Environ. Sci. Technol. 2007, 41, 3715-3721
Tobacco Waste/Industrial Sludge Based Desulfurization Adsorbents: Effect of Phase Interactions during Pyrolysis on Surface Activity MYKOLA SEREDYCH AND TERESA J. BANDOSZ* Department of Chemistry, The City College of New York, City University of New York, 138th Street and Convent Avenue, New York, New York 10031
Industrial waste derived adsorbents were obtained by pyrolysis of tobacco waste with either metal sludge or waste oil sludge from a shipyard. The materials were used as media to remove hydrogen sulfide at room temperature in the presence of moisture. The initial and exhausted adsorbents after the breakthrough tests were characterized using sorption of nitrogen, thermal analysis, XRD, elemental analysis, and surface pH measurements. It was found that mixing tobacco and industrial sludges results in a strong synergy, enhancing the catalytic properties of adsorbents. This synergy is observed in both surface chemistry and porosity. During pyrolysis, new mineral phases are formed as a result of solid-state reactions between the components of the sludges. They are highly dispersed on the surface of mesopores. A high volume of these pores is a result of activation of the carbon phase in the composite by alkaline earth metals and also by the release of water from the decomposition of an inorganic phase that is in the predominant quantity. A high temperature of pyrolysis is beneficial for the adsorbents due to the enhanced activation of the carbonaceous phase and the chemical stabilization of the inorganic phase. Samples obtained at low temperatures are sensitive to water, which deactivates their catalytic centers.
Introduction Activated carbons are the adsorbents of choice to remove pollutants either from gas or from liquid phases (1). This is owing to their large surface area and high volume of pores. Often, these characteristics of activated carbons are not potent enough to retain certain molecules, especially small ones, for which the dispersive interactions with the carbon surface are rather weak. In such cases, the carbon surface has to be modified to impose the specific interactions (2). These interactions include hydrogen bonding, complexation, acid/base reactions, or redox processes. Fortunately, in the case of carbon, various technologies leading to the modified surfaces exist and are relatively easy to achieve. Examples are oxidations with various oxidants such as strong acids, ozone, or air; impregnation with catalytic metals or reducing/ oxidizing compounds; heat treatment in the presence of heteroatom sources such as chlorine or nitrogen compounds; and others (3). * Corresponding author phone: (212)650-6017; fax: (212)650-6107; e-mail:
[email protected]. 10.1021/es0624624 CCC: $37.00 Published on Web 04/14/2007
2007 American Chemical Society
In some cases, modifications of a carbon surface besides being a challenge are associated with high expenses, especially when noble or catalytic metals are involved. This directed our attention to industrial sludges. Some of them, like those coming from shipyards or other heavy metal industries, are rich in catalytic transition metals (4-7). By pyrolysis of these materials, not only is the volume of waste reduced but those environmentally detrimental wastes can be recycled and converted into valuable products. These products, when used, can be safely disposed since the leaching of metals is significantly reduced by their mineralization via high temperature solid-state reactions. All of the factors mentioned previously resulted in the exploration of various wastes, municipal or industrial, as sources of adsorbents (4-14). As a result of various treatments, surface areas between 100 and 500 m2/g were obtained, and the materials’ applicability was shown for the removal of hydrogen sulfide, sulfur dioxide, basic or acidic dyes, phenol, or mercury. It is important to mention that their performance was reported as comparable to or better than that of activated carbons (12, 13). In some cases, the wastes were mingled and, owing to the synergy between the components, more efficient adsorbents were obtained. Examples are mixtures of sewage sludge with waste paper or industrial sludges (5-7, 14). The objective of this paper is to evaluate the performance of materials obtained by pyrolyses of industrial sludge/ tobacco waste as desulfurization media. While the tobacco waste is expected to provide the carbonaceous filler and dispersion phase, the industrial sludges are the sources of catalytic metals for desulfurization (5-7). This includes calcium, magnesium, and iron. Calcium and magnesium provide the basicity needed for H2S dissociation, and iron is a redox catalyst for the oxidation of HS- ions to elemental sulfur. To achieve our objective, the materials were prepared using two different sludges with two different compositions and two different pyrolysis temperatures. The performance of the materials is linked to their surface features, which are the consequences of the preparation technology applied.
