Use of Waste Sludge from the Tannery Industry - American Chemical

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Use of Waste Sludge from the Tannery Industry Ismail Cem Kantarli† and Jale Yanik*,‡ Ataturk Medical Technology Vocational Training School, and Department of Chemistry, Faculty of Science, Ege UniVersity, BornoVa/Izmir 35100, Turkey ReceiVed December 21, 2008. ReVised Manuscript ReceiVed March 31, 2009

In this study, the conversion of waste sludge from a tannery industry into useful materials was the aim. For this purpose, the thermal behavior of waste sludge was investigated using thermogravimetry (TG) and a vertical fixed-bed reactor. Thermogravimetric analysis was performed at different heating rates to the temperature of 750 °C. The main pyrolytic decomposition step was observed in the temperature range of 248-500 °C. In the fixed-bed reactor, waste sludge was pyrolyzed at 450 and 600 °C in a nitrogen atmosphere. Pyrolysis products were separated as gas, liquid, and char. The temperature was found to be slightly effective on the product distribution. Pyrolysis gas had a considerable gross calorific value (23.22 MJ N m-3). Although the pyrolytic oils had reasonable calorific value, they contained a high amount of nitrogenated and oxygenated compounds. Using waste sludge as a raw material for activated carbon production was also investigated. For this purpose, activated carbons were produced from waste sludge by physical and chemical activation methods. To check the potential of activated carbons to be used as adsorbents, methylene blue, phenol, and Cr(VI) adsorption studies were carried out with selected activated carbons.

1. Introduction The tanning industry is known as a potentially pollutionintensive industry. It globally generates 4 million tons of solid waste per year.1 Most of the operations in the tannery are performed in water. As a result, wastewater effluent is one of the major problems in tanneries and needs to be treated before discharged into surface water. It is reported that approximately 0.2 kg of waste sludge (WS) (on a dry basis) per kilogram of wet salted hides/skins is generated in a tannery as a result of wastewater treatment.2 Sludge is generated directly from the initial treatment stages, a full treatment of wastewater in the tannery, or the wastewater treatment plant to which they are released. For the disposal of sludge, options in practice are landfilling, anaerobic digestion, application in agriculture, and incineration. However, some of these options will not be available in the future or might require further pretreatment steps because of legislation prepared for disposal of wastes, imposing strict criteria for industries. These strict criteria for waste disposal require efficient recycling of resources without supply of harmful substances to humans and the environment and have brought additional costs to production. Thus, industries have started to consider recycle and use of wastes for both environmental and economical concerns. Also, in the recent years, a worldwide interest has awoken for use of any kind of waste to reuse them and take advantage of their energy content. Several * To whom correspondence should be addressed. Telephone/Fax: +90232-388-82-64. E-mail: [email protected]. † Ataturk Medical Technology Vocational Training School. ‡ Department of Chemistry, Faculty of Science. (1) Booth, S.; Long, A. J.; Addy, V. L. Converting tannery waste to energy. Visual Display Presentation, V-21, IULTCS II: Eurocongress, I˙stanbul, Turkey, May 24-27, 2006 (accessed at http://www.aaqtic.org.ar/ congresos/istanbul2006). (2) Integrated Pollution Prevention and Control (IPPC). Reference document on best available techniques for the tanning of hides and skins. Institute for Prospective Technological Studies, Technologies for Sustainable Development, European IPPC Bureau, 2001; p 26 (accessed at http:// eippcb.jrc.es/pages/FActivities.htm).

technologies presenting an alternative to conventional combustion processes are currently being developed. One of these technologies is pyrolysis. Pyrolysis produces more useful products; gas, oil, and solid char, which may be used as fuels or a feedstock for petrochemicals and other applications. In general, the chars obtained from pyrolysis of sewage sludge could also be either incinerated, disposed of by landfilling, or used as adsorbents depending upon their properties. An increasing demand for adsorption processes in the water treatment industry has encouraged research into the production of activated carbon from alternative precursors, such as industrial wastes and agricultural byproduct. Because WS has a carbonaceous nature, it is suitable for producing activated carbon. The use of sludge for the production of activated carbon offers the combined benefits of reducing the volume of sludge and producing a valuable adsorbent with low cost. In this context, use of WS from the tannery industry was the aim in this study. For this purpose, pyrolysis and activation of WS from a tannery located in Izmir, Turkey, were carried out. From pyrolysis and activation, conversion of wastes to useful and harmless products was the aim. In addition, potential application of some of the produced activated carbons in methylene blue, phenol, and Cr(VI) removal was also investigated. This study has both environmental and economical importance that should be considered by the tannery industry. 2. Experimental Section 2.1. Material. The WS used in this study, which is a mixture of sludge obtained from both the chemical and biological treatment units of the wastewater treatment plant of the tannery industry and dewatered in belt press, was supplied by Sepiciler Co., Izmir, Turkey. Proximate and ultimate analyses of WS are given in Table 1. 2.2. Thermogravimetric Analysis. Thermogravimetric analysis of WS was performed in a thermogravimetric analyzer (PerkinElmer Diamond TG/DTA) under N2 atmosphere. The sample amount (particle size < 100 µm) was about 10 mg for each run.

