Environ. Sci. Technol. 2005, 39, 6217-6224
Inorganic-Organic Phase Arrangement as a Factor Affecting Gas-Phase Desulfurization on Catalytic Carbonaceous Adsorbents ADIL ANSARI AND TERESA J. BANDOSZ* Department of Chemistry, The City College of New York, 138th Street and Convent Avenue, New York, New York 10031
Dried sewage sludge was physically mixed with waste paper (paper-to-sludge ratios from 25% to 75%). To increase the catalytic activity, from 1% to 6% calcium hydroxide was added to the mixtures. Then the precursors were carbonized at 950 °C. The performance of materials as H2S adsorbents was tested using a home-developed dynamic breakthrough test. The samples, before and after the adsorption process, were characterized by adsorption of nitrogen, potentiometric titration, thermal analysis, XRF, and SEM. Differences in the performance were linked to the surface properties. It was found that mixing paper with sludge increases the amount of H2S adsorbed/oxidized in comparison with that adsorbed/oxidized by the adsorbents obtained from pure precursors (sludge or waste paper) and the capacity is comparable to those of the best activated carbons existing on the market. Although both sewage sludge and waste paper provide the catalytic centers for hydrogen sulfide oxidation, the dispersion of the catalyst and its location within accessible pores is an important factor. The presence of cellulose in the precursor mixture leads to the formation of a light macroporous char whose particles physically separate the inorganic catalytic phase of the sewage sludge origin, decreasing the density of the adsorbent and thus providing more space for storage of oxidation products. This, along with calcium, contributes to a significant increase in the capacity of the materials as hydrogen sulfide adsorbents. On their surface about 30 wt % H2S can be adsorbed, mainly as elemental sulfur or sulfates. The results demonstrate the importance of the composition and arrangement of inorganic/ organic phases for the removal of hydrogen sulfide. The interesting finding is that although some microporosity is necessary to increase the storage area for oxidation products, the carbonaceous phase does not need to be highly microporous. It is important that it provides space for deposition of sulfur, which is formed on the inorganicphase catalyst. That space can be in meso- and macropores as shown in the case of char derived from the waste paper.
Introduction One of the reasons for removal of hydrogen sulfide from air is increasing concerns about the acid rain pollution (1). In * Corresponding author phone: (212) 650-6017; fax: (212) 6506107; e-mail:
[email protected]. 10.1021/es050053m CCC: $30.25 Published on Web 07/16/2005
2005 American Chemical Society
the atmosphere H2S is easily oxidized to SO2 and SO3, which is followed by formation of H2SO4 when scavenging by rain droplets occurs. The precipitation of acid rain causes environmental damage in ecosystems (soil, water) and in infrastructures (detoriation of concrete and corrosion of steal). The Acid Rain Program of Clean Air Act of 1990 requires reduction and continuous monitoring of sulfur-containing gas emission (2). One of the ways to efficiently remove hydrogen sulfide is its adsorption on activated carbons. The adsorbents, owing to their large surface area, high pore volume, and catalytic surface properties (3), are able to retain a significant amount of hydrogen sulfide (4). Via various paths of surface reactions governed mainly by surface pH (5, 6), the pollutant is oxidized to elemental sulfur and/or sulfuric acid. When the former is desirable, sulfuric acid deposition may cause problems of hazardous waste disposal when a recovery of the reaction products is not applied. Some carbons, such as Calgon’s Centaur, are able to convert a significant amount of H2S to H2SO4 (7). Moreover, after this process the surface is regenerated using just simple water washing. Another commercial carbonaceous adsorbent worth mentioning here is catalytic carbon, which converts hydrogen sulfide to elemental sulfur. An example is US Filter’s Midas (8) on which about 60 wt % sulfur can be stored before a breakthrough of hydrogen sulfide occurs (9). This material is prepared by physical mixing of alkaline-earth-metal oxides (calcium and/or magnesium) with coal-based activated carbons. As a result of this process high dispersion of the inorganic phase is achieved. That basic inorganic phase acts as a catalyst for hydrogen sulfide oxidation to sulfur. On oxide particles, in