activated montmorillonite nanocomposite

activated montmorillonite nanocomposite

CLAY-03482; No of Pages 9 Applied Clay Science xxx (2015) xxx–xxx Contents lists available at ScienceDirect Applied Clay Science journal homepage: w...

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CLAY-03482; No of Pages 9 Applied Clay Science xxx (2015) xxx–xxx

Contents lists available at ScienceDirect

Applied Clay Science journal homepage:

Adsorption of methylene blue onto Fe3O4/activated montmorillonite nanocomposite Jiali Chang a, Jianchao Ma b,1, Qingliang Ma c, Duoduo Zhang a, Nannan Qiao a, Mengxiao Hu a, Hongzhu Ma a,⁎ a b c

School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an 710119, Shaanxi, PR China College of Mining Technology, Taiyuan University of Technology, Taiyuan 030024, Shanxi, PR China Key Laboratory of Coal Science and Technology, Taiyuan University of Technology, Ministry of Education and Shanxi Province, Taiyuan 030024, Shanxi, PR China

a r t i c l e

i n f o

Article history: Received 22 November 2014 Received in revised form 26 June 2015 Accepted 28 June 2015 Available online xxxx Keywords: Fe3O4/Mt nanocomposite Methylene blue Adsorption Kinetics Isotherm

a b s t r a c t A Fe3O4/activated montmorillonite (Fe3O4/Mt) nanocomposite was prepared by a coprecipitation method. The morphology and structure of the Fe3O4/Mt nanocomposite are explored using X-ray powder diffraction, Fourier transform infrared spectroscopy, N2 adsorption–desorption isotherm and scanning electron microscopy. The adsorption capacity of Fe3O4/Mt for methylene blue (MB) was evaluated. Within 25 min, the Fe3O4/Mt nanocomposite (0.5 g) removed 99.47% of the MB from a 120 mg L−1 solution at 293 K and at an original solution pH of 7.37. The experimental adsorption data followed a pseudo-second-order kinetic model and Langmuir isotherm. The reusability of the Fe3O4/Mt was tested and still over 83.73% color removal of MB was obtained after five cycles. The Fe3O4/Mt nanocomposite has good stability and reusability. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Dyes and pigments are the main organic pollutant compounds in wastewater released from various industries, especially the textile dyeing and processing factories. The presence of dyes in the effluents, even at low concentrations, is very harmful to human beings and microorganisms (Weng and Pan, 2007). Various technologies developed for the removal of dye contaminants from wastewater, include adsorption, coagulation/flocculation, advanced oxidation processes, ozonation, membrane filtration and biological treatment (Fatimah et al., 2011; Zhou et al., 2012; Peng et al., 2013; Cottet et al., 2014). Due to the high efficiency, cost effectiveness, and simple operation, adsorption of dye contaminants from wastewater has received a great deal of attention (Sivakumar and Palanisamy, 2009). Some adsorbents, such as activated carbon, clays, zeolites and other porous materials, are widely investigated (Nandi et al., 2009; Houndonougbo et al., 2013). Especially activated carbon is frequently used since it is effective for the removal of dissolved organic matter. However, difficulties in regenerating it and its higher processing cost limit its applications (Lin et al., 2014). Clay minerals are ubiquitous in nature (Zhang et al., 2010; Bergaya and Lagaly, 2013). Most clay minerals are composed of tetrahedral and

⁎ Corresponding author. E-mail address: [email protected] (H. Ma). The author contributed equally to this work. SNNU and TYUT contributed equally to this work. 1

