Adsorption kinetics of methylene blue onto Fe-doped sulfated titania

Adsorption kinetics of methylene blue onto Fe-doped sulfated titania

Colloids and Surfaces A: Physicochem. Eng. Aspects 389 (2011) 12–17 Contents lists available at SciVerse ScienceDirect Colloids and Surfaces A: Phys...

782KB Sizes 0 Downloads 24 Views

Colloids and Surfaces A: Physicochem. Eng. Aspects 389 (2011) 12–17

Contents lists available at SciVerse ScienceDirect

Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Adsorption kinetics of methylene blue onto Fe-doped sulfated titania Ying Yang a,∗ , Congxue Tian a , Xiangpo Zhao b a b

College of Biological and Chemical Engineering, Panzhihua University, Panzhihua 617000, China PetroChina Southwest Oil and Gas Field Company, Chengdu 610051, China

a r t i c l e

i n f o

Article history: Received 6 August 2011 Received in revised form 2 September 2011 Accepted 4 September 2011 Available online 10 September 2011 Keywords: Adsorption Fe-doped sulfated titania Methylene blue Kinetics

a b s t r a c t Fe-doped sulfated titania (FST) photocatalysts with high photocatalytic activity were prepared from industrial titanyl sulfate solution and characterized using N2 adsorption–desorption technique. Adsorption kinetics and mechanism of methylene blue onto FST samples were studied at different temperatures (298, 303 and 308 K). The kinetic experimental data appropriately correlate with the pseudo-second order model. The overall rate of the adsorption process appears to be influenced by both boundary layer diffusion and intraparticle diffusion. The low adsorption activation energy (in the range of 15.59–19.31 kJ mol−1 ) suggests that the adsorption of methylene blue onto FST samples was conformed to the physisorption mechanism. With calcination temperature increases from 400 to 600 ◦ C, sulfur species gradually decomposes and desorbs from the surface of FST samples, which can enhance the affinity between methylene blue and FST samples. Moreover, the specific surface decreases and the pore volume and pore diameter increase with rise in calcining temperature. All these have a significant influence on the adsorption properties of FST samples. © 2011 Elsevier B.V. All rights reserved.

1. Introduction In recent years, semiconductor photocatalysis has attracted considerable scientific and practical interest because it’s promising applications in environmental pollution remediation such as the removal of inorganic or organic pollutants from air or wastewater. Among various semiconductor metal oxides, titania has proven to be the most potential photocatalyst due to its high photoreactivity, cost effectiveness, nontoxicity, chemical and biological inertness, long-term stability against photocorrosion [1–4]. Like other semiconductors, titania has some fatal drawbacks (the relatively large band gap, the rapid recombination rate of photogenerated electron–hole pairs, etc.) [5]. Thus, many modification methods including metal or non-metal doping, surface sensitization, semiconductor coupling, precious metal deposition and increasing crystal defects have been developed in order to enhance the photocatalytic activity and efficiencies of titania photocatalysts especially under visible light in the past few decades [6–9]. However, little information about the absorption in photocatalysis has been reported especially in photodegradation of organic pollutants in wastewater, although absorption of organic pollutants onto the surface of photocatalysts is a critical step in photocatalysis [10]. In the previous work of our group, using low-cost industrial titanyl sulfate solution as raw material, which contains abundant

∗ Corresponding author. Tel.: +86 812 3371021; fax: +86 812 3371000. E-mail address: [email protected] (Y. Yang). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.09.003

iron and sulfate, Fe-doped sulfated titania (denoted as FST) photocatalysts were prepared by one-step thermal hydrolysis method, and the effects of the volume ratio of pre-adding water to TiOSO4 on the photocatalytic activity of FST photocatalysts have been studied [11,12]. In this work, it is aimed to investigate the adsorption kinetics of methylene blue onto FST photocatalysts at three different temperatures (298, 303 and 308 K). This study was also undertaken to evaluate kinetic parameters of the adsorption process at different temperatures above for each FST photocatalyst. In addition, the effects of the preparation condition (calcination temperature) of FST samples on its adsorption kinetics were carried out as well. 2. Experimental 2.1. FST samples preparation and characterization FST samples were prepared through one-step thermal hydrolysis method using industrial titanyl sulfate solution as materials. The more detailed procedure can be found in references [11,12]. In a typical synthesis, 150 ml TiOSO4 solution and 34.5 ml preadding water were preheated to 96 ± 1 ◦ C, respectively. The heated TiOSO4 solution was dropped into the pre-adding water under stirring and reflux at a feeding speed of 8.55 ml/min. After feeding off, the mixture solution was heated to boiling point (called the first boiling point) at a heating rate of 0.82 ◦ C/min. Heating and stirring were stopped immediately when the mixture reached a gray color (called gray point) and then were turned on again after 30 min, then the mixture was heated to the boiling point again (called the

