Reactive Adsorption of NO2 at Dry Conditions on Sewage Sludge

Oct 3, 2007 - Department of Chemistry, The City College of the City University of New York, 138th Street at Convent Avenue, New York, New York 10031...
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Environ. Sci. Technol. 2007, 41, 7516-7522

Reactive Adsorption of NO2 at Dry Conditions on Sewage Sludge-Derived Materials ROBERT PIETRZAK† AND TERESA J. BANDOSZ* Department of Chemistry, The City College of the City University of New York, 138th Street at Convent Avenue, New York, New York 10031

Composite inorganic-carbonaceous adsorbents were obtained by pyrolysis of sewage sludge at 500, 650, and 950 °C for various periods of time. They were used as media for reactive adsorption of NO2. The surface structure and chemistry of the initial and exhausted materials were analyzed using adsorption of nitrogen, XRD, FTIR, and thermal analysis. The results indicate the high level of conversion of NO2 to NO with the retention of both species on the surface depending on its chemistry. At 650 °C as the pyrolysis temperature, the most efficient adsorbents were obtained owing to a high reactivity of their oxides toward the formation of nitrites and nitrates. When the pyrolysis temperature is low, NO2 is reduced to NO on surface sulfides and reacts with surface oxides, forming nitrites and nitrates. When adsorbents are obtained at 950 °C, the chemically stable surface prevents the formation of nitrites and nitrates, and the majority of NO2 is reduced to NO in the highly carbonized carbonaceous phase.

Introduction NOx emissions create significant environmental problems owing to the main role of nitric oxides in photochemical smog formation (1). The emission of NOx can be decreased either by limiting the source of nitrogen or via chemical reduction or oxidation in a gaseous or liquid phase or on solid adsorbents. Thus far, the most commonly used method is a selective catalytic reduction (SCR) (2). The disadvantage of this method is in its high corrosivity and the toxicity of ammonia, which is used as a reductant. The alternative method is the utilization of adsorptive and reductive properties of the carbonaceous materials without using reducing agents. This process was studied by Klose and Rinco´n (3), who applied activated carbons obtained from oil palm shells as adsorbents of NO between 100 and 150 °C. Their results showed that the reactions occurring in the system include simultaneous adsorption, reduction, and catalytic oxidation of NO accompanied by adsorption of formed NO2. Suzuki and co-workers (4) found that certain amounts of oxygen significantly increase the rate of carbon-nitric oxide reactions on the surface of carbon. They also showed that the formation of surface oxygen complexes is essential for the C-NO reaction. On the basis of the numerous papers focusing on the adsorption of NO (5-7), it can be stated that in the * Corresponding author phone: (212)650-6017; fax: (212)650-6107; e-mail: [email protected]. † Permanent address: Laboratory of Coal and Technology, Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan ˜ , Poland. 7516

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presence of oxygen, both adsorption and reduction of NO are enhanced. Even though the majority of NOx is emitted as NO, the amount of NO2 in the emitted NOx can reach up to 30% (8). Nitrogen dioxide is a toxic species, and its detrimental effects on the environment are much more severe than those of CO and SO2. Heterogeneous conversion of NO2 on commercial soot was studied by Kleffmann and co-workers (9). They found that surface reactions/NO2 consumption caused the deactivation of active sites. The main products of these reactions were nitric oxide and nitrous acid (HONO). When the removal process occurred in dry conditions, NO was the only reaction product. Although the mechanism of heterogeneous reaction of NO2 on carbonaceous materials was studied mainly on soot and carbon blacks (10-12), there were also numerous investigations addressing the removal of NOx on activated carbons (13-15). On the basis of those studies, it was established that as a result of carbon surface-NO2 interactions and/or interactions of NO and NO2 in the presence of oxygen at low temperatures, various surface complexes were formed, such as C-NO2, C-ONO, C-ONO2, and C-O. The effects of impregnation of activated carbons with potassium hydroxide on the efficiency of NOx removal was studied by Lee and co-workers (16, 17). They found that KOH creates selective adsorption sites (increases the basicity of carbons by the presence of OH-) for NOx adsorption. Besides caustic and nitrogen modifications of the carbon surfaces (16-22), the reduction of NOx was extensively studied on carbons impregnated with transition metals (14, 23, 24), such as Ni, Fe, Co, or Cu. From all of those metals, copper was found to be the most efficient catalyst toward the reduction of NO into N2 and O2 either with or without oxygen. The mechanism of NOx removal was also investigated on inorganic oxides such as alumina, KOH impregnated alumina, MgO, Fe2O3, or TiO2. The results indicated chemisorption via nitrite and nitrate formation on the surface (25). The objective of this work was to study the removal of NO2 on composite adsorbents formed via the pyrolysis of sewage sludge. They consist of inorganic and carbonaceous phases with the majority being the former. Their surface is mesoporous and basic in its nature. The origin of basicity is in a few percent of calcium and magnesium oxides (26, 27). Moreover, the surface has oxidation-reduction properties caused by the presence of iron and chromium oxides. On the basis of the literature study (25), all of this is expected to result in an enhanced adsorption of NO2 and its strong chemisorption on the surface. Although the small surface of sludge-derived materials (26, 27) can be a limiting factor for NO2 retention, the results obtained can shed some light toward the development of high surface area activated carbon-sludge composite adsorbents. On the basis of the results obtained and the surface characteristics, the mechanism of adsorption is analyzed.

