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water interface
Energy Conversion and Management 46 (2005) 33–46 www.elsevier.com/locate/enconman
Investigation of applicability of the various adsorption models of methylene blue adsorption onto lignite/water interface S. Karaca a b
a,*
, A. G€ urses b, R. Bayrak
b
Department of Chemistry, Atat€urk University, Fen-Edebiyat Fak€ ultesi, 25240 Erzurum, Turkey Department of Chemistry, Atat€urk University, K.K. E gitim Fak€ ultesi, 25240 Erzurum, Turkey Received 5 December 2003; accepted 4 February 2004 Available online 18 March 2004
Abstract Methylene blue adsorption isotherms for raw and pyrolysed coal samples were determined at 20 °C. The raw coal sample showed the highest adsorption capacity. It was observed that adsorption capacity generally decreased with increasing pyrolysis temperature. The sample pyrolysed at 700 °C exhibited the lowest adsorption capacity. The experimental data obtained were applied to the Freundlich, Langmuir, BET, Halsey, Harkins-Jura, Smith and Henderson isotherm equations to test the fitness of these equations to raw and pyrolysed coal samples. By considering the experimental results and adsorption models applied in this study, it can be concluded that adsorption of methylene blue occurs through physical interactions, and the lignite sample has a mesoporous structure. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Adsorption isotherms; Lignite; Pyrolysis; Functional groups; Cationic dye
1. Introduction Industrial growth and associated sophistication have resulted in many environmental problems. Effluents discharged from various industries pose disposal problems. It is essential to dispose of them safely. Among the various treatment processes, adsorption has been found to be an efficient and economic process to remove dyes, pigments and other colourants and also to control biochemical oxygen demand [1,2]. Activated carbon, inorganic oxides, natural adsorbents (such as *
Corresponding author. Tel.: +90-442-231-4435; fax: +90-442-236-0948. E-mail address: [email protected] (S. Karaca).
0196-8904/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2004.02.008
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clays and clay minerals, cellulosic materials, chitin and chitosan) have been extensively used as adsorbents [3–7]. Porous carbons, which have a collective name of activated carbon, have been widely used as adsorbents in the separation and purification of gas or liquid streams. Activated carbon adsorption systems are playing an important role in the cleaning and treatment of industrial effluents and municipal wastewater. Despite its profilic use in water and waste water industries, commercial activated carbon remains as an expensive material. This has led to a search for low cost material as alternative adsorbent materials [4,5]. Because of its availability and cheapness, coal is the most commonly used precursor for activated carbon production [8–11]. In industrial water pollution, the colour produced by minute amounts of organic dyes in water is considered very important because, besides having possible harmful effects, the colour in water is aesthetically unpleasant. Although not strongly hazardous, methylene blue can have various harmful effects. On inhalation, it can give rise to short periods of rapid or difficult breathing, while ingestion through the mouth produces a burning sensation and may cause nausea, vomiting, diarrhea and gastritis. Accidental large doses create abdominal and chest pain, severe headache, profuse sweating, mental confusion, painful micturition, and methemoglobinemia [12,13]. The most important thing to design and run an industrial adsorption plant is the knowledge of adsorption kinetics and isotherms. Adsorption isotherms have been studied by many researchers [12–16]. Most practical applications of adsorbents require sorbents having a large volume of very fine pores. The presence of micropores substantially influences its sorption properties because the amount adsorbed on the macropore surface is negligible in comparison to that for the micropores [17]. Therefore, characterization of the activated carbons has become one of the most important problems in adsorption technology. For this, the lignite sample was pyrolysed under a CO2 atmosphere at different temperatures, and the changes in surface properties of these samples were determined. Determinations of pore structure are made by fitting either the Langmuir or BET equation [18] to the isothermal equilibrium data obtained. However, these values are not a true indication of the adsorption capacity of a charcoal during liquid phase adsorption studies [19]. The literatures [12,13,20] indicate that the adsorption of methylene blue from the aqueous phase is a useful tool for product control in the manufacture of activated carbon. In this study, aqueous solutions of a basic dye, methylene blue, were used as a model compound in an attempt to use raw and pyrolysed Turkish lignite under a CO2 atmosphere at different temperatures as an adsorbent. The present work investigates the adsorption capacities, pore and surface structures of raw and pyrolysed coal samples. It involves the adsorption from an aqueous methylene blue solution. The purpose of our study was to determine the methylene blue adsorption isotherms of coal samples at 20 °C and also to investigate the applicability of six known isotherm equations, to accommodate sorption data obtained at 20 °C. Information was also obtained about the pore and surface structure of the adsorbent by using the adsorption models. 2. Experimental 2.1. Material The coal sample used in this study was obtained from the Balkaya coal mines in Turkey. The sample was air dried, ground and then sieved to give )88 + 150 lm size fraction using ASTM
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(American Society for Testing and Materials) standard sieves. The proximate and ultimate analyses were performed using Turkish and ASTM standards. The average results of duplicate analyses performed according to ASTM procedures for the coal sample are given in Table 1. 2.2. Apparatus Pyrolysis experiments were conducted in a fixed bed reactor as shown schematically in Fig. 1. The system consists of a porcelain furnace placed in a packed column, sample holder, temperature control unit and some necessary facilities (Electrical power unit, transformer, moisture holder, flowmeter). The sample holder, made of stainless steel with 3.0 cm inner diameter and 14.0 cm length having a sample room, was placed in the middle of the porcelain furnace, which was heated by electrical power. The temperature was measured and controlled in the middle of the porcelain furnace and in the coal sample by two Ni–Cr thermocouples. The furnace is safely held at the desired temperature by this controlling system.
Table 1 Proximate and ultimate analysis contents of sulfur forms and heating values of Balkaya lignite Proximate analysis (wt.%)a
Ultimate analysis (daf)b
Forms of sulfur (wt.%)a
Ash Volatile matter Fixed carbon Moisture
32.5 38.9 21.0 7.6
Carbon Hydrogen Sulfur Nitrogen Oxygen (diff.)
Pyritic Sulfate Organic Total
Heating value (cal/g)
4040
a b
65.5 4.1 4.7 1.9 23.8
2.2 1.3 1.2 4.7
As received. Dry, ash-free basis.
Fig. 1. Schematic diagram of the apparatus used in pyrolysis experiments: PF––programmable furnace, K––controlling system, O––recorder, SH––sample holder, T––thermocouple, F––flowmeter, NK––driver, R––pressure regulator, V–– valve, V2––transformer.
