Adsorption Thermodynamics and Kinetics of p-Xylene on Activated

Article pubs.acs.org/jced

Adsorption Thermodynamics and Kinetics of p-Xylene on Activated Carbon Ting Qiu,* Yu Zeng, Changshen Ye, and Hui Tian College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou 350108, Fujian, China ABSTRACT: A good activated carbon adsorbent, KC-8, was used to remove residual p-xylene (PX) effectively after the extraction process of treating pure terephthalic acid (PTA) wastewater. The adsorption thermodynamics and kinetics of PX on activated carbon KC-8 were investigated completely and systematically. A series of adsorption equilibrium experiments were conducted under temperatures of (313.15, 323.15, and 333.15) K. The adsorption equilibrium data were fitted to Langmuir and Freundlich isothermal equations. The results showed that the adsorption equilibrium data were agreed with the Freundlich isothermal equation well. Thermodynamic analysis suggested ΔH > 0, ΔG < 0, and ΔS > 0. The adsorption of PX on KC-8 was a spontaneous physical and endothermic adsorption process. Kinetic studies indicated that the adsorption process of PX on activated carbon KC-8 could be described well by the pseudosecond-order kinetic model, and particle diffusion was the main rate-controlling step in the adsorption process.

1. INTRODUCTION Pure terephthalic acid (PTA) is a very important organic chemical raw material and is widely used in polyester industry and other chemical industries. A large amount of wastewater is produced in the PTA refined section. The wastewater contains terephthalic acid (TA), methyl benzoic acid (P-TOL), benzoic acid (BA), Fe, Co, Mn, and other metal ions and few other organisms. If the wastewater is reused directly, iron ions will cause catalyst poisoning, and high P-TOL content will reduce the cleaning effect.1−4 At present, the technology of treating PTA wastewater by the extraction−ultrafiltration−reverse osmosis membrane process is successfully developed.5 p-Xylene (PX) is used as an extraction agent to extract organic acid from the wastewater in the extraction process. It solves membrane pollution and blocking effectively. But the wastewater contains (110 to 120) ppm PX after extraction. The widely used material of the reverse osmosis membrane in industry is polyamide, which is dissolved easily in PX and then shortens the life of the membranes. Therefore, removing PX from wastewater is an urgent problem. In this study, PX was removed by activated carbon adsorption, and the adsorption thermodynamics and kinetics were also investigated. The research not only obtained basic data but also had very important practical significance for further perfecting the technology of treating PTA wastewater by the extraction−ultrafiltration−reverse osmosis membrane method.

vibrator (SHA-C, Zhengzhou, China) and gas chromatograph (Varian CP-3900, Corona, CA, USA). Experimental materials were as follows: macroporous adsorption resins (AB-8, XAD-1, H-103, XAD-200, Qinshi Technology Co., Ltd., Zhengzhou, China), activated carbons (coal, nut shell, KC-6, coconut shell, KC-8, Kecheng Guanghua New Technology Co., Ltd., Beijing), PX (analytical reagent (AR) grade), and toluene (AR grade). 2.2. Methods. 2.2.1. Analysis Method. Because of the low solubility of PX in water, the concentration of PX was determined by extraction.6 Toluene was used as an extractant, and the extraction ratio was 10:1 (PX aqueous solution: toluene). PX and toluene were mixed and stirred and then settled to two layers for a half hour in a separating funnel. A 1 μL sample was taken from the upper layer and analyzed with the gas chromatograph. The conditions of gas chromatography were as follows: The analytical column was a capillary column (0.25 mm × 0.25 μm × 30 m) with a flame ionization detector. The carrier gas was high pure N2; vaporizing chamber temperature: 493.15 K; detector temperature: 493.15 K; splitting ratio: 50:1; injection volume: 1 μL. The column temperature was initially kept at 353.15 K for 1 min and then gradually increased to 423.15 K with a rate of 12 K·min−1 and held for 1 min. 2.2.2. Selection of the Absorbents. Samples of 0.04 g of different adsorbents and 57 g of aqueous solution containing 170 ppm of PX were mixed in 50 mL tapered bottles. The bottles were enclosed and vibrated at 250 rpm and 313.15 K. The experimental temperature was maintained at ± 0.1 K.