10.1021/ef8011068 CCC: $40.75  2009 American Chemical Society Published on Web 05/07/2009

Use of Waste Sludge from the Tannery Industry

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Table 1. Properties of WS Proximate Analysis (wt %) moisture

ash

volatiles

4.0

33.1

50.5

Ultimate Analysis (wt %) C H N S Al Cr

34.2 4.4 5.2 2.3 1.5 2.4

Zn Pb Cd Cu Ni

0.6 2.5 × 10-3 3.3 × 10-4 7.3 × 10-3 8.5 × 10-4

The flow rate of purge gas (pure N2, 99.99%) was kept at 200 mL min-1. The sample was heated from the ambient temperature up to 750 °C, with heating rates of 5, 10, and 20 °C min-1. Both the thermogravimetric (TG) and differential thermogravimetric (DTG) data were used to differentiate the pyrolysis behavior of waste samples at different heating rates. 2.3. Pyrolysis Experiments. Pyrolysis experiments were carried out in a vertical reactor. The pyrolysis reactor was a fixed-bed design and stainless-steel, with 6 cm in diameter and 21 cm high. Before pyrolysis experiments, WS was dried in an oven at 105 °C to a constant weight and then ground below 0.1 cm. In a typical run, WS of 100 g was placed into the reactor. The system was heated to the desired pyrolysis temperature, at a heating rate of 5 °C min-1, and held at this temperature for 2 h. The volatile products were swept by nitrogen gas (25 mL min-1) from the reactor to collection flasks cooled with ice, where the liquid products were condensed in the traps. Following the traps, noncondensable volatiles were passed through the lead nitrate solution (33 wt %) containing traps to absorb H2S and then the remaining gases were collected in a Tedlar bag. The liquid product collected in traps contained the aqueous and organic phase (pyrolytic oil). The aqueous phase in the condensate was separated from the organic phase by centrifugation. In each experiment, char and liquid (oil and aqueous fraction) yields were determined by weight and the gas fraction yield was calculated by weight difference. 2.4. Gas Product Analyses. The amount of hydrogen sulfur in the gaseous product was determined as a lead sulfur precipitate, which was formed from the reaction between H2S and lead nitrate in the traps. The lead sulfur precipitate was filtered, washed with distilled water, dried at 110 °C, and weighed. The pyrolysis gas product collected in the Tedlar bag was analyzed by gas chromatography using a HP model 5890 series II with a thermal conductivity detector. A stainless-steel packed column (6.0 m × 1/8 in. Porapak Q, 2.0 m × 1/8 in. 5A molecular sieve, serially connected to each other) was used. The separation of CO2, C1, C2, C3, and C4 hydrocarbons was carried out with a Porapak Q column, and the separation of O2, N2, and CO was achieved by the MS 5A (molecular sieve) column. 2.5. Pyrolytic Oil Analyses. The viscosity of the oil fraction (pyrolytic oil) was determined according to American Society for Testing and Materials (ASTM) D445. C, H, N, and S contents of pyrolytic oil were determined using an elemental analyzer LECO CHNS 932. The GCV of the oils was calculated by the following correlation proposed by Channiwala and Parikh3

GCV ) 0.3491C + 1.1783H + 0.1005S - 0.1034O 0.015N (MJ/kg) where C is the carbon percent, H is the hydrogen percent, S is the sulfur percent, O is the oxygen percent, and N is the nitrogen percent of oil. The asphaltenes of the pyrolytic oils were precipitated by the addition of n-hexane. Oil portions soluble in n-hexane were fractioned by column chromatography into aliphatic, aromatic, and polar fractions using hexane, toluene, and methanol, respectively.4 2.6. Demineralization and Activation of Pyrolytic Chars. The pyrolytic chars were demineralized to decrease its inorganic content. (3) Channiwala, S. A.; Parikh, P. P. Fuel 2002, 81 (8), 1051–1063.

The chars were treated with HCl solution (10 wt %) at 100 °C for 2 h. After HCl treatment, they were washed with hot water until no chlorine ions could be detected and then dried. The activation process was carried out by carbon dioxide. In the activation process, about 50 g of pyrolytic char, either demineralized or not, was loaded into the pyrolysis reactor and heated to 900 °C under a flowing nitrogen atmosphere (25-30 mL/ min). When 900 °C was reached, the inert atmosphere was rapidly substituted by flowing (350 mL/min) carbon dioxide. The activation times changing between 2 and 14 h were tested. At the end of the desired activation time, the reactor was cooled to room temperature under a nitrogen atmosphere. The resulting carbons (activated carbon) from the activation process were weighted to calculate the burnoff value. 2.7. Production of Activated Carbon by Chemical Activation. As the first step of chemical activation, 50 g of the dried WS was mixed in a baker with 250 mL of ZnCl2 solution with varying concentrations. ZnCl2 solution was used in impregnation ratios of 1:1 and 1.5:1 of the weight of impregnation reagent/ weight of waste (referred to as 100 and 150 wt % loading). The concentrations of the ZnCl2 solutions used for activation are typical of those used for the commercial production of activated carbons by chemical activation. The slurry was then dried in a moisture oven at 105 °C overnight. The impregnated WS sample was placed into the pyrolysis reactor. The system was heated at a rate of 5 °C min-1 to 450 or 600 °C and held at this temperature for 2 h. The reactor was continuously purged with nitrogen at a flow rate of 25 mL min-1. After pyrolysis, the reactor was cooled to room temperature in a nitrogen gas stream overnight. The char was boiled with 200 mL of 10% HCl solution for 120 min, filtered in a vacuum flask, and washed with hot water and finally cold water to remove the chloride ions and other inorganics. After washing, activated carbon samples were dried at 110 °C for 24 h and weighed (m2). The yield of activated carbon from chemical activation was calculated by

