Correspondence Comment on “Impact of Sulfur Oxides on Mercury Capture by Activated Carbon” The recent ASAP article by Presto and Granite (1) on investigating the impact of sulfur oxides on mercury capture by activated carbon (AC) was quite interesting and provided a lot of valuable data for the Hg-removal performance evaluation of AC application in the simulated flue gas. However, there are several questions related to their research await answers.
What Kind of Mercury Species is Adsorbed? The temperature, volume of the flue gas, type of AC and flue gas composition can have an impact on the form (Hg2+ or elemental mercury, Hg0) and subsequent capture of mercury (2). These factors are not independent of one another, but are synergistic and are very dependent on the composition of flue gas (including water and oxygen). According to the experimental apparatus provided by the authors, mercury is liberated as Hg0(g) because of its volatility, but it will readily react with HCl(g) to form HgCl2 (3). In addition, the homogeneous phase reaction of mercury with oxygen is important in the flue gas. Thermodynamic calculations (4) suggest that ∼30% of the mercury can be present as HgO(g) at 200 °C. Hg0 is difficult to capture and must be converted to an oxidized form for capture (3). According to Table 1 from Presto and Granite (1), the concentrations of O2 and HCl in the simulated flue gas are 5.25% and 50 ppm, respectively. Therefore, the reactions between mercury and HCl and/or O2 are unavoidable, which will change the form of mercury. What kind of mercury species is adsorbed, Hg0, HgCl2 or HgO? Does the capture of HgCl2 and HgO(g) by AC compete with sulfur oxides for the same binding sites on the carbon surface in the same way as the authors suggest? The authors did not give any discussion about these questions.
What is the Mechanism of Mercury Adsorption on the AC Surface? Figures 2-4 from a bench-scale study in a U.S. Environmental Protection Agency (EPA) report (2) illustrates the effect of SO2 and HCl on the equilibrium adsorption capacity of the lignite-based AC for Hg0 and HgCl2. This figure is transcribed and illustrated as Figure 1. As shown in Figure 1, removing HCl from the flue gas did not affect the adsorption of the carbon for mercury chloride; however, it did prevent the carbon from adsorbing elemental mercury (2). The latter result suggests that HCl participates in the adsorption mechanism of elemental mercury, and the adsorption mechanism of mercury is not purely physical, i.e., interactions between elemental mercury and HCl on the carbon surface are also important. According to Table 1 from Presto and Granite (1), the concentration of HCl in the simulated flue gas is 50 ppm. The presence of HCl may affect the capture mechanism of mercury by AC. The authors should carry out comparative experiments without addition of HCl in the simulated flue gas to investigate the effect of HCl on mercury capture and discuss its mechanism. Hall et al. (5) reported the homogeneous gas phase reaction of mercury and oxygen and the corresponding reactions on AC. At temperatures between 100 and 250 °C, 970
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FIGURE 1. Equilibrium adsorption capacity of Hg0 and HgCl2 by a lignite-based AC (transcribed from reference (2)). Equilibrium adsorption capacities were determined for fixed beds of the carbon at 275 °F and three flue gas compositions: one containing 1600 ppm SO2 and 50 ppm HCl (the baseline composition); a second containing no SO2 and 50 ppm HCl; and a third containing 1600 ppm SO2 and no HCl (All three compositions of flue gas had the same concentration of elemental mercury, mercuric chloride, CO2, water, and O2). oxygen has a significant effect on the mercury adsorption by AC. Several papers (6, 7)concerning the oxidation of various types of carbon (for example, graphite and carbon black) have proposed that the mechanism involves dissociation of oxygen molecules forming chemisorbed oxygen atoms on the carbon surface. This reaction species may then react with mercury atoms forming HgO, so the elemental mercury is captured by carbon. Reactions between HgO adsorbed on AC and SO3 which is adsorbed on the AC surface or in the flue gas may lead to the formation of HgSO4(s) (3).
