Applied Clay Science 95 (2014) 25–31
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Adsorption characteristics of montmorillonite clay modiﬁed with iron oxide with respect to methylene blue in aqueous media L. Cottet a,b,⁎, C.A.P. Almeida a, N. Naidek a, M.F. Viante a, M.C. Lopes a, N.A. Debacher b a b
Departamento de Química, Universidade Estadual do Centro-Oeste, 85040-080 Guarapuava, PR, Brazil Departamento de Química, Universidade Federal de Santa Catarina, 88040-900 Florianópolis, SC, Brazil
a r t i c l e
i n f o
Article history: Received 30 September 2013 Received in revised form 14 March 2014 Accepted 17 March 2014 Available online 24 April 2014 Keywords: Adsorption Montmorillonite clay Methylene blue Kinetics Isotherms Thermodynamics
a b s t r a c t Montmorillonite clay modiﬁed with iron oxide (MtMIO) was prepared for use as an adsorbent of methylene blue dye. Structural characterization of MtMIO was performed by ﬁeld-emission scanning electron microscopy, energy-dispersive spectroscopy, surface area measurements, zeta potential analysis and Fourier transform infrared spectroscopy. Batch experiments were carried out under different conditions of initial dye concentration, contact time and temperature to investigate the adsorption of methylene blue onto MtMIO. Adsorption isotherms were experimentally determined and the Freundlich and Langmuir models were used to ﬁt the data, better results being obtained for the latter with a maximum adsorption capacity of 71.12 mg g−1 at 333 K. Thermodynamic parameters, such as standard enthalpy, standard entropy and standard free energy changes were determined, indicating an endothermic and non-spontaneous process. Also, pseudo-ﬁrst-order and pseudo-second-order kinetic models were applied, and the experimental data ﬁtted the pseudo-second order model. The activation energy of +19.32 kJ mol−1 was determined using the Arrhenius equation. © 2014 Elsevier B.V. All rights reserved.
1. Introduction In the manufacturing of fabrics, leather, paper, cosmetics, polymers and food, among other items, large amounts of dyes are used to color the products, which are partly discharged in efﬂuents with no previous treatment. The presence of dyes in efﬂuents, even in low concentrations, is a major concern because they are highly visible, toxic to microorganisms and harmful to human health (Auta and Hameed, 2013; Kabra et al., 2013; Verma et al., 2012). In recent decades, various methods have been developed for the removal of dyes from wastewaters, such as chemical oxidation, biodegradation, membrane separation, electrochemical processes, coagulation/ ﬂocculation and adsorption (Mondal et al., 2013; Vergili et al., 2012; Verma et al., 2012). Adsorption is considered an effective way to remove dyes from wastewater, since it is not destructive and easy to apply (Ahmad and Kumar, 2010; Anirudhan et al., 2010). The use of activated carbon as an adsorbent has a long history and it has been widely utilized for dye removal in wastewaters. However, due to its high cost, researchers have been studying alternative adsorbents (Ahmaruzzaman and Gayatri, 2010; Duman and Ayranci, 2010). Magnetic adsorbents, which combine the adsorbent capacity of an adsorbent with the magnetic property of iron oxide, have been studied due to their great potential for application in several areas, such as the biological, medical, industrial and environmental ﬁelds (Durdureanu⁎ Corresponding author. E-mail address: [email protected]
http://dx.doi.org/10.1016/j.clay.2014.03.023 0169-1317/© 2014 Elsevier B.V. All rights reserved.
Angheluta et al., 2012; Zhao et al., 2012). Magnetic adsorbents have active sites with afﬁnity for organic and/or inorganic compounds, being composed of an iron oxide core coated with organic or inorganic molecules. After the adsorption, magnetic adsorbents can be separated from the medium by a magnetic ﬁeld (Mahdavian and Mirrahimi, 2010; Yamaura et al., 2004). In this study, a magnetic adsorbent was obtained by modifying montmorillonite clay with iron oxide (MtMIO). The MtMIO produced was characterized and applied to methylene blue (MB) adsorption using the bath technique. The uptake time, initial concentration and temperature effects were investigated, as well as the equilibrium, kinetics and thermodynamics.
