Pore structure and surface chemistry of adsorbents obtained by pyrolysis of sewage sludge-derived fertilizer

Carbon 39 (2001) 1971–1979

Pore structure and surface chemistry of adsorbents obtained by pyrolysis of sewage sludge-derived fertilizer Andrey Bagreev a ,1 , Teresa J. Bandosz a , *, David C. Locke b a

Department of Chemistry and Center for Water Resources and Environmental Research, The City College of New York, New York, NY 10031, USA b Department of Chemistry, Queens College, CUNY, Flushing, NY 11367, USA Received 28 August 2000; accepted 3 January 2001

Abstract Terrene  , a fertilizer product derived from New York City municipal sewage sludge, was pyrolyzed at various temperatures between 400 and 9508C. The pore structure and surface chemistry of the adsorbent materials obtained were characterized using nitrogen adsorption, thermal analysis, potentiometric titration and FTIR. The adsorbents contain a high percentage of inorganic matter (up to 70%) and only up to 30% carbon. The results show that microporosity is developed within the carbon deposit and at the organic / inorganic interface with increasing temperature of heat treatment. An increase in the pyrolysis temperature also results in significant changes in the surface chemistry towards the development of basic nitrogen centers. In particular there was an increase in the pH values of adsorbents’ surfaces from 7 to 11.  2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon precursor; B. Pyrolysis; C. Adsorption; D. Microporosity; Chemical structure

1. Introduction The abundance of raw sewage sludge produces one of the major environmental problems of contemporary civilization. Various methods have been proposed for its disposal [1]. Ocean dumping was popular until recently, but it is no longer an option because of stricter environmental regulations [2]. Among the most often used methods of disposal are landfilling, cropland application, and incineration [1]. The incineration residue can be used in construction materials or road surfacing. Although incineration is effective in reducing the volume of sludge and produces useful end products, cleaning of the flue gases generated requires effective and expensive scrubbers. The application of raw sewage sludge as a fertilizer produces odor problems, and is also associated with the risk of contamination of the soil by heavy metals and pathogens. A more *Corresponding author. Tel.: 11-212-650-6017; Fax: 11-212650-6107. E-mail address: [email protected] (T.J. Bandosz). 1 Permanent address: Institute for Sorption and Problems of Endoecology, Ukraine.

efficacious and safer alternative is the pyrolytic carbonization of sludge to obtain useful sorbents [3–7]. The preparation of adsorbents by carbonization of sludge was first patented by Hercules, Inc. [8]. The process was further investigated by Chiang and You [4], and Lu et al. [5,6]. Both simple pyrolysis and pyrolysis after addition of chemical activation agents such as zinc chloride or sulfuric acid were used. The sorbents obtained had a relatively high surface area (100–200 m 2 / g upon physical activation and up to 400 m 2 / g upon chemical activation) and developed microporosity. As suggested by Chiang and You, the high content of inorganic matter, usually around 75% wt., together with the microporosity, promotes the adsorption of organic species such as methyl ethyl ketone or toluene [4]. Lu and coworkers used the sorbents obtained from sludge by chemical activation as media for the removal of hydrogen sulfide [6]. Their removal capacity was only 25% of that of Calgon activated carbons and the mechanism and efficiency of the process were not studied in detail. Since hydrogen sulfide is the main source of odor from sewage treatment plants, the possibility of using sewage sludge as a source of adsorbents for H 2 S is appealing. The idea is even more attractive when the mechanism of

0008-6223 / 01 / $ – see front matter  2001 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 01 )00026-4