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2.3. Methods Approximately 15.0 g of the coal sample were placed in the sample room of the sample holder. Firstly, with the aim of preheating, the sample holder content was held at a region of the reactor, determined with experiments, where there is no desulfurization. Then, it was inserted into the furnace as soon as the desired temperature had been reached. To provide thermal equilibrium between the gas and coal sample, the gas is preheated to the bed temperature, and it was introduced into the coal sample from the underside of the sample holder by passing it through the moisture holder and the flowmeter. In all experiments, the gas flow rate was held constant. The experiments were conducted at determined temperatures (350, 450, 500, 600 and 700 °C) and at a definite gas flow rate (1700 ml min1 ). At the end of treatment, the sample holder content is pulled towards the coldest end of the reactor, and it was allowed to cool. Then, the sample holder content was removed from the reactor and was weighed and analysed. In order to determine the change of the adsorptive capacities of the pyrolysed coal samples, adsorption experiments were conducted by using a colouring matter (methylene blue). The adsorption experiments were made in 100 ml, glass stoppered, round bottom flasks immersed in a thermostated shaker bath. For this aim, 0.1 g of the coal was mixed with 100 ml of an aqueous solution of known concentration of methylene blue and shaken vigorously for 2 min by hand to wet the coal. Then, the flask with its contents was shaken by the shaker for 5 min at 20 °C. It was determined that the adsorption time of 5 min is enough to reach adsorption equilibrium. At the end of the adsorption period of 5 min, the supernatant was centrifuged for 2 min at 3750 min1 . The concentration of methylene blue in the supernatant solution after and before adsorption was determined using a double beam UV spectrophotometer at 666 nm. It was found that the supernatant from the coals did not exhibit any absorbance at this wavelength and also that the calibration curve was very reproducible and linear over the concentration range used in this work. The amount of methylene blue adsorbed was calculated from the concentrations in the solution before and after adsorption. Blanks containing no methylene blue were used for each series of experiments.
Table 2 Oxygen functional groups, specific surface areas, total pore surface areas, average pore diameters and weight losses for the coal samples used in the experiments Carboxylic groupsa
Phenolic groupsa
SBET b
Pyrolysis temperature (°C)
meq g
wt.%O
meq g
wt.%O
m g
Raw 350 450 500 600 700
0.29 0.04 – – – –
0.93 0.13 – – – –
2.85 2.17 1.99 1.17 2.43 2.30
4.56 3.47 3.18 1.87 3.89 3.68
7.60 2.82 1.05 0.26 0.14 0.00
a
1
Dry, ash-free basis. BET, N2 . c Measured by Hg porosimeter. b
1
2
1
rc
Weight losses
m g
nm
wt.%
10.33 9.68 7.16 12.34 14.43 0.23
7.6 7.4 7.2 8.9 8.5 3.6 Æ 103
3.40 10.30 12.20 22.31 31.40
Stotal c 2
1
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To investigate the effect of surface functional groups on the adsorption capacity of raw and pyrolysed coal samples, the two principal oxygen containing functional groups, carboxylic and phenolic, were determined in all coal samples used in this study. Total acidity (carboxylic plus phenolic) was determined (by reaction with barium hydroxide) via the method used by Miller et al. [21]. Carboxylic groups have been determined (by reaction with calcium acetate) according to the method of Blom et al. [22]. The phenolic group concentration was then calculated by subtracting the value for the carboxylic content from the total acidity. The BET surface area of the raw and pyrolysed coal samples was measured from N2 isotherms at 77 K with a Sorptiometer (Micromeritics Flow Sorb II-2300). The total pore surface area and average pore diameters of the same coal samples were determined with a mercury porosimeter (Autopore II 9220, maximum PHg ¼ 2721:8 atm). The obtained results are displayed in Table 2.
3. Results and discussion The isotherms belonging to the methylene blue adsorption on the raw and pyrolysed coal samples under a CO2 atmosphere at different temperatures are given in Fig. 2. As seen from the figure, the raw coal sample showed the highest adsorption capacity, which was reduced with increasing pyrolysis temperature after pyrolysis. The adsorbed amounts of methylene blue on the pyrolysed coal sample under CO2 atmosphere at 450 °C are higher in comparison with those on the pyrolysed coal sample at 350 °C in low equilibrium concentrations. The adsorption efficiency is clearly reduced in pyrolysed coal samples under CO2 atmosphere 500, 600 and 700 °C. The 50
Amount adsorbed (mg/g)
40
30
Temperature (C) raw
20
700 600 500
10
450 350
0 0.00
.02
.04
.06
.08
.10
Ceq (g/L)
Fig. 2. The adsorption isotherms at adsorption temperature of 20 °C for the raw and pyrolysed coal samples under CO2 atmosphere at different temperatures (pyrolysis time: 10 min).
