Optimization and Modeling of Photocatalytic Degradation of Azo Dye

Feb 22, 2012 - Department of Chemical Engineering, Tarbiat Modares University, Tehran, Iran ... access to clean potable water and that up to half of t...
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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 Mohammad Vaez, Abdolsamad Zarringhalam Moghaddam,* and Somayeh Alijani Department of Chemical Engineering, Tarbiat Modares University, Tehran, Iran S Supporting Information *

ABSTRACT: The experimental design methodology was used to model and optimize the operational parameters of the photocatalytic degradation of Acid Red 73 using immobilized TiO2 nanoparticles. Four experimental parameters were chosen as independent variables: pH, initial dye concentration, H2O2 concentration, and anion concentration. A multivariate experimental design was used to establish a quadratic model as a functional relationship between the efficiency of Acid Red 73 degradation (response) and four independent variables. The degradation efficiency was significantly affected by the initial dye concentration and the pH. The optimal values of the parameters were found to be a pH of 3, an initial dye concentration of 25 mg/L, an H2O2 concentration of 0.5 mg/L, and an anion concentration of 0.69 mg/L. The degradation efficiency approached 92.24% under optimal conditions. Regression analysis with an R2 value of 0.9785 indicated a satisfactory correlation between the experimental data and predicted values.

1. INTRODUCTION It is estimated that more than 1 billion people have little or no access to clean potable water and that up to half of the people worldwide have a disease associated with poor drinking water and inadequate sanitation. These figures are expected to increase in the near future due to the overwhelming discharge of contaminants into the natural water cycle.1 To slow the growth of the clean water shortage, the development of low-cost and effective technologies for wastewater treatment is needed. Most conventional water treatment methods, such as adsorption, coagulation, ultrafiltration, and reverse osmosis, merely concentrate pollutants and convert them to other phases.2−5 Other methods, such as sedimentation, filtration, chemical treatments, and membrane technologies, have high operating costs and could release toxic secondary pollutants into the ecosystem.5 Heterogeneous photocatalysis utilizing titanium dioxide, one of the advanced oxidation processes (AOPs), has recently attracted tremendous attention due to its potential to mineralize a wide range of organic pollutants at ambient temperature and pressure into CO2 and water6−8 under UV light irradiation. Recently, the photocatalytic activity of TiO2 in presence of UV-LED light irradiation has been also reported.9,10 Various investigations have been reported for the enhancement of the photocatalytic activity of the pure TiO2.11 Studies of photocatalytic processes are widely carried out in slurry systems operating with titanium dioxide suspensions.9,12−14 The main problem in these systems is the separation and recycling of TiO2 nanoparticles after treatment, which can be a timeconsuming and expensive process. Supported TiO2 nanoparticles have been developed to solve this problem.10,15−18 This kind of photocatalytic system is highly advantageous for water treatment on a large scale. However, to date, little progress has been made in the development of a photocatalytic technology for water treatment in large applications.19 The presence of a number of system factors that require rapid © 2012 American Chemical Society

coverage testing is one of the technical barriers to scaling up photocatalytic technology.19 Previous studies have typically used the traditional one-factor-at-a-time (OFAT) approach to optimize photocatalytic processes.15,20−22 When process factors are independent (which is rare), the most common approach is OFAT.23 However, this univariate approach does not show the interactions between the operational factors of the process.19,24 Moreover, the OFAT approach is time-consuming and expensive due to reagent costs.23 There is a current trend toward replacing this inefficient practice with effective chemometric methods, such as response surface methodology (RSM), based on statistical designs of experiments (DOEs).23 This experimental strategy for seeking the optimum conditions is an efficient technique for use with a multivariable system.25,26 RSM has been successfully applied to various processes to achieve optimization using experimental designs, including TiO2-coated/UV oxidation.12,27,28 However, there have been no reports of research on the optimization of influencing factors and their interactions using experimental design methodology during the degradation of Acid Red 73 with immobilized TiO2. In previous work, we reported the decolorization and degradation of acid dye with immobilized TiO2 nanoparticles on sackcloth fiber.5 Sackcloth fiber was used as an effective and environmentally friendly support in the decolorization and degradation of colored wastewater. In the present work, supported TiO2 particles on sackcloth fiber are applied to the photocatalytic treatment of Acid Red 73. The main aim of this study is the optimization of the efficiency of the degradation of Acid Red 73 using immobilized Received: Revised: Accepted: Published: 4199

