Kinetic and thermodynamic studies on the adsorption of xylenol orange onto MIL-101(Cr)

Chemical Engineering Journal 183 (2012) 60–67

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Kinetic and thermodynamic studies on the adsorption of xylenol orange onto MIL-101(Cr) Chen Chen, Meng Zhang, Qingxin Guan, Wei Li ∗ Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin 300071, China

a r t i c l e

i n f o

Article history: Received 26 September 2011 Received in revised form 6 December 2011 Accepted 7 December 2011 Keywords: Chromium-benzenedicarboxylates (MIL-101) Xylenol orange Kinetic studies Thermodynamic studies

a b s t r a c t A highly porous metal-organic framework (MOF) material based on chromium-benzenedicarboxylates (MIL-101) was applied to the adsorption of xylenol orange (XO) from aqueous solution. Adsorption kinetics and isotherms were determined from the experimental data, and the results showed that pseudosecond-order kinetic model and Langmuir adsorption isotherm matched well for the adsorption of XO onto MIL-101. Thermodynamic parameters including free energy, enthalpy, and entropy of adsorption were obtained, and all the results were in favor of the adsorption. It was found that the adsorbed amounts decreased with increasing pH value of the XO solution, which indicates that the mechanism may be the charge interactions between the dye stuffs and the adsorbents. The used MIL-101 could be regenerated by washing with a dilute concentration of NaOH solution. Compared with other adsorbents like active carbon and MCM-41, especially in high concentrations of XO, MIL-101 demonstrated a superior dye adsorption capability. © 2011 Elsevier B.V. All rights reserved.

1. Introduction In our daily life, more than 100,000 types of commercial dyes are used with a production of over 7 × 105 tonnes annually [1,2]. Many of them are considered to be toxic and even carcinogenic [3]. In particular, synthetic dyes in an effluent, even in a small amount, are highly visible and have undesired effects not only on the environment, but also on living creatures. However, most toxic dyestuffs are stable to light and oxidants, which makes them difficult to degrade [4]. There are a number of technologies available for the removal of dyestuffs, such as physical, chemical and biological methods [5–7]. Adsorption technology is regarded as one of the most competitive methods because it does not need a high operation temperature and several coloring materials can be removed simultaneously [4]. Activated carbon, as a traditional adsorbent that has been used extensively in industry, is inadequate for the adsorption of dyes [2,8]. Porous metal-organic framework materials (MOFs) containing nanometric pores and channels, currently receive a considerable amount of attention because of their potential applications in a number of industrially important areas, such as gas storage, separation and heterogeneous catalysis areas, which are traditionally

∗ Corresponding author. Tel.: +86 22 23508662; fax: +86 22 23508662. E-mail address: [email protected] (W. Li). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.12.021

attributed to mineral oxides such as zeolites [9,10]. Among the numerous MOFs reported so far, one of the most topical solids is a porous chromium-benzenedicarboxylates (Cr-BDCs) namely MIL101 [11,12] (MIL stands for Material of Institute Lavoisier), which is a very important material because of its mesoporous structure and huge porosity. MIL-101 has demonstrated good performance in hydrogen storage [10]. There derives a series application of gas storage/adsorption on MIL-101, such as CO2 [13], CH4 [13,14], long-chain alkanes [15]. Recently, progressively more research on MIL-101 has been applied to the field of catalysis [16,17], and its catalytic activity is comparable to commercial catalysts. At the same time, MIL-101 can also be applied to drug delivery where the drug content is larger and the delivery rate is slower compared to similar materials with comparable cage sizes, such as MCM-41 [18,19]. However, there are only a few articles about aqueous solution adsorption onto MIL-101. Haque et al. [5] introduced MIL-101 for the adsorption of methyl orange (MO) from aqueous solution, in which MIL-101 demonstrated excellent adsorption properties. We have found experimentally that MIL-101 is much better at adsorbing xylenol orange (XO, Fig. 1) than MO. MIL-101 also shows excellent adsorption of XO over a wide concentration range, moreover, the adsorption saturation was much greater than traditional adsorbents like MCM-41 and active carbon. In addition, the material can be reused after washing with lye. A series of experiments was carried out to understand the characteristics of

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Fig. 1. The structure of reactive dye.