Experimental Procedures Materials. Two industrial sludges, waste oil sludge (WO) and metal sludge (M) from Newport News Shipyard, were mixed with dry tobacco compost (TC), homogenized, dried at 120 °C for 48 h, and then carbonized at 650 and 950 °C for 1 h under a nitrogen atmosphere in a horizontal furnace. The heating rate was 10 °C/min. The weight of the wet industrial sludges (they contain 75% water) was adjusted to have 10 and 50% of the industrial sludge component based on the dry mass. Tobacco waste is referred to as TC. The names of samples 1 and 2 refer to a 90:10 and 50:50 tobacco/sludge ratio (TC/WO or TC/M), respectively, and A and B refer to pyrolysis at 650 and 950 °C, respectively. (The names of the adsorbents obtained and their compositions along with the yield, ash content, and bulk density are collected in Table A of the Supporting Information). Methods. Evaluation of H2S Sorption Capacity. 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 pre-humidified with moist air (relative humidity 80% at 25 °C) for 1 h (15). The amount of water adsorbed was estimated from an increase in the sample weight. Moist air (relative humidity 80% at 25 °C) containing 0.3% (3000 ppm) of H2S with a flow rate of 0.5 L/min was passed through the adsorbent. The breakthrough of H2S was monitored using a MultiRae photoionization sensor. The test VOL. 41, NO. 10, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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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. Besides H2S, the content of SO2 in the outlet gas was also monitored using a MultiRae photoionization sensor. The adsorbents exhausted after H2S adsorption are designated by adding an additional letter E to their names. Characterization of Pore Structure of Adsorbents. The sorption of nitrogen on at the adsorbents at -196 °C was carried out using ASAP 2010 (Micromeritics). Before the experiments, the samples were outgassed at 120 °C to a 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 micropores, Vmic; mesopore volume, Vmes; and pore size distributions were calculated according to density functional theory (DFT) (16)). pH Measurement. A dry adsorbent powder of 0.4 g 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 as follows: 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. For analysis of the results, the differential thermogravimetric curves (DTG curves) were used to evaluate the results. The ash content was determined from the residue left after heating the samples at 800 °C in the presence of air. Catalytic Metal Content. Metal content in the adsorbents was determined using ICP in LSL labs, Syracuse, NY and VTEC Labs, Bronx, NY. X-Ray Diffraction (XRD). XRD measurements were conducted using standard powder diffraction procedures. Adsorbents were ground with methanol in a small agate mortar. The mixture was smear-mounted onto the zerobackground quartz window of a Phillips specimen holder and allowed to air dry. Samples were analyzed by Cu KR radiation generated in a Phillips XRG 300 X-ray diffractometer.
Results and Discussion The H2S breakthrough curves for our samples are presented in Figure 1. As seen based on the steep rise in the breakthrough curves, all tobacco based materials have short diffusion zone, and almost immediately after H2S was detected in the outlet gas, the adsorbents stop to work, allowing the challenge gas to pass chemically undisturbed through the bed. It is interesting that no SO2 was detected, which indicates that all H2S is converted to sulfur. In the case of metal and oil sludge derived materials, small concentrations of sulfur dioxide, up to few ppm, were measured at the same time when hydrogen sulfide appeared in the outlet gas. It is interesting that even in the case of 50% tobacco waste and 50% waste oil sludge, the mechanism of hydrogen sulfide retention characteristic to tobacco is still predominant since the slope of the curve does not resemble the one obtained for the waste oil sludge derived adsorbent. The results of the H2S breakthrough capacity measurements are summarized in Table 1, where besides the capacity expressed in unit mass per gram of the adsorbent and per unit volume of the bed, the amount of water adsorbed during the prehumidification and the pH of the surface before and after the adsorption process are reported. As seen from Table 1, the highest capacity is found for tobacco waste oil sludge compositions pyrolyzed at 950 °C. A higher content of oil sludge is beneficial for the performance and only 10% waste oil sludge increases the performance by about 100% as compared to the pure tobacco waste based 3716
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FIGURE 1. H2S breakthrough capacity curves for adsorbents studied.
