Effect of Surface Characteristics on Adsorption of Methyl Mercaptan on

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SEPARATIONS Effect of Surface Characteristics on Adsorption of Methyl Mercaptan on Activated Carbons Svetlana Bashkova,† Andrey Bagreev,‡ and Teresa J. Bandosz*,‡ Department of Chemistry and The International Center for Water Resources and Environmental Research of The City College of New York and The Graduate School of The City University of New York, New York, New York 10031

Activated carbons of different origins were studied as methyl mercaptan adsorbents in wet conditions. To broaden the spectrum of surface acidity, carbons were oxidized with hydrogen peroxide. The surface properties of adsorbents were evaluated using nitrogen adsorption, Boehm titration, potentiometric titration, and thermal analysis. The study was focused on the role of surface chemistry and porosity in the adsorption/oxidation of methyl mercaptan. The results showed that the amount adsorbed depends on the surface features. Although the main product of methyl mercaptan oxidation is dimethyl disulfide, in some cases, oxidation proceeds further, likely leading to the formation of methyl methanethiosulfonate as detected using GC/MS. Introduction Mercaptans (thiols) are sulfur-containing organic species that are well-known for their disagreeable odors of rotten cabbage. Among thiols, methyl mercaptan (CH3SH) is one commonly used to “odorize” natural gas. This is done because of its low odor threshold (one part in 5 × 1010 parts1). At high concentration, methyl mercaptan has a significant toxicity. To remove methyl mercaptan from air, activated carbons have been applied.2-8 Methyl mercaptan has been reported to be converted to dimethyl disulfide (DMDS) on the surface of activated carbons. Dimethyl disulfide is much more strongly adsorbed at room temperature then methyl mercaptan (MM) because of the differences in the physicochemical constants of these species.9 Moreover, the oxidation of methyl mercaptan to dimethyl disulfide can be enhanced by the presence of different functional groups on the surface and metal ions such as iron present as constituents of ash.2-5,10 It has also been demonstrated that activated carbons impregnated with various chemicals such as potassium iodide, potassium iodate, potassium carbonate, and ammonia work well as MM adsorbents.5 The adsorption of methyl mercaptan in wet conditions on virgin activated carbon fibers in the presence of hydrogen sulfide and dimethyl sulfides has been studied and described in the literature.2-8 According to the authors, in the first step disulfides are created on the carbon surface, and then the formation of sulfonic acid as the final product of the oxidation process takes place. * To whom correspondence should be addressed. E-mail: [email protected]. Tel.: (212) 650-6017. Fax: (212) 650-6107. † The Graduate School of The City University of New York. ‡ Department of Chemistry and The International Center for Water Resources and Environmental Research of The City College of New York.

When the adsorption of mercaptans is performed under dry conditions at elevated temperatures (∼200 °C), (CH3)2S2, H2O, and small amounts of S8 and CO2 are expected as oxidation products.3 Although the results described in the literature clearly indicate the importance of water in the oxidation of methyl mercaptan, the exact role of the carbon surface from the point of view of porosity and surface chemistry has not been discussed in detail. An objective of this study is to link the performance of activated carbons as methyl mercaptan adsorbents to their surface characteristics. The carbons used are washed before the experiments to ensure that the breakthrough capacity is not affected by water-soluble species. Experimental Section Materials. Four commercial activated carbons of various origins were used in this study. They are as follows: BAX (wood-based, Westvaco), S208 (coconut shell, Waterlink Barnabey and Sutcliffe), BPL (bituminous coal, Calgon), and PCB (coconut shell, Norit). Before further testing, the carbons were washed in a Soxhlet apparatus to a constant pH of a lechate. The subsamples were oxidized with hydrogen peroxide. Briefly, 10 g of carbon was treated with 100 mL of 35% H2O2 and left on the stirrer for 22 h. Then, the samples were washed in a Soxhlet apparatus to remove excess oxidant and other water-soluble species. After oxidation, the samples are referred to as BAX-O, S208-O, BPL-O, and PCB-O. The exhausted samples after methyl mercaptan adsorption and purging with 80% humidified air are designated with an additional letter E. One percent methyl mercapatan in nitrogen was used for the preparation of air-MM mixtures. Methods. CH3SH Breakthrough Capacity. Dynamic tests were carried out at room temperature to evaluate the capacity of the sorbents for CH3SH removal under