Experimental Procedures Materials. Adsorbents were prepared from dewatered sludge from New York City via pyrolysis of dry sludge (1-2 mm particle size) in a horizontal furnace under nitrogen flow (flow rate 100 mL/min). As indicated in our previous studies (26, 27), the sludge from all New York City water treatment plants is processed in one facility where the addition of significant quantities of calcium and iron species is an important technological step. This results in dewatered sludge with a relatively stable composition with a content of catalytic metals within a 15% margin. The samples were heated 10.1021/es071863w CCC: $37.00

 2007 American Chemical Society Published on Web 10/03/2007

(10 °C/min) from room temperature to final pyrolysis temperatures of 500, 650, and 950 °C. At the final temperatures, the samples were maintained for 30, 60, and 120 min and then cooled in an inert gas atmosphere. The samples are referred to as CS-A-B, where A represents the holding time (30, 60, or 120 min) and B -represents the pyrolysis temperature (650 or 950 °C). Thus, CS-30-650 represents the sample obtained at 650 °C with a 30 min holding time. Methods. Evaluation of NO2 Sorption Capacity. A home designed dynamic test was used to evaluate NO2 adsorption from gas streams (27). Samples were packed into a glass column (length 370 mm, i.d. 9 mm, bed volume 3 cm3) and used as received. Dry air with 0.1% NO2 was passed through the column of adsorbents at 0.450 L/min. The flow rate was controlled using Cole Parmer flow meters. The breakthrough of NO2 was monitored using an electrochemical sensor. The tests were stopped at a breakthrough concentration of 20 ppm. The interaction capacities of each sorbent in terms of milligrams of NO2 per gram of adsorbent were calculated by integration of the area above the breakthrough curves and from the NO2 concentration in the inlet gas, flow rate, breakthrough time, and mass of sorbent. To check the NO2 reduction, the concentration of NO was also monitored until 150 ppm (electrochemical sensor limit). After exhaustion, each sample was identified by adding the letter E to its name. Characterization of Pore Structure of Adsorbents. Adsorption isotherms were measured on Micrometric ASAP 2010 instrument at -196 °C. Before the experiments, the samples were outgassed at 120 °C to constant vacuum (10-4 Torr). The isotherms were used to calculate the specific surface area (SBET), total pore volume (Vt), and micropore volume (Vmic). The pore size distributions were evaluated using density functional theory (DFT) (28, 29). pH. 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 as follows: heating rate 10 °C/min and a nitrogen atmosphere at 100 mL/min flow rate. For each measurement, about 30 mg of a ground adsorbent was used. FTIR. FTIR spectroscopy was carried out using a Nicolet Magna-IR 830 spectrometer using attenuated total reflectance (ATR). The spectra were collected 16 times and corrected for background noise. The experiments were performed on the powdered samples, without KBr addition.