TABLE 1. H2S Breakthrough Capacity, Amount of Water Adsorbed, and pH Values of Adsorbent Surfaces H2S breakthrough capacity sample
(mg/g)
(mg/cm3)
CWO-B CM-B CTC-A CTC-B CTCWO-1A CTCWO-2A CTCWO-1B CTCWO-2B CTCM-1A CTCM-2A CTCM-1B CTCM-2B
40.2 5.0 6.6 23.1 16.1 0.9 42.6 90.2 13.0 22.5 23.1 18.9
21.1 2.9 4.2 12.1 6.7 0.4 17.8 36.4 7.2 11.7 13.5 5.7
water pH adsorbed (mg/g) initial exhausted 11 0 51.8 38.2 45.4 82.0 35.4 43.3 29.6 11.2 21.5 10.8
10.7 11.2 11.2 11.3 10.6 9.2 10.0 10.3 10.6 9.4 11.2 10.8
10.2 11.2 10.7 11.3 9.6 9.2 9.8 9.3 10.5 9.3 11.1 10.6
material. For the CTC material, the pyrolysis at high temperatures also significantly enhances the capacity. These results suggest the predominant influence of the tobacco waste on the performance. It has been previously reported that the waste oil sludge derived adsorbents had the best capacity at low temperatures, reaching 30 wt % (6, 7). In fact, comparison of the capacity obtained for both tobacco and waste oil sludge based materials obtained at 950 °C clearly shows a synergetic effect; the capacity obtained for the mixture is much higher than for either one of its components.
FIGURE 2. Comparison of the measured and calculated (assuming the physical mixture of components) H2S breakthrough capacities. It is interesting that pyrolysis of the waste oil sludge/ tobacco mixture at 650 °C with a high content of the waste oil sludge component has a detrimental effect on the H2S removal capacity. Although a large amount of water is adsorbed on the surface of this sample, the H2S removal capacity is negligible. Since the material obtained from waste oil sludge pyrolyzed at 650 °C had a very high capacity (reaching 30 wt % (6)), the tobacco component apparently hinders the capacity when low temperature treatment is applied. On the other hand, when the mixture contained metal sludge was pyrolyzed at low temperatures, the capacity was enhanced as compared to pure tobacco or pure metal sludge. Pyrolysis of those two mixtures at high temperatures enhances the capacity of the low sludge content material, indicating once again the importance of the tobacco phase for hydrogen sulfide removal on the composite adsorbents. Taking into account variations in the behavior of the samples with their pyrolysis temperature, the relationship between the amount of water preadsorbed and the H2S breakthrough capacity was analyzed (see Figure A, Supporting Information). For the samples pyrolyzed at low temperatures, water has a detrimental effect on the H2S breakthrough capacity. This might be linked to the low degree of mineralization and high reactivity of the surface. It is likely that exposure to water causes a hydroxylation reaction, which was observed previously (17). If the small pores are present, those hydroxides may block their entrances and thus decrease the available space for H2S adsorption and the storage of elemental sulfur. This problem is readdressed further in this paper when the porosity is discussed. In the case of samples pyrolyzed at 950 °C, water apparently enhances the capacity for H2S removal. This might be linked to its physical retention on the surface and the formation of a water film with basic pH (15, 18). This enables a high concentration of HS- ions and thus their oxidation to elemental sulfur. All samples have a basic pH, which should help with their performance as hydrogen