2. Materials and methods 2.1. Materials Methylene blue (MB) dye, which has the chemical structure shown in Fig. 1, was used as the adsorbate. MB (basic blue 9, C.I. 52015) is a cationic dye with the molecular formula C16H18ClN3S·3H2O and a molar mass of 373.90 g mol−1. Its electronic spectrum has a maximum absorbance peak in the visible region at a wavelength of 664 nm. All solutions used were prepared with deionized water and all reagents were of analytical grade. Working solutions of MB were prepared from a stock solution of 2000 mg L−1 to give the required initial concentrations (Co = 100 to 1000 mg L−1) for each experimental run.
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2.4. Batch adsorption experiments
Fig. 1. The chemical structure of MB.
The montmorillonite clay (Mt) used in this study was supplied by the company Agetec Engenharia (Florianópolis, Santa Catarina — Brazil). Before the experiments the Mt was sieved to give a particle size range of 53–105 μm. The chemical composition was then determined by X-ray ﬂuorescence spectroscopy and the major components of the Mt were SiO2 (65.75%), Al2O3 (13.76%), Fe2O3 (6.80%), MgO (2.38%) and CaO (1.60%), with a loss on ignition of 7.64%.
2.2. Modiﬁed adsorbent Samples of MtMIO were prepared using iron oxide (IO) obtained by a method based on co-precipitation. For the iron oxide preparation, a solution of NH4OH (1.0 mol L−1) was slowly added to a solution comprising a mixture of FeCl3 (2.0 mol L− 1) and FeSO4 (1.0 mol L− 1) under mechanical stirring at 600 rpm, until the solution reached a pH of between 11 and 12. This procedure was adopted in order to obtain mainly magnetite instead of maghemite or hematite particles (Compéan-Jasso et al., 2008; Petcharoen and Sirivat, 2012). The magnetite particles (iron oxide — Fe3O4) were then removed from the solution and washed several times. In the next step, they were dispersed in 150 mL of deionized water, under agitation, with the addition of Mt, for 1 h. The resultant material (MtMIO) was vacuum-ﬁltered, washed with deionized water until pH of around 7, dried in an oven at 50 °C for 24 h, and then sieved to give a standard particle size in the range of 125–250 μm (120–60 mesh) (Petcharoen and Sirivat, 2012). The MtMIO was stored in a desiccator for characterization and batch adsorption experiments.
2.3. Characterization of adsorbent Morphological studies were carried out using ﬁeld-emission scanning electron microscopy (FE-SEM; JEOL microscope model JSM6701F) coupled with energy-dispersive X-ray spectroscopy (EDS) for elemental analysis. The samples were sprinkled over a piece of conducting carbon tape and coated with a thin gold layer to give them conductive properties. Fourier transformed infrared (FTIR) spectroscopy (ABB Bomem spectrometer, model 120) analysis was performed in the range of 4000–400 cm− 1. The samples were ground with KBr (analytical grade) and the mixture was pressed under vacuum conditions to produce disks for the FTIR analysis. Zeta potential measurements were carried out using a Malvern Zetasizer Nano ZS analyzer (Malvern Instrument Ltd., Malvern Worcestershire, UK) equipped with a multipurpose autotitrator (model MPT-2, Marvern Instruments, Worcestershire, UK). Titration was performed from pH 10.0 to 2.0 with 0.1 mol L−1 NaOH or 0.1 mol L−1 HCl solutions under constant stirring. The speciﬁc surface areas (SSA) of the samples were determined using an AUTOSORB-1 surface area analyzer, Quantachrome (Brunauer– Emmett–Teller — BET). Prior to the measurements, the samples were degassed at 573 K for 2 h, and then the nitrogen adsorption and desorption were measured at 77 K.
