Environ. Sci. Technol. 2004, 38, 345-351
Efficient Hydrogen Sulfide Adsorbents Obtained by Pyrolysis of Sewage Sludge Derived Fertilizer Modified with Spent Mineral Oil ANDREY BAGREEV† AND T E R E S A J . B A N D O S Z * ,‡ Department of Chemistry and The International Center for Water Resources and Environmental Research of The City College of New York and The Graduate School of The City University of New York, New York, New York 10031
Terrene, sewage sludge derived granulated fertilizer, was impregnated with spent mineral oil and then pyrolyzed at 600, 800, and 950 °C. Materials obtained were characterized from the point of view of the pore structure and surface chemistry. Then the H2S breakthrough capacity was measured using a lab designed test. The results showed that the new adsorbents overperform by 30% materials obtained by simple thermal treatment of Terrene and by 230% virgin coconut shell based activated carbon. The surface reaction products were evaluated using thermal analysis. On the surface of new adsorbents hydrogen sulfide is oxidized mainly to elemental sulfur which is then deposited within the pore system. The breakthrough occurs when all small pores available to promote catalytic oxidation (caused by the inorganic sludge component) are filled with sulfur. An increase in pyrolysis temperature leads to an improvement in the performance of materials as hydrogen sulfide adsorbents. This is caused likely by changes in an inorganic phase and inorganic/carbonaceous phase interactions during pyrolysis.
Introduction Contemporary society, especially densely populated urban environments, produce abundant quantity of wastes (1). One of such waste materials is municipal sewage sludge, which is also called biosolid (2). It consists of organic material, mainly dead bacterial cells, and an inorganic component in the form of various oxides and salts (3-5). It is expected that about 70% of sewage sludge will be beneficially utilized. Sewage sludge is used for landfilling and road paving, production of fertilizer, and conversion onto adsorbents, or it is disposed of by incineration (2). Agricultural application of sewage sludge as a fertilizer is questionable, especially in Europe where standards for heavy metals in fertilizers are up to 100 times lower than those in the United States (3). One of the ways of efficient and environmentally friendly utilization of sewage sludge is its conversion into adsorbents. Several patents have proposed carbonization of sewage sludge (6-10) and application of carbonized material to * Corresponding author: phone: (212) 650-6017; fax: (212) 6506107; e-mail:
[email protected]. † Department of Chemistry and The International Center for Water Resources and Environmental Research of The City College of New York. ‡ The Graduate School of The City University of New York. 10.1021/es0303438 CCC: $27.50 Published on Web 11/21/2003
2004 American Chemical Society
removal of organics in the final stages of water cleaning (8) and removal of chlorinated organics (9). Sludge-derived adsorbents have been also tested to remove acidic gases such as sulfur dioxide and hydrogen sulfide (11, 12). Adsorbents obtained by pyrolysis of sewage sludge can be considered as complex pseudocomposite materials. However, the process of carbonization of biosolids has been studied in detail previously and it is described in the literature (6-14). So far the most promising results were obtained in our laboratory (15-18). It has been recently shown that by simple pyrolysis of sewage sludge derived fertilizer, Terrene, exceptionally good adsorbents for removal of sulfurcontaining gases can be obtained. Their removal capacity is twice that of coconut shell based activated carbon (16). Although it was attributed to the specific combination of inorganic oxides of such metals as iron, copper, zinc, or calcium, we are still not able to find the unambiguous chemical and physical reasons for the extraordinary performance of these materials. The predominant influence of the inorganic phase or combination of oxides, which are also quite commonly used as catalysts for hydrogen sulfide oxidation or sulfur dioxide adsorption (19), was ruled out on the basis of the performance of a pure inorganic phase (ash) in the removal of sulfur-containing gases. The capacity of pure inorganic phase heated at 950 °C was negligible. The data also showed that the oxidation of hydrogen sulfide occurs until all micropores, likely within carbonaceous deposit or on the carbon/oxide interface, are filled with the reaction products (16, 18, 20). The form of that carbonaceous deposit is not known. Recent discovery of a new form of quasi-onedimensional carbon, Carbolite (21), indicates that carbon still has a possibility of formation of a wide variety of allotropes. Since the capacity of sewage sludge derived materials obtained using heat treatment of Terrene was exhausted when the micropores were filled by the oxidation product, sulfur, an increase in the volume of micropores seems to be a logical step. One of the possible ways to increase the micropore volume is an increase in the amount of the carbonaceous phase. It was thought that spent mineral oil, from the point of view of carbon content, can be good as a precursor for this purpose due to the simplicity of the technological process (just impregnation), its availability, and the content of heavy metals which may act as additional catalysts. The objective of this paper is to demonstrate the performance of adsorbents obtained by carbonization of Terrene impregnated with spent mineral oil. However, although the content of carbonaceous phase did not increase noticeably, the removal capacity is much better than that obtained on Terrene pyrolyzed without oil. The results emphasize the role of surface chemistry and pore volume in the process of hydrogen sulfide removal.