10.1021/ie020137t CCC: $22.00 © 2002 American Chemical Society Published on Web 07/24/2002

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wet conditions. Adsorbent samples were ground (1-2mm particle size), packed into a glass column (length 370 mm, internal diameter 9 mm, bed volume 6 cm3), and prehumidified with moist air (relative humidity 80% at 25 °C) for 1 h. The amount of water adsorbed was estimated from the increase in the sample weight. Moist air (relative humidity 80% at 25 °C) containing 0.3% (3000 ppm) CH3SH was then passed through the column of adsorbent at 0.5 L/min. The flow rate was controlled using Cole Parmer flow meters. The breakthrough of CH3SH was monitored using a MicroMax monitoring system (Lumidor) with an electrochemical sensor. The test was stopped at the breakthrough concentration of 50 ppm. The adsorption capacities of each sorbent in units of milligrams of CH3SH per gram of carbon were calculated by integration of the area above the breakthrough curves, using the CH3SH concentration in the inlet gas, flow rate, breakthrough time, and mass of sorbent. For each sample, the CH3SH test was repeated at least twice. The determined capacities agreed to within 4%. The amount of weakly adsorbed CH3SH was evaluated by purging the adsorbent column with breathinggrade air at 0.35 L/min immediately after the breakthrough experiment. The CH3SH concentration was monitored until it dropped to 1 ppm. The process took about up to 3 h depending on the type of adsorbent. pH of Carbon Surface. Carbon powder (0.4 g) was placed in 20 mL of water and equilibrated overnight. Then, the pH of the suspension was measured using an Accumet Basic pH meter (Fisher Scientific). For the exhausted samples, an additional letter E is added (pHE). Sorption of Nitrogen. Nitrogen isotherms were measured using an ASAP 2010 instrument (Micromeritics) at -196 °C. Before the experiment, the samples were heated at 120 °C and then outgassed overnight at this temperature under a vacuum of 10-5 Torr to constant pressure. The isotherms were used to calculate the specific surface areas (S), micropore volumes (Vmic), total pore volumes (Vt), average micropore sizes (Lmic),11 and pore size distributions (PSDs) using density functional theory (DFT).12,13 This statistical mechanical approach is based on the calculation of equilibrium density profile for an adsorbed phase by minimization of a free energy of the system in regularly shaped pores of different sizes. Thermal Analysis. Thermal analysis was carried out using TA Instruments thermal analyzer. The instrument settings were a heating rate of 10 °C/min in nitrogen atmosphere with a flow rate of 100 mL/min. Boehm Titration. One gram of carbon sample was placed in 50 mL of each of the following 0.05 N solutions: sodium hydroxide, sodium carbonate, sodium bicarbonate, and hydrochloric acid. The vials were sealed and shaken for 24 h; then, 10 mL of each filtrate was pipetted, and the excess of base or acid was titrated with HCl or NaOH. The numbers of acidic sites of various types were calculated under the assumption that NaOH neutralizes carboxyl, phenolic, and lactonic groups; Na2CO3, carboxyl and lactonic; and NaHCO3, only carboxyl groups.14 The number of surface basic sites was calculated from the amount of hydrochloric acid that reacted with the carbon. Potentiometric Titration. Potentiometric titration measurements were performed with a DMS Titrino 716 automatic titrator (Metrohm).15-17 The instrument was