Results and Discussion The measured NO2 breakthrough curves along with the NO emission curves are presented in Figures 1and 2, respectively. The calculated capacities with the standard deviations calculated from three measurements are collected in Table 1. Even though the values listed in Table 1 are similar for all samples, with an increase in pyrolysis time and pyrolysis temperature, a change in the shapes of both the NO2 breakthrough curve and the NO emission curve is seen. From the point of view of the performance as adsorbents, the breakthrough curve with zero emissions for a long period of time, which is followed by a steep rise in the concentration of the emitted gas, can be considered as favorable. Moreover, the side products of surface reaction are undesirable owing to the secondary pollution problems. In the case of our materials, the shapes of the breakthrough curves indicate an increase in efficiency of adsorption with an increase in the carbonization temperature and time for the samples obtained at 650 and 950 °C. For the samples obtained at 500 °C, the best performance was found for the sample that was pyrolyzed for the shortest period of time. On the other hand, the NO emission curves indicate surface reduction, and the

FIGURE 1. NO2 breakthrough curves for samples pyrolyzed at 500 °C (A), 650 °C (B), and 950 °C (C). worst samples from the point of view of secondary pollution were samples obtained at the highest temperature. Considering practical applications, a high concentration of NO creates pollution problems, and thus, another type of chemistry would have to be employed in NO reduction. For NO2 removal, the pyrolysis time does not affect the performance significantly. The most significant effects of pyrolysis time on the surface activity/reactivity are found for the samples obtained at 650 °C. For this series of materials, an increase in holding time has a positive effect on both NO2 removal capacity and existence of secondary pollution. If NO either is not formed at the beginning of the removal process or if it is formed, it is efficiently retained on the surface for over 15 min, probably via its involvement in surface reactions. In the case of SC-60-500, SC-120-500, and SC-30-650, similarities in the specific shapes of the breakthrough curves VOL. 41, NO. 21, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 2. Structural Parameters for Initial and Exhausted Samples sample SC-30-500 SC-30-500-E SC-60-500 SC-60-500-E SC-120-500 SC-120-500-E SC-30-650 SC-30-650-E SC-60-650 SC-60-650-E SC-120-650 SC-120-650-E SC-30-950 SC-30-950-E SC-60-950 SC-60-950-E SC-120-950 SC-120-950-E

FIGURE 2. NO emission curves for samples pyrolyzed at 500 °C (A), 650 °C (B), and 950 °C (C).

TABLE 1. NO2 Breakthrough Capacity and Surface pH Values NO2 breakthrough capacity

pH

sample

mg/g adsorbent

mg/cm3 a adsorbent

initial

exhausted

SC-30-500 SC-60-500 SC-120-500 SC-30-650 SC-60-650 SC-120-650 SC-30-950 SC-60-950 SC-120-950

11.2 ( 1.5 13.4 ( 1.8 12.8 ( 1.7 15.8 ( 2.1 14.4 ( 1.9 12.7 ( 1.7 10.5 ( 1.4 11.8 ( 1.6 10.7 ( 1.4

6.8 ( 1.0 8.0 ( 1.2 7.9 ( 1.1 9.8 ( 1.4 8.4 ( 1.2 8.1 ( 1.2 6.4 ( 0.9 7.0 ( 1.0 6.4 ( 0.9

8.68 8.77 8.62 10.94 11.04 10.56 10.82 10.20 10.63

6.48 5.71 6.12 6.40 10.00 9.87 7.04 9.11 9.36

can be noticed. They show an increase in the concentration of NO2, relatively early in the experimental run, followed by a slow down in its release demonstrated as a plateau or even 7518

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SBET (m2/g) Vmic (cm3/g) Vmeso (m2/g) Vt (cm3/g) Vmic/Vt 69 25 74 19 75 23 82 14 84 15 82 15 114 74 116 81 106 79

0.032 0.011 0.037 0.009 0.035 0.010 0.040 0.007 0.041 0.007 0.040 0.007 0.049 0.029 0.049 0.032 0.044 0.031

0.054 0.047 0.051 0.042 0.055 0.044 0.056 0.041 0.058 0.043 0.062 0.042 0.090 0.083 0.100 0.091 0.104 0.097

0.086 0.058 0.088 0.051 0.090 0.054 0.096 0.048 0.099 0.050 0.102 0.049 0.139 0.112 0.149 0.123 0.148 0.128