Batch adsorption experiments were carried out in a set of glass ﬂasks (80 mL) containing 50 mL of unbuffered MB solutions (pH 5.5–7.5) with different initial concentrations at three temperatures (308, 318 and 333 K) and with 0.1 g of adsorbent added to each solution. The pH measurements were monitored by using a laboratory pH meter model 827, Metrohm (USA). The MB solutions were kept under stirring using a mechanical stirrer built in our laboratory, with the speed calibrated by a manual tachometer at 450 rpm. The solutions were kept in thermal equilibrium by a thermostated system, with an outer circulating water bath (Nova Ética). Aliquots of 0.1 mL were withdrawn from the solutions over a period of 4 h, at predetermined time intervals during each run. These aliquots were then diluted, homogenized and centrifuged before the measurement of the supernatant absorbance. The absorbance was measured using a UV–vis spectrophotometer (Varian Cary 50 BIO spectrophotometer, USA) and a 1-cm path-length cell to monitor the absorbance at λmax = 664 nm, corresponding to the maximum absorbance. The MB concentrations at each time were estimated from a previously prepared calibration curve obtained by plotting the absorbance against the MB concentration of the solution. The batch adsorption was conducted for a contact period of 4 h, a time sufﬁcient for the concentration of the solutions to reach a constant value. The amount of MB adsorbed by MtMIO in each time interval, t, was calculated through the following mass balance equation: qt ¼ ðC 0 −C t Þ
where C0 is the initial liquid-phase MB concentration (mg L−1); Ct is the liquid-phase MB concentration at time t (mg L−1); V is the volume of the MB solution (ca. 0.05 L) and m is the mass of MtMIO used (ca. 0.1 g). 3. Results and discussions 3.1. Surface characterization Fig. 2 shows micrographs of the Mt, IO and MtMIO. As can be seen in Fig. 2a, the Mt particles have irregular surfaces with undulations, indicating a crystalline-type order and stacking of sheets of the material. In the case of the IO (Fig. 2b), in the synthesis of the magnetite nanoparticles aggregates with relatively homogeneous distribution were formed. The magnetite nanoparticles have an average size of around 25 nm. Fig. 2c shows aggregates of the IO homogeneously distributed on the MtMIO surface, suggesting magnetite nanoparticles supported on the Mt surface. From Table 1, it can be observed that there is a substantial increase in the amount of iron on the surface of the clay after the chemical modiﬁcation. The presence of iron on the MtMIO surface veriﬁes that a thin layer of IO formed on the Mt surface due to electrostatic interactions. Also, according to Table 1 there was a lack of the elements Na, Mg and Al and a decrease in the Si content in the MtMIO, which also indicates that IO was adsorbed by the Mt surface covering the elements previously observed on the unmodiﬁed Mt surface. The IO particles are positively charged while the surface of Mt is negatively charged resulting in electrostatic interactions. These interactions may lead to a good dispersion of IO on the surface of Mt, as found by other authors (Yuan et al., 2009). 3.2. Surface properties In this study, for IO, Mt and MtMIO the behavior of the surface charge was investigated in detail. Since the adsorption process is primarily driven by electrostatic forces, zeta potential ζ measurements provide key information to understanding the interaction between the adsorbent and the adsorbate. When the zeta potential is positive, the
L. Cottet et al. / Applied Clay Science 95 (2014) 25–31
Fig. 2. FE-SEM images. a) Mt; b) IO; c) MtMIO.
