Oxidation on Materials Obtained Using Sulfuric Acid Activation of Sewage Sludge-Derived Fertilizer

Journal of Colloid and Interface Science 252, 188–194 (2002) doi:10.1006/jcis.2002.8419

H2 S Adsorption/Oxidation on Materials Obtained Using Sulfuric Acid Activation of Sewage Sludge-Derived Fertilizer Andrey Bagreev and Teresa J. Bandosz1 Department of Chemistry and The International Center for Environmental Resources and Development, City College of New York, City University of New York, New York, New York 10031 E-mail: [email protected] Received February 4, 2002; accepted April 8, 2002; published online July 1, 2002

Sewage sludge-derived fertilizer, Terrene, was used as a precursor of adsorbents tested for removal of hydrogen sulfide from moist air. The adsorbents were obtained by pyrolysis of sulfuric acid-treated granular fertilizer at 600, 800, and 950◦ C in a nitrogen atmosphere. The highest H2 S removal capacity was obtained for the sample carbonized at 950◦ C. This is a result of a combined effect of the specific chemistry of the inorganic phase and the development of microporosity within the carbon deposit. On the surface of the materials studied hydrogen sulfide is converted to elemental sulfur, sulfides, and sulfates as a result of the reaction with salts/oxides and the presence of an oxidizing atmosphere. The pores are gradually filled as the surface reactions proceed. The removal of H2 S occurs until all the small micropores are filled with the reaction/oxidation products. C 2002 Elsevier Science (USA) Key Words: sewage sludge; adsorption of hydrogen sulfide; porosity; surface chemistry; sulfuric acid activation; pyrolysis.

INTRODUCTION

The minimization, reuse, and recycling of wastes are contemporary approaches toward preservation of our natural environment (1). The common municipal waste produced in abundant quantity is sewage sludge. It is a biosolid that consists of an organic material, mainly dead bacterial cells, inorganic components in the form of various oxides and salts (aluminum, silicon, calcium, magnesium, iron, etc.), and heavy metal contaminants such as lead and copper from plumbing. Other metals include nickel and zinc (2–4). Since environmental regulation no longer allows for dumping it to the bottom of the ocean, other ways of minimizing or reusing it must be developed. So far sewage sludge has been utilized in landfilling and road paving, and converted into adsorbents for water treatment (5). The most common way of utilizing sewage sludge is its conversion to fertilizer and further application in agriculture. This approach is used mainly in the USA since in Europe the standards for heavy metal con1

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tent in fertilizer are about 100 times stricter than those in the USA (2). Sewage sludge can be also incinerated (5, 6) or used as low-efficiency fuel. The conversion of sewage sludge into adsorbents has attracted the attention of researchers since the 1970s (7–11). Among the proposed applications of these new adsorbents are the removal of organics in the final stages of water cleaning (9), the removal of chlorinated organics (10), and the removal of acidic gases such as hydrogen sulfide or sulfur dioxide (4, 12–14). The adsorption capacity reported varies, depending on the nature of the sludge, activation method (12, 13, 15), and temperature treatment (4, 14, 15). It has been demonstrated recently that materials obtained by pyrolysis of sewage sludge-derived organic fertilizer, Terrene, can perform very well as adsorbents of hydrogen sulfide (4, 15) and sulfur dioxide (14). Their removal capacity is comparable to the capacity of coconut shell-based activated carbon, which is considered as an alternative material to using caustic impregnated carbons extensively for odor control in sewage treatment plants (16, 17). However, activation with zinc chloride results in a significant increase in the porosity of the materials; it does not increase their performance as hydrogen sulfide adsorbents (15). The objective of this study is to compare the efficiency of H2 S removal on materials obtained by chemical activation of Terrene using sulfuric acid to that on those obtained using simple pyrolysis (3, 4). The method of preparation of activated carbonaceous material from sewage sludge and sulfuric acid proposed in U.S. Patent 3,998,757 (7) was also applied by Lu and coworkers (12, 13). As a result of pyrolysis in the presence of H2 SO4 the microporosity significantly increases and so does the yield of the carbonaceous phase. The results emphasize the role of an activation agent in the development of porosity. The presence of sulfuric acid as an activation agent does not significantly change the chemical composition of an inorganic phase compared to simple pyrolysis. Probably, the favorable combination of the microporous carbonaceous phase and basic inorganic oxides is a factor governing the performance of materials as hydrogen sulfide adsorbents.

