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Treatment of pollutants in wastewater: Adsorption of methylene blue onto olive-based activated carbon
Journal of Industrial and Engineering Chemistry 18 (2012) 780–784
Contents lists available at SciVerse ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Treatment of pollutants in wastewater: Adsorption of methylene blue onto olive-based activated carbon Mo´nica Berrios, Marı´a A´ngeles Martı´n *, Antonio Martı´n University of Cordoba (Spain), Department of Inorganic Chemistry and Chemical Engineering, Campus Universitario de Rabanales, Edificio Marie Curie (C3), Planta Baja, 14071 Cordoba, Spain
A R T I C L E I N F O
Article history: Received 16 May 2011 Accepted 5 August 2011 Available online 12 November 2011 Keywords: Activated carbon Adsorption isotherms Kinetics studies Methylene blue Olive stones
A B S T R A C T
This study used olive stone-based activated carbon for the removal of methylene blue from wastewater in order to evaluate the adsorption capacity of the carbon. The equilibrium and kinetics of adsorption were examined at 258, 308, 358 and 40 8C and several agitation speeds. Type III adsorption isotherms corresponding to physical adsorption in a multilayer system were used for the methylene blue system. The equilibrium data for methylene blue adsorption showed a good fit to the Freundlich equation. The kinetic data was analysed to determine kinetic constants and order of reaction. Kinetics was evaluated by means of an n-order model, showing that the reaction was a first-order reaction. The results indicated that olive stone-based activated carbon could be used as a low-cost alternative to commercial activated carbon for the removal of organic compounds from wastewater. However, due to its microporosity, the application of this type of activated carbon was found to be suitable for molecules smaller than methylene blue. ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction The textile industry requires large amounts of water and produces highly polluted wastewater containing different types of dyes [1]. The main problem involved in decontaminating textile wastewaters is the removal of colour, since no single process is currently capable of generating adequate effluents [2]. Most dyes have an adverse impact on the environment as they are considered toxic and have carcinogenic properties, which make the water inhibitory to aquatic life [3]. Biological treatment processes are reported to be efficient in chemical oxygen demand reduction, but are largely ineffective in removing colour from wastewater. Hence, research has been conducted on physico-chemical methods for colour removal in textile effluent. These studies include the use of coagulants, oxidising agents, photocatalysis, ultrafiltration, electrochemical and adsorption techniques [4,5]. Among the various treatment technologies available, adsorption onto activated carbon has proven to be one of the most effective and reliable physico-chemical treatments. However, commercially available activated carbons are very expensive [6]. The carbon derived from agricultural wastes is gaining importance
* Corresponding author. Tel.: +34 957 21 86 24; fax: +34 957 21 86 25. E-mail address: [email protected] (M.n).
due to its low price and suitability for the removal of organic and inorganic pollutants from wastewater [7]. Although, these agricultural wastes can be also used as biosorbents directly [8– 10]. In this sense, Nieto et al. [11] studied the ability of crude olive stones, a residue of the olive-oil industry, for the adsorption of iron present in the industrial wastewaters. Researchers have studied the production of activated carbon from palm-tree cobs, plum kernels, cassava peel, bagasse, jute fiber, rice husks, date pits, nutshells, wood, maize cob, cotton seed shell, rubber seed coat, apricot stone, almond shell, pongam seed coat, coconut shell, orange peel, walnut stone, bamboo dust, sunflower seed hull and peach stone as has been detailed in the literature [6,7,12–18]. Little information has been reported about the particular case of activated carbon from olive stones [19–21]. In Mediterranean countries, olive stones and residues are a cheap and quite abundant agricultural waste [21]. Activated carbon has a porous structure with a large internal surface area. Four consecutive mass transport steps are associated with the adsorption of solute from solution by porous adsorbent as follows: the adsorbate migrates through the solution to the exterior surface of the adsorbent particles, molecular diffusion takes place in the boundary layer, solute is moved from the particle surface into the interior site by pore diffusion and finally the adsorbate is adsorbed into the active sites at the interior of the adsorbent particle. This phenomenon takes relatively long contact time [6,21].
