Environ. Sci. Technol. 2001, 35, 2785-2791
Effects of Sulfur Impregnation Temperature on the Properties and Mercury Adsorption Capacities of Activated Carbon Fibers (ACFs) H S I N G - C H E N G H S I , † M A R K J . R O O D , * ,† M A S S O U D R O S T A M - A B A D I , * ,†.‡ SHIAOGUO CHEN,‡ AND RAMSAY CHANG§ Department of Civil and Environmental Engineering, 205 North Mathews Avenue, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, Illinois State Geological Survey, 615 East Peabody Drive, Champaign, Illinois 61820, and EPRI; 3412 Hillview Avenue, Palo Alto, California 20036
Laboratory studies were conducted to determine the role of sulfur functional groups and micropore surface area of carbon-based adsorbents on the adsorption of Hg0 from simulated coal combustion flue gases. In this study, raw activated carbon fibers that are microporous (ACF-20) were impregnated with elemental sulfur between 250 and 650 °C. The resulting samples were saturated with respect to sulfur content. Total sulfur content of the sulfur impregnated ACF samples decreased with increasing impregnation temperatures from 250 and 500 °C and then remained constant to 650 °C. Results from sulfur K-edge X-ray absorption near-edge structure (S-XANES) spectroscopy showed that sulfur impregnated on the ACF samples was in both elemental and organic forms. As sulfur impregnation temperature increased, however, the relative amounts of elemental sulfur decreased with a concomitant increase in the amount of organic sulfur. Thermal analyses and mass spectrometry revealed that sulfur functional groups formed at higher impregnation temperatures were more thermally stable. In general, sulfur impregnation decreased surface area and increased equilibrium Hg0 adsorption capacity when compared to the raw ACF sample. The ACF sample treated with sulfur at 400 °C had a surface area of only 94 m2/g compared to the raw ACF sample’s surface area of 1971 m2/g, but at least 86% of this sample’s surface area existed as micropores and it had the largest equilibrium Hg0 adsorption capacities (2211-11343 µg/ g). Such a result indicates that 400 °C is potentially an optimal sulfur impregnation temperature for this ACF. Sulfur impregnated on the ACF that was treated at 400 °C was in both elemental and organic forms. Thermal analyses and CS2 extraction tests suggested that elemental sulfur was the main form of sulfur affecting the Hg0 adsorption capacity. These findings indicate that both the presence of elemental sulfur on the adsorbent and a microporous structure are important properties for improving the performance of carbon-based adsorbents for the removal of Hg0 from coal combustion flue gases.
Introduction Carbon-based adsorption processes (e.g., direct injection and fixed bed) have the potential to remove very low gas-phase 10.1021/es001794k CCC: $20.00 Published on Web 05/18/2001
2001 American Chemical Society
concentrations (e.g., ≈µg/m3 levels) of mercury species that occur in coal combustion flue gas streams (1). Recent studies have shown that activated carbon adsorbents can be developed with larger adsorption capacities for mercury by impregnating the samples with elemental sulfur at temperatures between 200 and 650 °C (2-7). In one study, the total surface area of these activated carbons decreased by ∼83% and the sulfur content increased to 36-39 wt % after sulfur impregnation at 250 °C (5). In contrast, sulfur impregnation at 600-650 °C had reduced total surface area by
