Article pubs.acs.org/jced
Multiwall Carbon Nanotube for Adsorption of Acetic Acid Ö nder Ö zcan,† Iṡ mail Iṅ ci,‡ and Yavuz Selim Aşci̧ ‡,* †
MN Pharmaceuticals, Gayrettepe 34349, Istanbul Chemical Engineering Department, Engineering Faculty, Istanbul University, 34320, Istanbul
‡
ABSTRACT: The adsorption capabilities of multiwall carbon nanotubes (MWCNTs) for acetic acid have been investigated experimentally and theoretically. Carbon nanotubes (CNTs) are an important new material in the carbon family. Adsorption time required to attain the steady state and influences of quantities of alumina, effects of temperature, and effects of initial values of acetic acid concentrations have been determined experimentally. The most common used adsorption isotherms have been drawn by using experimental results. These are selected as Langmuir, Freundlich, and Temkin. In experimental works, the equilibrium is considerably affected by initial concentration values of acetic acid at various temperatures (278 K, 298 K, 318 K). Langmuir isotherm has been found the best suitable for acetic acid. Also kinetic models for adsorption such as Elovich and pseudo-first and -second orders have been used. The equilibrium results fit well within the Elovich model and while the pseudo-second order model is a good representation of the adsorption, the pseudo-first order approach does not fit with experimental data. The model parameters have been calculated.
1. INTRODUCTION Carboxylic acids, especially in the food industry, are used intensively, as ready-sweetened and antimicrobial protection, and as a reactant in many chemical manufacturing processes, as well as in pharmaceutical, cosmetic, metal cleaning, textile, paper, and gelatin industries. Carboxylic acid is usually obtained in bioengineering processes by converting its aqueous solutions. Its transformation and purification are performed in many stages. Consequently, recovery of carboxylic acid from its solution in water is an important research area in chemical engineering. Separations and purifications of bitechnolgical acids from diferrent aqueous media are very arduous, because distillation of these nonvolatile solutions requires a large amount of energy.1−6 Acetic acid is a very popular and important chemical for the chemical industry. Acetic acid extensively has been produced by fermentation processes with different methods. The important and common problem of this process is the low concentration values of carboxylic acid (approximately 10%). Recently, the separation of acids that have biotechnological importance from their solutions in water has been extensively studied, with focus on the extraction and distillation processes. Because of the high solvent consumption, difficulties of solvent recovery, and the challenges of high energy requirements during these processes, alternative separation methods are being tested today.7−9 Adsorption is an available and popular separation method for the recovery of organic compounds. This method widely used for separation of fermentation products has a low energy consumption, produces a high product purity, and requires a simple regeneration.10−13 A new allotrope of carbon (CNT) was discovered by Iijima in 1991, and researchers have begun to investigate its potential applications. One of these applications, the adsorption of biotechnological product, is a relatively inexpensive, practical, and usable technique. Because of their special structures, CNTs © XXXX American Chemical Society
have huge adsorption capacities making them a preferred new material as adsorbent for different chemicals.14−16 There has been much research in the separation of acids from their solutions in water by using adsorption. Bi et al. studied the separation of lactic acid from fermentation media with a special silica,17 Soni et al. studied the recovery of glycolic acid with bagasse flyash.18 Aljundi reported the separation of lactic acid from aqueous solution obtained by fermentation with zeolite.19 In the present study, we study the adsorption capability of multiwall carbon nanotubes (MWCNTs) for the separation of acetic acid from diluted solutions.
2. MATERIAL AND METHODS Acetic acid was purchased from Merck. MWCNTs were obtained from Sigma-Aldrich. MWCNTs are (40 to 60) wt % carbon base with diameter × length being (10−15) nm × (0.1−10) μm. The purity of the substances studied are provided in Table 1 as Table 1. The Purity of Substances Studied chemical name acetic acid MWCNTb a
source Merck SigmaAldrich
initial mass fraction purity
purification method
analysis method
0.99 0.40
none none
HPLCa
High performance liquid chromatography. bMultiwall carbon nanotube.
mass fraction. Acetic acid solutions with known concentration have been prepared by using distilled water and were added to different amounts of MWCNT. All experimental solutions were Received: September 14, 2012 Accepted: January 24, 2013
A
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taken for each acetic acid solutions and added to 20 mg of CNT. Influence of Temperature. To explain the influences of temperature, experiments were performed at three temperatures for various acetic acid concentrations. A 20 mL portion of the samples was taken for each acetic acid solution and added 20 mg of CNT. These experiments were performed at (278, 298, and 318) K. Results of the temperature effects on adsorption with various initial quantities of acid were listed in Table 3.
shaken a sufficient time for equilibration with a GFL-3031 shaker. The shaker was operated at 278 K, 298 K, and 318 K. All solutions were percolated with 0.45 μm sterile filter paper. For acetic acid concentration determination the Agilent 1100 series HPLC was used.20,21
3. RESULTS AND DISCUSSION In the present work, effects of various conditions such as adsorption equilibrium time, initial concentration values of acetic acid, and temperature have been studied. These experimental results were used to determine the kinetics and equilibrium of acetic acid adsorption onto MWCNTs. Influence of Adsorption Time. The effect of adsorption time for acetic acid by MWCNTs has been investigated for the determination of equilibrium time of a period of 150 min for 123.2 mg.kg−1 initial acetic acid concentrations at 298 K. In these experiments, 50 mL of the samples was taken for each acetic acid solution and the MWCNT dosage was 15 mg. Experimental results of equilibrium time for the adsorption of acetic acid are presented in Table 2, also these values are drawn in Figure 1.