octahedral sheets, which are normally classified into two groups with structural arrangements in 1:1 or 2:1 layers. For example, montmorillonites are a class of 2:1 clay minerals consisting of one Al3+ octahedral sheet sandwiched between two Si4+ tetrahedral sheets (Bailey et al., 1980; Bergaya and Lagaly, 2013). The isomorphous substitution of Al3+ for Si4+ in the tetrahedral layer and the substitution of Mg2+ or Zn2+ for Al3+ in the octahedral layer results in permanent negative charge on the clay mineral surface (Zhou, 2011a). The layers are held together by van der Waals forces (Lee and Tiwari, 2012). Due to these weak forces and the charge deficits in the structure, water can easily penetrate into the layers and some inorganic cations (Na+, Ca2+ ions, etc.) balance the negative charge (Kaya and Oren, 2005; Zhou et al., 2011b). With lower charge density types of surfaces, clay minerals can play an important role in the attraction of neutral organic contaminants, thus, there has been an interest in utilizing clay minerals such as montmorillonite, kaolinite, illite, and saponite as adsorbents (DiazGomez-Trevino et al., 2013; Zhou and Keeling, 2013). However, because of the hydrophilicity of their interlayer surfaces, the adsorption capacity of natural clay minerals for hydrophobic organic compounds is limited. So, surfactant-modified clay minerals have been extensively studied to improve the hydrophobicity of their surfaces (Tong et al., 2010; Wu et al., 2013; Zaghouane-Boudiaf et al., 2014). It has also been demonstrated that Fe3+ and Cu2+-modified montmorillonite interacts strongly with organic matter (Polubesova et al., 2008). Magnetic separation is a quick and effective technique for separating magnetic particles (Ai et al., 2011a). Recently, a magnetic [email protected] graphene composite was developed and then utilized for dye removal 0169-1317/© 2015 Elsevier B.V. All rights reserved.

Please cite this article as: Chang, J., et al., Adsorption of methylene blue onto Fe3O4/activated montmorillonite nanocomposite, Appl. Clay Sci. (2015),


J. Chang et al. / Applied Clay Science xxx (2015) xxx–xxx

(Yao et al., 2012). Fe3O4 coated with different amounts of soluble biobased materials were prepared for the removal of the cationic azo dye crystal violet (Magnacca et al., 2014). Magnetic γ-Fe2O3 carbon composite was synthesized through the thermal pyrolysis of hydrochar and used for pollutant removal (Zhu et al., 2014). Thus, combining clay materials with magnetic property to fabricate magnetic adsorbents is a promising method to remove dyes from wastewater. The main objective of this research was to investigate the ability of a magnetite Fe3O4/activated montmorillonite (Fe3O4/Mt) nanocomposite to remove the cationic dye methylene blue (MB) from aqueous solutions. The Fe3O4/Mt nanocomposite was synthesized by a coprecipitation method and characterized by X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Brunauer–Emmett–Teller (BET) specific surface area and scanning electron microscopy (SEM). The effects of temperature, initial pH, contact time, adsorbent dosage and initial MB concentration on the MB removal efficiencies and the lifespan of the Fe3O4/Mt nanocomposite were investigated. Finally, the physical and chemical characteristics of the Fe3O4/Mt nanocomposite in batch adsorption experiments were examined in order to gain a deeper understanding of the adsorption process.

2. Materials and methods 2.1. Materials Activated montmorillonite (Huangshan, Baiyue activated montmorillonite Co. Ltd, China), methylene blue (Zhengzhou, Paini Chemical Reagent Co. Ltd, China), 30% ammonia solution, HCl, HNO3, NaNO3, FeCl3·6H2O, FeCl2·4H2O, ethanol and all other chemicals were reagent grade and purchased from Sinopharm Chemical Reagent Co. Ltd. Hexaammine cobalt chloride was prepared from ammonium chloride and cobalt chloride hexahydrate (Bjerrum and McReynolds, 1946). The Fe3O4/Mt nanocomposite was synthesized by a coprecipitation of FeCl3·6H2O and FeCl2·4H2O with ammonia solution in the presence of Mt. In detail, 30% ammonia solution was added dropwise to the iron solution (nFe2+: nFe3+ = 1:1.8) at 363 K with constant stirring to adjust the pH to 9–10 so that Fe2+/Fe3+ ions precipitated. The Mt powder was then added and the solution was stirred for 4 h. Then the solution was cooled to room temperature, a dark black solution formed which was then filtered. The precipitate was washed with distilled water/ethanol (1/1) and dried in vacuum at 343 K, to obtain the Fe3O4/Mt nanocomposite. The weight ratio of Fe3O4 to Mt was 1:3.