Discussion The adsorption of mercury on the surface of AC may proceed in the following paths (the expression “ad” in the parentheses is the abbreviation of “adsorption”): Path 1: Hg(g) + O2(g) f HgO (g) HgO(g) f HgO(s, ad) Path 2: Hg(g, ad) + O2(g, ad) f HgO(s, ad) Path 3: Hg(g) + HCl(g) f HgCl2 (g) HgCl2 (g) f HgCl2(s, ad) Path 4: Hg(g) + HCl(g, ad) f HgCl2 (s, ad) Path 5: Hg(g, ad) + HCl(g, ad) f HgCl2 (s, ad) Path 6: Hg(g) f Hg(g, ad) As shown in these paths, the possible adsorption processes on the AC surface involve Hg(g). HgO(g), O2(g), HCl(g) and HgCl2(g), among which O2(g) and HCl(g) are the reactive species for mercury transformation. If the binding sites for these compounds are occupied by other compounds present in the flue gas (for example, SO3 and vapor), the capture of mercury by AC will be inevitably decreased. If the occupied binding sites are much increased, it will lead to the rapid decrease of the capture of mercury by AC. The results from Figure 3 of this ASAP article by Presto and Granite (1) demonstrated this conclusion. The mercury content is reduced as sulfur content increases, because many more binding sites for Hg(g). HgO(g), O2(g), HCl(g), and HgCl2(g) are preoccupied by sulfur. However, the data shown in Figure 3 are not evidence for the direct competition between mercury and sulfur for the same biding sites on the AC surface (the authors’ conclusion), but that for the competition between sulfur and the species Hg(g). HgO(g), O2(g), HCl(g) and HgCl2(g) for the binding sites on the surface of AC. Due to the low vapor pressure and the tendency to condense on AC, SO3 (and H2SO4) may occupy 10.1021/es7021347 CCC: $40.75
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the binding sites for the species that are mentioned above so that the possible adsorption processes illustrated in Paths 1-6 are inhibited, resulting in the decrease of Hg capture by AC. The scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM-EDS), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR) analyses should be conducted to investigate the chemisorption processes between AC and various mercury species. These analyses can prove the existence of mercury compounds (HgCl2, HgO, HgSO4, etc.) (8).
Literature Cited (1) Presto, A. A.; Granite, E. J. Impact of sulfur oxides on mercury capture by activated carbon. Environ. Sci. Technol. 2007, 42, 972–973. (2) U.S. Environmental Protection Agency. Mercury Study Report to Congress;EPA-452/R-97–003; Office of Air Quality Planning and Standards, Office of Research and Development: Washington, DC, 1997; Vol. 3. (3) Olson, E. S.; Miller, S. J.; Sharma, R. K.; Dunham, G. E.; Benson, S. A. Catalytic effects of carbon sorbents for mercury capture. J. Hazard. Mater. 2000, 74, 61–79.
(4) Hall, B.; Lindqvist, O.; Ljungström, E. Mercury chemistry in simulated flue gas related with waste incineration conditions. Environ. Sci. Technol. 1990, 24, 108–111. (5) Hall, B.; Schager, P.; Weesmaa. The homogeneous gas phase reaction of mercury with oxygen, and the corresponding heterogeneous reactions in the presence of activated and fly ash. Chemosphere 1995, 30 (4), 611–627. (6) Hart, P. J.; Vastola, F. J.; Walker, P. L. Electron-microscopy study of the ferric chloride/graphite compound. Carbo n 1967, 5, 383–384. (7) Kelemen, S. R.; Freund, H. O2 oxidation studies of the edge surface of graphite. Carbo n 1985, 23, 619–625. (8) Lee, S. J.; Seo, Y.-C.; Jurng, J. S.; Lee, T. G. removal of gas-phase elemental mercury by iodine- and chlorine-impregnated activated carbons. Atmos. Environ. 2004, 38, 4887–4893.
Yangsheng Liu Department of Environmental Engineering, Peking University, Beijing 100871, China, Peking University Shenzhen Graduate, Shenzhen, 518055, China,
[email protected] ES7021347
VOL. 42, NO. 3, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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