Optimization and modeling of preparation conditions of TiO2 nanoparticles coated on hollow glass microspheres using response surface methodology

Separation and Purification Technology 125 (2014) 156–162

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Optimization and modeling of preparation conditions of TiO2 nanoparticles coated on hollow glass microspheres using response surface methodology Lei Sun a,b,⇑, Shungang Wan b, Zebin Yu a, Lijun Wang c a b c

School of Environment, Guangxi University, Nanning 530004, PR China Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, PR China Xiamen Fuman Pharmaceuticals Co., Ltd., Xiamen 361022, PR China

a r t i c l e

i n f o

Article history: Received 7 November 2013 Received in revised form 12 January 2014 Accepted 29 January 2014 Available online 6 February 2014 Keywords: Response surface methodology Sol–gel method TiO2 thin film Hollow glass microspheres Photocatalytic

a b s t r a c t Response surface methodology was used to find and optimize the effect of the variables on the preparation, morphology and catalytic activity of nano-crystalline TiO2 thin films coated on hollow glass microspheres (HGMs) by sol–gel method. Three experimental parameters were chosen as independent variables: the calcination temperature, the amount of titanium butoxide and template F127. Morphology analysis showed that structure of TiO2–HGMs prepared was core shell, and the surface of HGMs was coated by TiO2 thin films with an average layer thickness of 728 ± 12 nm. A quadratic model was established as a functional relationship between three independent variables and the degradation efficiency of methyl orange (MO). The results of model fitting and statistical analysis demonstrated that the amount of titanium butoxide and F127 played a key role in discoloration for MO. The optimum conditions for maximum MO degradation efficiency (98.6%) were titanium butoxide of 18.64 g, F127 of 3.12 g and calcination temperature of 501.89 °C. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Advanced oxidation processes (AOPs) can complete decompose the less reactive organic pollutants in aqueous solution quickly and non-selectively by generation of reactive species including hydroxyl radicals [1,2]. Among AOPs, the heterogeneous photocatalytic process is an effective approach that it has been successfully employed for the decomposition of various organic pollutants such as antibiotics [3], pesticide [4,5], dye [6,7] and chlorophenols [8] at ambient temperature and pressure into CO2 and water under UV light irradiation with the aid of photocatalysts. Compared with various semiconductors photocatalysts such as ZnO, WO3, CdS and NiO during the photocatalysis process, TiO2 in the anatase form seems to have the most interesting attributes such as high stability, good performance and low cost [1,9]. At present, studies of photocatalytic processes are widely carried out in slurry systems operating with TiO2 suspensions [10,11]. However, to date, little progress has been made in the development of a photocatalytic technology

⇑ Corresponding author. Addresses: School of Environment, Guangxi University, Nanning 530004, PR China, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, PR China. Tel./fax: 86 0771 3270672. E-mail address: [email protected] (L. Sun). http://dx.doi.org/10.1016/j.seppur.2014.01.042 1383-5866/Ó 2014 Elsevier B.V. All rights reserved.

for water treatment in large applications. The major problem in these systems is the separation and recycling of TiO2 nanoparticles from the slurry after wastewater treatment, which can be a time consuming and expensive process [12]. Thus, the TiO2 nanoparticles coated on supporting substrate, such as indium–tin oxide conducting glass [13], glass [14] and stainless steel [15] may be preferred to solve this problem for its commercialization. TiO2 can be immobilized on various substrates to form thin film by many techniques, such as sol–gel method [16,17], radio frequency magnetron sputtering [18,19], chemical vapor deposition [20,21], cathodic arc deposition [22,23] and electron beam evaporation [24]. Among these techniques, the sol–gel method is one of the simplest and cost-effective synthetic techniques with several advantages, such as high purity materials, low processing temperature, homogeneity, and easy formation of large-area coating on different substrates [25,26]. Previous investigations have typically used the traditional one-factor-at-a-time (OFAT) approach to optimize preparation conditions and photocatalytic processes, however, this univariate approach does not show the interactions between the operational factors of the process. Moreover, the OFAT approach is time-consuming and expensive due to reagent costs. There is a current trend toward replacing this inefficient practice with effective chemometric methods, such as response surface