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A. Bagreev et al. / Carbon 39 (2001) 1971 – 1979

adsorption of hydrogen sulfide is taken into account. As proposed elsewhere [9–11], H 2 S is first adsorbed in the water film present on the carbon surface, followed by dissociation and adsorption of HS 2 in the micropores. In the next step HS 2 is oxidized to various sulfur species. The speciation of the final products of oxidation depends on the pH of the activated carbon surface [11–13]. This mechanism was derived from a study of unmodified carbons [10–12]. In the case of catalytic carbons containing nitrogen, it was proposed that nitrogen-containing basic centers located in the micropores are the high energy adsorption sites playing an important role in the oxidation of hydrogen sulfide to sulfuric acid [13]. The latter as the final product makes the regeneration feasible using simple methods such as washing with water [14,15]. In the case of catalytic carbons such as Centaur  the basic centers are introduced using a special urea modification process [16]. Since sewage sludge contains a considerable amount of organic nitrogen, carbonization of such species can lead to the creation of basic nitrogen groups within the carbon matrix, which again have been proven to be important in the oxidation of H 2 S [13,16]. Another advantage to the use of sludge as a starting material is the presence of significant amounts of iron added to the raw sludge as a dewatering conditioner; iron is also considered to be a catalyst for H 2 S oxidation [17–19]. The objective of this paper is to characterize the surface features of adsorbents obtained by pyrolysis of Terrene  , produced by the New York Organic Fertilizer Company (NYOFCo) by thermally drying and pelletizing dewatered New York City municipal sewage sludge. Since the cost of production of adsorbents for this application should be kept low, no chemical activation is used in this stage of our study. A series of adsorbents was obtained by carbonization at various temperatures. They differ significantly in porous structure and surface chemistry. The surface characteristics are analyzed from the point of view of the possible changes in the chemistry of the sludge components during pyrolysis.

2. Experimental

2.1. Materials Terrene  was obtained from NYOFCo in the form of 3 mm diameter granules with about 5% wt. water content. The results of the chemical analysis (Pace Analytical Services) are presented in Table 1. The adsorbents studied were prepared by pyrolysis of Terrene  at temperatures between 400 and 9508C in a nitrogen atmosphere in a fixed bed (horizontal furnace). The samples, experimental pyrolysis conditions, and adsorbent yield calculated as the weight ratio after and before pyrolysis are given in Table 2. To determine the effect of acid washing on the pore structure and chemistry of the organic carbonaceous phase,

Table 1 Results of selected chemical and physical analyses of Terrene  Element / quantity

Content

Aluminum Arsenic Cadmium Calcium Chromium Cobalt Copper Iron Lead Magnesium Molybdenum Nickel Potassium Selenium Silver Zinc

7410 ppm 5.97 ppm 3.52 ppm 20300 ppm 73.2 ppm 10.2 ppm 932 ppm 26600 ppm 280 ppm 5550 ppm 13.2 ppm 44.6 ppm 0.09 ppm 6.09 ppm ND a 1290 ppm

Sulfur Nitrogen Phosphorus pH Bulk density Density Fixed solids Volatile solids

0.70% wt. 1.82% wt. 3.17% wt. 7.08 0.78 g / cm 3 1.21 g / cm 3 65.3% wt. 34.7% wt.

a

Not detected.

the SLC-1 and SLC-3 samples were subsequently treated with 16% hydrochloric acid to remove acid-soluble inorganic matter and then washed with distilled water to remove excess acid. The samples obtained in this way are designated as C-1 and C-3, respectively.

2.2. Methods Nitrogen adsorption isotherms were measured using an ASAP 2010 analyzer (Micromeritics, Norcross, GA, USA) at 21968C. Before the experiment the samples were degassed at 1208C to a constant pressure of 10 25 Torr. The isotherms were used to calculate the specific surface area, SN 2 , micropore volume, Vmic , total pore volume, Vt , and pore size distribution. All the parameters were determined using Density Functional Theory (DFT) [20,21], the Dubinin–Radushkevich (DR) method [22] and the Brunauer–Emmet–Teller (BET) method. The relative microporosity was calculated as the ratio of micropore volume (DR) to total pore volume. Potentiometric titration measurements were performed with a DMS Titrino 716 automatic titrator (Metrohm, Brinkmann Instruments, Westbury, NY, USA). The instrument was set in the equilibrium mode when the pH was collected. Approximately 0.1 g samples were placed in a container thermostatted at 258C with 50 ml of 0.01 M NaNO 3 and equilibrated overnight. To eliminate any

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Table 2 Names of samples, conditions of preparation and yields of carbonaceous materials obtained by pyrolysis of Terrene  in flowing nitrogen Sample name

Temperature [8C]

Heating rate [8C / min]

Hold time [min]

Yield of adsorbents a [%wt.]

