The adsorption behavior of mercury on the hematite (1-102) surface from coal-fired power plant emissions

One of the biggest environmental concerns caused by coal-fired power plants is the emission of mercury (Hg). Worldwide, 475 tons of Hg are released from coal-burning processes annually, comprising 24% of total anthropogenic Hg emissions. Because of the high toxicity of Hg species, US Environmental Protection Agency (EPA) proposed a standard on Hg and air toxic pollutants (Mercury and Air Toxics Standards, MATS) for new and existing coal-fired power plants in order to eliminate Hg in flue gas prior to release through the stack. To control the emission of Hg from coal-derived flue gas, it is important to understand the behavior, speciation of Hg as well as the interaction between Hg and solid materials, such as fly ash or metal oxides, in the flue gas stream. In this study, theoretical investigations using density functional theory (DFT) were carried out in conjunction with experiments to investigate the adsorption behavior of oxidized Hg on hematite (α-Fe₂O₃), an important mineral component of fly ash which readily sorbes Hg from flue gas.

For DFT calculation, the two α-Fe₂O₃ (1-102) surfaces modeled consisted of two different surface terminations: (1) M2-clean, which corresponds to the oxygen-terminated surface with the first layer of cations removed and with no hydroxyl groups and (2) M2-OH₂-OH, which has bihydroxylated top oxygen atoms and a second layer of hydroxylated oxygen atoms. These surface terminations were selected because both surfaces are highly stable in the temperature range of flue gases. The most probable adsorption sites of Hg, Cl and HgCl on the two α-Fe₂O₃ surface terminations were suggested based on calculated adsorption energies. Additionally, Bader charge and projected density of states (PDOS) analyses were conducted to characterize the oxidation state of adsorbates and their bonding interactions with the surfaces. Results indicate that oxidized Hg physically adsorbs on the M2-clean surface with a binding energy of -0.103 eV and that chlorine added to the Hg-adsorbed surface enhances Hg adsorption, which is evidenced by a shorter Hg-surface interaction distance compared to a surface with singly bound Hg. Bader charge and PDOS analyses also suggest that a higher surface charge transfer and bonding interaction occur when we added Cl to the system, which increases the stability of Hg the surface.