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Adsorption and desorption of antimony acetate on sodium montmorillonite
Journal of Colloid and Interface Science 345 (2010) 154–159
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Adsorption and desorption of antimony acetate on sodium montmorillonite Zhenlu Zhao, Xiaoqun Wang *, Chuan Zhao, Xiaoguang Zhu, Shanyi Du School of Material Science and Engineering, Beihang University, Beijing 100191, China
a r t i c l e
i n f o
Article history: Received 23 August 2009 Accepted 15 January 2010 Available online 6 February 2010 Keywords: Antimony acetate Montmorillonite Adsorption Desorption
a b s t r a c t The adsorption of antimony acetate (Sb(OAc)3) on sodium montmorillonite (Na-MMT) was studied at five different initial concentrations, and data from the adsorption isotherm were modeled using the Langmuir, Freundlich and D–R isotherm equations. The kinetics of adsorption was also discussed using three kinetic models: the pseudo-first-order, the pseudo-second-order and the intraparticle diffusion model. The rate constants of pseudo-first-order, pseudo-second-order and intraparticle diffusion kinetics, and the amount of Sb(OAc)3 adsorbed at equilibrium were determined. Moreover, the desorption of Sb(OAc)3 from several kinds of Sb-MMT (Na-MMT was intercalated by antimony acetate) was investigated at room temperature and 180 °C. The results show that according to the maximum amounts of adsorbate and correlation coefficients calculated from the three isotherm equations mentioned above, the corresponding data from adsorption experiments fit fairly well to the Langmuir isotherm. The adsorption data show a good compliance with the pseudo-second-order kinetic model and also follow the intraparticle diffusion model up to 30 min. The equilibrium adsorption capacity of Sb(OAc)3 on MMT is close to the cation exchange capacity (CEC) of the montmorillonite. The desorption amount of Sb(OAc)3 is correlated with both the temperature of desorption and the drying temperature of Sb-MMT. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction Montmorillonite (MMT) is a 2:1 layer type phyllosilicate composed of two silica tetrahedral sheets and one alumina octahedral sheet. Due to its high swelling capacity, large specific surface area, high cation exchange and adsorption capacity, excellent mechanical and thermal resistance characters [1–3], montmorillonite is widely used in many and diverse fields such as paints, cosmetics, catalysis, and fixation of pollutants. There is great interest in polymer/montmorillonite nanocomposite because of its tremendously improved properties such as excellent mechanical properties [4–6], thermal stability [5–8], gas barrier [7,9], and flame retardation [6,8,9] compared with conventional composites. It is well known that there are two kinds of nanocomposites, intercalated or exfoliated, depending on the dispersion state of MMT layers in polymer matrix. Usually the exfoliated nanocomposite has better properties than the intercalated one because of its better structural uniformity. Montmorillonite is usually modified with various organic chemicals to expand the interlayer distance and modify the surface polarity of MMT layers before polymer/MMT nanocomposite is prepared by in situ polymerization, melt blending or solution blending [6,10]. Montmorillonite could not be exfoliated into individual layers in the poly(ethylene terephthalate)/montmorillonite (PET/MMT) * Corresponding author. Fax: +86 10 82338827. E-mail address: [email protected] (X. Wang). 0021-9797/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2010.01.054
nanocomposite by these traditional methods because there is no driving force provided that facilitates absorption of the monomers or oligomers between adjacent silicate layers during the polymerization process. The conventional treatment causes the PET/MMT nanocomposite to become only an intercalated dispersion of MMT instead of an exfoliated dispersion in the polymer substrate [11]. In order to obtain the exfoliated PET/MMT nanocomposite, some researchers [7,11] applied a catalyst to intercalate into the gallery spaces of MMT. The function of MMT-supported catalyst is to create active sites between the MMT layers while the polymerization occurs, and the chain growth in the MMT galleries accelerates the layers exfoliation and nanocomposite formation. During the period of preparing the exfoliated PET/MMT nanocomposite, the author and her co-workers [12] found that the catalyst not only needed to be intercalated into MMT layers but also needed to remain within interlayer during polymerization. The aim of this work was to investigate the adsorption of catalyst antimony acetate (Sb(OAc)3) onto sodium montmorillonite (Na-MMT) and the desorption of Sb(OAc)3 from Sb-MMT (Na-MMT intercalated by Sb(OAc)3). To meet these aims, a batch of adsorption and desorption experiments were conducted. The adsorption and kinetic studies of Sb(OAc)3 on Na-MMT were investigated. The experimental data were analyzed by different models. The desorption of Sb(OAc)3 from Sb-MMT was investigated at room temperature and 180 °C, respectively. Furthermore, the relationship between the desorption efficiency and the drying temperature of Sb-MMT was researched.
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3.2
2. Materials and methods
10-4 (mol/g)
All chemicals were used as received unless stated otherwise. Sodium montmorillonite (Na-MMT) was purchased from Zhejiang Fenghong Clay Chemicals Co., Ltd., and the cation exchange capacity (CEC) was 100 mmol/100 g MMT. Antimony acetate (Sb(OAc)3) and ethylene glycol (EG) were obtained from Liaoyang catalyst Co. and Liaoyang petrochemical fiber Co., respectively.
3.0 2.8 2.6 2.4 2.2
Cads
2.1. Materials
2.0 1.8 1.6 0
5
2.2. Adsorption experiments Na-MMT (2.5 g) was added to each 50 ml of EG solution of Sb(OAc)3. The initial concentrations of Sb(OAc)3 were 0.05 mol/l, 0.0333 mol/l, 0.025 mol/l, 0.0167 mol/l and 0.01 mol/l, and the adsorption experiments were carried out at 120 °C for 4 h at a constant stirring speed. Five gram of Na-MMT and 3 g Sb(OAc)3 were added to 200 g EG. The adsorption experiments were carried out at 120 °C at a constant stirring speed for various adsorption times (5, 15, 30, 60, 120, 240 and 480 min). The above-mentioned modified clay was isolated by filtration and washed several times with EG to remove unreacted catalyst and dried. The concentration of Sb(OAc)3 in filtrate was then measured by iodometry. And the adsorbed amount of Sb(OAc)3 was calculated from the concentrations in solution before and after adsorption. Iodometry is a redox titration where the appearance or disappearance of elementary iodine indicates the end point, starch as an indicator. The equation is as follows, I2 + Sb3+ = 2I + Sb5+. 2.3. Desorption experiments For the desorption experiments, 30 g of Na-MMT and 9 g Sb(OAc)3 were added to 600 g EG. The adsorption experiments were carried out at 120 °C for 4 h at a constant stirring speed. Then the modified clay (Sb-MMT) was dried at 100 °C in vacuum for 48 h, 240 °C for 4 h and 450 °C for 2 h, respectively. Five gram of Sb-MMT prepared by above method was shearing dispersed in 50 g EG at room temperature for 1 h, and the clay was isolated by filtration and washed, then the Sb-MMT was stirred in 50 g EG at 180 °C for 2 h. The desorbed amount of Sb(OAc)3 was measured. 3. Results and discussion
10
15
Cw
20
25
30
35
10-3 (mol/l)
Fig. 1. Adsorption isotherm for Sb(OAc)3 on Na-MMT from experimental data.
