Environ. Sci. Technol. 2000, 34, 686-692
Analysis of the Relationship between H2S Removal Capacity and Surface Properties of Unimpregnated Activated Carbons FOAD ADIB,† ANDREY BAGREEV,‡ AND TERESA J. BANDOSZ* Department of Chemistry and the Center for Water Resources and Environmental Research, of the City College of New York, New York, New York 10031
The H2S breakthrough capacity was measured on two series of activated carbons of a coconut shell and a bituminous coal origins. To broaden the spectrum of surface features the samples were oxidized using nitric acid or ammonium persulfate under conditions chosen to preserve their pore structures. Then the carbons were characterized using Boehm titration, potentiometric titration, thermal analysis, temperature programmed desorption, sorption of nitrogen, and sorption of water. It was found that the choice of unimpregnated carbon for application as H2S adsorbent should be made based on parameters of its acidity such as number of acidic groups, pH of surface, amount of surface groups oxygen, or weight loss associated to decomposition of surface oxygen species. The results obtained from the analyses of six unimpregnated carbon samples suggest that there are certain threshold values of these quantities which, when exceeded, have a dramatic effect on the H2S breakthrough capacity.
Introduction Problems associated with removal of odor from air have become controversial issues, especially in urban areas. One of the leading malodorants arising from sewage treatment facilities is hydrogen sulfide (1). Activated carbons used for removal of H2S from sewage treatment plants are generally impregnated with caustic materials such as NaOH or KOH or are otherwise modified (2). Air currents around odorgenerating facilities are initially washed in scrubbers, during which they intake high levels of humidity, and are then blown through activated carbon vessels. The residual H2S quickly reacts with strong base and is immobilized. The presence of humidity facilitates the reaction (3, 4). The carbon bed is mostly used as a passive support for the caustic. Furthermore, due to the energetic reactions of caustic materials, the impregnation decreases the ignition temperature of the carbon and poses a hazard of self-ignition. The application of virgin (unimpregnated) activated carbon for removal of H2S from air has been investigated (5-12) ,and considerable removal capacities have been reported for carbons at temperatures around 200 °C. The * Corresponding author e-mail:
[email protected]. edu; telephone: (212)650-6017; fax: (212)650-6107. † Present address: The Graduate School of the City University of New York, Department of Civil Engineering. ‡ Present address: Institute for Sorption and Problems of Endoecology, Kiev, Ukraine. 686
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use of virgin activated carbon for H2S removal at the ambient temperatures, although the subject of recent studies (3, 4, 13-16), is not common yet. The major difficulty seems to lie with the fact that little is known with certainty about the mechanism of reaction on the carbon surface. Unlike caustics-impregnated carbons, the reactions on virgin carbons are very complex since they involve the broad spectrum of physical and chemical properties of the adsorbent. Most of the results reported so far have been based on an empirical analysis of specific types of carbon, and sometimes they appear to be contradictory (3-5, 9-12). The objective of this research is to identify the surface features of unimpregnated activated carbons of different origins that are important for their performance as adsorbents of hydrogen sulfide. Although many studies published so far have explored the effect of dynamic conditions such as concentration of H2S, content of air, and humidity, we underline here the role of carbon surface. The search for surface characteristics of unimpregnated carbons is driven by disadvantages of caustics-impregnated carbons that have been used so far as effective H2S adsorbents. These disadvantages are as follows: (i) low temperature of self-ignition, (ii) low capacity for physical adsorption, (iii) special precautions used with caustics, and (iv) high costs of adsorbents. The advantage of caustics-impregnated carbons used as H2S adsorbents is the fast kinetic of the oxidation reaction (2), but they are efficient only till KOH or NaOH is not exhausted (15). The oxidation occurs in the liquid phase where high pH (around 10) provides high concentration of hydrogen sulfide ions. The mechanism of H2S immobilization by the unimpregnated carbons significantly differs, but when the conditions for completion are attained (low concentration of hydrogen sulfide in air), the capacities of two categories of carbons are comparable (13). To broaden the spectrum of materials, the carbons were oxidized with nitric acid and ammonium persulfate (17-19). On the basis of the results obtained, the relationships between the observed H2S breakthrough capacity and the major properties of carbons are analyzed. We explore the general features that either govern the behavior of carbons as H2S adsorbents and/or are common for the two types of materials chosen for this study. The carbons were oxidized since all commercial carbons have some amount of oxygen-containing groups on the surface. Those groups are known to significantly change the catalytic activity and/or selectivity of carbons in various oxidation reactions (20, 21). Moreover, storage of activated carbon results in its oxidation, which affects the performance as adsorbent and catalyst.