December 1, 2011 February 13, 2012 February 22, 2012 February 22, 2012 dx.doi.org/10.1021/ie202809w | Ind. Eng. Chem. Res. 2012, 51, 4199−4207

Industrial & Engineering Chemistry Research

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TiO2 particles on sackcloth fiber. Several previous investigations have reported that the initial dye concentration, pH, H2O2 concentration, and the dissolved anion concentration have significant impacts on the disappearance of dye in the photocatalytic degradation process.12,23,27,29−39 The inorganic ions dissolved in dye-containing industrial wastewater may compete for the active sites on the TiO2 surface or deactivate the photocatalyst, causing a decrease in the degradation efficiency of the target dyes.40 NaCl salt was selected to investigate the effect of dissolved inorganic anion on the degradation efficiency. Moreover, the previous investigation reported that pH value, initial dye concentration, oxidant H2O2 concentration, and the concentration of dissolved inorganic anions are important parameters influencing the photocatalytic degradation of acid dye.5 Therefore, these parameters were considered as independent factors and RSM was used to study their effects on and to optimize the efficiency of the degradation of Acid Red 73.

Figure 2. SEM images of bare sackcloth fiber (a) and sackcloth with immobilized TiO2 (b).

fiber system retained the structure of bare sackcloth fibers with rod-like particles. This fibrous structure is an aid to the accessibility of photons to the TiO241 and can improve UV light penetration into the photocatalyst to an appreciable depth, as demonstrated in the previous work.5 Therefore, in this study, sackcloth fiber was applied as a support in the decolorization and degradation of Acid Red 73. The experiments were carried out in batch mode in an immersion rectangular reactor made of Pyrex glass. Four UV-A lamps (9W, Philips, 31250−25000 cm−1) were used as the irradiation source. The scheme of the photocatalytic reactor is shown in Figure 3. A UV radiometer (UVA 365 Lutron) was used to measure the UV irradiation intensity of lamps. The measured intensity was approximately 1.8 mW/cm2. Two air pumps with 1 L/min flow rates were used for the mixing and aeration of the dye solution. The heating effect of the lamps was eliminated by a fan placed at the box. The total volume of reactor was 1 L. To evaluate the effect of adsorption on acid dye decolorization, adsorption (dark) experiments were carried out for Acid Red 73 under gentle air agitation in a photocatalytic reactor with immobilized TiO2 nanoparticles (dye 56.25 mg/L, H2O2 0.55 mg/L, pH 6.5). The concentration of acid dye was approximately 90% of its initial concentration after 70 min (Figure 4), which indicates that the effect of adsorption on the dye concentration was insignificant. Photocatalytic degradation processes were performed at 298 K with 700 mL of solution and a known initial concentration of Acid Red 73. The pH of the solution was adjusted either with 0.1 M HNO3 or with 0.1 M NaOH. The results of blank studies indicated that the initial pH of dye solutions has a negligible effect on the chemistry of Acid Red 73. The maximum absorbance wavelength (λmax (nm)) of Acid Red 73 at three pH values, one acidic, one basic, and one natural, are shown in Table 1. The changes in the maximum absorbance wavelength of Acid Red 73 at different pH values were negligible, so the effect of pH on the absorbance of the acid dye can be ignored. Samples were taken from the sample point at certain time intervals and were filtered (pore size < 0.22 μm) to remove the unattached TiO2 particles present in the solution. The samples were analyzed for residual dye concentration using an Optizen 3220UV Double Beam spectrophotometer. The maximum wavelength (λmax) of Acid Red 73 is 545 nm. All of the experiments were carried out at fixed radiation time of 60 min. Before implementing an experimental design, it is necessary to study the extents of adsorption and photocatalytic degradation in Acid Red 73 decolorization. Figure 5 shows the photocatalytic degradation of Acid Red 73 under four different

2. MATERIAL AND METHODS 2.1. Reagents. Acid Red 73 (C22H14N4Na2O7S2, Mw = 556.48 g/mol) was obtained from the Ciba Company and applied as a model acid dye. The chemical structure of the acid dye is shown in Figure 1. Titanium dioxide nanoparticles

Figure 1. Chemical structure of Acid Red 73.

