KCl supported by γ-Al2O3 for selective adsorption of gaseous oxidized mercury: Preparation and adsorption mechanism

The separation of Hg-0 and HgCl2 with selective adsorbents is crucial for mercury speciation detection. KCl is commonly used but has a limited surface area, a short lifespan, and an unclear mechanism for Hg-0/HgCl2 adsorption. Hence, KCl supported by gamma-Al2O3 to create a high-surface-area selective adsorbent to enhance practical performance was proposed and the selective adsorption mechanisms of Hg-0 and HgCl2 through experimental investigations and density functional theory (DFT) calculations were elucidated. Compared to the total breakthrough of pure KCl, that of KCl/gamma-Al2O3 exhibits markedly selective adsorption efficacy on HgCl2, with a nearly complete breakthrough rate on Hg-0 at a KCl-to-gamma-Al2O3 molar ratio of 1, attributed to their favorable porosity and dispersed active sites. The DFT results verify a weak van der Waals force between KCl and Hg-0, indicating physisorption, while covalent bonding occurs between KCl and HgCl2, suggesting chemisorption. High-resolution transmission electron microscopy (HRTEM) identifies two crystalline surfaces of 001 and 011 on the developed adsorbent. This clarifies three stable adsorption configurations: 001 Top-Cl, 011 Top-Cl, and 011 Cl-Cl bridge, with adsorption energies of -87.11, -110.74, and -174.91 kJ/mol, respectively. Analyses of the electronic structure and interaction region indicator (IRI) reveal that the Hg atom within HgCl2 forms covalent bonds with unsaturated Cl atoms on the KCl surface. Additionally, integrated crystal orbital Hamilton population (ICOHP) analysis and electron transfer results demonstrate that the interaction strength of the three configurations with HgCl2 follows the order of 011 Cl-Cl bridge > 011 Top-Cl > 001 Top-Cl. This matches the three different temperature peaks of 276, 231, and 120 degrees C, respectively, in the analysis of the temperature-programmed desorption of HgCl2 (HgCl2-TPD). This study offers a novel insight on mercury speciation partitioning between Hg-0 and HgCl2.