sulfide removal media. It is interesting that the lowest pH is found for the CTCWO-2A sample, which also has a very low H2S removal capacity. That pH is much lower than the pH of the single component adsorbents. The reason for this might be either in oxidation of the carbon phase or in specific chemistry formed as a result of synergetic effects between the composite components. To check the synergetic effect on the H2S breakthrough capacity, the measured values were compared to those calculated assuming the physical mixtures of the pyrolyzed components. The results are presented in Figure 2. While in
the case of metal sludge containing samples only a slight enhancement in the capacity was observed as a result of mixing, for the waste oil sludge/tobacco composites, a significant synergetic effect was found in the H2S breakthrough capacity. That synergetic effect might be the result of either new catalytic phases formed when the materials are mixed and exposed to high temperature, formation of new pores enhancing physical adsorption and storage of oxidation products, increased dispersion of the catalytic phase, or more likely, a combination of all of these factors. Using X-ray diffraction, one may see both the changes in the degree of crystallinity of the adsorbents and the formation of new phases as a result of solid-state reactions (6-8). Figure 3a shows the comparison of XRD patterns for CTC adsorbents obtained at 650 and 950 °C. As can be seen from the analysis of the ash content (Table A, Supporting Information), all adsorbents, even those derived from only tobacco waste, have the majority of the inorganic phase. In the case of CTCA, only quartz and magnesium ferrosilite ((Fe,Mg)SiO3) are identified. Heating at 950 °C results in the formation of more crystalline phases identified as bayerite (Al(OH)3), ordered anorthite (CaAl2Si2O8), anthophyllite ((Mg, Fe)7Si8O22(OH)2), and barrigerite (Fe2P). Some of these minerals such as barrigerite were also identified in sewage sludge derived materials in which enhanced H2S adsorption was found (6, 7). Magnesium, calcium, and iron from these minerals can contribute to catalytic oxidation of hydrogen sulfide to sulfur (17). In the case of CWOB, metallic iron, bornite (Cu5FeS4), hibonite (CaAl12O19), zincite (ZnO), and ankerite (Ca(Fe,Mg)(CO3)2) were detected (Figure 3b). Heating metal sludge to 950 °C resulted in the formation of numerous crystalline phases (multi-peak pattern) from which pyrrohotite (Fe1-xS), troilite (FeS), pyrope (Mg3Al2(SiO4)3), and metallic copper, zinc, and iron have a high probability to exist. A multi-peak pattern is also observed for the mixtures of tobacco with metal sludge of various compositions and pyrolyzed at two different temperatures. For these series of materials, the new phases are detected on X-ray diffraction patterns. In the case of CTCM-1A, they are spinel (MgAl2O4), margarite (CaAl2(Si2Al2)O10(OH)2), malachite (Cu2CO3(OH)2, calcite (CaCO3), cordierite (Mg2Al4Si5O18), pigeonite ((Fe,Mg,Ca)SiO3), corundum (Al2O3), tenorite (CuO), magnesioferrite (MgFe2O4), moissanite (SiC), and metallic iron. Pyrolyzing the mixture containing a more metal sludge derived phase at 950 °C results in even more complex chemistry with predominant structures of mixed calcium iron and magnesium silicates and aluminosilicates. Some of them, as ferrocilite or anorthite, were