(AEROXIDE TiO2 P25) with greater than 97% purity (average primary particle size: 21 nm; anatase-to-rutile: 80:20) were purchased from Evonik. All other chemicals were of analytical grade and supplied by Merck 2.2. Photoreactor and Experimental Procedure. Titanium dioxide-coated sackcloth fiber was used as the photocatalyst for the degradation of Acid Red 73. TiO2 nanoparticles were immobilized on the sackcloth fiber by a procedure described in detail elsewhere.5 The coating solution contained 180 mL of ethanol, 5 g of TiO2, and an appropriate amount of diluted nitric acid, which is essential for better dispersion of titania powder. The solution was sonicated for 10 min in an ultrasonic bath. Pieces of sackcloth fiber (250 mm × 250 mm × 3 mm) were used as catalyst supports. The main component of sackcloth is cellulose. The surface area of the support before and after coating were 1.1123 and 4.909 m2/g, respectively, measured by nitrogen adsorption−desorption at 77 K using the Brunauer− Emmett−Teller (BET) method with a Micromeritics 2000 instrument (ASAP 2000, Micromeritics, USA). These values indicate that good photocatalytic efficiency can be expected from the coated samples, as demonstrated in previous experiments.5 The XRD results from the previous work showed that minor changes in the structure of the titania nanoparticles occurred due to the immobilization on the sackcloth fiber.5 SEM images of the sackcloth fiber before and after immobilization of TiO2 (Figures 2a and b) confirmed that the TiO2-coated 4200

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Figure 3. Schematic diagram of the photocatalytic reactor.

Figure 5. Effect of photocatalyst on the degradation rate of Acid Red 73 for (a) bare sackcloth fiber with UV irradiation, (b) immobilized TiO2 without UV irradiation, (c) pure TiO2 slurry with UV irradiation, and (d) immobilized TiO2 with UV irradiation, with the initial conditions being Acid Red 73 concentration 56.25 mg/L, pH 6.5, and concentration H2O2 0.55 mg/L.

Figure 4. Adsorption of dye on the TiO2-coated sackcloth without UV irradiation.

Table 1. Maximum Absorbance Wavelengths (λmax (nm)) of Acid Red 73 at Different pH Values pH

λmax (nm)

acidic natural basic

Acid Red 73 458 457 456

in comparison with the fresh sackcloth fiber (Figure 5, curve b). The affinity of the sackcloth fiber for acid dye after the deposition of TiO2 nanoparticles and the higher surface area of the coated sample could be responsible for the higher catalytic activity of the TiO2-coated fiber. The differences between Acid Red 73 removal with (Figure 5, curves c and d) and without (Figure 5, curve b) radiation indicate that the disappearance of Acid Red 73 is due to photocatalytic degradation instead of adsorption alone. On the basis of data shown in Figure 5 (curves c and d), the percentage degradation of acid dye with the immobilized TiO2 particles was higher than with TiO2 powder. This can be attributed to the improvement of UV light penetration in the immobilized system. Two explanations can be offered for the increased light penetration: (1) a thinner fluid layer on the surface of the immobilized TiO2 and/or (2)

conditions (dye 56.25 mg/L, H2O2 0.55 mg/L, pH 6.5): bare sackcloth fiber with UV irradiation (a), immobilized TiO2 without UV irradiation (b), pure TiO2 slurry with UV irradiation (c), and immobilized TiO2 with UV irradiation (d). After 60 min, the concentration of Acid Red 73 on bare sackcloth fiber did not decrease further with prolonged UV irradiation. This indicates that bare sackcloth fiber does not exhibit photocatalytic activity (Figure 5, curve a) and that the degradation of Acid Red 73 in the 60 min period can be ascribed to UV−H2O2 degradation. TiO2-coated fiber without UV irradiation exhibited a higher saturated adsorption capacity 4201

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the reduction of the light penetration depth due to the scattering of UV light by TiO2 particles in the slurry system.42,43 During the photocatalytic degradation of Acid Red 73, various organic intermediates were produced. Consequently, the degradation of both the parent dye and its intermediates should be evaluated as an overall process. The ability of TiO2coated sackcloth fiber to decompose all of the organic compounds in the dye solution was investigated through chemical oxygen demand (COD) analysis. Changes in COD for the Acid Red 73 solution (56.25 mg/L, pH 6.5) using H2O2 (0.55 mg/L) in the immobilized TiO2 photoreactor are shown in Figure 6.

Figure 7. Cyclic photocatalytic performance of titania nanoparticles immobilized on sackcloth for Acid Red 73 degradation (dye 56.25 mg/L, H2O2 0.55 mg/L, pH 6.5).

Acid Red 73 as the response (dependent variable). The ranges and the levels of the independent variables are given in Table 2. Table 2. Experimental Range and Levels of the Independent Test Variables ranges and levels

Figure 6. COD removal during photocatalytic degradation of Acid Red 73 (dye 56.25 mg/L, H2O2 0.55 mg/L, pH 6.5, COD0 is the initial COD, and COD is the COD at time t).

