Environ. Sci. Technol. 2006, 40, 3378-3383
Municipal Sludge-Industrial Sludge Composite Desulfurization Adsorbents: Synergy Enhancing the Catalytic Properties TERESA J. BANDOSZ* AND KARIN 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
Mixtures of sewage sludge, waste oil sludge, and metal oil sludge were prepared and carbonized at 950 °C in an inert atmosphere. Dynamic adsorption of H2S was measured on the materials obtained, and the breakthrough capacity was calculated. The initial and exhausted adsorbents after the breakthrough tests were characterized using sorption of nitrogen, thermal analysis, and XRF, XRD, and surface pH measurements. Mixing sludges leads to very high capacity adsorbents on which hydrogen sulfide is oxidized to elemental sulfur. Although the micropore volume of the adsorbents obtained is not high, their high volume of mesopores contributes significantly to reactive adsorption and provides space to store the oxidation products. The H2S breakthrough capacity on the new materials reaches 10 wt %. These adsorbents work until all active pores are filled and the catalytic centers are exhausted. The reason for such high capacity is in the formation of catalytically active mineral like phases during pyrolysis in the presence of nitrogen and carbon. This highly dispersed phase provides basicity and catalytic centers for hydrogen sulfide dissociation and its oxidation to sulfur.
Introduction One category of the wastes produced in abundant quantity by contemporary society is sludges, either sewage-based or industrial waste-based (1). The former are a mixture of exhausted biomass generated in the aerobic and anaerobic digestion of the organic constituents of municipal sewage along with inorganic materials such as sand and metal oxides. Industrial sludges include wastes from industries such as shipyards, motor vehicle production plants, airplane manufacturing facilities, foundries, paper mills, etc. It is estimated that about 10 million dry tons of sewage sludge is produced in the United States annually (2). Moreover, Sweden alone (3) contributes 220 000 dry tons of sludge to 8-10 million tons of dry sludge produced by the European Union (4). The process of carbonization of sewage sludges has been studied in detail previously and is described extensively in the literature (2-27). Materials obtained as a result of the * Corresponding author. Department of Chemistry, The City College of New York, City University of New York. Phone: (212) 6506017; fax: (212) 650-6107; e-mail:
[email protected]. † Department of Earth and Environmental Sciences, The Graduate School and University Center of the City University of New York. 3378
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various treatments have surface areas between 100 and 500 m2/g. Their performance as adsorbents of hydrogen sulfides, sulfur dioxide, basic or acidic dyes, phenol, or mercury has been reported as comparable to or better than that of activated carbons (19-24). In many processes an excellent sorption ability of these materials is linked to the catalytic action of metals present in various forms in the final products. Their chemical forms along with their location on the surface were reported as important factors governing the pollutant removal capacities. It has been shown that by simple pyrolysis of sewage sludge derived fertilizer, Terrene, exceptionally good adsorbents for removal of sulfur-containing gases can be obtained (18). Their removal capacity is twice that of coconut shellbased activated carbon. It happens although the carbon content is small (about 20%) and the pore volume is much smaller than that of carbons. Oxidation of hydrogen sulfide occurs until all micropores (mainly about 6 Å in size), likely within carbonaceous deposit or on the carbon/oxide interface, are filled with the reaction products. The form of that carbonaceous deposit is important (28) and that deposit may play a role in adsorption capacity. The products of oxidation immobilized on the surface are stored there. Recent studies showed that the pore volume active in the removal of such compounds as hydrogen sulfide does not need to be in pores similar in size to the adsorbent molecule (26). Since the catalytic oxidation is the predominant mechanism of adsorption, the larger pores, meso- and macropores, where the product of oxidation are stored were found to be beneficial. Another important factor is the chemistry of a catalytic phase, its dispersion, location on the surface, compatibility with the carbon phase, and the effects of both phases on the removal process (adsorption/catalytic oxidation/storage). It was found that excellent capacity of an expensive desulfurization catalyst, US Filter carbon Midas (29), is linked to the presence of calcium and magnesium oxides dispersed within the microporous activated carbon (29). On this catalyst, hydrogen sulfide is oxidized on basic centers of alkali earth metal oxides and sulfur is formed. The fact that this carbon is able to retain up to 60 wt % sulfur is linked to a limited reactivity of MgO and CaO. On their surface, due to the basic pH and the presence of moisture, sulfur is formed and, owing to the close proximity of the carbon phase, that sulfur migrates to the high-energy adsorption centers, small pores. In this way the catalytic centers are renewed and the adsorbents work until all small pores are filled by sulfur. The objective of this paper is to demonstrate that industrial sludges such as waste oil sludge and metal sludge can be utilized to enhance the properties of sewage sludge-based adsorbents (35). Although only waste oil sludge can lead to adsorbents with a good performance in desulfurization with 10 wt % removal capacity, we consider presence of sewage sludge as an economically feasible method of utilization of this abundant material. The performance of the adsorbents is demonstrated in conjunction with their surface features, which include the surface chemistry, porosity, and the synergetic effects between them.
