Influence of Surface Properties on the Mechanism of H₂S Removal by Alkaline Activated Carbons

Abstract

Alkaline activated carbons are widely used as adsorbents of hydrogen sulfide (H2S), one of the major odorous compounds arising from sewage treatment facilities. Although a number of studies have explored the effects of various parameters, mechanisms of H2S adsorption by alkaline carbons are not yet fully understood. The major difficulty seems to lie in the fact that little is known with certainty about the predominant reactions occurring on the carbon surface. In this study, the surface properties of alkaline activated carbons were systematically investigated to further exploit and better understand the mechanisms of H2S adsorption by alkaline activated carbons. Two commercially available alkaline activated carbons and their representative exhausted samples (8 samples collected at different height of the column after H2S breakthrough tests) were studied. The 8 portions of the exhausted carbon were used to represent the H2S/carbon reaction process. The surface properties of both the original and the exhausted carbons were characterized using the sorption of nitrogen (BET test), surface pH, Boehm titration, thermal and FTIR analysis. Porosity and surface area provide detailed information about the pore structure of the exhausted carbons with respect to the reaction extent facilitating the understanding of potential pore blockages. Results of Boehm titration and FTIR both demonstrate the significant effects of surface functional groups, and identification of oxidation products confirmed the different mechanisms involved with the two carbons. From the DTG curves of thermal analysis, two well-defined peaks representing two products of surface reactions (i.e., sulfur and sulfuric acid) were observed from the 8 exhausted portions with gradually changing patterns coinciding with the extent of the reaction. Surface pH values of the exhausted carbons show a clear trend of pH drop along the reaction extent, while pH around 2 was observed for the bottom of the bed indicating sulfuric acid as the predominant products. Although both carbons are coal-based and of KOH impregnated type, performances of different carbons differ significantly. A correlation is well established to link the reaction extent with various surface properties. In summary, not only the homogeneous alkali impregnation and physical porosity but also the carbon surface chemistry are significant factors influencing the performances of alkaline activated carbons as H2S adsorbents.

Investigation of Impregnated Activated Carbon Properties Used in Hydrogen Sulfide Fine Removal

“…It is seen that water presence enhanced the adsorption of H 2 S. Previous results stated that water is very important in H 2 S removal (Primavera et al, 1998;Bandosz et al, 2000;Yang et al, 1998;Yan et al, 2004;Flytzani-Stephanopoulos et al, 2006;Li et al, 2008;Klein and Henning, 1984;Mikhalovsky and Zaitsev, 1997). According to Primavera et al (1998), water presence may affect the removal reaction path in two ways: (a) H 2 S dissolution to HS -ions occurs in the water film inside the adsorbent pores; thus, the removal reaction proceeds faster in water than on the catalyst surface; (b) water continuously removes sulfur from the active sites and promotes sulfur adsorption on different carbon sections (Xiao et al, 2008, Primavera et al, 1998, Klein and Henning, 1984).…”

Fuel quality issues in stationary fuel cell systems.

“…There are many other choices to remove H 2 S, such as impregnated activated carbon [48][49][50][51], and other proprietary iron oxide media, such as Sulfur-Rite®, Sulfa-Bind®, and Iron Sponge, as well. The manufacturer of the particular media often states a maximum adsorption capacity that is equivalent to saturation conditions (i.e., the concentration in the exit gas equals the concentration in the feed gas).…”

Application of biological activated carbon as a low pH biofilter medium for gas mixture treatment

“…Physical adsorption was a reversible process, involving only weak forces of van der Waals. Chemical adsorption involved a chemical reaction and formation of new chemical bonds between the gas molecules and the surface of the adsorbent (Yan et al, 2004). Figure 7b shows the DTG curves of the BAC in different packing length of the low pH biofilter.…”