Ash content b [%wt.]

Carbon yield c [%wt.]

SLC-1 SLC-2 SLC-3 SLC-4 Ash d

400 600 800 950 600

10 10 10 10 5

30 60 60 60 120

51.9 46.3 41.8 39.3 33.1

61.7 69.1 76.8 80.7 –

19.9 14.3 9.7 7.6 –

a

Calculated as the ratio of a sample weight after carbonization to the initial weight of dry sludge. Obtained as a residue after TG analysis of carbonized samples in air. c Calculated from thermal analysis as a difference in the residue after heating of the carbonized samples in nitrogen and in air to a specific carbonization temperature. d Obtained by heating Terrene  in air at 6008C for 2 h. b

interference by dissolved CO 2 , the suspension was continuously saturated with N 2 . The carbon suspension was stirred throughout the measurement. 0.1 M NaOH was used as a titrant. Experiments were carried out in the pH range 3–10 [23–25]. Diffuse reflectance IR spectra were obtained using a Nicolet Impact 410 FT-IR spectrometer equipped with a diffuse reflectance unit (Nicolet Instrument Corp., Madison, WI, USA). Adsorbent carbon powders were placed in a micro-sample holder. A 0.4 g sample of dry adsorbent was added to 20 ml of water and the suspension was stirred overnight to reach equilibrium. The sample was filtered and the pH of solution was measured using a Accumet Basic pH meter (Fisher Scientific). Thermal analysis was carried out using a TA Instruments Thermal Analyzer (New Castle, DE, USA). The heating rate was 108C / min in a nitrogen atmosphere at 100 ml / min flow rate. An average sample size was about 20 mg. Carbon, hydrogen and nitrogen analyses were performed by Huffman Laboratories, Golden, CO, USA.

3. Results and discussion The analytical data in Table 1 shows that the thermally dried sludge pellets (Terrene  ) have a high content of inorganic matter, especially metals such as iron and calcium, which can be beneficial for the catalytic oxidation of hydrogen sulfide [17]. As expected for this digested municipal sludge product, the organic nitrogen, phosphorus and sulfur contents are also high. The analysis does not include carbon, but, based on the results of previous studies, the total organic matter in the sludge product is expected to be about 70% (w / w) [4–6]. The ash content of the sample was evaluated using thermal analysis. The residue after heating the sludge pellets to 10008C in air is 31.4% of the initial sample mass, which is in agreement

with previous studies [4–6]. About 80% of the ash is expected to consist of inorganic oxides such as Al 2 O 3 , SiO 2 and Fe 2 O 3 [5]. The results of the thermal analysis of the sludge pellets in air and in nitrogen are presented in Fig. 1. The TGA curves in air and nitrogen are similar up to about 2758C (Fig. 1a and b). The first peak at about 1258C is the result of the removal of water (around 4%). The weight loss observed between ca. 200 and 4008C is the result of the emission of volatile organic compounds responsible for the strong unpleasant odor during carbonization. In air, the sharp decrease in weight at 2758C correlates with the first, smaller DTA exotherm (Fig. 1c), caused by oxidation and / or volatilization of easily oxidized volatiles in the sludge pellets. The corresponding weight change in nitrogen atmosphere maximizing at about 3408C reflects volatilization of these compounds. A second, similar event is observed at ca. 4258C. More significant differences occur at temperatures greater than 4508C. In the air atmosphere, ignition of the refractory organic material occurs as indicated by the sharp exotherm on the DTA and weight derivative curve on the TGA scan. In contrast, the DTA curve in nitrogen (Fig. 1c) is almost featureless. The weight losses (Fig. 1b) are quantified in Table 3, and show a 7.7% higher yield of carbon after heating Terrene  to 10008C in nitrogen than in air. The total yield of adsorbent in nitrogen is 39.1%, with an 80.3% ash content. The yield of carbonaceous phase in nitrogen is up to 19.7%. The results of the thermal analysis of Terrene  were used to choose the experimental conditions and, as shown in Table 2, 400, 600, 800 and 9508C were chosen as pyrolysis temperatures. As expected, the yield of carbon material (calculated from thermal analysis as a difference in the residue after heating of the carbonized samples in nitrogen and in air to a specific carbonization temperature) decreases with increasing temperature of heat treatment and ranges from 19.9% for SLC-1 to 7.6% for SLC-4. The results of CHN analyses in Table 4 show that the