XO adsorption onto MIL-101 and the possibility of using MOFs as adsorbents for the removal of dye materials from wastewater.

Fig. 2. Powder XRD patterns of MIL-101 (A) fresh MIL-101; (B) 1st reused MIL-101; (C) 2nd reused MIL-101.

3. Results and discussion 3.1. Characterization results

2. Experimental The MIL-101(Cr) was synthesized by a hydrothermal method as described previously [11]. Powder X-ray diffraction (XRD) patterns were collected on a Bruker D8 focus diffractometer, with Cu K␣ at 40 kV and 40 mA between 2◦ and 40◦ (2) with a scanning speed of 12◦ /min. Nitrogen adsorption–desorption isotherms of samples at 77 K were measured with a BEL-MINI adsorption analyzer. The surface area was calculated using a multipoint Brunauer–Emmett–Teller (BET) model. Zeta potential was measured with Malvern Zetasizer (Nano series). Before the measurement, MIL-101 was dissolved to water of different pH value from 2 to 12 with a certain concentration (0.05 wt%) under ultrasound. An aqueous stock solution of XO (2000 ppm) was prepared by dissolving XO (molecular formula: C31 H28 N2 Na4 O13 S) in deionized water. The aqueous XO solution was diluted to different concentrations from 100 to 400 ppm. The XO concentrations were determined using ultraviolet spectrophotometer at 273 nm. The calibration curve was obtained from the spectra of standard solutions (1–100 ppm). Before adsorption, MIL-101 was dried in a vacuum oven at 100 ◦ C for 3 h. The powder (0.0500 g) was added to the aqueous solution of XO (50 mL) with different concentrations from 100 to 400 ppm. The admixture was mixed well under magnetic stirring at a fixed power for a fixed time (1–180 min) at 298 K. After adsorption, the solution was separated from MIL-101 by a sintered filter (G6), and the concentration was determined by UV spectroscopy. To obtain the thermodynamic parameters of adsorption such as G (free energy change), H (enthalpy change) and S (entropy change), the adsorption was repeated at 308 and 318 K. To determine the adsorption capacity at various pH values, the pH value of the XO solution was adjusted with 1 M NaOH or 1 M HCl aqueous solution. To compare with active carbon and MCM-41, the same dose of active carbon (coconut-made charcoal, BET surface: 779 m2 /g) and MCM-41 (BET surface: 945 m2 /g) were added to the aqueous of XO (50 mL). The used MIL-101 was mixed with 0.01 M NaOH aqueous solution (about 0.05 g in 50 mL aqueous solution) and stirred by magnetic stirring for 1 h. After filtration, the filter cake of MIL-101 was washed with 0.01 M HCl aqueous solution and deionized water until the pH value of the filtrate was near 7. Then the powder was dried and reused for the next adsorption run.

Powder XRD patterns of MIL-101 are shown in Fig. 2. The diffraction peaks matched well with previous papers [11,12]. The peak intensity decreased with the number of times the material was reused, and from Fig. 2, we can see that the peak that at 2◦ disappeared, when we washed MIL-101 with NaOH solution, but the main peak at 8–10◦ remained the same. In our opinion, the nature of the structure was destroyed and the crystallinity decreased after the NaOH solution wash. The BET surface areas and pore volumes of the materials measured by N2 adsorption at 77 K are shown in Table 1. The values are in good agreement with most of those reported in previous papers [14,20,21], but much smaller than the results of Ferey et al. [11,12]. The values of BET surface areas steadily declined with number of the times MIL-101 was reused, while the pore volume and pore diameter increased. The results corresponded with the analysis of the powder XRD patterns. 3.2. Adsorption kinetics Adsorption was carried out at different temperature (298–318 K). The results are shown in Fig. 3. As can be seen, the adsorption curves kept the same trend, and all of the adsorption achieved equilibrium in 30 min, showing a rapid adsorption of XO onto MIL-101. To obtain the adsorption kinetics, the changes of adsorption amount with time were treated with pseudo-second-order kinetic model [22,23]: dqt = k2 (qe − qt )2 dt

(1)

1 1 t = + t qt qe k2 q2e

(2)

Table 1 The BET surface areas and pore volumes.

