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Ind. Eng. Chem. Res. 2006, 45, 3666-3672
Removal of Hydrogen Sulfide on Composite Sewage Sludge-Industrial Sludge-Based Adsorbents Teresa J. Bandosz*,† and Karin A. Block‡ Department of Chemistry, The City College of New York, City UniVersity of New York, 138th Street and ConVent AVenue, New York, New York 10031, and Department of Earth and EnVironmental Sciences, The Graduate School and UniVersity Center of the City UniVersity of New York, 365 Fifth AVenue, New York, New York 10016
The sludge-based adsorbents were obtained either from mixtures of sewage sludge, waste oil sludge, and metal sludge or single components by carbonization at 650 °C in an inert atmosphere. 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, ICP, and surface pH measurements. Although on all materials hydrogen sulfide is oxidized to elemental sulfur, exceptionally good performance is obtained on the waste oil sludge-based adsorbent. This is attributed to the combined effects of surface chemistry and porosity. High pore volume of the waste oil sludge-based adsorbent provides space to store 30 wt % elemental sulfur formed when hydrogen sulfide undergoes oxidation on the surface. Mixing sludges and carbonization of their mixtures result in adsorbents whose capacity, although smaller than that for the single-component waste oil sludge-based adsorbent, is high compared to that of conventional activated carbons. Moreover, when additional chemical heterogeneity is provided, the structural and chemical features of the mixed waste oil sludge/sewage sludge-based adsorbents are enhanced as a result of synergy between the individual components, which occurs during solid-state reactions. Introduction Nowadays, strict environmental regulations compel the search for new, efficient adsorbents and catalysts. While activated carbons are still considered the best conventional adsorbents, catalysts based on noble metals become more and more expensive, which leads to an increase in productions costs, especially in the petrochemical industry. Excellent adsorption capacities of activated carbons are linked to their high surface area and large pore volumes. Small sizes of pores are critical when carbons are used as adsorbents of small molecules at ambient conditions. Despite these assets, virgin/unmodified activated carbons are not very efficient as media for reactive adsorption or catalysis. Although they may have surface catalytic centers active in certain reactions,1 and their surface can contain heteroatom-based polar groups involved in specific sorbatesorbent interactions, this specificity is not enough in the situation when the only way to immobilize the pollutant is via its complete conversion to another compound which is strongly adsorbed on the surface. To enhance this so-called reactive adsorption, special surface modifications have to be carried out. The most common modifications of carbon surfaces are oxidation,2,3 incorporation of nitrogen,4,5 or impregnation with various salts, acids, or hydroxides.6,7 The latter can be done either via an incipient wetness method or via coprecipitation. As a result of these treatments, new functional groups/chemical species are introduced and immobilized on the surface. Their role is to impose the specific and/or chemical interactions with the species to be removed.8 Here, the chemical state of these species and their dispersion on the surface become crucial. The modifications should be done in such a way in which a minimal * To whom correspondence should be addressed. Tel.: (212) 6506017. Fax: (212) 650-6107. E-mail:
[email protected]. † Department of Chemistry. ‡ Department of Earth and Environmental Sciences.