Experimental Section Sorbent Precursors. Terrene samples were obtained from Synagro, Bronx, NY, and spent car oil was provided by Eli’s Auto Care, Teaneck, NJ. The chemical composition of Terrene is considered as quaziconstant. The standard deviation in the content of metals between batches is about 10-15%. Preparation of Adsorbents. Forty milliliters of dry Terrene was mixed with 10 mL (7.0 g) of spent car oil. Then the sample was saturated with oil for 24 h. The temperatures of saturation were 25 °C. After saturation, the samples were pyrolyzed at either 600, 800, or 950 °C for 1 h in a nitrogen atmosphere. The heating rate was 10 deg/min up to selected pyrolysis VOL. 38, NO. 1, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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temperature with a 1 h holding time at the final temperature. Then the samples were cooled in nitrogen to 30 °C. The samples are referred to as SLOL-600, SLOL-800, and SLOL950. Thermal Analysis. Thermal analysis was carried out using a TA Instruments thermal analyzer. The instrument was setting to a heating rate of 10 deg/min up to 1000 °C with flow rate 100 mL/min. Nitrogen or air atmosphere was used. pH of Adsorbents. The surface chemistry was evaluated based on the pH of the materials. Adsorbent powder (0.4 g) was placed in 20 mL of water and equilibrated overnight. Then the pH of suspension was measured. Elemental Analysis. The content of metals was determined using inductively coupled plasma at Microbac Laboratories, Inc. Characterization of Pore Structure of Adsorbents. Characterization of pore sizes and adsorption capacity of the carbonaceous materials was accomplished using physical sorption measurement. Equilibrium adsorption isotherms of N2 were measured using standard volumetric techniques (ASAP 2010, Micromeritics). From the isotherms the pore size distributions were evaluated using the recently developed density functional theory (DFT) (22, 23). The specific surface areas were calculated using the BET approach (BrunauerEmmett-Teller). In addition, micropore volumes and surface areas were evaluated using the Dubinin-Radushkevich equation (DR) (24). H2S Breakthrough Capacity. The dynamic tests were carried out at room temperature to evaluate the capacity of adsorbents for H2S removal. Adsorbent samples were packed into a column (length, 360 mm; diameter, 9 mm; bed volume, 6 cm3) and prehumidified with moist air (80% relative humidity at 25 °C) for an hour. The amount of adsorbed water was estimated from the increase in the sample weight. Moist air (80% relative humidity at 25 °C) containing 0.3% (3000 ppm) H2S was then passed through the column of adsorbent at 0.5 L/min. The elution of H2S was monitored using an Interscan LD-17 H2S continuous monitor system interfaced with a computer data acquisition program. The test was stopped at the breakthrough concentration of 500 ppm. The adsorption capacities of each sample in terms of milligrams of H2S per gram of adsorbent were calculated by integration of the area above the breakthrough curves and from the H2S concentration in the inlet gas, flow rate, breakthrough time, and mass of adsorbent. After exhaustion of its H2S adsorption capacity, each sample is identified by the letter “E” added to its designation.