set in the mode for the collection of equilibrium pH values. Subsamples of the carbons at concentrations of about 0.100 g in 50 mL of 0.01 M NaNO3 were placed in a container thermostated at 20 °C and equilibrated overnight with the electrolyte solution. To eliminate the influence of atmospheric CO2, the suspensions were continuously saturated with N2. The carbon suspensions were stirred throughout the measurements. Volumetric standard NaOH (0.1 M) was used as the titrant. The experiments were conducted in the pH range of 3-10. Each sample was titrated with base after acidification of the carbon suspension. GC/MS. The GC/MS study was performed using a Shimadzu gas chromatograph/mass spectrometer (model QP 5050). The separation was done on a Shimadzu XTI-5 column (bonded 5% phenyl) that was 30 m in length, 0.25 mm in internal diameter, and 0.025 µm in df. The GC oven heating program was as follows: 30 °C for 10 min, then heating from 30 to 250 °C at a rate of 10 °C/min and holding at 250 oC for 10 min. The injector temperature was 60 °C. The results were analyzed using an unrestricted library search. The samples for analysis were extracted by placing 1.2 mL of carbon in a vial to which 2 mL of methanol was added. Then, the suspensions were heated at 60 °C for 1 h. After separation of the liquid phase from the carbon, 0.5 µL of the extracted solution was injected into the GC column. The mass spectra were collected for m/z between 12 and 500. Results and Discussion Methyl mercaptan breakthrough curves obtained for the samples studied are collected in Figure 1. Judging from the breakthrough times, the performances of the carbons differ significantly. It is interesting that, for S208-O, an increase in the breakthrough time was found in comparison with that of the initial sample. The breakthrough time for BAX-O remained short and almost unchanged compared to the initial sample, whereas for BPL-O, a decrease was found. A summary of the breakthrough experiments is presented in Table 1. BPL and S208 carbons have high capacities. It is worth mentioning that the capacities of samples used in this study are smaller than those used “as received” without washing (difference of about 10100%) described elsewhere.8 This is likely caused by the removal of water-soluble inorganic matter, which might catalyze the oxidation of methyl mercaptan to DMDS and/or cause the basic pH of the carbon surface to be maintained. As indicated elsewhere, the high pH facilitates the dissociation of CH3SH as well as its oxidation.8 After treatment with hydrogen peroxide, the MM breakthrough capacity of the S208 sample increased by about 25%, which might be related to changes in the carbon surface chemistry. For BPL-O, the capacity was 2-fold smaller than that of the initial sample. Indeed the pH of S208-O increased compared to that of the initial sample, whereas the pH of BPL-O decreased by 1 unit. As indicated above, a small increase in the MM breakthrough capacity was also found for PCB-O. In the case of the BAX carbon, the most visible changes in the surface chemistry occurred after oxidation. They were manifested by a decrease of more than 3 units in the pH value of the surface. Oxidation also caused an increase in the amount of water adsorbed during prehumidification as a result of the increase in the degree of hydrophilicity of the carbon surface caused by the

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Figure 2. pKa distributions. Table 2. Results of Boehm Titration (mmol/g) Figure 1. Methyl mercaptan breakthrough curves for the carbons studied. Table 1. Surface pHs, Amounts of Water Preadsorbed, Breakthrough Times, and CH3SH Breakthrough Capacities for the Materials Studied

sample

pH

water prebreakCH3SH CH3SH adsorbed through capacity desorbed (mg/g) pHE (mg/g) time (min) (mg/g)

BAX BAX-O BPL BPL-O S208 S208-O PCB PCB-O

7.20 4.07 7.41 6.60 7.47 8.13 7.57 6.35

6.78 3.30 3.82 3.92 5.94 5.75 5.34 4.57

163.4 168.8 89.9 99.6 92.6 98.1 78.2 94.5

13 14 155 79 143 184 59 65

28.2 23.0 216.8 104.3 162.2 203.2 68.2 72.9

0.48 3.56 0.03 0.05 0.04 0.04 0.11 0.23

formation of new oxygen-containing groups.18 The amount of MM desorbed during purging was relatively high for BAX carbons (more than 10%). For BPL, S208, PCB, and their oxidized counterparts, only traces of methyl mercaptan were desorbed, suggesting strong adsorption of CH3SH and/or oxidation products. To link the behavior of carbons as methyl mercaptan adsorbents to the specific features of the materials, a chemical analysis of the surfaces was done using Boehm titration14 and potentiometric titration.15-17 The results of the former method are presented in Table 2. Oxidation with hydrogen peroxide is considered to be a weak oxidation that preserves the porous structure of the carbons.19 Indeed, in the cases of the BPL, PCB, and S208 carbons, only small changes in surface chemistry were found. They are demonstrated mainly in an increase in the number of carboxylic and lactonic groups. For BPL-O, a decrease in the number of phenols can be noticed, whereas the number of basic groups is