189

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EXPERIMENTAL

Materials Terrene was obtained from the New York Organic Fertilizer Company (Bronx, NY) in the form of 3-mm-diameter granules with about 5% water content. The detailed chemical composition is presented elsewhere (3, 4). It contains around 40% inorganic matter, mainly in the form of iron, calcium, alumina, silica oxides, and carbonates, and 60% organics (3, 4, 14). The adsorbents studied were prepared by mixing 30 g of Terrene with 18.4 g (10 mL) of concentrated sulfuric acid. Then lowtemperature carbonization in air was done at 300◦ C, which was accompanied by mechanical stirring for 30 min. The product was ground and pyrolyzed at temperatures 600, 800, and 950◦ C in a nitrogen atmosphere in a fixed bed (horizontal) furnace. The samples are referred to as SCS-300, SCS-600, SCS-800, and SCS-950. The performance of the samples was compared to those obtained using simple pyrolysis described elsewhere (3, 4). Those samples are referred to as SC-600, SC-800, and SC-950. For comparison with the commercially used materials the H2 S breakthrough capacity of our carbons was compared to the capacity of coconut shell-based S208 carbon supplied by Waterlink Barnabey and Sutcliffe. The prepared materials were studied as hydrogen sulfide adsorbents in the dynamic tests described below. After exhaustion of its adsorption capacity, each sample is identified by adding the letter “E” to its designation. Methods H2 S breakthrough capacity. The dynamic tests were carried out at room temperature to evaluate the capacity of sorbents for H2 S removal. Adsorbent samples were packed into a column (length 360 mm, diameter 9 mm, bed volume 6 cm3 ) and prehumidified with moist air (relative humidity 80% at 25◦ C) for an hour. The amount of adsorbed water was estimated from the increase in the sample weight. Moist air (relative humidity 80% at 25◦ C) containing 0.3% (3000 ppm) H2 S was then passed through the column of adsorbent at 0.5 L/min. The elution of H2 S was monitored using an Interscan LD-17 H2 S 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 H2 S per gram of adsorbent were calculated by integration of the area above the breakthrough curves, and from the H2 S concentration in the inlet gas, flow rate, breakthrough time, and mass of the sorbent. Thermal analysis. Thermal analysis was carried out using a TA Instruments thermal analyzer (TA Instruments, New Castle, DE). The instrument settings were heating rate 10◦ /min in either a nitrogen or air atmosphere with 100 mL/min flow rate. The content of the carbonaceous phase was evaluated based on the residues left after a TA run in the air atmosphere.

pH. A 0.4-g sample of dry, grounded adsorbent was added to 20 mL of deionized water, and the suspension was stirred overnight to reach equilibrium. The sample was filtered and the pH of the solution was measured using an Accumet Basic pH meter (Fisher Scientific, Springfield, NJ). Nitrogen adsorption. Nitrogen adsorption isotherms were measured using an ASAP 2010 analyzer (Micromeritics, Norcross, GA) at −196◦ C. Prior to the experiments the samples were degassed at 100◦ C at constant vacuum 10−5 Torr. The isotherms were used to calculate the specific surface area, SBET , micropore volume, Vmic , total pore volume, Vt , and pore size distributions (PSDs). The pore volumes and PSDs were calculated using the density functional theory (18, 19). RESULTS AND DISCUSSION

A yield of the preparation process (pyrolysis with sulfuric acid activation) and the content of the organic (carbonaceous) phase for the materials obtained are summarized in Table 1. The yield of the materials decreases with increasing temperature of carbonization due to the volatilization of the organic compounds, and decomposition and dehydroxylation of the inorganic components (3). On the other hand, the content of carbon increases with increasing carbonization temperature. It is around 10% higher than that for the samples obtained by simple pyrolysis at temperatures between 800 and 950◦ C (3). The yield of carbon being higher than that in the case of simple carbonization is a result of the dehydrating effect of sulfuric acid at low-temperature (300◦ C) pyrolysis (20). The pyrolysis process is shown as the TG and DTG curves presented in Fig. 1. The first peak on the DTG cure represents the removal of water. It is worth mentioning that the intensity of this peak is smaller than that for the carbonization of sludge without sulfuric acid (3), which is the result of the dehydrating action of H2 SO4 . The removal of volatile organic compounds occurs between 300 and 550◦ C. The peak is complex, indicating various stages of decomposition of an organic material and its conversion into carbonaceous materials. Another peak present between 600 and 800◦ C is likely to be a result of the decomposition of inorganic salts, such as aluminum sulfate, nickel sulfate, copper sulfate,

TABLE 1 Yield of Adsorbents and Carbonaceous Material (%) Sample

Yield of adsorbent

Carbon content

SCS-300 SCS-600 SCS-800 SCS-950

46.0a 35.6a 32.5a 29.1a

18.6 25.9 37.3 35.7

SC-600 SC-800 SC-950

46.3 41.8 39.3

27.1 26.4 24.9

a

Based on the sludge weight with H2 SO4 .