decrease in the surface area/pore volume is achieved. Very often this is considered a challenge. All the above resulted in exploration of the possibility of using wastes, municipal or industrial, as a source of adsorbents.8-28 Since disposal of such waste requires significant efforts and energy,29-31 their conversion to adsorbents is a promising alternative. It is estimated that about 10 million dry tons of sewage sludge is produced in the United States.30 Moreover, Sweden alone31 contributes 220000 dry tons of sludge to 8-10 million tons of dry sludge produced by the European Union.32 Conversion of sewage sludge to adsorbents via pyrolysis has been addressed in scientific literature.8-28 As a result of various treatments, surface areas between 100 and 500 m2/g were obtained. Materials were studied as adsorbents of hydrogen sulfides, sulfur dioxide, basic or acidic dyes, phenol, or mercury. Their performance was reported as comparable or better than that of activated carbons.20-25 Good performance of the sludge-derived materials has been linked not only to the presence of highly dispersed inorganic phases with catalytic metals such as copper, zinc, iron, or calcium but also to the carbon phase in which small micropores exist.17,18,28,33 It was found that an excellent capacity of industrial sludge-sewage sludge composite adsorbents for hydrogen sulfide removal is the result of the formation of new minerallike phase during carbonization of sludge mixtures, which is based mainly on zinc, iron, and calcium.34 The objective of this paper is to explore the possibility of utilization of industrial sludges such as waste oil sludge, metal sludge, and sewage sludge as precursors for the development of industrial waste-based adsorbents.34 An important and environmentally friendly aspect is simplicity of the process, low carbonization temperature (650 °C), and short time of carbonization (half an hour). The application of new adsorbents is demonstrated in the desulfurization of moist air where such exceptionally high capacities as 30 wt % can be obtained. Removal of hydrogen sulfide from air is important in wastewater
10.1021/ie0514152 CCC: $33.50 © 2006 American Chemical Society Published on Web 04/18/2006
Ind. Eng. Chem. Res., Vol. 45, No. 10, 2006 3667 Table 1. Adsorbents’ Composition, Yields, and Densities sample WO SS MS WOSS WOMS WOSSMS
wet composition
solid content
dry precursor composition
yield (dry mass)
bulk density [g/cm3]
WO: 100% SS: 100% MS: 100% WO: 50% SS: 50% WO: 50% MS: 50% WO: 40% SS: 40% MS: 20%
23.6 24.6 23.4
WO: 100% SS: 100% MS: 100% WO: 49% SS: 60% WO: 50% MS: 50% WO: 46% SS: 31% MS: 23%
32 47 52 40
0.26 0.52 0.47 0.36
58
0.38
46
0.38
treatment facilities, especially those located in populated areas. Although the concentration of H2S in air in such facilities reaches only a few hundred ppb, due to the practical reasons for testing the performance of adsorbents, so-called accelerated tests are used with high concentrations of hydrogen sulfide (between 3000 and 10000 ppm). Experimental Section Materials. Two industrial sludges, waste oil sludge (WO) and metal sludge (MS), from Newport News Shipyard were mixed with dewatered sewage sludge from Wards Island Water Pollution Control Plant, NYC DEP (SS), homogenized, dried at 120 °C for 48 h, and then carbonized at 650 °C in nitrogen in a horizontal furnace. The heating rate was 10 °C/min with a holding time of half an hour. The names of the adsorbents obtained, their compositions along with the yield, ash content, and bulk density are collected in Table 1. According to the information supplied by the shipyard, waste oil sludge was treated with CaCl2, Na3PO4, NaOH, and alum. Metal sludge treatment history includes addition of sulfuric acid and sodium hydroxide for pH adjustments, Al2SO4 for coagulation, anionic and cationic polymers, sodium bisulfide for chromium reduction, lime, and CaCl2. Thus, besides alkaline or alkaline earth element-containing compounds and iron, the waste oil sludge also contains 0.4% Cu, 2% Zn, and between 200 and 1000 ppm of chromium, lead, and nickel. In metal sludge there are less than 1% each of cadmium, chromium, copper, lead, manganese, selenium, vanadium, and zinc. The content of volatile compounds in both waste oil sludge and metal sludge reaches 40% of their dry mass, while the content of water in as-received materials is about 75%. The pH is neutral. The sewage sludge from NYC was characterized elsewhere.20 As in its industrial counterparts, it contains about 65% of fixed solids and about 35% of volatile solids and has neutral pH. Methods. (a) Evaluation of 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.35 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. 