2. EXPERIMENTAL SECTION 2.1. Apparatus and Materials. The experimental apparatus consisted of a water bathing constant temperature

Received: February 2, 2012 Accepted: March 27, 2012 Published: April 10, 2012

© 2012 American Chemical Society

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dx.doi.org/10.1021/je300138a | J. Chem. Eng. Data 2012, 57, 1551−1556

Journal of Chemical & Engineering Data

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When the concentration of PX and the adsorption of the activated carbon did not change again, the adsorption could be considered to reach equilibrium. Samples were taken and analyzed respectively. Then the equilibrium adsorption capacity of PX could be calculated. 2.2.3. Adsorption Equilibrium Experiments. A portion of 0.02 g of KC-8 activated carbon whose particle size was (30 to 40) mesh was put into 50 mL tapered bottles. The properties of the used KC-8 activated carbon are presented in Table 1. A 57

Table 2. Adsorption Characteristics of Adsorbents

Table 1. Properties of the Used KC-8 Activated Carbon properties

KC-8 −1

BET surface area (m ·g ) micropore area (m2·g−1) average pore diameter (nm) micropore volume (cm3·g−1) 2

769.39 603.89 2.15 0.32

n

∑ i=1

(Ci − 1 − Ci)Mi m

Qe

ppm

mg·g−1

KC-8 KC-6 coal coconut shell nut shell XAD-200 H-103 XAD-1 AB-8

13.42 15.96 22.08 32.65 45.78 44.05 37.90 64.40 106.57

234.90 233.67 218.96 203.94 191.60 146.82 137.60 121.95 97.58

activated carbon is selected as the suitable adsorbent for adsorbing PX from PTA wastewater. 3.2. Adsorption Thermodynamics. 3.2.1. Adsorption Equilibrium. The Langmuir and Freundlich equations are the most widely used isothermal adsorption equations. The Langmuir isothermal adsorption equation is:7−9

g aqueous solution with the initial PX concentrations of (40, 50, 60, 110, 140, and 170) ppm were added into each bottle. The shaking speed was controlled at 250 rpm under temperatures of (313.15, 323.15, and 333.15) K, respectively. Samples were taken and analyzed until reaching the adsorption equilibrium. The equilibrium concentration of PX was measured, and the equilibrium adsorption capacity was calculated. Then the adsorption equilibrium curves of (313.15, 323.15, and 333.15) K were obtained, respectively. 2.2.4. Adsorption Kinetics Experiments. A portion of 0.02 g of KC-8 activated carbon (30−40 mesh) was mixed with 57 g of aqueous solution containing 110 ppm of PX in 50 mL tapered bottles. Dynamics experiments were conducted under different temperatures of (313.15, 323.15, and 333.15) K at a steady shaking speed of 250 rpm. Samples were taken and analyzed until reaching the adsorption equilibrium. The adsorption capacity of the activated carbon Qt was obtained according to the mass balance eq 1. The curve between Qt and time t was plotted to get the adsorption rate curve. Qt =

Ce adsorbents

Ce C 1 = + e Qe qmKL qm

(3)

where KL is the Langmuir equilibrium constant; qm is the saturated adsorption capacity (mg·g−1). The Freundlich isothermal adsorption equation is:10,11 1 ln Q e = ln Ce + ln KF (4) n where KF and n are the Freundlich constants. Generally, when the value of n is 2 to 10, it means that it is easy to adsorb; when n is less than 0.5, it is difficult to adsorb. According to the Freundlich theory, n can also be used to determinate whether the adsorption is favorable. When n > 1, it is favorable adsorption; when n = 1, it is linear adsorption; when n < 1, it is unfavorable adsorption.12 The adsorption equilibrium data of PX on KC-8 were obtained through equilibrium experiments. The isothermal adsorption curves at different temperatures are depicted in Figure 1. Figure 1 shows that the equilibrium adsorption capacity increases with the increase of temperature and the equilibrium concentration of PX. It indicates that the adsorption of PX on

(1) −1

where Qt is the adsorption capacity at the time t (mg·g ); Ci−1 (ppm) and Ci (ppm) are the concentration of PX in the aqueous solution, before t and at t, respectively; m is the mass of activated carbon (g); Mi is the mass of the solution (g); n is the sampling number.