0.37 0.20 0.42 0.18 0.39 0.19 0.42 0.15 0.41 0.14 0.39 0.14 0.35 0.26 0.33 0.26 0.30 0.24

a decrease in the concentration consistent in all analyses on replicate samples (or SC-60-500 and SC-120-500) followed by a steep rise in the NO2 concentration. This shape must be related to surface chemistry, specifically the activation of the surface sites. These surface sites can include reactive centers originating in either carbonaceous or inorganic phases that are highly dispersed in the pores of adsorbents. They also can be formed as a result of exposure to NO2. Therefore, the surface of materials should be analyzed before and after the adsorption process. It is well-known that for the adsorbents, an important feature is their porosity. The structural parameters for our materials calculated from the adsorption of nitrogen are listed in Table 2. An increase in the pyrolysis temperature has a positive effect on the development of the surface area. Even though the materials cannot be considered to be highly porous, those obtained at 950 °C have a relatively high volume of mesopores. Their volume increases almost 70% with an increase in the pyrolysis temperature between 650 and 950 °C. A small increase is noticed also in the volume of the micropores. As indicated previously (30), these changes are caused by increases in the degree of carbonization of the organic phase and changes in the inorganic phase such as decomposition of various hydroxides and salts and the formation of spinel-like structures. After exposure to NO2 and NO, as a side product of surface reactions, the surface area and volume of pores decrease. The effect is the most pronounced for samples obtained at 650 °C whose micropores almost disappear. Since that decrease is much smaller for samples obtained at 950 °C, a different mechanism of adsorption, related to the surface chemistry of our materials, can be involved. As analyzed previously, the surface of the low temperature pyrolyzed samples is much more reactive owing to a lesser extent to the solid state reactions between the oxides of the sludge (Table 3). Nevertheless, the observed decrease in the structural parameters must be related to the deposition of NO2 and NO via their reaction with surface active species. Pore size distributions presented in Figure 3 provide more details about the changes in porosity as a result of NO2 exposure. While for the series of materials obtained at 500 and 650 °C mainly pores smaller than 100 Å in diameter are affected via a decrease in their volume, in the case of their counterparts obtained at 950 °C, an increase in the volume of pores larger than 100 Å is noticed. They must be either secondary pores formed in large macropores as a result of the reaction of carbon with NO2 or macropores decreased in their sizes owing to the deposit of surface reaction products. Even though in all cases the volume of the micropores

TABLE 3. Chemical Composition of Sewage Sludge element/oxide SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 Cr2O3 NiO V2O5 ZrO2 LOI Total

content (%) 8.35 0.86 3.43 7.27 0.07 1.39 4.20 0.40 0.50 5.54 0.12 0.01 0.02 0.15 68.04 100.36

FIGURE 4. XRD patterns for samples pyrolyzed for 2 h. Solid triangles indicate differences between initial and exhausted samples.

FIGURE 3. Pore size distributions for initial and exhausted samples pyrolyzed at 500 °C (A), 650 °C (B), and 950 °C (C). disappears or significantly decreases, in the case of the materials pyrolyzed at 950 °C, pores between 10 and 20 Å seem not to be affected. This can be the apparent effect related to the decrease in the sizes of smaller mesopores or the formation of new pores as a result of oxidation. As discussed previously (26, 27), the materials obtained are basic in their chemical nature. This is related to their high content of calcium and magnesium (Table 3). Although an increase in carbonization temperature and time has little effect on the average surface acidity expressed as pH (Table 1), the changes in the chemistry are seen on the X-ray

