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

Abstract

The separation of Hg⁰ 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 Hg⁰/HgCl₂ adsorption. Hence, KCl supported by γ-Al₂O₃ to create a high-surface-area selective adsorbent to enhance practical performance was proposed and the selective adsorption mechanisms of Hg⁰ and HgCl₂ through experimental investigations and density functional theory (DFT) calculations were elucidated. Compared to the total breakthrough of pure KCl, that of KCl/γ-Al₂O₃ exhibits markedly selective adsorption efficacy on HgCl₂, with a nearly complete breakthrough rate on Hg⁰ at a KCl-to-γ-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 Hg⁰, 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°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 Hg⁰ and HgCl₂.