Theoretical research on mercury-laden halogenated activated carbon adsorbent bonding nature

Halogenated activated carbon adsorbents exhibit superior mercury capture performance in the flue gas. Not only the fabrication of effective adsorbents, but the used adsorbents stability is also crucial. It is essentially determined by product bonding nature. Here, Density functional theory was employed to investigate spent halogenated activated carbon adsorbents bonding nature at the atomic level. Seven possible mercury-containing product molecular geometries and effects of halogen species (X = Cl/Br/I) were considered. Specifically, bond length value, and bond order of Hg-C, Hg-X and C-X bonds of interest were calculated to investigate product stability in the gaseous phase. It was found that mercury was attached on the carbonaceous surface with C-Hg Mayer bond order around 0.5 in product structures G and H, suggesting a weak interaction. In other possible product molecular geometries, C-Hg or X-Hg exhibited a single bond character. Moreover, effects of halogen atoms on X-Hg and C-X bond strength were discussed. Electron localization function (ELF) and Localized orbital locator (LOL) were utilized to determine electron high localization regions. Results indicated C-Hg and C-X on the carbonaceous surface are covalent bonds and formed X-Hg bonds are ionic bonds. Furthermore, atom composition in the localized molecular orbital was calculated via the Hirshfeld method. In summary, this study investigated the bonding nature of used halogenated activated carbon adsorbents. The results are beneficial for fabricating more environmental-friendly mercury removal adsorbents in the future.