Y. Yang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 389 (2011) 12–17

-1.5

-2.5 -3.0

(1)

where Ci represent the initial concentration of methylene blue (mg l−1 ). The Ct is the concentration of methylene blue in solution (mg l−1 ) at time t = t· V (L) and mass FST are the solution volume and the number of grams of FST sample used, respectively. Each experiment was performed individually at three different temperatures (298, 303 and 308 K) for each FST sample. But the same procedure was used for each as detailed above. 3. Results and discussion 3.1. Adsorption kinetics model The adsorption kinetics of methylene blue onto FST samples may be described by the pseudo-first order model suggested by Lagergren [13]. The equation is as follows: (2)

Integrating Eq. (2) with the boundary conditions t = 0 to t = t and qt = 0 to qt = qt gives the linearized form: ln(qe − qt ) = ln qe − k1 t

-4.0 Linear fitting 2

Y=-1.6046-0.01505X; R =0.9942 2 Y=-1.8854-0.01615X; R =0.9935 2 Y=-2.1771-0.01779X; R =0.9933

-5.0 -5.5 -20

0

20

40

60

80

100

120

t (min)

140

160

180

200

Fig. 1. Linear plots of the pseudo-first order kinetic mode of methylene blue adsorption onto different FST samples at 298 K.

-1 Experimental FST(400) FST(500) FST(600)

-2 -3

ln(qe-qt)

The adsorption experiments were carried out in a 250 ml threeneck flask containing 200 ml methylene blue solution (6 mg l−1 ) and 0.2 g FST sample and the three-neck flask was placed in a thermostatic electric jacket with a magnetic stirrer. After ultrasonic dispersion for 3 min, the adsorption experiments were conducted in dark under stirring at a rate of 150 rpm to prevent bulk diffusion as a controlling step of adsorption kinetics. At defined time points, 10 ml solution was taken out from the three-neck flask and immediately centrifuged for 5 min at 4000 rpm to separate the supernate and the FST sample. And then small amounts of the supernatant were taken to be analyzed by recording the absorbance at a wavelength of maximum absorbance of methylene blue (666 nm) using a spectrometer. The concentration of methylene blue remaining in solution at time t = t (Ct , mg l−1 ) can be calculated according to the calibration curve of methylene blue (A = 0.0068 + 0.1514C) and the corresponding adsorption capacity (qt , mg/g) was obtained using the following equation:

dqt = k1 (qe − qt ) dt

-3.5

-4.5

2.2. Adsorption of methylene blue on FST samples

(C − Ct ) · V qt = i mass FST

Experimental FST(400) FST(500) FST(600)

-2.0

ln(qe-qt)

second boiling point). Maintaining slight boiling 2.5 h after the second boiling point, the hydrolysis process is then finished. The slurry was promptly cooled to room temperature, then filtered and washed. The as-prepared metatitanic acid was dried at 80 ◦ C for 24 h, then calcined at different temperatures (400, 500 and 600 ◦ C) for 1.5 h in static air, Fe-doped sulfated titania photocatalysts were then obtained, denoted as FST(t), and the value in parentheses indicates the calcining temperature, ◦ C. The FST samples were characterized using N2 adsorption– desorption technique and the photocatalytic efficiencies of FST samples were evaluated by the photooxidation of methylene blue in aqueous solution under UV irradiation. More details can be found in Refs. [11,12].

