Optimization of sonocatalytic degradation of Rhodamine B in aqueous solution in the presence of TiO2 nanotubes using response surface methodology

Chemical Engineering Journal 166 (2011) 873–880

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Optimization of sonocatalytic degradation of Rhodamine B in aqueous solution in the presence of TiO2 nanotubes using response surface methodology Yean Ling Pang, Ahmad Zuhairi Abdullah ∗ , Subhash Bhatia School of Chemical Engineering, Universiti Sains Malaysia, Nibong Tebal, 14300 Penang, Malaysia

a r t i c l e

i n f o

Article history: Received 8 October 2010 Received in revised form 15 November 2010 Accepted 16 November 2010 Keywords: Sonocatalytic Titanium dioxide nanotubes Rhodamine B Response surface methodology Optimization Interactions

a b s t r a c t Efficiency in the sonocatalytic degradation of Rhodamine B dye catalyzed by TiO2 nanotubes as a function of initial concentration of dye, catalyst amount and power of ultrasonic irradiation at a frequency of 35 kHz was studied. A central composite design (CCD) was used for response surface modeling to evaluate the combined effects of these variables as well as to optimize the degradation efficiency of Rhodamine B. Satisfactory prediction based on a second-order model with high coefficient of determination (R2 ) of 0.98 was achieved for the optimized sonocatalytic degradation process. Lastly, the significance and adequacy of the model were analyzed using analysis of variance (ANOVA). The optimal conditions for sonocatalytic degradation efficiency of Rhodamine B were found at 44.8 mg/L of Rhodamine B, 2.14 g/L of TiO2 nanotubes and ultrasonic power of 68.9 W to achieve 94.6% dye removal under ultrasonic irradiation in 3 h. The significance of the findings at a confidence level of 95% was demonstrated. It was found that ultrasonic power, initial dye concentration and interaction between dye concentration and catalyst loading had important effects in the sonocatalytic degradation efficiency of the Rhodamine B by TiO2 nanotubes. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The annual production of organic dyes in the world is currently reported to be more than 700,000 tons and a significant portion of this amount will go to textile industry [1]. Considering water consumption utilized per kilogram of textile product which is around 100–200 L coupled with the wastewater generated from dyeing and finishing operations, textile industry is rated as one of the most polluting industries [2]. It is estimated that about 10–30% of the total world production of dyes is lost during the dyeing process and released together with effluents [2,3]. Rhodamine B is a basic dye of the xanthene class. It is widely used in industrial purposes and capable to causes irritation to the skin, eyes, gastrointestinal tract and respiratory tract [4]. Therefore, treatment of dye-containing effluents, i.e. Rhodamine B is a topic of significant interest among researchers. A lot of studies have been dedicated to treat and neutralize these wastewater by using advanced oxidation process (AOP) through the generation of highly reactive hydroxyl (• OH) radicals to mineralize the harmful organic compounds. Among the AOPs, sonocatalytic degradation of organic dyes has appeared as an emerging destructive technology. This oxidation process has strong penetration

∗ Corresponding author. Tel.: +60 4 599 6411; fax: +60 4 594 1013. E-mail addresses: [email protected], [email protected] (A.Z. Abdullah). 1385-8947/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2010.11.059

ability even in colored effluent [5]. Ultrasonic irradiation in the frequency range of 30–1000 kHz will result in the formation, growth and implosive collapse of cavities in a liquid. Subsequently, many local hot spots with extreme conditions (=5,000 K and =1,000 atm) are generated [4,6]. Under such extremely high temperature and pressure, acoustic cavitation is able to provide sufficient energy to produce many holes and create many • OH radicals to attack and degrade the organic compounds. Many researchers have shown that the presence of TiO2 nanoparticles could increase the sonocatalytic degradation of organic pollutants [3,5]. The potential applications of TiO2 nanotubes as photocatalysts have attracted much attention in recent years [7–9]. However, the performance of TiO2 nanotubes on the ultrasonic-assisted degradation of organics dyes is hardly reported in previous publications. Sonocatalytic degradation using titanium dioxide (TiO2 ) nanotubes has been recently investigated and it showed remarkable improvement in the degradation efficiency of Rhodamine B as compared to TiO2 powder [10]. TiO2 in the form of nanotubes have improved catalytic properties compared to TiO2 powder such as high aspect ratio, high surface to volume ratio, high sedimentation rate and versatile chemistry [11,12]. The special features of TiO2 nanotubes provide more channels for enhanced electron transfer and interpenetration of holes transport materials to help in increasing the degradation efficiency. Recent studies on the optimization of the operational factors in a sonocatalytic reactor system to degrade organic pollutants are