the presence of water, H2S dissociates and HS- ions are oxidized to sulfur, which immediately migrates to small pores of a neighboring carbon particle (9). A similar mechanism likely exists on sewage-sludgederived adsorbents. Contrary to Midas, their content of an organic phase is only about 20% (75% in Midas) (9-11). Both materials contain noticeable amounts of calcium (6-9%). Although based on the surface area of the catalysts, the performance of sewage-sludge-derived materials is 2 times better than that of Midas; the absolute values differ by almost an order of magnitude as do the surface areas of both materials. Taking this into account, it was hypothesized that an increase in the content of the carbonaceous phase, or its porosity, should increase the capacity for desulfurization. This was expected to happen as a result of providing additional pore space to store the oxidation products. The results obtained using various systems brought mixed conclusions (12, 13). Generally, the carbonaceous phase was able to increase the capacity; however, that increase was not as significant as one would expect. It was concluded that the reason for this is the high acidity of the specific carbon phase used (12). It provided a high surface area, but with the presence of a certain critical amount of acidic carbonaceous phase, the performance of the materials decreased and the H2S removal capacities were even smaller than those for sludge-derived adsorbents. The plausible explanation was that the addition of porous space is beneficial only if the sufficient basicity for H2S dissociation is maintained. When added acidic carbon exceeds the “buffer capacity” of the basic sludge-derived phase, the capacity diminishes as a result of the shift in the dissociation of hydrogen sulfide to the left. On the basis of the results obtained in our earlier work (12), the objective of this paper is to investigate the effect of an almost nonporous carbonaceous phase on the performance of sewage-sludge-waste-paper-based adsorbents. In VOL. 39, NO. 16, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Names of Samples, Yields on Various Stages, Bulk Density, and Ash Content
sample
composition
yield (%)
CS CC C-1Ca6 C-2Ca1 C-3Ca5 C-4Ca4 C-5
sludge waste paper 22/72/6 24.5/74.5/1 42.5/42.5/5 58/38/4 75/25/0
40 22 40 30 34 33 27
amt of dry sludge (%)
density (g/cm3)
ash content (%)
100 0 72 74.5 42.5 38 25
0.63 0.19 0.52 0.59 0.48 0.34 0.24
75 31 70 65 64 60 50
spite of the fact that this carbonaceous phase is basic in its chemical nature, to check whether addition of well-dispersed catalysts promoting the dissociation of H2S can influence the performance, various amounts of calcium were added to the precursor before carbonization. The results are discussed in terms of the content of the carbonaceous phase, the porosity, and the presence of calcium. Moreover, the results address the applications of carbon from wastes (14, 15), which are of interest to current carbon and environmental scientists.
Experimental Section Materials. Adsorbents were prepared from dewatered sludge (from Wards Island Water Pollution Control Plant, NYC DEP), waste paper, and mixtures of both precursors and calcium hydroxide with the following weight percentages of waste paper/dewatered sludge/calcium hydroxide: 22/72/6, 24.5/ 74.5/1, 42.5/42.5/5, 58/38/4, and 75/25/0. Waste paper was first ground to a fine powder, then mixed with powdered dried sludge, and homogenized using a small amount of water with an appropriate content of calcium hydroxide. Prepared pulp was extruded into 4 mm granules and dried. In all cases the pyrolysis was done in a horizontal furnace under a nitrogen atmosphere with a heating rate 10 deg/min. The final pyrolysis temperature was 950 °C with a holding time of 1 h. The composite adsorbents are referred to as C, which is followed by numbers representing an increasing ratio of the waste paper to sludge and the percentage of calcium hydroxide added to the precursor mixture. The adsorbent obtained from dewatered sludge is referred to as CS and carbon obtained from waste paper as CC. The names of the adsorbents and their yields are collected in Table 1. Methods. Evaluation of the H2S Sorption Capacity. A custom-designed dynamic test was used to evaluate the performance of adsorbents for H2S adsorption from gas streams as described elsewhere (10). Adsorbent samples were ground (1-2 mm 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) H2S was passed through the column of adsorbent at 0.5 L/min. The flow rate