adsorbent surface favors the adsorption of negative ions; when it is negative, positive ions will be attracted by the adsorbent surface. The electrostatic interactions will thus be the primary driving force for the adsorption between the adsorbent and a cationic adsorbate (MB). In Fig. 3 it can be observed that the ζ values for IO are positive below pH 9.1, while negative ζ values were obtained for Mt across the whole pH range. After the Mt had been modiﬁed with IO, the MtMIO obtained had positive ζ values below pH 6.5. Similar results were obtained for IO by Nosrati et al. (2009). The negative ζ value for Mt is attributed to the isomorphic substitutions of lattice-constituent metal ions by cations of lower charge and the deprotonated silanol and aluminol groups at the pH-dependent edge faces (Nosrati et al., 2009; Wang et al., 2011). The values for the point of zero charge (PZC) for IO and MtMIO were around pH 9.1 and pH 6.5, respectively, while a PZC was not observed for Mt (Hu and Luo, 2010). At acidic pH IO has positive ζ values since Fe(II,III)OH+ 2 is the predominant surface species while at alkaline pH it has negative ζ values since Fe(II,III)O− is the predominant surface species. At around the PZC IO has neutral ζ values, the predominant surface species being Fe(II,III)OH (Lassalle et al., 2011). The observed shift in the ζ toward a positive value for the binary-system of Mt and IO is consistent with previous observations (Tombácz et al., 2004). Experiments were performed at different pH values to analyze the effect of surface charge on the MtMIO removal from the MB. At pH 2, the adsorption capacity of the MB onto MtMIO was 3.10 mg g− 1, while at pH 8 the adsorption capacity increased to 12.63 mg g− 1. These results suggest that the dominant mechanism of the adsorption may occur by electrostatic attraction. In acidic pH, repulsion between the MtMIO positive surface and the molecular cations of MB occurs, and in basic pHs the amount adsorbed increases due to the negative surface charge of the adsorbent, which favors the attraction of the dye molecules. Therefore, basic pH values are more effective for the removal of cationic dyes.
attributed mainly to Fe\O stretching vibrations. For IO, similar results have been found by Zhao et al. (2012). Fig. 4b shows a peak at 3634 cm−1 for OH, and the peaks at 914 cm−1 and 792 cm−1 can be collectively assigned to the stretching vibration of Si\O (Costa et al., 2013). In the MtMIO spectrum (Fig. 4c) the presence of IO is veriﬁed by the peak at around 630 cm− 1. Also, in Fig. 4c, absorption bands at 914 cm−1 and 792 cm− 1 reveal the presence of Si\O bonds. The band at 561 cm−1 is assigned to Fe\O and is typical of magnetite. In Fig. 4a and b, bands at 3450 cm−1, 1641 cm−1 and 1650 cm−1 can be ascribed to the vibration of water molecules (Yamaura et al., 2004; Zhao et al., 2012).
3.4. Adsorption studies 3.4.1. Adsorption kinetics In order to investigate the process involved in the adsorption of MB dye onto MtMIO, a pseudo-ﬁrst-order model, also known as the Lagergren kinetic model (Ho, 2004), and a pseudo-second-order (Ho and McKay, 2000) model were applied. These are the models most frequently Xused to describe the adsorption in solid–liquid systems. Eqs. (2) and (3) represent the linear forms of the pseudo-ﬁrst and pseudo-second-order kinetics, respectively, after integrating the differential equations.
Inðqe −qt Þ ¼ In qe −k1 t
3.3. FTIR spectroscopy analysis
Table 1 Chemical composition of the surfaces of IO, Mt and MtMIO. Element (%)
O Fe Na Mg Al Si SSA (m2 g−1) Isoelectric point
56.90 43.10 – – – – – 8.95
66.54 1.93 3.11 1.24 6.49 20.79 62a –
57.06 41.95 – – – 0,99 118.1 6.47
(Almeida et al., 2009).
Zeta Potential (mV)
The FTIR spectra for IO, Mt and MtMIO are shown in Fig. 4. In Fig. 4a it can be observed that there is a peak at 561 cm−1, which can be
pH Fig. 3. Zeta Potential. (●) IO; (■) Mt; (▲) MtMIO.