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the organic dye in aqueous solution under ultrasonic irradiation. The detail procedures for preparing the catalyst were described earlier [10]. 2.2. Catalysts characterization

Fig. 1. Chemical structure of Rhodamine B.

merely limited to the conventional one-factor-at-a-time approach [3,4]. Meanwhile, studies on the use of response surface methodology (RSM) to investigate the interaction effects between the operational variables have been reported for sonochemical degradation method without the use of any catalyst [13,14]. It has been proven that RSM is a powerful experimental design tool and successfully employed for the optimization of degradation for different pollutants. Thus, the main aim of the present study was to optimize the design parameters of the sonocatalytic degradation of Rhodamine B as well as to investigate the interaction effects between the tested variables. The independent variables for the sonocatalytic degradation Rhodamine B in this study were initial dye concentration, amount of TiO2 nanotubes and power of ultrasonic irradiation. These variables were selected in precedence of other important variables because they could better reflect the fundamental effects of TiO2 nanotubes towards the degradation of Rhodamine B and laid a solid foundation to investigate for more complicated variables in the future. 2. Experimental 2.1. Experimental materials Rhodamine B (molecular weight = 479.01 g/mol) was obtained from Sigma–Aldrich as a commercially available dye. The chemical structure of this dye is shown in Fig. 1. Distilled water was used throughout the study. The catalyst used, i.e. TiO2 nanotubes was synthesized using a sol–gel process followed by a hydrothermal treatment and then calcined at 300 ◦ C for 2 h before use in treating

The tubular structure of TiO2 nanotubes were analyzed using a Phillips CM 12 transmission electron microscope equipped with an image analyzer and operated at 120 kV. The specific surface area, pore volume and pore size distribution properties of TiO2 nanotubes were measured by nitrogen adsorption isotherm at 77 K using a surface analyzer (Micromeritics ASAP-2020) by the Brunauer–Emmett–Teller (BET) method. 2.3. Sonocatalytic degradation of Rhodamine B An ultrasonic bath (Elma Transsonic series TI-H5) operated at a fixed frequency of 35 kHz and a maximum effective power output of 100 W through manual adjusting was adopted to irradiate the organic dye solution. The schematic diagram of experimental set up is illustrated in Fig. 2. In a typical experimental run, 200 mL of Rhodamine B solution was prepared in a glass reaction vessel and TiO2 nanotubes were added. Then, the suspension was stirred for 15 min to ensure a good dispersion of the TiO2 nanotubes. The reaction vessel was then placed inside the ultrasonic bath at around 30 ◦ C and irradiated under ultrasonic sound for 3 h. All the experimental runs were conducted in a batch mode. 2.4. Analysis of liquid sample After 3 h of sonocatalytic reaction, 5 mL of liquid sample was withdrawn from the reaction vessel and a centrifuge (Kubota 2100) operated at 3,500 rpm for 20 min was used to settle down any suspended TiO2 nanotubes. After that, Rhodamine B’ concentration was measured using AquamateTM Plus UV–vis spectrophotometer (Thermo Scientific Company). The concentration of Rhodamine B was measured based on absorbance at 554 nm. The degradation efficiency of Rhodamine B dye is defined as follows: Degradation efficiency(%) =

Fig. 2. Schematic diagram of the sonocatalytic experimental set up.