was controlled using Cole Palmer flow meters. The breakthrough of H2S was monitored using electrochemical sensors. The test was stopped at the breakthrough concentration of 100 ppm. The adsorption capacities of each adsorbent in terms of milligrams of hydrogen sulfide per gram of adsorbent were calculated by integration of the area above the breakthrough curves, and from the H2S concentration in the inlet gas, flow rate, breakthrough time, and mass of sorbent. For each sample the test was repeated at least twice. The adsorbents exhausted after H2S adsorption are designated by adding an additional letter E to their names. Characterization of the Pore Structure of the Adsorbents. On the materials obtained sorption of nitrogen at its boiling point was carried out using an ASAP 2010 (Micromeritics). 6218
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Before the experiments, the samples were outgassed at 120 °C to constant vacuum (10-4 Torr). From the isotherms, the surface areas (BET method), total pore volumes, Vt (from the last point of the isotherm at a relative pressure equal to 0.99), volumes of the micropores, Vmic, mesopore volume, Vmes, and pore size distributions were calculated. The last three quantities were calculated using density functional theory, DFT (16, 17). Study of Surface Chemistry. The surface properties were evaluated first using potentiometric titration experiments (18-20). Here, it is assumed that the population of sites can be described by a continuous pKa distribution, f(pKa). The experimental data can be transformed into a proton binding isotherm, Q, representing the total amount of protonated sites, which is related to the pKa distribution by the following integral equation:
Q(pH) )
∫
∞
-∞
q(pH,pKa) f(pKa) d(pKa)
(1)
The solution of this equation is obtained using the numerical procedure (21), which applies regularization combined with nonnegativity constraints. On the basis of the spectrum of acidity constants and the history of the samples, the detailed surface chemistry shall be evaluated. pH. The pH of a carbonaceous sample suspension provides information about the acidity and basicity of the surface. A sample of 0.4 g of dry carbon powder was added to 20 mL of distilled water, and the suspension was stirred overnight to reach equilibrium. Then the pH of the suspension was measured. Thermal Analysis. Thermal analysis was carried out using a TA Instruments thermal analyzer. The instrument settings were a heating rate of 10 °C/min and a nitrogen atmosphere with a 100 mL/min flow rate. For each measurement about 25 mg of a ground adsorbent sample was used. Calcium Content. X-ray fluorescence analysis was applied to study the calcium content in the samples. The SPECTRO model 300T benchtop analyzer from ASOMA Instruments, Inc. was used. The instrument has a titanium target X-ray tube and a high-resolution detector. The samples were studied in the solid phase after grounding and sieving to use matrixes with similar physical properties. The calcium content in the waste-paper-derived materials was determined quantitatively by dissolving the ash in hydrochloric acid followed by precipitation of calcium hydroxide and gravimetric analysis of its content. SEM. SEM analysis was done using a Ziess DSM 942 scanning electron microscope.
Results and Discussion The yields of the materials collected in Table 1 show the dependence on the content of the waste paper (cellulose). More paper in the composition leads to a smaller yield, which is in fact expected knowing the low efficiency of the cellulose carbonization in terms of the mass of the final product. Nevertheless, the yields of materials obtained in this research are much higher than those for sludge-organic polymer mixtures where washing was a necessary step to remove the deposited metals and to develop the porosity (12). The density also decreases with an increase in the content of paper. This is either related to a decrease in the content of ash with increasing amounts of waste paper (higher carbon content) or caused by an increase in the total porosity, which may have its origin in the carbonaceous phase. This aspect is addressed later in the discussion. The performance of the obtained materials as hydrogen sulfide adsorbents is presented in Figure 1 and Table 2. It is worth mentioning that the H2S removal capacity of these materials is better than that on virgin activated carbon (20 mg/cm3) and comparable to that of the catalytic carbon
FIGURE 1. H2S breakthrough capacity curves for selected samples.