without dissociation to definite sites of attachment on the surface of the adsorbent. (2) Each site of attachment can accommodate only one adsorbed molecule. (3) The energies of the states of any adsorbed molecule are independent of the presence or absence of any other adsorbed molecules on neighbouring points of attachment. Langmuir adsorption isotherm was examined according to the following equation [17]:
C w =C ads ¼ 1=Mb þ C w =M
ð1Þ
where Cw is the equilibrium concentration of Sb(OAc)3 in solution, Cads is the amount of Sb(OAc)3 adsorbed on MMT, M is a constant related to maximum amount of adsorbed solute, and b is a constant related to the binding energy of organic compounds, varied with temperature. The plot of Cw/Cads versus Cw is shown in Fig. 2. The values of the constants M and b are determined and listed in Table 1. The maximum amount of adsorbed Sb(OAc)3 is 3.19 104 mol/g according to the Langmuir model, which is close to the experimental value (0.0997 g/g MMT). 3.1.2. Dubinin–Radushkevich (D–R) isotherm The D–R isotherm is more general than the Langmuir isotherm, because it does not assume a homogeneous surface or constant adsorption potential [18]. The D–R isotherm was applied to distinguish between the physical and chemical adsorption of Sb(OAc)3 onto Na-MMT using the linear relationship [13,17],
ln C ads ¼ ln C m B2
ð2Þ
where Cads is the amount of Sb(OAc)3 adsorbed on MMT, Cm is the maximum amount of Sb(OAc)3 adsorbed, B is a constant related to the adsorption energy, is the Polanyi potential, and
3.1. Adsorption isotherm
¼ RT lnð1 þ 1=C w Þ
The adsorption of Sb(OAc)3 onto MMT was conducted with five initial Sb(OAc)3 concentrations at 120 °C. The driving force for the adsorption of Sb(OAc)3 onto Na-MMT is the cation exchange process [13,14]. The adsorption isotherm is given in Fig. 1. It is constructed by plotting the adsorbed amount of Sb(OAc)3 (Cads, mol/ g) versus the equilibrium concentration of Sb(OAc)3 in the solution (Cw, mol/l). In this study, data from the adsorption isotherm were modeled using the Langmuir, Freundlich and D–R isotherm equations.
where Cw is the equilibrium concentration of Sb(OAc)3 in solution.
3.1.1. Langmuir isotherm The Langmuir model was originally developed to describe the adsorption of gases onto solid surfaces but has since been used to model solute adsorption onto various adsorbents from aqueous solution [15]. The derivation of the Langmuir equation was based on three assumptions [15,16]: (1) The molecules are adsorbed
ð3Þ
120
Cw/Cads
100 80 60 40 20 0 0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035
Cw (mol/l) Fig. 2. Langmuir isotherm for adsorption of Sb(OAc)3 on Na-MMT.
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Table 1 Parameters of Langmuir isotherm. M, mol/g 4
3.19 10
Table 2 Dubinin–Radushkevich isotherm parameters.
b, l/g
Correlation coefficient (R2)
B, kJ2/mol2
Cm, mol/g
E, kJ/mol
Correlation coefficient (R2)
551.5
0.999
0.00193
0.0004
16.1
0.984
The plot of ln Cads versus 2 gives straight line as shown in Fig. 3, and the values of Cm and B are determined from the intercept and slope using the linear regression method. The adsorption energy, E, which is the free energy required to transfer 1 mol of Sb(OAc)3 from infinity in the solution to the surface of Na-MMT [19], is given by
E ¼ ð2BÞ1=2
ð4Þ
The magnitude of E is useful for estimating the type of adsorption process. The magnitude of adsorption energy may give an idea about the type of adsorption [18]. Depending on the nature of attractive forces existing between the adsorbate and adsorbent, adsorption can be classified as: (i) physical adsorption; (ii) chemical adsorption. In physical adsorption, the forces of attraction between the molecules of the adsorbate and the adsorbent are of the weak van der Waals’ type. Since the forces of attraction are weak, the process of physical adsorption can be easily reversed by heating or decreasing the pressure of the adsorbate, because the energy requirements are small (usually no more than 4.2 kJ/ mol). In chemical adsorption, the forces of attraction between the adsorbate and the adsorbent are very strong; the molecules of adsorbate form chemical bonds with the molecules of the adsorbent present in the surface. The value of E in the present case (Table 2) shows that the intercalation is due to the chemical adsorption based on the cation exchange process [13,17]. 3.1.3. Freundlich isotherm The Freundlich isotherm is an empirical expression that encompasses the heterogeneity of the surface and an exponential distribution of the sites and their energies [1,19,20]. The common form of the Freundlich isotherm is: 1=n C ads ¼ KC w
ð5Þ
and the linear form of the Freundlich isotherm is:
ln C ads ¼ ln K þ ð1=nÞ ln C w
ð6Þ
If a plot of ln Cads against ln Cw gives a straight line, the adsorption data obey the Freundlich equation. The slope of the straight line then give 1/n, the intercept gives ln K [16]. According to the Freundlich constant, 1/n, the shape of the adsorption isotherm can be determined [1]. It is concluded that the adsorbate is easily adsorbed when 1/n value is between 0.1 and 0.5, while it is difficult to adsorption when the 1/n is larger than 2.0. Fig. 4 shows the plot of ln Cads versus ln Cw, and the K and 1/n values are listed in Table 3. The values of 1/n is smaller than 0.5, indicating favorable adsorption processes for Sb(OAc)3. Furthermore, the agreement is poor, particularly at high concentration. In conclusion, the Langmuir isotherm fits quite well with the experimental data. The maximum amount of adsorbed solute M corresponds to the monolayer coverage, and it indicates the monolayer adsorption. Moreover, the value of the adsorption energy determined using the D–R isotherm confirms that the adsorption takes place by the cation exchange mechanism. According to the Freundlich constant, 1/n, it is concluded that the adsorbate is easily adsorbed. 3.2. Kinetic studies The kinetic behavior of organics on solid adsorbents including clays is fitted reasonably well with the pseudo-second-order kinetic model [21–23]. However, there is no study in the literature reported about adsorption kinetics of Sb(OAc)3 onto Na-MMT. In order to clarify the adsorption mechanisms, such as chemical reaction, diffusion control and mass transfer [24], several kinetic models were proposed. In this study, the adsorption kinetics of Sb(OAc)3 onto Na-MMT were studied in terms of pseudo-first-order kinetic, pseudo-second-order kinetic and intraparticle diffusion models. The adsorption isotherm of Sb(OAc)3 onto Na-MMT is given in Fig. 5. As seen from this figure, the adsorption reaches saturation in 2 h, and the equilibrium adsorption capacity of Na-MMT for Sb(OAc)3 is 0.0997 g/g MMT.