% yield ) (m2/M) × 100 where M is the initial mass of waste. 2.8. Characterization of Activated Carbons. The BrunauerEmmett-Teller (BET) surface area measurements were obtained from nitrogen adsorption isotherms at 77 K using a Quantachrome Autosorb 1-C surface area analyzer. The micropore and mesopore volumes were calculated by the t-plot and BJH methods, respectively. These analyses were carried out in TUBITAK-MAM research laboratory in Izmit, Turkey. The scanning electron micrograph (SEM) analyses were recorded using a PHILIPS XL30 S FEG SEM instrument. The amount of surface oxygen groups on the activated carbons has been determined by the Boehm titration method.5 2.9. Adsorption Experiments. The ability of the activated carbons to remove methylene blue, phenol, and Cr(VI) from aqueous solutions was determined under batch-mode conditions. Cr(VI) solutions were prepared by dissolving potassium dichromate in distilled water. The pH of each Cr(VI) solution was adjusted to an optimum value of 2. No pH adjustment was performed for methylene blue and phenol solutions. Test solutions (100 mL) of various concentration (ranging between 100 and 500 mg L-1 for methylene blue, 50-250 mg L-1 for phenol, and 50-250 mg L-1 for Cr(VI)) were added to the adsorbent (0.1 g) in flasks, and suspensions were shaken for an equilibrium time determined with preliminary studies (24, 4, and 2 h and for methylene blue, phenol, and Cr(VI), respectively). The filtrates were analyzed for residual methylene blue and phenol concentrations using the UV-vis spectrophotometer (UV-160A, Shimadzu) at 665 and 269 nm, respectively. For residual Cr(VI) concentrations, the filtrates were (4) Yanik, J.; Yuksel, M.; Saglam, M.; Olukcu, N.; Bartle, K.; Frere, B. Fuel 1995, 74 (1), 46–50. (5) Jindarom, C.; Meeyoo, V.; Kitiyanan, B.; Rirksomboon, T.; Rangsunvigit, P. Chem. Eng. J. 2007, 133, 239–246.

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Kantarli and Yanik Table 2. Product Yields from the Pyrolysis of WS (wt %) pyrolysis temperature (°C) gaseousa liquid oil aqueous char a

Figure 1. TG and DTG curves of WS at heating rates of 5, 10, and 20 °C min-1.

analyzed by a reaction with 1,5-diphenylcarbazide followed by absorbance measurement at 540 nm using the UV-vis spectrophotometer.

3. Results and Discussion 3.1. Thermogravimetry Results. Thermogravimetric analysis provides preliminary knowledge of the pyrolysis behavior of WS.6 To find out the optimal range of the pyrolyis temperature, TGA experiments were carried out with the WS at different heating rates. The results of TGA/DTG are shown in Figure 1. Both TGA and DTG curves can be divided into three regions, indicating the gradualness of the thermal decomposition process. The first region with a weight loss of approximately 5% corresponds to evaporation of water and does not involve pyrolysis. The main pyrolytic decomposition steps occur in the range of 248-500 °C. In this temperature range, peaks with a shoulder are observed in DTG curves (Figure 1). The main peak, because of decomposition of less complex organic structures, took place at lower temperatures, while the shoulder at higher temperatures was believed to be caused by the decomposition and devolatilization of more complex organic structures.7-9 The remaining residues at this step are 53-56% of total weight. The last step occurs at higher temperatures (>600 °C), and this step is accounted to be 12-15% of the total weight loss. The weight loss at this temperature might be due to the degradation of inorganic content, as proposed by Karayıldırım et al.,6 and/or both devolatilization and reactions between char and volatiles from previous stages, as proposed by Calvo et al.10 The residues were 36-40% of the total weight. Figure 1 shows that the peak temperatures (Tmax, temperature corresponding to the maximum mass loss rate) of each (6) Karayildirim, T.; Yanik, J.; Yuksel, M.; Bockhorn, H. Fuel 2006, 85, 1498–1508. (7) Thipkhunthod, P.; Meeyoo, V.; Rangsunvigit, P.; Kitiyanan, B.; Siemanond, K.; Rirksomboon, T. Chemosphere 2006, 64, 955–962. (8) Gascia-Perez, M.; Chaala, A.; Yang, J.; Roy, C. Fuel 2001, 80, 1245– 1258. (9) Sørum, L.; Grønli, M. G.; Hustad, J. E. Fuel 2001, 80, 1217–1227. (10) Calvo, L. F.; Otero, M.; Moran, A.; Garcia, A. I. Bioresour. Technol. 2001, 80 (2), 143–148.

450

600

17.5 26.8 14.3 12.5 55.7

21.9 24.3 12.9 11.4 53.8

Calculated from mass balance.