present in CTC-A. Examples are fosterite (Mg2SiO4), huntite (Mg3Ca(CO3)4), aragonite (CaCO3), wollastonite (CaSiO3), dolomite (CaMg(CO3)2), cohenite (Fe3C), fersilicite (FeSi), covelite (CuS), bornite (Cu5FeS4), grunerite (Fe7Si8O22(OH)2), hardystonite (Ca2ZnSi2O7), or akermanite (Ca2MgSi2O7). In this case, as compared to the sample pyrolized at 650 °C, more carbonates are present, likely the result of gasification of carbon, less aluminum is involved in the crystalline phases, and two more elemental compounds appear. A structure that is very complex and different from the parent compound structure was obtained for CTCWO-2B (Figure 3c). In this case, besides a significant amount of quartz, over 50 new compounds were detected. They are mainly aluminosilicates with magnesium, calcium, iron, sodium, copper, and lead. Examples include: sodium of anorthite ((Ca,Na)(Al,Si)2Si2O8), fosterite (Mg2SiO4), albite (CaAl2Si2O8), richterite (KNaCaMg5Si8O22(OH)2), renhahnite (Ca3(Si3O8(OH)2), dahlite (Ca9.35Na1.07(PO4)5.46CO3), and rockbridgeite (Fe5(PO4)3(OH)5). Although surface chemistry can play a crucial role in the process of hydrogen sulfide oxidation on the surface of the VOL. 41, NO. 10, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. XRD patterns for tobacco derived samples (a), metal and waste oil sludge derived adsorbents (b), and composite tobacco/metal sludge based adsorbents (c). materials studied, the porous structure where the active species are located should be discussed in detail. The structural parameters calculated from nitrogen adsorption isotherms are collected in Table 2. It is interesting that either a waste oil sludge or a metal sludge addition increases the 3718
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surface area of samples obtained at 950 °C in spite of the fact that the surface areas of both components pyrolyzed separately are much smaller. Once again, this clearly indicates the beneficial synergetic effect of mixing the wastes. That development of porosity can be caused by the gasification
TABLE 2. Structural Parameters Calculated from Nitrogen Adsorption sample
SBET (m2/g)
CTCA CTCAE CTCB CTCB-E CTCWO-1A CTCWO-1AE CTCWO-2A CTCWO-2AE CTCWO-1B CTCWO-1BE CTCWO-2B CTCWO-2BE CTCM-1A CTCM-1AE CTCM-2A CTCM-2AE CTCM-1B CTCM-1BE CTCM-2B CTCM-2BE
73 0.9 78 42 71 33 35 13 120 37 162 59 77 8 74 24 96 46 59 49
a
Vmic (cm3/g) 0.037 NPa 0.039 0.017 0.041 0.014 0.015 0.009 0.055 0.019 0.069 0.026 0.035 0.006 0.031 0.013 0.043 0.018 0.031 0.022
Vmeso (m2/g) 0.016 NP 0.020 0.039 0.051 0.088 0.165 0.127 0.096 0.072 0.180 0.163 0.071 0.047 0.144 0.115 0.113 0.097 0.061 0.109
Vmic/Vt
DBJH (Å)
DDA (Å)
Eo (kJ/mol)
0.053 NP 0.059 0.056 0.092 0.102 0.180 0.136 0.151 0.091 0.249 0.189 0.106 0.053 0.175 0.128 0.156 0.115 0.092 0.131
0.698
69
15
25.06
0.661 0.304 0.446 0.137 0.083 0.066 0.364 0.209 0.277 0.138 0.330 0.113 0.177 0.102 0.276 0.157 0.337 0.168
41 44 95 100 123 144 56 68 61 85 63 138 79 124 62 99 82 107
16 17 16 17 21 21 16 17 17 18 15 17 17 18 16 18 16 18
21.82 19.28 23.80 17.62 10.09 9.61 20.65 17.55 20.01 15.45 23.94 18.79 18.67 16.36 20.53 15.62 20.19 16.67
NP: nonporous.
TABLE 3. Content of Catalytic Metals in Samples Pyrolyzed at 950 °C sample
Fe (%)
Ca (%)
Mg (%)
Cu (%)
Zn (%)
CWOB CMB CTCB
3.7 22 145a
5.1 14 1.2
8.40 0.46 0.25
0.25 0.77 15.5a
0.51 0.16 20a
a
Vt (cm3/g)
Cr (ppm) 280 6700 ND
In ppm.