Results and Discussion Figure 1 shows DTG (differential thermal gravimetry) results for carbonization of Terrene impregnated with mineral oil. On the curve, various peaks related to the volatilization of organic compounds of sludge and oil are detected. The graph indicates that after heating at 600 °C the removal of volatile organic compounds no longer occurs and only an increase in the degree of carbonization and changes in the inorganic phase are expected with an increase in pyrolysis temperature. Table 1 summarizes the yield of adsorbents and bulk density of the materials obtained. For comparison the data for pure fertilizer (SL) and pure oil (OL) is included (Figure 1). With increasing pyrolysis temperature, the yield slightly decreases as a result of high-temperature decomposition of an organic and inorganic matter. It is interesting that the yield with oil present on the surface is smaller than that without oil. This may be the result of contribution of oil to the total mass of the initial material (oil content was around 20% for all samples). The vast majority of that oil is removed from the surface during carbonization. Since the pyrolysis of pure oil resulted in its almost 100% evaporation, we cannot expect 346
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FIGURE 1. DTG curves in nitrogen for Terrene impregnated with mineral oil (TOL), Terrene (SL), and mineral oil (OL).
TABLE 1. Carbonization Conditions and Yield of Adsorbents sample
carbonization temp (°C)
total conversiona (%)
total yieldb (%)
bulk density
SLOL-600 SLOL-800 SLOL-950 SL OL
600 800 950 950 1000
62.8 67.2 68.8 61 98.7
37.2 32.8 31.4 39 1.3
0.56 0.56 0.55 0.65
a Total conversion, X ) (W - W )/W × 100, where W is the initial t in fin in in weight of sample before carbonization (sludge plus oil) and Wfin is the weight of sample after carbonization. b Yield is calculated as Y ) 100 - Xt.
TABLE 2. Results of Thermal Analysis sample
ash content (wt %)
carboneous phase content (wt %)
ignition temp (°C)
SLOL-600 SLOL-800 SLOL-950
67.6 75 77.1
32.4 25 22.9
380 461 523
high contribution of carbonaceous material from carbonization of spent oil. Table 2 collects the results of thermal analysis in nitrogen (Figure 2) and air. Comparison of the weight loss in those two atmospheres leads to the calculation of the carbonaceous phase content. As expected, the carbon content decreases with an increase in the carbonization temperature. It is interesting that the ignition temperature for SLOL-950 is higher than that for SLOL- 800. This likely happens as a result of the removal of oxygen-containing functional groups from the carbonaceous matrix. Release of oxygen from those groups lowers the ignition temperature of carbon-based adsorbents (25). The nitrogen adsorption isotherms are collected in Figure 3. The shape of isotherms and the presence of hysteresis loops indicate that adsorbents have mixed micro- and mesoporous structure (26). From these isotherms the pore size distributions (PSDs) were calculated using DFT (22, 23). They are presented in Figure 4 along with the pore size distribution of the sample obtained by simple pyrolysis of sewage sludge, SC-950 (16). While SLOL-600 and SLOL-800 seem to have similar pore structure, the SLOL-950 differs significantly. The micropores are smaller than those for the other samples and their volume is higher. It is also interesting that the volume of pores bigger than 200 Å decreased
TABLE 3. Structural Parameters DR
DFT
sample
SBET (m2/g)
Vtot (cm3/g)
Vmic (cm3/g)
Smic (cm2/g)
Vmic (cm3/g)
Vmes (cm3/g)
Vtot (cm3/g)
Smic (m2/g)
Stot (m2/g)
SLOL-600 SLOL-800 SLOL-950 SLOL-600E SLOL-800E SLOL-950E
90 97 117 16 19 30
0.139 0.149 0.176 0.091 0.101 0.115
0.041 0.044 0.048 0.007 0.008 0.011
103 108 114 15 17 24
0.025 0.025 0.022 0.002 0.003 0.002
0.047 0.045 0.047 0.040 0.051 0.054
0.106 0.101 0.087 0.067 0.091 0.097
69 67 61 3 4 3
91 105 99 10 13 20
FIGURE 2. DTG curves in nitrogen for sewage sludge derived adsorbents.