decomposition step are shifted to higher temperatures with an increasing heating rate, possibly because of the effect of the kinetics of the decomposition, which resulted in a delayed degradation. This observation was consistent with that of other authors.11,12 The results also indicate that a higher heating rate gives a higher final residue amount and thus a lower conversion. 3.2. Pyrolysis of WS. 3.2.1. Pyrolysis Yields of WS. The product distribution from pyrolysis at two different temperatures (450 and 600 °C) is presented in Table 2. Pyrolysis temperatures were selected according to the result of TG analysis and were in the range where the pyrolytic reaction of sludge is reported to be significant in the literature.6,12 Table 2 shows that relatively high amounts of char were found, in keeping with the high initial ash contents. It must be noted that the yield of the chars showed similarity to those obtained by TGA. Pyrolysis liquids consisted of aqueous and oil (pyrolytic oil) fractions. The amounts of aqueous fraction from pyrolysis of WS were much more than the moisture content of feed. This is reasonable, because water originates from a dehydration reaction of organic compounds in addition to physically bonded and free water in the sludge.13,14 Aqueous fractions consist of mainly water and small amounts of organic substances soluble in water, such as alcohols, ethers, aldehydes, and carboxylic acids.6 Elemental analysis of the aqueous phase from pyrolysis of digested sludge showed that the aqueous phase contained 1-4% of the initial carbon.14 As seen from Table 2, the increase in the temperature from 450 to 600 °C led to a little change in the yield of products obtained from pyrolysis. The gaseous product yield increased, while the liquid product and char yield slightly decreased with the increase in the pyrolysis temperature. The difference in gas yields between 450 and 600 °C was 4.4%. However, in the pyrolysis of the digested and dried sewage sludge from a sewage treatment, with the increase in the temperature from 446 to 610 °C, the oil yield decreased at the ratio of ∼12% and the gas yield increased at the ratio of ∼7%.15 In another study related to the pyrolysis of an anaerobic sewage sludge,16 it was reported that the liquid yield increased slightly when the temperature increased from 450 to 650 °C then remained more or less constant above 650 °C. In the pyrolysis of an activated sewage sludge sample, it was observed that the oil yields increased initially with an increasing temperature and then decreased because of secondary decomposition reactions, which break the oil into lighter, gaseous hydrocarbons. The maximum oil yield was obtained at 525 °C, while the char yield, on the other hand, decreased steadily and the gas yield increased continuously with (11) Shie, J. L.; Chang, C. Y.; Lin, J. P.; Wu, C. H.; Lee, D. J. J. Chem. Technol. Biotechnol. 2000, 75 (6), 443–450. (12) Inguanzo, M.; Menendez, J. A.; Fuente, E.; Pis, J. J. J. Anal. Appl. Pyrolysis 2001, 58-59, 943–954. (13) Murwanashyaka, J. N.; Pakdel, H.; Roy, C. J. Anal. Appl. Pyrolysis 2001, 60, 219–231. (14) Lutz, H.; Romerio, G. A.; Damasceno, R. N.; Kutubiddin, M.; Bayer, E. Bioresour. Technol. 2000, 74, 103–107. (15) Park, E.; Kang, B.; Kim, J. Energy Fuels 2008, 22 (2), 1335–1340. (16) Inguanzo, M.; Dominguez, A.; Menendez, J. A.; Blanco, C. G.; Pis, J. J. J. Anal. Appl. Pyrolysis 2002, 63, 209–222.



Table 3. Composition of the Gaseous Products from Pyrolysis of WS (mol %) H2

COx

C1

C2

C3

C4

C5

C6

36.1

34.3

17.1

6.1

3.5

1.6

0.8

0.6

an increasing temperature.17 These differences in results given in the literature are reasonable because pyrolysis behavior of sewage sludge is greatly influenced by their composition as well as pyrolysis conditions.18-21 3.2.2. Composition of WS Pyrolysis Gases. The compositions of the gaseous products from pyrolysis at representative temperature (450 °C) detected by gas chromatography (GC) are shown in Table 3. As seen from the table, H2, COx, and methane are the major gaseous products. This result is in good agreement with results of pyrolysis studies carried out with WS and a variety of biomass.6,17,22 The formations of CO and CO2 up to 600 °C were generated from the degradation of organic compounds, mainly carboxylic groups.6 Besides decarboxylation reactions, the formation of COx and also H2 may be attributed to the following gasification reactions by the catalytic effect of inorganic materials in sludge: water gas reaction: C + H2O f CO + H2 ∆H ) 132 kJ mol-1 (1) water gas shift reaction: CO + H2O f CO2 + H2 ∆H ) -41.5 kJ mol-1 (2) Pyrolysis gas contained H2S by a weight percent of 5.11. It may be suggested that the hydrogen sulfide was formed by the thermal degradation of bacteria in sludge. The heating value of gas was calculated as 23.22 MJ N m-3 and seems to be sufficient for provision of some part of the pyrolysis plant energy requirements. This heating value is the mean heating value of the gas mixture, and it has been calculated from the concentration of each individual gas and its corresponding heating value. The heating value of gas is higher than those reported in the literature and is comparable to those of coke-oven gas (19-22 MJ N m-3).16 The heating value is related to the gas composition, and the composition of gas products depends upon various parameters, such as sludge constituents, temperature, heating rate. Shen et al.,17 who investigated the pyrolysis of activated sewage sludge in a fluidized bed, reported that the yields of all of the gases (hydrocarbon and nonhydrocarbon gases) increased steadily as the temperature increased from 300 °C, with the exception of CO. In the study by Dominguez et al.,23 the heating values of gases from conventional pyrolysis of sewage sludge at a high heating rate (74.3 °C/min) were calculated as 13.0-14.0 MJ N m-3. Besides, the heating values of gases from the microwave pyrolysis of sewage sludge were calculated between 7.5 and 9.5 MJ N m-3. In another study related to sludge pyrolysis carried out at different temperatures,16 a maximum heating value (25.0 MJ N m-3) was obtained around 455 °C with a heating (17) Shen, L.; Zhang, D. Fuel 2003, 82, 465–472. (18) Gasco, G.; Blanco, C. G.; Guerrero, F.; Mendez, A. J. Anal. Appl. Pyrolysis 2005, 74, 413–420. (19) Mendez, A.; Gasco, G.; Freitas, M. M. A.; Siebielec, G.; Stucznynski, T.; Figueiredo, J. L. Chem. Eng. J. 2005, 108, 169–177. (20) Gomez-Rico, M.; Font, R.; Fullana, A.; Martin-Gullon, I. J. Anal. Appl. Pyrolysis 2005, 74, 421–428. (21) Gasco, G.; Cueto, M. J.; Mendez, A. J. Anal. Appl. Pyrolysis 2007, 80, 496–501. (22) Phan, A. N.; Ryu, C.; Sharifi, V. N.; Swithenbank, J. J. Anal. Appl. Pyrolysis 2008, 81 (1), 65–71. (23) Dominguez, A.; Menendez, J. A.; Inguanzo, M.; Pis, J. J. Bioresour. Technol. 2006, 97 (10), 1185–1193.