where Cads is the amount of Sb(OAc)3 adsorbed on MMT, Cw is the equilibrium concentration of Sb(OAc)3 in solution, K and 1/n are constants. K is the adsorption capacity of the adsorbent, which is defined as the adsorption or distribution coefficient and represents the quantity of Sb(OAc)3 adsorbed onto adsorbent for a unit equilibrium concentration, and the magnitude of the exponent 1/n gives an indication of the favorability of adsorption [20].
ln Cads (mol/g)
-8.1
-8.1
-8.3 -8.4 -8.5 -8.6 -8.7
-8.2
ln Cads
-8.2
-8.3
-6.5 -6.0 -5.5 -5.0 -4.5 -4.0 -3.5 -3.0
ln Cw (mol/l)
-8.4 -8.5
Fig. 4. Freundlich adsorption isotherm for adsorption of Sb(OAc)3 on Na-MMT.
-8.6 -8.7 1.0
1.5
2.0
2.5
3.0
2
108
3.5
4.0
4.5
Fig. 3. Dubinin–Radushkevich isotherm for adsorption of Sb(OAc)3 on Na-MMT.
Table 3 Parameters of Freundlich isotherm. K, mol/g
1/n
Correlation coefficient (R2)
0.0006
0.200
0.955
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0.100
5000
t/qt (min g/g)
qt (g/g)
0.095 0.090 0.085 0.080 0.075
4000 3000 2000 1000
0.070
0
0.065 0
100
200
300
400
0
500
100
200
t (min)
300
400
500
t (min)
Fig. 5. The amount of adsorbed with adsorption time.
Fig. 7. The pseudo-second-order kinetic of Sb(OAc)3 adsorbed onto Na-MMT.
3.2.1. pseudo-first-order First, the kinetics of adsorption was analyzed by the pseudofirst-order equation given by Lagergren [25,26]. The pseudo-firstorder reaction model is expressed as follows,
Table 4 Comparison of the pseudo-first-order, pseudo-second-order and intraparticle diffusion models for Sb(OAc)3 adsorption onto MMT.
lnðqe qt Þ ¼ ln qe k1 t
qe,
exp
(g/g) Pseudo-first-order kinetic model
ð7Þ
where qe and qt are the amounts of Sb(OAc)3 adsorbed on Na-MMT at equilibrium and at time t, respectively, and k1 is the rate constant. The constant k1 was obtained from the plots of ln (qe qt) versus t (Fig. 6). 3.2.2. pseudo-second-order The adsorption data were also analyzed in terms of pseudo-second-order mechanism. The model described by
t=qt ¼ 1=k2 q2e þ t=qe
ð8Þ
where qe and qt are the amounts of Sb(OAc)3 adsorbed on Na-MMT at equilibrium and at time t, respectively, and k2 is the rate constant of pseudo-second-order adsorption. The straight line plot of t/qt versus t is used to obtain the constants k2 for pseudo-second-order adsorption. The values of correlation coefficient R2, obtained from the plot of pseudo-second-order kinetics given in Fig. 7 is greater than that of the pseudo-first-order and intraparticle diffusion models (Table 4). Moreover, the equilibrium adsorption capacity for pseudo-second-order shows a good agreement with the experimental value (Table 4), indicating the applicability of pseudo-second-order kinetics model to describe the adsorption process of Sb(OAc)3 onto Na-MMT. This suggests that the rate of the adsorption process appears to be controlled by the chemical process in this case in accordance with the pseudo second-order reaction mechanism [27], and the rate limiting step may be a chemical adsorption involving valency forces through sharing or exchange of electrons between Sb(OAc)3 molecules and MMT [27,28].
qe, cal (g/g) 0.0997
Pseudo-second-order Intraparticle kinetic model diffusion model
k1 R2 (min1)
qe, cal k2 (g/ R2 (g/g) g min1)
0.0335 0.0376 0.978 0.100 3.93
ki R2 (g1 min1/2)
0.999 0.0016
0.967
3.2.3. Intraparticle diffusion model Intraparticle diffusion can be described by three consecutive steps [29]: (1) The transport of adsorbate from bulk solution to outer surface of the adsorbent by molecular diffusion, known as external (or) film diffusion. (2) Internal diffusion, the transport of adsorbate from the particles surface into interior sites. (3) The adsorption of the solute particles from the active sites into the interior surface of the pores. The rate of the adsorption process will be controlled by the slowest step, the rate limiting step. The nature of the rate limiting step in a batch system can be determined from the properties of the solute and adsorbent. In adsorption systems, if intraparticle diffusion is the rate limiting step, the intraparticle diffusion approach described by [26,30]
qt ¼ ki t 1=2 þ c
ð9Þ
where qt is the amounts of adsorbate adsorbed on adsorbent at time t, ki is the intraparticle diffusion rate constant, and c is the intercept. According to the intraparticle diffusion model, if the plot of qt versus t1/2 is a straight line passing through the origin, it can be assumed that the mechanism involves the intraparticle diffusion, and the intraparticle diffusion is the rate limiting step, and the ki is the rate constant of intraparticle transport [25,30].
0.100 0.095 0.090
-4.0
qt (g)
ln qe-qt (g/g)
-3.5
-4.5
0.085 0.080 0.075
-5.0
0.070 -5.5
0.065 0
10
20
30
40
50
60
t (min) Fig. 6. The pseudo-first-order kinetic of Sb(OAc)3 adsorbed onto Na-MMT.
0
5
10 1/2
t
15 1/2
(min
20
25
)
Fig. 8. The intraparticle diffusion kinetic of Sb(OAc)3 adsorbed onto Na-MMT.