C0 − Ct × 100% C0

(1)

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Table 1 Actual values of the variables and their respective coded levels used in central composite rotatable design (CCRD). Variables

Factors

Dye concentration, [Dye]0 (mg/L) Catalyst loading, [TiO2 ]0 (g/L) Ultrasonic power (W)

x1 x2 x3

Actual values for the coded levels −˛ (−1.68)

−1

0

+1

+˛ (+1.68)

16.36 1.2 26.36

30.00 1.5 40.00

50.00 2.0 60.00

70.00 2.5 80.00

83.64 2.8 93.64

where C0 = initial concentration of Rhodamine B (mg/L) and Ct = concentration of Rhodamine B at reaction time t (min) (mg/L). The chemical oxygen demand (COD) test was conducted using commercially available test kits provided by Hach Company based on the standard method of potassium dichromate oxidation in the COD range of 0–150 mg/L [15]. Samples were added into COD test tubes that contained potassium dichromate in acidic medium and heated at 150 ◦ C for 2 h in a digestion reactor (Hach Digital Reactor Block DRB 200). Oxidizable organic compounds would react and 3+ reduce the dichromate ion (Cr2 O2− 7 ) to green chromic ion (Cr ). After cooling the COD test tubes to room temperature, COD levels were measured by a colorimeter (Hach DR/890). 2.5. Experimental design and optimization by using response surface methodology

variable (response). The ranges and levels of independent variables studied were determined through a series of preliminary tests and present in Table 1. The value of ˛ was fixed √ at 1.68 for rotatable design. Lower or higher value of ˛ (˛ = k, where k is number of process variables) would cause the independent variables to become negative and lower range of the axial point, respectively [17]. Each variable in the design was studied at five different coded levels (−1.68, −1, 0, 1, 1.68). All variables were taken at a central coded value considered which was at the zero level. Table 2 represents the experiment matrix and the experimental response (degradation efficiency of Rhodamine B). The experiment sequence was randomized in order to minimize systematic errors while the responses were evaluated. 2.6. Statistical analysis

In this study, RSM was used for the experimental design and optimization of the process which were influenced by several independent variables with minimum runs of experiment. This could eliminate the time consuming phase which could not be achieved using conventional method (one-factor-at-a-time approach). Besides, the central composite design (CCD) is well suited for fitting a quadratic surface, which usually works well for the process optimization [16]. By using CCD, linear, quadratic, cubic and cross-product effects of operating condition variables on the degradation efficiency were investigated. A set of 20 experiments were designed to optimize the sonocatalytic degradation efficiency of Rhodamine B using TiO2 nanotubes. The 6 replications at the design center point were utilized to provide information on the variation of the responses about the average and the residual variance. The tree independent variables studied were the initial concentration of Rhodamine B (x1 ), amount of TiO2 nanotubes (x2 ) and power of ultrasonic irradiation (x3 ) while the degradation efficiency of Rhodamine B was chosen as the output

The Design Expert software (version 6.0.6, Stat-Ease, Inc., Minneapolis, USA) was used for regression and graphical analyses of the data. In the optimization process, the experimental responses can be analyzed with the following second-order polynomial Eq. (2) [18]. ypred = ˇ0 +

k 

ˇi xi +

k 

i

ˇii xi2 +

i

k k−1   i

ˇij xi xj + ε

(2)

j=i+1

where ypred is the predicted variable, xi and xj are the independent variables, ˇ0 is the constant coefficient, ˇi , ˇii and ˇij are the interaction coefficients of linear, quadratic and the second-order terms, respectively, and ε is the error. Response surfaces and contour plots were developed using the fitted quadratic polynomial equation obtained from regression analysis. The experiments were run by changing of two variables while holding the other variable at a constant value. In this study, data for the degradation efficiency of Rhodamine B were processed based on Eq. (3) including analysis

Table 2 Experimental results for the three independent variables with six replicates of the center points. Standard order

Point type

Coded independent variable levels

Degradation efficiency, % (y)

[Dye]0 , mg/L (x1 )

[TiO2 ]0 , g/L (x2 )

Power, W (x3 )