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 was passed through the column of adsorbent at 0.5 L/min. The flow rate was controlled using Cole Parmer flowmeters. The breakthrough of H2S was monitored using electrochemical sensors. The test was stopped at the breakthrough concentration of 350 ppm. The adsorption capacities of each adsorbent in terms of mg of hydrogen sulfide per g 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. Besides H2S the content of SO2 in the outlet gas was also monitored using a MicroMax electrochemical sensor. The adsorbents exhausted after H2S adsorption are designated by adding an additional letter, E, to their names. (b) Characterization of Pore Structure of Adsorbents. On the materials obtained, sorption of nitrogen at its boiling point was carried out using ASAP 2010 (Micromeritics). 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 relative pressure equal to 0.99), volumes of micropores, Vmic (DR36), mesopore volume Vmes, and total pore volume, Vt, along with pore size distributions, were calculated (DFT37,38). (c) 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. (d) Thermal Analysis. Thermal analysis was carried out using a TA Instrument Thermal Analyzer. The instrument settings were as follows: heating rate 10 °C/min and a nitrogen atmosphere with 100 mL/min flow rate. For each measurement about 25 mg of a ground adsorbent sample was used. For analysis of the results the derivative thermogravimetric curves (DTG curves) are used. (e) Elemental Analysis. Metal content in the adsorbents was determined using ICP in LSL labs., Syracuse, NY. (f) XRD. X-ray diffraction measurements were conducted using a standard powder diffraction procedure.39 Adsorbents were ground with methanol in a small agate mortar. Grinding of the adsorbents by hand ensures particle sizes between 5 and 10 µm, which prevents line broadening in diffraction peaks. The mixture was smear-mounted onto the zero-background 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. A quartz standard slide was run to check for instrument wander and to obtain accurate location of 2θ peaks. Results and Discussion The main structural characteristics of adsorbents are obtained from their nitrogen adsorption isotherms. As seen from Figure 1, the materials differ in either a nitrogen uptake or shapes of the isotherms. All samples obtained from single components are mainly mesoporous with hysteresis loops related to complex pore shapes. Mixing waste oil sludge with two other sludges results in a decrease in the nitrogen uptake of adsorbents and thus in a decrease in their porosity. The structural parameters of the adsorbents obtained are collected in Table 2. The highest surface area is found for the WO sample. This sample also has exceptionally high volumes of pores, mainly mesopores. The SS sample has a twice smaller surface area and volume of micropores than those for WO and its volume of mesopores is much lower. This must contribute to the density of materials and the WO sample is expected to be very light. Indeed, as listed in Table 1, its density is half of that obtained for the sewage sludge-derived material. Mixing other sludges with waste oil results in the value of structural parameters located between those found for the singlecomponent materials. All materials obtained have a very small
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Figure 2. X-ray diffraction patterns. Table 3. Content of Catalytically Important Metals in the Adsorbents Studied
Figure 1. Nitrogen adsorption isotherms. Table 2. Structural Parameters sample WO WO-E SS SS-E MS MS-E WOSS WOSS-E WOMS WOMS-E WOSSMS WOSSMS-E
Vmic ∆Vmica Vmes ∆Vmesb Vt Vmic/Vt SBET [m2/g] [cm3/g] [cm3/g] [cm3/g] [cm3/g] [cm3/g] [%] 202 83 92 79 34 25 154 72 92 65 110 59
0.074 0.032 0.037 0.029 0.014 0.011 0.058 0.027 0.036 0.026 0.042 0.023
-0.42 -0.008 -0.003 -0.031 -0.010 -0.011
0.765 0.434 0.113 0.106 0.122 0.160 0.459 0.281 0.270 0.265 0.372 0.250
-0.321 -0.007 0.038 -0.178 -0.005 -0.122
0.839 0.517 0.150 0.135 0.136 0.171 0.517 0.308 0.306 0.291 0.415 0.273
10 6 25 27 11 6 12 10 12 9 10 8
a
Calculated from the difference between the Vmic of the initial material and the exhausted one. The negative values indicate a decrease in the volume of pores after H2S adsorption. b Calculated from the difference between the Vmes of the initial material and the exhausted one. The negative values indicate a decrease in the volume of pores after H2S adsorption and the positive values an increase in the volume of pores after H2S adsorption.