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observed decrease in adsorption capacities with increasing pyrolysis temperature can be attributed to the changes occurring in the surface morphology and pore size distribution of the lignite samples during pyrolysis [10]. On the other hand, the observed decrease in efficiency of the pyrolysed coal samples at temperatures beyond the temperature 450 °C shows that the number of available sites for methylene blue adsorption decreased. As can be seen from Table 2, in the range of 500–700 °C temperature, the functional group contents of the samples (especially carboxylic oxygen) decreased, and the specific surface area of the same coal samples (BET, N2 ) rapidly decreased. Average pore diameters measured with the mercury porosimeter (see Table 2) show that the raw and pyrolysed lignite samples used in this study have a mesoporous structure in which the pores of small diameter are dominant. The pores of adsorbents are generally classified into three groups, micropore (pore size <2 nm), mesopore (2–50 nm) and macropore (>50 nm). The porosity of the coal sample pyrolysed at 700 °C almost vanished, and the total specific pore surface area of the same coal sample fell to the value of 0.23 m2 g1 . Thus, it can be said that methylene blue adsorption occurs through physical interactions, such as ion–dipole and/or dipole–dipole interactions. Here, it can be suggested that carboxylic groups have the most effect on the methylene blue adsorption. In order to determine the mechanism of methylene blue adsorption on the raw and pyrolysed coal samples and evaluate the relationship between adsorption and the pyrolysis temperature, the experimental data were applied to the Freundlich, Langmuir, BET, Halsey, Harkins-Jura, Smith and Henderson isotherm equations. The constant parameters of the equations for this system were calculated by regression using the linear form of the isotherm equations and SPSS 10.0 Software. The results are given in Table 3, together with the isotherm equations. Fig. 3 shows the Freundlich isotherms of raw and pyrolysed coal samples under a CO2 atmosphere at different temperatures. As seen from this figure, the obtained adsorption data for raw and pyrolysed coal samples at 350, 500 and 600 °C fit well with the Freundlich isotherm. It is seen from Table 3 that the highest value of k (a measure of adsorbent capacity) for the raw coal sample and the highest value of n (a measure of the intensity of adsorption) for the pyrolysed coal sample at 500 °C are obtained among the fitted samples to the Freundlich isotherm quite well. Although the increase of pyrolysis temperature generally caused a decrease in methylene blue adsorption, it is clear that available sites for methylene blue adsorption, from the point of view of morphological and steric factors, appear. As seen in Table 2, the carboxyl contents of the coal samples are decreased, while their average pore diameter and total specific pore surface area are increased during pyrolysis at temperatures after 450 and 500 °C. Fig. 4 shows the Langmuir isotherms of raw and pyrolysed coal samples under a CO2 atmosphere at different temperatures. The Langmuir equation and Langmuir constants belonging to the samples are given in Table 3. As seen from Fig. 4, all of the obtained isotherms fit the Langmuir model quite well. When the monolayer capacities ðYm Þ in Table 3 of raw and pyrolysed coal samples at various temperatures are compared, it is seen that the highest Ym values are obtained for the raw coal sample and the pyrolysed coal sample at 500 °C among the pyrolysed coal samples. This situation clearly shows that methylene blue adsorption depends on both morphological and pore structures and functional group contents of the samples. As a result, it may be said that the pyrolysis at 500 °C makes the surface of coal more available for methylene blue adsorption in terms of surface morphology and functional group distribution. The adsorption of methylene blue is importantly affected by structural transformations depending on the secondary
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Table 3 Applicability of isotherm equations to methylene blue adsorption data at 20 °C of raw and pyrolysed coal samples under CO2 atmosphere at different temperatures and their constant parameters Isotherm equations Freundlich ln y ¼ ln k þ n ln C
Langmuir C=y ¼ 1=kym þ ð1=ym ÞC
BET C=yð1 CÞ ¼ 1=ðym kÞ þ ½ðk 1Þ=ðym kÞC
Halsey ln y ¼ ½ð1=nÞ ln k ð1=nÞ ln½lnð1=CÞ
Harkins-Jura 1=y 2 ¼ ðB=AÞ ð1=AÞ log C
Smith y ¼ Wb W lnð1 CÞ
Pyrolysis temperature (°C)
Constant parameter
Raw 350 450 500 600 700
n ¼ 0:2 n ¼ 0:5 n ¼ 0:4 n ¼ 0:7 n ¼ 0:7 n ¼ 0:3
Raw 350 450 500 600 700
ym ym ym ym ym ym
Raw 350 450 500 600 700
ym ym ym ym ym ym
Raw 350 450 500 600 700
Regression coefficients k k k k k k
¼ 76:8 ¼ 0:3 ¼ 44:6 ¼ 77:0 ¼ 64:4 ¼ 5:8
0.9970 0.9935 0.9497 0.9996 0.9988 0.8831
¼ 0:045 ¼ 0:027 ¼ 0:021 ¼ 0:030 ¼ 0:026 ¼ 0:003
K K K K K K
¼ 76:30 ¼ 26:88 ¼ 60:12 ¼ 9:28 ¼ 8:55 ¼ 65:95
0.9902 0.9946 0.9896 0.9817 0.9726 0.9793
¼ 41:83 ¼ 22:57 ¼ 18:52 ¼ 22:58 ¼ 19:34 ¼ 2:89
k k k k k k
¼ 227:61 ¼ 33:96 ¼ 77:85 ¼ 12:53 ¼ 11:91 ¼ 98:10
0.9936 0.9974 0.9879 0.9907 0.9839 0.9767
n ¼ 1:10 n ¼ 0:62 n ¼ 0:87 n ¼ 0:45 n ¼ 0:45 n ¼ 1:19
k k k k k k
¼ 181:32 ¼ 15:76 ¼ 32:13 ¼ 8:09 ¼ 7:41 ¼ 8:77
0.9977 0.9793 0.9175 0.9937 0.9944 0.8474
Raw 350 450 500 600 700
A ¼ 858:86 A ¼ 61:38 A ¼ 101:70 A ¼ 16:90 A ¼ 11:96 A ¼ 4:28
B ¼ 0:817 B ¼ 0:978 B ¼ 0:835 B ¼ 1:032 B ¼ 1:021 B ¼ 0:571
0.9880 0.9325 0.9238 0.9319 0.9424 0.8976
Raw 350 450 500 600 700
Wb Wb Wb Wb Wb Wb
¼ 6:52 ¼ 6:28 ¼ 4:68 ¼ 5:91 ¼ 5:04 ¼ 0:68
0.9397 0.9642 0.8245 0.9939 0.9920 0.7356
¼ 59:60 ¼ 34:11 ¼ 29:65 ¼ 27:53 ¼ 23:28 ¼ 4:58
W W W W W W
(coninued on next page)
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Table 3 (continued) Isotherm equations Henderson ln½ lnð1 CÞ ¼ ln k þ n ln y
Pyrolysis temperature (°C)
Constant parameter
Raw 350 450 500 600 700
n ¼ 4:71 n ¼ 2:03 n ¼ 2:73 n ¼ 1:45 n ¼ 1:43 n ¼ 3:18
Regression coefficients k k k k k k
¼ 1:35 109 ¼ 2:12 104 ¼ 2:83 105 ¼ 2:04 103 ¼ 2:76 103 ¼ 2:85 103
0.9973 0.9929 0.9478 0.9995 0.9988 0.8804
y, adsorption capacity of methylene blue (mg/g); ym , monolayer adsorption capacity; C, equilibrium concentration; n, k, K, A, B, Wa and W are constant parameters for the isotherm equations. 4.0
Temperature (C) 3.5
raw Rsq = 0.9970
3.0
700 Rsq = 0.8831
2.5
lny
600 Rsq = 0.9988
2.0 500 Rsq = 0.9996 1.5 450 Rsq = 0.9497
1.0
350 Rsq = 0.9935
.5 -6.0
-5.5
-5.0
-4.5
-4.0
-3.5
-3.0
-2.5
-2.0
lnC
Fig. 3. Freundlich isotherms for the adsorption of methylene blue at 20 °C on raw and pyrolysed coal samples at different temperatures (pyrolysis time: 10 min).
reactions of sulfur containing gases and conversion reactions of pyrite occurring between 450 and 500 °C [10,23]. Figs. 5–9 show the isotherms belonging to BET, Halsey, Harkins-Jura, Smith and Henderson adsorption models of raw and pyrolysed coal samples at different temperatures, respectively. All these equations are suitable for multilayer adsorption. Especially, the fitting of these equations can be seen in heteroporous solids [24,25]. As seen from these figures and Table 3, the raw coal sample and all of the pyrolysed coal samples at different temperatures fit into the BET equation. However, the raw and pyrolysed coal sample at 500 °C fit quite well into the Halsey equation. From Table 3, it was also seen that the experimental data obtained for the Harkins-Jura model show the best fitting for raw coal and the poorest fitting for the pyrolysed coal sample at 700 °C.