Experimental values

Predicted values

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

Fact Fact Fact Fact Fact Fact Fact Fact Axial Axial Axial Axial Axial Axial Center Center Center Center Center Center

30 (−1) 30 (−1) 30 (−1) 30 (−1) 70 (1) 70 (1) 70 (1) 70 (1) 16.36 (−1.68) 50 (0) 50 (0) 50 (0) 83.64 (+1.68) 50 (0) 50 (0) 50 (0) 50 (0) 50 (0) 50 (0) 50 (0)

1.5 (−1) 1.5 (−1) 2.5 (1) 2.5 (1) 1.5 (−1) 1.5 (−1) 2.5 (1) 2.5 (1) 2.0 (0) 1.16 (−1.68) 2.0 (0) 2.84 (+1.68) 2.0 (0) 2.0 (0) 2.0 (0) 2.0 (0) 2.0 (0) 2.0 (0) 2.0 (0) 2.0 (0)

40 (−1) 80 (1) 40 (−1) 80 (1) 40 (−1) 80 (1) 40 (−1) 80 (1) 60 (0) 60 (0) 26.36 (−1.68) 60 (0) 60 (0) 93.64 (+1.68) 60 (0) 60 (0) 60 (0) 60 (0) 60 (0) 60 (0)

80.61 91.77 71.32 89.89 55.03 71.24 60.05 87.92 94.86 71.45 56.27 79.27 67.13 94.48 93.58 92.52 94.11 92.84 94.24 90.94

80.30 92.17 70.79 92.19 52.98 72.02 59.90 88.47 93.87 72.27 58.20 78.10 67.77 92.20 93.05 93.05 93.05 93.05 93.05 93.05

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Fig. 3. TEM images of (a) TiO2 powder and (b) TiO2 nanotubes.

of variance (ANOVA). Data analysis through ANOVA is much more scientific than direct observation analysis [19]. 3. Results and discussion 3.1. Characterization of TiO2 catalyst The TEM images were carried out in order to understand the transformation from TiO2 powder to TiO2 nanotubes through a hydrothermal process in the presence of 10 M NaOH followed by acid washing. Fig. 3(a) shows that the original TiO2 particles had dimensions of about 15–25 nm and Fig. 3(b) shows that the TiO2 nanotubes presented an average diameter of 10–15 nm with a layered, hollow structure and open ended. BET surface area and average pore size for TiO2 powder and TiO2 nanotubes were ˚ respectively. According 77 m2 /g, 94.27 A˚ and 151 m2 /g, and 83.52 A, to the IUPAC classification, the pore sizes in both types of catalysts belong to the mesopore group. It is predicted that the inner and outer surface of layered-tubular structure is the major reason for the increase in surface area, which is one of the new quality of this nanocatalyst when it is compared with the start material. Other characterizations on TiO2 nanotubes such as SEM, EDX and XRD have been discussed previously [10].

3.2. Regression analysis The degradation efficiency of Rhodamine B was found to range from 55.0 to 94.9% in response to the variation in the experimental conditions. A regression analysis was performed to fit the response function and predict the outcome of degradation efficiency with a simple equation. The approximating function of degradation efficiency of Rhodamine B (in terms of coded units) obtained by the software is given in Eq. (3): ypred = 93.05 − 7.76x1 + 1.73x2 + 10.11x3 − 4.32x12 − 6.31x22 + 4.11x1 x2 + 1.79x1 x3 + 2.38x2 x3

(3)

Positive and negative signs in front of the terms in Eq. (3) indicate synergistic effect and antagonistic effect, respectively. Table 3 summarizes the ANOVA results. The quality of the model developed was evaluated based on the value of coefficient of determination (R2 ). The R2 value for Eq. (3) was 0.98 which was relatively high (close to unity). This implied that 98% of the variations for the degradation efficiency of Rhodamine B are explained by the independent variables and only 2% of the total variability in the response was not explained by the model. The high R2 value indicated that the model obtained was able to give a convincingly good estimate of response in the studied range.