L. Sun et al. / Separation and Purification Technology 125 (2014) 156–162

methodology (RSM), based on statistical designs of experiments [27]. This experimental strategy for seeking the optimum preparation conditions is an efficient technique for use with a multivariable system. RSM has been successfully applied to various processes to achieve optimization using experimental designs, such as photocatalytic [28,29] and Fenton’s peroxidation [30]. In addition, hollow glass microspheres (HGMs) is a kind of cheap material, and its characteristics is low density, low thermal conductivity, high strength and excellent chemical stability. However, no report has been published on the optimization of influencing factors and their interactions using experimental design methodology during the process of immobilized TiO2 on HGMs by sol–gel method. The present study focused on optimization the synthetic parameters for the sol–gel preparation of TiO2 thin film coated on HGMs (TiO2–HGMs) with central composite design (CCD), which is a widely used form of RSM. The synthesis parameters effect on the photocatalytic activity of prepared TiO2–HGMs is evaluated according to the degradation efficiency of methyl orange (MO). Furthermore, the predicted response values of degradation efficiency and amount of TiO2 coated using RSM were compared with experimental values. Finally, the synthesis parameters were optimized using RSM. 2. Materials and methods

157

RSM and more detailed information have been discussed elsewhere [31–33]. A second-order (quadratic) polynomial equation was used to fit the experimental results of CCD as follows:

Yð%Þ ¼ b0 þ b1 x1 þ b2 x2 þ b3 x3 þ b12 x1 x2 þ b13 x1 x3 þ b23 x2 x3 þ b11 x211 þ b22 x222 þ b33 x233

ð1Þ

where Y represents the response of degradation efficiency of MO or amount of TiO2 coated, where b0 is constant, b1, b2, and b3 is linear coefficients, b12, b13, and b23 is cross product coefficients, b11, b22, and b33 is quadratic coefficients. Analysis of variance (ANOVA) was applied to evaluate the adequacy of the model. In addition, the Pareto analysis was also employed to demonstrate the importance of the factors in Eq. (1) by calculation the percentage effect of each factor on the response according to the following relation [34]: 2

b Pn ¼ 100  P n 2 bn

! ðn – 0Þ

ð2Þ

where Pn represents the percentage effect of each factor, and bn represents statistically significant coefficients of polynomial including both first order term and quadratic term in Eq. (1). The high levels of Pn values indicated that corresponding variable exert a statistically significant influence on the response such as degradation efficiency of MO.

2.1. Materials MO (C14H14N3NaO3S, Mw = 327.33 g mol1) was selected as a model dye and which was purchased from the Sinopharm Chemical Reagent Co., Ltd., China. Surfactant pluronicÒ F127 (Product number: P2443) used as template was purchased from Sigma–Aldrich. Titanium butoxide (Ti(OC4H9)4, TBT) with greater than 97% purity was purchased from Sinopharm Chemical Reagent Co., Ltd., China. Additionally, all other chemicals used were analytical grade without further purification. The HGMs (N40, U300– 1000 lm, floating rate > 95%) were provided by Qinhuangdao Qinhuang Glass Microsphere Co., Ltd., China. 2.2. CCD experimental design The RSM using the free Design-Expert 8.0 software (Stat-Ease, Inc., MN, USA) was employed to find the optimum synthesis conditions and further assess the ability of decolorization for the prepared TiO2–HGMs. The analysis focused on the influence of independent variables, namely, amount of titanium butoxide (A), amount of F127 (B), and calcination temperature (C), on the degradation efficiency of MO and the amount of TiO2 coated according to our preliminary experiments. CCD, the most frequently used form of RSM, was employed to evaluate the influence of three independent variables in 20 sets of experiments. Table 1 summarizes the ranges and levels of the independent variables involved in the design strategy. Accordingly, 20 experiments which determined by consisting 8 (2n) full factorial points, 6 (2n) axial points located at the central and both extreme levels and 6 center points designed as replications were showed in Table 2. The fundamental assumptions of Table 1 Experimental ranges and levels of independent variables. Variable