of a carbon phase by alkaline earth metals present in the sludges, which can be considered as self-activation. Adding more waste oil sludge increases the surface area, volume of micropores, and volume of mesopores. Although the latter are present in much higher volumes in the CWOB adsorbent, the new volume of micropores is certainly the result of activation during pyrolysis. On the other hand, the addition of metal sludge, even in only small quantities, seems to be the most beneficial for tobacco/metal sludge mixtures. These materials have a new volume of mesopores formed, which do not exist in either tobacco or metal sludge single component materials. In this case, gasification may also play an important role. Much more alkaline earth metals than in waste oil sludge result (Table 3) in the formation of larger pores in the carbonaceous deposit. It is interesting that the smallest surface area and pore volume are obtained for the metal sludge/tobacco mixture with a 50:50 ratio of components pyrolyzed at 650 °C. In the case of materials obtained at 650 °C, the carbonization temperature was not high enough to promote decomposition of the inorganic phase and gasification of the carbonaceous phase mentioned previously. This is consistent with this sample’s low capacity for hydrogen sulfide removal. Since both tobacco derived samples have almost identical structural parameters, the differences in their performance as the hydrogen sulfide adsorbents must be attributed to the differences in surface chemistry mentioned previously. After H2S removal, the surface area and volumes of micropores significantly decrease. But for CTC-BE, CTCWO1AE, and CTCM-2BE, the volume of the mesopores increases, while the surface area and volume of the micropores decrease. This phenomenon has been observed in previous studies and has been attributed to the formation of new pores within either sulfur deposits in large pores (6, 7), if capacity was high, or/and in the formation of hydroxides on the surface
as a result of exposure to water during prehumidification (17). Although in the case of CTCM-2BE only a small amount of water was adsorbed with relatively high amounts of H2S, taking into account the small surface area of these samples, a significant, almost 100%, increase in the volume of the mesopores can be attributed to that sulfur deposit. The surface of the large pores of this material must be active since extensive gasification helped in high dispersion of the catalysts on the surface. For CTCWO-1AE, that increase can be attributed rather to the formation of hydroxides since the surface is active and a large amount of water is adsorbed, as well as to the sulfur deposit. Those hydroxides can completely block the porosity in the carbon deposit when more sludge derived phases are present and the sample is exposed to moisture from the atmosphere. This likely happens in the case of CTCWO-2A, which was totally inactive in the process of H2S adsorption, contrary to the waste oil sludge based sample whose capacity was found previously to be very high (6, 7), and it was linked, besides surface chemistry, to the high volume of mesopores. Owing to their large sizes, those pores cannot be blocked by hydroxides. As seen from Table 2, the average pore sizes calculated using the DubininAstakhov method are related to the values of the characteristic energy of adsorption, which is the highest for CTC-A, CTCWO1A, and CTCM-1A. These materials are obtained at low temperatures so that they can be considered as chars or underactivated carbons. More details about the differences in the porosity of our samples can be seen in PSD, examples of which are presented in Figure 4. For all samples, two regions of pore sizes can be distinguished: micro- and mesopores. After H2S adsorption, the smallest pores are not seen anymore, indicating that sulfur is deposited either there, or at their entrances, and that the new pores appear, especially with a range of sizes between 50 and 200 Å. In same cases, it happens with the expense of macropores. This shows the importance of a large porosity with a catalytically active surface to the process of hydrogen sulfide oxidation. If only physical adsorption were predominant, those pores would not play any role and would have a negative effect on the performance of materials based on the unit volume of the bed. Thus, in the case of this group of materials, very light adsorbents can be used that may increase the cost effectiveness of the removal process. The synergetic effect of the porosity development in our materials is presented in Figure 5, where the measured VOL. 41, NO. 10, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Pore size distributions for samples pyrolyzed at 950 °C. volumes of micro- and mesopores are compared to those calculated assuming the physical mixture of the components. As discussed previously, the synergetic effects of the sludge components on the activation of the final products is clearly seen with the most pronounced effects of waste oil sludge on the volume of micropores and metal sludge- on the volume of mesopores. To check the role of porosity for the H2S adsorption, the