Table 3 ANOVA results for sonocatalytic degradation efficiency of Rhodamine B dye. Factors

Squares sum

Freedom degrees

Square average

F-Value

Probability, P (P > F-value)

Quadratic model x1 x2 x3 x12 x22 x32 x1 x2 x1 x3 x2 x3 Residual Lack of fit Pure error Corrected total

3659.05 822.52 41.07 1395.91 269.28 574.69 573.73 135.05 25.74 45.46 30.87 23.27 7.60 3689.92

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

406.56 822.52 41.07 1395.91 269.28 574.69 573.73 135.05 25.74 45.46 3.09 4.65 1.52

131.70 266.45 13.30 452.20 87.23 186.17 185.86 43.75 8.34 14.73

<0.0001 <0.0001 0.0045 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 0.0062 0.0033

3.06

0.1223

R2 = 0.98; adequate precision = 32.92.

Significant

Insignificant

Y.L. Pang et al. / Chemical Engineering Journal 166 (2011) 873–880

Fig. 4. Actual and predicted values of sonocatalytic degradation of Rhodamine B (R2 = 0.98).

The model F-value of 131.70 implied that the model was significant for the degradation efficiency of Rhodamine B and there was only a 0.01% chance that a model F-value of this large could occur due to noise. Besides, a ratio of adequate precision that was greater than 4 and it was desirable as it generally measures the signal to noise ratio. Therefore, the ratio of 32.92 in the quadratic model of degradation efficiency of Rhodamine B indicated that adequate signal for the model was used to navigate the design space. For Eq. (3), lack of fit F-value of 3.06 implied that the model of degradation efficiency of Rhodamine B developed was insignificance. The non-significant lack of fit indicates good predictability of the model. There was a 12.23% chance that a lack of fit F-value could occur due to noise. The value of probability (P > F-value) indicated that the probability equals the proportion of the area under the curve of the F-distribution that lies beyond the observed F-value. The significance of the particular model term that affected the measured response (degradation efficiency in this case) of the system was proven by the small value of P > F (less than 0.05). Meanwhile, the value of P which was greater than 0.10 indicated that the model term was insignificant [18]. Thus, this analysis confirmed the all the significant terms of individual variables effects (x1 , x2 and x3 ) and interaction terms (x12 , x22 , x32 , x1 x2 , x1 x3 , and x2 x3 ) for the degradation efficiency of Rhodamine B. Fig. 4 shows the predicted output values versus actual experimental values for the degradation efficiency of Rhodamine B. From this figure, it is noted that the values calculated using the predictive second-order model was in good agreement with the experimental values with satisfactory correlation between these values. Therefore, the model developed is suitable for predicting the degradation efficiency of Rhodamine B in the conditions investigated. 3.3. Optimization conditions and response surface analysis From ANOVA results, sonocatalytic degradation of Rhodamine B was affected by the main factors: initial dye concentration (x1 ), amount of TiO2 nanotubes (x2 ), power of ultrasonic irradiation (x3 ) and their respective higher-order terms (x12 , x22 , and x32 ). Significant interactions terms were found to exist between the main factors (x1 x2 , x1 x3 and x2 x3 ). Fig. 5 represents the three-dimensional

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Fig. 5. Three-dimensional response surface plot of degradation efficiency (%) of Rhodamine B: effect of initial dye concentration and catalyst loading at a ultrasonic power of 60 W.