1226-086X/$ – see front matter ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2011.11.125
M. Berrios et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 780–784
Determinations of surface area can be made by fitting the BET equation to the isothermal equilibrium data obtained. However, these values are not a true indication of the adsorption capacity of an activated carbon applied in liquid-phase adsorption studies. It is therefore more logical to determine the porous structure by combining both the gas-phase and the liquid-phase adsorption equilibrium data. The literature indicates that the adsorption of phenol, methylene blue, caffeine and iodine from the aqueous phase is a useful tool for product control in the manufacture of activated carbon [22–24]. In addition to determining the porous structure of activated carbon, methylene blue can be employed as a thiazine (cationic or basic) dye; the most commonly used dye for colouring among all other dyes of its category. It is generally used for dyeing cotton, wool, and silk [25] and has a number of biological uses. However, given that methylene blue has various harmful effects on human beings, it is of utmost importance to remove it from wastewater. Methylene blue dissociates in aqueous solution as electrolytes into methylene blue cation and the chloride ion. Because the coloured cation is retained at great length by several adsorbents [3,26], methylene blue was selected as the adsorbate in this study. This research study aimed to evaluate the adsorption potential of olive stone-based activated carbon for methylene blue from a synthetic wastewater as olive stones are a very abundant and inexpensive material in Mediterranean countries. The kinetic and equilibrium data of adsorption studies were processed to understand the adsorption behaviour of the dye molecules onto the activated carbon. Although this activated carbon is commercial, it was selected because no data about this behaviour has yet been reported. 2. Materials and methods 2.1. Materials Methylene blue (MB) supplied by Panreac (Spain) was used as an adsorbate and was not purified prior to use. The molecular weight (g/mol), molecular volume (cm3/mol) and molecular diameter (nm) of MB are 319.85, 241.9 and 0.8, respectively [27]. Olive stone-based activated carbon (AC) was supplied by Ibe´rica de Carbones Activos S.A. Textural characterization of the AC was carried out by N2 adsorption at 77 K using Micromeritic ASAP 2020 in our laboratory. The BET surface area, total pore volume and average pore diameter of the AC were found to be 587 m2/g, 0.333 cm3/g and 2.27 nm, respectively.
8000 rpm for 5 min and filtered prior to analysis in order to remove suspended particles of AC. 2.4. Kinetic experiments The kinetic tests were carried out in a similar way to the previous equilibrium tests. Several amounts of AC (1, 2, 4 and 8 g) were added to the MB solutions (initial concentration at 5 mg/L) and kept in an isothermal shaker SI 50 GIRALT (STUART SCIENTIFIC, UK) at different temperatures (258, 308, 358 and 40 8C) and agitation speeds (50, 100, 150, 200 and 250 rpm). The aqueous samples (10 mL) were taken at time intervals of up to 95 min. The samples were centrifuged at 8000 rpm for 5 min and filtered prior to analysis in order to remove suspended particles of AC. The amount of adsorption or adsorption capacity q (mg/g) was calculated by q¼
The equilibrium tests were performed in 4 Erlenmeyer flasks (1 L) in which 500 mL of MB solution (initial concentration at 0.5 mg/L) were placed. The agitation speed was fixed at 50 rpm and the AC doses (0.25, 0.50, 1.00 and 2.00 g) were added to the MB solutions and kept in an isothermal shaker SI 50 GIRALT (STUART SCIENTIFIC, UK) at different temperatures (258, 308, 358 and 40 8C). The contact time to reach equilibrium between the solid phase and the liquid phase was approximately 11.5 h (that was checked in previous assays). The aqueous samples (10 mL) were centrifuged at
(1)
3. Results and discussion 3.1. Adsorption equilibrium The adsorption of dyes from the liquid to solid phase can be considered a reversible reaction with equilibrium established between the two phases [28]. The adsorption isotherm (qe vs. Ce) indicates how the adsorption molecules distribute between the liquid phase and the solid phase when the adsorption process reaches an equilibrium state. The analysis of the equilibrium data of the isotherm models is very important for the use of adsorbents [12]. Fig. 1 shows the adsorption isotherms at four temperatures for the MB solution and olive stone-based AC system. According to Brunauer et al. [29] and Hinz [30], the adsorption isotherms have the same shape that the type III isotherms or S1 isotherms (Giles classification), which correspond to physical adsorption in a multilayer system where no difference is noticed between the filling of the first layer and the other layers. The Freundlich isotherm is the earliest known relationship describing the adsorption equation. This fairly satisfactory 1.0
C0 (MB) = 0.508 mg/L 0.8
qe (mg/g)
2.3. Equilibrium experiments
C0V 0 CiV i W
where C0 and Ci (mg/L) are the liquid-phase concentrations of MB at initial and any time (Ct) or equilibrium (Ce), respectively, obtaining the amount of adsorption at time (qt) or the amount of adsorption at equilibrium (qe). V0 is the initial volume of the solution (0.5 L), Vi is the real volume when sampling and W is the mass of the AC used (g).
2.2. Analysis of methylene blue concentration The MB concentration in the supernatant solution before and after adsorption was determined using a UV–vis spectrophotometer S-20 (BOECO, Germany) at 664 nm. The calibration curve was very reproducible and linear over the concentration range used in this work.
781
0.6
0.4 25ºC 30ºC 35ºC 40ºC
0.2
0.0 0.06
0.08
0.10
0.12
Ce (mg/L) Fig. 1. Adsorption isotherms for the MB solution and olive stone-based AC system.