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Table 4 Carbon, hydrogen and nitrogen contents [%wt.] (obtained by elemental analysis) Sample

C

H

N

SLC-1 SLC-2 SLC-3 SLC-4 C-1 C-3

28.19 27.14 26.37 24.89 36.92 31.97

2.04 1.14 0.42 0.35 2.46 0.63

3.83 3.19 1.61 0.94 4.79 1.90

degree of carbonization. Moreover, the organic nitrogen that is probably present as amine functionalities in the material carbonized at low temperature is gradually transformed into pyridine-like compounds (see below) which should result in an increased basicity of the surface. After washing with hydrochloric acid the weight % contents of C and N increase because of the partial removal of inorganic matter. It is worth noting that the N content of the acid washed sample C-3 increased 18% compared to that of unwashed precursor, which may be beneficial for the application of this material as a sorbent for acidic gases. The dissolution of inorganic matter by acid treatment of SLC-1 and SLC-3 resulted in overall weight losses of 26% and 19% wt., respectively. The adsorption isotherms presented in Fig. 2 show that the total sorption uptake increases with increasing pyrolysis temperature. The isotherms are characteristic of

Fig. 1. TG (a), DTA (b) and DTG (c) curves in air (thick lines) and nitrogen (thin lines).

sorbents obtained have about 30% carbon which seems to be a sufficient amount for the development of the surface features responsible for physical adsorption [26]. Also, as expected, sample SLC-1 has the highest content of nitrogen, almost 4% by weight. With increasing carbonization temperature both N and H contents decrease because of both the loss of volatile species and an increase in the

Fig. 2. Nitrogen adsorption isotherms for ash and the sludge derived materials.

Table 3 Weight loss during thermal analysis of Terrene  [%wt.] Atmosphere

30–2008C

200–5008C

500–10008C

Total

Yield

nitrogen air

7.2 7.4

42.8 48.1

10.9 13.1

60.9 68.6

39.1 31.4

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Table 5 Structural parameters calculated from nitrogen adsorption isotherms at 21968C Sample

SBET [m 2 / g]

Vmic (DR) [cm 3 / g]

Smic (DR) [m 2 / g]

SDFT [m 2 / g]

Vmic (DFT) [cm 3 / g]

Vt (0.99 a) [cm 3 / g]

Vmic /Vt

SLC-1 SLC-2 SLC-3 SLC-4 C-1 C-3 Ash b

41 99 104 122 17 139 20

0.016 0.044 0.048 0.051 0.006 0.058 0.008

35 108 118 120 13 135 17

21 92 106 104 15 150 13

0.006 0.030 0.033 0.028 0.002 0.030 0.003

0.084 0.131 0.132 0.158 0.063 0.201 0.077

0.19 0.33 0.36 0.32 0.09 0.29 0.10

a b

Total pore volume filled at a relative pressure p /po equal to 0.99. Obtained by heating Terrene  in air at 6008C for 2 h.

predominantly mesoporous solids with some contribution by the micropores. The BET surface areas and pore volumes are given in Table 5. The surface area increases by a factor of almost three as the pyrolysis temperature is increased from 400 to 9508C. The biggest difference exists between samples SLC-1 and SLC-2, thereby suggesting that significant development of the porosity occurs between 400 and 6008C. Similar changes are observed for micropore volume, which reaches 0.051 cm 3 / g for SLC-4 (DR method). The relative microporosity, defined as the ratio of the volume of micropores to total pore volume, is almost constant for samples heated to temperatures higher than 6008C. Its values near 30% indicate a significant contribution of mesoporosity in the sludge-derived adsorbents. The mesopores may have their origin in the high (|80%) content of inorganic matter, which consists mainly of silica, alumina, and iron oxides [4]. To determine the contribution of the mesopores, the adsorption isotherms were measured on the ash obtained after heating the untreated sample in air at 6008C (Fig. 2). The ash surface area is quite small, which reflects the presence mainly of mesopores (Table 5). The development of porosity with increasing pyrolysis temperature is illustrated in Fig. 3. The pore size distributions (PSDs) calculated for our