dependence of the H2S removal capacity on the volume of pores was analyzed. A good relationship is found for the volume of micropores (see Figure B, Supporting Information). They have an origin likely in the tobacco derived carbon phase, and thus, their contribution to the H2S adsorption process has to have a similar mechanism on the all tobacco containing samples. A linear trend is also noticed for the volume of mesopores but only for the materials obtained at 950 °C. As was shown previously, water has a detrimental effect on the chemistry of low temperature pyrolyzed samples; thus, the linear trend in this case is not expected. The linear relationship between capacity and volume of mesopores indicates the activity of large pores in the process of hydrogen sulfide catalytic oxidation. The examples of the comparison of DTG curves before and after adsorption of hydrogen sulfide is presented in Figure 6 (see also Figure C, Supporting Information). The peaks on the curves represent weight loss due to the decomposition/ desorption of the surface species. For some initial samples such as CTCB, CTCM-1A, and CTCM-1B, an increase in weight (negative peaks) is observed between 150 and 400 °C and between 600 and 800 °C. The latter negative peak is also found for CTCWO-2B. This strange behavior was noticed previously for some metal sludge, waste oil sludge, and even sewage sludge based adsorbents (6, 7). Since only nitrogen was present, the formation of nitrides was given as the only 3720
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FIGURE 5. Comparison of the volume of micropores and mesopores measured and calculated assuming a physical mixture of the components. plausible explanation of these phenomena (19). After H2S adsorption, a negative peak was revealed only at a high temperature range for CTCM-2BE. For other samples, it was compensated by weight loss between 200 and 400 °C caused by the removal of deposited sulfur. However, this weight loss/peak intensity should be proportional to the amount of hydrogen sulfide deposited on the surface, and in the case of materials pyrolyzed at 650 °C, an additional weight loss occurs as a result of dehydroxylation of the surface at temperatures lower than 600 °C. These hydroxides were formed when samples were exposed to water during prehumidification and H2S adsorption. Pyrolysis of waste tobacco compost and industrial sludges from heavy industries leads to the development of effective catalysts for the desulfurization of waste gas streams. An important contribution of waste tobacco for the composite materials is in providing additional carbon content. That carbon content contributes to the development of porosity in both micro- and mesopore ranges. This happens via selfactivation of the carbon material by alkaline earth metals and water released from the decomposition of inorganic matter during heat treatment. As a result of solid-state reactions at high temperatures, new catalytic species are formed on the surface of adsorbents as a result of synergy between the components of sludges. Location of these species in mesopores is beneficial for the desulfurization process. The surface of those pores retains a water film where hydrogen sulfide can dissociate in the basic environment. Sulfur formed in the oxidation reaction can be stored there
(2) Leon y Leon, C. A.; Radovic, L. R. Interfacial Chemistry and Electrochemistry of Carbon Surfaces. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1992; Vol. 24, pp 213-310. (3) Bandosz, T. J.; Ania, C. O. Surface Chemistry of Activated Carbon and its Characterization. In Activated Carbon Surfaces in Environmental Remediation; Bandosz, T. J., Ed.; Elsevier: Amsterdam, 2006; pp 159-229. (4) Martin, M. J.; Serra, E.; Ros, A.; Balaguer, M. D.; Rigola, M. Carbonaceous Adsorbents from Sewage Sludge and Their Application in a Combined Activated Sludge-Powered Activated Carbon (AS-PAC) Treatment. Carbon 2004, 42 (7), 1389-94. (5) Bandosz, T. J.; Block, K. Municipal Sludge-Industrial Sludge Composite Desulfurization Adsorbents Synergy Enhanced Catalytic Properties. Environ. Sci. Technol. 2006, 40 (10), 337883. (6) Bandosz, T. J.; Block. K. Removal of Hydrogen Sulfide on Composite Sewage Sludge-Industrial Sludge Based Adsorbents. Ind. Chem. Eng. Res. 2006, 45, 3666-72. (7) Bandosz, T. J.; Block, K. Effect of Pyrolysis Temperature and Time on Catalytic Performance of Sewage/Industrial Sludge Based Composite Adsorbents. Appl. Catal., B 2006, 67, 77-85. (8) Ros, A.; Montes-Moran, M. A.; Fuente, E.; Nevskaia, D. M.; Marin, M. J. Dried Sludges and Sludge Based Chars for H2S Removal at Low Temperature, Influence of Sewage Sludge Characteristics. Environ. Sci. Technol. 2006, 40, 302-9. (9) Lu, G. Q.; Low, J. C. F.; Liu, C. Y.; Lau, A. C. Surface Area Development of Sewage Sludge during Pyrolysis. Fuel 1995, 74, 3444. (10) Khalili, N. R.; Arastoopour, H.; Walhof, L. K. Synthesizing Carbon from Sludge. U.S. Patent 6,030,922, 2000. (11) Bagreev, A.; Bashkova, S.; Locke, D. C.; Bandosz, T. J. Sewage Sludge Derived Materials as Efficient Adsorbents for Removal of Hydrogen Sulfide. Environ. Sci. Technol. 2001, 35 (7), 153743.