M. Berrios et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 780–784
782
Table 1 Freundlich constants for the MB solution and olive stone-based AC system at different temperatures. Temperature (8C)
KF 103 (mg/g)(L/g)1/n n r2
25
30
35
40
936.9 0.165 0.999
734.1 0.163 0.999
1744.1 0.150 0.998
132.7 0.172 0.997
empirical isotherm can be used for non-ideal adsorption that involves heterogeneous surface energy systems and is expressed by the following equation: 1=n
qe ¼ K F Ce
(2)
where KF is a rough indicator of the adsorption capacity and (1/n) is the adsorption intensity. In general, the KF value increases as the adsorption capacity of adsorbent for a given adsorbate increases. The magnitude of the exponent (1/n) indicates easy uptake of adsorbate from aqueous solution. A value for (1/n) below one indicates a normal Langmuir isotherm, while (1/n) above one is indicative of cooperative adsorption [12]. The Freundlich constants (KF and n) were calculated by nonlinear regression using Sigmaplot1 11.0 software. The results and the regression coefficients are shown in Table 1. As can be observed in the regression coefficients, the adsorption of MB from wastewater on olive stone-based AC follow the Freundlich isotherm at all tested temperatures. According to Hameed et al. [12], our (1/n) values above 1 were indicative of cooperative adsorption. This means that the binding of a MB molecule to one site on AC influences the affinity of other sites. Similar values of n were found by Avom et al. [24] for palm tree cobs-based AC, indicating the heterogeneity of the AC surface. When the temperature increased, the MB amount in solid phase decreased in the equilibrium (Fig. 1). Hence, the effectiveness of the adsorption process decreased at a high temperature because the desorption process took place at higher temperatures. Therefore, the MB concentration in liquid phase increased. Based on the experimental data, an empirical equation was obtained to relate qe to temperature (T) and Ce for the olive stonebased AC. This relationship is shown in the three-dimensional graph in Fig. 2.
It is believed that adsorption of organics onto AC depends on both the pore structure and surface chemical properties of carbon as well as the adsorbate. Dye adsorption tests help to determine the capacity of carbon to adsorb molecules of a particular size. The MB molecule has a minimum molecular diameter of 0.8 nm and cannot enter pores with a diameter of less than 1.3 nm [28,31]. Therefore, it can only enter the larger micropores, but most of it is likely to be adsorbed in mesopores. Despite the high surface area of olive stone-based AC (587 m2/g) as other authors have reported for olive stone-based activated carbon [32], the adsorption capacity of MB in aqueous solution (for example qe = 0.858 mg/g at 25 8C with an AC dose of 0.5 g/L) was poor due to the molecular diameter of MB and the AC pore size distribution as described in Section 2. Although this AC has a high surface area, its application could be more suitable for molecules smaller than MB due to the high microporosity of AC. 3.2. Adsorption kinetics The kinetic adsorption data was evaluated to understand the dynamics of the adsorption process. The MB molecules must overcome three stages before coming into contact with the active sites of AC. These stages are the migration of the solute through the solution to the exterior surface of the adsorbent particles, molecular diffusion in the boundary layer and solute movement from particle surface into the interior site by pore diffusion. Adsorption itself could be considered almost instantaneous if the phenomenon was purely a physical process, while mass transfer could be minimized by means of suitable agitation speeds. In order to evaluate the influence of agitation speed (rpm) on the external mass transfer, several experiments were carried out in the range of 50–250 rpm for all the temperature conditions and AC doses selected. Fig. 3 shows the influence of agitation speed on the adsorption capacity at 25 8C and an AC dose of 8 g/L. As can be observed, no differences were detected between 100 and 250 rpm. However, when the agitation speed was increased from 50 to 100 rpm, the adsorption capacity (q) was enhanced. This increase highlighted that the mass transfer through the solution and in the boundary layer did not limit the adsorption process when an agitation speed of 100 or higher was selected. Once the external mass transfer did not limit the adsorption process, the kinetics was evaluated at 100 rpm. As can be observed in Fig. 4, the q vs. t plots for all temperatures and AC doses were found to rise exponentially to the maximum for an AC dose of 4 g/L. Temperature did not significantly influence the adsorption
0.6
q (mg/g)
0.5
0.4
50 rpm 100 rpm 150 rpm 200 rpm 250 rpm
25ºC, 8 g AC/L 0.0 0
20
40
60
80
100
time (min) Fig. 2. Three-dimensional graph of the relationship between qe, temperature (T) and Ce .
Fig. 3. Effect of agitation speed on the adsorption capacity at 25 8C, 8 g AC/L and an initial MB concentration of 5 mg/L.
M. Berrios et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 780–784
783
Table 2 Kinetic constants of the n-order model.