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As can be seen from Fig. 8, the plot does not pass through the origin which indicates that the intraparticle diffusion is not the only rate limiting step, but also other kinetic models may control the rate of adsorption, all of which may be operating simultaneously [24]. The plot presents multilinearity, indicating that three steps take place and this deviation from the origin might be due to the difference in the mass transfer rate in the initial and final stages of adsorption [25]. In Fig. 8, the first sharper portion may be considered as an external surface adsorption or faster adsorption stage. The second portion describes the gradual adsorption stage, where intraparticle diffusion is rate controlled. The third portion is attributed to the final equilibrium stage, where intraparticle diffusion starts to slow down due to the extremely low adsorbate concentrations in the solution. In the intermediate stage where the adsorption is gradual, the process may be controlled by intraparticle diffusion [31]. The value of intraparticle diffusion rate is estimated from the slopes of the plot of qt against t1/2. The values of R2, obtained from the plot of intraparticle diffusion kinetics is much lower than that of the pseudo-second-order model (Table 4), but this model indicates that the adsorption of Sb(OAc)3 onto Na-MMT may be followed by an intraparticle diffusion model up to 30 min. This indicates that although intraparticle diffusion is involved in the adsorption process, it is not the only rate controlling step [25,32]. Kinetic parameters from linear plots of three kinetic models are given in Table 4. The pseudo-second-order equation provides the best correlation for the adsorption process, and the calculated value of qe is close to the experimental value. The pseudo-first-order and intraparticle equations do not give a good fit to the experimental data for the adsorption of Sb(OAc)3. 3.3. Desorption studies
desorption efficiency (%)
In order to prepare the exfoliated PET/MMT nanocomposite, the catalyst Sb(OAc)3 needs to remain at the interlayer during the polymerization process as long as possible. In this paper, the desorption of Sb(OAc)3 from MMT was conducted. The desorption experiments were performed on Sb-MMT dried at three different temperatures, which were mentioned in experimental section. The desorption efficiency of Sb(OAc)3 is expressed as a percentage of the Sb(OAc)3 desorbed from MMT. The results are listed in Fig. 9. Within 1 h about 11–16% of Sb(OAc)3 has been released into solution at room temperature by shear dispersing, but subsequently desorption begins to increase sharply, and more than 35% of adsorbed Sb(OAc)3 has been released within 2 h stirring at 180 °C. The enhancement of desorption capacity at high temperature is related to the type of adsorption. According to the pseudo-second-order, the rate limiting step may be a chemical adsorption involving valency forces through sharing or exchange of electrons between Sb(OAc)3 molecules and MMT. Sb(OAc)3 is
desorption temperature Fig. 9. The desorption efficiency of Sb(OAc)3 from Sb-MMT.
difficult to be desorbed because the energy of making chemical bonds broken provided by simple mechanical agitation at room temperature is not sufficient. Moreover, the values of Sb-MMT desorption efficiency at room temperature are similar, whereas after 2 h stirring at 180 °C, the desorption of Sb(OAc)3 from SbMMT dried at 100 °C is higher than the other two MMTs. This can be explained by that with the increase of drying temperature, Sb(OAc)3 and MMT may be somewhat chemically bonded. It is concluded that the desorption amount of Sb(OAc)3 is positive correlated with the desorption temperature and significantly influenced by the drying temperature of Sb-MMT. It turns out that when the desorption temperature is 180 °C, the desorption amounts of Sb(OAc)3 from Sb-MMT dried at 240 °C and 450 °C are lower than that dried at 100 °C (Fig. 9). It is reported that [7] if the catalyst is intercalated in abundance into the MMT interlayer, the exfoliated PET/MMT nonocomposite may be obtained. Therefore, the drying temperature of Sb-MMT should be higher than 240 °C in order to make abundant catalyst remain at interlayer during polymerization. 4. Summary In this study, it is found that the adsorption of Sb(OAc)3 on MMT is very fast and the equilibrium is reached within 2 h. The equilibrium adsorption amount of adsorbed Sb(OAc)3 is 0.0997 g/g MMT which is closed to the CEC of Na-MMT. The adsorption isotherm experimental data fit the Langmuir isotherm model better than the Freundlich and D–R models. Moreover, the value of the adsorption energy determined using the D–R isotherm confirms that the adsorption takes place by the cation exchange mechanism. The pseudo-second-order equation provides the best correlation for the adsorption process, and the calculated value of qe is close to the experimental value. Also, the rate of the adsorption process appears to be controlled by the chemical process in accordance with the pseudo-second-order reaction mechanism and the adsorption fits well to the intraparticle diffusion model up to 30 min, but diffusion is not the only rate controlling step. The desorption of Sb(OAc)3 is correlated with the temperature of desorption and significantly influenced by the drying temperature of Sb-MMT. The drying temperature of Sb-MMT should be 240 °C or higher to make enough Sb(OAc)3 to stay in the interlayer during polymerization of PET. References [1] T.S. Anirudhan, P.S. Suchithra, P.G. Radhakrishnan, Appl. Clay Sci. 43 (2009) 336. [2] M. Matzke, K. Thiele, A. Müller, J. Filser, Chemosphere 74 (2009) 568. [3] G. Abate, J.C. Masini, J. Agric. Food Chem. 55 (2007) 3555. [4] G.Z. Zhang, T. Shichi, K. Takagi, Mater. Lett. 57 (2003) 1858. [5] L. Zhu, R.P. Wool, Polymer 47 (2006) 8106. [6] G.H. Guan, C.C. Li, D. Zhang, J. Appl. Polym. Sci. 95 (2005) 1443. [7] W.J. Choi, H.J. Kim, K.H. Yoon, O.H. Kwon, C.I. Hwang, J. Appl. Polym. Sci. 100 (2006) 4875. [8] Z.L. Zhao, X.Q. Wang, Z.Y. Li, S.Y. Du, J. Mater. Eng. Z1 (2008) 280. [9] M. Alexandre, P. Dubois, Mater. Sci. Eng. 28 (2000) 1. [10] T.J. Pinnavaia, G.W. Beall, Polymer–Clay Nanocomposites, Wiley, Chichester, 2000. [11] T.Y. Tsai, C.H. Li, C.H. Chang, W.H. Cheng, C.L. Hwang, R.J. Wu, Adv. Mater. 17 (2005) 1769. [12] Z.Y. Li, Preparation and Property Investigation of High Barrier PET, Beihang University, Beijing, 2008. [13] A.H. Gemeayl, J. Colloid Interface Sci. 251 (2002) 235. [14] C. Blachier, L. Michot, I. Bihannic, O. Barrès, A. Jacquet, M. Mosquet, J. Colloid Interface Sci. 336 (2009) 599. [15] M.M. Saeed, A. Ghaffar, J. Radioanal. Nucl. Chem. 232 (1998) 171. [16] S. Brunauer, The Adsorption of Gases and Vapors: Physical Adsorption, Princeton University Press, Princeton, 1943. [17] A.H. Gemeay, A.S.E. Sherbiny, A.B. Zaki, J. Colloid Interface Sci. 245 (2002) 116. [18] M. Akçay, J. Colloid Interface Sci. 296 (2006) 16. [19] A.B. Karima, B. Mounira, M. Hachkara, M. Bakassec, A. Yaacoubi, J. Hazard. Mater. 168 (2009) 304.