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41
40
Temperature (C) raw Rsq = 0.9902 30 700
C/Y
Rsq = 0.9793 600 20
Rsq = 0.9726 500 Rsq = 0.9817
10
450 Rsq = 0.9896 350 Rsq = 0.9946
0 0.00
.02
.04
.06
.08
.10
Ceq (g/L)
Fig. 4. Langmuir isotherms for the adsorption of methylene blue at 20 °C on raw and pyrolysed coal samples at different temperatures (pyrolysis time: 10 min).
.04
Temperature (C) raw Rsq = 0.9936 .03 700
C/[Y(1-C)]
Rsq = 0.9767 600 .02
Rsq = 0.9839 500 Rsq = 0.9907
.01
450 Rsq = 0.9879 350 Rsq = 0.9974
0.00 0.00
.02
.04
.06
.08
.10
Ceq (g/L)
Fig. 5. BET isotherms for the adsorption of methylene blue at 20 °C on raw and pyrolysed coal samples at different temperatures (pyrolysis time: 10 min).
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Fig. 6. Halsey isotherms for the adsorption of methylene blue at 20 °C on raw and pyrolysed coal samples at different temperatures (pyrolysis time: 10 min).
Fig. 7. Harkins-Jura isotherms for the adsorption of methylene blue at 20 °C on raw and pyrolysed coal samples at different temperatures (pyrolysis time: 10 min).
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Fig. 8. Smith isotherms for the adsorption of methylene blue at 20 °C on raw and pyrolysed coal samples at different temperatures (pyrolysis time: 10 min).
-2
Temperature (C) raw Rsq = 0.9973 -3 700
ln[-ln(1-C)]
Rsq = 0.8804 600 -4
Rsq = 0.9988 500 Rsq = 0.9995
-5
450 Rsq = 0.9478 350 Rsq = 0.9929
-6 .5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
lny
Fig. 9. Henderson isotherms for the adsorption of methylene blue at 20 °C on raw and pyrolysed coal samples at different temperatures (pyrolysis time: 10 min).
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As seen from Figs. 7 and 8 and Table 3, the best fitting for the Smith and Henderson equations appeared in the pyrolysed coal sample at 500 °C, whereas the poorest fitting occurred in the pyrolysed coal sample at 700 °C. Figs. 5–7 show that the adsorption of methylene blue on the raw coal was fitted well with the BET, Halsey and Harkins-Jura models. This situation can be explained that the raw coal sample has a mesoporous structure. As seen in Table 2, the average pore diameter of the raw coal sample is 7.6 nm, which is importantly smaller than 50 nm that is the limit value of macroporosity, implying that the lignite sample is a mesoporous material exhibiting microporous properties. Especially, both the significant decrease in the adsorption capacity and the loss of fitting for the BET, Halsey and Harkins-Jura models as a result of pyrolysis at higher temperatures (700 °C) support this thinking. It clearly shows that the methylene blue adsorption capacity of the lignite sample is decreased because of the change of average pore diameter and specific surface area of the coal samples depending on the pyrolysis temperature. Although the methylene blue adsorption in the coal surface after pyrolysis under a CO2 atmosphere at 500 °C for the three models is reduced in comparison with that of the raw coal sample, the fitting to these defined models continues with a high correlation. The average pore diameter and specific surface area of the pyrolysed coal sample at this temperature were 8.9 nm and 12.34 m2 g1 , respectively. Carboxylic groups are completely removed from the coal, while the phenolic groups are decreased to the value of 1.17 meq g1 during pyrolysis at this temperature. As a result of this, the number of available sites for methylene blue adsorption increase because of the formation of relatively open pores at this temperature. As seen from Figs. 8 and 9 and Table 3, the adsorption of methylene blue in raw and pyrolysed coal samples at different temperatures fits well for the Henderson and Smith adsorption models, but this fitting is importantly reduced in the pyrolysed coal sample at 700 °C in both models. These results show that the pyrolysis at 700 °C leads to a loss of porosity as a result of sintering. The good fitting of the obtained results for the raw and pyrolysed coal samples at 350, 500 and 600 °C and partially at 450 °C with the Henderson equation show that the microporous texture is dominant in the lignite sample. Also, Table 2 shows that the weight losses gradually increase with increasing pyrolysis temperature, implying that the mineral matters of coal change and part of their CaCO3 converts to oxides.
4. Conclusion In this study, methylene blue adsorption isotherms for raw and pyrolysed coal samples were determined at 20 °C. The two oxygen functional groups, carboxylic and phenolic, were determined for all coal samples and their specific surface area (BET–N2 ) and total pore surface area were measured. The raw coal sample showed the highest adsorption capacity,which was generally decreased with increasing pyrolysis temperature. The sample pyrolysed at 700 °C exhibited the lowest adsorption capacity. In the range of 500–700 °C temperature range, the functional group contents of the samples (especially carboxylic oxygen) decreased, and the specific surface area of the same coal samples (BET, N2 ) decreased greatly. The porosity of the coal sample pyrolysed at 700 °C almost vanished, and the total specific pore surface area of the same coal sample fell to the value of 0.23 m2 g1 . Thus, it can be said that methylene blue adsorption occurs through physical,
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such as ion–dipole and/or dipole–dipole interactions. Carboxylic groups have a large effect on the methylene blue adsorption. The experimental data obtained were applied to the Freundlich, Langmuir, BET, Halsey, Harkins-Jura, Smith and Henderson isotherm equations to test the fitness of these equations to raw and pyrolysed coal samples. By considering the experimental results and adsorption models applied in this study, it can be concluded that adsorption of methylene blue occurs through physical interactions, and the lignite sample has a mesoporous structure.