Experimental Section Materials. Two activated carbons of different origins are chosen for this study. The first sorbent, RB3, is a peat-based, pellet-shaped carbon (pellet size: 2 × 1 mm), manufactured by Norit Americas Inc. The second, S208c, supplied by Waterlink Barnebey Sutcliffe, is a granular activated carbon (irregular granules of about 5 × 1 mm) obtained from coconut shell. Each carbon was washed in a Soxhlet apparatus to a constant pH of the leachate to remove the water-soluble impurities and then dried in oven at 120 °C. These initial samples are referred to as N0 (Norit) and S0 (Waterlink Barnebey Sutcliffe). The initial samples were oxidized with 15 M nitric acid with a ratio of 5 mL of acid/g of carbon at room temperature. Then the carbons were washed, dried, and designated as N1 and S1. Two other oxidized samples were prepared using a saturated solution of ammonium persulfate as an oxidant in 10.1021/es990341g CCC: $19.00
2000 American Chemical Society Published on Web 01/12/2000
1 M sulfuric acid at a ratio of 10 mL/g of carbon at room temperature. These samples are referred to as N2 and S2. Methods. H2S Breakthrough Capacity. Moist air (relative humidity 80% at 25 °C) containing 0.3% (3000 ppm) H2S was passed through a column of either palletized or granular carbon (length 370 mm, diameter 9 mm) at 0.5 L/min at room temperature. The column was widened at the top 60 mm to a diameter of 23 mm to minimize the wall effect. The H2S emission was monitored by an Interscan LD-17 H2S continuous monitor system (electrochemical detector) interfaced with a computer data acquisition program. The test was stopped at a breakthrough concentration of 500 ppm. The breakthrough capacity of carbon was then calculated using the integrated area above the breakthrough curve (difference between inlet 3000 ppm and breakthrough concentration curves), the mass of carbon, and the flow rate. Boehm Titration. The oxygenated surface groups were determined according to the method of Boehm (22). One gram of carbon sample was placed in 50 mL 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 5 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 neutralizes carboxyl and lactonic groups; and NaHCO3 neutralizes only carboxyl groups. 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). The instrument was set at the mode when the equilibrium pH was collected. Subsamples of the carbons of about 0.100 g in 50 mL of 0.01 M NaNO3 were placed in a container maintained at 25 °C and equilibrated overnight with the electrolyte solution. To eliminate the influence of atmospheric CO2, the suspension was continuously saturated with N2. The carbon suspension was stirred throughout the measurements. Volumetric standard NaOH (0.1 M) was used as titrant. The experiments were done in the pH range of 3-10. Each sample was titrated with base after acidifying the carbon suspension. pH of Carbon Surface. The sample of 0.4 g of dry carbon powder was added to 20 mL of water, and the suspension was stirred overnight to reach equilibrium. Then the sample was filtered, and the pH of the solution was measured. Thermal Analysis. Thermal analysis was carried out using TA Instruments thermal analyzer. The instrument settings were as follows: heating rate, 10 deg/min; and either air or nitrogen atmosphere with 50 mL/min flow rate. Temperature-Programmed Desorption. Temperatureprogrammed desorption (TPD) was conducted on a Pulse ChemiSorb 2705 (Micromeritics) using helium as a carrier gas. The dried carbon samples of about 0.2 g were placed in a U-shaped quartz reactor and heated at 120 °C for 3 h under a helium flow (40 mL/min). Then the flow was changed to 18 mL/min, and after the baseline was stabilized, the temperature was raised to 1000 °C at a rate of 10 deg/min. The decomposition products of surface oxygen-containing groups (CO, CO2, and H2O) were measured by a thermoconductivity detector. Three series of experiments were carried out for each carbon sample in order to distinguish the products of desorption. In the first step, the combined amount of CO, CO2, and H2O was determined. In the second step, the TPD curve for CO was measured using a cold trap with liquid nitrogen located before the detector. Then the curve representing CO and CO2 was recorded using a cold trap with dry ice. The instrument was calibrated by injection of precise volumes of the pure gases (CO and CO2).