13

-4 -5

Linear fitting 2

Y=-1.0912-0.01854X; R =0.9853 2 Y=-1.8854-0.01940X; R =0.9959 2 Y=-2.1771-0.02488X; R =0.9945

-6 -7 -20

0

20

40

60

80

100

120

140

160

180

200

t (min) Fig. 2. Linear plots of the pseudo-first order kinetic mode of methylene blue adsorption onto different FST samples at 303 K.

the plot. Figs. 1–3 show the plots ln(qe − qt ) versus t for adsorption of methylene blue onto different FST samples (FST(400), FST(500) and FST(600)) at different temperatures (298, 303 and 308 K), respectively. The rate constants, k1 for different FST samples were obtained from Figs. 1–3 at different temperatures and presented in Table 1 along with the corresponding correlation coefficients, R12 . Another model for the analysis of sorption kinetics is the pseudo-second order model put forward by Ho et al. [14]. The rate law for this system is expressed as dqt = k2 (qe − qt )2 dt

(4)

(3)

where qt and qe are the adsorption capacity (mg/g) of methylene blue onto FST samples at time and at equilibrium, respectively. k1 is the rate constant (min−1 ) of pseudo-first order sorption model. Although this mode is the simplest one, it was extensively used to describe the kinetics of sorption of solutes from a liquid solution. If the adsorption kinetics follows a pseudo-first order model, the plot of ln(qe − qt ) versus t should be linear. At the same time, the rate constant, k1 and the coefficient, R12 can be calculated from

Table 1 Pseudo-first order kinetic model rate constants for adsorption of methylene blue onto different FST samples at different temperatures. Samples

298 K

303 K −1

k1 (min FST(400) FST(500) FST(600)

0.01505 0.01854 0.02272

)

308 K −1

R12

k1 (min

0.9942 0.9935 0.9933

0.01615 0.01940 0.02451

)

R12

k1 (min−1 )

R12

0.9853 0.9959 0.9945

0.01779 0.02488 0.03189

0.9938 0.9936 0.9956

14

Y. Yang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 389 (2011) 12–17

0 1400

-1

Experimental FST(400) FST(500) FST(600)

2

-1

-3

ln(qe-qt)

Y=268.64+4.5001X; R =0.9979 2 Y=242.92+4.2611X; R =0.9997 2 Y=278.17+5.8824X; R =0.9979

1200

t/qt (min.g.mg )

-2

Linear fitting

-4 -5

1000

800

600

Experimental FST(400) FST(500) FST(600)

Linear fitting

-6

2

Y=-0.8105-0.02272X; R =0.9938 2 Y=-1.3023-0.02451X; R =0.9936 2 Y=-1.7974-0.03189X; R =0.9956

-7

400

200

-8

0

-20

0

20

40

60

80

100

120

140

160

180

t (min) Fig. 3. Linear plots of the pseudo-first order kinetic mode of methylene blue adsorption onto different FST samples at 308 K.

Integrating Eq. (4), for the boundary conditions t = 0 to t = t and qt = 0 to qt = qt gives the following equation: 1 1 = + k2 t qe qe − qt

(5)

Eq. (5) can be rearranged to obtain a linear form, t 1 1 = + t qt qe k2 q2e

20

40

60

80

(6)

where k2 is the rate constant (g mg−1 min−1 ) of pseudo-second order sorption model. The meanings of the other parameters as mentioned above. A plot of t/qt against t gives a straight line with the slope of 1/qe and the intercept of 1/(k2 q2e ). So the sorption rate constant, k2 can be calculated from the slope and intercept. Figs. 4–6 show the plots of t/qt against t for adsorption of methylene blue onto different FST samples (FST(400), FST(500) and FST(600)) at different temperatures (298, 303 and 308 K), respectively. The rate constants, k2 for different FST samples were obtained from Figs. 4–6 at different temperatures and presented in Table 2 along with the corresponding correlation coefficients, R22 . Based on the correlation coefficients for k1 and k2 as presented in Tables 1 and 2, respectively, the adsorption of methylene blue onto

100

120

140

160

180

200

t (min)

200

Fig. 5. Linear plots of the pseudo-second order kinetic mode of methylene blue adsorption onto different FST samples at 303 K.

