||This study investigated the single and dual adsorption of mercury chloride (HgCl2) and elemental mercury (Hg0) using an innovative composite sulfur-impregnated activated carbon prepared by combining two sulfur impregnation methods with aqueous-phase sodium sulfide (Na2S) and vapor-phase elemental sulfur (S0). This study aims to prepare an innovative composite sulfur-impregnated PACs under different impregnation conditions, such as various sulfur species and impregnation temperatures, and to investigate the changes of its pore size distribution and specific surface area (SSA) before and after sulfur impregnation. Thermogravimetric analysis (TGA) technology was further applied to determine the saturated adsorptive capacities and adsorption mechanisms of single and dual Hg0 and/or HgCl2 adsorption.|
Sulfur impregnation results indicated that, the more sulfur was impregnated, the lower SSA and pore volume of sulfur-impregnated PACs were obtained. These results showed that part of the micropores, mesopores, and even marcopores were probably plugged by sulfur species. Energy Dispersive Spectrometer (EDS) results showed that the composite sulfur impregnation process could greatly enhance the sulfur content of PACs. The sulfur content of the composite sulfur-impregnated PACs (Na2S and S0) was almost the sum of the sulfur content of two separate single sulfur-impregnated PACs (Na2S or S0). Furthermore, the higher the sulfur content of sulfurized PACs, the lower the specific surface area. However, since the mesopores and marcopores were the main channels for adsorbates entering the micropores, the amount of mesopores and marcopores could influence the saturated adsorptive capacity of PACs.
The adsorption of gas-phase HgCl2 showed that the influent concentration of HgCl2 could enhance its adsorptive capacity as the adsorption temperature increased. It suggested that the composite sulfur-impregnated PACs tended to be chemisorption and was a favorable adsorpation at elevated temperatures. In addition, when the adsorption occurs at 200-300℃, the saturated adsorptive capacity of Hg0 for the composite sulfur-impregnated PACs always exceled that of HgCl2. Using coal-fired power plant as an example, the saturated adsorptive capacity of HgCl2 or Hg0 was higher than that of dual Hg0 and HgCl2 adsorption when the molar ratio of influent HgCl2 and Hg0 was 5:5. The breakthrough time became shorter and the saturated adsorptive capacity decreased significantly, suggesting a competition between HgCl2 and Hg0 on the activated sites. Using municipal solid waste incinerator as an example, when the molar ratio of influent HgCl2 and Hg0 was 6:4, the adsorptive capacity of Hg species for coal-fired power plants was prominently lower than that of municipal solid waste incinerators at the adsorption temperatures of 150~300℃. It suggested that HgCl2 was a more competitive adsorbate than Hg0 which was tentatively replaced by HgCl2 on the adsorption sites.
Results from thermodynamic analysis showed negative ∆G for both single and dual adsorption of HgCl2 and/or Hg0 at the adsorption temperatures of 150~300℃. Experimental results confirmed the feasibility of adsorption process and the spontaneous nature for the adsorption of HgCl2 and Hg0. The negative ∆H for the adsorption HgCl2 and/or Hg0 at the adsorption temperatures of 150~300℃ suggested that the adsorption of HgCl2 and/or Hg0 was an exothermic process. The Langmuir isotherm results showed that the higher the adsorption temperatures, the greater the qm, with the qm of 33.789-98.118, and thus concurred with the theory of Langmuir isotherms. Moreover, the equilibrium adsorption constant KL ranged from 0.008 to 0.070 which highly related to the adsorption affinity or free energy. The Freundlich isotherm results showed that the value of n＞1, suggesting the adsorption of HgCl2 and/or Hg0 was favorable adsorption. The n values of 1.949-10.288 for the single adsorption of vapor-phase HgCl2 or Hg0 by the composite sulfurized activated carbon suggested that the sulfurized activated carbons were thought as good adsorbents.