Effect of pH and surface chemistry on the mechanism of H2S removal by activated carbons

The performances of three wood-based activated carbons as adsorbents of hydrogen sulfide were evaluated by dynamic breakthrough testing. The subsequent products of H2S oxidation on the carbon surfaces were analyzed. The adsorbents were studied using sorption of nitrogen, thermal analysis, Boehm titration, FTIR, ion chromatography, and temperature programmed desorption. Based on the results, the effects of surface chemistry and structural features on the yield of water soluble products and on the regenerability of the exhausted carbons were evaluated. The results showed that the breakthrough capacity and the yield on regeneration depend on the average pH of the carbon surface related to the pH in local pore environment. When the surface is very acidic, the dissociation of H2S is suppressed resulting in a very small concentration of hydrogen sulfide ions and thus in the formation of highly dispersed sulfur. Such conditions are favorable for oxidation of sulfur to S4+ and S6+, When the surface is less acidic the degree of dissociation is higher and the creation of polymeric elemental sulfur species resistant to further oxidation is more favorable. A small increase in pH (half a unit) in the acidic range results in a 15-fold increase in hydrogen sulfide breakthrough capacity accompanied by only a one third decrease in the yield of sulfur oxides. (C) 1999 Academic Press.