Experimental Section Materials. Two industrial sludges, waste oil sludge (WO) and metal sludge (MS) from Newport News Shipyard were mixed with dewatered sewage sludge (SS) from Wards Island Water Pollution Control Plant, NYC DEP, homogenized, dried at 120 °C for 48 h, and then carbonized at 950 °C in nitrogen in a horizontal furnace. The heating rate was 10 °C/min with holding time half an hour. The names of the adsorbents 10.1021/es052272d CCC: $33.50
2006 American Chemical Society Published on Web 04/05/2006
TABLE 1. Adsorbents’ Composition, Yields, Their Ash Contents, and Bulk Densities
sample
wet composition
solid content
dry composition
WO SS MS WOSS WOMS WOSSMS
WO 100% SS 100% MS 100% WO 50%; SS 50% WO 50%; MS 50% WO 40%; SS 40%; MS 10%
23.6 24.6 23.4
WO 100% SS 100% MS 100% WO 49%; SS 51% WO 50%; MS 50% WO 46%; SS 31%; MS 23%
yielda (dry mass) [%] 29 45 47
ash contentb
∆c [g/cm3]
92 80 NDd ND ND ND
0.48 0.46 0.47 0.46 0.47 0.46
a Yield: Percentage of the product in relation to the amount of the precursor (dry). b Determined as mass left at 950 °C after TA run in air. c ∆: Bed density (for the same particle size and the same packing for all adsorbents). d ND: Undeterminable as a result of reaction with air at high temperature.
obtained and their compositions, along with the yield, ash content, and bulk density are collected in Table 1. Methods. Evaluation of H2S Sorption Capacity. A customdesigned dynamic test was used to evaluate the performance of adsorbents for H2S adsorption from gas streams as described elsewhere (31-33). Adsorbent samples were ground (1-2 mm particle size) and 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 Palmer flow meters. 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. 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 isotherm at relative pressure equal to 0.99), volumes of micropores, Vmic (DR (34)), mesopore volume Vmes, and total pore volume, Vt, along with pore size distributions were calculated (DFT (35, 36). 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 Instrument thermal analyzer. The instrument settings were 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. XRF. X-ray fluorescence analysis was applied to study the calcium and iron 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 a solid phase after grinding and sieving in order to use the matrices with similar physical properties.