FST samples is best described by the pseudo-second order model. This is quite consistent with the theoretical analysis results, i.e., the sorption of solute from the solution obeys pseudo-first order kinetics model at high initial concentration of solute, while it obeys pseudo-second order kinetics model at low initial concentration of solute [15]. The pseudo-second order rate constant for the adsorption of methylene onto FST samples could be expressed as a function of temperature by the Arrhenius type relationship, as shown in the following equation: ln k2 = ln A −

Ea RT

(7)

where A is the frequency factor (g mg−1 min−1 ), Ea is the Arrhenius activation energy of adsorption (J/mol), representing the minimum energy that reactants must have for the reaction to proceed, R is the universal gas constant (8.314 J mol−1 K−1 ), and T is the absolute temperature of solution (K). As shown in Fig. 7, the slopes of the linear plots of ln k2 versus 1/T for the adsorption of methylene blue onto different FST samples were constructed to calculate the adsorption activation energy, which are 17.31, 15.59 and 19.31 kJ mol−1 for FST(400), FST(500) and FST(600), respectively. Low activation energies (5–40 kJ mol−1 ) 1200

1800

2

2

Y=427.62+5.3884X; R =0.9981 2 Y=315.04+4.6197X; R =0.9980 2 Y=436.97+6.9428X; R =0.9979

1600 1400

-1

t/qt (min.g.mg )

1200 1000 800

Experimental FST(400) FST(500) FST(600)

600 400 0

20

40

60

80

100

Y=157.00+3.6573X; R =0.9979 2 Y=113.04+3.0693X; R =0.9979 2 Y=171.73+4.9485X; R =0.9982

1000

-1

t/qt (min.g.mg )

Linear fitting

Linear fitting

120

140

160

180

800

600

400

Experimental FST(400) FST(500) FST(600)

200

200

t (min) Fig. 4. Linear plots of the pseudo-second order kinetic mode of methylene blue adsorption onto different FST samples at 298 K.

0

20

40

60

80

100

120

140

160

180

200

t (min) Fig. 6. Linear plots of the pseudo-second order kinetic mode of methylene blue adsorption onto different FST samples at 308 K.

Y. Yang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 389 (2011) 12–17

15

Table 2 Pseudo-second order kinetic model rate constants for adsorption of methylene blue onto different FST samples at different temperatures. Samples

298 K

FST(400) FST(500) FST(600)

303 K

308 K

k2 (g mg−1 min−1 )

R22

k2 (g mg−1 min−1 )

R22

k2 (g mg−1 min−1 )

R22

0.06790 0.06774 0.11031

0.9981 0.9980 0.9979

0.07538 0.07474 0.12439

0.9979 0.9997 0.9979

0.08520 0.08334 0.14259

0.9979 0.9979 0.9982

7

are characteristic of physical adsorption, while higher activation energies (40–800 kJ mol−1 ) suggest chemical adsorption [16]. Thus the adsorption of methylene blue onto FST samples may be conformed to physisorption mechanisms.

5

3.2. Adsorption mechanism

Rt = − ln(1 − F)

For intraparticle or pore diffusion :

(8) Bt = − ln(1 − F) − 0.4977 (9)

where F = qt /qe ; qt and qe are the adsorption capacity (mg/g) of methylene blue onto FST samples at time and at equilibrium, respectively. R is the rate constant for film diffusion; B = 2 Di/r2 (Di is the inter diffusion coefficient and r is the particle radius). Plots of −ln(1 − F) and Bt versus time, t, according to Eqs. (8) and (9) for FST samples at 308 K, are shown in Figs. 8 and 9, respectively, which are similar to the results at 298 and 303 K (not shown here). Straight lines were obtained when −ln(1 − F) was plotted against

4

-ln(1-F)

The adsorption of methylene blue onto FST samples from aqueous solution may involve the following steps: (i) migration of methylene blue from the bulk solution to the external surface of FST samples (bulk diffusion), (ii) film diffusion of methylene blue through a hypothetical boundary layer to the external surface of FST samples (film diffusion or boundary layer diffusion or external diffusion), (iii) adsorption of methylene blue at an active site on the surface of FST samples (adsorption), and (iv) the diffusion of methylene blue within the pore volume of FST samples and/or along the pore wall surface (pore diffusion or intraparticle diffusion or internal diffusion). The rates of bulk diffusion and adsorption are generally considered to be very fast and they cannot be the rate determining step. Therefore, film and intraparticle diffusion may be the rate controlling steps in the adsorption of methylene blue onto FST samples. Following equations were used to ascertain the rate controlling step [17]. For film diffusion :