Titanium butoxide (g) F127 (g) Temperature (°C)

Symbol

A B C

Coded levels Low (1)

Center (0)

High (+1)

12.0 2.40 450

16.0 4.40 500

20.0 6.40 550

2.3. Preparation of TiO2–HGMs HGMs were cleaned by pure water rinse for 5 min, in 20% sodium hydroxide solution for corrosion 30 min, rinsed with pure water, and then air-drying. A series of TiO2–HGMs were prepared via the sol–gel method according to our previous study [13]. In brief, given amount of titanium butoxide was added drop wise to 120 mL 20% (v/v) acetic acid aqueous solution under vigorous magnetic stirring. The well-mixed solution was sealed and kept stirring at room temperature for 3 h to obtain the solution A. In a separated beaker, given amount of F127 was dissolved in 64 mL absolute ethanol thoroughly under vigorous magnetic stirring to obtain the solution B. The solution B was slowly added to the solution A, and the mixed solution was continuously stirring at ambient temperature for another 3 h. The resultant sol was homogenized by ultrasonic vibration for 3 min. The pretreatment HGMs were dipcoated with the prepared TiO2 colloidal solution, and then dried at 120 °C for 10 min. Such coating procedure was repeated five times. Finally, the as-synthesized thin films were calcined in a muffle furnace at a given temperature for 3 h at a heating rate of 3 °C /min in air. Twenty prepared TiO2–HGMs are defined as no. 1#–20#, respectively. 2.4. Characterization of activity and experimental procedure The degradation experiments were carried out in batch mode in a quartz reactor (300 mL) with a double-walled cooling-water jacket to keep the constant temperature of solutions throughout the experiments. A high-pressure mercury lamp (GGZ-125, Shanghai Yaming Lighting, kmax = 365 nm) with a power consumption of 125 W, placed in one side of the quartz reactor as the irradiation source. The distance between the surface of the reactor and the lamp was 15 cm. 3.0 g TiO2–HGMs was added into 100 mL MO aqueous solutions at fixed initial concentration 10 mg L1 under stir mixing. The photocatalytic degradation of MO was carried out for 180 min under the UV irradiation. Prior to irradiation, all the reactions were kept stiring in the dark for 30 min to reach the adsorption equilibrium of MO onto the TiO2–HGMs.

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Table 2 The experiment design of RSM for the three independent variables. Standard order

Variables in uncoded levels A: Titanium butoxide (g)

B: F127 (g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

12.00 20.00 12.00 20.00 12.00 20.00 12.00 20.00 9.27 22.73 16.00 16.00 16.00 16.00 16.00 16.00 16.00 16.00 16.00 16.00

2.40 2.40 6.40 6.40 2.40 2.40 6.40 6.40 4.40 4.40 1.04 7.76 4.40 4.40 4.40 4.40 4.40 4.40 4.40 4.40

Response Y1 Degradation efficiency (%)

Response Y2 Titanium dioxide (mg g1)

C: Temperature (°C)

Actual value

Predicted value

Actual value

Predicted value

450.00 450.00 450.00 450.00 550.00 550.00 550.00 550.00 500.00 500.00 500.00 500.00 415.91 584.09 500.00 500.00 500.00 500.00 500.00 500.00

86.7 91.8 71.4 83.4 75.1 96.4 70.7 90.2 74 91.2 94.2 80.7 85.8 81.6 95.4 93.9 97.5 95.1 93.6 95.4

87.14 92.66 74.24 82.31 76.77 94.14 70.43 90.34 71.50 92.88 94.06 80.02 84.27 82.31 95.17 95.17 95.17 95.17 95.17 95.17