FIGURE 1. H2S breakthrough results for N (A) and S (B) series of samples. Sorption of Nitrogen. Nitrogen isotherms were measured using an ASAP 2010 (Micromeritics) at -196 °C. Before the experiment, the samples were heated at 120 °C and then outgassed at this temperature under a vacuum of 10-5 Torr to constant pressure. The isotherms were used to calculate the specific surface area, SN2; micropore volume, Vmic; volume of mesopores, Vmes; and total pore volume, Vt. All of the above parameters were calculated using Density Functional Theory (DFT) (23, 24). Sorption of Water. Water sorption experiments were carried out at 20 °C using Micromeritics ASAP 2010 with a vapor sorption kit. The instrument was equipped with a homemade thermostated system controlled by a Fisher Scientific Isotemp Refrigerated Circulator. Samples were first heated at 120 °C and outgassed to 10-5 Torr. HPLC-grade water used as an adsorbate was free of any dissolved gases. Using ASAP 2010, water uptake is able to be measured starting from very low relative pressure (p/po ∼ 10-3) (4, 19, 25). Each point of the isotherm is recorded after equilibrium is reached. The isotherms were measured to relative pressure about 0.3.
Results and Discussion The breakthrough performance of initial and oxidized carbons is summarized in Figure 1. The breakthrough capacities measured for N0, N1, N2, S0, S1, and S2 are 95.8 ( 5.11, 47.7 ( 0.64, 28.2 ( 1.83, 111.8 ( 4.5, 14.5 ( 2.3, and 11.5 ( 1.4 mg of H2S/g of carbon, respectively. However, the time periods for initial carbons after which 500 ppm is reached are similar, so the shape of the breakthrough curves shows slightly better performance in the case of S0 material. Oxidation with nitric acid and ammonium persulfate significantly decreases the capacity, especially for S series of carbons. In both cases, ammonium persulfate oxidation caused the most dramatic decrease in the performance. This decrease was expected due to a decrease in pH as a result of the introduction of more acidic groups. It supports the proposed pathways of H2S oxidation on unimpregnated wood-based carbons presented elsewhere, where the pH of surface was pointed out as an important factor (16). To link the measured capacity of unimpregnated carbons to their VOL. 34, NO. 4, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. pH and Numbers of Functional Groups from Boehm Titration (mequiv/g) sample
pH
carboxylic
lactonic
phenolic
basic
N0 N1 N2 S0 S1 S2
7.72 4.47 3.59 6.89 4.58 3.66
0.00 0.31 0.56 0.17 0.29 0.65
0.12 0.41 0.81 0.02 0.32 0.42
0.00 0.32 0.33 0.12 0.35 0.65
1.00 0.45 0.49 0.51 0.26 0.12
surface features, the detailed analysis of carbons was carried out with the stress on features important for the sorption process such as surface chemistry and structural parameters. To check the extent of oxidation process, Boehm titration was performed (22). The used bases enable us to identify the number of carboxyls, lactones, and phenols. The results obtained along with the pH values of carbon surface are collected in Table 1. Comparison of the initial materials shows that the S0 sample is more heterogeneous than N0. In the case of N0 carbon, only lactonic groups represent strong acids. The samples also differ in the amount of basic species; the N0 sample has twice the basic groups as the S0. Oxidation with nitric acid significantly increases the number of acidic groups, decreases the number of basic groups for both carbons, and creates carboxylic acids in the case of N1 material. The decrease in the number of basic species as compared to the initial samples is likely due to the conversion of insoluble components of inorganic matter such as carbonates into soluble nitrates. On the other hand, oxidation with ammonium persulfate is more effective for S2 than for N2. The S2 sample shows a significant increase in the number of carboxylic and phenolic groups along with a 75% decrease in the number of basic species. More details about the strength of acidic and basic groups present on the surface of our carbons are derived from potentiometric titration experiments. The method of analysis, assumptions, and numerical approach used (SAIEUS) are described elsewhere (17, 18, 26). The distributions of acidity constants for species on the surface of carbons are shown in Figure 2. As proposed elsewhere, the species having pKa smaller than 8 are assigned to strong carboxylic