FIGURE 1. H2S breakthrough curves. XRD. X-ray diffraction measurements were conducted using standard powder diffraction procedure. 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 zerobackground quartz window of a Phillips specimen holder and allow 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 H2S breakthrough curves obtained on our sewage/industrial sludge based adsorbents are presented in Figure 1. Their various slopes indicate the differences in the kinetics of the surface reaction in the single particle of the adsorbent. It is interesting that the curves for SS and WOSS are much steeper than those for other adsorbents. This suggests the sewage sludge components affect the chemistry of the removal process. Although the WOSSMS seems to work the best (longest time), at low concentration the crossover with the curve for WOSS is noticed. This indicates that till 50 ppm WOSS is the most efficient adsorbent. Knowing that oxidation of hydrogen sulfide may also lead to the undesirable formation of SO2, the concentration of sulfur dioxide eluted from the bed was monitored and the concentration curves are presented in Figure 2. For all adsorbents but WO less than 1 ppm of SO2 was recorded and the emission occurred almost simultaneously with that of H2S, which is a positive factor from the point of view of the performance of these materials. Only in the case of the material derived from waste oil sludge does the concentration of SO2 reach 7 ppm and then decrease to 5 ppm. This phenomenon may be caused by the presence of a strong oxidizing agent which is able to oxidize SO2 to SO3. After its reaction with water, sulfuric acid is formed. That small VOL. 40, NO. 10, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. SO2 emission curves.
TABLE 2. H2S Breakthrough Capacities, Adsorption of Water, and Surface pH Before and After H2S Adsorption
sample WO SS MS WOSS WOMS WOSSMS
breakthrough breakthrough water capacity capacity adsorbed [mg/g] [mg/cm3] [mg/g] 109 45 5 108 86 121
52 21 3 50 40 56
0 26 0 11 3 4
pH
pHE
9.9 9.4 10.9 10.0 11.2 11.2 10.8 9.1 9.9 8.8 10.5 9.4
quantity of H2SO4 either remains on the surface or reacts with the inorganic phase. From the breakthrough curves, the H2S breakthrough capacities were calculated and the results are summarized in Table 2. The results per a unit bed volume for all samples but that derived from sewage sludge and metal sludge are between 40 and 50 mg/cm3 which is over two times more than the capacity of the coconut shell- based activated carbon (18). Smaller capacity for the sewage sludge-based material than reported previously is the result of different carbonization conditions. In this research holding time of 0.5 h was used as opposed to 1 h applied before (18). When the capacity is recalculated per unit mass of an adsorbent, over 10 wt % of H2S is retained on the surface. The results clearly show that adding sewage sludge to waste oil sludge does not decrease the capacity of the latter. Moreover, the capacity of the resulting material is now higher than that of sewage sludge carbonized for twice as long at 950 °C, which is a significant saving in the cost of the process. A very interesting observation is the high level of surface hydrophobicity (Table 2). WO does not adsorb any water during prehumidification. Mixtures of waste oil with sewage sludge or sewage and metal sludge are also very hydrophobic. Only on SS is about 2.5% of water adsorbed. These results suggest nonreactivity and stability of inorganic phase. From the point of view of overall surface chemistry, the materials obtained are basic and H2S oxidation on the surface decreases the pH values only slightly indicating formation of sulfur and its salts. In the case of sewage sludge-based materials, the source of their basicity is calcium. The calcium is also present in industrial sludges. According to the information supplied by the shipyard, the 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 oil waste also contains 0.4% of Cu, 2% of Zn, 3380
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FIGURE 3. XRF spectra for the initial samples. 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. Since for catalytic oxidation of hydrogen sulfide, calcium and iron were indicated previously as the most important (18, 33), their relative content was evaluated using XRF (Figure 3). As indicated iron and calcium are present in large quantity with the ratio of intensity of iron peak to calcium peak about 2. In WO and WOSS a significant amount of zinc is also detected. The changes on the surface of the materials after H2S adsorption can be seen from analysis of DTG curves in nitrogen where peaks represent the weight losses as a result of heat treatment (Figure 4). The peak at less than 200 °C represents the removal of water and weakly adsorbed SO2, which was formed on the surface. Based on the practically unchanged pH values (Table 2), the second broad peak located between 200 and 450 °C represents the removal of elemental sulfur. Its intensity is related to the breakthrough capacity of adsorbents and broadness - to the sizes of pores. To remove sulfur from a small pore more energy is needed. In fact the most complex porous structure is expected to be found for the WOSS and WOSSMS samples. The second peak centered at about 550 °C represents either removal of sulfur from very small micropores or decomposition of sulfates formed on the surface. Its absence on the DTG curves can be misleading for the three-component sample. A closer look at TG curves (Figure 5), from which DTG curves were calculated, indicates an unusual increase in weight between 300 and 800 °C, especially for the metal sludge containing materials. Since samples were obtained at 950 °C and the experiments were done in nitrogen, the only plausible explanation is reaction with nitrogen and formation of complex nitrides. The literature describes that the relatively low temperatures are required for nitridation in the presence of char (37, 38). Although those reactive centers are difficult to identify, they may play a role in catalytic oxidation of hydrogen sulfide. The enhanced performance of the composite sludgebased adsorbents can have its origin in porous structure
FIGURE 4. DTG curves in nitrogen. FIGURE 6. Pore size distributions.
