Theoretical and experimental investigations of mercury adsorption on hematite surfaces
Creators: name: Jung, Ji-Eun
name: Liguori, Simona
name: Jew, Adam D.
name: Brown, Gordon E.
name: Wilcox, Jennifer
Publisher: name: Taylor & Francis
Year: 2017
Subjects: subject: Biophysics
subject: 29999 Physical Sciences not elsewhere classified
subject: FOS: Physical sciences
subject: FOS: Physical sciences
subject: Cell Biology
subject: Physiology
subject: FOS: Biological sciences
subject: FOS: Biological sciences
subject: 59999 Environmental Sciences not elsewhere classified
subject: FOS: Earth and related environmental sciences
subject: FOS: Earth and related environmental sciences
subject: 39999 Chemical Sciences not elsewhere classified
subject: FOS: Chemical sciences
subject: FOS: Chemical sciences
subject: Ecology
subject: 69999 Biological Sciences not elsewhere classified
subject: Hematology

description: One of the biggest environmental concerns caused by coal-fired power plants is the emission of mercury (Hg), which is toxic metal. To control the emission of Hg from coal-derived flue gas, it is important to understand the behavior and speciation of Hg as well as the interaction between Hg and solid materials in the flue gas stream. In this study, atomic-scale theoretical investigations using density functional theory (DFT) were carried out in conjunction with laboratory-scale experimental studies to investigate the adsorption behavior of Hg on hematite (α-Fe<sub>2</sub>O<sub>3</sub>). According to the DFT simulation, the adsorption energy calculation proposes that Hg physisorbs to the α-Fe<sub>2</sub>O<sub>3</sub>(0001) surface with an adsorption energy of −0.278 eV, and the subsequent Bader charge analysis confirms that Hg is slightly oxidized. In addition, Cl introduced to the Hg-adsorbed surface strengthens the Hg stability on the α-Fe<sub>2</sub>O<sub>3</sub>(0001) surface, as evidenced by a shortened Hg-surface equilibrium distance. The projected density of states (PDOS) analysis also suggests that Cl enhances the chemical bonding between the surface and the adsorbate, thereby increasing the adsorption strength. In summary, α-Fe<sub>2</sub>O<sub>3</sub> has the ability to adsorb and oxidize Hg, and this reactivity is enhanced in the presence of Cl. For the laboratory-scale experiments, three types of α-Fe<sub>2</sub>O<sub>3</sub> nanoparticles were prepared using the precursors Fe(NO<sub>3</sub>)<sub>3</sub>, Fe(ClO<sub>4</sub>)<sub>3</sub>, and FeCl<sub>3</sub>, respectively. The particle shapes varied from diamond to irregular stepped and subrounded, and particle size ranged from 20 to 500 nm depending on the precursor used. The nanoparticles had the highest surface area (84.5 m<sup>2</sup>/g) due to their highly stepped surface morphology. Packed-bed reactor Hg exposure experiments resulted in this nanoparticles adsorbing more than 300 μg Hg/g. The Hg L<sub>III</sub>-edge extended X-ray absorption fine structure spectroscopy also indicated that HgCl<sub>2</sub> physisorbed onto the α-Fe<sub>2</sub>O<sub>3</sub> nanoparticles. <i>Implications</i>: Atomic-scale theoretical simulations proposes that Hg physisorbs to the α-Fe<sub>2</sub>O<sub>3</sub>(0001) surface with an adsorption energy of −0.278 eV, and the subsequent Bader charge analysis confirms that Hg is slightly oxidized. In addition, Cl introduced to the Hg-adsorbed surface strengthens the Hg stability on the α-Fe<sub>2</sub>O<sub>3</sub>(0001) surface, as evidenced by a shortened Hg-surface equilibrium distance. The PDOS analysis also suggests that Cl enhances the chemical bonding between the surface and the adsorbate, thereby increasing the adsorption strength. Following laboratory-scale experiment of Hg sorption also shows that HgCl<sub>2</sub> physisorbs onto α-Fe<sub>2</sub>O<sub>3</sub> nanoparticles which have highly stepped structure.