TABLE 2. H2S Breakthrough Capacity Results
sample
H2S brth cap (mg/cm3)
H2S brth cap (mg/g)
water adsorption (mg/g)
pHin
pHE
CS CC C-1Ca6 C-2Ca1 C-3Ca5 C-4Ca4 C-5
72 17 64 50 105 120 57
115 90 124 85 211 351 237
55 89 59 59 61 64 74
10.6 12.6 12 11.5 12.0 12.0 12.3
9.6 12.2 10.7 10.9 11.5 11.2 9.8
Centaur (60 mg/cm3) and caustic impregnated carbons (140 g/cm3) determined using the standard ASTM test (22). Also, taking into account their smaller density than that of the majority of coal-derived activated carbon, the capacity per unit mass is exceptionally high. It is interesting that there is only a slight increase in the capacity per unit volume with a noticeable increase in the content of calcium for two samples having similar sludge contents, C-1 and C-2. On the other hand, an increase in the content of waste paper when calcium is added resulted in a huge capacity, comparable to that on caustic impregnated activated carbons, for example, C-3C5 and C-4Ca4 samples, with over 30% sulfur adsorbed on the surface of the C-4Ca4 sample. Since these two samples have similar amounts of calcium added, the higher capacity of C-4Ca4 can be linked to the higher content of the phase derived from waste paper. Nevertheless, when no calcium is added, the capacity of the adsorbent is still over 2 times higher than that of virgin activated carbon and over 22% sulfur is deposited in its pore system. To interpret these findings, detailed analysis of the surface porosity and chemistry is needed. Calcium is added to our samples to compensate for smaller contents of the catalytic phase having its origin in a sewage sludge precursor (9, 10). As shown elsewhere, CS contains 6.6% calcium. Calcium oxide provides the basicity of the surface needed for efficient hydrogen sulfide dissociation and can react with H2S, forming sulfides. When well dispersed, it contributes to the specific catalytic function of microreactors formed in the pore systems where hydrogen sulfide is oxidized to elemental sulfur (9). The high capacity of the C-5 sample suggests that the basicity of the matrix without calcium catalyst added might be sufficient for feasible H2S removal. A previous study showed that the high capacity can be achieved with a pH as high as 10 (5). On the basis of the high pH of the CC sample, it is important to mention here that adding waste paper to sludge does not necessarily decrease the total content of calcium in the adsorbents. It is well-known that calcium carbonate and minerals containing calcium are added to paper to improve its
FIGURE 2. XRF peaks for calcium in the samples studied.
FIGURE 3. Dependence of the H2S breakthrough capacity on the intensity of the calcium XRF peak. quantity and interactions with the ink. In fact 30% ash in the CC sample must have its origin in the paper production process. The analysis of the ash indicated that its majority is in the form of calcium carbonate and a total of 12% calcium is present in the CC sample. This can explain the higher pH than in the case of the CS adsorbent, which contains only 6.6% calcium. Taking into account the differences in the contents of the precursor mixtures, yields of adsorbents, and the amount of calcium added, the content of calcium in the composite samples varies from 7% to 10% and increases with an increase with the waste-paper-derived phase. The relative content of calcium in the samples was estimated using XRF. The results are presented in Figure 2. The intensity of the calcium peak from XRF analysis can be related to the Ca content if the matrixes of the analytes and their volumes and particles sizes are comparable. Thus, the dependence of the H2S capacity on the intensity of calcium peaks for the samples containing total calcium originated from cellulose, sludge, and that added as hydroxide to the precursor is presented in Figure 3. The results for CC and CS are not included due to the apparent differences in the physical features of the matrixes. The distinct linear trend shows the importance of calcium-containing components of the wastes to the process of desulfurization from the gas phase. VOL. 39, NO. 16, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. pKa distributions.
TABLE 3. Structural Parameters of the Adsorbents Studied sample
SN2 (m2/g)
Vmic (cm3/g)
Vmes (cm3/g)
Vt (cm3/g)
CC CCE CS CSE C-1Ca1 C-1Ca1E C-2Ca6 C-2Ca6E C-3Ca5 C-3Ca5E C-4Ca4 C-4-Ca4E C-5 C-5E
66 15.7 75 20 91 29 108 20 72 20 67 14 73 16
0.017 0 0.011 0.0 0.016 0 0.024 0 0.012 0. 0.012 0 0.018 0
0.092 0.058 0.097 0.070 0.099 0.097 0.103 0.071 0.084 0.068 0.089 0.047 0.101 0.057
0.128 0.074 0.111 0.088 0.108 0.115 0.112 0.085 0.097 0.077 0.102 0.052 0.119 0.064
The amounts of water adsorbed on the samples during prehumidification indicate the hydrophilic nature of an inorganic matter having its origin in waste paper. With an increasing content of the paper-based carbonaceous phase the amount of water preadsorbed significantly increases, indicating a higher activity of calcium-containing compounds for binding water than that in the case of sludge-derived adsorbents. In the case of CC calcium is present as calcium oxide, which adsorbs water as a result of hydration. In CS calcium, besides oxide, is expected to be in the form of inorganic salts. All samples containing waste paper, either with calcium addition or without, have a pH greater than 12, which is even higher than that for the sewage-sludge-derived sample. That high pH enables dissociation of adsorbed H2S in the film of water and provides buffer capacity for the adsorbents for reaction with hydrogen sulfide. After H2S adsorption the materials are still basic, which suggests formation of either salts or elemental sulfur. In fact, on the basis of the adsorbent composition, two reaction products are expected. The sewage sludge component and addition of calcium oxide should contribute to the formation of sulfur (9). Some calcium and metal oxides present in sludge will react with H2S, forming sulfide and sulfates. The biggest decrease in the pH is found for the sample which has the highest content of the carbonaceous phase, C-5 (69% carbon). As described previously, the carbonaceous phase, when small micropores are present, preferably oxidizes H2S to H2SO4 (4). Formation of a small amount of sulfuric acid (depending on the porosity of the carbonaceous phase) may contribute to a 3 pH unit increase in the surface acidity. The distributions of acidity constants presented in Figure 4 show differences in the chemistries of CC and CS and the adsorbents resulting from their mixtures. For CC two welldefined peaks at pKa around 4.3 and 7.2 likely represent the presence of calcium carbonate. Those peaks are also present 6220
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FIGURE 5. Nitrogen adsorption isotherms.