Fresh 1st reused 2nd reused

Specific surface (m2 /g)

Total pore volume (cm3 /g)

Mean pore diameter (nm)

2663.7 1918.7 1176.7

1.47 0.81 0.56

2.52 2.78 3.00

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Fig. 4. Plots of pseudo-second-order kinetics of XO adsorption onto MIL-101: (a) T = 298 K; (b) T = 308 K; (c) T = 318 K.

Fig. 3. Effect of contact time and initial XO concentration on the adsorption of MO onto MIL-101: (a) T = 298 K; (b) T = 308 K; (c) T = 318 K.

where qt : amount adsorbed at time (t) (mg/g); qe : amount adsorbed at equilibrium (mg/g); k2 : pseudo-second-order kinetic constant (g/(mg min)). Eq. (2) is derived from Eq. (1) by integrating for the boundary conditions t = 0 to t = t and qt = 0 to qt = qe . There is no need to know any parameter beforehand and the equilibrium adsorption density, qe , can be calculated from Eq. (3) [23,24]. Fig. 4 shows the plots of pseudo-second-order kinetics of XO adsorption onto MIL-101 at different temperatures. From the plots, k2 and qe were calculated with the intercepts and slopes. The results are shown

in Table 2. As can be seen in Fig. 4 and Table 2, the data fit quite well for the adsorption of XO onto MIL-101 under the experimental conditions employed (correlation coefficient, R2 > 0.999). With temperature increasing, k2 increased, that is, the initial adsorption rate increased with temperature. However, at a constant temperature, k2 decreased with the initial concentration of XO. The data of adsorption amount can also be treated with pseudofirst-order kinetic model [23,25]: dq = k1 (qe − qt ) dt

(3)

ln(qe − qt ) = ln qe − k1 t

(4) (min−1 ).

where k1 : pseudo-first-order kinetic constant Eq. (4) is derived from Eq. (3) by integrating for the boundary conditions t = 0 to t = t and qt = 0 to qt = qe . The equilibrium

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Table 2 Pseudo-first-order and Pseudo-second-order kinetics constants of XO adsorption onto MIL-101. T (K)

Pseudo-first-order kinetics qe,cal (mg/g)

Pseudo-second-order kinetics

qe,exp (mg/g)

k1 (min−1 )

R2

qe,cal (mg/g)

k2 (g/(mg min))