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Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Adsorption and desorption of antimony acetate on sodium montmorillonite Zhenlu Zhao, Xiaoqun Wang *, Chuan Zhao, Xiaoguang Zhu, Shanyi Du School of Material Science and Engineering, Beihang University, Beijing 100191, China
a r t i c l e
i n f o
Article history: Received 23 August 2009 Accepted 15 January 2010 Available online 6 February 2010 Keywords: Antimony acetate Montmorillonite Adsorption Desorption
a b s t r a c t The adsorption of antimony acetate (Sb(OAc)3) on sodium montmorillonite (Na-MMT) was studied at five different initial concentrations, and data from the adsorption isotherm were modeled using the Langmuir, Freundlich and D–R isotherm equations. The kinetics of adsorption was also discussed using three kinetic models: the pseudo-first-order, the pseudo-second-order and the intraparticle diffusion model. The rate constants of pseudo-first-order, pseudo-second-order and intraparticle diffusion kinetics, and the amount of Sb(OAc)3 adsorbed at equilibrium were determined. Moreover, the desorption of Sb(OAc)3 from several kinds of Sb-MMT (Na-MMT was intercalated by antimony acetate) was investigated at room temperature and 180 °C. The results show that according to the maximum amounts of adsorbate and correlation coefficients calculated from the three isotherm equations mentioned above, the corresponding data from adsorption experiments fit fairly well to the Langmuir isotherm. The adsorption data show a good compliance with the pseudo-second-order kinetic model and also follow the intraparticle diffusion model up to 30 min. The equilibrium adsorption capacity of Sb(OAc)3 on MMT is close to the cation exchange capacity (CEC) of the montmorillonite. The desorption amount of Sb(OAc)3 is correlated with both the temperature of desorption and the drying temperature of Sb-MMT. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction Montmorillonite (MMT) is a 2:1 layer type phyllosilicate composed of two silica tetrahedral sheets and one alumina octahedral sheet. Due to its high swelling capacity, large specific surface area, high cation exchange and adsorption capacity, excellent mechanical and thermal resistance characters [1–3], montmorillonite is widely used in many and diverse fields such as paints, cosmetics, catalysis, and fixation of pollutants. There is great interest in polymer/montmorillonite nanocomposite because of its tremendously improved properties such as excellent mechanical properties [4–6], thermal stability [5–8], gas barrier [7,9], and flame retardation [6,8,9] compared with conventional composites. It is well known that there are two kinds of nanocomposites, intercalated or exfoliated, depending on the dispersion state of MMT layers in polymer matrix. Usually the exfoliated nanocomposite has better properties than the intercalated one because of its better structural uniformity. Montmorillonite is usually modified with various organic chemicals to expand the interlayer distance and modify the surface polarity of MMT layers before polymer/MMT nanocomposite is prepared by in situ polymerization, melt blending or solution blending [6,10]. Montmorillonite could not be exfoliated into individual layers in the poly(ethylene terephthalate)/montmorillonite (PET/MMT) * Corresponding author. Fax: +86 10 82338827. E-mail address: [email protected] (X. Wang). 0021-9797/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2010.01.054
nanocomposite by these traditional methods because there is no driving force provided that facilitates absorption of the monomers or oligomers between adjacent silicate layers during the polymerization process. The conventional treatment causes the PET/MMT nanocomposite to become only an intercalated dispersion of MMT instead of an exfoliated dispersion in the polymer substrate [11]. In order to obtain the exfoliated PET/MMT nanocomposite, some researchers [7,11] applied a catalyst to intercalate into the gallery spaces of MMT. The function of MMT-supported catalyst is to create active sites between the MMT layers while the polymerization occurs, and the chain growth in the MMT galleries accelerates the layers exfoliation and nanocomposite formation. During the period of preparing the exfoliated PET/MMT nanocomposite, the author and her co-workers [12] found that the catalyst not only needed to be intercalated into MMT layers but also needed to remain within interlayer during polymerization. The aim of this work was to investigate the adsorption of catalyst antimony acetate (Sb(OAc)3) onto sodium montmorillonite (Na-MMT) and the desorption of Sb(OAc)3 from Sb-MMT (Na-MMT intercalated by Sb(OAc)3). To meet these aims, a batch of adsorption and desorption experiments were conducted. The adsorption and kinetic studies of Sb(OAc)3 on Na-MMT were investigated. The experimental data were analyzed by different models. The desorption of Sb(OAc)3 from Sb-MMT was investigated at room temperature and 180 °C, respectively. Furthermore, the relationship between the desorption efficiency and the drying temperature of Sb-MMT was researched.
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3.2
2. Materials and methods
10-4 (mol/g)
All chemicals were used as received unless stated otherwise. Sodium montmorillonite (Na-MMT) was purchased from Zhejiang Fenghong Clay Chemicals Co., Ltd., and the cation exchange capacity (CEC) was 100 mmol/100 g MMT. Antimony acetate (Sb(OAc)3) and ethylene glycol (EG) were obtained from Liaoyang catalyst Co. and Liaoyang petrochemical fiber Co., respectively.
3.0 2.8 2.6 2.4 2.2
Cads
2.1. Materials
2.0 1.8 1.6 0
5
2.2. Adsorption experiments Na-MMT (2.5 g) was added to each 50 ml of EG solution of Sb(OAc)3. The initial concentrations of Sb(OAc)3 were 0.05 mol/l, 0.0333 mol/l, 0.025 mol/l, 0.0167 mol/l and 0.01 mol/l, and the adsorption experiments were carried out at 120 °C for 4 h at a constant stirring speed. Five gram of Na-MMT and 3 g Sb(OAc)3 were added to 200 g EG. The adsorption experiments were carried out at 120 °C at a constant stirring speed for various adsorption times (5, 15, 30, 60, 120, 240 and 480 min). The above-mentioned modified clay was isolated by filtration and washed several times with EG to remove unreacted catalyst and dried. The concentration of Sb(OAc)3 in filtrate was then measured by iodometry. And the adsorbed amount of Sb(OAc)3 was calculated from the concentrations in solution before and after adsorption. Iodometry is a redox titration where the appearance or disappearance of elementary iodine indicates the end point, starch as an indicator. The equation is as follows, I2 + Sb3+ = 2I + Sb5+. 2.3. Desorption experiments For the desorption experiments, 30 g of Na-MMT and 9 g Sb(OAc)3 were added to 600 g EG. The adsorption experiments were carried out at 120 °C for 4 h at a constant stirring speed. Then the modified clay (Sb-MMT) was dried at 100 °C in vacuum for 48 h, 240 °C for 4 h and 450 °C for 2 h, respectively. Five gram of Sb-MMT prepared by above method was shearing dispersed in 50 g EG at room temperature for 1 h, and the clay was isolated by filtration and washed, then the Sb-MMT was stirred in 50 g EG at 180 °C for 2 h. The desorbed amount of Sb(OAc)3 was measured. 3. Results and discussion
10
15
Cw
20
25
30
35
10-3 (mol/l)
Fig. 1. Adsorption isotherm for Sb(OAc)3 on Na-MMT from experimental data.