1.2
AC dose (g/L) 1.0
K¯ (1/min)
q (mg/g)
0.8
2
4
8
16
0.020 0.002
0.030 0.009
0.057 0.004
0.050 0.003
0.6
0.4
0.2
25ºC 30ºC 35ºC 40ºC
4 g AC/L, 100 rpm
0.0 0
20
40
60
80
100
Table 2. These kinetic constants were similar to the results obtained by Santhy and Selvapathy [7] for the adsorption of reactive dyes. The kinetic constant increased from 0.020 to 0.030 and 0.057 (1/min) for AC doses of 2, 4 and 8 g/L, respectively. At higher AC doses, the kinetic constant remained approximately stable, indicating that the adsorption rate did not increase when the AC dose was higher than 8 g/L.
time (min)
4. Conclusions Fig. 4. Fitting of experimental data to exponential function at 100 rpm and AC dose of 4 g/L.
capacity in the range studied for temperature and time (25–40 8C and 100 min). Several mathematical expressions explain the increase in MB concentration in a dose of AC with greater adsorption time. Most studies (i.e. Langergen and Svenska) use first- and second-order kinetic equations to model adsorption capacity [12]. Lee [33] applied pseudo second-order kinetic equations to study the adsorption of erythrosine dye from aqueous solution using activated carbon. In order to determine the exact order of reaction, the following generalised kinetic equation was employed: dq ¼ Kðqmax qÞn dt
This study showed that olive stone-based activated carbon can be used for the removal of organic compounds from aqueous solution under a wide range of conditions. Type III adsorption isotherms were used for the MB system. These isotherms correspond to physical adsorption in a multilayer system where no difference is noticed between the filling of the first layer and the other layers. Adsorption behaviour was described by the Freundlich isotherm with n values that demonstrate the heterogeneity of the AC surface. Kinetics was evaluated by means of an n-order model, showing that the reaction was a first-order reaction. The olive stone-based AC characterization showed high microporosity, thus indicating that the poor adsorption capacity results for MB are due to the molecular properties of this compound.
(3)
where K is the kinetic constant and qmax is the maximum adsorption capacity for each temperature and AC dose. The kinetic constant and the order of reaction can be calculated by linearising Eq. (3): dq ¼ log K þ n log ðqmax qÞ (4) log dt as shown in Fig. 5 for an AC dose of 2 g/L. The order of reaction (n) was found to be 1 or close to 1 in all the experiments. This result coincides with other authors [4,6,7,34]. The mean kinetic constants (K, 1/min) for each AC dose (2, 4, 8 and 16 g/L) can be observed in
0.1
Acknowledgements The authors are very grateful to Iberica de Carbones and Junta de Andalucia (PAIDI group RNM-271) for funding this research. We also wish to express our gratitude to laboratory technician Inmaculada Bellido Padillo for her help. References [1] [2] [3] [4] [5] [6] [7] [8]
(dq/dt) (mg/g·min)
[9] [10] [11] [12] [13] [14] 0.01
[15] [16] [17] 25ºC 30ºC 35ºC 40ºC
[18] [19]
2 g AC/L, 100 rpm 0.001 0.01
0.1
1
10
(qmax - q) (mg/g) Fig. 5. Kinetic plots for the removal of MB by adsorption on olive stone-based AC.
[20] [21] [22] [23] [24] [25]
F. Banat, N. Al-Bastaki, Desalination 170 (2004) 69. M.F.R. Pereira, S.F. Soares, J.J.M. O´rfa˜o, J.L. Figueiredo, Carbon 41 (2003) 811. E.N. El Qada, S.J. Allen, G.M. Walter, Chem. Eng. J. 124 (2006) 103. N. Kannan, M.M. Sundaram, Dyes Pigments 51 (2001) 25. V. Gomez, M.S. Larrechi, M.P. Callao, Chemosphere 69 (2007) 1151. P.K. Malik, J. Hazard. Mater. B113 (2004) 81. K. Santhy, P. Selvapathy, Bioresour. Technol. 97 (2006) 1329. A.R. Binupriya, M. Sathishkumar, D. Kavitha, K. Swaminathan, S. Yun, S. Mun, Clean 35 (2007) 143. H. Parab, M. Sudersanan, N. Shenoy, T. Pathare, B. Vaze, Clean 37 (2009) 963. P. Saha, S. Chowdhury, S. Gupta, I. Kumar, R. Kumar, Clean 38 (2010) 437. L.M. Nieto, S.B.D. Alami, G. Hodaifa, C. Faur, S. Rodrı´guez, J.A. Gime´nez, J. Ochando, Ind. Crops Prod. 32 (2010) 467. B.H. Hameed, A.T.M. Din, A.L. Ahmad, J. Hazard. Mater. 141 (2007) 819. D. Mohan, K.P. Singh, V.K. Singh, J. Hazard. Mater. 152 (2008) 1045. N. Thinakaran, P. Baskaralingam, K.V. Thiruvengada Ravi, P. Panneerselvam, S. Sivanesan, Clean 36 (2008) 798. A. Dermibas, J. Hazard. Mater. 167 (2009) 1. M. Rafatullah, O. Sulaiman, R. Hashim, A. Ahmad, J. Hazard. Mater. 177 (2010) 70. S.B. Hartono, S. Ismadji, Y. Sudaryanto, W. Irawaty, J. Ind. Eng. Chem. 11 (2005) 864. M. Soleimani, T. Kaghazchi, J. Ind. Eng. Chem. 14 (2008) 28. M.H. Alaya, M.A. Hourieh, A.M. Youssef, F. El-Seiariah, Adsorpt. Sci. Technol. 18 (2000) 27. W.K. Lafi, Biomass Bioenerg. 20 (2001) 57. G.G. Stavropoulos, A.A. Zabaniotou, Micropor. Mesopor. Mater. 82 (2005) 79. R.S. Juang, R.L. Tseng, F.C. Wu, Adsorption 7 (2001) 65. M.K.N. Yenkie, G.S. Natarajan, Sep. Sci. Technol. 28 (1993) 1177. J. Avom, J.K. Mbadcam, C. Noubactep, P. Germain, Carbon 35 (1997) 365. S. Senthilkumaar, P.R. Varadarajan, K. Porkodi, C.V. Subbhuraam, J. Colloid Interface Sci. 284 (2005) 78.