FIGURE 3. Nitrogen adsorption isotherms. Solid points represent the desorption part of isotherms. significantly and new pores in the mesopore range (from 30 to 100 Å) are created. This is likely due to the thermal changes in the inorganic matter. During carbonization at 950 °C in reducing atmosphere, inorganic salts and oxides decompose (27, 28) releasing oxygen which acts as a pore former. This is reflected in an increase in the volume of very small pores smaller than 7 Å. Those pores are likely formed within the carbonaceous deposit and/or on the interface between the carbonaceous and inorganic phase. As indicated from the previous research, they should be active in the process of hydrogen sulfide removal (16, 18, 20). A decrease in the volume of mesopores is probably due to the melting of an inorganic phase and an increase in the density of the material
FIGURE 4. Pore size distributions for the materials studied.
FIGURE 5. H2S breakthrough curves. (28). There are no striking differences in the porosity of SLOL950 and SC-950 samples. The structural parameters calculated from the isotherms are listed in Table 3. Indeed the highest surface area and pore volume are found for SLOL-950. However, on the basis of DFT calculation there are no big differences between the porosity of all three samples studied. When only simple pyrolysis was done, the differences in the pore volume and surface were more noticeable (15, 16). On the samples prepared, the H2S breakthrough tests were done. The breakthrough curves are collected in Figure 5. For comparison, the results obtained for the SC-950 sample (simple pyrolysis without oil) and coconut shell based activated carbons, S-208, are included (16). From the data it is clearly seen that despite similarities in the pore structure VOL. 38, NO. 1, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 4. H2S Breakthrough Results
sample SLOL-600 SLOL-800 SLOL-950 SC-950
TABLE 5. Metal Content in Samples
breakwater breakthrough pH pH preadsorption through capacity initial exhausted (mg/g) time (min) (mg/g) 10.83 11.05 11.26 10.84
9.9 10.2 9.5 9.6
50 48 65 64
59 84 186 160
35.2 49.6 115 86
the performance of materials differs and it gets better with an increasing pyrolysis temperature. An important finding is an observed increase in the breakthrough time for the sample treated with oil compared to its untreated counterpart. It is also clearly seen that SLOL-950 performs much better than activated carbon, S208. The breakthrough results along with the values of surface pH are collected in Table 4. It is interesting that the surface pH values for all samples are basic, which should have a positive effect on the hydrogen sulfide removal from moist air, as proposed elsewhere (29-33). After the breakthrough test, the pH values slightly decrease, likely due to reactions of H2S with surface oxides (16, 18); however, they still remain in the basic range. This is an indication that a strong acid (H2SO4) is not formed on the surface as observed for some activated carbons (29-33). The H2S breakthrough capacities of samples obtained from sludge-oil precursor are higher than those obtained by simple pyrolysis without the presence of waste oil. In the case of SLOL-950, the performance is better by about 30% when compared to SC-950. Such a significant increase, consistent for the samples obtained in all three temperatures (16), must have its origin either in porosity or in the surface chemistry of materials. Since waste oil was used, one could assume that catalytic metal content could have an effect on the final performance of materials. The metal analysis results for samples pyrolyzed without and with mineral oil are collected in Table 5. The results show that treatment with spent oil did not change significantly the chemistry of materials from the point of view of various metal