FIGURE 5. TG curves in nitrogen. where the H2S has to initially be adsorbed and the products of its oxidation are stored. The pore size distributions (PSDs) calculated from nitrogen adsorption isotherms are collected in Figure 6. It important to mention that despite the low temperature of outgassing for the exhausted samples, a significant amount of elemental sulfur (likely from larger pores) was removed from the samples and condensed on the walls of the sample tubes. This means that nitrogen uptake on the exhausted samples, and thus pore volumes, are overestimated. This should be kept in mind when analyzing the PDSs. While WO and SS are porous materials with a high contribution of mesopores, MS cannot be considered as a porous adsorbent. The differences between SS and WO are seen in all ranges of pore sizes. The waste oil sludge-based material has a much higher volume of mesopores larger than 100 Å and a smaller volume of micropores with sizes less than 10
Å. Although after H2S adsorption new pores are formed, likely between elemental sulfur deposits, they differ in sizes for these two materials. In the case of SS-E the new pores are those between 20 and 100 Å and for WO-E they are between 200 and 1000 Å. The fact that they are similar in sizes to those already existing indicates that these particular pores are active in the adsorption process. Also very active for the oxidation reaction and storage of sulfur are pores smaller than 20 Å, which practically disappear after exposure to hydrogen sulfide. A similar trend is noticed on PSDs for the sludge composite samples. Although generally speaking the distributions look very similar to each other, with the predominant volume of big meso- and macropores coming from WO sample, addition of sewage sludge results in an increase in the intensity of the peak representing small micropores. In all range of pores larger than 20 Å adsorption of H2S causes an increase in their volume. On the other hand, the volume of micropores is significantly smaller after exhaustion. The quantitative information about all of the changes described above is collected in Table 3. Although all samples but MS have surface area between 120 and 150 m2/g, mixing waste oil sludge and sewage sludge definitely increases the microporosity of the composite adsorbent. As expected from the shape of the isotherms, the samples have a high volume of mesopores. The highest degree of mesoporosity is found for WOSS. After H2S adsorption the volume of micropores decreased significantly (about 50%) with a noticeable increase in the volume of micropores for all metal sludge-free samples. Assuming that sulfur filled the micropores gradually, and assuming sulfur density being 2 g/cm3, between 32 and 64 mg of sulfur is stored in micropores, which accounts for about 50% of the sulfur present on the surface based on H2S breakthrough capacity. Other sulfur deposited in larger pores or was removed during outgassing. VOL. 40, NO. 10, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 3. Structural Parameters sample WO WO-E SS SS-E MS MS-E WOSS WOSS-E WOMS WOMS-E WOSSMS WOSSMS-E
SBET Vmic Vmes Vt ∆Vmic ∆Vmes [m2/g] [cm3/g] [cm3/g] [cm3/g] [cm3/g] [cm3/g] Vmic/Vt 132 96 141 121 10 4 150 89 70 60 144 59
0.050 0.034 0.058 0.032 0.002 0.001 0.061 0.030 0.022 0.017 0.053 0.022
0.314 0.364 -0.16 0.355 0.041 0.389 0.151 0.209 -0.26 0.190 0.039 0.222 0.015 0.017 -0.01 0.005 -0.010 0.006 0.163 0.224 -0.31 0.258 0.096 0.288 0.144 0.166 -0.05 0.154 0.010 0.171 0.267 0.320 -0.21 0.183 -0.085 0.205
14 8 28 17 12 17 41 31 13 11 20 11
FIGURE 8. Comparison of the predicted and measured H2S breakthrough capacity.