acids, and those with pKa greater than 8 are assigned to phenols (18). The results obtained for initial carbons indicate the existence of a larger number of strong acids in the case of S0 as compared to N0, which is in agreement with the results of Boehm titration. However, the surface groups seem to be more heterogeneous in the case of the N0 sample where more peaks are revealed (at pKa about 4.2, 5.6, 6.8, and 7.5). After oxidation with nitric acid, a slightly larger number of groups is detected for N1 carbon with disappearance of the peak at pKa of about 6.8 present for the initial material (Figure 2A). This peak is likely related to the presence of inorganic matter, which was removed from the carbon during acid treatment. The changes in the acidity after oxidation are more pronounced in the S series, as shown from the Boehm titration results. The S1 sample shows a significant increase in the population of groups with pKa smaller than 5.5 (Figure 2B), along with the creation of new strongly acidic species, beyond the limits of our experimental window (17). After oxidation with ammonium persulfate, even though peaks at similar positions are revealed for both carbons, the number of groups is larger in the case of S2 material. Those results indicate similarity in the quality of groups created using the same oxidant (18). The results obtained from acid/base titration are in general agreement with the pH values collected in Table 1. They represent the average acidity of the carbon surface indicating that S0 is more acidic than N0, but after oxidation only small differences in pH remain between both carbons. 688
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FIGURE 2. Acidity constant distributions for N (A) and S (B) series of carbons. The presence of oxygen groups on the surface of carbons is also detected by means of TPD. The obtained TPD curves are collected in Figure 3. The most acidic groups (carboxyls, lactones) are desorbed as CO2 in the temperature range of 200-650 °C, while less acidic (phenols, carbonyls) and basic groups (pyrones) are desorbed mainly as CO or CO + CO2 at the temperature range between 500 and 1000 °C (27, 28). The amounts of volatile products formed during TPD experiments are collected in Table 3. As expected, the total amounts of CO and CO2 desorbed from oxidized carbons significantly increases as a result of oxidation (27). The presence of water is a result of the decomposition of carboxylic groups. The TPD data confirm the results of titrations and clearly show that oxidation with ammonium persulfate creates more oxygen groups on the carbon surface than does oxidation with nitric acid. The peak at 730 °C representing CO2 on the TPD curve for the N0 sample is likely related to the presence of carbonate impurities. The titration results showed that these species disappear after oxidation. Another method providing fast and meaningful information about surface chemistry is thermogravimetric analysis (TG). The TG curves along with the weight derivatives (DTG) obtained for both series of samples from the experiments performed in nitrogen appear in Figure 4. Although tests are carried out starting from room temperature, curves are plotted from 120 °C to take away the adsorbed water interference. The data are corrected accordingly so that the weight at 120 °C is considered as 100%. The weight losses for our carbons in the temperature ranges related to the presence of strong and weak acids (28, 29) are reported in Table 2. For the initial carbon, N0, the total weight loss is around 5% with a significant peak between 500 and 800 °C (Figure 3). This peak likely represents the decomposition of the basic species and/or inorganic matter that significantly contribute to the chemistry of this material (highest ash content). After oxidation, the total weight loss for both N1 and N2 carbons is similar, but small differences exist at the low temperature range where strong acids are expected to decompose (28). Although similar behavior is observed for the S series, no significant weight loss in the temperature range of 500-800
TABLE 3. Weight Loss in Different Temperature Ranges and Ash Content (%) sample
120-500 °C
500-800 °C
120-1000 °C
ash content
N0 N1 N2 S0 S1 S2
1.36 4.05 3.55 0.59 4.95 5.58
2.61 4.08 3.77 1.60 8.70 6.64
5.70 10.73 9.60 4.21 17.85 16.26
3.40 3.01 1.71 0.00 0.00 0.00
FIGURE 3. Thermodesorption spectra.