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[23] Chen H, Li B, Zhang B. Decomposition of pyrite and the interaction of pyrite with coal organic matrix in pyrolysis and hydropyrolysis. Fuel 2000;79:1627–31. [24] Ertugay MF, Certel M, G€ urses A. Moisture adsorption isotherms of Tarhana at 25 °C and 35 °C and the investigation of fitness of various isotherm equations to moisture sorption data of Tarhana. J Sci Food Agric 2000;80:2001–4. [25] Rosen MJ. Surfactants and interfacial phenomena. New York: John Wiley; 1978. p. 32–76.
Investigation of applicability of the various adsorption models of methylene blue adsorption onto lignite/water interface S. Karaca a b
a,*
, A. G€ urses b, R. Bayrak
b
Department of Chemistry, Atat€urk University, Fen-Edebiyat Fak€ ultesi, 25240 Erzurum, Turkey Department of Chemistry, Atat€urk University, K.K. E gitim Fak€ ultesi, 25240 Erzurum, Turkey Received 5 December 2003; accepted 4 February 2004 Available online 18 March 2004
Abstract Methylene blue adsorption isotherms for raw and pyrolysed coal samples were determined at 20 °C. The raw coal sample showed the highest adsorption capacity. It was observed that adsorption capacity generally decreased with increasing pyrolysis temperature. The sample pyrolysed at 700 °C exhibited the lowest adsorption capacity. The experimental data obtained were applied to the Freundlich, Langmuir, BET, Halsey, Harkins-Jura, Smith and Henderson isotherm equations to test the fitness of these equations to raw and pyrolysed coal samples. By considering the experimental results and adsorption models applied in this study, it can be concluded that adsorption of methylene blue occurs through physical interactions, and the lignite sample has a mesoporous structure. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Adsorption isotherms; Lignite; Pyrolysis; Functional groups; Cationic dye
1. Introduction Industrial growth and associated sophistication have resulted in many environmental problems. Effluents discharged from various industries pose disposal problems. It is essential to dispose of them safely. Among the various treatment processes, adsorption has been found to be an efficient and economic process to remove dyes, pigments and other colourants and also to control biochemical oxygen demand [1,2]. Activated carbon, inorganic oxides, natural adsorbents (such as *
Corresponding author. Tel.: +90-442-231-4435; fax: +90-442-236-0948. E-mail address: [email protected] (S. Karaca).
0196-8904/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2004.02.008
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S. Karaca et al. / Energy Conversion and Management 46 (2005) 33–46
clays and clay minerals, cellulosic materials, chitin and chitosan) have been extensively used as adsorbents [3–7]. Porous carbons, which have a collective name of activated carbon, have been widely used as adsorbents in the separation and purification of gas or liquid streams. Activated carbon adsorption systems are playing an important role in the cleaning and treatment of industrial effluents and municipal wastewater. Despite its profilic use in water and waste water industries, commercial activated carbon remains as an expensive material. This has led to a search for low cost material as alternative adsorbent materials [4,5]. Because of its availability and cheapness, coal is the most commonly used precursor for activated carbon production [8–11]. In industrial water pollution, the colour produced by minute amounts of organic dyes in water is considered very important because, besides having possible harmful effects, the colour in water is aesthetically unpleasant. Although not strongly hazardous, methylene blue can have various harmful effects. On inhalation, it can give rise to short periods of rapid or difficult breathing, while ingestion through the mouth produces a burning sensation and may cause nausea, vomiting, diarrhea and gastritis. Accidental large doses create abdominal and chest pain, severe headache, profuse sweating, mental confusion, painful micturition, and methemoglobinemia [12,13]. The most important thing to design and run an industrial adsorption plant is the knowledge of adsorption kinetics and isotherms. Adsorption isotherms have been studied by many researchers [12–16]. Most practical applications of adsorbents require sorbents having a large volume of very fine pores. The presence of micropores substantially influences its sorption properties because the amount adsorbed on the macropore surface is negligible in comparison to that for the micropores [17]. Therefore, characterization of the activated carbons has become one of the most important problems in adsorption technology. For this, the lignite sample was pyrolysed under a CO2 atmosphere at different temperatures, and the changes in surface properties of these samples were determined. Determinations of pore structure are made by fitting either the Langmuir or BET equation [18] to the isothermal equilibrium data obtained. However, these values are not a true indication of the adsorption capacity of a charcoal during liquid phase adsorption studies [19]. The literatures [12,13,20] indicate that the adsorption of methylene blue from the aqueous phase is a useful tool for product control in the manufacture of activated carbon. In this study, aqueous solutions of a basic dye, methylene blue, were used as a model compound in an attempt to use raw and pyrolysed Turkish lignite under a CO2 atmosphere at different temperatures as an adsorbent. The present work investigates the adsorption capacities, pore and surface structures of raw and pyrolysed coal samples. It involves the adsorption from an aqueous methylene blue solution. The purpose of our study was to determine the methylene blue adsorption isotherms of coal samples at 20 °C and also to investigate the applicability of six known isotherm equations, to accommodate sorption data obtained at 20 °C. Information was also obtained about the pore and surface structure of the adsorbent by using the adsorption models. 2. Experimental 2.1. Material The coal sample used in this study was obtained from the Balkaya coal mines in Turkey. The sample was air dried, ground and then sieved to give )88 + 150 lm size fraction using ASTM
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35
(American Society for Testing and Materials) standard sieves. The proximate and ultimate analyses were performed using Turkish and ASTM standards. The average results of duplicate analyses performed according to ASTM procedures for the coal sample are given in Table 1. 2.2. Apparatus Pyrolysis experiments were conducted in a fixed bed reactor as shown schematically in Fig. 1. The system consists of a porcelain furnace placed in a packed column, sample holder, temperature control unit and some necessary facilities (Electrical power unit, transformer, moisture holder, flowmeter). The sample holder, made of stainless steel with 3.0 cm inner diameter and 14.0 cm length having a sample room, was placed in the middle of the porcelain furnace, which was heated by electrical power. The temperature was measured and controlled in the middle of the porcelain furnace and in the coal sample by two Ni–Cr thermocouples. The furnace is safely held at the desired temperature by this controlling system.
Table 1 Proximate and ultimate analysis contents of sulfur forms and heating values of Balkaya lignite Proximate analysis (wt.%)a
Ultimate analysis (daf)b
Forms of sulfur (wt.%)a
Ash Volatile matter Fixed carbon Moisture
32.5 38.9 21.0 7.6
Carbon Hydrogen Sulfur Nitrogen Oxygen (diff.)
Pyritic Sulfate Organic Total
Heating value (cal/g)
4040
a b
65.5 4.1 4.7 1.9 23.8
2.2 1.3 1.2 4.7
As received. Dry, ash-free basis.