3. RESULTS AND DISCUSSION 3.1. Selection of Adsorbents. The screening experiment of the adsorbents was conducted according to the method in section 2.2.2. The equilibrium adsorption capacity Qe of PX was calculated as follows: Qe =

(C0 − Ce)M m

(2) −1

where Qe is the equilibrium adsorption capacity (mg·g ); C0 (ppm) and Ce (ppm) are the initial and equilibrium concentration of PX, respectively; M is the mass of the solution (g). The results are listed in Table 2. Table 2 shows that the equilibrium adsorption capacity of PX on KC-8 activated carbon is the largest. Therefore, KC-8

Figure 1. Adsorption isotherms of PX on activated carbon. ■, 313.15 K; ●, 323.15 K; ▲, 333.15 K; , Langmuir model; - - -, Freundlich model. 1552



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Figure 1 indicates that the Freundlich isotherms can represent the adsorption equilibrium data better than the Langmuir isotherms. Although all of the R2 are higher than 0.99 of both equations in Tables 3 and 4, the Langmuir model does not fit the experimental data well especially at low values of Ce. Hence, the adsorption equilibrium of PX on KC-8 can be described with the Freundlich isotherm. Moreover, n > 1 demonstrates that it is a favorable adsorption. 3.2.2. Enthalpy Change ΔH. The Clapeyron−Clausius equation is:13

KC-8 is an endothermic process, and the temperature increase is beneficial to the adsorption process. To assess the nature of the adsorption process of PX on KC8, the adsorption equilibrium data were fitted to Langmuir and Freundlich isothermal equations. The results are shown in Figures 2 and 3, respectively. The fitted curves are shown in

ln Ce,cal =

ΔH − ln K 0 RT

(5)

where Ce,cal is the adsorption capacity calculated by the obtained Freundlich equation at given values of Qe; T is the adsorption temperature (K); K0 is the Clapeyron−Clausius constant; R is the gas constant, 8.314 J·mol−1·K−1. Equation 5 shows that ln Ce has a linear relationship with 1/ T, and the slope is ΔH/R. The relationships between ln Ce and 1/T were plotted to get the values of ΔH. The results are shown in Table 5 and Figure 4. 3.2.3. Free Energy Change ΔG. The equation of calculating ΔG was deduced according to the Gibbs adsorption isothermal equations.14−16 When the adsorption equilibrium equation is the Freundlich equation, the Gibbs free energy change of the adsorption process was related to the Freundlich constant n by the following equation:

Figure 2. Fitted Langmuir adsorption isotherms. ◆, 313.15 K; ●, 323.15 K; ▲, 333.15 K.

ΔG = −nRT

(6)

3.2.4. Entropy Change ΔS. The Gibbs−Helmholtz equation is:14 ΔS =

Figure 1. The correlative isothermal equations along with relevant parameters are listed in Tables 3 and 4. Table 3. Langmuir Isotherm Constants at Different Temperatures KL

K

Langmuir isotherm

313.15 323.15 333.15

Ce/Qe = 0.0044Ce + 0.0162 Ce/Qe = 0.0041Ce + 0.0171 Ce/Qe = 0.0038Ce + 0.0149

qm

g·mg

−1

0.27 0.24 0.26

mg·g−1

R2

227.27 243.90 263.16

0.997 0.994 0.997

Table 4. Freundlich Isotherm Constants at Different Temperatures T/K

Freundlich isotherm

KF

n

R2

313.15 323.15 333.15

ln Qe = 0.1663 ln Ce + 4.6658 ln Qe = 0.1778 ln Ce + 4.6781 ln Qe = 0.2004 ln Ce + 4.6756

106.20 107.51 107.24

6.01 5.62 4.99

0.995 0.998 0.995

(7)