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[26] S.S. Barton, Carbon 25 (1987) 343. [27] E.N. El Qada, S.J. Allen, G.M. Walter, Chem. Eng. J. 135 (2008) 174. [28] S. Wang, Z.H. Zhu, A. Coomes, F. Haghseresht, G.Q. Lu, J. Colloid Interface Sci. 284 (2005) 440. [29] S. Brunauer, L.S. Deming, W.E. Deming, E. Teller, J. Am. Chem. Soc. 62 (1940) 1723. [30] C. Hinz, Geoderma 99 (2001) 225.
[31] H. Valdes, M. Sanchez-Polo, J. Rivera-Utrilla, C.A. Zaror, Langmuir 18 (2002) 2111. [32] A. Wahby, Z. Abdelouahab-Reddam, R. El Mail, M. Stitou, J. Silvestre-Albero, A. Sepu´lveda-Escribano, F. Rodrı´guez-Reinoso, Adsorption 17 (2011) 603. [33] J.J. Lee, J. Ind. Eng. Chem. 22 (2011) 224. [34] K.V. Kumar, J. Hazard. Mater. B137 (2006) 1538.
Contents lists available at SciVerse ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Treatment of pollutants in wastewater: Adsorption of methylene blue onto olive-based activated carbon Mo´nica Berrios, Marı´a A´ngeles Martı´n *, Antonio Martı´n University of Cordoba (Spain), Department of Inorganic Chemistry and Chemical Engineering, Campus Universitario de Rabanales, Edificio Marie Curie (C3), Planta Baja, 14071 Cordoba, Spain
A R T I C L E I N F O
Article history: Received 16 May 2011 Accepted 5 August 2011 Available online 12 November 2011 Keywords: Activated carbon Adsorption isotherms Kinetics studies Methylene blue Olive stones
A B S T R A C T
This study used olive stone-based activated carbon for the removal of methylene blue from wastewater in order to evaluate the adsorption capacity of the carbon. The equilibrium and kinetics of adsorption were examined at 258, 308, 358 and 40 8C and several agitation speeds. Type III adsorption isotherms corresponding to physical adsorption in a multilayer system were used for the methylene blue system. The equilibrium data for methylene blue adsorption showed a good fit to the Freundlich equation. The kinetic data was analysed to determine kinetic constants and order of reaction. Kinetics was evaluated by means of an n-order model, showing that the reaction was a first-order reaction. The results indicated that olive stone-based activated carbon could be used as a low-cost alternative to commercial activated carbon for the removal of organic compounds from wastewater. However, due to its microporosity, the application of this type of activated carbon was found to be suitable for molecules smaller than methylene blue. ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction The textile industry requires large amounts of water and produces highly polluted wastewater containing different types of dyes [1]. The main problem involved in decontaminating textile wastewaters is the removal of colour, since no single process is currently capable of generating adequate effluents [2]. Most dyes have an adverse impact on the environment as they are considered toxic and have carcinogenic properties, which make the water inhibitory to aquatic life [3]. Biological treatment processes are reported to be efficient in chemical oxygen demand reduction, but are largely ineffective in removing colour from wastewater. Hence, research has been conducted on physico-chemical methods for colour removal in textile effluent. These studies include the use of coagulants, oxidising agents, photocatalysis, ultrafiltration, electrochemical and adsorption techniques [4,5]. Among the various treatment technologies available, adsorption onto activated carbon has proven to be one of the most effective and reliable physico-chemical treatments. However, commercially available activated carbons are very expensive [6]. The carbon derived from agricultural wastes is gaining importance
* Corresponding author. Tel.: +34 957 21 86 24; fax: +34 957 21 86 25. E-mail address: [email protected] (M.n).