degree of mesoporosity (Vmic/Vt) with the overwhelming majority of pores with diameter greater than 20 Å. Mixing sludges also has an effect on the overall chemistry of the final product. The contents of catalytically active metals in our adsorbents is presented in Table 3. As seen, and based on the origin of raw sludges and the technology applied for their treatment, the materials have high contents of iron, zinc, and copper. Their high pH is linked to the present of a few percent of calcium and magnesium. Iron, zinc, and copper were identified as catalytically important for H2S immobilization on sewage sludge-derived adsorbents19 and calcium and magnesium contribute to high pH and thus to the high degree of H2S dissociation, which facilities its oxidation.33 Figure 2 shows X-ray diffraction patterns for the samples studied. Although the degree of crystallinity is not high in WO,
sample
Fe [%]
Ca [%]
Mg [%]
Cu [%]
Zn [%]
Cr [ppm]
SS650 WO650 MS650 WOSS650 WOSSMS950
4.9 3.2 2.3 4.0 4.5
4.8 4.0 14.0 4.4 7.0
1.3 11.0 0.50 6.15 3.90
0.13 0.20 0.80 0.16 0.34
0.19 0.54 0.20 0.36 0.30
58 140 6700 99 1488
the following minerals identified are crystallographic phases: barringerite (Fe2S), digenite (Cu1.8S), and metallic iron. In SS, besides metallic aluminum and iron, magnesium calcite, sapphirine ((Mg4Al4)Al4Si2)O20), and quartz are present. In MS650 moissanite (SiC), margarite (CaAl2(Si2Al2)O10(OH)2), almandine (Fe3Al2(SiO4)3), and metallic aluminum and iron are present. When sewage and waste oil sludge precursors are mixed and pyrolyzed at 650 °C, iron is still detected, and anorthite (CaAl2Si2O8) and diaspore (AlO(OH)) appear as new species. All of these identified minerals are calcium and magnesium aluminosilicates, which must be formed in solid-state reactions during pyrolysis. Mixing three sludges results in the formation of troilite (FeS), moganite (SiO2), pyrrhotite (Fe1-xS), sphalerite (ZnS), ferrisilicite (FeSi), zhanghengite (CuZn), and metallic aluminum, copper, and iron. In Figure 3 the appearance of new peaks can be observed; however, they are not assigned to the specific compounds. Doing this would make the figure unreadable due to the peak overlapping. The DTG curves obtained for the initial individual components and their mixtures are presented in Figure 3. Although at our pyrolysis temperature all volatile organic compounds are already removed from the samples, some changes still occur at higher temperatures. They must be related to decomposition on an inorganic phase (salts/oxides/hydroxides) and solid-state reactions. In the case of sewage sludge and waste oil sludge, broad complex peaks with a shoulder at about 450 °C are revealed. The curve for MSin looks complex; however, the intensity of the peaks is much smaller than that for waste oil sludge and sewage sludge due to the small content of organic matter. Nevertheless, weight loss is seen in the entire experimental window as a result of decomposition of inorganic salts. When the materials are mixed, a new feature found for all samples is an increase in the weight loss between 100 and 200 °C, which was not so well-pronounced for the single compo-
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Figure 4. H2S breakthrough curves.
Figure 3. DTG pyrolysis curves for single sludges and their mixtures. Table 4. H2S Breakthrough Capacities (Brth Capacity), Adsorption of Water, and Surface pH before and after H2S Adsorption
sample
brth capacity [mg/g]
brth capacity [mg/cm3]
water adsorbed [mg/g]
pH
pHE
WO SS MS WOSS WOMS WOSSMS
315 9 79 146 130 78
82 5 37 53 49 33
48 18 0 21 14 20
9.3 10.9 7.8 9.2 9.8 9.7
9.3 11.1 7.1 9.1 9.4 9.2
nents. This effect is especially seen for the WOMSin mixture. The evolution of gases/vapors, which is revealed as well-defined shoulders between 450 and 500 °C, is possibly responsible for the development of an additional porosity in the mixtures of sludges. The presence of alkali and alkaline earth metals used in sludge treatment technology is demonstrated by basic pH listed in Table 4. In Table 4 we also summarize the performance of materials as H2S adsorbents. The H2S breakthrough capacities were obtained from the breakthrough curves collected in Figure 4. Following the breakthrough time, WO and its two-component mixtures with sewage sludge or metal sludge have the best performance, although WO outperforms all other materials developed in this research task. The slopes of the curves are similar to each other, which suggests similarity in the reactive adsorption process. The capacities were calculated either per unit volume of the adsorbent bed or per unit mass of the adsorbent. When the performance per unit volume of the bed is considered, WO has a capacity similar to that obtained on caustic impregnated activated carbons and better than that of Centaur catalytic carbon (between 0.12 and 0.20 g/g depending on the type of Centaur).33,40 The high capacity comparable to that of Centaur is also obtained for the two-component mixtures. The very low capacity of sewage sludge-based material, much
