Effect of pH and Surface Chemistry on the Mechanism of H 2 S Removal by Activated Carbons Foad Adib,* Andrey Bagreev, 1 and Teresa J. Bandosz 2 The City College of New York, Department of Chemistry and *Department of Civil Engineering, The Graduate School of the City University of New York, and Center for Water Resources and Environmental Research of the City College of New York Received January 27, 1999; accepted May 20, 1999 The performances of three wood-based activated carbons as adsorbents of hydrogen sulfide were evaluated by dynamic break- through testing. The subsequent products of H 2 S 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 struc- tural features on the yield of water soluble products and on the regenerability of the exhausted carbons were evaluated. The re- sults 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 H 2 S is suppressed resulting in a very small concentration of hydrogen sulfide ions and thus in the formation of highly dispersed sulfur. Such conditions are favor- able for oxidation of sulfur to S 4 and S 6 . 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 break- through capacity accompanied by only a one third decrease in the yield of sulfur oxides. © 1999 Academic Press Key Words: activated carbons; hydrogen sulfide; oxidation; sur- face chemistry; porosity; breakthrough capacity. INTRODUCTION Activated carbons are used as adsorbents of gases and va- pors, catalysts supports, and separation media (1). Their per- formance is governed by the unique features of their surface such as developed microporosity, high pore volume, and large specific surface area (2). When the molecules interacting in a specific way are to be adsorbed or separated, surface chemistry starts to play an important role (3). It is widely assumed that pores of activated carbons are created between parallel oriented graphite-like crystallites (slit-shape pores). At the edges of these crystallites atoms other than carbon can be built into the activated carbon matrix (3– 6). The most common heteroatoms are hydrogen, oxygen, nitrogen, and phosphorus. Those atoms are arranged in the forms of common organic functional groups (5, 6), such acidic ones as carboxylic, lactonic, phenolic, or basic species, such as pyranes. The number and strength of those groups influence the apparent acidity or basicity of the carbon surface. Moreover, the presence of a few percent of inorganic matter from the organic precursor such as coal or wood is also considered important for the performance of carbons, especially when they are used as catalyst supports (7). For applications where specific interactions are important the chemical features of activated carbons surface are tailored to some extent (3). For instance, carbons impregnated with noble metals or with transition metal oxides can catalyze the air oxidation of VOCs. Activated carbon can be impregnated with caustics in order to increase its capacity for acidic gases (8, 9). Another treatment is impregnation of carbons with certain nitrogen compounds such as urea and then calcination at high temperature (10), which builds basic nitrogen compounds into an activated carbon matrix. All of the above modifications, especially chemical, raise the cost of sorbents and, in some cases, create hazardous condi- tions for their applications. Very often the risk of in situ fire is high (11). An example of the application of activated carbons impregnated with caustics is the removal of hydrogen sulfide from vent air in sewage treatment plants. The disadvantages of such carbons directed our attention toward studying virgin carbons as adsorbents of hydrogen sulfide (11–14). Although the catalytic oxidation of H 2 S by virgin carbons at high tem- perature (400 K) is known (15–17), their performance at the conditions close to ambient is still not fully understood and the factors which govern it are not well defined (13, 18 –21). This study is a continuation of our research on the perfor- mance of wood-based activated carbons as adsorbents of hy- drogen sulfide (11–14). The comparison of the breakthrough results along with the proposed general mechanism of oxida- tion was presented elsewhere (13). The objective of this paper is to relate the oxidation products to the differences in the surface features of wood-based carbons. It is very important from the point of view of regeneration of exhausted carbons to 1 Permanent address: Institute for Sorption and Problems of Endoecology, General Naumova Str. 13, Kiev-164, 252180, Ukraine. 2 To whom correspondence should be addressed. E-mail: tbandosz@ scisun.sci.ccny.cuny.edu. Journal of Colloid and Interface Science 216, 360 –369 (1999) Article ID jcis.1999.6335, available online at http://www.idealibrary.com on 360 0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.