sample BAX BAX-O BPL BPL-O S208 S208-O PCB PCB-O

carboxylic lactonic phenolic acidic basic total groups groups groups groups groups groups 0.20 1.20 0.00 0.10 0.00 0.05 0.00 0.05

0.21 0.05 0.05 0.20 0.05 0.10 0.00 0.18

0.49 1.15 0.45 0.20 0.31 0.25 0.25 0.28

0.90 2.40 0.50 0.50 0.36 0.40 0.25 0.51

0.35 0.60 0.40 0.40 0.45 0.40 0.45 0.35

1.25 3.00 0.90 0.90 0.81 0.80 0.70 0.86

unchanged. For the S208-O carbon, in addition to an increase in the number of strong acid groups, a small increase in the number of basic groups occurred. For PCB-O, only slight changes occurred in comparison with the initial sample. On the other hand, the BAX carbon is very susceptible to oxidation.20,21 For this material, the number of carboxylic and phenolic groups increased significantly; the total number of acidic groups increased nearly 3-fold, and the number of basic groups increased nearly 2-fold. These changes are reflected as changes in the pH values listed in Table 1. Figure 2 presents the pKa distributions of the species present on the carbons’ surfaces calculated using the SAIEUS22 procedure, which applies regularization combined with nonnegativity constraints. The choice of the degree of regularization/smoothing is based on an analysis of a measure of the effective bias introduced by the regularization and a measure of the uncertainty of the solution. SAIEUS has been tested using simulated data and experimental titration data of organic standards, and it was demonstrated that this method can completely resolve peaks that are less than 1 pKa unit apart.15 As expected, the curves obtained from the Boehm titrations of BPL and BPL-O do not differ significantly. The main effect of oxidation is an increase in the numbers of species having a pKa of about 7.6.

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They can be considered as carboxylic acids.23,24 For the S208 series of samples, the trend is similar to that for BPL; however, the pKa of weakly acidic groups is slightly larger after oxidation. For the PCB series, only a small increase in the number of groups is noticed, especially in the range of strong acids. Noticeable differences exist between the distributions obtained for BAX and BAX-O. Although the peaks have similar positions (similar pKa’s), the number of groups significantly increased after oxidation, especially in the range of carboxylic acids (pKa < 8), which was also observed from the results of the Boehm titration. The amount of phenols, represented by the last peak, increased slightly, which is also consistent with the data listed in Table 2. Changes in surface chemistry likely affect the adsorption of methyl mercaptan as seen from Table 1. As demonstrated elsewhere,2-8 MM when adsorbed on activated carbons is oxidized to DMDS. The proposed reaction pathways (arrows represent chemical equilibrium) should depend on the apparent pH of the system because MM is able to dissociate (pKa ) 10.3)

At pH < pKa KH

CH3SHgas 98 CH3SHads KS

CH3SHads 98 CH3SHads-L

(1) (2)

Figure 3. Mass spectra for species extracted from S208-E: (A) dimethyl disulfide, (B) methyl methanethiosulfonate.

2CH3SHads + O*ads 98 CH3SSCH3 ads + H2O (3)

Another possible scenario is the formation of methyl methanethiosulfonate (C2H6O2S2) (MMTS) through the oxidation of DMDS

KR1

At pH > pKa Ka

CH3SHads-L 98 CH3S-ads + H+ KR2

(4)

2CH3S-ads + O*ads 98 CH3SSCH3 ads + O2-

(5)

2H+ + O2- f H2O

(6)

C2H6S2ads + 2O*ads + H+ f CH3SO2Hads + CH3S/ads (7)

(8)

It is important to indicate here that sulfonic acid is a very strong acid and its effect on pH should be greater than a decrease of 3 or 4 pH units.