response surface plot of degradation efficiency at an ultrasonic power of 60 W to investigate the interactive effect of dye concentration and catalyst loading. By comparing run 2 and run 8, it was showed that low initial concentration of Rhodamine B would require low amount of catalyst to obtain the optimum degradation efficiency and vice versa. However, the surface area provided by TiO2 nanotubes (151 m2 /g) was higher than TiO2 powder (77 m2 /g), thereby, the former type of catalyst could treat higher concentration of dye molecules. At different catalyst loading, the degradation efficiency was decreased with decreasing initial concentration. Initially, higher concentration might cause excessive adsorption of Rhodamine B molecules on the surface of TiO2 nanotubes. This phenomenon would eventually prohibit TiO2 particles from absorbing heat and energy that were generated by acoustic cavitation [20]. As a result, it would limit the generation of • OH radicals which was the main oxidant in this process to oxidize the organic dye and subsequently reduced the degradation efficiency of Rhodamine B. Another possible reason might be due to the interference from intermediates formed during the oxidation of Rhodamine B [21]. This would cause higher competition between dye molecules and intermediate products to react with • OH radicals at higher concentration of Rhodamine B. Consequently, it led to the reduction in the degradation efficiency of Rhodamine B. Fig. 6 shows the interaction between ultrasonic power and initial concentration of Rhodamine B in the three-dimensional response surface plot at a catalyst loading of 2 g/L. It was interesting to observe that ultrasonic power has a higher effect on the degradation efficiency of Rhodamine B at high initial dye concentration than that at low initial concentration. It could be observed by comparing the run 1, 2 (increased 11%) versus run 5, 6 (increased 16%) when increasing ultrasonic power at a fixed dosage of TiO2 nanotubes. This phenomenon can be explained by • OH radicals generated through a stronger acoustic cavitation which in turn can oxidize more organic dyes molecules on the surface of TiO2 nanotubes. Besides, the higher output power could also disperse TiO2 NTs in the solution more evenly. Consequently, it would enhance the degradation efficiency of Rhodamine B. However, the degradation rate at high power levels could level off to indicate the increase in the rate of the competing electron–hole recombination

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Fig. 6. Three-dimensional response surface plot of degradation efficiency (%) of Rhodamine B: initial dye concentration and ultrasonic power at a catalyst loading of 2 g/L.

reaction step. It should be noted that a higher optimum ultrasonic power (>200 W) was required to degrade the organic dyes, i.e. Acid Green 20 without the presence of catalyst [14]. The optimum ultrasonic power (69.8 W) used in this study is significantly lower than that used in the sonocatalytic degradation of Methylene Blue (200 W) as reported by Shimizu et al. [22]. This is because the reduction of ultrasonic power used in this study was influenced by the catalyst–organic molecule system (TiO2 pellet versus TiO2 nanotubes in this case). The response surface plot of degradation efficiency at 50 mg/L of Rhodamine B with a different catalyst loading and ultrasonic power is shown in Fig. 7. It showed that ultrasonic power had a

significant effect on degradation efficiency of Rhodamine B; however, the degradation efficiency was less affected by the catalyst loading. The effect of TiO2 nanotubes loading was more significant when applying at high ultrasonic power. It can be seen that the degradation efficiency increased slightly with increasing amount of TiO2 nanotubes from 1.5 to 2.0 g/L and then decreased slightly at higher loading. The increasing amount of catalyst will provide more available active sites for the generation of free radicals for the oxidation of dye molecules. However, it showed a detrimental effect if excessive amount of catalyst was added. This was due to the screening effect by those excess particles that would hinder the optimum generation of • OH free radical on the surface of the catalyst. In a secondary degradation reaction, it also shielded Rhodamine B molecules from receiving sonic waves to result in a drop in sonolysis reaction. It was highlighted here that the optimum catalyst loading was lower in this sonocatalytic degradation process as compared to most reported photocatalytic applications [21,23]. It could be associated with the shorter wavelength of ultraviolet wave as compared to that in ultrasonic wave as used in this study. Shorter wavelength will undergo more deflection to impart its energy on new catalyst surfaces. Muruganandham and Swaminathan [23] and Tangestaninejad et al. [24] used 4 g/L and 3.33 g/L TiO2 dosage for photocatalytic and sonocatalytic reaction of organic compounds, respectively. The lower amount of optimum catalyst loading (2.14 g/L) in this study might be due to the formation of active sites on the inner and outer surface of the layered-tubular TiO2 nanotubes. In short, TiO2 nanotubes could provide channels for enhanced charge separation and higher surface area. Thereby, it could help to increase the efficiency of the sonocatalytic reaction with lower catalyst loading to degrade higher concentration of dye. For instance, Wang et al. [25] reported a lower sonocatalytic degradation efficiency of 10 mg/L Rhodamine B (50%) in the catalyzed by nanosized powder after 2 h. However, the degradation efficiency of Rhodamine B found in this study was significantly higher (55–95%) even though higher concentration of dyes molecules was used after 3 h. The higher sonocatalytic activity with the use of TiO2 nanotubes as compared to nanosized powder was directly attributed by the higher surface area of its tubular structure. As explained earlier, the layered-structure of TiO2 nanotubes provide larger specific surface area will allow more Rhodamine B reactants to be adsorbed and oxidized on the surface as compared to nanoparticles. At the same time, higher pore volume (0.32 cm3 /g for TiO2 nanotubes versus 0.24 cm3 /g for TiO2 powder) results in a more rapid diffusion of various products during the sonocatalytic reaction [26]. The sonocatalytic degradation mechanism is often related to the theory of ‘hot spot’ and phenomena of sonoluminescence [3]. The extreme localized condition and formation of the light flash with comparative band gap energy are able to produce reactive radicals and excite the electrons from the valence band to the conduction band, leaving behind holes at the valence band. Here, the increased delocalization of charge carriers in TiO2 nanotubes is able to reduce the probability of recombination of electrons and holes [27]. The electrons are then captured by the surface adsorbed O2 molecules • • to yield O− 2 and OOH radical ions. On the other hand, these holes not only could directly decompose the adsorbed dye molecules on the surface of the nanotubes, it could also accelerate the generation of • OH radicals through oxidizing H2 O molecules in the bulk solution [28]. Then, Rhodamine B molecules could be mineralized at an accelerated rate by these reactive radical ions. 3.4. Model validation and experimental confirmation