ΔS can be obtained according to eq 7. All of the results are summarized in Table 5. When the adsorption force is van der Waals force, the adsorption heat is (4 to 10) kJ·mol−1; when the force is a hydrogen bonding force, the adsorption heat is (2 to 40) kJ·mol−1. As the force is the exchange of dentate, dipole−dipole interaction, and chemical bonds force, the adsorption heat is about 40 kJ·mol−1, (2 to 29) kJ·mol−1, and above 60 kJ·mol−1, respectively.17 From Table 5, it is known that ΔH > 0 which indicates that the adsorption of PX on the activated carbon is endothermic. It also indicates that the adsorption force cannot be a van der Waals force or chemical bonds force. Generally, the physical adsorption force is less than the chemical adsorption force. The free energy change of physical adsorption is (−20 to 0) kJ·mol−1, and the free energy change of chemical adsorption is (−400 to −80) kJ·mol−1.17 Table 5 shows that the adsorption free energy change ΔG is negative and in the range of (−20 to 0) kJ·mol−1, so the adsorption of PX on KC-8 is a spontaneous physical adsorption process. The adsorption entropy change ΔS is positive in Table 5; thus, it can be deduced that the adsorption process of PX exists as a desorption process of the solvent. Although the freedom decreases with the adsorption of PX, which leads to an entropyreducing process, the desorption process of the solvent (water) is an entropy-increasing process. The molecular weight of water is less than that of PX; therefore, the adsorption of each PX molecule will cause desorption of multiple water molecules. Although adsorption reduces the entropy, the entropy still

Figure 3. Fitted Freundlich adsorption isotherms. ◆, 313.15 K; ●, 323.15 K; ▲, 333.15 K.

T

ΔH − ΔG T

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Table 5. Thermodynamics Parameters for the Adsorption of PX on KC-8 mg·g

ΔG/(kJ·mol−1)

ΔH

Qe −1

254.23 226.02 193.26 159.35 133.88 114.44

−1

kJ·mol 44.99 39.21 31.53 22.05 13.50 5.80

± ± ± ± ± ±

0.05 0.04 0.01 0.02 0.04 0.07

313.15 K

−15.66 ± 4.84

323.15 K

−15.11 ± 3.66

ΔS/(J·mol−1·K−1) 333.15 K

−13.82 ± 1.93

313.15 K 193.66 175.22 150.67 120.42 93.12 68.53

± ± ± ± ± ±

15.63 15.58 15.50 15.52 15.60 15.68

323.15 K 185.98 168.11 144.32 115.01 88.55 64.72

± ± ± ± ± ±

11.50 11.45 11.37 11.39 11.47 11.54

333.15 K 176.53 159.19 136.12 107.69 82.03 58.91

± ± ± ± ± ±

5.96 5.91 5.84 5.85 5.93 6.00

adsorption such as internal diffusion, external diffusion, and adsorption were lumped together; it was assumed that the difference between the average solid phase concentration and the equilibrium concentration was the driving force of the adsorption, and the overall adsorption rate was proportional to either the driving force as in the pseudofirst-order equation or the square of the driving force as in the pseudosecond-order equation. The pseudofirst-order model is:18 ln(1 − F ) = −k1t

(8)

where F = Qt/Qe, k1 is the pseudofirst-order rate constant (min−1); Qe is the equilibrium adsorption capacity (mg·g−1); Qt is the adsorption capacity at time t (mg·g−1) . The pseudosecond-order model is:19,20

Figure 4. Enthalpy changes of adsorption PX on KC-8. ◆, Qe = 254.23; ■, Qe = 226.02; ▲, Qe = 193.26; ×, Qe = 159.34; ∗, Qe = 133.88; -, Qe = 114.44.

t 1 1 = t+ Qt Qe k 2Q e 2

increases because of the desorption of more water molecules in the activated carbon. 3.3. Adsorption Kinetics. 3.3.1. Adsorption Rate Curves of Different Temperatures. According to the method in section 2.2.4, the obtained adsorption rate curves are shown in Figure 5. The equilibrium adsorption capacity of PX are (193.72,

(9)

where k 2 is the pseudosecond-order rate constant (g·mg−1·min−1). The fitted curves are depicted in Figures 6 and 7, and the relevant parameters are presented in Table 6.

Figure 6. Pseudofirst-order model PX on KC-8. ◆, 313.15 K; ●, 323.15 K; ▲, 333.15 K. Figure 5. Adsorption rate curves of different temperatures of PX on KC-8. ◆, 313.15 K; ●, 323.15 K; ▲, 333.15 K.

