [6].…”

“…with brominated AC likely involves Hg interaction with the surface-bound Br by possibly detaching/replacing it from the graphene edge site. The effect of O on the binding of Hg in the presence of Br atom has been investigated in our previous work [10]. It was found that the edge C binds to Hg more tightly when it is located next to a surface O atom, but not immediately next to a bound Br atom [10].…”

“…High Hg concentrations were used to decrease testing time and increase signal intensity of the Hg4f spectrum in XPS analysis. The powdered AC sorbents, prepared from lignite-based coal, contain Si by nature, which interferes with the spectra of oxidized Hg compounds due to the similarity in binding energies of Hg4f and Si2p electrons [10]. Mercury adsorption tests were also conducted at a lower temperature, i.e.…”

“…Using a 9-ring deep graphene ribbon model, electronic structure calculations have been carried out to determine the most stable concentrations and configurations of hydrogen (H), bromine (Br), and Hg atoms on the edge sites. Details of the model are described elsewhere [10]. The partial density of states (DOSs) were calculated by projecting the electronic wave functions onto spherical harmonics centered on Hg, carbon (C), and Br atoms to determine the potential mechanisms associated with Hg binding to edge sites of graphene.…”