21.9471 20.4817 14.8717 23.7229 19.6138 19.5991 15.9601 19.3891 14.4421 31.732 26.6754 19.1419 21.6795 21.7251 27.0891 21.944 19.9812 22.334 22.774 23.3945

20.46 26.30 17.48 23.32 19.53 25.37 16.55 22.39 16.51 26.34 23.93 18.92 22.21 20.64 21.42 21.42 21.42 21.42 21.42 21.42

2.5. Analytical method Aqueous solution samples were taken at fixed time intervals, and they were filtered by filter membrane (pore size < 0.22 lm). The concentration of MO was analyzed by using an UV–vis spectroscopy at its maximum absorption wavelength of 463 nm. The degradation efficiencies of MO were calculated by comparing the loss concentration after 180 min illumination with the initial concentration. The crystal phase of the obtained TiO2 thin film was measured by X’Pert Pro (Netherlands). X-ray diffraction (XRD) for the TiO2–HGMs was measured using Cu Ka radiation at 40 kV and 40 mA over the range 5–80° (2h) at a scan speed of 6° min1. The surface characteristics of TiO2–HGMs were analyzed using field emission-scanning electron microscopy (FE-SEM, Hitachi S-4800, Japan). The amount of TiO2 coated was measure according to the method reported by Bo et al. [35]. 3. Results and discussion 3.1. Characterization of TiO2–HGMs The XRD spectra of the TiO2–HGMs were shown in Fig. 1. The XRD pattern showed that no peak existed for HGMs without coating TiO2. However, some minor peaks appeared for TiO2–HGMs

Intensity (a.u.)

JCPD No. 01-073-1764

80

60

48.2 55.2

3.2. Model fitting and statistical analysis

100

TiO2-HGM HGM

40

62.8

Intensity (%)

25.4

due to the immobilization of titania nanoparticles on the surface of HGMs, which showed a high degree of crystallinity with characteristic peaks at 25.4°, 48.2°, 55.2° and 62.8°. These values match the characteristic peaks of TiO2 according to the database of the Joint Committee on Powder Diffraction Standards (reference code: 01-073-1764). It was shown that the crystal phase of TiO2 prepared as supported in the surface of HGMs was anatase form. The SEM analysis was also applied to investigate the TiO2 particles distribution and structure on the surface of HGMs. As shown in Fig. 2a, the overall outer surface of the TiO2–HGMs was rough and uneven with many pits. Fig. 2b and c demonstrated a continuous and homogeneous distribution of TiO2 nanoparticles on the outer surface of HGMs, and it also indicated that the film thickness is fairly uniform. To visualize and determine the thickness of film, a TiO2–HGM was crushed and a complete cross-section was obtained as shown in Fig. 2d. The cleaved TiO2–HGM showed the inner of HGM was hollow with a very dense shell wall. In addition, Fig. 2e was the magnification of a part of the shell wall (the red box marked in Fig. 2d), and it demonstrated the outer of shell wall was covered by a layer of TiO2 film with an average layer thickness of 728 ± 12 nm. The thickness of front end sharply thinned due to breakage of the film during the electron microscopy sample preparation process. Fig. 2f was the magnification of TiO2 film from side profile (the blue box marked in Fig. 2e) to give a sense of hierarchy of TiO2 nanoparticles stacked.

The experimental matrix design, the experimental values and predicted values based on experiments proposed by RSM for the degradation efficiency of MO and amount of TiO2 coated on the HGMs were all summarized in Table 2. On the basis of the experimental design presented in Table 2, a second-order polynomial equation in terms of actual factors was found which demonstrated the empirical relationships between the independent variables and the response Y1:

20

Y 1 ð%Þ ¼ 262:708 þ 3:015A  5:221B þ 1:360C þ 0:080AB 0 0

10

20

30

40

50

60

70

2 (degrees) Fig. 1. X-ray diffraction pattern of TiO2–HGMs.

80

þ 0:015AC þ 0:016BC  0:287A2  0:719B2  1:681  103 C 2 where Y1 was the degradation efficiency of MO.