TABLE 2. Amounts of Gases Thermodesorbed at Different Temperature Ranges (mmol/g) 120-500 °C
500-1000 °C
sample
CO
CO2
H2O
CO
CO2
H2O
total
N0 N1 N2 S0 S1 S2
0.03 0.10 0.06 0.08 0.09 0.11
0.06 0.45 0.73 0.35 0.54 0.99
0.14 0.16 0.17 0.14 0.10 0.27
0.55 1.68 2.79 2.08 2.58 4.22
0.22 0.40 0.54 0.31 0.53 0.49
0.05 0.01 0.00 0.02 0.01 0.00
1.05 2.80 4.29 2.98 3.85 6.08
°C is found for the initial sample, S0 (Figure 4, Table 3). This suggests that unlike N0, S0 is low in basic species and/or inorganic matter (practically zero ash). In the process of hydrogen sulfide adsorption on activated carbons, structural parameters should play an important role (4, 5, 14). They are calculated from nitrogen isotherms
FIGURE 4. Thermal analysis results for TG (A and C) and DTG (B and D). measured at -196 °C using density functional theory (DFT) (23, 24). The specific surface area, volume of micropores (Vmic), volume of mesopores (Vmes), and total pore volume VOL. 34, NO. 4, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 4. Structural Parameters Calculated from Nitrogen Adsorption (DFT) sample
SN2 (m2/g)
Vmic (cm3/g)
Vmes (cm3/g)
Vt (cm3/g)
N0 N1 N2 S0 S1 S2
790 790 800 1030 970 880
0.31 0.31 0.33 0.39 0.37 0.32
0.06 0.08 0.05 0.02 0.03 0.04
0.47 0.50 0.49 0.52 0.50 0.46
FIGURE 6. Water adsorption isotherms at 20 °C. that the sorption capacity of S0 carbon is much higher than N0. This result is expected since the S carbon has more oxygenated groups and more developed microporosity (25, 26). After oxidation with nitric acid, water uptake significantly increases for the N1 carbon, while for the S1 only a small enhancement is observed. This difference follows the effect of oxidation described above. After oxidation with ammonium persulfate, the amounts adsorbed for N2 and S2 are almost the same due to similar surface chemistry and structural parameters (Table 1, Table 4).