Fig. 1. X-ray diffraction patterns of (a) Mt, (b) Fe3O4 and (c) Fe3O4/Mt.

2.2. Characterization The Fe3O4/Mt nanocomposite was analyzed by XRD using a Rigaku apparatus with Cu Kα radiation. The morphologies were identified using JCPDS data files. SEM measurements were collected using a Philips Quanta 200 scanning electron microscope at 20 kV. The BET specific surface areas and pore structures were obtained from nitrogen adsorption data at 77 K using a Micromeritics ASAP2020 system. FTIR spectra were taken on a Tensor 27 (Bruker, Germany) instrument in the range of 4000–400 cm − 1 . The cation-exchange capacities (CEC) and zero charge points (pHzpc) of the nanocomposite were determined using a titrimetric method, according to the literature (Davranche et al., 2003). 2.3. Adsorption isotherms Adsorption experiments were carried out in a batch equilibrium mode. Typically, 0.5 g of Fe3O4/Mt nanocomposite and 200 mL of dye solution (120 mg L−1 MB) were added to a 250 mL glass flask at 293 K under an agitation speed of 200 rpm. At predetermined time intervals, the Fe3O4/Mt nanocomposite was removed from the solution using a magnet and MB concentration in the solution was then determined by measuring the absorbance of the solution at 665 nm (λmax for MB), using a UV–vis spectrophotometer (UVT6, Beijing Purkinje General Instrument Co. Ltd, China). The equilibrium amount of adsorption (qe), the amount of adsorption (qt) and the color removal were calculated based on the following equations:

qe ¼

ðC 0 −C e ÞV W


qt ¼

ðC 0 −C t ÞV W


color removalð%Þ ¼

ðC 0 −C t Þ  100% C0


where C0 (mg L−1), Ce (mg L−1) and Ct (mg L−1) are MB concentrations at the initial, equilibrium time and time t (min), V (L) is the volume of the solution, and W (g) is the weight of the Fe3O4/Mt nanocomposite. All the experimental data were averages of duplicate or triplicate determinations.

Fig. 2. FT-IR spectra of Fe3O4, Mt and Fe3O4/Mt.

Please cite this article as: Chang, J., et al., Adsorption of methylene blue onto Fe3O4/activated montmorillonite nanocomposite, Appl. Clay Sci. (2015),

J. Chang et al. / Applied Clay Science xxx (2015) xxx–xxx


Fig. 3. Nitrogen adsorption–desorption isotherms and pore size distributions of (a) Fe3O4, (b) Mt and (c) Fe3O4/Mt.

2.4. Adsorption isotherm models

Table 1 Porosity parameters of Fe3O4, Mt and Fe3O4/Mt. Samples

BET specific surface area (m2 g−1)

Total pore volume (cm3 g−1)

Average pore size (nm)

Fe3O4 Mt Fe3O4/Mt

91.92 187.30 147.92

0.2967 0.2261 0.3169

22.50 47.41 46.39

To determine the interactive behavior between the adsorbent and the adsorbate, adsorption isotherm models, including the Langmuir, Freundlich and Temkin models were tested. The nonlinear mathematical expression of the Langmuir model (Langmuir, 1916) can be expressed as:

qe ¼

qm K L C e 1 þ K L Ce


Fig. 4. SEM images of (a) Fe3O4, (b) Mt and (c) Fe3O4/Mt.