contents. Although small variations exist between samples pyrolyzed with and without mineral oil, it is difficult to link them to the quality and quantity of metals usually present in the spent oil (1). While the samples pyrolyzed at 600 and 800 °C without mineral oil have more chromium, copper, iron, lead, and potassium than their oil-treated counterparts, the general trend is reversed for the samples pyrolyzed at 950 °C. This may be related to the increase in the content of organic phase after oil treatment. This finding suggests that an increase in the capacity should have its origin in the porosity of the materials. However, not the volume of pores but their shape and distributions seem to be important features. This is due to the fact that, as described above, the differences in the pore volume between SC-950 and SLOL-950 are not visible enough to account for an observed enhancement in the performance of materials as hydrogen sulfide adsorbents. This problem requires further studies of the nature and properties of the carbonaceous phase. It is seen that the water uptakes increase with an increase in the pyrolysis temperature. Since the water uptake differs about 40% for SLOL-800 and SLOL-950, it supports our hypothesis about significant changes in chemistry of the material pyrolyzed at high temperature. It is likely that new dehydrated forms of oxides were formed. Those oxides adsorb water and become active in the process of removal of sulfur-containing gases. Since there is no significant difference in the micropore volume between SLOL348
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samplea
aluminum (ppm)
chromium (ppm)
copper (ppm)
iron (ppm)
lead (ppm)
Terrene SC-600 SC-800 SC-950
6536 19900 21550 21300
57 106 65 114
669 1385 1845 1785
20102 54450 61250 66350
211 504 304 20
sample
potassium (ppm)
nickel (ppm)
zinc (ppm)
calcium (ppm)
total metals (%)
Terrene SC-600 SC-800 SC-950
961 2715 3796 3449
45 64 63 102
903 2340 1720 42
18143 41200 45700 47400
4.76 12.28 13.64 14.07
sample
aluminum (ppm)
chromium (ppm)
copper (ppm)
iron (ppm)
lead (ppm)
SLOL-600 SLOL-800 SLOL-950
18200 20000 23100
83 47 124
1270 1430 1790
50400 57150 67250
437 209 17
sample
potassium (ppm)
nickel (ppm)
zinc (ppm)
calcium (ppm)
total metals (%)
SLOL-600 SLOL-800 SLOL-950
2511 3336 3441
42 60 97
2410 1780 32.4
40450 41450 49000
11.59 12.56 14.49
a SC represents samples obtained by pyrolysis of sewage sludge derived fertilizer without mineral oil.
800 and SOL-950, the increase in the uptake of water cannot be related to the changes in the physical structure of the materials. The mechanism of H2S removal on the sludge-derived adsorbents is much more complex than that on virgin activated carbons. This is due to the presence of at least two types of active sites: one type on the surface of porous carbon and the second one on the surface of metal oxides or carbonates. The mechanisms on those sites were discussed separately (16,17, 20). They consist of a few steps: (1) adsorption (physical or ionic) of H2S on carbonaceous phase in dry or wet conditions
H2Sgas f H2Sads
(1)
H2Sliq f HS-ads + H+
(2)
where the subscripts gas, liq, and ads correspond to H2S in gas, dissolved in liquid, and adsorbed phases, respectively. (2) chemisorption of H2S on iron, zinc, and copper oxides with sulfide formation:
ZnO + H2S f ZnS + H2O
(3)
CuO + H2S f CuS + H2O
(4)
Fe2O3 + 3H2S f FeS + FeS2 + 3H2O
(5)
(3) chemisorption on calcium or potassium oxides and carbonates due to neutralization reactions
CaO + H2S f CaS + H2O
(6)
2CaCO3 + 2H2S f Ca(HCO3)2 + Ca(HS)2
(7)
(4) oxidation of H2S by molecular oxygen on the carbon surface (eqs 8 and 9) or on the surface of catalytically active
FIGURE 7. Sulfur adsorbed on the surface of sewage sludge derived materials. Comparison of sulfur content in samples determined by TA and by H2S breakthrough tests.