FIGURE 6. DTG curves in nitrogen for the samples pyrolyzed at 950 °C. in large quantities without rapid deactivation of the catalytic centers by steric hindrances. A high temperature of pyrolysis is beneficial for the adsorbents due to the enhanced activation of the carbonaceous phase and chemical stabilization of the inorganic phase. Samples obtained at low temperatures are sensitive to water, which deactivates their catalytic centers.
Acknowledgments This research was partially supported by Gateway Consulting, Inc. The authors are grateful to Drs. Karin Block and Elizabeth Rudolph for help with XRD analyses.
Supporting Information Available Figures of dependence of H2S breakthrough capacity on the amount of water adsorbed; dependence of H2S breakthrough capacity on the volume of pores (micropores and mesopores for samples pyrolyzed at two temperatures); and DTG curves for tobacco sludge composites pyrolyzed at 650 °C. Table A showing names of samples, their compositions, pyrolysis temperature, yield, bulk density, and ash content. This material is available free of charge via the Internet at http:// pubs.acs.org.
Literature Cited (1) Bandosz, T. J. Activated Carbon Surfaces in Environmental Remediation; Elsevier: Amsterdam, 2006; pp 1-571.
(12) Zang, F.; Nriangu, J. O.; Itoh, H. Photocatalytic Removal and Recovery of Mercury from Water using TiO2-Modified Sewage Sludge Carbon. J. Photochem. Photobiol., A 2004, 167, 223-8. (13) Rio, S.; Faur-Brasquet, C.; Le Coq, L.; Courcoux, P.; Le Cloirec, P. Experimental Design Methodology for the Preparation of Carbonaceous Sorbents from Sewage Sludge by Chemical ActivationsApplication to Air and Water Treatments. Chemosphere 2005, 58, 423-37. (14) Ansari, A.; Bandosz, T. J. Inorganic-Organic Phase Arrangement as a Factor Affecting Gas Phase Desulfurization on Catalytic Carbonaceous Adsorbents. Environ. Sci. Technol. 2005, 39 (16), 6217-6224. (15) Bandosz, T. J. On the Adsorption/Oxidation of Hydrogen Sulfide on Unmodified Activated Carbon at Temperatures Near Ambient. J. Colloid Interface Sci. 2002, 246 (1), 1-20. (16) Lastoskie, C. M.; Gubbins, K. E.; Quirke, N. Pore Size Distribution Analysis of Microporous Carbons, a Density Functional Theory Approach. J. Phys. Chem. 1993, 97, 4786-96. (17) Seredych, M.; Bandosz, T. J. Removal of Copper on Composite Sewage Sludge-Industrial Sludge Based Adsorbents, Role of Surface Chemistry. J. Colloid Interface Sci. 2006, 302, 379-88. (18) Bandosz, T. J. Desulfurization on Activated Carbons. In Activated Carbon Surfaces in Environmental Remediation; Bandosz, T. J., Ed.; Elsevier: Amsterdam, 2006; pp 231-292. (19) Yamamoto, O.; Ishida, M.; Saitoh, Y.; Sasamoto, T.; Shimada, S. Influence of Mg2+ on the Formation of R-SiAlON by the Carbothermal Reduction-Nitridation of Homogeneous Gel. Int. J. Inorg. Mater. 2001, 3 (7), 715-9.
Received for review October 13, 2006. Revised manuscript received January 13, 2007. Accepted March 8, 2007. ES0624624
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