without dissociation to definite sites of attachment on the surface of the adsorbent. (2) Each site of attachment can accommodate only one adsorbed molecule. (3) The energies of the states of any adsorbed molecule are independent of the presence or absence of any other adsorbed molecules on neighbouring points of attachment. Langmuir adsorption isotherm was examined according to the following equation [17]:
C w =C ads ¼ 1=Mb þ C w =M
ð1Þ
where Cw is the equilibrium concentration of Sb(OAc)3 in solution, Cads is the amount of Sb(OAc)3 adsorbed on MMT, M is a constant related to maximum amount of adsorbed solute, and b is a constant related to the binding energy of organic compounds, varied with temperature. The plot of Cw/Cads versus Cw is shown in Fig. 2. The values of the constants M and b are determined and listed in Table 1. The maximum amount of adsorbed Sb(OAc)3 is 3.19 104 mol/g according to the Langmuir model, which is close to the experimental value (0.0997 g/g MMT). 3.1.2. Dubinin–Radushkevich (D–R) isotherm The D–R isotherm is more general than the Langmuir isotherm, because it does not assume a homogeneous surface or constant adsorption potential [18]. The D–R isotherm was applied to distinguish between the physical and chemical adsorption of Sb(OAc)3 onto Na-MMT using the linear relationship [13,17],
ln C ads ¼ ln C m B2
ð2Þ
where Cads is the amount of Sb(OAc)3 adsorbed on MMT, Cm is the maximum amount of Sb(OAc)3 adsorbed, B is a constant related to the adsorption energy, is the Polanyi potential, and
3.1. Adsorption isotherm
¼ RT lnð1 þ 1=C w Þ
The adsorption of Sb(OAc)3 onto MMT was conducted with five initial Sb(OAc)3 concentrations at 120 °C. The driving force for the adsorption of Sb(OAc)3 onto Na-MMT is the cation exchange process [13,14]. The adsorption isotherm is given in Fig. 1. It is constructed by plotting the adsorbed amount of Sb(OAc)3 (Cads, mol/ g) versus the equilibrium concentration of Sb(OAc)3 in the solution (Cw, mol/l). In this study, data from the adsorption isotherm were modeled using the Langmuir, Freundlich and D–R isotherm equations.
where Cw is the equilibrium concentration of Sb(OAc)3 in solution.
3.1.1. Langmuir isotherm The Langmuir model was originally developed to describe the adsorption of gases onto solid surfaces but has since been used to model solute adsorption onto various adsorbents from aqueous solution [15]. The derivation of the Langmuir equation was based on three assumptions [15,16]: (1) The molecules are adsorbed
ð3Þ
120
Cw/Cads
100 80 60 40 20 0 0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035
Cw (mol/l) Fig. 2. Langmuir isotherm for adsorption of Sb(OAc)3 on Na-MMT.
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Table 1 Parameters of Langmuir isotherm. M, mol/g 4
3.19 10
Table 2 Dubinin–Radushkevich isotherm parameters.
b, l/g
Correlation coefficient (R2)
B, kJ2/mol2
Cm, mol/g
E, kJ/mol
Correlation coefficient (R2)
551.5
0.999
0.00193
0.0004
16.1
0.984
The plot of ln Cads versus 2 gives straight line as shown in Fig. 3, and the values of Cm and B are determined from the intercept and slope using the linear regression method. The adsorption energy, E, which is the free energy required to transfer 1 mol of Sb(OAc)3 from infinity in the solution to the surface of Na-MMT [19], is given by
E ¼ ð2BÞ1=2
ð4Þ
The magnitude of E is useful for estimating the type of adsorption process. The magnitude of adsorption energy may give an idea about the type of adsorption [18]. Depending on the nature of attractive forces existing between the adsorbate and adsorbent, adsorption can be classified as: (i) physical adsorption; (ii) chemical adsorption. In physical adsorption, the forces of attraction between the molecules of the adsorbate and the adsorbent are of the weak van der Waals’ type. Since the forces of attraction are weak, the process of physical adsorption can be easily reversed by heating or decreasing the pressure of the adsorbate, because the energy requirements are small (usually no more than 4.2 kJ/ mol). In chemical adsorption, the forces of attraction between the adsorbate and the adsorbent are very strong; the molecules of adsorbate form chemical bonds with the molecules of the adsorbent present in the surface. The value of E in the present case (Table 2) shows that the intercalation is due to the chemical adsorption based on the cation exchange process [13,17]. 3.1.3. Freundlich isotherm The Freundlich isotherm is an empirical expression that encompasses the heterogeneity of the surface and an exponential distribution of the sites and their energies [1,19,20]. The common form of the Freundlich isotherm is: 1=n C ads ¼ KC w
ð5Þ
and the linear form of the Freundlich isotherm is:
ln C ads ¼ ln K þ ð1=nÞ ln C w
ð6Þ
If a plot of ln Cads against ln Cw gives a straight line, the adsorption data obey the Freundlich equation. The slope of the straight line then give 1/n, the intercept gives ln K [16]. According to the Freundlich constant, 1/n, the shape of the adsorption isotherm can be determined [1]. It is concluded that the adsorbate is easily adsorbed when 1/n value is between 0.1 and 0.5, while it is difficult to adsorption when the 1/n is larger than 2.0. Fig. 4 shows the plot of ln Cads versus ln Cw, and the K and 1/n values are listed in Table 3. The values of 1/n is smaller than 0.5, indicating favorable adsorption processes for Sb(OAc)3. Furthermore, the agreement is poor, particularly at high concentration. In conclusion, the Langmuir isotherm fits quite well with the experimental data. The maximum amount of adsorbed solute M corresponds to the monolayer coverage, and it indicates the monolayer adsorption. Moreover, the value of the adsorption energy determined using the D–R isotherm confirms that the adsorption takes place by the cation exchange mechanism. According to the Freundlich constant, 1/n, it is concluded that the adsorbate is easily adsorbed. 3.2. Kinetic studies The kinetic behavior of organics on solid adsorbents including clays is fitted reasonably well with the pseudo-second-order kinetic model [21–23]. However, there is no study in the literature reported about adsorption kinetics of Sb(OAc)3 onto Na-MMT. In order to clarify the adsorption mechanisms, such as chemical reaction, diffusion control and mass transfer [24], several kinetic models were proposed. In this study, the adsorption kinetics of Sb(OAc)3 onto Na-MMT were studied in terms of pseudo-first-order kinetic, pseudo-second-order kinetic and intraparticle diffusion models. The adsorption isotherm of Sb(OAc)3 onto Na-MMT is given in Fig. 5. As seen from this figure, the adsorption reaches saturation in 2 h, and the equilibrium adsorption capacity of Na-MMT for Sb(OAc)3 is 0.0997 g/g MMT.