ð3Þ

L. Sun et al. / Separation and Purification Technology 125 (2014) 156–162

159

Fig. 2. SEM images of (a) whole TiO2–HGMs (150); (b) outer surface of TiO2–HGMs (100 k); (c) outer surface of TiO2–HGMs (200 k); (d) cleaved TiO2–HGMs: inner concave surface (ICS) and shell wall (SW) (300); (e) shell wall (SW) and TiO2 film coated (50 k); (f) TiO2 nanoparticles from side profile (200 k).

In addition, a polynomial equation was also found for the response Y2:

Y 2 ð%Þ ¼ 17:701 þ 0:730A  0:746B  9:350  103 C

ð4Þ

where Y2 was the amount of TiO2 coated on the surface of HGMs. The negative and positive signs of regression coefficients represent the antagonistic effect and synergistic effect of each variable on the response, respectively. The results of ANOVA for degradation efficiency of MO (Y1) and amount of TiO2 coated (Y2) were listed in Tables 3 and 4, respectively. The model F-value was 40.33 for response Y1 and 4.72 for response Y2, respectively, which was much greater than the value of F(9, 10) (3.02) and F(3, 16)(3, 24) at 95% significance, implied the models were significant. In particularly, the model p value was <0.0001 for Y1 and <0.05 for Y2, which also indicated that the model were significant. The ‘‘Predicted R-Squared’’ of 0.8259 was in reasonable agreement with the ‘‘Adjusted R-Squared’’ of 0.9491 for Y1. Adequate precision measures the signal to noise ratio, and a ratio greater than 4 is desirable. The ratio of 17.444 indicated an adequate signal, and the model can be used to navigate the design space. Compared with Y1, the ‘‘Predicted R-Squared’’ of 0.1127 was not as close to the ‘‘Adjusted R-Squared’’ of 0.3703 as one might normally expect. This might indicate a large block effect or a possible problem with model of Y2. However, adequate precision measured the signal to noise ratio was 6.756, and it indicated this model could also be used to navigate the design space. As shown in Fig. 3a, the good

correlation (R2 = 0.9732) for degradation efficiency of MO indicated that this model could well explain the variables range studied. However, the correlation R2 = 0.4696 for the amount of TiO2 coated was far from 1, and it indicated that there are difference between the real values and the theoretical values as shown in Fig. 3b. Therefore, the model represented by Eq. (4) for the amount of TiO2 coated was not appropriate for further discussion. To further evaluate the quantitative effect of three variables on degradation efficiency of MO, the p-values were calculated (Table 3). If p-value is less than 0.05, and it means the variable is significant. The p-value of the model for the degradation efficiency of MO was <0.0001, which implied this model was very significant. In addition, the independent variables, including A, B, AC, BC, A2, B2, C2 are significant model terms for Y1 model. In particularly, the A-titanium butoxide, B-F127, and the second-order effect of A-titanium butoxide and C-calcination temperature are highly significant parameters with p-values <0.0001. Furthermore, the second-order effects of B-F127, and the synergistic effect of A-titanium butoxide and C-calcination temperature, B-F127 and C-temperature are significant with p < 0.05. More information about the importance of the variables in Eq. (3) was provided by Pareto analysis in the form of Pareto Chart. As shown in Fig. 4, the most important variable for the degradation efficiency of MO was the amount of B-F127 (70.25%), followed by A- titanium butoxide (23.42%) and then C-calcination temperature (4.77%). In addition, the degradation efficiency of MO was also affected by two variables interactions effect of B2 (1.33%) and A2 (0.21%).