(9)

or the disproportionation of sulfonic acid1

3CH3SO2H f C2H6O2S2 + CH3SO3H + H2O

where CH3SHgas, CH3SHads-L, CH3SHads, and CH3SSCH3 ads represents the species in the gas, liquid, and adsorbed phases, respectively; KH, KS, Ka, KR1, and KR2 are the equilibrium constants for related processes (adsorption, gas solubility, dissociation, and surface reactions, respectively); and O*ads is dissociatively adsorbed oxygen. Analyzing the equilibria of these reactions, it is apparent that, when pH > pKa, the concentration of thiolate ions, CH3S-ads, is much greater than the concentration of CH3SHads, leading to an increase in the amount of methyl mercaptan adsorbed on the carbons. A noticeable decrease in the pH for all carbons after MM adsorption suggests that DMDS is not the only product of the surface reactions. Because water is present in the system and the carbon surface is able to produce and sustain active oxygen radicals and hydroxyl radicals,2,25 the further oxidation of disulfides to Rsulfinic acid, CH3SO2H, and the formation of methane sulfonic acid are possible

CH3S*ads + 3O*ads + H+ f CH3SO3Hads

C2H6S2 + 2O* f C2H6O2S2

(10)

Taking into account the possible reaction products and their apparent effect on the pH of carbon, it is also possible that the decrease in pH is an effect of interactions between the carbon surface and sulfonic acid. As a result, some derivatives of sulfonic acid might be introduced or strongly bonded to the carbon matrix, thus decreasing its pH. A GC/MS analysis of the species extracted from the carbon surface with methanol supports the proposed reaction pathways. Two peaks were observed on the chromatograms for the majority of samples: the first with a retention time equal to 7.8 min and the second with a retention time of 18.8 min. The intensity of the first peak is significantly (10-50 times) greater than that of the second peak, indicating the formation of DMDS as the main reaction product. The examples of mass spectra for two peaks revealed in the chromatogram of S208 are collected in Figure 3. An analysis of the fragment composition and an unrestricted library search indicate the presence of DMDS as the first peak and MMTS as the second peak with similarity indices of 90 and 77, respectively. The products of the surface reactions were evaluated using the TA method.8,20,21,26,27 The DTG curves for our carbons are collected in Figures 4 and 5. The common features in the DTG curves for all samples are two peaks at temperatures lower than 300 °C. The first, centered at about 80 °C, represents the desorption of water,8 and the second, centered between 180 and 300 °C (depending

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Figure 4. DTG curves in nitrogen for the BAX and PCB series carbons.

on the sample), is assigned to the desorption of DMDS. For all oxidized samples, the intensity of the first peak increases with increasing degree of hydrophilicity of the carbon and its higher affinity to adsorb water.18 A comparison of the second peaks for the BPL, PCB, and S208 series exhausted samples, initial and oxidized, reveals differences in their positions and shapes. For the S208 samples, the peak is complex and can be considered as an overlapping of two peaks, one centered at about 220 °C and the second centered at 300 °C. After oxidation, the intensity of the first component of the peak is slightly changed, whereas the shoulder at 300 °C almost disappears. This suggests that the formation of strongly adsorbed compounds (desorbed at higher temperatures) is affected by the oxidation of the carbon surface. A behavior slightly different from that for the S208 series of samples is observed for the BPL carbons. For BPL-O, the intensity of the water desorption peak increases, and the intensity of the DMDS peak decreases, which is consistent with the breakthrough results presented in Table 1. The intensity of the high temperature shoulder does not change after oxidation. This shoulder, although present, is not as well pronounced as that for S208. This species is likely MMTS because it was detected using GC/MS analysis of the adsorption products. Another important difference between the S208 and BPL carbons is the width of the peak, which spreads between 100 and 350 °C. The lower temperature of desorption for BPL suggests weaker adsorption of DMDS than in the case of S208, which might be caused by differences in the pore sizes of the two carbons (Figure 6). The PSD for BPL is broader

Figure 5. DTG curves in nitrogen for the S208 and BPL series of carbons.