From Figures 6 and 7, we can see that the adsorption kinetics data fit the pseudosecond-order model better. Table 6 shows that the R2 values of the pseudo second-order model are larger than 0.997, and the fitted values Qe,fit of the pseudosecondorder kinetic equation have small deviations with the experimental data. This further indicates that the adsorption process can be described well by the pseudosecond-order kinetic model. The plot of ln k2 versus 1/T is shown in Figure 8. The graph shows that ln k2 has a good linear relationship with 1/T, and the R2 is 0.99. The fitted equation is

203.26, and 220.37) mg·g−1, respectively, at different temperatures. From Figure 5, we can see that the equilibrium adsorption capacity of PX increases with the increasing temperature. 3.3.2. Adsorption Kinetics Models. To analyze the adsorption rate behavior of PX on KC-8 activated carbon under different temperatures, both the pseudofirst-order kinetic model and the pseudosecond-order kinetic model were used to describe the kinetic data. In the two models, all of the steps of 1554



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t=

ar0 2Q [3 − 3(1 − F )2/3 − 2F ] 6Dc0

(12)

where a is the chemical measurement constant (8.315); r0 is the particle diameter of the activated carbon (cm); Q is the adsorption capacity of the activated carbon (mg·g−1); c0 is the solution concentration of main body (ppm); kf is the liquid film mass transfer coefficient (cm·s−1); D is the diffusion coefficient of particles (cm2·s−1). If film diffusion is the rate-controlling step in the adsorption of PX on the KC-8 activated carbon surface, the values of F (F=Qt/Qe) have a linear relationship with t. If particle diffusion is the rate-limiting step, then the values of [3 − 3(1 − F)2/3 − 2F] have a linear relationship with t. The curves of F versus t and [3 − 3(1 − F)2/3 − 2F] versus t according to the experimental data are plotted in Figure 9.

Figure 7. Pseudosecond-order model PX on KC-8. ◆, 313.15 K; ●, 323.15 K; ▲, 333.15 K.

Table 6. Pseudosecond-Order Kinetics Parameters parameters Qe,fit T/K

mg·g

313.15 323.15 333.15

k2

−1

Qe,exp

g·(mg·min) −3

175.86 191.31 221.07

4.72·10 4.45·10−3 3.98·10−3

−1

R

2

0.998 0.997 0.999

mg·g−1 175.74 187.18 219.76

Figure 9. Liquid film diffusion and particle diffusion model of PX. ■, [3 − 3(1 − F)2/3 − 2F]; ◆, F.

The linearity of the plot of [3 − 3(1 − F)2/3 − 2F] and t indicates that the particle diffusion of PX on the KC-8 activated carbon is the main rate-limiting step. With the kinetic data, the linear fits obtained by plotting [3 − 3(1 − F)2/3 − 2F] and t are shown in Figure 10. It shows good linear relationships at Figure 8. Relationship between adsorption rate constant and temperature of PX on KC-8.

ln k 2 = −1220.886

1 − 4.888 T

according to the following Arrhenius equation: ln k 2 = −

E + ln A RT

(10)

The slope of plot of ln k2 versus 1/T is used to evaluate adsorption activation energy E, which is found to be 10.15 kJ·mol−1. 3.3.3. Controlling Step. The mechanism of the PX adsorption on the studied activated carbon was based on the previous work on the equilibrium of the adsorption together with kinetic and thermodynamic data. The mass transfer effect on the adsorption kinetics can be analyzed by the following eqs 11 and 12.21 When it is liquid film diffusion: t=

ar0Q F 3c0k f

Figure 10. Particle diffusion model fitted curves of PX under different temperatures. ◆, 313.15 K; ●, 323.15 K; ▲, 333.15 K.

different temperatures. So the adsorption process is controlled by particle diffusion because the values of R2 are higher than 0.99.

4. CONCLUSIONS (1) In all studied adsorbents, KC-8 is a suitable adsorbent for removing PX from PTA wastewater.

(11)

When it is particle diffusion: 1555



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(2) The adsorption of PX on KC-8 is a spontaneous physical adsorption process, and it is also an endothermic and entropy-increasing process. The adsorption activation energy of the adsorption of PX on KC-8 is 10.15 kJ·mol−1. (3) The adsorption equilibrium data of PX on KC-8 agree with the Freundlich isothermal equation well. The adsorption process can be described well by the pseudosecond-order kinetic model. Particle diffusion is the main rate-controlling step under the shaking speed of 250 rpm.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Funding

The authors would like to thank the National Natural Science Foundation of China for their financial support (No. 21176049). Notes

The authors declare no competing financial interest.

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REFERENCES

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Adsorption Thermodynamics and Kinetics of p-Xylene on Activated

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