FIGURE 5. Pore size distributions for the N (A) and S (B) carbon series. (Vt) are collected in Table 4. The initial carbons differ in surface areas and pore volumes; the porous structure of the S material is more developed than the N. Comparison of the data indicates that in the case of N carbon oxidation does not affect the pore structure, while for the S series a small decrease in the surface area and micropore volume is noted along with a slight increase in the volume of mesopores. Detailed changes in the pore structure of the S carbons are seen in the pore size distributions presented in Figure 5. It is interesting to note that while oxidation with nitric acid decreased the volume of pores smaller than 10 Å (a peak centered at around 7 Å), oxidation with ammonium persulfate left these pores almost intact with a significant decrease in the volume of pores represented by peak at about 15 Å. The results reported in the literature indicate that in most cases the presence of moisture enhances the adsorption and oxidation of hydrogen sulfide (3, 4, 13). According to the mechanism proposed by Hedden et al., dissociative adsorption of hydrogen sulfide has a significant impact on its oxidation (10). A similar conclusion was reported in our previous study (16). It is well-known that the affinity of activated carbon to retain water depends on the number of oxygenated groups present on the surface (19, 25, 30), which are considered as primary adsorption centers. Further adsorption of water on the already adsorbed water molecules leads to the condensation in small pores even at relatively low humidity. To investigate the affinity of our carbons to retain moisture, water adsorption isotherms were measured at relative pressure smaller than 0.3 at 20 °C. Figure 6 shows 690
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To identify the surface parameters crucial for the enhancement of the hydrogen sulfide adsorption capacity, the dependence of the measured capacity on the surface features described above was analyzed. No direct relationship between the performance of carbons as H2S adsorbents and the parameters of pore structure was found; however, higher micropore volume along with smaller pores result in enhanced capacity (Figure 1, Figure 5, and Table 4). As we indicated elsewhere for wood-based carbons (16), immobilization of hydrogen sulfide within pore structure is a result of its dissociation in the preadsorbed water film to HS- ion and adsorption of the latter followed by its oxidation. The extent of dissociation depends on the local pH in the pore system, which is governed by surface chemistry. Taking into account these findings, the dependence of breakthrough capacity on six quantities affecting dissociation of hydrogen sulfide in the pore structure has been analyzed based on the performance of the six samples studied. These are pH of carbon surface, number of basic groups, total number of acidic groups, water adsorption at 30% relative humidity, weight loss between 500 and 800 °C [associated with the destruction of weak acids such as phenols and carbonyls (20, 21)], and amount of surface groups’ oxygen (Figure 7). In all cases, a good linear correlation was not found. Analysis of the plots presented in Figure 7 suggests that there is a certain threshold value in each of those parameters beyond which significant changes in the capacity occur. An estimate of the threshold values may be as follows: a surface pH of more than 4.5, 0.5 basic groups/g of carbon, 0.85 mequiv of acidic groups/g of carbon, around 3 mmol of adsorbed water/g of the carbon material, around 3% of weight loss between 500 and 800 °C, and 2 µmol/m2 of surface groups oxygen. An increase in pH and the number of basic groups beyond the reported values increase the capacity, whereas an increase in the other four parameters (number of acidic groups, weight loss, surface groups oxygen, and water adsorption) significantly decreases the performance of carbons as H2S adsorbents.