Table 3. The Initial Acid Concentration Changes at Three Different Temperaturesa temp. T
initial acid concn
amount of MWCNT
equilibrium concn Ce
amount of adsorbed acid Qe
K
mg·kg−1
g
mg·kg−1
mg·g−1
278
37.584 56.376 78.300 93.960 117.450 37.584 56.376 78.300 93.960 117.450 37.584 56.376 78.300 93.960 117.450
0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02
31.32 50.54 71.51 86.65 109.92 32.21 51.21 71.98 86.91 110.96 33.67 51.66 73.74 89.30 112.26
5.72 5.81 6.56 7.24 7.35 5.09 5.33 6.32 6.47 6.49 4.04 4.84 4.63 4.83 5.01
298
Table 2. The Equilibration Time for the Adsorption of Acetic Acid onto MWCNT Particles at 298 Ka initial acid concn, C0
time
equilibrium concn Ct
amount of adsorbed acid Qt
g
h
mg·kg−1
mg·g−1
0.15 0.15 0.15 0.15 0.15 0.15 0.15
0.17 0.34 0.50 1.00 1.50 2.00 2.50
121.63 120.24 120.23 120.07 119.45 119.35 119.29
1.047 1.973 1.980 2.087 2.500 2.567 2.613
temp. T
amount of MWCNT
mg·kg−1
K
123.2 123.2 123.2 123.2 123.2 123.2 123.2
298 298 298 298 298 298 298
318
a Standard uncertainties u are u(Ce) = 0.01 Ce, u(T) = 0.02 K, and the combined expanded uncertainty Uc is Uc(Qt) = 0.001 Qt .
Adsorption Isotherms. For the engineering of the adsorption, the most important step is to find the adsorption isotherms. In the present work, we have studied the Langmuir, Freundlich, and Temkin isotherm to obtain equilibrium charateristics of acetic acid adsorption by MWCNT. Langmuir Isotherm. The Langmuir isotherm equation is22,23
a
Standard uncertainties u are: u(Ct) = 0.01 Ct, u(T) = 0.02 K, u(t) = 0.001 h, and the combined expanded uncertainty Uc is Uc(Qt) = 0.001 Qt.
Qe =
KAQ oCe 1 + KACe
(1)
where Qe and Qo denote the acetic acid concentration values at MWCNT and saturation capacity, respectively. Ce means equilibrium concentration. In this isotherm, KA is used as a constant for the adsorption ability of the adsorbent. The values of KA and Qo are found by using Ce = −KL + Ce
Qo Qe
KL =
1 KA
(2)
where KL and Qo are the intercept and slope of the adsorption line drawn at different temperatures in Figure 2. By using these data, Langmuir adsorption parameters have been calculated for 278 K, 298 K, and 308 K. The values calculated are given in Table 4. Freundlich Isotherm Equation. The Freundlich isotherm can be determined from24−26
Figure 1. The equilibration time for the adsorption of acetic acid onto MWCNT particles at 298 K.
Influence of Initially Concentration of Acetic Acid. Acetic acid adsorption by MWCNT experiments have been conducted at five different initial concentration values of acetic acid. These concentrations are (37.58, 56.38, 78.30, 93.96 and 117.45) mg·kg−1. In these experiments, 20 mL of samples were
Q e = K f Ce1/ n
(3)
Equation 3 can be linearized to calculated the n and Kf values; as a result of this linearization eq 4 is found. The new form can B
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Table 5. Freundlich Isotherm Parameters for Adsorption of Acetic Acid onto MWCNT Freundlich Isotherm T
Kf
K
n
mg·g−1 kg·mg−1·n−1
R2
278 298 318
4.55 5.31 5.31
2.42 2.09 3.05
0.6717 0.7894 0.7097
of adsorption with binding energies with the same distribution, up to maximum value.27,28 Temkin isotherm for acetic acid adsorption can be written as Qe =
RT ln(KTCe) b
(5)
Figure 2. Langmuir isotherms of acetic acid MWCNT adsorption at different temperatures: ■, 278 K; •, 298 K; ▲, 318 K.