Fig. 7. Three-dimensional response surface plot of degradation efficiency (%) of Rhodamine B: catalyst loading and ultrasonic power on a initial dye concentration of 50 mg/L.

The use of RSM for optimization of sonolysis oxidation process without the presence of catalyst to degrade organic dyes has

Y.L. Pang et al. / Chemical Engineering Journal 166 (2011) 873–880 Table 4 Experimental results for model validation conducted at the optimum conditions as obtained from RSM. Runs

x1

x2

x3

Experimental values

Predicted values

1 2 3 4 5

44.84 45.97 44.30 32.71 36.26

2.14 1.82 2.00 2.24 2.27

68.88 64.00 64.49 65.31 69.91

94.58 91.41 93.12 93.78 94.15

97.21 94.87 96.74 96.37 97.40

been reported by several researchers [13,14]. In this study, three independent variables were optimized using RSM to obtain the highest possible degradation efficiency of Rhodamine B by using TiO2 nanotubes. In order to confirm the validity of predicted model and optimize the variables, ten solutions for the optimum conditions were generated by the DOE software according to the order of suitability. The first five solutions for the optimum conditions were chosen as shown in Table 4. The experimental values obtained for sonocatalytic degradation efficiency of Rhodamine B were found to be within 5% accuracy to those predicted values using Eq. (3). For the optimization of process variables in sonocatalytic degradation efficiency of Rhodamine B at a frequency of 35 kHz, the theoretical predicted degradation efficiency closer to 97.2% was obtained with the initial concentration of Rhodamine B of 44.8 mg/L, TiO2 nanotubes amount of 2.14 g/L and ultrasonic power of 68.9 W. The experimental sonocatalytic degradation efficiency of 94.6% was achieved. The experimental value obtained was in good agreement with the value predicted from the quadratic model. This result confirmed that the RSM was an effective and reliable method for optimizing the sonocatalytic degradation of Rhodamine B. In addition, changes in the absorption spectrum of Rhodamine B under optimum conditions are shown in Fig. 8. The significant drop in the absorbance within visible wavelengths confirmed the sonocatalytic degradation of Rhodamine B. However, specific trend in UV wavelengths (200–400 nm) was not observed in this study because the small (colorless) degradation products such as ultimate gaseous and volatile products would simply escape from the reaction mixture. On the other hand, it was found that only 4.5% of COD removal was achieved after 3 h of ultrasonic irradiation in the presence of TiO2 nanotubes. It has to be pointed out that reducible species such as H2 O2 that was generated during the ultrasonication of water and excess of chloride ions from the Rhodamine B molecules could interfere into the COD test leading to erroneous COD results. The reactions between H2 O2 and Cl− leading to the reduction of dichromate 3+ ion (Cr2 O2− 7 ) in the COD reagent to chromic ion (Cr ) are as fol-