Fig. 1. Schematic diagram of the apparatus used in pyrolysis experiments: PF––programmable furnace, K––controlling system, O––recorder, SH––sample holder, T––thermocouple, F––flowmeter, NK––driver, R––pressure regulator, V–– valve, V2––transformer.
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2.3. Methods Approximately 15.0 g of the coal sample were placed in the sample room of the sample holder. Firstly, with the aim of preheating, the sample holder content was held at a region of the reactor, determined with experiments, where there is no desulfurization. Then, it was inserted into the furnace as soon as the desired temperature had been reached. To provide thermal equilibrium between the gas and coal sample, the gas is preheated to the bed temperature, and it was introduced into the coal sample from the underside of the sample holder by passing it through the moisture holder and the flowmeter. In all experiments, the gas flow rate was held constant. The experiments were conducted at determined temperatures (350, 450, 500, 600 and 700 °C) and at a definite gas flow rate (1700 ml min1 ). At the end of treatment, the sample holder content is pulled towards the coldest end of the reactor, and it was allowed to cool. Then, the sample holder content was removed from the reactor and was weighed and analysed. In order to determine the change of the adsorptive capacities of the pyrolysed coal samples, adsorption experiments were conducted by using a colouring matter (methylene blue). The adsorption experiments were made in 100 ml, glass stoppered, round bottom flasks immersed in a thermostated shaker bath. For this aim, 0.1 g of the coal was mixed with 100 ml of an aqueous solution of known concentration of methylene blue and shaken vigorously for 2 min by hand to wet the coal. Then, the flask with its contents was shaken by the shaker for 5 min at 20 °C. It was determined that the adsorption time of 5 min is enough to reach adsorption equilibrium. At the end of the adsorption period of 5 min, the supernatant was centrifuged for 2 min at 3750 min1 . The concentration of methylene blue in the supernatant solution after and before adsorption was determined using a double beam UV spectrophotometer at 666 nm. It was found that the supernatant from the coals did not exhibit any absorbance at this wavelength and also that the calibration curve was very reproducible and linear over the concentration range used in this work. The amount of methylene blue adsorbed was calculated from the concentrations in the solution before and after adsorption. Blanks containing no methylene blue were used for each series of experiments.
Table 2 Oxygen functional groups, specific surface areas, total pore surface areas, average pore diameters and weight losses for the coal samples used in the experiments Carboxylic groupsa
Phenolic groupsa
SBET b
Pyrolysis temperature (°C)
meq g
wt.%O
meq g
wt.%O
m g
Raw 350 450 500 600 700
0.29 0.04 – – – –
0.93 0.13 – – – –
2.85 2.17 1.99 1.17 2.43 2.30
4.56 3.47 3.18 1.87 3.89 3.68
7.60 2.82 1.05 0.26 0.14 0.00
a
1
Dry, ash-free basis. BET, N2 . c Measured by Hg porosimeter. b
1
2
1
rc
Weight losses
m g
nm
wt.%
10.33 9.68 7.16 12.34 14.43 0.23
7.6 7.4 7.2 8.9 8.5 3.6 Æ 103
3.40 10.30 12.20 22.31 31.40
Stotal c 2
1
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37
To investigate the effect of surface functional groups on the adsorption capacity of raw and pyrolysed coal samples, the two principal oxygen containing functional groups, carboxylic and phenolic, were determined in all coal samples used in this study. Total acidity (carboxylic plus phenolic) was determined (by reaction with barium hydroxide) via the method used by Miller et al. [21]. Carboxylic groups have been determined (by reaction with calcium acetate) according to the method of Blom et al. [22]. The phenolic group concentration was then calculated by subtracting the value for the carboxylic content from the total acidity. The BET surface area of the raw and pyrolysed coal samples was measured from N2 isotherms at 77 K with a Sorptiometer (Micromeritics Flow Sorb II-2300). The total pore surface area and average pore diameters of the same coal samples were determined with a mercury porosimeter (Autopore II 9220, maximum PHg ¼ 2721:8 atm). The obtained results are displayed in Table 2.
3. Results and discussion The isotherms belonging to the methylene blue adsorption on the raw and pyrolysed coal samples under a CO2 atmosphere at different temperatures are given in Fig. 2. As seen from the figure, the raw coal sample showed the highest adsorption capacity, which was reduced with increasing pyrolysis temperature after pyrolysis. The adsorbed amounts of methylene blue on the pyrolysed coal sample under CO2 atmosphere at 450 °C are higher in comparison with those on the pyrolysed coal sample at 350 °C in low equilibrium concentrations. The adsorption efficiency is clearly reduced in pyrolysed coal samples under CO2 atmosphere 500, 600 and 700 °C. The 50
Amount adsorbed (mg/g)
40
30
Temperature (C) raw
20
700 600 500
10
450 350
0 0.00
.02
.04
.06
.08
.10
Ceq (g/L)
Fig. 2. The adsorption isotherms at adsorption temperature of 20 °C for the raw and pyrolysed coal samples under CO2 atmosphere at different temperatures (pyrolysis time: 10 min).