Table 4. Some Properties of Pyrolytic Oils Obtained at Different Pyrolysis Temperatures Oil-450 viscosity at 40 °C (cSt) 22.98 GCV (kJ/kg) 35129 elemental composition (wt %) C 71.30 H 9.52 N 8.61 S 1.03 O 9.55 H/C 1.60

Oil-600 nd 32794 67.52 9.01 9.03 1.01 13.43 1.60

rate of 5 °C min-1. This corresponded to the maximum release of C2H4 and C2H6. At a heating rate of 60 °C min-1, the release of these gases took place over a broader temperature interval (>600 °C) and the maximum heating value (20.0 MJ N m-3) was obtained at 600 °C. 3.2.3. Pyrolytic Oil. The chemical and physical properties of oils from pyrolysis of WS are shown in Table 4. Pyrolytic oils from pyrolysis at 450 and 600 °C were donated as Oil-450 and Oil-600, respectively. The C/O atomic ratio of oils decreased as the pyrolysis temperature increased from 450 to 600 °C, indicating that the number of oxygenated compounds has increased. A similar result was observed by Inguanzo et al.16 Because of the high carbon content, the calorific value of Oil-450 is higher than that of Oil-600. Heating values of both oils are comparable to some conventional fuels, such as wood and coal, and pyrolytic oils obtained from WS6,15,23 and even higher than those of pyrolytic oils from other solid wastes.22 The viscosity of oil lies in the viscosity range of fuel oil. Consequently, the low sulfur content and the high calorific value of pyrolysis oils reflect the potential of these oils for use as fuel besides being used as chemical feedstock. Knowledge of oil composition is necessary for their use as either a raw material or fuel. Pyrolytic oils are complex mixtures consisting of organic compounds from a wide variety of chemical groups. To characterize the pyrolytic oil, they were fractionated as asphaltenes, aliphatics, aromatics, and polars using the liquid adsorption chromatography. The compositions of pyrolytic oils are shown in Figure 2. Both Oil-450 and Oil-600 contain a high amount of asphaltenes and polar compounds. The possible reason for the excessive amount of polar compounds compared to others in pyrolytic oils is the existence of proteins and lipids in the sludge.24 On the other hand, Shen and Zhang reported that the sewage sludge had a higher proportion of oxygenated aromatics and these oxygenated aromatics form the oil during lowtemperature pyrolysis.25 Generally, pyrolysis of sludge yielded liquid products that contained significant amounts of oxygenated compounds and unacceptable amounts of nitrogen.6,15,16,25 The aromatic content of oils obtained in this study was 9-10 wt %. Usually low aromatic and polar content is preferred in

Figure 2. Composition of oils from sludge pyrolysis.

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Table 5. Ash Content of WS-Derived Pyrolytic Chars before and after Demineralization pyrolysis temperature (°C) demineralization ash content (wt %)

54.3

450

+ 12.1

60.7

600

+ 15.0

fuels. On the other hand, these contents have importance from an industrial point of view, if they are used as chemical feedstock. What is critical for the pyrolytic oil to be used as fuel is the aliphatic content. A high content of aliphatic compounds is ideal for oil, because they have a high heating value and lower viscosity. The aliphatic content was measured to be considerably low (8-10 wt %). Therefore, these pyrolytic oils with also a high asphalthene amount should be upgraded to be used as fuel and chemical feedstock. The little differences in oil compositions are supposed to depend upon the pyrolysis temperature. 3.3. Production of Activated Carbon from WS. The production of carbon adsorbents from WS has been investigated by several authors.26-28 Moreover, both chemical29,30 and physical31 activation of carbonaceous residues from the pyrolysis of sewage sludge have already been studied, showing that activation (especially chemical activation) of the solid residues improves the relatively poor textural properties of the pyrolysis residue. In this study, activated carbons were prepared from WS by both physical and chemical activation methods. In the physical activation method, the chars obtained from the pyrolysis at both 450 and 600 °C were treated with CO2 gas at 900 °C for 2 h. At the end of 2 h, excessive burning of the both chars occurred and only the inorganic content of chars remained in the reactor. This result was not surprising. Because char reactivity is related to the ash content of char (Table 5), high ash content had a catalytic effect on the activation of chars32 and led to the complete burning of chars. To obtain activated carbons from pyrolytic chars by the physical activation method, the chars were demineralized by treating with HCl solution. By demineralization, the ash content of chars decreased considerably. Demineralized chars were activated with CO2 at 900 °C for hours changing between 2 and 14. Figure 3 shows the influence of the activation time on the degree of burnoff in CO2 achieved for chars. The char obtained at 450 °C (C-450) was more reactive than the char obtained at 600 °C (C-600). This reason is due to the high pyrolysis temperature, which led to more complete carbonization (less volatile matter content, etc.).33 Similar behavior has been reported by other authors.12,34 The effect of the activation time on the BET surface area and pore volumes of C-450 and C-600 is given in Table 6. The BET surface area of activated carbons obtained from demineralized C-450 (surface area of 45 m2 g-1) showed a high increase (24) Boocock, D. G. B.; Konar, S. K.; Leung, A.; Ly, L. D. Fuel 1992, 71 (11), 1283–1289. (25) Shen, L.; Zhang, D. Fuel 2005, 84, 809–815. (26) Jeyaseelan, S.; Lu, G. Q. Water Sci. Technol. 1996, 34, 499–505. (27) Martin, M. J.; Balaguer, M. D.; Rigola, M. EnViron. Technol. 1996, 17, 667–672. (28) Chen, X.; Jeyaseelan, S.; Graham, N. Waste Manage. 2002, 22, 755–760. (29) Chiang, P. C.; You, J. H. Can. J. Chem. Eng. 1987, 65, 922–927. (30) Lu, G. Q.; Lau, D. D. Gas Sep. Purif. 1996, 10 (2), 103–111. (31) Lu, G. Q. EnViron. Prog. 1996, 15 (1), 12–18. (32) Iniesta, E.; Sanchez, F.; Garcia, A. N.; Marcilla, A. J. Anal. Appl. Pyrolysis 2001, 58-59, 983–994. (33) Lu, G. Q.; Low, J. C. F.; Liu, C. Y.; Lua, A. C. Fuel 1995, 74 (3), 344–348. (34) de la Puente, G.; Fuente, E.; Pis, J. J. J. Anal. Appl. Pyrolysis 2000, 53 (1), 81–93.