Please cite this article as: Chang, J., et al., Adsorption of methylene blue onto Fe3O4/activated montmorillonite nanocomposite, Appl. Clay Sci. (2015),


J. Chang et al. / Applied Clay Science xxx (2015) xxx–xxx

where Ce is the equilibrium concentration of the adsorbate (mg L− 1), qe (mg g− 1) is the amount of dye adsorbed by the nanocomposite at equilibrium, qm (mg g−1) signifies the maximum adsorption capacity and the Langmuir constant KL (L mg−1) relates to the free energy and affinity of the adsorption. The feasibility of the adsorption process is determined using the separation factor (RL) (Fan et al., 2012), which is defined by: 1 RL ¼ 1 þ K L C0


The value of RL indicates the category of the isotherm: unfavorable (RL N 1), linear (RL = 1), irreversible (RL = 0) or favorable (0 b RL b 1). The nonlinear form of the Freundlich model can be expressed as: qe ¼ K F C e1=n


where KF (mg g−1) represents the adsorption capacity and n represents the degree of dependence of the adsorption at equilibrium concentration (Freundlich, 1906).

Fig. 5. Effect of parameters on MB removal by the Fe3O4/Mt nanocomposite. (a) pH, (b) contact time, (c) temperature, (d) adsorbent dosage and (e) initial dye concentration.

Please cite this article as: Chang, J., et al., Adsorption of methylene blue onto Fe3O4/activated montmorillonite nanocomposite, Appl. Clay Sci. (2015),

J. Chang et al. / Applied Clay Science xxx (2015) xxx–xxx


2.5. Adsorption kinetics Kinetic models can predict the adsorption rate (k) and equilibrium adsorption capacity (qe), which play an important role in the adsorption mechanism. Two kinetic models were used to investigate the adsorption kinetics, the Lagergren pseudo-first-order model and the pseudo-secondorder model. The former is based on the assumption that the ratelimiting step is a physical adsorption process that occurs via van der Waals forces, π–π interactions, and hydrogen bonding between the adsorbent and the adsorbate. The latter is based on the assumption that the rate-limiting step is a chemical adsorption that occurs via the sharing or exchange of electrons between the adsorbent and the adsorbate (Li et al., 2013). The Lagergren equation describes the adsorption of solid–liquid phases and can be expressed as follows: logðqe −qt Þ ¼ log qe −


where qe and qt are the amounts of MB (mg g−1) adsorbed on the adsorbent at equilibrium and at a given time t (min), respectively, and k1 is the rate constant of adsorption (g mg−1 min−1) (Lagergren, 1898). The pseudo-second-order rate model can be expressed as:

Fig. 6. Recycling of Fe3O4/Mt.

The nonlinear Temkin isotherm is represented as: qe ¼ A þ B lnC e

k1 t 2:303


where B = RT/β, is related to the heat of adsorption β (J mol−1), A is the Temkin constant and Ce is the equilibrium concentration of the adsorbate (mg L−1) (Temkin and Pyzhev, 1940).

t 1 t ¼ þ qt k2 q2e qe


where k2 is the rate constant for the pseudo-second-order adsorption process (Ho and McKay, 1999).

Fig. 7. Modeling of MB adsorption kinetics using pseudo-first-order (a–b) and pseudo-second-order (c–d) models.

Please cite this article as: Chang, J., et al., Adsorption of methylene blue onto Fe3O4/activated montmorillonite nanocomposite, Appl. Clay Sci. (2015),


J. Chang et al. / Applied Clay Science xxx (2015) xxx–xxx

Table 2 Kinetic parameters of the pseudo-first-order and pseudo-second-order models for the adsorption of MB onto Fe3O4/Mt. Pseudo-first-order model

Pseudo-second-order model

T (K)

co (mg L−1)

qe,exp (mg g−1)

q1e (mg g−1)

k1 (min−1)



q2e (mg g−1)

k2 (L mg−1 min−1)




