Experimental and theoretical investigations of mercury adsorption on hematite surfaces [electronic resource]
Keywords: Energy Resources Engineering.

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. Considering the high toxicity of Hg species, Hg emission from the coal-fired power plants should be reduced. 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, atomic-scale theoretical investigations using density functional theory were carried out in conjunction with lab-scale experimental studies to investigate the adsorption behavior of Hg on hematite (α-Fe2O3), an important mineral component of fly ash, which readily sorbs Hg from flue gas. Oxygen terminated α-Fe2O3(1-102), which is r-cut, and (0001), which is c-cut, surfaces were chosen as representative α-Fe2O3 models based upon a previous ab initio thermodynamics study showing high surface stability in the temperature range of typical flue gases. In order to see the effect of chlorine (Cl) during Hg adsorption, the most probable adsorption sites of Hg, Cl, and HgCl on the α-Fe2O3 surface were found based on adsorption energy calculations, and the oxidation states of the adsorbates are determined by Bader charge analysis. Additionally, the projected density of states (PDOS) analysis characterized the surface-adsorbate bonding mechanism. Regarding the α-Fe2O3(1-102) surface, the adsorption energy of -0.103 eV indicated that Hg physisorbs to the α-Fe2O3 surface, and the subsequent Bader charge analysis confirmed that Hg is oxidized. Adding Cl to the Hg-adsorbed surface further enhanced the strength of Hg adsorption as evidenced by a shortened Hg-surface equilibrium distance. Bader charge and PDOS analyses also suggested that the presence of Cl enhances the charge transfer between the α-Fe2O3 surface and the adsorbate, thereby increasing adsorption strength. Compared to the results using the α-Fe2O3(1-102) surface, the results using the α-Fe2O3(0001) surface showed stronger adsorption energy of Hg (-0.278 eV) in the same coverage conditions. Chlorine introduced to the Hg-adsorbed surface also strengthens Hg stability on the α-Fe2O3(0001) surface. In theoretical calculations, therefore, Hg interacted with α-Fe2O3 surface especially in the presence of Cl. Additionally, nano-sized α-Fe2O3 particles were synthesized and characterized for their abilities to uptake Hg in order to understand the factors affecting Hg adsorption on a lab-scale. Three α-Fe2O3 nanoparticles, i.e. NP1, NP2, and NP3, were prepared using different precursors Fe(NO3)3, Fe(ClO4)3, and FeCl3, respectively. The different compositions of precursors and aging durations resulted in dissimilar sizes and morphologies of α-Fe2O3, which have been shown to impact Hg sorption ability. The nanoparticle shapes varied from rhombohedra (NP1) to irregular stepped (NP2) to subrounded (NP3). In addition, the nanoparticle sizes ranged from 20 to 500 nm depending on the precursor used. The Brunauer-Emmett-Teller surface area method was applied to the NP2, which has numerous stepped surfaces, revealing a surface area of 84.5 m2/g. This surface area was higher than that of the other two particles, which were 58.5 and 22.0 m2/g, for NP1 and NP3, respectively. From the packed-bed reactor Hg exposure experiments, the NP2 physically adsorbed more than 300 ppm of HgCl2, while the NP1 and NP3 only adsorbed less than 100 ppm. The equilibrium distances of Hg-Fe, Hg-Cl and Hg-O measured by extended x-ray absorption fine structure indicate that HgCl2 physisorbed onto NP2 nanoparticles. Overall, the NP2 performed best in terms of surface area and sorption ability of Hg.