879

lows: K2 Cr2 O7 + 3H2 O2 + 4H2 SO4 → K2 SO4 + Cr2 (SO4 )3 + 7H2 O + 3O2 (4) + 3+ + 7H2 O 6Cl− + Cr2 O2− 7 + 14H → 3Cl2 + 2Cr

(5)

Small amount of chloride was not likely to affect the COD value due to the presence of mercuric sulphate in the digestion solution [15]. However, as one chloride ion is present in the Rhodamine B molecule, the interference could not be totally ruled out. Minero et al. [6] had proven that small amounts of H2 O2 could be produced during ultrasonic irradiation. Thus, the most reliable reason of small decrement in COD value was attributed to the presence of H2 O2 in the final reaction mixture after the ultrasonic reaction. This justified further study to be carried out in order to quench the H2 O2 in the solution prior to COD measurement. The right approach to optimize the performance is important in order to minimize the operation cost for an industrial wastewater treatment processes. Hence, a figure-of-merit, electric energy per order (EE/O) was used to evaluate the cost efficiency of the optimum degradation process [29]. EE/O is the electrical energy (in kWh) required to degrade organic pollutant by one order of magnitude in 1000 US gal (3785 L) of water. It can be defined as: EE P (t/3600)3785 2.42P = elec = O Vkapp V (log(C0 /C))

(6)

where P is the electrical power (kW), t is the irradiation time (s), V is the reactor volume (L), kapp is the pseudo-first-order rate constant, C0 and C are the initial and final concentrations, respectively. Most researchers have observed the kinetics for ultrasonic degradation of organic pollutants to follow a pseudo-first-order kinetics [3,6,20]. The pseudo-first-order rate constants of Rhodamine B before and after optimization were 8.49 × 10−3 min−1 (83%) [10] and 0.016 min−1 (94.6%) (figure not shown), respectively. Thus, the calculated EE/O values before and after optimization were 855.1 kWh/gallon-order and 625.0 kWh/gallon-order, respectively. This indicated that the optimum conditions for sonocatalytic degradation could reduce the EE/O value. 4. Conclusions TiO2 nanotubes appeared to be very active catalyst for sonocatalytic degradation of Rhodamine B dye in aqueous solution. The proposed second-order polynomial model showed high coefficient of determination and the experimental values accurately fitted the predicted values under various process conditions. This finding proved the suitability of the model and the success of RSM in optimizing the sonocatalytic process conditions for maximizing the sonocatalytic degradation efficiency of Rhodamine B. Relative effects of interactions between process variables were successfully analyzed. The optimized variables for the degradation efficiency of Rhodamine B determined in this study were found at an initial concentration of 44.8 mg/L, an amount of TiO2 nanotubes of 2.14 g/L and an ultrasonic power of 68.9 W. Under these conditions, the degradation efficiency of Rhodamine B was at 94.6% in 3 h. In the present study, ultrasonic power, initial dye concentration and interaction between dye concentration and catalyst loading exhibited the most significant effects on the sonocatalytic degradation of Rhodamine B by TiO2 nanotubes. Acknowledgements

Fig. 8. Changes in absorbance during sonocatalytic degradation of Rhodamine B (initial dye concentration = 44.8 mg/L, amount of TiO2 nanotubes = 2.14 g/L, ultrasonic power = 68.9 W, ultrasonic frequency = 35 kHz, and reaction time = 3 h).

Financial supports provided by the Research University (RU) Grant, Postgraduate Research Grant Scheme and a Fellowship from Universiti Sains Malaysia are gratefully acknowledged.

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