100 150 200 250 120 120 120

39.96 59.96 79.95 99.85 47.97 47.96 47.97

0.0388 0.0948 1.9191 7.3672 0.0508 0.0304 0.0550

0.0859 0.1113 0.1599 0.1843 0.1576 0.1293 0.1461

0.8139 0.8668 0.7754 0.9644 0.9436 0.9564 0.9649

0.1645 0.1747 0.3449 0.1420 0.0820 0.0340 0.0850

40.00 59.88 80.00 100.00 47.98 47.96 47.96

11.1607 2.0738 0.2418 0.0902 1.2884 1.6756 4.2333

0.9999 0.9999 0.9999 0.9999 0.9999 0.9999 0.9999

0.0000 0.0003 0.0013 0.0006 0.0002 0.0000 0.0000

293 303 313

Several stages are involved in the adsorption process, including the diffusion of the dye from the aqueous phase to the adsorbent surface and then into the interior of the adsorbent pores. The rate-limiting step of the adsorption can be determined using an intraparticle diffusion model, which was developed by Weber and Morris (Weber and Morris, 1963), the most commonly used intraparticle diffusion model and represented as: qt ¼ kid t 0:5 þ C


where kid is the rate constant (mg g−1 min−0.5) and C (mg g−1) is the boundary layer thickness. 3. Results and discussion 3.1. Characterization The raw material is mainly composed of montmorillonite (JCPDS file no. 29-1498) and quartz (JCPDS file no.46-1045, Fig. 1a). The reflection of Fe3 O4 (Fig. 1b) can be indexed as cubic phase Fe3O 4 (JCPDS file no.19-0629) (Zhang et al., 2008). In the XRD pattern of the Fe3O4/Mt nanocomposite, the reflections corresponding to Mt are present at 2θ = 19.891°, 20.859°, 26.639°, 39.464°, 50.138° and 54.231°, but with lower intensities than those in the pure Mt. The reflections at 17.170°, 35.022° and 36.543° are not seen and no significant shifts in the Fe3O4 reflections are observed. More importantly, loading Fe3O4 onto the Mt changed the d001-value from 1.54 nm for the parent Mt to 1.23 nm, indicating that Mg2 + and Ca2 + metal ions were replaced by hydrogen ions which intercalated into the silicate layers and that the interlayer water was lost during the synthesis procedure (Lee and Tiwari, 2012). In addition, the ferriferrous oxide particles may be dispersed on the external surfaces of Mt instead of being in the interlayer spaces. FT-IR spectroscopy was employed to further confirm the structure of the Fe3O4/Mt nanocomposite (Fig. 2). The absorption bands at 3430,

2370 and 1640 cm− 1 can be ascribed to the vibrations of the water molecules (Wu et al., 2014a). The three absorption bands at 1030, 791 and 528 cm−1 are Si\\O stretching vibrations (Hajjaji and Arfaoui, 2009; Wu et al., 2014b). The characteristic bands at 573 and 663 cm−1 are due to Fe\\O stretching vibrations (Magnacca et al., 2014). The intensity of the Si\\O bands in the Fe3O4/Mt nanocomposite is weaker than those in Mt, indicating that the Fe\\O bonds may interact with the Si\\O bonds on the surface of Mt. Fe3O4 is probably bonded to the surface of Mt during the synthesis procedure. The average pore diameter of Mt and Fe3O4/Mt suggests that both Mt and the Fe3O4/Mt nanocomposites comprised macropores and mesopores (Randelovic et al., 2014) (Fig. 3). The type IV isotherms with apparent hysteresis loops in the P/Po range of 0.4–1.0 are observed, indicating that mesopores are present (Liu et al., 2014). The total pore volumes in the Fe3O4/Mt nanocomposite are bigger than those in the Mt (Table 1), which is beneficial for adsorption efficiency. The micrographs of Mt and Fe3O4/Mt are very similar (Fig. 4), the Fe3O4 particles are well dispersed on the surface of Mt with no agglomeration (Zhao et al., 2014). 3.2. Optimization of the adsorption conditions A high MB removal efficiency was achieved for the entire pH range 3–11 (Fig. 5a). This can be explained by the electrostatic interactions between the cationic dye MB and the negatively charged surface of the Fe3O4/Mt nanocomposite (Da Fonseca et al., 2006). A little lower MB removal was obtained at acidic pH values than at alkaline pH values. This is caused by the fact that excess H+ ions compete with the dye cations for the available adsorption sites at acidic pH values (Bestani et al., 2008). Moreover, the adsorption of MB onto the Fe3O4/Mt nanocomposite was most favorable at pH N pHpzc (7.56), which is consistent with the results reported (Salleh et al., 2011). Since color removal was not significantly affected by pH, the original solution pH (7.37) was selected for all subsequent MB adsorption experiments.