FIGURE 6. Pore size distributions.
FIGURE 7. Dependence of the surface area on the composition of the precursor. The solid line represents theoretical calculations of the volume assuming the physical mixture of the compounds. for C-5Ca4 and C-5; however their intensity, especially for the first species, is smaller due to the mass dilution effect of the sludge-derived phase. The chemistry of the SC looks very complex. On the pKa distributions five peaks are revealed. A mixture of both CC and CS increases the surface chemical heterogeneity. For the sludge-waste-paper-derived samples a new feature is present as an offset of the peak at pKa over 10 which represents very basic species, either organic groups or inorganic compounds which are apparently the result of the synergy of the surface components after treatment at high temperature (high pH of the samples). Information about the changes in the porosity is provided in Table 3 and Figures 5 and 6, where examples of the nitrogen adsorption isotherms and pore size distributions, respectively, are plotted. At first glance, the specific surface areas and volumes of the pores are similar to those obtained from
croporosity. The opposite was found to be true when adsorbents were obtained from the mixture of sludge with an organic polymer (12). To analyze the effect of the twophase mixture, the surface areas measured were compared to those expected when two components contribute to the surface according to their weight contents and yields. The results are plotted in Figure 7.
FIGURE 8. Density changes with the content of the waste-paperderived phase. the pure precursors. These findings indicate that the carbon phase does not contribute to the development of mi-
It is very interesting that for adsorbents with a small content of paper-derived phase a significant, about 30%, increase in the surface area is observed. Moreover, the surface is higher when the content of calcium increases. The latter can be explained by the contribution of calcium to the development of porosity as a gasification catalyst (23). When the waste paper component of the precursor reaches about 50%, the trend in the surface area follows the expected one for the physical mixture of the components and is independent of the addition of calcium. It is likely that the small content of carbon from waste paper (in fact, after recalculation of yields, it contributes to only about 15% of the total mass of the C-1 and C-2 adsorbents) is able to create a thin
FIGURE 9. SEM images for CS (A), CC (B, C), and C-5 (D-F). VOL. 39, NO. 16, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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deposit between the bulky granules of sewage and on that interface additional pores exist for penetration by nitrogen molecules. This deposit is susceptible to the catalytic role of calcium in the gasification, and thus, small pores are developed in the thin layer. With an increase in the content of waste paper, the aggregates of the carbonaceous phase are bigger, their level of dispersion significantly decreases, and calcium is not able to promote the extended gasification. Hence, the surface area is a summary of the surfaces of both coexisting, next to each other, phases. The carbon phase from waste paper is almost nonporous and can be considered as a char since no activation was used. Its location within the granules of the composite results in a decrease in the bulk density of the adsorbent with an increase in the content of waste paper. The presence of these bulky low-density “inserts” between the sludge-derived phases increases the accessibility of catalysts for providing basic pH and thus for H2S oxidation. Support for the hypothesis described above is the density values for the adsorbents studied. In Figure 8 the comparison of the measured densities with those predicted assuming the physical mixture of the components without any synergetic effect is presented. For adsorbents with a small content of the waste-paper-derived phase the densities measured followed the predicted ones. The situation changes when the content of waste papers reaches 50% of the total precursor mass. For these samples, a well-defined loop exists between the densities predicted and measured. The measured ones are much lower than those predicted, which clearly indicates the existence of the secondary porosity effect. The effect described above can be seen in SEM images presented in Figure 9, where the CS sample appears as the most dense (Figure 9A) and CC as the least dense (Figure 9B,C). C-5 reveals the features of both phases with the apparent