due to its low price and suitability for the removal of organic and inorganic pollutants from wastewater [7]. Although, these agricultural wastes can be also used as biosorbents directly [8– 10]. In this sense, Nieto et al. [11] studied the ability of crude olive stones, a residue of the olive-oil industry, for the adsorption of iron present in the industrial wastewaters. Researchers have studied the production of activated carbon from palm-tree cobs, plum kernels, cassava peel, bagasse, jute fiber, rice husks, date pits, nutshells, wood, maize cob, cotton seed shell, rubber seed coat, apricot stone, almond shell, pongam seed coat, coconut shell, orange peel, walnut stone, bamboo dust, sunflower seed hull and peach stone as has been detailed in the literature [6,7,12–18]. Little information has been reported about the particular case of activated carbon from olive stones [19–21]. In Mediterranean countries, olive stones and residues are a cheap and quite abundant agricultural waste [21]. Activated carbon has a porous structure with a large internal surface area. Four consecutive mass transport steps are associated with the adsorption of solute from solution by porous adsorbent as follows: the adsorbate migrates through the solution to the exterior surface of the adsorbent particles, molecular diffusion takes place in the boundary layer, solute is moved from the particle surface into the interior site by pore diffusion and finally the adsorbate is adsorbed into the active sites at the interior of the adsorbent particle. This phenomenon takes relatively long contact time [6,21].
1226-086X/$ – see front matter ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2011.11.125
M. Berrios et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 780–784
Determinations of surface area can be made by fitting the BET equation to the isothermal equilibrium data obtained. However, these values are not a true indication of the adsorption capacity of an activated carbon applied in liquid-phase adsorption studies. It is therefore more logical to determine the porous structure by combining both the gas-phase and the liquid-phase adsorption equilibrium data. The literature indicates that the adsorption of phenol, methylene blue, caffeine and iodine from the aqueous phase is a useful tool for product control in the manufacture of activated carbon [22–24]. In addition to determining the porous structure of activated carbon, methylene blue can be employed as a thiazine (cationic or basic) dye; the most commonly used dye for colouring among all other dyes of its category. It is generally used for dyeing cotton, wool, and silk [25] and has a number of biological uses. However, given that methylene blue has various harmful effects on human beings, it is of utmost importance to remove it from wastewater. Methylene blue dissociates in aqueous solution as electrolytes into methylene blue cation and the chloride ion. Because the coloured cation is retained at great length by several adsorbents [3,26], methylene blue was selected as the adsorbate in this study. This research study aimed to evaluate the adsorption potential of olive stone-based activated carbon for methylene blue from a synthetic wastewater as olive stones are a very abundant and inexpensive material in Mediterranean countries. The kinetic and equilibrium data of adsorption studies were processed to understand the adsorption behaviour of the dye molecules onto the activated carbon. Although this activated carbon is commercial, it was selected because no data about this behaviour has yet been reported. 2. Materials and methods 2.1. Materials Methylene blue (MB) supplied by Panreac (Spain) was used as an adsorbate and was not purified prior to use. The molecular weight (g/mol), molecular volume (cm3/mol) and molecular diameter (nm) of MB are 319.85, 241.9 and 0.8, respectively [27]. Olive stone-based activated carbon (AC) was supplied by Ibe´rica de Carbones Activos S.A. Textural characterization of the AC was carried out by N2 adsorption at 77 K using Micromeritic ASAP 2020 in our laboratory. The BET surface area, total pore volume and average pore diameter of the AC were found to be 587 m2/g, 0.333 cm3/g and 2.27 nm, respectively.
8000 rpm for 5 min and filtered prior to analysis in order to remove suspended particles of AC. 2.4. Kinetic experiments The kinetic tests were carried out in a similar way to the previous equilibrium tests. Several amounts of AC (1, 2, 4 and 8 g) were added to the MB solutions (initial concentration at 5 mg/L) and kept in an isothermal shaker SI 50 GIRALT (STUART SCIENTIFIC, UK) at different temperatures (258, 308, 358 and 40 8C) and agitation speeds (50, 100, 150, 200 and 250 rpm). The aqueous samples (10 mL) were taken at time intervals of up to 95 min. The samples were centrifuged at 8000 rpm for 5 min and filtered prior to analysis in order to remove suspended particles of AC. The amount of adsorption or adsorption capacity q (mg/g) was calculated by q¼
The equilibrium tests were performed in 4 Erlenmeyer flasks (1 L) in which 500 mL of MB solution (initial concentration at 0.5 mg/L) were placed. The agitation speed was fixed at 50 rpm and the AC doses (0.25, 0.50, 1.00 and 2.00 g) were added to the MB solutions and kept in an isothermal shaker SI 50 GIRALT (STUART SCIENTIFIC, UK) at different temperatures (258, 308, 358 and 40 8C). The contact time to reach equilibrium between the solid phase and the liquid phase was approximately 11.5 h (that was checked in previous assays). The aqueous samples (10 mL) were centrifuged at
(1)
3. Results and discussion 3.1. Adsorption equilibrium The adsorption of dyes from the liquid to solid phase can be considered a reversible reaction with equilibrium established between the two phases [28]. The adsorption isotherm (qe vs. Ce) indicates how the adsorption molecules distribute between the liquid phase and the solid phase when the adsorption process reaches an equilibrium state. The analysis of the equilibrium data of the isotherm models is very important for the use of adsorbents [12]. Fig. 1 shows the adsorption isotherms at four temperatures for the MB solution and olive stone-based AC system. According to Brunauer et al. [29] and Hinz [30], the adsorption isotherms have the same shape that the type III isotherms or S1 isotherms (Giles classification), which correspond to physical adsorption in a multilayer system where no difference is noticed between the filling of the first layer and the other layers. The Freundlich isotherm is the earliest known relationship describing the adsorption equation. This fairly satisfactory 1.0
C0 (MB) = 0.508 mg/L 0.8
qe (mg/g)
2.3. Equilibrium experiments
C0V 0 CiV i W
where C0 and Ci (mg/L) are the liquid-phase concentrations of MB at initial and any time (Ct) or equilibrium (Ce), respectively, obtaining the amount of adsorption at time (qt) or the amount of adsorption at equilibrium (qe). V0 is the initial volume of the solution (0.5 L), Vi is the real volume when sampling and W is the mass of the AC used (g).