lower than that reported previously,18 is the result of a lowtemperature treatment and short carbonization time, which limit the chemical transformations (mineralization) needed for a good performance.34 When values per unit mass are analyzed, it is clearly seen that on the surface of WO about 30 wt % hydrogen sulfide can be adsorbed. High values are also found for the twocomponent mixtures and even the material obtained from metals sludge, despite its low surface area, can store around 8 wt % hydrogen sulfide. Adsorption of water during prehumidification is linked to surface hydrophilicity and porosity. The most reactive toward water is WO and least-MS. These differences are the result of chemical transformations during pyrolysis. In MS mainly an inorganic dehydroxylated phase is present, whereas in WO and SS, between 10 and 30% carbon phase was detected. The unchanged pH values for adsorbents before and after desulfurization, even when such high capacity as 30 wt % is revealed, indicate the deposition of elemental sulfur.18,34 Indeed on DTG curves for exhausted samples (Figure 5) the peaks between 200 and 400 °C are predominant. They represent the removal of sulfur from large pores of adsorbents. On the curve for the WO sample and its mixtures with two other sludges also, a broad peak between 400 and 650 °C is noticed. It might represent the removal of sulfur from micropores originating from a waste oil sludge-derived phase. The peaks over 650 °C cannot be interpreted here since the materials were not exposed to such a high temperature during pyrolysis. The DTG results obtained from TA analysis are collected in Figure 6. While for WO, SS, and their mixture a gradual weight loss is observed with an increasing temperature, for MS and its mixture with waste oil sludge and sewage sludge an unusual increase in the weight is found between 200 and 600 °C. Since the samples were exposed only to nitrogen, the only plausible explanation at this stage of our study is nitride formation, which can happen at relatively low temperatures when char is present in the system.41 It is interesting that this weight increase was not found for the WOMS material. In this combination, the active ingredients in MS must be chemically bonded with the WO components. However, it does not happen when three components are present in the precursor. From TG curves the weight losses at various temperature ranges were calculated and compared to the amount of sulfur expected on the surface based on the breakthrough experiments. The results are presented in Table 5. ∆ represents the difference in the weight loss between the initial and exhausted sample. With an assumption that elemental sulfur is the only reaction product, very good agreement between the total weight loss and the amount of sulfur expected on the surface is found. Discrepancies exist for the samples where metal sludge is
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Ind. Eng. Chem. Res., Vol. 45, No. 10, 2006 Table 5. Weight Losses [%] in Various Temperature Ranges and Amount of Sulfur Adsorbed from H2S Breakthrough Capacity Test (SBrth) [%]a sample
20-150
WO WO-E SS SS-E WOSS WOSS-E MS MS-E WOMS WOMS-E WOSSMS WOSSMS-E
4.70 5.42 1.86 3.08 3.66 4.48 1.01 1.16 3.31 2.86 1.52 4.65
D 0.72 1.22 0.82 0.15 0 3.13
150-400 1.71 23.35 0.46 1.03 1.37 13.3 0 6.62 0 6.13 0 8.2
D 21.64 0.57 11.93 6.62 6.13 8.2
∆ total 400-650 150-650 ∆b SBrth 0.77 3.41 0.68 1.48 0.78 2.28 2.49 2.13 0.93 2.78 3.23 3.16
2.64 0.80 1.5
31.6 1.38 15.2
31 0.9 14.3
0
7.14
7.7
1.85
9.00 12.7
0
9.20 12.0
a
∆ represents the differences in the weight loss between the exhausted and initial sample. b Weight loss is corrected for the amount adsorbed in H2S breakthrough test.
Figure 5. DTG curves in nitrogen.
Figure 7. SO2 emission concentrations.