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FIG. 2.

FIG. 1. TG and DTG curves for pyrolysis in nitrogen atmosphere of sewage sludge-derived fertilizer in the presence of sulfuric acid.

and iron sulfate (21) and calcium carbonate, respectively (15). This leads to an increase in the pH after carbonization at 800◦ C as listed in Table 2. H2 S breakthrough capacity tests were run on the samples prepared using sulfuric acid activation. The results are collected in Table 2 along with the pH of the materials’ surface. Figure 2 presents the breakthrough capacity curves. The breakthrough capacity of the sample obtained at 950◦ C is significant and better than those obtained for well-performing, unmodified activated carbons with large surface areas and high pore volumes (17, 22, 23). All sorbents are basic in their nature and basicity significantly increases between 600 and 800◦ C, suggesting changes in an inorganic phase (4). It is likely related to the decomposition of salts and the formation of basic oxides for which the greatest contribution should have calcium oxide and iron oxide based on the elemental analysis of sludge (2% of calcium and 2.7% of iron) (3, 4). Those changes are also reflected in the amount of preadsorbed water, which significantly increases for SCS-950. This may be a result of the reaction of water with basic oxides, such as calcium, and the formation of active hydroxides (14). After H2 S adsorption the surface pH decreases, suggestTABLE 2 H2 S Breakthrough Capacity, Amount of Preadsorbed Water, and the pH Values for the Samples Studied Sample

H2 S breakthrough capacity (mg/g)

H2 O ads. (mg/g)

pH

pHEa

SCS-300 SCS-600 SCS-800 SCS-950

1 40.7 43 114

39.3 52.7 42.1 67.4

4.15 9.97 11.52 11.62

— 7.99 8.83 7.46

a

pHE—for exhausted samples after the breakthrough tests.

H2 S breakthrough curves.

ing chemical reaction with the formation of neutral compounds. They may be either sulfides, sulfates, or elemental sulfur (4, 14, 15). As indicated elsewhere (4), dissociated hydrogen sulfide (hydrogen sulfide anion) can be oxidized on the adsorbents surface according to the reactions + H2 Sads-liq → HS− ads + H

HS− ads SO2ads +

+

O∗ads

→ Sads + OH

−

[1] [2]

∗ − HS− ads + 3Oads → SO2ads + OH

[3]

O∗ads

[4]

+ H2 Oads → H2 SO4ads

H+ + OH− → H2 O.

[5]

H2 Sads-liq and H2 Sads correspond to H2 S in the liquid and adsorbed phases; O∗ads is dissociatively adsorbed oxygen; and Sads , SO2ads , H2 SO4ads represent sulfur, SO2 and H2 SO4 as the end products of the surface oxidation reactions. When metal oxides are present, H2 S removal at low temperatures mainly occurs due to gas–solid reactions in a thin hydrated lattice of metal oxides. This process leads to sulfide and sulfate formation: ZnO + H2 S → ZnS + H2 O

[6]

CuO + H2 S → CuS + H2 O

[7]

Fe2 O3 + 3H2 S → FeS + FeS2 + 3H2 O

[8]

Fe2 O3 + 3H2 S → Fe2 S3 + 3H2 O

[9]

2Fe2 S3 + 3O2 → 2Fe2 O3 + 6S

[10]

Fe2 O3 + 3H2 S + 6O2 → Fe2 (SO4 )3 + 3H2 O

[11]

CaO + H2 S + 2O2 → CaSO4 + H2 O.

