Theoretical research on role of sulfur allotropes on activated carbon surface in adsorbing elemental mercury

Sulfur impregnated activated carbon has been proven to be promising adsorbents for mercury (Hg-0) removal in the flue gas. However, the active sites and detailed reaction pathway happened on the carbonaceous surface still remain uncertain. Here, we investigated the interaction behavior of various Sn allotropes (n = 2-8) with carbonaceous surface and the subsequent Hg-0 adsorption process by quantum chemistry calculation. The results indicate all Sn molecules could be attached to the carbonaceous surface by chemisorption with calculated adsorption energy lower than -400 kJ/mol. Moreover, the Hg-0 binding on virgin activated carbon surface takes place by physisorption with calculated adsorption energy around -20 kJ/mol. Sn allotropes (n = 2-3,6-8) attached on the carbonaceous surface have a negligible influence on improving Hg-0 removal performance. In contrast, S-4 and S-5 attached to the carbonaceous surface have a positive effect on improving Hg-0 adsorption performance and form S-Hg and C-Hg bonds in the products. Moreover, electronic wavefunction analysis was also conducted to investigate chemical bonds breaking and formation process in this research. Both bond lengths analysis and Mayer bond order value analysis were employed and localized molecular orbital analysis results indicate the formed S-Hg and C-Hg bonds exhibit typical bond character in the products. In addition, steam, a major component in the flue gas, could greatly accelerate the Hg-0 adsorption on carbonaceous surface by lowering reaction energy barriers. This research would be helpful for the development of efficient sulfur impregnated carbon based adsorbents in the future.