R2

100

298 308 318

6.24 ± 5.64 7.98 ± 4.70 4.30 ± 2.85

6.80 × 10−2 1.31 × 10−1 6.99 × 10−2

0.603 0.924 0.720

97.27 ± 0.07 96.71 ± 0.06 96.33 ± 0.03

5.71 × 10−2 7.53 × 10−2 9.88 × 10−2

1.000 1.000 1.000

200

298 308 318

76.96 ± 21.90 69.76 ± 32.52 61.68 ± 32.12

1.01 × 10−1 9.37 × 10−2 8.25 × 10−2

0.928 0.905 0.852

195.3 ± 0.1 193.4 ± 0.7 191.2 ± 0.8

4.25 × 10−3 4.55 × 10−3 4.73 × 10−3

0.999 0.999 0.999

193.5 191.9 189.9

300

298 308 318

89.78 ± 2.97 80.51 ± 6.32 75.54 ± 11.04

5.94 × 10−2 5.09 × 10−2 5.87 × 10−2

0.998 0.986 0.966

271.0 ± 0.9 269.5 ± 0.9 265.2 ± 0.9

2.68 × 10−3 2.76 × 10−3 3.34 × 10−3

0.999 0.999 0.999

268.8 267.6 263.7

400

298 308 318

159.3 ± 16.5 123.9 ± 17.1 113.4 ± 19.5

6.61 × 10−2 5.00 × 10−2 3.74 × 10−2

0.986 0.958 0.893

311.5 ± 0.7 304.8 ± 0.9 303.9 ± 0.9

1.42 × 10−3 1.65 × 10−3 1.73 × 10−3

0.999 0.999 0.999

307.1 301.8 301.6

adsorption density qe is required to fit the data, but in many cases qe remains unknown due to slow adsorption processes. The first-order equation of Lagergren does not fit well to the whole range of contact time and is generally applicable over the initial stage of the adsorption process [24]. From the plots, k1 and qe can be calculated with intercepts and slopes. The results are shown in Table 2. This model did not fit the experimental data, as the correlation coefficient was poor. Under most conditions, k1 decreased with temperature, that is, the initial adsorption rate decreased with temperature, which was inconsistent with theory or experimental observation. Comparing the correlation coefficient (R2 ), pseudo-secondorder kinetic model matched well with this work. Additionally, from Table 2, the qe s value calculated with this model is in good agreement with experimental data. On the contrary, the qe s value calculated with pseudo-first-order kinetic model differed significantly from the results of experiment. The study in Ref. [26] indicated that the sorption process obeys pseudo-firstorder kinetics at high initial concentrations of solute, while it obeys pseudo-second-order kinetics model at lower initial concentrations of solute. Moreover, that study [26] determined that rate constants are not only dependent on the temperature, but also on the initial concentration of the solution.

3.3. Adsorption isotherms To describe the adsorption isotherm more scientifically, the Langmuir and Freundlich model equations were selected for use in this study. The Langmuir adsorption isotherm has been successfully applied to many pollutant adsorption processes from aqueous solution. The equation is expressed as: Qe =

Q0 KL Ce 1 + KL Ce

(5)

where Qe : the equilibrium adsorption capacity of XO on the adsorbent (mg/g); Ce : the equilibrium XO concentration in solution (mg/L); Q0 : the maximum monolayer capacity of adsorbent (mg/g); KL : the Langmuir adsorption constant (L/mg), related to the free energy of adsorption. A linear plot of (Ce /Qe ) versus Ce is obtained from the model as shown in Fig. 5. KL and Q0 were calculated from the slope and intercept of the different straight lines representing the different temperature. Table 3 lists the calculated results. The data fit quite well for the adsorption of XO onto MIL-101 under the experimental conditions (correlation coefficient, R2 > 0.999). These results indicated that the adsorption of XO onto MIL-101 was a typical monomolecular-layer adsorption, and the maximum monolayer capacity Q0 was stable (Q0 = 322–326 mg/g). In addition, the

97.17 96.69 96.67

Fig. 5. Langmuir plots of the isotherms for XO adsorption onto MIL-101.

Langmuir constant KL demonstrated an opposite trend with temperature. The Freundlich isotherm used for isothermal adsorption is a special case for heterogeneous surface energy in which the energy term in the Langmuir equation varies as a function of surface coverage strictly due to variation of the sorption. The Freundlich equation is given as: 1/n

Qe = KF Ce

(6)

(mg/g(L/mg)1/n )

where KF and 1/n represents the Freundlich constants corresponding to adsorption capacity and adsorption intensity, respectively [3]. KF and 1/n can be determined from the linear plot of ln (Qe ) versus ln (Ce ). Table 3 lists the calculated results. The magnitude of the exponent 1/n gives an indication of the favorability of adsorption. Values, n > 1 represented favorable adsorption condition [23], while the correlation coefficient R2 < 0.90 shows a poor agreement with the experimental data. The Langmuir equation was developed originally to describe individual chemical adsorbents, and is applicable to physical adsorption (monolayer) within a low concentrations range. The Freundlich equation is an empirical approach for adsorbents with Table 3 Adsorption isotherms of XO adsorption onto MIL-101. T (K)

Langmuir adsorption isotherm KL (mg

298 308 318

0.171 0.150 0.129

−1

)

2

Freundlich adsorption isotherm

Q0 (mg/g)

R

KF (mg/g(L/mg)1/n )

n

R2

325 ± 7 322 ± 5 324 ± 3

0.999 0.999 0.999

86 ± 27 79 ± 16 74 ± 20

3.69 3.58 4.36

0.779 0.794 0.868

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Table 4 Adsorption thermodynamics constants of XO adsorption onto MIL-101.