[12]

The breakthrough capacities of samples obtained using sulfuric acid activation are 30 to 50% higher than those obtained

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H2 S ADSORPTION/OXIDATION

FIG. 3. Comparison of the changes in the surface area for adsorbents obtained using simple pyrolysis and activated with sulfuric acid at various temperatures.

using simple pyrolysis (4). Since the yield of carbon is about 10% higher than that for the simple pyrolyzed samples the reason for an increased capacity might be the development of porosity. As indicated elsewhere (3), micropores are likely created within the carbon deposit or in the interface between the organic and inorganic phases. A higher content of carbon should result in an increase in the pore volumes and surface areas. Figure 3 presents the dependence of the surface area on the pyrolysis temperature. For comparison, the data obtained for the samples obtained by simple pyrolysis are included (3). From the plots it is clearly seen that the surface areas are 30–50% higher for the sulfuric acid activated samples than for the simple pyrolyzed ones. The exact values of the structural parameters for the samples analyzed here are collected in Table 3. Although those parameters slightly vary, the differences are too small to account for a significant increase in the capacity of the SCS-950 sample. After exhaustion the surface areas decreased significantly and micropores almost disappeared. A similar situation was observed for samples obtained using simple pyrolysis where capacity lasted until all the pores were filled with the reac-

TABLE 3 Structural Parameters Calculated from Nitrogen Adsorption Isotherms Sample

SBET (m2 /g)

Vmic (DFT) (cm3 /g)

Vmes (DFT) (cm3 /g)

Vt (0.995a ) (cm3 /g)

Vmic /Vt

SCS-300 SCS-600 SCS-600E SCS-800 SCS-800E SCS-950 SCS-950E

26 197 24 172 11 205 31

0.003 0.060 0.002 0.053 0.002 0.050 <0.001

0.035 0.074 0.041 0.071 0.039 0.099 0.069

0.068 0.202 0.077 0.190 0.064 0.250 0.105

0.04 0.30 0.03 0.28 0.03 0.20 0.03

a

At the relative pressure p/ po equal to 0.995.

FIG. 4.

Pore size distributions for initial and exhausted samples.

tion/oxidation products (4, 14). Since the pore volumes of the SCS materials are about 30% higher than those for the SC series, the samples last longer than the hydrogen sulfide adsorbents and the mechanism of the process should be similar to that for the SC series of adsorbents (4). Figure 4 shows pore size distributions for the initial and exhausted samples. A comparison of the plots indicates that with increasing pyrolysis temperature the degree of heterogeneity in the sizes of the micropores increases. It happens likely to be the a result of the decomposition of the inorganic matter. Released species (seen as peaks on the DTG curves) act as pore formers ˚ com(15). After H2 S adsorption the pores smaller than 10 A pletely disappear, and the volume of the larger pores is noticeably reduced. It is the result of deposition of the reaction/oxidation products. Since the pore volume is a factor likely to differentiate the samples obtained using sulfuric acid activation from those obtained using simple pyrolysis, the specific capacity, normalized for the pore volume, is plotted in Fig. 5. For comparison the data obtained for the SC samples are included. The similarity of the data supports our hypothesis about the role of surface chemistry in the removal of hydrogen sulfide and the role of pore volume

192

FIG. 5. perature.

BAGREEV AND BANDOSZ

Dependence of specific H2 S adsorption on the pyrolysis tem-

as a storage space. The active surface chemistry is formed after heating the samples at 950◦ C (4). To study the products of surface reactions thermal analysis was done for the initial and exhausted samples (4, 14, 15, 17, 24–27). DTG curves differ, depending on the temperature of pyrolysis. As seen from Fig. 6 the common features for all the exhausted samples are peaks representing weight losses between 20 and 150◦ C and 150 and 600◦ C. The first peak represents the removal of water and very weakly adsorbed SO2 as a product of H2 S oxidation (4, 14), whereas the second peak centered at about 250◦ C was previously assigned to the evolution of SO2 from the decomposition of sulfuric acid created during surface oxidation of H2 S (strong adsorption) (4, 14, 24–26). Since in the case of our materials the pH values of the exhausted samples do not support the presence of strong acid this peak is likely to be related to the presence of forms of sulfur other than sulfuric acid. Its intensity increases with an increasing carbonization temperature, which reflects the differences in the amounts adsorbed. Support for this thermal analysis carried out in air is presented in Fig. 7 as DTG and DTA curves. For all the exhausted samples the weight loss occurs between 150 and 300◦ C (as in nitrogen), which is likely related to the oxidation process during thermal analysis. This is confirmed by the presence of exotherm on the DTA curves. If sulfur were present as SO2 and/or sulfuric acid this process would not occur. Based on this, the hypothesis about the presence of elemental sulfur can be formulated. To check it, TA experiments were carried out for sulfur powder and three different adsorbents impregnated previously with sulfur at 150◦ C: SCS-950, S208, and BAX-1500. It is important to mention here that the carbons chosen differ significantly in their porosity (17). The carbon S208 is homogeneously microporous, whereas BAX-1500 contains, besides micropores, a large volume of mesopores. The results of the experiments carried out in nitrogen are presented in Fig. 8. The