materials using the density functional theory [20,21] are shown in Figs. 4 and 5. The development of micropores ˚ is observed with increasing carboniza(smaller than 20 A) tion temperature. Moreover, the volume of mesopores ˚ significantly increases with temperature smaller than 100 A which is a result of chemical changes in the inorganic matter [27,28] and the carbon–inorganic matter interface.

Fig. 3. Development of porosity with increasing pyrolysis temperature.

Fig. 5. Comparison of pore size distributions of SLC-1, SLC-3 and their acid-washed counterparts.

Fig. 4. Pore size distributions for the ash and the sludge derived materials.

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These PSDs also confirm our hypothesis about the source of the mesoporosity being the inorganic matter. Fig. 5 shows the changes in the distribution of pore sizes for selected samples before and after acid washing. For the sample carbonized at 4008C, a significant decrease in the volume of pores of all sizes is observed after acid washing, as a result of the chemical reactivity of this sample, which can be considered as undercarbonized. This is due to the fact that 4008C is not high enough to dehydroxylate inorganic oxides and fully carbonize organic matter. It follows that after pyrolysis at low temperature both organic and inorganic structures are reactive toward acids, resulting in a significant change in the physical state of the material upon acid washing. On the other hand, for the sludge pellet sample treated at 8008C, acid washing produces a significant increase in the volume of micro- and small mesopores along with a decrease in the volume of mesopores. It is likely that acid washing removes species such as iron oxides [27], creating new pores within the inorganic matter and at the interface between the inorganic and organic phases. An increase in the total porosity is also a result of the increasing contribution of the carbonaceous material in this sample. All the structural parameters reported in Table 5 are much smaller than those for commercial activated carbons [29,30]. Considering that our sorbents contain 25 to 30% carbon, the porous structure responsible for an increase in the surface area has to be developed within this organic deposit. Since a higher pore volume is generated simultaneously with a lower carbon content, the increase in pyrolysis temperature must produce subtle chemical changes resulting in the gasification of carbon and the creation of more pore volume. The small peaks in the DTG curves in nitrogen at 7008C and 8008C may reflect these changes. The chemical changes may be related to an increase in the degree of aromatization and the incorporation of nitrogen and other heteroatoms into the carbon matrix [13,31,32]. Moreover, water released by dehydroxylation of the inorganic material can act as a pore former and activation agent creating very small (Angstrom-size) pores in the carbon deposit [33–35]. The suggested chemical changes described above are manifested first through changes in the pH values of the adsorbents (Table 6). It is interesting that the pH of sample SLC-1 is close to neutral and that pyrolysis at higher

temperatures causes a significant increase in the pH values to .11. The pH of the ash sample is also close to neutral. Since the primary dehydroxylation of inorganic species is expected to occur at ca. 4008C [27,28], an increase of 3.8 pH units between SLC-1 and SLC-2 must be related to chemical changes in the carbon phase. These samples have high organic nitrogen contents. When the pyrolysis temperature is .6008C this nitrogen is probably incorporated into the carbon matrix as heteroatoms such as pyridine-like structures [13,31,32]. The basicity of these centers probably contributes to the high pH of the sludge-derived adsorbents. Changes in the surface chemistry were further studied using potentiometric titrations. The procedure applied and the mathematical treatment of the data [23–25,36] yield the distributions of acidity constants. The results are presented in Fig. 6. Comparisons of the peak intensities and peak positions indicate the predominant effect of the inorganic matrix on the acidity of the samples. The peaks for the high temperature sludge-derived samples are similar to those present in their ash. Heat treatment results in changes in the acidity due to the dehydroxylation of inorganic matter and the rearrangement in coordination of oxides of metals such as alumina or iron [37–39]. It is

Table 6 pH values of adsorbents’ surface (equilibrium pH values of aqueous slurries of selected samples) Sample

pH

SLC-1 SLC-2 SLC-3 SLC-4 Ash

7.72 11.51 11.29 10.96 8.27

Fig. 6. pKa distributions for the materials studied.