FST(400); Y=0.49767+0.0224X FST(500); Y=0.49708+0.0245X FST(600); Y=0.49479+0.0319X

6

3

2

1 0

20

40

60

80

100

120

140

160

180

200

t (min) Fig. 8. Plots of −ln(1 − F) versus t for the adsorption of methylene blue onto different FST samples at 308 K.

time, t (Fig. 8) which did not pass through the origins. This indicates that film diffusion is not limiting step of the overall adsorption process kinetics. Fig. 9 indicates that straight lines were obtained on plotting Bt versus time, t which nearly passes through the origins. This shows that intraparticle diffusion may be the rate controlling step [18]. Adsorption kinetic data was further processed to confirm whether intraparticle diffusion is the rate limiting and to find out the rate parameter for intraparticle diffusion. For such purpose Morris–Weber equation [19]: qt = kid (t)1/2 + I

(10)

-1.4 -1.6 -1.8

Experimental FST(400) FST(500) FST(600)

6

Linear fitting

5

4

Bt

lnk2

-2.0 -2.2

3

-2.4

2

-2.6

1

-2.8 0.00324

FST(400); Y=-2.77E-5+0.0224X FST(500); Y=-6.23E-4+0.0245X FST(600); Y= 3.19E-3+0.0319X

2

Y=4.2929-2081.88X; R =0.9985 2 Y=3.8554-1875.55X; R =0.9966 2 Y=5.5929-2322.42X; R =0.9960

0.00326

0.00328

0.00330

0.00332

0.00334

0.00336

-1

T (K ) Fig. 7. Arrhenius plots of the pseudo-second order kinetic mode for the adsorption of methylene blue onto different FST samples.

0 0

20

40

60

80

100

120

140

160

180

200

t (min) Fig. 9. Plots of Bt versus t for the adsorption of methylene blue onto different FST samples at 308 K.

16

Y. Yang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 389 (2011) 12–17

Table 3 Calculated parameters for adsorption of methylene blue onto different FST samples at different temperatures. Samples

298 K

FST(400) FST(500) FST(600)

303 K

308 K

kid1

kid2

I1

I2

kid1

kid2

I1

I2

kid1

kid2

I1

I2

0.0134 0.0156 0.0113

0.0070 0.0065 0.0049

0.0239 0.0211 0.0170

0.0351 0.0608 0.0410

0.0177 0.0183 0.0149

0.0073 0.0079 0.0040

0.0254 0.0275 0.0101

0.0683 0.0665 0.0869

0.0234 0.0285 0.0179

0.0072 0.0075 0.0029

0.0220 0.0189 0.0074

0.1229 0.1674 0.1393

0.22

FST(400)

0.20 0.18

-1

qt (mg.g )

0.16 0.14 0.12 0.10 0.08

298 K 303 K 308 K

0.06 0.04 4

6

8

10

t

1/2

12

14

1/2

(min )

Fig. 10. Morris–Weber plots of methylene blue onto FST(400) sample at different temperatures.

was applied to the kinetic data. The parameter kid is the rate constant for intraparticle diffusion (mg g−1 min−1/2 ). Values of I (mg g−1 ) give an idea about the thickness of the boundary layer, i.e., the larger the intercept, the greater is the boundary layer effect [20]. Plots of qt against (t)0.5 are shown in Figs. 10–12 for FST(400), FST(500) and FST(600), respectively. Figs. 10–12 indicate that two distinct regions were observed for all FST samples. The initial linear portion was ascribed to the boundary layer diffusion effects and the second linear portion was due to the intraparticle diffusion [17]. The values of kid1 , kid2 and I1 , I2 for methylene blue adsorption onto different FST samples at different temperatures are listed in Table 3, which obtained from the slopes and intercepts of the two portion straight lines, respectively. It was also observed that all lines do not pass through the origin, indicating that there is a boundary layer