298 K 308 K 318 K

G (kJ/mol)

H (kJ/mol)

S (J/mol/K)

R2

−28.89 −29.52 −30.08

−11.0 ± 0.6

60 ± 2

0.994

very uneven adsorbing surfaces [27,28]. It can be seen that the linear correlation coefficient (R2 ) using the Langmuir model are higher than those for the Freundlich isotherm for XO, which suggested that the Langmuir model describes the adsorption of XO onto MIL-101 better. 3.4. Adsorption thermodynamics The adsorption thermodynamics function obtained from the isotherms will further reveal the adsorption mechanism. As shown in Table 3 and Fig. 5, the experiment was carried out over the temperature range of 298–318 K, and the Langmuir equation was selected to fit the adsorption isotherm. In this work, there were few differences between the results of Q0 , which can be considered approximately equal, at the same time, KL decreased with the temperature.The Gibbs free energy change G can be calculated by the following equation: G = −RT ln KL

(7)

where KL : the Langmuir adsorption constant (L/mol). The Langmuir constant KL can be obtained from the slope/intercept of the Langmuir plot of Fig. 5. The adsorption under the experiment condition is a spontaneous process because of the negative free energy change (G) shown in Table 4. The enthalpy change H and S can be obtained from the van’t Hoff equation: ln KL =

S H − R RT

Fig. 7. (a) The pH effect on the adsorption of XO onto MIL-101 (initial concentration: 400 ppm, T = 298 K); (b) the pH effect on the Zeta potential of MIL-101.

(8)

A linear plot of ln KL versus 1/T is obtained from the model as shown in Fig. 6.The enthalpy change (H) and entropy change (S) can be calculated from the slope and intercept of the van’t Hoff plot, respectively. As shown in Table 4, the negative enthalpy change (H) suggests that the adsorption of this work is an exothermic reaction. However, in literature [5], the adsorption of MO onto MIL101 obtained a positive value; the reason may be due to a stronger interaction between preadsorbed water and the adsorbent than the interaction between MO and the adsorbent. In addition, the entropy change (S) is positive, suggesting the process results in an

increase in entropy. In the solid–liquid adsorption system, adsorption of solute onto the adsorbent and desorption of solvent from the adsorbent both exist; the former one is an entropy reduction process, and the latter is a contrary process. The entropy change of the adsorption is the sum of the two processes. In this system of the adsorption of XO onto MIL-101, probably, desorbed water molecule is larger than that of the adsorbed XO molecule as XO molecule is giant compared with water molecule; therefore several water may be desorbed by adsorption of XO molecule. Therefore, the driving force for XO adsorption (negative G) onto MIL-101 is due to both enthalpy effect and entropy effect. 3.5. Effect of pH value and reuse

Fig. 6. van’t Hoff plots to get the H and S of the XO adsorption onto MIL-101.

The adsorption of a dye usually depends highly on the pH value of the dye solution [29]. In this work, the pH value effect on the adsorption was carried out at 298 K. Selecting the concentration of 400 ppm as an example, the pH value (2–12) of the solution was adjusted with NaOH or HCl solution. As shown in Fig. 7a, the adsorbed amounts decreased with increasing pH value of the XO solution, which is quite similar to previously reported results of MO adsorbed onto various adsorbents [5]. When the pH values were 4–10, the adsorbed amounts were much closer. As the acidity enhanced to pH value was 2, the percentage removal of XO reached almost 90%; however, in comparison, when the alkalinity increased to pH value was 12, the amount of adsorption was zero. As shown in Fig. 7b, the Zeta potential increased with the increase of the pH value in the acidic area, however, in the alkalinity area the Zeta potential decreased with the increase of the pH value. That might be explained by the diffuse double layer theory; when MIL-101 was

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Fig. 8. The possible mechanism of the adsorption of XO onto MIL-101 (R-SO3 − representatives XO).

added to the solution, the particle surface was surrounded by a layer of positive charge, therefore, it showed a positive Zeta potential. However, as the concentration of H+ increased in the solution, the Zeta potential decreased. As to the alkalinity area, there is a reaction between OH− and carboxyl, and the oxygen anions are exposed. As the concentration of OH− increased this trend was more obvious. When the pH value increased to 12, it demonstrated a sharp negative Zeta potential, and the structure was destroyed as can be seen from the XRD patterns, maybe that is why the adsorption amount was zero at this pH value. The mechanism might be described by Fig. 8, in which a microstructure is stripped from the structure of MIL-101, and the R-SO3 − group of XO are responsible for the pH value effect on the adsorption of XO onto MIL-101. This might explain the behavior of all dye stuffs with a –SO3 − group that adsorbed onto MIL-101. More experimental data are needed to verify specific mechanism. Regeneration of an adsorbent is very important for industrial applications. In this work, the used adsorbent was regenerated with 0.01 M NaOH solution, because a simple physical method like ultrasound [5] could not remove the XO from the adsorbent. The results are shown in Fig. 8 with an initial concentration of 200 ppm. From the patterns of powder XRD, we know that NaOH solution destroys the nature of the structure and decreases the specific surface, but the reused MIL-101 still had the capacity to adsorb XO, moreover, the performance was kept above 90% even when reused for the