A. Bagreev et al. / Carbon 39 (2001) 1971 – 1979

interesting that the sample carbonized at 6008C, SLC-2, differs significantly in acidity from the other samples. This is not consistent with the variation in pH values listed in Table 6. The reason for the discrepancy may lie in a limitation of the potentiometric titration method, which is able to detect species having pKa ’s between 3 and 11 (in our case titration was carried out to pH about 10 to avoid dissolution of silica in the basic medium [40]). Species present with pKa ’s beyond the experimental window will affect the average pH of the sample. In the case of SLC-2 it is likely that a significant change in both the inorganic and organic phases occurs at its pyrolysis temperature, which also results in a significant increase in the porosity evaluated from nitrogen sorption data. Comparison of the pKa distributions for SLC-1 and SLC-3 reveals differences in the intensities of peaks at pKa ’s of approximately 3.5, 8.6, and 5.5. At the higher carbonization temperature the number of species having pKa ’s near 5.5 significantly decreased relative to the species with pKa ’s near 3.5 or 8.6. The observed effect is related to changes in the acidity of inorganic oxides. After acid washing the distributions become more consistent with each other except for the peak at pKa near 7. In the case of C-1 this peak almost disappears whereas in C-3 the amount of detected species is much lower than for SLC-3. The peak probably represents soluble iron oxides [38]. In C-1 the intensity of this peak decreased about 97% vs. a decrease of about 62% for C-3, relative the corresponding peaks for the non-acid washed material. The surface chemistry of the sewage sludge-derived samples was also studied using DRIFT spectroscopy. The results are presented in Figs. 7 and 8. In the spectra obtained for the sludge-derived samples carbonized at the lowest temperature, the well-defined features of the inorganic matter are seen. For higher carbonization tempera-

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Fig. 8. DRIFT spectra for SLC-1, SLC-3 and their acid-washed counterparts (C-1 and C-3).

tures the spectra become more similar to those characteristic of activated carbons, reflecting the combined effects of the ‘screening’ action of carbon, dehydroxylation of the mineral matter and possible formation of spinel-like compounds. Washing with acid changes the chemistry, which is reflected in changes in the relative intensities of the peaks. A distinctive feature of the low temperature carbonized samples, SLC-1 and C-1, is the presence of a peak between 2750 and 3000 cm 21 which likely represents the nitrogen in amine functionalities. If, as suggested above, amine nitrogen is converted into pyridine form at high temperatures, peaks around 1575 cm 21 should be apparent [41,42]. Although in the spectra for our samples this peak is present, we cannot rule out other species such as –COO– bonds [42] which absorb in this region.

4. Conclusions

Fig. 7. DRIFT spectra for the initial sludge (SL), ash, SLC-2 and SLC-4.

Pyrolysis of a thermally dried sewage sludge fertilizer product, Terrene  , results in adsorbents having chemical and physical features potentially useful for adsorption of acidic gases. Sorbents having surface areas up to 140 m 2 / g can be derived using a simple carbonization method. The materials have broad pore size distributions with about 30% of total pore volume located in narrow micropores. Their unique surface chemistry related to chemical heterogeneity of sewage sludge is believed to result from a combination of acidity from metal oxides such as silica, alumina, or iron, and basicity from carbonaceous matter and organic nitrogen in the form of amine or pyridine-like groups. The presence of basic nitrogen and iron can offer significant advantages for the application of these materials

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as sorbents for acidic gases. This possibility is being explored in our ongoing research.

Acknowledgements This study was supported by CUNY Collaborative Research grant [91906. The assistance of Ms. Betty Mei-Ki Chan and Anna Kleyman is appreciated. TJB thanks Dr. Jacek Jagiello for SAIEUS software. The authors are grateful to Mr. Thomas Murphy of NYC DEP and Mr. Peter Scorziello, the manager of the NYOFCo facility, for supplying Terrene  .

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