presence of large pores (Figure 9D-F). It is interesting that after H2S adsorption the volume of the micropores disappears completely, making the removal process even more efficient than in the case of sewage-sludgebased materials by utilizing the pore volume at 100%. It is obvious that about 30% sulfur cannot be stored in as small a volume of micropores as that existing in our materials. Assuming the density of sulfur to be equal to 2 g/cm3, only about 10% of the total sulfur can be located there. Thus, other pores, meso- and big macropores, between the particles must be active in retaining the oxidation products. Indeed, the analysis of the pore size distribution presented in Figure 6 shows a decreasing trend in the volume of pores smaller than 20 Å after an increase in the content of carbon from waste paper. After H2S adsorption, those pores completely disappear. An increase in the volume of other pores after H2S removal can be caused by deposition of sulfur in macropores, which results in the formation of a new mesoporous phase. All the features listed above contribute to the measured exceptional H2S breakthrough capacity of our adsorbents. To see the synergetic/catalytic effect of the samples’ composition, the capacities expected for the physical mixtures of the components were calculated (without the contribution of calcium as H2S adsorbent/oxidant). They are plotted in Figure 10 in comparison with the measured values. Contrary to the trend found for the development of porosity, for adsorbents with a low content of waste paper, regardless of the amount of calcium, the measured capacities more or less match the expected ones. The difference arises for the samples with a content of waste paper in the precursor of about 50% or higher. Although the performance of adsorbents has already been linked to the content of calcium (Figure 2), on the basis of the results presented in Figure 10, chemistry cannot be the only governing factor. The C-4Ca4 and C-3Ca5 samples have similar added contents of calcium. Although 6222
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FIGURE 10. Measured and predicted H2S breakthrough capacities.
FIGURE 11. Schematic representation of the H2S removal on sewagesludge-waste-paper-based adsorbents. Dark gray particles represent the waste-paper-derived phase (CC) and light gray particles the sludge-derived phase (CS). On the latter, oxidation of H2S to sulfur occurs and sulfur is adsorbed either in the pores of the carbon phase (CC) or in the macropores between the particles. the contribution of calcium from the waste-paper-derived phase is higher for the former sample than for the latter, that difference cannot account for such significant differences in the H2S breakthrough capacity. The plausible explanation is that those bulky inserts of low-density char mentioned above in the discussion of porosity enhance the capacity. Although they do not provide the high volume of micropores, inside them the space for sulfur deposition exists. They are in close contact with a catalytic phase of sludge origin, and this enables the migration of sulfur. Support for this is the very high capacity of the C-5 sample, where all calcium comes from waste paper. Its high carbon content and limited inorganic phase from sludge lower the “buffer capacity” of the adsorbent. This causes a small fraction of hydrogen sulfide to be oxidized to sulfuric acid as shown by a decrease in the pH. The simplification of the H2S adsorption/oxidation process on our materials is presented in Figure 11. First, H2S is adsorbed on the inorganic-phase-originated active centers of either sewage sludge or waste paper origin (light gray phase) where, in the presence of water, its dissociation followed by oxidation occurs (4, 9). The resulting sulfur migrates either to the micropores and mesopores of the neighboring carbon phase (dark phase) or to the macropores between phases where external, nonporous carbonaceous surface is exposed. Some information about the speciation of H2S surface reaction products can be obtained from the analysis of the pH values and their changes after H2S adsorption. As indicated above, on the basis of the values collected in Table 2, we expect mainly sulfur on the surface with some contribution of salts such as sulfides or sulfates and carbonates. Figure 12 shows DTG curves in nitrogen for the selected initial and exhausted samples. The peaks represent the
TABLE 4. Weight Losses (%) in Nitrogen in Various Temperature Ranges, Total Weight Loss, ∆W, and the Expected Content of Sulfur Based on the H2S Breakthrough Capacity Results ([S]brth)a
sample