Figure 3. Carbon burnoff in CO2 for the demineralized WS-derived pyrolytic chars at different activation times.

up to 4 h and then a slightly high increase from 4 to 6 h. The micropore volume of activated carbons remained almost unchanged, while the mesopore volume increased with the increase of the activation time. In the case of demineralized C-600 (surface area of 56 m2 g-1), the BET surface area of activated carbons increased up to 10 h and then slightly decreased. On the other hand, the micropore volume decreased with increasing activation time, while the mesopore volume increased. It is noted that the activated carbons obtained from C-450 had a smaller BET surface area but higher micropore volume than that obtained from C-600 and the activated carbons obtained from C-600 had almost total mesoporous character. Generally, the activated carbons produced from sewage sludge by physical activation have a surface area smaller than 400 m2 g-1.19 Activated carbons produced from both C-450 and C-600 have a higher BET surface area than the activated carbons produced from WS by other researchers. Mendez et al. obtained the activated carbon having 102 m2 g-1 from sewage sludge by air activation.19 On the other hand, CO2 activation of sewage sludge produced the activated carbons having 26035 and 269 m2 g-1.36 In the chemical activation method, the WS was impregnated with ZnCl2 in impregnation ratios of 1:1 and 1.5:1 of the impregnation reagent weight/biomass weight (referred to as 100 and 150 wt % reagent concentration). The impregnated samples were pyrolyzed at 450 and 600 °C. The effect of the pyrolysis temperature and reagent concentration on the yield, ash content, and BET surface area of activated carbon is given in Table 7. It seems that, in the case of pyrolysis at 600 °C, impregnation with ZnCl2 increased the yield of activated carbon. However, this increase originated mainly from the ash content of activating carbons. While the ash contents of activated carbons obtained at 450 °C were around 11-13 wt %, these values were around 31-35 wt % for activated carbons obtained at 600 °C. This high ash content of 600 °C activated carbons could be due to the formation of water- and HCl-insoluble compounds, which originated from the combination of the inorganic constituent of the raw material with zinc chloride. On the other hand, the impregnation ratio appears to have no effect on the yield of activated carbon for both pyrolysis temperatures. The increase in the pyrolysis temperature resulted in the increase of the surface area of resulting activated carbons. In addition, the increase in the concentration of impregnation agent led to a decrease in the surface area in the case of pyrolysis at 450 °C, whereas it led to an increase in the surface area in the case of pyrolysis at 600 °C. Surface areas of activated carbons obtained in this study are higher than that of activated carbons (35) Rio, S.; Faur-Brasquet, C.; Le Coq, L.; Le Cloirec, P. Adsorption 2005, 11, 793–798. (36) Ros, A.; Lillo-Rodenas, M. A.; Fuente, E.; Montes-Moran, M. A.; Martin, M. J.; Linares-Solano, A. Chemosphere 2006, 65, 132–140.



Table 6. BET Surface Area, Pore Volumes, and Average Pore Size of Activated Carbons Produced from C-450 and C-600 C-450 activation time (h) 2

-1

BET surface area (m g ) micropore volume (cm3 g-1) mesopore volume (cm3 g-1) total pore volume (cm3 g-1) average pore size (Å)

C-600

2

4

6

4

6

8

10

14

711 0.212 0.470 0.679 39.1

957 0.208 0.626 0.869 36.2

974 0.213 0.634 0.882 36.5

874 nd nd nd nd

1096 0.113 1.067 1.227 44.8

1052 0.102 1.178 1.306 49.6

1213 0.088 1.163 1.312 43.3

1194 0.046 1.368 1.436 48.1

Table 7. Variances in the Yield and Some Properties of Activated Carbons with the Concentration of Impregnation Agent and Pyrolysis Temperature pyrolysis reagent yield ash BET surface temperature (°C) concentration (wt %) (wt %) (wt %) area (m2/g) 450 600