than that for S208, and the contribution of larger pores is significant. It is well-known that the adsorption forces in such pores are weaker than those in small micropores. An analysis of the dependence of the position of the main peak on the amount of MM adsorbed shows a decreasing trend with increasing MM uptake. Indeed, the large amount adsorbed must be related to filling of the larger pores by DMDS. DMDS can be removed more easily from those pores than from small micropores. The lowest temperature of the peak is expected to be close to the boiling point of DMDS. For PCB, DMDS desorbs at the highest temperature compared to the other carbons studied, which is likely due to adsorption in the small pores of this carbon (Figure 7). It is interesting that the shoulder observed for BPL and S208 can be also noticed on the DTG curves for the PCB series of carbons and that its intensity is relatively high in comparison with that of the peak representing DMDS. For this carbon, a noticeable decrease in the pH after exhaustion is found, despite the relatively low adsorption of MM. For the PCB series of carbons, differences in the DTG curves exist between the initial and oxidized samples. The curve for the initial sample has high-temperature peaks centered at about 350 and 450 °C. Those peaks are not present in the DTG curves for the exhausted oxidized sample. Judging from the positions of these peaks and the pH differences between the initial and exhausted carbons, it is likely that these peaks represent elemental sulfur adsorbed in small pores. After oxidation, the surface losses the ability to oxidize MM to sulfur polymers, and only organic compounds can be detected as the products of surface reactions.

Ind. Eng. Chem. Res., Vol. 41, No. 17, 2002 4351 Table 3. Balance of Sulfur Species from the Breakthrough Experiments and Thermal Analysis sample

MM ads (%)

∆W 100-550 °C (%)

BAX-E BAX-OE BPL-E BPL-OE S208-E S208-OE PCB-E PCB-OE

3.3 2.6 21.7 10.4 16.2 20.4 6.8 7.3

1.3 0.0 22.6 13.3 15.9 19.1 7.7 8.00

Table 4. Structural Parameters of Carbons Calculated from Adsorption of Nitrogen

Figure 6. Changes in PSDs for the S208 and BPL series of carbons before and after adsorption of methyl mercaptan.

Figure 7. Changes in PSDs for the BAX and PCB series of carbons before and after adsorption of methyl mercaptan.

The validity of the TA method for evaluating the quantity of adsorbed compounds is demonstrated as a

sample

S (m2/g)

Vmic (cm3/g)

Vt (cm3/g)

Vmic/Vt

Lmic (Å)