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Table 3 ANOVA results for the response surface quadratic model of Y1. Source

Sum of squares

Degree of freedom

Mean squares

F-value

P-value

Remarks

Model A B C AB AC BC A2 B2 C2 Residual Lack of fit Pure error Cor total

1461.18 552.02 237.94 4.64 3.25 70.21 21.45 303.77 119.23 254.48 40.25 30.64 9.62 1501.43

9 1 1 1 1 1 1 1 1 1 10 5 5 19

162.35 552.02 237.94 4.64 3.25 70.21 21.45 303.77 119.23 254.48 4.03 6.13 1.92

40.33 137.14 59.11 1.15 0.81 17.44 5.33 75.47 29.62 63.22

<0.0001 <0.0001 <0.0001 0.3080 0.3899 0.0019 0.0436 <0.0001 0.0003 <0.0001

Significant Significant Significant

3.19

0.1146

Significant Significant Significant Significant Significant Not significant

R-Squared: 0.9732; Adj R-Squared: 0.9491; Pred R-Squared: 0.8259; Adeq precision: 17.444.

effect for titanium butoxide, it indicated that the titanium butoxide had an obvious positive effect on the photocatalytic activity of TiO2–HGMs. For instance, the degradation efficiency increased from 74.0% to 97.5% with increasing amount of titanium butoxide from 9.27 g to 16.0 g while amount of F127 was kept constant at 4.40 g. It was attributed to more TiO2 particles coated on the surface of HGMs. Compared with titanium butoxide, the F127 showed obviously negative effect on the photocatalytic activity of TiO2– HGMs. For instance, the degradation efficiency decreased from 94.2% to 80.7% with increasing amount of F127 from 1.04 g to 7.76 g while titanium butoxide was kept constant at 16.0 g. This was because the amount of TiO2 coated decreased with increasing amount of F127 as listed in Table 2. Meanwhile, Fig. 5a also indicates that the negative effect of F127 could be reduced by the increase of the amount of titanium butoxide. The highest value of degradation efficiency could achieve at the minimum amount of F127 with amount of titanium butoxide approximately closed to 20 g. Fig. 5b shows the combined effects of variables A and C on degradation efficiency at fixed F127 of 4.40 g. Degradation efficiency increased smoothly with increasing amount of titanium butoxide and calcination temperature. From the response surface analysis of the effect for titanium butoxide and calcination temperature, it indicated that the amount of titanium butoxide had an obvious positive effect on the photocatalytic activity of TiO2–HGMs. For calcination temperature, degradation efficiency firstly increased while calcination temperature increased from 450 °C to 500 °C,

Table 4 ANOVA results for the response surface quadratic model of Y2. Source

Sum of squares

Degree of freedom

Mean squares

Fvalue

pValue

Model Residual Lack of fit Pure error Total

149.81 169.10 141.54

3 16 11

49.94 10.57 12.87

4.72

0.0152

2.33

0.1802

27.56

5

5.51

318.90

19

R-Squared: 0.4698; Adj R-Squared: 0.3703; Pred R-Squared: 0.1127; Adeq precision: 6.756

3.3. RSM analysis In order to better understand the relationship between the degradation efficiency of MO and the independent variables A, B and C, three-dimensional surfaces plots were formed based on the model polynomial function (Fig. 5). Fig. 5a shows the combined effect of variables A and B on degradation efficiency at constant calcination temperature of 500 °C. It was observed that degradation efficiency increased with increasing amount of titanium butoxide and decreased with increasing amount of F127. This effect was more obvious at higher levels of F127 and at lower levels of titanium butoxide as observed. From the response surface analysis of the

Predicted vs. Actual

Predicted vs. Actual 35.00

100.00 2

30.00

90.00

Predicted

Predicted

95.00

85.00

25.00

20.00

80.00

15.00 75.00

10.00

70.00

70.00

75.00

80.00

85.00

90.00

95.00

100.00

10.00

15.00

20.00

25.00

Actual

Actual

(a)

(b)

30.00

35.00

Fig. 3. The relationship between the predicted values and actual values. (a) Degradation efficiency of MO; (b) amount of TiO2 coated on the surface of HGMs.