2.2. Analysis of methylene blue concentration The MB concentration in the supernatant solution before and after adsorption was determined using a UV–vis spectrophotometer S-20 (BOECO, Germany) at 664 nm. The calibration curve was very reproducible and linear over the concentration range used in this work.
781
0.6
0.4 25ºC 30ºC 35ºC 40ºC
0.2
0.0 0.06
0.08
0.10
0.12
Ce (mg/L) Fig. 1. Adsorption isotherms for the MB solution and olive stone-based AC system.
M. Berrios et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 780–784
782
Table 1 Freundlich constants for the MB solution and olive stone-based AC system at different temperatures. Temperature (8C)
KF 103 (mg/g)(L/g)1/n n r2
25
30
35
40
936.9 0.165 0.999
734.1 0.163 0.999
1744.1 0.150 0.998
132.7 0.172 0.997
empirical isotherm can be used for non-ideal adsorption that involves heterogeneous surface energy systems and is expressed by the following equation: 1=n
qe ¼ K F Ce
(2)
where KF is a rough indicator of the adsorption capacity and (1/n) is the adsorption intensity. In general, the KF value increases as the adsorption capacity of adsorbent for a given adsorbate increases. The magnitude of the exponent (1/n) indicates easy uptake of adsorbate from aqueous solution. A value for (1/n) below one indicates a normal Langmuir isotherm, while (1/n) above one is indicative of cooperative adsorption [12]. The Freundlich constants (KF and n) were calculated by nonlinear regression using Sigmaplot1 11.0 software. The results and the regression coefficients are shown in Table 1. As can be observed in the regression coefficients, the adsorption of MB from wastewater on olive stone-based AC follow the Freundlich isotherm at all tested temperatures. According to Hameed et al. [12], our (1/n) values above 1 were indicative of cooperative adsorption. This means that the binding of a MB molecule to one site on AC influences the affinity of other sites. Similar values of n were found by Avom et al. [24] for palm tree cobs-based AC, indicating the heterogeneity of the AC surface. When the temperature increased, the MB amount in solid phase decreased in the equilibrium (Fig. 1). Hence, the effectiveness of the adsorption process decreased at a high temperature because the desorption process took place at higher temperatures. Therefore, the MB concentration in liquid phase increased. Based on the experimental data, an empirical equation was obtained to relate qe to temperature (T) and Ce for the olive stonebased AC. This relationship is shown in the three-dimensional graph in Fig. 2.