100 150 100 150

23.1 23.7 31.2 28.5

11.0 13.1 35.6 31.1

682 594 737 871

produced from WS in the literature. Chen et al.,28 Zhang et al.,37 and Zhai et al.38 obtained activated carbons with a surface area of 647, 555, and 550 m2 g-1, respectively, using ZnCl2. However, Ros et al. produced activated carbons having a surface area up to 1300 m2 g-1 by alkaline hydroxide activation.39 When the results from chemical activation are compared to that from physical activation, it can be concluded that chemical activation with ZnCl2 was not as effective as CO2 activation in the production of activated carbon with a high surface area. SEM micrographs of activated carbons produced from WS by physical and chemical activation are given in Figure 4. From a comparison of panels a and b of Figure 4, it can be said that activated carbon obtained by chemical activation has a cloudy surface that represents a thick wall structure and a less-developed porosity possibly because of the acid-insoluble inorganic compounds incorporated into the carbon matrix. On the other hand, for the activated carbons obtained by physical activation, some small cavities can be observed, implying the existence of some macroporosity. To evaluate adsorption behavior of the activated carbons obtained from WS chars by the physical activation method, the amount of surface oxygen groups on the activated carbons having acidic and basic properties has been determined by the Boehm titration method. In Table 8, total amounts of acidic surface oxygen groups (carboxylic, lactonic, and phenolic) and basic surface oxygen groups (chromene and pyrone) of actitaved carbons are given. Activated carbons obtained from the activation of C-450 for 6 h and activation of C-600 for 8 and 14 h were donated as C-450-6, C-600-8, and C-600-14, respectively. The results presented in Table 8 revealed that the predominant functional groups at the surface of the activated carbons produced from C-600 are basic, while those of activated carbons produced from C-450 are acidic. 3.4. Adsorption from Solution. Aqueous adsorption tests were conducted on selected activated carbons with the aim of assessing potential applications in the water-treatment industry. Methylene blue, phenol, and Cr(VI) were used as adsorbates in this study. Methylene blue and phenol cover a range of molecular sizes, which makes them useful for the investigation of adsorption in pores of different dimensions. Phenol is preferentially adsorbed in small- and medium-sized micropores, (37) Zhang, F.; Nriagu, J. O.; Itoh, H. Water Res. 2005, 39, 389–395. (38) Zhai, Y.; Wei, X.; Zeng, G.; Zhang, D.; Chu, K. Sep. Purif. Technol. 2004, 38, 191–196. (39) Ros, A.; Lillo-Rodenas, M. A.; Canals-Batlle, C.; Fuente, E.; Montes-Moran, M. A.; Martin, M. J.; Linares-Solano, A. EnViron. Sci. Technol. 2007, 41 (12), 4375–4381.

while methylene blue is mainly adsorbed in medium- and largesized micropores. On the other hand, the adsorption of Cr(VI) depends upon the surface chemistry as well as surface area and porosity. Cr(VI) is often found in industrial waste streams. It is a strong oxidant and known to be both acutely and chronically toxic to human beings and animals.40 The Freundlich and Langmuir adsorption isotherm models were used to describe adsorption equilibrium data and determine adsorption capacity for methylene blue, phenol, and Cr(VI) of activated carbons. Parameters of the Langmiur and Freundlich adsorption models of methylene blue, phenol, and Cr(VI) are given in Table 9. For methylene blue adsorption, the Langmuir isotherms generated a satisfactory fit to the experimental data (R2 > 0.99)

Figure 4. SEM micrographs of activated carbon produced by (a) physical activation of WS-derived demineralized 600 °C pyrolytic char for 10 h and (b) chemical activation with ZnCl2 (150 wt %) at 600 °C. Table 8. Total Amounts of Surface Acidic and Basic Sites of Activated Carbons activated carbon

total acidic sites (mmol of H+/g of AC)

total basic sites (mmol of OH-/g of AC)

C-450-6 C-600-8 C-600-14

1.80 0.75 1.00

0.65 1.20 1.35

3132


Kantarli and Yanik

Table 9. Parameters of the Langmiur and Freundlich Adsorption Models of Methylene Blue, Phenol, and Cr(VI) Langmuir model Freundlich model activated -1 -1 2 carbon SM (mg g ) KL (L mg ) R 1/n (mg g-1) Kf (L mg-1) R2 C-450-6 C-600-8

344.8 454.5

Methylene Blue 1.933 0.99 0.080 0.786 0.99 0.153

C-450-6 C-600-8 C-600-14

109.9 172.4 163.9

0.056 0.054 0.043

Phenol 0.97 0.99 0.96

C-450-6 C-600-8 C-600-14

108.7 109.9 112.4

0.034 0.259 0.144

Cr(VI) 0.96 0.99 0.99

263.1 272.6

0.96 0.99

0.306 0.413 0.383

22.8 23.6 23.1

0.98 0.99 0.99

0.384 0.188 0.224

14.3 46.3 39.0

0.99 0.93 0.99

for both of the activated carbons. This demonstrates monolayer coverage of methylene blue at the outer surface of the adsorbent. In comparison to the activated carbons derived from sludges in the literature, activated carbons obtained in this study showed considerably high adsorption capacity for methylene blue.10,41 For phenol adsorption, both equations were found to fit the data well. The differences observed on the adsorption capacities of these carbons could not be explained from their different BET surface areas. In addition, the micropore volume alone does not appear sufficient for the understanding of the trend observed on the adsorption capacities of the different carbons. In fact, the C-450-6 sample, which presents the highest micropore volume, yields a somewhat lower capacity than C-600-8 and C-600-14. Oda et al.42 determined the adsorption isotherms of phenol on six commercially available activated carbons and their heat-treated products containing varying amounts of surface acidic groups. They observed that the presence of acidic surface groups reduced the adsorption of phenol. Our results are consistent with the observation of Oda et al.42 As given in Table 9, C-450-6 had a higher amount of acidic surface groups than C-600-8 and C-600-14. Thus, the lower phenol adsorption capacity of C-450-6 can be related to its higher amount of acidic surface groups. In comparison to the activated carbons derived from sludge (by both physical and chemical activation) in the literature, activated carbons obtained from sludge in this study showed considerably high adsorption capacity for phenol.28,35,43,44 In addition to this, activated carbons obtained by physical activation from 600 °C pyrolytic char had higher adsorption capacity than some commercial activated carbons.28,44 Cr(VI) adsorption equilibrium experiments were carried out at pH 2 and for an equilibrium time of 2 h, which were determined as optimum according to the preliminary studies. pH dependence of metal adsorption can be largely related to the type and ionic state of the functional group present in the adsorbent and also to the metal chemistry in the solution.45 High adsorption of Cr(VI) at low pH can be explained by the species of the Cr and the adsorbent surface. At acidic pH, the predominant species of Cr are Cr2O72-, HCrO4-, Cr3O102-, and (40) Bayrak, Y.; Yesiloglu, Y.; Gecgel, U. Microporous Mesoporous Mater. 2006, 91 (1-3), 107–110. (41) Otero, M.; Rozada, F.; Calvo, L. F.; Garcia, A. I.; Moran, A. Biochem. Eng. J. 2003, 15, 59–68. (42) Oda, H.; Kishida, M.; Yokokawa, C. Carbon 1981, 19 (4), 243– 248. (43) Tay, J. H.; Chen, X. G.; Jeyaseelan, S.; Graham, N. Chemosphere 2001, 44 (1), 53–57. (44) Martin, M. J.; Artola, A.; Balaguer, M. D.; Rigolab, M. J. Chem. Technol. Biotechnol. 2002, 77, 825–833. (45) Gupta, V. K.; Shrivastava, A. K.; Jain, N. Water Resources 2001, 35 (17), 4079–4085.