Fig. 8. Modeling of MB adsorption kinetics using intraparticle diffusion model.

Please cite this article as: Chang, J., et al., Adsorption of methylene blue onto Fe3O4/activated montmorillonite nanocomposite, Appl. Clay Sci. (2015),

J. Chang et al. / Applied Clay Science xxx (2015) xxx–xxx Table 3 Kinetic parameters of the intraparticle diffusion model for the adsorption of MB onto Fe3O4/Mt. T (K)

c0 (mg L−1)

C (mg g−1)

Kid (mg g−1 min−0.5)




100 150 200 250 120 120 120

39.91 59.84 77.04 94.17 47.87 47.76 47.70

0.0090 0.0228 0.5657 1.0722 0.0173 0.0360 0.0499

0.7593 0.6147 0.5228 0.6696 0.8772 0.7529 0.6439

0.0089 0.8613 0.8133 1.1635 0.0100 0.0320 0.0573

293 303 313

In the first 10 min the color removal increased sharply and thereafter gradually slowed down as the equilibrium approached in about 25 min (Fig. 5b). Initially there was a very rapid uptake of MB molecules on the external surfaces which was then followed by slow intraparticle diffusion into the interior of the Fe3O4/Mt nanocomposite (Yagub et al., 2012). After 25 min, a dynamic equilibrium was reached between the adsorption and desorption of the dye. A contact time of 25 min was used for all further experiments. With the temperature increased from 293 to 303 K, the color removal decreased from 99.47% to 99.05% (Fig. 5c). The slight drop of adsorption capacity at higher temperature may be attributed to the weakening of the physical interactions between the dye and the adsorbent (Cottet et al., 2014). Since there is only a slight change in the color removal with temperature, it is most practicable to adsorb the dye at room temperature. An initial rapid increase in color removal from 65.23% to 99.41% with increasing adsorbent dosage from 0.625 to 2.5 g L− 1 was detected


(Fig. 5d), maybe due to the availability of more adsorption sites. As the dosage of adsorbent increased further (N 2.5 g L−1), the color removal did not change much, indicating the splitting effect of flux (concentration gradient) between adsorbent and adsorbate (Zhang et al., 2012). In order to make full use of the adsorbent, 2.5 g L−1 was considered to be the optimum dose. The MB removal declined from 99.28% to 79.92% with the initial concentration of MB increased from 120 to 1000 mg L− 1 (Fig. 5e). This trend might be explained by the fact that at lower MB concentrations, a maximum amount of dye molecules can adsorb onto the Fe3O4/Mt nanocomposite, which ensured a higher MB removal. In contrast, at higher dye concentrations, the active adsorption sites became saturated and lower removal efficiency was observed (Toor and Jin, 2012). Based on all these results, the optimized conditions for the maximum removal of MB were: the solution's original pH (7.37); contact time, 25 min; temperature, 293 K; adsorbent dosage, 2.5 g L−1 Fe3O4/Mt; initial dye concentration, 120 mg L−1. The color removal of MB by Fe3O4/Mt was still over 83.73% after five times (Fig. 6). Furthermore, the Fe3O4/Mt nanocomposite maintained good magnetic separation effect in an external magnetic field (Fig. 6 inset), indicating that Fe3O4/Mt has relatively good stability and reusability. 3.3. Adsorption kinetics The adsorption kinetics were investigated using two kinetic models (Fig. 7). Table 2 summarized the pseudo-first-order and pseudo-secondorder kinetic constants that were obtained from a linear regression analysis. For the pseudo-second-order kinetics model, high correlation

Fig. 9. Various isotherm models for the adsorption of MB on Mt and Fe3O4/Mt nanocomposite.