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observed decrease in adsorption capacities with increasing pyrolysis temperature can be attributed to the changes occurring in the surface morphology and pore size distribution of the lignite samples during pyrolysis [10]. On the other hand, the observed decrease in efficiency of the pyrolysed coal samples at temperatures beyond the temperature 450 °C shows that the number of available sites for methylene blue adsorption decreased. As can be seen from Table 2, in the range of 500–700 °C temperature, the functional group contents of the samples (especially carboxylic oxygen) decreased, and the specific surface area of the same coal samples (BET, N2 ) rapidly decreased. Average pore diameters measured with the mercury porosimeter (see Table 2) show that the raw and pyrolysed lignite samples used in this study have a mesoporous structure in which the pores of small diameter are dominant. The pores of adsorbents are generally classified into three groups, micropore (pore size <2 nm), mesopore (2–50 nm) and macropore (>50 nm). The porosity of the coal sample pyrolysed at 700 °C almost vanished, and the total specific pore surface area of the same coal sample fell to the value of 0.23 m2 g1 . Thus, it can be said that methylene blue adsorption occurs through physical interactions, such as ion–dipole and/or dipole–dipole interactions. Here, it can be suggested that carboxylic groups have the most effect on the methylene blue adsorption. In order to determine the mechanism of methylene blue adsorption on the raw and pyrolysed coal samples and evaluate the relationship between adsorption and the pyrolysis temperature, the experimental data were applied to the Freundlich, Langmuir, BET, Halsey, Harkins-Jura, Smith and Henderson isotherm equations. The constant parameters of the equations for this system were calculated by regression using the linear form of the isotherm equations and SPSS 10.0 Software. The results are given in Table 3, together with the isotherm equations. Fig. 3 shows the Freundlich isotherms of raw and pyrolysed coal samples under a CO2 atmosphere at different temperatures. As seen from this figure, the obtained adsorption data for raw and pyrolysed coal samples at 350, 500 and 600 °C fit well with the Freundlich isotherm. It is seen from Table 3 that the highest value of k (a measure of adsorbent capacity) for the raw coal sample and the highest value of n (a measure of the intensity of adsorption) for the pyrolysed coal sample at 500 °C are obtained among the fitted samples to the Freundlich isotherm quite well. Although the increase of pyrolysis temperature generally caused a decrease in methylene blue adsorption, it is clear that available sites for methylene blue adsorption, from the point of view of morphological and steric factors, appear. As seen in Table 2, the carboxyl contents of the coal samples are decreased, while their average pore diameter and total specific pore surface area are increased during pyrolysis at temperatures after 450 and 500 °C. Fig. 4 shows the Langmuir isotherms of raw and pyrolysed coal samples under a CO2 atmosphere at different temperatures. The Langmuir equation and Langmuir constants belonging to the samples are given in Table 3. As seen from Fig. 4, all of the obtained isotherms fit the Langmuir model quite well. When the monolayer capacities ðYm Þ in Table 3 of raw and pyrolysed coal samples at various temperatures are compared, it is seen that the highest Ym values are obtained for the raw coal sample and the pyrolysed coal sample at 500 °C among the pyrolysed coal samples. This situation clearly shows that methylene blue adsorption depends on both morphological and pore structures and functional group contents of the samples. As a result, it may be said that the pyrolysis at 500 °C makes the surface of coal more available for methylene blue adsorption in terms of surface morphology and functional group distribution. The adsorption of methylene blue is importantly affected by structural transformations depending on the secondary
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Table 3 Applicability of isotherm equations to methylene blue adsorption data at 20 °C of raw and pyrolysed coal samples under CO2 atmosphere at different temperatures and their constant parameters Isotherm equations Freundlich ln y ¼ ln k þ n ln C
Langmuir C=y ¼ 1=kym þ ð1=ym ÞC
BET C=yð1 CÞ ¼ 1=ðym kÞ þ ½ðk 1Þ=ðym kÞC
Halsey ln y ¼ ½ð1=nÞ ln k ð1=nÞ ln½lnð1=CÞ
Harkins-Jura 1=y 2 ¼ ðB=AÞ ð1=AÞ log C
Smith y ¼ Wb W lnð1 CÞ
Pyrolysis temperature (°C)
Constant parameter
Raw 350 450 500 600 700
n ¼ 0:2 n ¼ 0:5 n ¼ 0:4 n ¼ 0:7 n ¼ 0:7 n ¼ 0:3
Raw 350 450 500 600 700
ym ym ym ym ym ym
Raw 350 450 500 600 700
ym ym ym ym ym ym
Raw 350 450 500 600 700
Regression coefficients k k k k k k
¼ 76:8 ¼ 0:3 ¼ 44:6 ¼ 77:0 ¼ 64:4 ¼ 5:8
0.9970 0.9935 0.9497 0.9996 0.9988 0.8831
¼ 0:045 ¼ 0:027 ¼ 0:021 ¼ 0:030 ¼ 0:026 ¼ 0:003
K K K K K K
¼ 76:30 ¼ 26:88 ¼ 60:12 ¼ 9:28 ¼ 8:55 ¼ 65:95
0.9902 0.9946 0.9896 0.9817 0.9726 0.9793
¼ 41:83 ¼ 22:57 ¼ 18:52 ¼ 22:58 ¼ 19:34 ¼ 2:89
k k k k k k
¼ 227:61 ¼ 33:96 ¼ 77:85 ¼ 12:53 ¼ 11:91 ¼ 98:10
0.9936 0.9974 0.9879 0.9907 0.9839 0.9767
n ¼ 1:10 n ¼ 0:62 n ¼ 0:87 n ¼ 0:45 n ¼ 0:45 n ¼ 1:19
k k k k k k
¼ 181:32 ¼ 15:76 ¼ 32:13 ¼ 8:09 ¼ 7:41 ¼ 8:77
0.9977 0.9793 0.9175 0.9937 0.9944 0.8474
Raw 350 450 500 600 700
A ¼ 858:86 A ¼ 61:38 A ¼ 101:70 A ¼ 16:90 A ¼ 11:96 A ¼ 4:28
B ¼ 0:817 B ¼ 0:978 B ¼ 0:835 B ¼ 1:032 B ¼ 1:021 B ¼ 0:571
0.9880 0.9325 0.9238 0.9319 0.9424 0.8976
Raw 350 450 500 600 700
Wb Wb Wb Wb Wb Wb
¼ 6:52 ¼ 6:28 ¼ 4:68 ¼ 5:91 ¼ 5:04 ¼ 0:68
0.9397 0.9642 0.8245 0.9939 0.9920 0.7356
¼ 59:60 ¼ 34:11 ¼ 29:65 ¼ 27:53 ¼ 23:28 ¼ 4:58
W W W W W W
(coninued on next page)
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Table 3 (continued) Isotherm equations Henderson ln½ lnð1 CÞ ¼ ln k þ n ln y
Pyrolysis temperature (°C)
Constant parameter
Raw 350 450 500 600 700
n ¼ 4:71 n ¼ 2:03 n ¼ 2:73 n ¼ 1:45 n ¼ 1:43 n ¼ 3:18
Regression coefficients k k k k k k
¼ 1:35 109 ¼ 2:12 104 ¼ 2:83 105 ¼ 2:04 103 ¼ 2:76 103 ¼ 2:85 103
0.9973 0.9929 0.9478 0.9995 0.9988 0.8804
y, adsorption capacity of methylene blue (mg/g); ym , monolayer adsorption capacity; C, equilibrium concentration; n, k, K, A, B, Wa and W are constant parameters for the isotherm equations. 4.0
Temperature (C) 3.5
raw Rsq = 0.9970
3.0
700 Rsq = 0.8831
2.5
lny
600 Rsq = 0.9988
2.0 500 Rsq = 0.9996 1.5 450 Rsq = 0.9497
1.0
350 Rsq = 0.9935
.5 -6.0
-5.5
-5.0
-4.5
-4.0
-3.5
-3.0
-2.5
-2.0
lnC
Fig. 3. Freundlich isotherms for the adsorption of methylene blue at 20 °C on raw and pyrolysed coal samples at different temperatures (pyrolysis time: 10 min).
reactions of sulfur containing gases and conversion reactions of pyrite occurring between 450 and 500 °C [10,23]. Figs. 5–9 show the isotherms belonging to BET, Halsey, Harkins-Jura, Smith and Henderson adsorption models of raw and pyrolysed coal samples at different temperatures, respectively. All these equations are suitable for multilayer adsorption. Especially, the fitting of these equations can be seen in heteroporous solids [24,25]. As seen from these figures and Table 3, the raw coal sample and all of the pyrolysed coal samples at different temperatures fit into the BET equation. However, the raw and pyrolysed coal sample at 500 °C fit quite well into the Halsey equation. From Table 3, it was also seen that the experimental data obtained for the Harkins-Jura model show the best fitting for raw coal and the poorest fitting for the pyrolysed coal sample at 700 °C.