Fig. 9. The reuse of MIL-101 on adsorption of XO (concentration 200 ppm, T = 298 K).

second time, showing an excellent capacity for regeneration. Further research is needed to find better physical methods for regeneration. 3.6. Adsorption onto different adsorbents Active carbon shows excellent capacity for dye adsorption, due to its large specific surface and pore volume [30,31]. The mesoporous molecular sieve MCM-41 is a traditional adsorbent material

Fig. 10. Adsorption of XO onto different adsorbents (a) compared MIL-101 to MCM41 and active carbon, initial concentration: 100 ppm; (b) compared MIL-101 to active carbon for different initial concentration, T = 298 K.

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as it exhibits hexagonal arrays of uniform channels, high BET surface area and large pore volume. In this work, MIL-101was compared with active carbon (AC, coconut-made charcoal, BET surface: 779 m2 /g) and MCM-41 (BET surface: 945 m2 /g), for the adsorption of XO at the same concentration (100 ppm) and at different temperatures for 1 h. The results are shown in Fig. 10. The same dose of active carbon was also added to different concentrations of XO to compare further with MIL-101 at 298 K. From the data in Fig. 9, MCM-41 showed weak adsorption of XO, while the adsorption of AC was slightly higher than MIL-101. In addition, the adsorption of XO onto the three adsorbents showed the same decreased trend with temperature. However, when AC and MIL-101 were both used in solution of higher dye concentrations, MIL-101 showed superior adsorption of XO. The saturated adsorption of AC increased a little with the increase of initial XO concentration, whereas MIL-101 demonstrated a rapid growth. The saturated adsorption of MIL-101 grew to nearly twice that of AC when the initial concentration was increased to 400 ppm. That is, MIL-101 has a strong dye adsorption capacity over a wide concentration range, while AC is only suitable for dye adsorption at lower concentrations. Comparing the adsorption of these three materials, we initially considered that surface area or pore volume were not the key to adsorption of XO from wastewater. Contact with the effect of pH on the adsorption of MIL-101, the key might be the charge interactions between dye stuffs and adsorbents.

4. Conclusions In this work, MIL-101(Cr) was introduced for the adsorptive removal of XO from wastewater. The results indicate that pseudo-second-order kinetic model matched much better with the adsorption of XO onto MIL-101 compared with pseudo-secondorder kinetic model. The Langmuir model fits the data better compared with the Freundlich model in terms of regression coefficients. The thermodynamic parameters, including free energy, enthalpy, and entropy of adsorption, were calculated from the result of isotherms, suggesting that the adsorption of XO onto MIL-101 was a process with negative free energy change, negative enthalpy change, and positive entropy change. The adsorbed amounts decrease upon increasing the pH value of the XO solution, and when the pH value increased to 12, the adsorption was zero. Combination of the Zeta potential data with pH value, the key factor of the adsorption might be the charge interactions between dye stuffs and adsorbents. MIL-101 could be regenerated with a more dilute concentration of NaOH solution, and reserved the adsorption ability. However, the structure may be destroyed using the NaOH solution, and the specific surface decreased with the increase in the number of the reuse cycles. Comparing the adsorption capacity of MIL-101 to MCM-41 and active carbon, the amount of adsorption of MCM-41 was much less than the other two adsorbents; active carbon was only suitable for dye adsorption at low concentration, whereas MIL-101 showed good capacity for dye adsorption over a wide concentration range. Therefore, the material of MIL-101 has a great prospect in the dye adsorption area, and is ready for reuse.

Acknowledgements The authors acknowledge financial support from the National Natural Science Foundation of China (Grant No. 21073098), the Natural Science Foundation of Tianjin (11JCZDJC21600), and the Research Fund for MOE (IRT-0927), the Doctoral Program of Higher Education (20090031110015), and the Program

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