FIGURE 9. XRD patterns for the SS, WO, and WOSS samples. FIGURE 7. Comparison of the predicted and measured volumes of micro- and mesopores. The fact the surface area and volume of micropores for WOSS is greater than those for the single component adsorbents suggests a synergetic effect of the mixture. Figure 7 shows the comparison of the volumes of pores measured and predicted for the physical mixture of waste oil sludge (WO), sewage sludge (SS), and metal sludge (MS). A generally observed trend indicates that mixing sludges results in the development of an additional pore volume. Besides porosity, alteration of surface chemistry during pyrolysis of the sludge mixture can also affect the performance. Figure 8 shows the comparison of the measured and predicted capacity based on the performance of the individual components assuming the physical mixture. The large, nearly 3382
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100% enhancement observed must be the result of changes in the composition and surface dispersion of an inorganic phase. The sludges studied contain iron, copper, nickel, zinc, calcium, chromium, and other metals in significant quantities. Their high-temperature reaction in the presence of carbon phase likely leads to unique spinel/mineral-like components active in the oxidation reactions. To support the hypothesis presented above the X-ray diffraction experiments were performed on single components, WO and SS, and on their mixture, WOSS. Comparison of the diffraction patterns presented in Figure 9 clearly shows the synergetic effect in the chemical composition of materials. In SS minerals such as wurtzite (ZnS), ferroan (Ca2(Mg, Fe)5 (SiAl)8O22(OH)2), chalcocite (Cu1.96S), spinel (MgAl2O4), and feroxyhite (FeO(OH)) were found, while in WO besides
metallic iron, bornite (Cu5FeS4), hibonite (CaAl12O19), zincite (ZnO), and ankerite (Ca(Fe, Mg)(CO3)2) are present. In the case of the WOSS, besides bornite, wurtzite, and spinel common for both the single component adsorbents, new minerals such as sapphirine (Mg3.5Al9Si1.5O20), maghemite (Fe2O3), cohenite (Fe3C), lawsonite (CaAl2Si2O7(OH)2H2O), and smithsonite (ZnCO3) are present. Indeed, the new components formed have their origin on addition of silica (coming from sewage sludge), and iron and zinc from waste oil sludge. The results presented in this paper clearly show the new technology of utilization of municipal and industrial wastes as adsorbents of hydrogen sulfide from moist air, as for instance from sewage treatment plants. Mixing sludges and their carbonization not only results in minimization and reuse of wastes and delivering new quality materials which may be used as adsorbents for desulfurization, but it also causes formation of new active surface chemistry and porosity crucial for hydrogen sulfide oxidation to elemental sulfur. This new chemistry may prove to be beneficial for other catalytic processes. The materials obtained are stable in air and can be easily disposed in landfills when used. Since elemental sulfur is the oxidation product and the capacity is exhausted when the significant volume of pores is filled their regeneration by heating to moderately high temperatures should be explored.
Acknowledgments This work was partially supported by NYC DEP. We are grateful to Vyacheslav Starkov of Stuyvesant High School for his contribution to the experimental work during his summer 2005 internship.
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Received for review November 10, 2005. Revised manuscript received March 13, 2006. Accepted March 16, 2006. ES052272D VOL. 40, NO. 10, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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