If this equation would be linearized,
Table 4. Langmuir Isotherm Parameters for Adsorption of Acetic Acid onto MWCNTs
In this final equation,
Q e = B1 ln(KT) + B1 ln(Ce)
B1 =
Langmuir Isotherm T
Q0
KL
K
mg·g−1
kg·mg−1
R2
278 298 318
7.76 5.69 8.70
0.058 0.068 0.054
0.9571 0.9833 0.9786
(6)
RT b
(7)
Kinetics. The Temkin isotherm coefficients B1 and KT can be found by using slope and the interception of a graphic of Qe vs ln(Ce). In this isotherm KT is called the coefficient of equilibrium binding (l mol−1) and corresponds to a maximum. The other coefficient concerns the adsorption heat. Figure 4
be written as log Q e = log K f + (1/n)log Ce
(4)
Plots of log Qe and log Ce were used for the aim of obtaining Kf and 1/n. Freundlich isotherms for 278 K, 298 K, and 308 K are shown in Figure 3. Results of the Freundlich isotherm are summarized briefly in Table 5.
Figure 4. Temkin isotherms of acetic acid-MWCNT adsorption at different temperatures: ■, 278 K; ●, 298 K; ▲, 318K.
Table 6. Temkin Isotherm Parameters for Adsorption of Acetic Acid onto MWCNTs Temkin Isotherm T
Figure 3. Freundlich isotherms of acetic acid MWCNT adsorption at different temperatures: ■, 278 K; ●, 298 K; ▲, 318 K.
Temkin Isotherm Equation. This isotherm, mathematically explains the adsorptive−adsorbent mutual behavior. In this isotherm, it is assumed that molecular adsorption heat in one molecular layer declines as the coating of adsorbent during the adsorption because of adsorbent−adsorbate mutual behaviors. Another assumption of the Temkin isotherm is characterization
K
B1
KT
R2
278 298 318
1.31 0.84 1.27
1.60 3.64 3.06
0.6645 0.7847 0.7272
gives the Temkin isotherm plot for the MWCNT. The Temkin isotherm data is given in Table 6. In this study the Langmuir isotherm fit experimental results for acetic acid, R2 values of 0.9577 at 278 K. But the Freundlich C
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and Temkin isotherms do not fit the experimental results of CNT adsorption. It can be concluded that by using these results adsorption advances as a monolayer form. In the past decades, adsorption reaction models have been widely developed to understand the adsorption kinetics. Much research has been used for a kinetic model to describe adsorption data. To determine adsorption kinetics of acetic acid by MWCNTs, pseudo-second order and Elovich models are used. The Elovich Kinetic Model Equation. We can write the Elovich kinetic model as18 dQ = α exp( −β Q) dt
in linear form from eq 11; in that case eq 11 can be written as t 1 1 = + t 2 Qt Qe k 2Q e
where Qe is founded by using the t/Qt vs t plot. It may be easily determined from the slope; k2 is found from the intercept of that line.28 Pseudo-second-order kinetic models calculations were performed by using a linear regression technique. It has been found that the best representation of experimental data for acetic acid adsorption onto MWCNT is pseudo-second-order. Acetic acid adsorption onto MWCNT does not obey pseudofirst-order. The correlation coefficient of pseudo-second-order is 0.9940 as presented in Figure 6. Parameters of the model can be seen in Table 8.
(9)
In this model, α is known initially as the rate of adsorption with the unit of mg·g−1·min−1. β is known as coefficient of desorption as g·mg−1. We can rewrite the Elovich kinetic model equation, with assumptions of αβt ≫ t and by using two boundary conditions. These conditions are Q = 0 for time of t = 0 and Q = Qt for time of t = tt. With these assumptions, the Elovich model equation can be written as Q t = 1β ln(αβ) +
1 ln(t t) β
(12)
(10)
If acetic acid adsorption with using MWCNT obeys the Elovich kinetic model, Qt vs ln(tt) must give a line with a slope of values of (1/β); ln(αβ) can be found as the intercept. This procedure has been applied in Figure 5. Coefficients of the Elovich equation were given in Table 7.
Figure 6. Pseudo-second-order kinetic of acetic acid−MWCNT adsorption.
Table 8. Pseudo-Second-Order Kinetic Parameters for the Removal of Acetic Acids by MWCNT Pseudo-second-order
Table 7. Elovich Equation Kinetic Parameters for the Removal of Acetic Acid by MWCNT Elovich Equation 1.03 · 10
4
β
R2
1.94
0.8837
Pseudo-Second-Order Kinetic Model. This approach is given as follows:29 dQ t dt
= k 2(Q e − Q t)2
K2
R2
0.048
86.2
0.9940
4. CONCLUSIONS This study deals with adsorption of acetic acid onto MWCNTs. Different parameters which influence the adsorption have been found. The Langmuir equation gave an acceptable fitting with experimental results to anticipate adsorption data with a R2 value higher than 0.95 at various temperatures. It has been concluded that from experimental and calculated results, the adsorption processes advance as monolayer adsorption. In this study, we investigated different kinetic equations to explain the rate of reaction and kinetics for adsorption of acetic acid on MWCNT, which were best represented by second-order kinetic models.
Figure 5. Elovich model of acetic acid−MWCNT adsorption.
α
Qe
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
(11)
where k2 is known as coefficient of rate of pseudo-secondorder (g·mg−1·min−1). It is possible to obtain an equation
Notes
The authors declare no competing financial interest. D
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