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CC

amount of F127 as calcination temperature was kept below 500 °C. The reason was that the amount of TiO2 coated decreased with increasing amount of F127 as listed in Table 2. In addition, the degradation efficiency of MO increased with increasing calcination temperature, but once it exceeded a certain level, it was accompanied by a decrease in degradation efficiency due to the transformation of crystal phase of TiO2 as discussed above. Fig. 5c also demonstrates that the highest value of degradation efficiency of MO was obtained at the minimum amount of F127 with calcination temperature approximately closed to 500 °C.

0.00

BB

1.33

AA

0.21

BC

0.00

AC

0.00

AB

0.02

C

4.77

B

70.25

A

23.42 0

1

2

3

4

5

6

7

20

3.4. Optimization of influencing factors

40

60

80

100

Optimization was performed on the basis of the desirability function to determine the optimal synthesis conditions for TiO2– HGMs prepared. The optimum conditions for maximum degradation efficiency of MO (98.6%) were titanium butoxide of 18.64 g, F127 of 3.12 g and calcination temperature of 501.89 °C at fixed 64 mL absolute ethanol and 120 mL 20% (v/v) acetic acid, respectively, based on the settings and boundaries listed in Table 1. Three replicate verification experiments were carried out in the identical photocatalytic reactor using the TiO2–HGMs prepared under the optimum conditions, and an average degradation efficiency of 97.3 ± 0.93% was obtained. The good agreement between the predicted value and the experimental value confirmed the validity of the model for photocatalytic degradation of MO.

Percentage (%) Fig. 4. Pareto graphic analysis for percentage effect of each factor on degradation efficiency of MO.

In this study, TiO2 nanoparticles were successfully coated on the surface of HGMs in the form of core–shell structure by sol–gel method. The RSM was used for optimization of synthesis

90 85 80 12

A:T

95

Degradation eff

95

90

85

6.4 13

itan

14

5.6 15

ium

4.8 16

but

17

oxi

4.0 18

de

19

(g)

3.2

B:

20 2.4

27

A:

) (g

F1

80 12

550 13

Tit 14 15 ani um 16 17 bu tox 18 19 ide (g) 20 450

(b)

525 500 475

C:

o ) (C

at

r

pe

e ur

m Te

100

Degradation

efficiency (%

)

(a)

4. Conclusions

iciency (%)

100

Degradation eff

iciency (%)

and then decreased gradually as the calcination temperature further increased beyond 500 °C. The reason of this observation was that the major crystal phase of TiO2 was anatase form as calcination temperature was less than 500 °C, and then the crystal phase of TiO2 gradually transited from anatase form to rutile form as calcination temperature further increased [36]. It was well known that the photocatalytic activity of TiO2 with anatase form was stronger than rutile phase. Fig. 5b indicates that titanium butoxide and calcination temperature had the synergistic effect on the degradation efficiency of MO, and the highest value was achieved at the maximum amount of titanium butoxide with calcination temperature approximately closed to 500 °C. Fig. 5c shows the combined effects of variables B and C on degradation efficiency at fixed titanium butoxide of 16.0 g. It showed that the degradation efficiency of MO decreased with increasing

95 90 85 80 550 2.4

(c)

525

3.2

500

4.0

B:F

127 4.8 (g)

475 5.6 6.4

450

C:

m Te

o

e(

C)

r tu

ra

pe

Fig. 5. Three-dimensional surfaces plots (a) effects of titanium butoxide and F127 on degradation efficiency of MO (calcination temperature = 500 °C); (b) effects of titanium butoxide and calcination temperature on degradation efficiency of MO (F127 = 4.40 g); (c) effects of F127 and calcination temperature on degradation efficiency of MO (titanium butoxide = 16.0 g).