L. Cottet et al. / Applied Clay Science 95 (2014) 25–31
60 50 40 30 20 10 0
4000 35 30 25 20 15 10 5 0 -5 4000 40 35 30 25 20 15 10 5 0 -5 4000
Wavenumber cm-1 Fig. 4. FTIR spectra. a) IO; b) Mt; c) MtMIO.
t 1 1 ¼ þ t qt k2 qe 2 qe
where qe and qt are the amounts of MB adsorbed (mg g−1) at equilibrium and after contact time t (h) of adsorption, respectively; k1 (h−1) and k2 (g mg− 1 h− 1) are the pseudo-ﬁrst and pseudo-second-order rate constants, respectively. If these kinetic models are applicable, the plot of ln(qe − qt) versus t and t/qt versus t should give a linear relationship, allowing the determination of the k1 and k2 constants from the slope of the straight lines. The best model is chosen considering the linear regression correlation coefﬁcient value closest to 1.0000 (r1 — pseudoﬁrst order or r2 — pseudo-second order). The results for the pseudo-second and pseudo-ﬁrst-order kinetic models, based on Eqs. (2) and (3), are shown in Fig. 5. Table 2 shows the kinetic parameters obtained from Fig. 5. As can be seen, the high values for r2 indicate that the adsorption data conform well to pseudo0,14 0,12
t/qt (h g mg-1)
second-order kinetics. Similar results were obtained for all of the other initial dye concentrations and temperatures, suggesting that the adsorption process may be best described by the pseudo-second-order model for the adsorbance of MB onto MtMIO. In the literature, it is reported that the pseudo-second-order kinetics model ﬁtted several other adsorption systems, including the adsorption of methyl violet onto stevensite-rich clay from Morocco (Elass et al., 2011) and the adsorption of methylene blue onto a Mt/CoFe2O4 composite (Ai et al., 2011). 3.4.2. Adsorption isotherms The Langmuir and Freundlich models are commonly used to describe the removal of pollutants from water and efﬂuents (Freundlich, 1906; Langmuir, 1917; Verma et al., 2012; Yu et al., 2009). The assumptions of the Langmuir model include considering that the surface is uniform and there are no interactions among adsorbed molecules on the surface. Also, each molecule occupies a speciﬁc site, with only one molecule per site, and is unable to move over the surface, each having the same energy, until the formation of a monolayer (Barnes and Gentle, 2005). The Freundlich equation is based on an empirical relationship between the adsorption and adsorbate concentration (Barnes and Gentle, 2005). The linear form of the Langmuir equation is represented by Eq. (4):
Ce 1 C ¼ þ e qe Q K L Q
Table 2 Kinetic parameters of the pseudo-ﬁrst order and pseudo-second order models for the adsorption of MB onto MtMIO.
Tempo (h) Fig. 5. Fit of the experimental data to pseudo-second order model. Initial concentration from 100 to 500 mg L−1. Temperature 308 K. (■) 100 mg L−1; (●) 200 mg L−1; (Δ) 300 mg L−1; (○) 400 mg L−1; (□) 500 mg L−1.
C0 (mg L−1)
qe (mg g−1)
k1 102 (h−1)
k2 102 (h g mg−1)
100 200 300 400 500
30.49 49.26 60.52 65.30 66.94
86.73 79.82 71.79 59.44 75.88
0.8476 0.8987 0.9685 0.9565 0.9661
26.69 18.56 6.32 5.22 8.52
0.9999 0.9993 0.9996 0.9995 0.9993
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Table 4 Amount of MB adsorbed onto MtMIO at equilibrium.
308 318 333
Initial concentration of MB (mg L−1) 100
30.49 31.34 31.58
49.26 50.60 50.82
60.52 62.83 62.87
65.30 65.41 67.50
66.94 68.46 68.13
66.64 69.64 68.14
66.59 69.31 69.14
66.99 69.46 68.63
67.89 69.03 68.88
67.40 69.39 69.39
interactions. When the temperature was raised from 308 to 333, the pseudo-second-order rate constants also increased, implying a kinetically-controlled process (Almeida et al., 2009). A plot of ln k2 against 1/T provided a linear curve. Using the Arrhenius equation (Eq. (6)) the parameters of activation could be estimated from the slope and the intercept of the straight line passing through the points.