FIGURE 6. (A) Comparison of the DTG curves in nitrogen for SLOL950 and for model materials: SLOL-950-SO2E (the exhausted sample after SO2 adsorption), SLOL-950-S-IMP (the sample impregnated with sulfur vapors). (B) DTG results in nitrogen for exhausted samples after hydrogen sulfide adsorption. oxides of transition metals (eqs 10 and 11) with formation of elemental sulfur, sulfur dioxide, or even sulfuric acid
Cf + 0.5O2 f C(O)
(8)
C(O) + H2Sads f Cf + Sads + H2O
(9)
2Fe3+ + H2Sads f 2Fe2+ + Sads + 2H+
(10)
2Fe2+ + 0.5O2 + 2H+ f 2Fe3+ + H2O
(11)
Sads + O2 f SO2 ads
(12)
SO2 ads + 0.5O2 + H2O f H2SO4 ads
(13)
where Cf is a free active site of carbon for oxygen chemisorption and C(O) is this site with chemisorbed oxygen (an occupied active site). The exhausted samples after H2S adsorption (E) were analyzed using TA (29-33). To identify the species desorbed from the surface (their desorption temperature depends on the porosity and chemical nature of the materials (20)), SO2 and sulfur (from the vapor phase) were adsorbed on the surface of SLOL-950 and then thermal analyses were done. The DTG curves obtained for those samples are collected in Figure 6A. For the sample exposed to SO2 adsorption (SLOL950-SO2E) the peaks at 240 and 860 °C are revealed. The former represents desorption of sulfur dioxide, and the latter represents calcium sulfate, which is formed as a results of
the reaction of SO2 with calcium hydroxide and calcium carbonate in the presence of water (17). The small intensity of the SO2 peak is the result of a small amount of sulfur dioxide adsorbed on the surface. When the sample was exposed to sulfur vapor, the position of the peak is identical to that on the sample exhausted after H2S adsorption. The DTG results for all exhausted samples are collected in Figure 6B, and they should be analyzed in comparison with the curves presented in Figure 2. In the case of SLOL950, a significant weight loss between 150 and 400 °C occurs. On the basis of the data presented in Figure 6A and our previous study (20), we assign it to the desorption of elemental sulfur. When present in small pores, elemental sulfur likely does not form S6 or S8 chain or ring polymers and vaporizes at a lower temperature than solid sulfur (36). A small weight loss at about 480 °C is related to decomposition of iron sulfate (28). The DTG curve for SLOL-800 is much more complicated that that for SLOL-950. For this sample many peaks are revealed. The peak between 200 and 350 °C represents removal of elemental sulfur. The other peaks at 300, 450, and 680 °C are likely the results of the decomposition of salts formed during reaction of H2S and sulfuric acid with surface oxides and hydroxides. These oxides have to be in other chemical states than those in SLOL-950. It is likely than after heating at 950 °C and melting the spinel-like compounds are formed with high catalytic activity toward hydrogen sulfide oxidation to elemental sulfur whereas in the case of the sample heated at 800 °C still separate inorganic oxides are present. For SLOL-600 after H2S adsorption only a small peak representing sulfur at 220 °C and a broad shoulder expanded to 400 °C are present. It has to be mentioned here that the amount adsorbed was the smallest for this material. Although heating at 600 °C is able to carbonize organic matter, it is not enough for dehydroxylation and decompostion of the majority of inorganic salts (28), as for instance carbonates; thus inorganic species are not in their catalytically active forms. Assuming that the peak between 200 and 400 °C represents sulfur, the amount of sulfur present on the surface was evaluated based on the integration of the area under the peak (weight loss in selected temperature range in TA). This amount was compared to the expected amount based on the breakthrough capacity results multiplied by the ratio of molecular weight of sulfur and hydrogen sulfide, equal to 0.94. In Figure 7 the correlation between these two sets of the data is presented. A good linear fit with a slope close to VOL. 38, NO. 1, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 8. Pore size distributions for samples after H2S adsorption. 1 validates our hypothesis about the presence of elemental sulfur on the surface of exhausted materials. Sulfur, as a major surface reaction product, should be deposited in the pore system of adsorbents. Figure 8 presents the pore size distributions for the exhausted (E) samples. They should be analyzed along with the PSDs presented in Figure 4. For all samples, the micropores,