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conditions of TiO2–HGMs, and a second-order polynomial equation was developed for describing the influence of key variables on degradation efficiency of MO. The results of model fitting and statistical analysis demonstrated that variable A and B played a key role on the photocatalytic activity of prepared TiO2–HGMs. The interactions of AB, AC and BC demonstrated a positive effect, while A2, B2 and C2 demonstrated a negligible effect on the photocatalytic activity of prepared TiO2–HGMs. The optimum synthesis conditions of TiO2–HGMs were titanium butoxide/F127/absolute ethanol/20% acetic acid/calcination temperature = 18.64 g/3.12 g/ 64 mL/120 mL/501.89 °C, and the maximum photocatalytic degradation efficiency of MO was 98.6%. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51368004, 51208492 and 21367002) and the China Postdoctoral Science Foundation (2013M530747). In addition, authors gratefully acknowledge Manager Yongbin Huo of Qinghuangdao Qinhuang Glass Microsphere Co., Ltd. China for kindly providing of HGMs. References [1] R. Andreozzi, V. Caprio, A. Insola, R. Marotta, Advanced oxidation processes (AOP) for water purification and recovery, Catal. Today 53 (1999) 51–59. [2] C. Comninellis, A. Kapalka, S. Malato, S.A. Parsons, I. Poulios, D. Mantzavinos, Advanced oxidation processes for water treatment: advances and trends for R&D, J. Chem. Technol. Biotechnol. 83 (2008) 769–776. [3] E.S. Elmolla, M. Chaudhuri, Degradation of amoxicillin, ampicillin and cloxacillin antibiotics in aqueous solution by the UV/ZnO photocatalytic process, J. Hazard. Mater. 173 (2010) 445–449. [4] E. Evgenidou, I. Konstantinou, K. Fytianos, T. Albanis, Study of the removal of dichlorvos and dimethoate in a titanium dioxide mediated photocatalytic process through the examination of intermediates and the reaction mechanism, J. Hazard. Mater. 137 (2006) 1056–1064. [5] D. Sud, P. Kaur, Heterogeneous photocatalytic degradation of selected organophosphate pesticides: a review, Crit. Rev. Environ. Sci. Technol. 42 (2012) 2365–2407. [6] H.L. Liu, Y.R. Chiou, Optimal decolorization efficiency of reactive red 239 by UV/TiO2 photocatalytic process coupled with response surface methodology, Chem. Eng. J. 112 (2005) 173–179. [7] I.K. Konstantinou, T.A. Albanis, TiO2-assisted photocatalytic degradation of azo dyes in aqueous solution: kinetic and mechanistic investigations: a review, Appl. Catal. B: Environ. 49 (2004) 1–14. [8] U.I. Gaya, A.H. Abdullah, Heterogeneous photocatalytic degradation of organic contaminants over titanium dioxide: a review of fundamentals, progress and problems, J. Photochem. Photobiol. C 9 (2008) 1–12. [9] V. Augugliaro, M. Bellardita, V. Loddo, G. Palmisano, L. Palmisano, S. Yurdakal, Overview on oxidation mechanisms of organic compounds by TiO2 in heterogeneous photocatalysis, J. Photochem. Photobiol. C 13 (2012) 224–245. [10] P.M. Álvarez, J. Jaramillo, F. López-Piñero, P.K. Plucinski, Preparation and characterization of magnetic TiO2 nanoparticles and their utilization for the degradation of emerging pollutants in water, Appl. Catal. B: Environ. 100 (2010) 338–345. [11] Z. Zhang, C.C. Wang, R. Zakaria, J.Y. Ying, Role of particle size in nanocrystalline TiO2-based photocatalysts, J. Phys. Chem. B 102 (1998) 10871–10878. [12] M. Vaez, A. Zarringhalam Moghaddam, S. Alijani, Optimization and modeling of photocatalytic degradation of azo dye using a response surface methodology (RSM) based on the central composite design with immobilized titania nanoparticles, Ind. Eng. Chem. Res. 51 (2012) 4199–4207. [13] L. Sun, T. An, S. Wan, G. Li, N. Bao, X. Hu, J. Fu, G. Sheng, Effect of synthesis conditions on photocatalytic activities of nanoparticulate TiO2 thin films, Sep. Purif. Technol. 68 (2009) 83–89.

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