resistance and the magnitude of the intercepts are proportional to the extent of the boundary layer thickness. Similar observations of double nature plots were also reported previously by other workers on various adsorbate–adsorbent systems studied [20–22]. The change in the intercepts of the plots suggests that the mechanism of the adsorption of methylene blue onto FST samples is predominantly diffusion, and the intraparticle diffusion played a significant role in rate determining, but it was not the sole rate-controlling step through out the adsorption process. Namely, both intraparticle and boundary layer diffusion seem significant in the rate determining step. Initially, the methylene blue was adsorbed by the exterior surface of FST samples at the beginning, so the adsorption rate was very fast. Upon the saturation of the exterior surface due to the adsorption, the methylene blue entered into the particle of FST samples through pores and was adsorbed by the interior surface of the particle. As a result of diffusion resistance, the intraparticle diffusion rate become slow and is therefore the rate determining step. 3.3. Effect of calcination temperature on kinetics Fig. 13 shows the effect of calcination temperature on adsorption activation energy, Ea, and adsorption rate constant, k2 of pseudo-second order kinetic model. This figure indicates that all adsorption rate constant (k2 ) at different system temperatures have the similar variation trend with increasing the calcination temperature. With the calcination temperature increases from 400 to 500 ◦ C, all adsorption rate constant (k2 ) at different system temperatures decreases slightly, which may be due to the decrease in specific surface (158.3 m2 g−1 for FST(400) and 120.7 m2 g−1 for FST(500)). However, with the calcination temperature further increases to 600 ◦ C, all adsorption rate constant (k2 ) at different system temperatures increase distinctly. This is probably a consequence of the rise in pore volume and pore diameter (0.4076 ml g−1 and 12.21 nm for FST(500) and 0.5793 ml g−1 and 33.84 nm for FST(600)), which can lower the resistance of intra-particle 0.20

0.24

FST(600)

FST(500) 0.16

-1

qt (mg.g )

-1

qt (mg.g )

0.20

0.16

0.12

298 K 303 K 308 K

0.08

0.04 4

6

8

10

t

1/2

12

0.12

0.08

298 K 303 K 308 K

0.04

14

1/2

(min )

Fig. 11. Morris–Weber plots of methylene blue onto FST(500) sample at different temperatures.

4

6

8

10

t

1/2

12

14

1/2

(min )

Fig. 12. Morris–Weber plots of methylene blue onto FST(600) sample at different temperatures.

Y. Yang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 389 (2011) 12–17

20

0.12

0.13

0.11

0.12

0.10

0.11

0.09

0.10

0.08

0.09

0.07

0.08

0.06

0.07

-1

Ea (kJ.mol ) -1 -1 k2 (at 298 K, g.mg .min ) -1 -1 k2 (at 303 K, g.mg .min ) -1 -1 k2 (at 308 K, g.mg .min )

19

18

17

0.15 0.14 0.13

17

0.12 0.11 0.10 0.09

16

15 400

450

500

550

0.08 0.07

600

Calcination temperature (ºC) Fig. 13. Effect of calcination temperature on Ea and k2 of pseudo-second order kinetic model.

diffusion. At the same time, with the calcination temperature increases from 400 to 600 ◦ C, the adsorption activation energy (Ea) also decreases first, and then increases obviously. This is in good agreement with the photocatalytic efficiencies (94.32, 98.08 and 90.92% for FST(400), FST(5400) and FST(600), respectively), i.e., the larger the adsorption activation energy, the lower is the photocatalytic efficiencies. One possible reason responsible for this is that the sulfur species gradually decomposes and desorbs from the surface of FST samples with the calcining temperature increases, which can enhance the affinity between methylene blue and FST samples [23]. 4. Conclusions Adsorption kinetics and mechanism of methylene blue onto FST samples were studied at different temperatures (298, 303 and 308 K). The kinetics experimental data appropriately correlate with the pseudo-second order model. The overall rate process appears to be influenced by both boundary layer diffusion and intraparticle diffusion. The adsorption activation energy calculated from Arrhenius equation was 17.31, 15.59 and 19.31 kJ mol−1 for FST(400), FST(500) and FST(600), respectively, suggesting that the adsorption of methylene blue onto FST samples follows the physisorption mechanism. With calcination temperature increases from 400 to 600 ◦ C, sulfur species gradually decomposes and desorbs from the surface of FST samples, which can enhance the affinity between methylene blue and FST samples. Moreover, the specific surface decreases and the pore volume and pore diameter increases with rise in calcining temperature. All these have a significant influence on the adsorption properties of FST samples. Acknowledgements This work was supported by the National Natural Science Foundation of China (50804025), the Applied and Basic Research Program of Sichuan Province, China (2008JY0140), the Youths Foundation of Sichuan Province, China (09ZQ026067), the Talents Innovation Project of Panzhihua City, China (2009TX-5(1)) and the Promoting Industrialization Program of Panzhihua City, China (2011CY-G-23).