5 4 3 2 1 0
InK 2 ¼ InA−
Fig. 6. Linearized Langmuir isotherm. (□) 308 K, (Δ) 318 K and (●) 333 K.
where KL (L mg−1) is the Langmuir constant related to the equilibrium of adsorption, Ce (mg L−1) is the equilibrium concentration and Q (mg g−1) is the maximum amount of MB for complete monolayer coverage. The logarithmic form of the Freundlich isotherm is shown in Eq. (5): 1 Inqe ¼ InK F þ InC e n
where KF (L g−1) and n are Freundlich constants related to the capacity and intensity of adsorption, respectively (Wang et al., 2011). Fig. 6 shows the linear Langmuir isotherm applied to the data at 308, 318 and 333 K. The values for KL and Q were obtained from the linear plot of Ce/qe versus Ce and they are given in Table 3 together with the correlation coefﬁcients. Parameters for the linear Freundlich isotherm are also shown in Table 3, where the high values for the correlation coefﬁcient indicated that the experimental data were well correlated by the Langmuir model. The Q values are consistent with the experimental data. 3.4.3. Effect of temperature Changing the temperature can have two different effects on the adsorption process: before equilibrium is reached it can alter the dye adsorption rate while after the equilibrium has been reached it can affect the adsorption equilibrium of the adsorbent for a particular adsorbate (Almeida et al., 2009). Table 4 shows an increase in the amount of MB adsorbed, after equilibrium was reached, when the temperature was raised from 308 to 333 K for most of the initial MB concentrations, indicating an endothermic adsorption process (Ahmad and Kumar, 2010). For the higher concentrations of MB, i.e., C0 values of 500 to 1000 mg L−1, at 333 K the amount of MB adsorbed decreased. The decrease in the amount of MB adsorbed at the highest temperature is related to the change in the equilibrium toward desorption, with the MB molecules leaving the MtMIO surface due to weakening of the MB–MtMIO Table 3 Langmuir and Freundlich isotherm parameters.
Langmuir isotherm KL (L mg−1) Q (mg g−1) r Freundlich isotherm KF (L mg−1) 1/n r
0.0539 69.11 0.9998
0.0552 71.12 0.9998
0.0628 70.57 0.9999
where Ea is the activation energy (kJ mol−1), k2 is the pseudo-secondorder rate constant of adsorption, A is the pre-exponential factor, R is the gas constant (8.314 J K−1 mol−1) and T is the temperature (K). The magnitude of the activation energy can indicate whether or not the adsorption mechanism is physisorption (5 at 40 kJ mol−1) or chemisorption (40 at 800 kJ mol−1). The value of 19.32 kJ mol−1 for the activation energy indicates that the adsorption has a potential barrier corresponding to a physisorption process (Anirudhan and Suchithra, 2010). 3.4.4. Thermodynamic parameters Thermodynamic parameters, such as the change in enthalpy (ΔH°), entropy (ΔS°) or Gibbs free energy (ΔG°), are important as indicators for the practical application of a process (Chen et al., 2011a,b). The values of these parameters provide information regarding the spontaneity of a system. The ΔH° and ΔS° values can be determined from the slope and the intercept of the van't Hoff equation (Eq. (7)) while ΔG° can be determined from Eq. (8), as follows. Δads H B Δads SB þ RT R
Δads GB ¼ Δads H B−TΔads SB
InK L ¼ −
In Eq. (7), KL is the Langmuir constant, T is the temperature (K) and R is the ideal gas constant (8314 J K−1 mol−1). In this study, the experiments were carried out at three temperatures (308 K, 318 K and 333 K), with ten systems containing different concentrations of MB dye, generating three KL values to be inserted into Eq. (7) in order to plot ln KL versus T−1. As can be seen in Table 5 the ΔH° value is positive conﬁrming the endothermic nature of the adsorption process (Errais et al., 2011; Gupta et al., 2006). The negative value of ΔS° indicates an increase in the molecular organization of the adsorption process, and a decrease in the randomness at the solid/liquid interface (Hu et al., 2010). The positive ΔG° value indicates that the process is not spontaneous with increasing temperature (Ozcan and Ozcan, 2004). The ΔG° value increases on raising the temperature from 308 K to 318 K and Table 5 Thermodynamic parameters associated with the adsorption process. Adsorbent Adsorbate Adsorption thermodynamic parameters ΔS°ads/J K−1 mol−1 ΔH°ads/kJ mol−1 ΔG°ads/kJ mol−1 Ref.
