||This study investigated the adsorptive and desorption capacity of HgCl2 onto powdered activated carbon derived from carbon black of pyrolyzed waste tires (CPBAC) via thermogravimetric analysis (TGA). |
Due to incomplete classification and recycling of municipal solid wastes (MSW), they still mix with a lot of hazardous materials, which unfortunately can not be removed by incinerators and air pollution control devices(APCDs). Among them, mercury and its pollutants attract more attention by people. Mercury and its pollutants emitted from the incineration of municipal solid wastes could cause severely adverse effects on human health and ecosystem since they exist mainly in vapor phase due to high vapor pressure. If they can not be remove by the air pollution control devices, they will be emitted to the atmosphere and cause serious effects on environmental ecology via various routes.
Activated carbon has been widely applied to the treatment of organic compounds and heavy metals in wastewater and waste gas stream. However, the adsorptive capacity of activated carbon decreases with adsorption temperature. The low adsorptive capacity of activated carbon at high temperature (>150 oC) can be overcome by impregnated activated carbons. Previous study reported that sulfur impregnated powdered activated carbons could effectively remove the vapor-phase elemental mercury (Hgo) emitted from MSW incinerators and utility power plants. However, the impregnated typically used is sulfur (S) which is solely applied for the adsorption of elemental mercury (Hgo). Besides, these studies seldom investigate the distribution of impregnated sulfur in the inner pores of activated carbon and its effects on the specific surface area and pore size distribution. Thus, this study was to investigate the fundamental mechanisms for the adsorption/desorption of HgCl2 by/from sulfur impregnated PAC.
Experimental results indicated that the sulfur content of sulfur impregnated CBPAC decreased with increasing impregnation temperatures form 400 to 650 oC; while the surface area of sulfur impregnated CBPAC increased with impregnation temperatures. In this study, TGA was applied to obtain the adsorptive capacity of HgCl2 onto CBPAC with adsorption temperature (150oC) and influent HgCl2 concentration (100~500 μg/m3). Experimental results indicated that the adsorptive capacity of CBPAC increased with the increase of influent HgCl2 concentration and surface area of the activated carbon. This study revealed that the impregnation of sulfur on CBPAC could increase its adsorption capacity at high temperatures.
Desorption experimental parameters included desorption temperature (400, 500, and 600 oC), heating rate (10, 15, and 20 oC /min) and regeneration cycle (1~7 cycles). In probing into the regeneration efficiency of CBPAC, experiments were conducted at the desorption times of 60 and 30 min. The results suggested the regeneration efficiency of carbon under 30 min was generally highter than that under 60 min. Because the desorption time was more longer and the sulfur content was lesser. Therefore, the regeneration times was reduce. Experimental results indicated that the mechanism of HgCl2 desorption from the spent CPBAC was strongly affected by desorption temperature. Both the desorption efficiency and the desorption rate of HgCl2 increased dramatically with desorption temperature. The desorption heat of HgCl2 (823 KJ/mole) was much higher than the vaporization heat of HgCl2 (59.2 KJ/mole), indicating that the adsorption of HgCl2 on sulfur impregnated CBPAC was chemical adsorption. Consequently, raising desorption temperature could enhance the desorption of HgCl2 and shorten the duration for HgCl2 desorption. Moreover, the formation of HgS during the desorption of HgCl2 from activated carbons can be proved by the surface characteristics of sulfur impregnated activated carbons. Results obtained from the regeneration of sulfur impregnated activated carbons indicated that the regeneration cycles decreased as the desorption duration increased. It was attributed to the potential desorption of sulfur from actived carbons, which thus decreased the adsorptive capacity and the regeneration cycles.