where Cads is the amount of Sb(OAc)3 adsorbed on MMT, Cw is the equilibrium concentration of Sb(OAc)3 in solution, K and 1/n are constants. K is the adsorption capacity of the adsorbent, which is defined as the adsorption or distribution coefficient and represents the quantity of Sb(OAc)3 adsorbed onto adsorbent for a unit equilibrium concentration, and the magnitude of the exponent 1/n gives an indication of the favorability of adsorption [20].
ln Cads (mol/g)
-8.1
-8.1
-8.3 -8.4 -8.5 -8.6 -8.7
-8.2
ln Cads
-8.2
-8.3
-6.5 -6.0 -5.5 -5.0 -4.5 -4.0 -3.5 -3.0
ln Cw (mol/l)
-8.4 -8.5
Fig. 4. Freundlich adsorption isotherm for adsorption of Sb(OAc)3 on Na-MMT.
-8.6 -8.7 1.0
1.5
2.0
2.5
3.0
2
108
3.5
4.0
4.5
Fig. 3. Dubinin–Radushkevich isotherm for adsorption of Sb(OAc)3 on Na-MMT.
Table 3 Parameters of Freundlich isotherm. K, mol/g
1/n
Correlation coefficient (R2)
0.0006
0.200
0.955
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0.100
5000
t/qt (min g/g)
qt (g/g)
0.095 0.090 0.085 0.080 0.075
4000 3000 2000 1000
0.070
0
0.065 0
100
200
300
400
0
500
100
200
t (min)
300
400
500
t (min)
Fig. 5. The amount of adsorbed with adsorption time.
Fig. 7. The pseudo-second-order kinetic of Sb(OAc)3 adsorbed onto Na-MMT.
3.2.1. pseudo-first-order First, the kinetics of adsorption was analyzed by the pseudofirst-order equation given by Lagergren [25,26]. The pseudo-firstorder reaction model is expressed as follows,
Table 4 Comparison of the pseudo-first-order, pseudo-second-order and intraparticle diffusion models for Sb(OAc)3 adsorption onto MMT.
lnðqe qt Þ ¼ ln qe k1 t
qe,
exp
(g/g) Pseudo-first-order kinetic model
ð7Þ
where qe and qt are the amounts of Sb(OAc)3 adsorbed on Na-MMT at equilibrium and at time t, respectively, and k1 is the rate constant. The constant k1 was obtained from the plots of ln (qe qt) versus t (Fig. 6). 3.2.2. pseudo-second-order The adsorption data were also analyzed in terms of pseudo-second-order mechanism. The model described by
t=qt ¼ 1=k2 q2e þ t=qe
ð8Þ
where qe and qt are the amounts of Sb(OAc)3 adsorbed on Na-MMT at equilibrium and at time t, respectively, and k2 is the rate constant of pseudo-second-order adsorption. The straight line plot of t/qt versus t is used to obtain the constants k2 for pseudo-second-order adsorption. The values of correlation coefficient R2, obtained from the plot of pseudo-second-order kinetics given in Fig. 7 is greater than that of the pseudo-first-order and intraparticle diffusion models (Table 4). Moreover, the equilibrium adsorption capacity for pseudo-second-order shows a good agreement with the experimental value (Table 4), indicating the applicability of pseudo-second-order kinetics model to describe the adsorption process of Sb(OAc)3 onto Na-MMT. This suggests that the rate of the adsorption process appears to be controlled by the chemical process in this case in accordance with the pseudo second-order reaction mechanism [27], and the rate limiting step may be a chemical adsorption involving valency forces through sharing or exchange of electrons between Sb(OAc)3 molecules and MMT [27,28].
qe, cal (g/g) 0.0997
Pseudo-second-order Intraparticle kinetic model diffusion model
k1 R2 (min1)
qe, cal k2 (g/ R2 (g/g) g min1)
0.0335 0.0376 0.978 0.100 3.93
ki R2 (g1 min1/2)
0.999 0.0016
0.967
3.2.3. Intraparticle diffusion model Intraparticle diffusion can be described by three consecutive steps [29]: (1) The transport of adsorbate from bulk solution to outer surface of the adsorbent by molecular diffusion, known as external (or) film diffusion. (2) Internal diffusion, the transport of adsorbate from the particles surface into interior sites. (3) The adsorption of the solute particles from the active sites into the interior surface of the pores. The rate of the adsorption process will be controlled by the slowest step, the rate limiting step. The nature of the rate limiting step in a batch system can be determined from the properties of the solute and adsorbent. In adsorption systems, if intraparticle diffusion is the rate limiting step, the intraparticle diffusion approach described by [26,30]
qt ¼ ki t 1=2 þ c
ð9Þ
where qt is the amounts of adsorbate adsorbed on adsorbent at time t, ki is the intraparticle diffusion rate constant, and c is the intercept. According to the intraparticle diffusion model, if the plot of qt versus t1/2 is a straight line passing through the origin, it can be assumed that the mechanism involves the intraparticle diffusion, and the intraparticle diffusion is the rate limiting step, and the ki is the rate constant of intraparticle transport [25,30].
0.100 0.095 0.090
-4.0
qt (g)
ln qe-qt (g/g)
-3.5
-4.5
0.085 0.080 0.075
-5.0
0.070 -5.5
0.065 0
10
20
30
40
50
60
t (min) Fig. 6. The pseudo-first-order kinetic of Sb(OAc)3 adsorbed onto Na-MMT.
0
5
10 1/2
t
15 1/2
(min
20
25
)
Fig. 8. The intraparticle diffusion kinetic of Sb(OAc)3 adsorbed onto Na-MMT.