It is believed that adsorption of organics onto AC depends on both the pore structure and surface chemical properties of carbon as well as the adsorbate. Dye adsorption tests help to determine the capacity of carbon to adsorb molecules of a particular size. The MB molecule has a minimum molecular diameter of 0.8 nm and cannot enter pores with a diameter of less than 1.3 nm [28,31]. Therefore, it can only enter the larger micropores, but most of it is likely to be adsorbed in mesopores. Despite the high surface area of olive stone-based AC (587 m2/g) as other authors have reported for olive stone-based activated carbon [32], the adsorption capacity of MB in aqueous solution (for example qe = 0.858 mg/g at 25 8C with an AC dose of 0.5 g/L) was poor due to the molecular diameter of MB and the AC pore size distribution as described in Section 2. Although this AC has a high surface area, its application could be more suitable for molecules smaller than MB due to the high microporosity of AC. 3.2. Adsorption kinetics The kinetic adsorption data was evaluated to understand the dynamics of the adsorption process. The MB molecules must overcome three stages before coming into contact with the active sites of AC. These stages are the migration of the solute through the solution to the exterior surface of the adsorbent particles, molecular diffusion in the boundary layer and solute movement from particle surface into the interior site by pore diffusion. Adsorption itself could be considered almost instantaneous if the phenomenon was purely a physical process, while mass transfer could be minimized by means of suitable agitation speeds. In order to evaluate the influence of agitation speed (rpm) on the external mass transfer, several experiments were carried out in the range of 50–250 rpm for all the temperature conditions and AC doses selected. Fig. 3 shows the influence of agitation speed on the adsorption capacity at 25 8C and an AC dose of 8 g/L. As can be observed, no differences were detected between 100 and 250 rpm. However, when the agitation speed was increased from 50 to 100 rpm, the adsorption capacity (q) was enhanced. This increase highlighted that the mass transfer through the solution and in the boundary layer did not limit the adsorption process when an agitation speed of 100 or higher was selected. Once the external mass transfer did not limit the adsorption process, the kinetics was evaluated at 100 rpm. As can be observed in Fig. 4, the q vs. t plots for all temperatures and AC doses were found to rise exponentially to the maximum for an AC dose of 4 g/L. Temperature did not significantly influence the adsorption
0.6
q (mg/g)
0.5
0.4
50 rpm 100 rpm 150 rpm 200 rpm 250 rpm
25ºC, 8 g AC/L 0.0 0
20
40
60
80
100
time (min) Fig. 2. Three-dimensional graph of the relationship between qe, temperature (T) and Ce .
Fig. 3. Effect of agitation speed on the adsorption capacity at 25 8C, 8 g AC/L and an initial MB concentration of 5 mg/L.
M. Berrios et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 780–784
783
Table 2 Kinetic constants of the n-order model.
1.2
AC dose (g/L) 1.0
K¯ (1/min)
q (mg/g)
0.8
2
4
8
16
0.020 0.002
0.030 0.009
0.057 0.004
0.050 0.003
0.6
0.4
0.2
25ºC 30ºC 35ºC 40ºC
4 g AC/L, 100 rpm
0.0 0
20
40
60
80
100
Table 2. These kinetic constants were similar to the results obtained by Santhy and Selvapathy [7] for the adsorption of reactive dyes. The kinetic constant increased from 0.020 to 0.030 and 0.057 (1/min) for AC doses of 2, 4 and 8 g/L, respectively. At higher AC doses, the kinetic constant remained approximately stable, indicating that the adsorption rate did not increase when the AC dose was higher than 8 g/L.
time (min)
4. Conclusions Fig. 4. Fitting of experimental data to exponential function at 100 rpm and AC dose of 4 g/L.
capacity in the range studied for temperature and time (25–40 8C and 100 min). Several mathematical expressions explain the increase in MB concentration in a dose of AC with greater adsorption time. Most studies (i.e. Langergen and Svenska) use first- and second-order kinetic equations to model adsorption capacity [12]. Lee [33] applied pseudo second-order kinetic equations to study the adsorption of erythrosine dye from aqueous solution using activated carbon. In order to determine the exact order of reaction, the following generalised kinetic equation was employed: dq ¼ Kðqmax qÞn dt
This study showed that olive stone-based activated carbon can be used for the removal of organic compounds from aqueous solution under a wide range of conditions. Type III adsorption isotherms were used for the MB system. These isotherms correspond to physical adsorption in a multilayer system where no difference is noticed between the filling of the first layer and the other layers. Adsorption behaviour was described by the Freundlich isotherm with n values that demonstrate the heterogeneity of the AC surface. Kinetics was evaluated by means of an n-order model, showing that the reaction was a first-order reaction. The olive stone-based AC characterization showed high microporosity, thus indicating that the poor adsorption capacity results for MB are due to the molecular properties of this compound.
(3)
where K is the kinetic constant and qmax is the maximum adsorption capacity for each temperature and AC dose. The kinetic constant and the order of reaction can be calculated by linearising Eq. (3): dq ¼ log K þ n log ðqmax qÞ (4) log dt as shown in Fig. 5 for an AC dose of 2 g/L. The order of reaction (n) was found to be 1 or close to 1 in all the experiments. This result coincides with other authors [4,6,7,34]. The mean kinetic constants (K, 1/min) for each AC dose (2, 4, 8 and 16 g/L) can be observed in
0.1
Acknowledgements The authors are very grateful to Iberica de Carbones and Junta de Andalucia (PAIDI group RNM-271) for funding this research. We also wish to express our gratitude to laboratory technician Inmaculada Bellido Padillo for her help. References [1] [2] [3] [4] [5] [6] [7] [8]
(dq/dt) (mg/g·min)
[9] [10] [11] [12] [13] [14] 0.01
[15] [16] [17] 25ºC 30ºC 35ºC 40ºC
[18] [19]
2 g AC/L, 100 rpm 0.001 0.01
0.1
1
10
(qmax - q) (mg/g) Fig. 5. Kinetic plots for the removal of MB by adsorption on olive stone-based AC.
[20] [21] [22] [23] [24] [25]
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