FIGURE 7. Dependence of H2S breakthrough capacity on: (A) pH of carbon surface, pH of carbon suspension (with dissolved H2S); (B) number of basic groups; (C) number of acidic groups; (D) water uptake at p/po ) 0.3 and 20 °C; (E) weight loss at 500-800 °C; (F) amount of surface groups oxygen. To explore the existence of a threshold in the pH value of the carbon surface at our experimental conditions, simplified calculations were performed based on the mechanism of H2S oxidation at low temperature (4.2, concentration of HS- in the adsorbed state will be equal to H2S in a gas phase (100% adsorption + dissociation), which is required for effective H2S removal. The dependence of the ratio of concentration VOL. 34, NO. 4, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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of HS- in the adsorbed state to the concentration of H2S in a gas phase (HS-ads/H2Sgas) on the pH of carbon suspension in water (with dissolved H2S) calculated using eq 6 is plotted in Figure 7A. The estimated value of pH is lower than the first dissociation constant (pKa) of H2S. This supports our hypothesis presented elsewhere that a pH value only high enough for mild dissociation of H2S is sufficient for its effective removal (16). In such a situation physically adsorbed H2S, upon dissociation in the water film to hydrogen sulfide ion (HS-), becomes vulnerable to attack by oxygen and can be oxidized to elemental S and sulfur oxides (8, 16). As seen from Figure 7A, both calculated and experimental results show the same trend expressed by a sharp increase of adsorption capacity in the range of pH between 4 and 5. Although calculated above threshold pH is equal to 4.2, the threshold estimated from the data presented in Figure 7A seems to be higher than 4.5. The discrepancy with the calculated value of pH is due to the fact that pH >4.2 was obtained from the simplified expression without the exact value of the adsorption constant, KH. The calculation presented above suggests the existence of the threshold; however, it cannot be taken as an absolute value (N1 and S1 carbons have pH >4.2 but are not able to effectively remove hydrogen sulfide). When the number of basic groups increases, the capacity increases (Figure 7B) due to enhancement in the hydrogen sulfide dissociation. However, the capacity increases only in accordance with the pore volume of carbons (Table 4), which is the limiting factor for the adsorption process. When the number of acidic groups exceeds 0.85 mequiv/g (Figure 7C), the surface becomes too acidic to promote the dissociation of hydrogen sulfide, and only its physical adsorption can occur (not high at room temperature). With such a high number of groups, sorption of water is enhanced even at low relative pressure (25, 26). Although tests carried out under dry conditions have generally demonstrated negligible capacities (13), it is seen from our experiments that water uptake higher than the threshold value does not increase the breakthrough capacities of unimpregnated carbons (Figure 7D). In the case of carbons chosen for this study, the affinity for water adsorption should not be greater than 3 mmol/g to reach the maximum capacity. It is likely that, when the affinity of carbons to adsorb water is very high, the small pores are filled by condensed adsorbate (25, 26) and the direct contact of HS- with carbon surface in the smallest pores is limited. Another two parameters related to carbon acidity analyzed in this study are the weight loss from TA experiments associated with weak acids (Figure 7E) and the amount of surface groups oxygen calculated from TPD (Figure 7F). As in the cases presented above, threshold values can be noticed. They are related to the acidity of surface and its effect on dissociation of hydrogen sulfide. Analysis of the results obtained for two series of carbons (coconut shell and bituminous coal origins) as adsorbents of hydrogen sulfide indicates that the choice of unimpregnated activated carbons for application as H2S adsorbents should be made based on surface parameters related to its acidity. While at pH values above 5 considerable capacities are observed, a more acidic environment, which decreases the dissociation of H2S, quickly suppresses the process. The consequent decrease in the concentration of HS- ions inhibits H2S adsorption and its further oxidation. The capacity significantly drops when the number of acidic groups exceeds approximately 0.85 mequiv/g of carbon. Although some humidity is crucial for effective removal of H2S, water effect beyond this threshold value is secondary to the surface chemistry of carbon. A reduced capacity is observed when the affinity for water adsorption is too high and exceeds about 3 mmol/g of carbon. This is due to the filling of small pores 692
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and its negative effect on the contact of dissociated HS- ions with the carbon surface in micropores. Although we realize that the number of data analyzed in this paper is too small to formulate the well-defined new carbon specification for industrial applications, the indication of the existence of the threshold in surface acidity has been demonstrated. This would suggest that, if unimpregnated carbons are to be used as hydrogen sulfide adsorbents, their long storage after the manufacturing process is not recommended due to air oxidation of the carbon surface and a decrease in the surface pH.
Acknowledgments This research was supported by The New York City Department of Environmental Protection. The authors thank Professor Amos Turk for encouragement and discussions. The help of Ms. Anna Kleyman in performing experiments is appreciated. T.J.B. wishes to thank Dr. Jacek Jagiello for providing SAIEUS program.
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Received for review March 25, 1999. Revised manuscript received August 27, 1999. Accepted December 1, 1999. ES990341G