diffraction pattern. Because of the complexity of the structure and limitation of figure space, we are not able to show the specific peak assignment. Examples of the results obtained for the samples obtained for 2 h are presented in Figure 4. In Figure 4, we direct the attention of the readers to the differences in the spectra before and after interactions with NO2 (marked as solid triangles), which must be caused by the chemical reactivity of the surface. As can be seen, a higher pyrolysis temperature increases the number, intensity, and sharpness of the peaks. The same is noticed with an increase in the pyrolysis time. This can be linked to the formation of new crystallographic phases as a result of high temperature reactions between the components of the sludge. In the case of materials obtained at 500 °C, the most pronounced crystallographic phases identified are quartz (SiO2), bornite (Cu5FeS4), cuprite (Cu2O), chalcite (Cu2S), cohenite (Fe3C), hematite (F2O3), and digenite (Cu9S5). Heating to 650 °C results in silica; sulfides of zinc, copper, and calcium; iron carbide (cohenite, Fe3C); and ferrisilicite (FeSi). After contact with NO2, sulfides are oxidized to oxides or metals on zero valency, as Zn or Fe. The most severe pyrolysis conditions lead to the formation of barringerite (Fe2P), fersilicite (FeSi), xifengite (Fe5Si3), bornite (Cu5FeS4), digenite (Cu1.8S), moissanite (FeC), lime (CaO), pyrrhotite (Fe1-xS), hematite (Fe2O3), tenorite (CuO), and other mixed valency copper oxides. Besides these compounds, various aluminosilicates with calcium, magnesium, or iron substitutions are present (30-32). It is interesting that after exposure of low temperature pyrolyzed samples to NO2, elemental sulfur and copper oxides on higher VOL. 41, NO. 21, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. FTIR spectra for samples pyrolyzed at 500 °C (A), 650 °C (B), and 950 °C (C). oxidation states are identified. This suggests the involvement of copper sulfides in the reduction of NO2 to NO. This effect is not seen in the case of high temperature pyrolyzed samples, where much less variety of copper sulfide species is present. Although after exposure to NO2 the formation of calcium nitrate and aluminum nitrate can be hypothesized as peaks at 2Θ 26.6 and 40.3 (for Ca(NO3)2) and at 29.4 (for Al(NO3)3‚ 9H2O), owing to the complexity of the chemistry of the materials studied, the diffraction patterns cannot bring firm evidence of their existence. More information about changes in surface chemistry after exposure to NO2 can be obtained from the analysis of the FTIR spectra presented in Figure 5. Besides an intense peak in the range between 900 and 1200 cm-1 where high intensities of bands representing Si-O-Al, Al-OH, SiO-Si, and Si-O are detected, new bands appear at 1430 and 1330 cm-1 for the series of samples pyrolyzed at 500 and 650 °C. They are related to the presence of nitrogen as nitrates (NO3-) and nitrites (NO2-), respectively (33, 34). The spectra are different for the samples obtained at 950 °C where only very small peaks of nitrites can be seen, and due to the reduction of the surface and the involvement of silica in compounds with iron and carbons, alumina is seen as a peak at 1050 cm-1 (35, 36). Carbonates in this sample are revealed at about 900 cm-1. For this sample, after exposure to NO2, a band at about 1300 cm-1 can be noticed, which represents nitro groups on carbons, which can be formed as a result of carbon-NO2 interactions. Although an indication of small bands in the carboxylic group region between 1100 and 1300 cm-1 can be seen (37, 38), the small amount of carbon in this sample limits the signals measured. The differences in chemistry between the series of samples after exposure to NO2 are clearly seen on DTG curves obtained in nitrogen (Figure 6). While the patterns for the exhausted samples pyrolyzed at 500 and 650 °C are similar, the samples obtained at 950 °C reveal a different trend with almost no weight loss (represented as peaks) over 400 °C. For the other two series of samples, well-defined peaks at about 100, 200, and 350 °C are seen. For the samples obtained at 650 °C, additional peaks at 450 and 600 °C appear, especially for the materials obtained at longer holding times. The first peak must represent the decomposition of unstable nitrites and 7520

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nitrates of such metals as magnesium, calcium, iron, copper, or manganese (39). The peak between 150 and 200 °C present on all DTG curves can be linked to the decomposition of Al(NO3)3 (39) and the removal of elemental sulfur (27), which is formed as a result of NO2 reduction on sulfides. It is the most pronounced for the samples obtained at 500 °C, where we do not expect the involvement of alumina in the formation of mineral-like structures (27) and where various sulfides are present as seen from XRD analysis. The peak between 300 and 400 °C we assign to the decomposition of sodium and potassium nitrates and nitrites (39). Only traces of these species are seen on DTG curves in the case of the samples obtained at 950 °C owing to the high probability of the presence of carbonates in these samples (31, 32) and the engagement of alkaline metals into new structures. Nevertheless, small amounts of nitrates and nitrites decomposed at low temperatures in agreement with the FTIR results. The analyses of surface chemistry and the structure of our samples show a clear difference in the mechanism of NO2 interactions with the surface of materials obtained at 500 and 650 °C and those pyrolyzed at 950 °C. On the surface of the latter, nitrates are not formed, and NO2 is likely reduced to NO via the involvement of the carbonaceous surface. In fact, in the case of this material, this phase is supposed to have the highest level of carbonization and aromatization in all of the samples studied. The inorganic phase of this material is also very stable owing to high temperature solid state reactions (26, 27). On the other hand, in the case of samples obtained at 500 or at 650 °C with the shortest pyrolysis time where the inorganic phase is expected to be the most reactive, NO2 reacts with oxides of alkaline earth and aluminum oxide and forms nitrites that become further oxidized to nitrates. The observed change in the slope of the breakthrough curve is likely caused by enhanced retention of NO2 via its involvement in the formation of nitrates from nitrites previously formed on the surface. Besides this, other species such as copper sulfides are involved in the reduction of NO2 with oxides and elemental sulfur as reaction products. It is interesting that the best performing sample, SC-120650, shows almost zero emission of NO2 and NO for about 15 min of the experimental run. This indicates that on its surface, NO2 is not reduced during that time and that its