Please cite this article as: Chang, J., et al., Adsorption of methylene blue onto Fe3O4/activated montmorillonite nanocomposite, Appl. Clay Sci. (2015),


J. Chang et al. / Applied Clay Science xxx (2015) xxx–xxx

Table 4 Isotherm parameters for the adsorption of MB onto Fe3O4/Mt at 293 K. Langmuir




qmax (mg g−1)


KL (L mg−1)



KF (mg g−1)








Mt Fe3O4/Mt

64.43 106.38

0.0054–0.0430 0.0025–0.0208

0.185 0.393

0.9953 0.9580

0.8336 2.2328

14.24 31.69

2.63 3.01

0.9878 0.9358

0.9992 3.4938

13.13 35.70

10.66 15.51

0.9324 0.8826

4.0154 8.1453

coefficients (R2 N 0.999) and low root mean square errors (RMS b 0.0015) were obtained for all dye concentrations and temperatures. Moreover, the theoretical qe values agreed well with the experimental data, indicating that this adsorption process conformed to pseudo-second-order kinetics. The qe values increased with an increase in the initial dye concentration, while the rate constants (k2) decreased. This might be attributed to less competition for the sorption surface sites at lower concentrations. At higher concentrations, the competition for the surface active sites was high, and consequently lower k2 values were obtained. The rate constants (k2) increased with temperature, indicating that the reaction rate was accelerated at higher temperatures. The Weber intraparticle diffusion model was employed to identify the steps that occurred during the adsorption process. The linear plots at various initial concentrations did not pass through the origin (Fig. 8), suggesting that intraparticle diffusion is not the only rate-controlling step (Huang et al., 2011). The values of the intercept, C, are an indication of the thickness of the boundary layer, which play a significant role in the uptake of the dye (Eskandarian et al., 2014). Generally, a larger C value represents a greater boundary layer effect (Dogan et al., 2007). The C values (39.91–94.17 mg g−1) increased with initial dye concentration (100–250 mg L−1,Table 3) increasing, demonstrating that an increase in the initial dye concentration promoted a boundary layer diffusion effect. Thus it can be concluded that surface adsorption (pseudosecond-order) and intraparticle diffusion took place concurrently (Ghorai et al., 2013). 3.4. Adsorption isotherms The regression coefficient (R2) obtained from the Langmuir isotherm is higher than those from the Freundlich and Temkin isotherms (Fig. 9 and Table 4), and the relative value of RMS is lower. This indicates that the Langmuir isotherm is a better fit and that adsorption occurred in the interlayer pores (Bouzid et al., 2015). The RL values are 0.0054–0.043 and 0.0025–0.021 for Mt and Fe3O4/Mt, respectively, since the values are 0 b RL b 1, this indicates that the adsorptions of MB on Mt and Fe3O4/Mt are favorable adsorption processes. Based on Langmuir isotherm, the maximum MB adsorption amount qm is 64.43 mg g−1 for Mt and 106.38 mg g−1 for Fe3O4/Mt. Modifying the Mt with Fe3O4 was beneficial for the adsorption of MB. A comparison with other reported adsorbents (Table 5) shows that the qm value for the Fe3O4/Mt nanocomposite is one of the highest, suggesting that the Fe3O4/Mt nanocomposite is a good candidate for dye removal from aqueous solutions.

Table 5 Comparison of adsorption capacity (qm, mg g−1) of MB on various adsorbents. Material Chitosan/bentonite Raw ball clay Phoenix tree leaves Montmorillonite modified with iron oxide Mt/CoFe2O4 composite Spent activated clay Mt Fe3O4/Mt

Maximum adsorption capacity qm (mg g−1) 95.24 34.65 80.90 69.11 97.75 127.50 64.43 106.38

References Bulut and Karaer (2015) Auta and Hameed (2012) Han et al. (2007) Cottet et al. (2014) Ai et al. (2011b) Weng and Pan (2007) This study This study

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