References [1] X. Chen, S.S. Mao, Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications, Chem. Rev. 107 (2007) 2891–2959. [2] X. Chen, S. Shen, L. Guo, S.S. Mao, Semiconductor-based photocatalytic hydrogen generation, Chem. Rev. 110 (2010) 6503–6570. [3] U. Diebold, The surface science of titanium dioxide, Surf. Sci. Rep. 48 (2003) 53–229. [4] A. Fujishima, X. Zhang, D.A. Tryk, TiO2 photocatalysis and related surface phenomena, Surf. Sci. Rep. 63 (2008) 515–582. [5] M. Anpo, P.V. Kamat, Environmentally Benign Photocatalysts: Applications of Titanium Oxide-based Materials, 1st ed., Springer, New York, 2010. [6] A.L. Linsebigler, G. Lu, J.T. Yates, Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results, Chem. Rev. 95 (1995) 735–758. [7] M. Anpo, M. Takeuchi, The design and development of highly reactive titanium oxide photocatalysts operating under visible light irradiation, J. Catal. 216 (2003) 505–516. [8] S. Rehman, R. Ullah, A.M. Butt, N.D. Gohar, Strategies of making TiO2 and ZnO visible light active, J. Hazard. Mater. 170 (2009) 560–569. [9] Y. Yang, H. Zhong, C. Tian, Photocatalytic mechanisms of modified titania under visible light, Res. Chem. Intermed. 37 (2011) 91–102. [10] D.A. Sverjensky, Physical surface-complexation models for sorption at the mineral–water interface, Nature 364 (1993) 776–780. [11] Y. Yang, C. Tian, Synergistic effects of sulfation and Fe-doping on the photocatalysis of titania, Res. Chem. Intermed. 36 (2010) 889–895. [12] Y. Yang, H. Zhong, C. Tian, Z. Jiang, Single-step preparation, characterization and photocatalytic mechanism of mesoporous Fe-doped sulfated titania, Surf. Sci. 605 (2011) 1281–1286. [13] S. Lagergren, About the theory of so-called adsorption of soluble substances, K. Sven. Vetenskapsakad. Handl. 24 (1898) 1–39. [14] Y.-S. Ho, G. McKay, Pseudo-second order model for sorption processes, Process Biochem. 34 (1999) 451–465. [15] S. Azizian, Kinetic models of sorption: a theoretical analysis, J. Colloid Interface. Sci. 276 (2004) 47–52. [16] H. Nollet, M. Roels, P. Lutgen, P. Van der Meeren, W. Verstraete, Removal of PCBs from wastewater using fly ash, Chemosphere 53 (2003) 655–665. [17] R. Qadeer, Adsorption behavior of ruthenium ions on activated charcoal from nitric acid medium, Colloids Surf. A 293 (2007) 217–223. [18] M.S. Shaukat, M.I. Sarwar, R. Qadeer, Adsorption of strontium ions from aqueous solution on Pakistani coal, J. Radioanal. Nucl. Chem. 265 (2005) 73–79. [19] W.J. Weber Jr., J.C. Morris, Kinetics of adsorption on carbon from solution, J. Sanit. Eng. Div. ASCE 89 (1963) 31–60. [20] V.S. Mane1, I.D. Mall, V.C. Srivastava, Kinetics and equilibrium isotherm studies for the adsorptive removal of Briliant Green dye from aqueous solution by rice husk ash, J. Environ. Manage. 84 (2007) 390–400. [21] L. Zhang, N. Liu, L. Yang, Q. Liu, Sorption behavior of nano-TiO2 for the removal of selenium ions from aqueous solution, J. Hazard. Mater. 170 (2009) 1197–1203. [22] H. Al-Johani, M.A. Salam, Kinetics and thermodynamic study of aniline adsorption by multi-walled carbon nanotubes from aqueous solution, J. Colloid Interface. Sci. 360 (2011) 760–767. [23] M.L. Fetterolf, J.V. Patel, J.M. Jennings, Adsorption of methylene blue and acid blue 40 on titania from aqueous solution, J. Chem. Eng. Data 48 (2003) 831–835.