For the 500 °C pyrolyzed samples

NO2(g) f tMNO2

(1)

2tMNO2 f tMNO3 + NO(g)

(2)

tMNO2 + NO2(g) f tMNO3 + NO(g)

(3)

where M represents a metal from any of the surface oxides. Reaction 2 describes the possible Langmuir-Hinshelwood mechanism where nitrite reacts with another nitrites, and reaction 3 describes the Eley-Rideal-type mechanism with nitrites reacting with NO2 from the gas phase. Those oxides can be present in sludge-derived adsorbents or formed from sulfides via their reaction with NO2. For simplification, reactions for stoichiometric sulfides are shown

CaS + NO2 f CaO + NO + S

(4)

Fe2S3 + 2NO2 f Fe2O3 + 2NO + 3S

(5)

CuS + NO2 f CuO + NO + S

(6)

ZnS + NO2 f ZnO + NO + S

(7)

Formation of these oxides can also contribute to the change in the slope of the breakthrough curve, which represents the change in the mechanism of the surface reaction (Figure 1). For the 950 °C pyrolyzed samples, the role of the carbonaceous surface seems to be predominant, and the following mechanism likely occurs following the results of Jequirim and co-workers (10):

-C* + NO2 f C(O) + NO

(8)

-C(O) + NO2 f -C(ONO2)

(9)

-C-C(ONO2) f CO2 + NO + -C*

(10)

-C-C(ONO2) f CO + NO2 + -C*

(11)

where -C* represents the carbon active site and -C(O) -the carbon-oxygen complexes. In the overall reaction, the oxidation of the carbon surface occurs

FIGURE 6. DTG curve in nitrogen for samples pyrolyzed at 500 °C (A), 650 °C (B), and 950 °C (C). significant amount is as NO3- or NO2-. This is not the case for the samples obtained at 950 °C, even though their breakthrough curves look similar to one another. That high surface reactivity of SC-120-950 toward reduction on NO2 can be linked to the reducing effect of its carbonaceous phase. On the basis of the results described previously and following the study of Underwood and co-workers (25) and Miletic and co-workers (40) who studied the interaction of NO2 with mineral oxides (Al2O3, Fe2O3, and TiO2) and magnesium oxide, respectively, we can propose the following surface reactions:

C + 2NO2 f CO2 + 2NO

(12)

C + NO2 f CO + NO

(13)

The 650 °C pyrolyzed samples have intermediate surface properties, and the two mechanisms mentioned previously likely occur, enhancing the removal process. The results presented in this paper demonstrate the effects of pyrolysis conditions on the performance of sewage sludgederived material as adsorbents of NO2 at room temperature and dry conditions. Owing to the chemical transformation occurring within inorganic and carbonaceous phases and between them, the reactivity of the surface toward NO2 reduction and retention on the surface changes. When adsorbents are obtained at low temperatures, mainly inorganic sulfides and oxides are responsible for reaction with NO2 via its reduction to NO and/or the formation of nitrates and nitrites. In the case of samples pyrolyzed at 950 °C, the stable inorganic phase is much less reactive, and it is a carbonaceous phase on which reduction of NO2 to NO occurs. The best adsorbents are obtained at 650 °C. On their surface, VOL. 41, NO. 21, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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both inorganic and carbonaceous phases seem to be involved in the surface reactions.

Acknowledgments This work was supported by ARO Grant W911NF-05-1-0537.

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Received for review July 26, 2007. Accepted August 27, 2007. ES071863W