Figure 6. TG curves in nitrogen.
present. The reason for this is in inaccuracy of the weight calculation in the ranges where an increase in the weight is observed for the initial samples and also in the possibility of sulfide formation in the reaction of hydrogen sulfide with heavy
metals. Those sulfides can decompose at temperatures higher than 650 °C or even higher than 1000 °C.18 For efficient H2S adsorbents the emission of SO2 should also be monitored to avoid the secondary pollution problems. The SO2 concentration curves are present in Figure 7. As seen, the SO2 emissions are very small and they start at almost the same time when H2S is detected in the outlet gas. This indicates the high efficiency of desulfurization and no secondary pollution problems. The role of mesoporosity in our materials is seen when the nitrogen adsorption isotherms for the exhausted samples are analyzed and pore size distributions are calculated.18 Figure 8 compares the PSDs for adsorbents before and after H2S adsorption. The effect of sulfur deposition is clearly seen in pores between 20 and 1000 Å (meso- and macropores). As suggested previously, these sizes of pores can be crucial for H2S adsorption when the whole surface is catalytically active.27 Indeed, their volume decreased significantly and, as listed in Table 2, that decrease reaches 0.3 cm3/g for WO material and 0.17 cm3/g for WOSS and is caused by the deposition of sulfur. We must mention here that the effect of the sulfur deposition on a decrease in porosity is even underestimated here since some sulfur, likely from the biggest pores, was removed from the surface during outgassing. This was seen as a yellow deposit on the sample tube internal walls. Even if we assume that all sulfur is present on the surface and it occupies all space equal to a decrease in the pore volume, the calculated average density of sulfur is approximately 1 g/cm3. On the other hand, sulfur can also be relocated during outgassing and some pores can be
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Figure 8. Pore size distributions. Figure 10. Comparison of surface area and volume of mesopores, measured and predicted.
(Figure 10). These findings support the earlier conclusion that without a proper chemistry of the surface the high pore volume is not going to compensate for the capacity.35 As indicated based on XRD, adding metal sludge causes the formation of the transition metal-based minerals (with copper, zinc, and iron) and also the formation of metallic copper which may be active in the reaction with hydrogen sulfide. Conclusions
Figure 9. Comparison of the measured and predicted H2S breakthrough capacity.
blocked by the sulfur deposit. This may cause the real density of sulfur to be higher than 1 g/cm3, which, in fact, seems to be more realistic. Moreover, the deposited sulfur in large pores can contribute to a new porosity development as seen for the samples containing metal sludge. The effect of mixing sludges on the performance of the materials is presented in Figure 9 where measured and predicted H2S breakthrough capacities are compared. The prediction was done taking into account the yields of single-component adsorbents and assuming the physical mixture of the components with surface characteristics and performance of the singlecomponent materials. Even though the error bars suggest the similarity in the values, the only beneficial effect noticed is when the waste oil sludge is mixed with sewage sludge. Adding metal sludge impedes the capacity of the mixture. On the other hand, mixing has an overall positive effect on the structural parameters; the surface areas and volume of pores are higher than those predicted, assuming the physical mixture of components
Low-temperature carbonization of industrial sludges results in the development of adsorbents with an exceptionally high capacity for hydrogen sulfide oxidation to elemental sulfur. On the materials obtained up to 30 wt % sulfur can be stored in the whole range of pores. Especially active in the adsorption process are large pores, meso- and macropores, and the new materials are very rich in them. The source of that porosity is in a waste oil sludge component. Moreover, mixing of waste oil sludge with sewage and metal sludge results in the formation of an additional porosity compared to the single-components adsorbents. Besides the high pore volume, the large H2S removal capacity is attributed to the specific chemistry of the inorganic phase where alkali and alkaline earth metals are present along with the transition metals (iron), which can contribute to redox reactions. Pyrolysis of sludges results in a partial mineralization of their components. Those minerals, particularly calcium aluminosilicates, can also enhance the catalytic activity via their basicity. Acknowledgment This work was partially supported by PSC-CUNY Grant No. 67284-0036. The authors are grateful to Vyacheslav Starkov
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ReceiVed for reView December 20, 2005 ReVised manuscript receiVed March 18, 2006 Accepted March 22, 2006 IE0514152