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As can be seen from Fig. 8, the plot does not pass through the origin which indicates that the intraparticle diffusion is not the only rate limiting step, but also other kinetic models may control the rate of adsorption, all of which may be operating simultaneously [24]. The plot presents multilinearity, indicating that three steps take place and this deviation from the origin might be due to the difference in the mass transfer rate in the initial and final stages of adsorption [25]. In Fig. 8, the first sharper portion may be considered as an external surface adsorption or faster adsorption stage. The second portion describes the gradual adsorption stage, where intraparticle diffusion is rate controlled. The third portion is attributed to the final equilibrium stage, where intraparticle diffusion starts to slow down due to the extremely low adsorbate concentrations in the solution. In the intermediate stage where the adsorption is gradual, the process may be controlled by intraparticle diffusion [31]. The value of intraparticle diffusion rate is estimated from the slopes of the plot of qt against t1/2. The values of R2, obtained from the plot of intraparticle diffusion kinetics is much lower than that of the pseudo-second-order model (Table 4), but this model indicates that the adsorption of Sb(OAc)3 onto Na-MMT may be followed by an intraparticle diffusion model up to 30 min. This indicates that although intraparticle diffusion is involved in the adsorption process, it is not the only rate controlling step [25,32]. Kinetic parameters from linear plots of three kinetic models are given in Table 4. The pseudo-second-order equation provides the best correlation for the adsorption process, and the calculated value of qe is close to the experimental value. The pseudo-first-order and intraparticle equations do not give a good fit to the experimental data for the adsorption of Sb(OAc)3. 3.3. Desorption studies
desorption efficiency (%)
In order to prepare the exfoliated PET/MMT nanocomposite, the catalyst Sb(OAc)3 needs to remain at the interlayer during the polymerization process as long as possible. In this paper, the desorption of Sb(OAc)3 from MMT was conducted. The desorption experiments were performed on Sb-MMT dried at three different temperatures, which were mentioned in experimental section. The desorption efficiency of Sb(OAc)3 is expressed as a percentage of the Sb(OAc)3 desorbed from MMT. The results are listed in Fig. 9. Within 1 h about 11–16% of Sb(OAc)3 has been released into solution at room temperature by shear dispersing, but subsequently desorption begins to increase sharply, and more than 35% of adsorbed Sb(OAc)3 has been released within 2 h stirring at 180 °C. The enhancement of desorption capacity at high temperature is related to the type of adsorption. According to the pseudo-second-order, the rate limiting step may be a chemical adsorption involving valency forces through sharing or exchange of electrons between Sb(OAc)3 molecules and MMT. Sb(OAc)3 is
desorption temperature Fig. 9. The desorption efficiency of Sb(OAc)3 from Sb-MMT.
difficult to be desorbed because the energy of making chemical bonds broken provided by simple mechanical agitation at room temperature is not sufficient. Moreover, the values of Sb-MMT desorption efficiency at room temperature are similar, whereas after 2 h stirring at 180 °C, the desorption of Sb(OAc)3 from SbMMT dried at 100 °C is higher than the other two MMTs. This can be explained by that with the increase of drying temperature, Sb(OAc)3 and MMT may be somewhat chemically bonded. It is concluded that the desorption amount of Sb(OAc)3 is positive correlated with the desorption temperature and significantly influenced by the drying temperature of Sb-MMT. It turns out that when the desorption temperature is 180 °C, the desorption amounts of Sb(OAc)3 from Sb-MMT dried at 240 °C and 450 °C are lower than that dried at 100 °C (Fig. 9). It is reported that [7] if the catalyst is intercalated in abundance into the MMT interlayer, the exfoliated PET/MMT nonocomposite may be obtained. Therefore, the drying temperature of Sb-MMT should be higher than 240 °C in order to make abundant catalyst remain at interlayer during polymerization. 4. Summary In this study, it is found that the adsorption of Sb(OAc)3 on MMT is very fast and the equilibrium is reached within 2 h. The equilibrium adsorption amount of adsorbed Sb(OAc)3 is 0.0997 g/g MMT which is closed to the CEC of Na-MMT. The adsorption isotherm experimental data fit the Langmuir isotherm model better than the Freundlich and D–R models. Moreover, the value of the adsorption energy determined using the D–R isotherm confirms that the adsorption takes place by the cation exchange mechanism. The pseudo-second-order equation provides the best correlation for the adsorption process, and the calculated value of qe is close to the experimental value. Also, the rate of the adsorption process appears to be controlled by the chemical process in accordance with the pseudo-second-order reaction mechanism and the adsorption fits well to the intraparticle diffusion model up to 30 min, but diffusion is not the only rate controlling step. The desorption of Sb(OAc)3 is correlated with the temperature of desorption and significantly influenced by the drying temperature of Sb-MMT. The drying temperature of Sb-MMT should be 240 °C or higher to make enough Sb(OAc)3 to stay in the interlayer during polymerization of PET. References [1] T.S. Anirudhan, P.S. Suchithra, P.G. Radhakrishnan, Appl. Clay Sci. 43 (2009) 336. [2] M. Matzke, K. Thiele, A. Müller, J. Filser, Chemosphere 74 (2009) 568. [3] G. Abate, J.C. Masini, J. Agric. Food Chem. 55 (2007) 3555. [4] G.Z. Zhang, T. Shichi, K. Takagi, Mater. Lett. 57 (2003) 1858. [5] L. Zhu, R.P. Wool, Polymer 47 (2006) 8106. [6] G.H. Guan, C.C. Li, D. Zhang, J. Appl. Polym. Sci. 95 (2005) 1443. [7] W.J. Choi, H.J. Kim, K.H. Yoon, O.H. Kwon, C.I. Hwang, J. Appl. Polym. Sci. 100 (2006) 4875. [8] Z.L. Zhao, X.Q. Wang, Z.Y. Li, S.Y. Du, J. Mater. Eng. Z1 (2008) 280. [9] M. Alexandre, P. Dubois, Mater. Sci. Eng. 28 (2000) 1. [10] T.J. Pinnavaia, G.W. Beall, Polymer–Clay Nanocomposites, Wiley, Chichester, 2000. [11] T.Y. Tsai, C.H. Li, C.H. Chang, W.H. Cheng, C.L. Hwang, R.J. Wu, Adv. Mater. 17 (2005) 1769. [12] Z.Y. Li, Preparation and Property Investigation of High Barrier PET, Beihang University, Beijing, 2008. [13] A.H. Gemeayl, J. Colloid Interface Sci. 251 (2002) 235. [14] C. Blachier, L. Michot, I. Bihannic, O. Barrès, A. Jacquet, M. Mosquet, J. Colloid Interface Sci. 336 (2009) 599. [15] M.M. Saeed, A. Ghaffar, J. Radioanal. Nucl. Chem. 232 (1998) 171. [16] S. Brunauer, The Adsorption of Gases and Vapors: Physical Adsorption, Princeton University Press, Princeton, 1943. [17] A.H. Gemeay, A.S.E. Sherbiny, A.B. Zaki, J. Colloid Interface Sci. 245 (2002) 116. [18] M. Akçay, J. Colloid Interface Sci. 296 (2006) 16. [19] A.B. Karima, B. Mounira, M. Hachkara, M. Bakassec, A. Yaacoubi, J. Hazard. Mater. 168 (2009) 304.
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