3.17 0.396 0.9359
4.76 0.3494 0.9605
14.13 0.1767 0.6428
21.87 (20 °C)
71.80 (20 °C)
This work 28
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Table 6 Comparison of MB adsorption capacity of MtMIO with other reported low-cost adsorbents. Adsorbents
Adsorption capacity (mg g−1)
Bentonite 10% Ball Clay Palygorskyte Activated clay Mt MtMIO
303 303 303 298 308 308
95.15 25.01 48.39 91.23 289.12 69.11
Liu et al. (2014) Auta and Hameed (2012) Chen et al. (2011a,b) Weng and Pan (2007) Almeida et al. (2009) This work
then to 333 K, since the ΔS° value is negative, while ΔH° is also positive. This means that the negative value of ΔS° has a strong inﬂuence on the ΔG° values (considering ΔH° is positive). If the ΔS° value is positive, the tendency would change and the process would become more spontaneous on raising the temperature. 3.4.5. Comparison of results with others clay adsorbents Table 6 summarizes the comparison of the maximum monolayer adsorption capacity of MB of various adsorbents including MtMIO. From Table 6, it is observed that the adsorption capacity of MB onto MtMIO is average among the adsorbents listed. Mt takes advantage of many other adsorbents due to its low cost (about $350 per ton) and wide availability in many countries, mainly in Brazil. Magnetic materials, such as MtMIO used in this work, they have advantage over many other unmodiﬁed adsorbents, because their magnetic properties facilitate their easy separation from the medium by applying an external magnetic ﬁeld. Therefore, the MtMIO is a promising material for the MB removal from aqueous solutions, as well as other cationic dyes. 4. Conclusions The removal of MB from water through adsorption onto montmorillonite clay modiﬁed with iron oxide was studied to investigate the equilibrium, kinetics and thermodynamics of adsorption. The characterization of MtMIO revealed that IO particles are present on the surface of montmorillonite clay. The MB adsorption onto MtMIO occurred through an endothermic and non-spontaneous process, which was described by the Langmuir model, indicating the formation of a monolayer of the dye. The activation energy of 19.32 kJ mol−1 indicates that the potential barrier is not very high and the adsorption process is of a physical nature. The adsorption of MB onto MtMIO was rapid, and the kinetics was better described by the pseudo-second than by the pseudo-ﬁrstorder model. The results indicate that the MtMIO shows promise for application in the removal of cationic dyes from industrial efﬂuents. Acknowledgments The authors are grateful to the Brazilian government funding agencies CAPES, CNPq and Fundação Araucária, Universidade Estadual do CentroOeste and Universidade Federal de Santa Catarina (Laboratório Central de Microscopia Eletrônica — LCME-UFSC) for supporting this research. References Ahmad, R., Kumar, R., 2010. Adsorption studies of hazardous malachite green onto treated ginger waste. J. Environ. Manag. 91, 1032–1038 (http://www.sciencedirect.com/science/article/pii/S0301479709004253). Ahmaruzzaman, M., Gayatri, S.L., 2010. Batch adsorption of 4-nitrophenol by acid activated jute stick char: equilibrium, kinetic and thermodynamic studies. Chem. Eng. J. 158, 173–180 (http://www.sciencedirect.com/science/article/pii/S1385894709008833). Ai, L., Zhou, Y., Jiang, M., 2011. Removal of methylene blue from aqueous solution by montmorillonite/CoFe2O4 composite with magnetic separation performance. Desalination 266, 72–77 (http://www.sciencedirect.com/science/article/pii/S0011916410005692). Almeida, C.A.P., Debacher, N.A., Downs, A.J., Cottet, L., Mello, C.A.D., 2009. Removal of methylene blue from colored efﬂuents by adsorption on montmorillonite clay. J. Colloid Interface Sci. 332, 46–53 (http://www.sciencedirect.com/science/article/pii/ S0021979708016123).
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