DTG curves for unimpregnated carbons are not included since in this scale they appear featureless. Comparison of the data reveals the differences in the temperature of sulfur removal. When only sulfur powder is analyzed, the peak appears between 150 and 300◦ C and is relatively narrow. For carbon with a broad distribution of pores and pores of large sizes (BAX-1500) the peak becomes broader; however its maximum is at temperatures similar to those for elemental sulfur with a shoulder at 450◦ C. The weight loss pattern changes significantly for microporous carbon, S208, where the majority of sulfur requires heating up to 450◦ C to be removed. The SCS-950 sample impregnated with sulfur has the main peak at 270◦ C and a second peak at 440◦ C. It follows that the temperature of sulfur removal is dependent on the pore size distribution. In smaller pores the adsorption forces are strong and the removal of sulfur requires high energy to be supplied to the system. Based on the above results and the basic pH values for the exhausted samples the peaks between 150 and 500◦ C likely represent elemental sulfur adsorbed in pores of different sizes. Since the differences in the porosity between SCS-800 and SCS950 are not very large, the differences in the pattern of sulfur

FIG. 6.

DTG curves for initial and exhausted samples.

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H2 S ADSORPTION/OXIDATION

TABLE 4 Balance of Sulfur Based on TA Experiments and H2 S Breakthrough Capacity Tests

Sample SCS-600 SCS-600E SCS-800 SCS-800E SCS-950 SCS-950E

FIG. 7.

Results of DTG (A) and DTA (B) analysis in air.

removal between these two samples are likely related to differences in the chemistry of the surfaces in the small pores. In the case of SCS-950E the peak centered at 420◦ C is well pronounced. Following the arguments presented above, more sulfur

Breakthrough capacity test (%) 4.07 4.3 11.4

TA in nitrogen (%) Weight loss 150–1000◦ C 18.06 22.1 7.29 13.86 2.5 12.77

Sulfur content

4.21 7.03 11.45

TA in air (%) Weight loss 150–350◦ C −1.3 2.71 −1.33 2.37 −0.85 10.67

Sulfur content

4.18 3.84 13.02

is likely adsorbed in the very small pores of this material, and this process is enhanced by changes in the composition of the solid during heat treatment between 800 and 950◦ C. Since elemental sulfur is the predominant product of hydrogen sulfide oxidation on activated carbons (17, 25, 28–31) its presence on SCS-950E can be linked to a relatively high content of a highly carbonized (after treatment at 950◦ C) carbonaceous phase and micropores within it. All of these cause that adsorbent, besides unique features of the surface chemistry, to behave as an activated carbon. The common feature for all the exhausted samples is a weight loss between 500 and 600◦ C represented by a peak centered at about 530◦ C. Since the areas of these peaks seem to be similar for all the samples they likely represent the decomposition of mixed sulfates of iron, copper, and zinc whose quantity is limited by the quantity of these metals in the samples (the same for all (14)). In Table 4 the balance of sulfur is presented, assuming that the DTG peaks represent the removal of adsorbed sulfur. The sulfur content from TA in nitrogen and/or air was calculated by the difference of weight loss in a corresponding temperature range for exhausted and initial samples, which was normalized to the initial sample weight. Comparison of the amount of sulfur which should be adsorbed on the surface based on the breakthrough results and these data shows remarkably good agreement, supporting our hypothesis about sulfur as a predominant product of surface reaction. CONCLUSIONS

FIG. 8. DTG curves in nitrogen for model materials containing elemental sulfur (referred to with letter “S”).

The results presented in this paper show that activation with sulfuric acid improves the performance of the sewage sludgederived materials as hydrogen sulfide adsorbents. The dehydration capability of sulfuric acid results in an increase in the amount of the carbonaceous phase with preserved surface chemistry of the inorganic phase. This leads to an increase in the volume of the micropores where the surface reaction products can be stored. Since the process occurs until all pores are filled with the surface reaction/oxidation products an increase in the amount of organic phase with preservation of surface chemistry is the direction of our further research.

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ACKNOWLEDGMENTS This study was supported by CUNY Collaborative Research Grant 91906. The authors are grateful to Mr. Thomas Murphy of the New York City Department of Environmental Protection and to Mr. Peter Scorziello, the manager of the NYOFCo facility for supplying the organic fertilizer, Terrene. Experimental help of Ms. Savanna Thorn during her summer 2000 internship at CCNY is appreciated.

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