BAX BAX-E BAX-O BAX-OE BPL BPL-E BPL-O BPL-OE S208 S208-E S208-O S208-OE PCB PCB-E PCB-O PCB-OE

1351 1351 1055 1009 1000 856 1016 866 1006 844 990 823 964 877 853 797

0.50 0.50 0.44 0.40 0.40 0.34 0.40 0.34 0.42 0.34 0.37 0.34 0.40 0.36 0.34 0.32

1.29 1.32 0.77 0.71 0.71 0.59 0.72 0.59 0.45 0.36 0.39 0.34 0.43 0.41 0.37 0.35

0.39 0.38 0.57 0.57 0.57 0.60 0.56 0.58 0.93 0.94 0.95 0.99 0.93 0.86 0.91 0.93

13.2 13.2 12.1 11.9 11.6 11.7 11.7 11.6 9.9 9.4 8.8 9.5 9.3 10.2 9.7 9.8

balance of sulfur species summarized in Table 3. The last column lists the amount desorbed corrected for the initial weight of carbon. The data demonstrate good agreement between the adsorbed amount of methyl mercaptan calculated from breakthrough capacity experiments and the amount desorbed evaluated by the TA approach. The greatest discrepancy occurs for the BAX carbons because of the uncertainty in the TA data caused by the small amount adsorbed. To better understand the differences in the DTG results between the S208, PCB, and BPL series of samples, the surface structure was analyzed in terms of the adsorption of nitrogen at its boiling point. From nitrogen adsorption isotherms, pore size distributions (PSDs) were calculated using density functional theory.12,13 The results are presented in Figures 6 and 7 and Table 4. The data clearly show that BPL differs significantly from PCB and S208. Comparison of the initial and oxidized samples indicates that oxidation changes the PSDs for all carbons slightly. Nevertheless, S208-O and PCB-O remain predominantly microporous after oxidation, with almost all pores smaller than 20 Å, whereas BPL-O has a significant volume in mesopores. The surface area of BPL is about 20% smaller than that of S208; however, the total pore volume of BPL is almost twice greater than that of S208. A similar trend is preserved after oxidation. This might explain the weaker adsorption forces for dimethyl disulfide on BPL demonstrated by the shift to lower temperature of the second DTG peak. DMDS adsorbed in larger pores is easier to remove than DMDS adsorbed in pores similar in size to the adsorbed molecule. After oxidation, for most carbons, the maximum of the peak shifts to higher temperature, which might be related to an increase in the volume of small pores that can accommodate DMDS. The smaller amount adsorbed is likely the result of a decrease in the surface pH as indicated above. The PCB

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series of samples is the most microporous, and for these materials, the DMDS desorption peaks are shifted to higher temperatures compared to the peaks for the other carbons. Although, from the point of view of porosity, BAX resembles BPL more than S208, the adsorption of MM is very small. A significant difference between this carbon and the other three is its very small volume in pores smaller than 10 Å and the noticeable decrease in total pore volume and surface area after oxidation. Compared to the other carbons, the average micropore size is the greatest, and the specific adsorption energy is the lowest. Oxidation also results in a significant increase in the volume of very small pores. Although the pH of BAX is only slightly lower than that for BPL, it might be below the threshold value of pH for dissociation of methyl mercaptan on activated carbon that ensures an efficient removal process.27 After exhaustion, the volumes of small pores decreased for all carbons. This indicates that degassing was not able to remove some strongly adsorbed oxidation products. A significant decrease in the volume of pores smaller than 10 Å might be the result of incorporation of sulfur- and oxygen-containing species as a result of the reaction of carbon with sulfonic acid and/ or the formation of MMTS strongly adsorbed in the small pores. Conclusions The results reported in this paper indicate that the mechanism of methyl mercaptan adsorption on activated carbons is affected by both the surface chemistry of the adsorbents and the carbon pore structure. Because the adsorption occurs under wet conditions, surface chemistry controls the extent of dissociation of methyl mercaptan, which governs its effective removal. Porosity becomes important for strong adsorption of the product of methyl mercaptan oxidation, which is mainly dimethyl disulfide. Acknowledgment Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the ACS, for support of this research (Grant ACS-PRF 35449-AC5). The authors are grateful to Ms. Anna Kleyman and Mr. Tamer Amer for experimental help. T.J.B. thanks Dr. Jacek Jagiello for providing the SAIEUS software. Literature Cited (1) Karchmer, J. H. The Analytical Chemistry of Sulfur and Its Compounds; Wiley: New York, 1970; Vol. 1, p 466. (2) Katoh, H.; Kuniyoshi, I.; Hirai, M.; Shoda, M. Studies of the Oxidation Mechanism of Sulfur-Containing Gases on Wet Activated Carbon Fibre. Appl. Catal. B: Environ. 1995, 6, 255. (3) Dalai, A. K.; Tollefson, E. L.; Yang, A.; Sasaoka, E. Oxidation of Methyl Mercaptan over an Activated Carbon in a FixedBed Reactor. Ind. Eng. Chem. Res. 1997, 36, 4726. (4) Turk, A.; Sakalis, E.; Lessuck, J.; Karamitsos, H.; Rago, O. Ammonia Injection Enhances Capacity of Activated Carbon for Hydrogen Sulfide and Methyl Mercaptan. Environ. Sci. Technol. 1989, 23, 1242. (5) Shin, C. S.; Kim, K. H.; Ryu, S. K. Adsorption of Methyl Mercaptan and Hydrogen Sulfide on Impregnated Activated

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Received for review February 14, 2002 Revised manuscript received May 29, 2002 Accepted June 9, 2002 IE020137T