Cr4O132-,46,47 and above pH 8, only CrO42- is stable. As the pH decreases into the region 3-6, Cr2O72- becomes the predominant species. At still lower pH values, Cr3O102- and Cr4O132- species are formed. Thus, decreasing pH results in the formation of more polymerized Cr oxide species. On the other hand, under acidic conditions, the surface of the adsorbent becomes highly protonated and favors the uptake of Cr(VI) in the anionic form.48 Langmuir Cr(VI) adsorption capacities of activated carbons derived from sludge are almost similar, although they have different surface area and micropore volume. The reason may be attributed to the simultaneous effect of surface groups, pore structure, and surface area of activated carbons. It must be noted that these activated carbons had higher adsorption capacities than most of the commercial and lignocellulosic material-derived activated carbons found in the literature.48-56 4. Conclusion In this study, conversion of WS from the tannery industry to useful products by pyrolysis and activation methods has been investigated. Pyrolysis studies revealed that the temperature was effective on the pyrolysis product distribution. The gaseous product yield increased from 17.5 to 21.9 wt %, while the liquid product and char yield slightly decreased (from 26.8 to 24.3 wt % and 55.7 to 53.8 wt %, respectively) with the increase in the pyrolysis temperature from 450 to 600 °C. The result of pyrolysis gas analysis showed that this gas has a considerable gross calorific value. Therefore, it was suggested that pyrolytic gas from WS can provide some part of the energy requirements of the pyrolysis process. Although, the oils obtained from pyrolysis of WS had reasonable calorific value, they contained a high amount of nitrogenated and oxygenated compounds. For this, the oil obtained was suggested to be used as fuel or chemical feedstock after retreatment, such as steam cracking, hydrogenation, gasification, Fisher-Tropsch synthesis, etc. Physical activation studies using CO2 as a activating agent showed that pyrolytic chars because of their high ash content burnt excessively and did not yield active carbon, unless they were demineralized. Activated carbons obtained by CO2 activation of demineralized 450 °C pyrolytic char had a smaller BET surface area (up to 974 m2/g) but higher micropore volume than those obtained from 600 °C pyrolytic char (up to 1213 m2/g). On the other hand, chemical activation of WS with ZnCl2 (46) Weckhuysen, B. M.; Wachs, I. E.; Schoonheydt, R. A. Chem. ReV. 1996, 96, 3327–3349. (47) Raji, C.; Anirudhan, T. S. Water Resour. 1998, 32 (12), 3772– 3780. (48) Mor, S.; Ravindra, K.; Bishnoi, N. R. Bioresour. Technol. 2007, 98, 954–957. (49) Aggarwal, D.; Goyal, M.; Bansal, R. C. Carbon 1999, 37, 1989– 1997. (50) Alvarez, P.; Blanco, C.; Granda, M. J. Hazard. Mater. 2007, 144, 400–405. (51) Liu, S.; Chen, X. X.; Chen, X. Y.; Liu, Z. F.; Wang, H. L. J. Hazard. Mater. 2007, 141, 315–319. (52) Ranganathan, K. Bioresour. Technol. 2000, 73, 99–103. (53) Gonzalez-Serrano, E.; Cordero, T.; Rodriguez-Mirasol, J.; Cotoruelo, L.; Rodriguez, J. J. Water Res. 2004, 38, 3043–3050. (54) Kobya, M.; Demirbas, E.; Senturk, E.; Ince, M. Bioresour. Technol. 2005, 96, 1518–1521. (55) Mohanty, K.; Jha, M.; Meikap, B. C.; Biswas, M. N. Chem. Eng. Sci. 2005, 60 (11), 3049–3059. (56) Guo, Y.; Qi, J.; Yang, S.; Yu, K.; Wang, Z.; Xu, H. Mater. Chem. Phys. 2002, 78, 132–137.


yielded activated carbons with a smaller surface area (up to 871 m2/g) compared to the CO2 activation of demineralized pyrolytic chars. For the activated carbons derived in this study, the methylene blue, phenol, and Cr(VI) adsorption capacities were investigated and found to be higher than those of most of the commercial and lignocellulosic material-derived activated carbons found in the literature. The present study shows that WS from the tannery industry can be effectively used as a raw material for the preparation of activated carbon by both physical and chemical activations. In


addition, it was proposed that produced activated carbons show promise for the removal of dye, CrVI, and phenol from aqueous streams. Consequently, the production of activated carbon fulfilling the need for volume reduction and safe disposal alternatives for WS seems to be a promising solution for WS disposal from an environmental and economical viewpoint. ¨ BI˙TAKAcknowledgment. The financial support from the TU C¸AYDAG group under contract 105Y111 and Ege University under contract 2005 Fen 025 is highly appreciated. EF8011068

Use of Waste Sludge from the Tannery Industry - American Chemical

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