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41
40
Temperature (C) raw Rsq = 0.9902 30 700
C/Y
Rsq = 0.9793 600 20
Rsq = 0.9726 500 Rsq = 0.9817
10
450 Rsq = 0.9896 350 Rsq = 0.9946
0 0.00
.02
.04
.06
.08
.10
Ceq (g/L)
Fig. 4. Langmuir isotherms for the adsorption of methylene blue at 20 °C on raw and pyrolysed coal samples at different temperatures (pyrolysis time: 10 min).
.04
Temperature (C) raw Rsq = 0.9936 .03 700
C/[Y(1-C)]
Rsq = 0.9767 600 .02
Rsq = 0.9839 500 Rsq = 0.9907
.01
450 Rsq = 0.9879 350 Rsq = 0.9974
0.00 0.00
.02
.04
.06
.08
.10
Ceq (g/L)
Fig. 5. BET isotherms for the adsorption of methylene blue at 20 °C on raw and pyrolysed coal samples at different temperatures (pyrolysis time: 10 min).
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Fig. 6. Halsey isotherms for the adsorption of methylene blue at 20 °C on raw and pyrolysed coal samples at different temperatures (pyrolysis time: 10 min).
Fig. 7. Harkins-Jura isotherms for the adsorption of methylene blue at 20 °C on raw and pyrolysed coal samples at different temperatures (pyrolysis time: 10 min).
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Fig. 8. Smith isotherms for the adsorption of methylene blue at 20 °C on raw and pyrolysed coal samples at different temperatures (pyrolysis time: 10 min).
-2
Temperature (C) raw Rsq = 0.9973 -3 700
ln[-ln(1-C)]
Rsq = 0.8804 600 -4
Rsq = 0.9988 500 Rsq = 0.9995
-5
450 Rsq = 0.9478 350 Rsq = 0.9929
-6 .5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
lny
Fig. 9. Henderson isotherms for the adsorption of methylene blue at 20 °C on raw and pyrolysed coal samples at different temperatures (pyrolysis time: 10 min).
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As seen from Figs. 7 and 8 and Table 3, the best fitting for the Smith and Henderson equations appeared in the pyrolysed coal sample at 500 °C, whereas the poorest fitting occurred in the pyrolysed coal sample at 700 °C. Figs. 5–7 show that the adsorption of methylene blue on the raw coal was fitted well with the BET, Halsey and Harkins-Jura models. This situation can be explained that the raw coal sample has a mesoporous structure. As seen in Table 2, the average pore diameter of the raw coal sample is 7.6 nm, which is importantly smaller than 50 nm that is the limit value of macroporosity, implying that the lignite sample is a mesoporous material exhibiting microporous properties. Especially, both the significant decrease in the adsorption capacity and the loss of fitting for the BET, Halsey and Harkins-Jura models as a result of pyrolysis at higher temperatures (700 °C) support this thinking. It clearly shows that the methylene blue adsorption capacity of the lignite sample is decreased because of the change of average pore diameter and specific surface area of the coal samples depending on the pyrolysis temperature. Although the methylene blue adsorption in the coal surface after pyrolysis under a CO2 atmosphere at 500 °C for the three models is reduced in comparison with that of the raw coal sample, the fitting to these defined models continues with a high correlation. The average pore diameter and specific surface area of the pyrolysed coal sample at this temperature were 8.9 nm and 12.34 m2 g1 , respectively. Carboxylic groups are completely removed from the coal, while the phenolic groups are decreased to the value of 1.17 meq g1 during pyrolysis at this temperature. As a result of this, the number of available sites for methylene blue adsorption increase because of the formation of relatively open pores at this temperature. As seen from Figs. 8 and 9 and Table 3, the adsorption of methylene blue in raw and pyrolysed coal samples at different temperatures fits well for the Henderson and Smith adsorption models, but this fitting is importantly reduced in the pyrolysed coal sample at 700 °C in both models. These results show that the pyrolysis at 700 °C leads to a loss of porosity as a result of sintering. The good fitting of the obtained results for the raw and pyrolysed coal samples at 350, 500 and 600 °C and partially at 450 °C with the Henderson equation show that the microporous texture is dominant in the lignite sample. Also, Table 2 shows that the weight losses gradually increase with increasing pyrolysis temperature, implying that the mineral matters of coal change and part of their CaCO3 converts to oxides.
4. Conclusion In this study, methylene blue adsorption isotherms for raw and pyrolysed coal samples were determined at 20 °C. The two oxygen functional groups, carboxylic and phenolic, were determined for all coal samples and their specific surface area (BET–N2 ) and total pore surface area were measured. The raw coal sample showed the highest adsorption capacity,which was generally decreased with increasing pyrolysis temperature. The sample pyrolysed at 700 °C exhibited the lowest adsorption capacity. In the range of 500–700 °C temperature range, the functional group contents of the samples (especially carboxylic oxygen) decreased, and the specific surface area of the same coal samples (BET, N2 ) decreased greatly. The porosity of the coal sample pyrolysed at 700 °C almost vanished, and the total specific pore surface area of the same coal sample fell to the value of 0.23 m2 g1 . Thus, it can be said that methylene blue adsorption occurs through physical,
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such as ion–dipole and/or dipole–dipole interactions. Carboxylic groups have a large effect on the methylene blue adsorption. The experimental data obtained were applied to the Freundlich, Langmuir, BET, Halsey, Harkins-Jura, Smith and Henderson isotherm equations to test the fitness of these equations to raw and pyrolysed coal samples. By considering the experimental results and adsorption models applied in this study, it can be concluded that adsorption of methylene blue occurs through physical interactions, and the lignite sample has a mesoporous structure.
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