The 96% reduction in the COD of the sample indicates the effective performance of immobilized TiO2 nanoparticles on sackcloth fiber in the complete decomposition of dye. The feasibility of repeated use of the TiO2-coated sackcloth fiber (dye 56.25 mg/L, H2O2 0.55 mg/L, pH 6.5) was investigated in four repeated experiments. The photocatalytic activity remained at approximately 80% of the initial activity after four cycles (Figure 7). The recyclability of photocatalytic surface after four cycles can indicate the stability of TiO2 coating on support. It was also observed that the color change of the support after repeated photocatalytic cycles was negligible. To study the stability TiO2-coated sample, TiO2 leaching after each photocatalytic experiment was determined through inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis using an Optima 3000 instrument (Perkin-Elmer). The dissolved Ti concentration in samples collected from the reactor after each experiment was always below 0.27 g/L, which is approximately 1% of the initial TiO2 loading. This indicates sufficient adhesion of the TiO2 particles to the sackcloth fiber surface. 2.3. Experimental Design and Optimization by Response Surface Methodology. RSM is an efficient technique for the optimization of a multivariable system.25,27 In this study, the optimal decolorization efficiency of Acid Red 73 was obtained by RSM using Design Expert 7.0. The four independent parameters chosen in this study were the initial dye concentration, pH, the anion concentration, and the oxidant H2O2 concentration, with the degradation percentage of

variables

−2

−1

0

+1

+2

pH (x1) initial dye concentration (x2) (mg/L) H2O2 concentration (x3) (mg/L) dissolved inorganic anion concentration (x4) (mg/L)

3 25 0.1 0.00

4.75 56.25 0.33 0.38

6.5 87.5 0.55 0.75

8.25 118.75 0.78 1.13

10 150 1.00 1.5

Central composite design (CCD), which is the most frequently used form of RSM, was employed to evaluate the influence of the four independent variables in 30 sets of experiments. The fundamental assumptions of RSM and more detailed information have been discussed elsewhere.23 A second-order (quadratic) polynomial equation was used to fit the experimental results of CCD as follows: Y (%) = b0 + b1x1 + b2x2 + b3x3 + b4x4 + b12x1x2 + b13x1x3 + b14x1x4 + b23x2x3 + b24x2x4 + b34x3x4 + b11x12 + b22x2 2 + b33x32 + b44x4 2

(1)

where Y represents the response variable (decolorization efficiency), bi, bii, and bij are the regression coefficients for linear and quadratic effects and the coefficients of the interaction parameters, respectively, and xi are the independent variables studied.

3. RESULTS AND DISCUSSION 3.1. Model Fitting and Statistical Analysis. The experimental matrix design and the responses based on experiments proposed by CCD for the degradation of Acid 4202

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Red 73 are given in Table S1 (see the Supporting Information). On the basis of the experimental design presented in Table S1, a second-order polynomial equation in terms of actual factors was found that demonstrates the empirical relationships between the independent variables and the response: % degradation = 135.99267 − 17.3387pH − 0.78301dye + 50.85597H2O2 + 11.17249anion − 0.018286pH × dye + 7.47198 × 10−15pH × H2O2 + 0.95238pH × anion − 0.1066dye × H2O2 + 0.01066dye × anion − 7.40741H2O2 × anion + 1.25170pH2 + 2.51733 × 10−3dye2 − 35.39095H2O2 2 − 11.85185anion 2 Figure 8. Experimental values plotted against the predicted values derived from the model.

To evaluate the adequacy of the model, analysis of variance (ANOVA) was applied. From the ANOVA of the empirical second-order polynomial model (Table 3), the F value for the model is 48.85, indicating

According to the monomial coefficient values of the regression model, p(x1) = 0.0001 (pH), p(x2) < 0.0001 (initial dye concentration), p(x3) = 0.3348 (H2O2 concentration), and p(x4) = 0.0647 (anion concentration), the order of importance among the factors is initial dye concentration (x2) > pH (x1) > anion concentration (x4) > H2O2 concentration (x3). This result can also be attributed to the experimental range of independent variables chosen in this study. 3.2. Response Surface Analysis. Three-dimensional surfaces can be presented as graphical representations of the regression equation applied to determine the optimum values of variables and are widely used to achieve better understandings of the interactions between variables within the range considered.30,31 The results of the interactions between the four independent variables and the response are shown in Figure 9. Figure 9a shows that the acid dye degradation efficiency decreases with increasing dye concentration and pH value. More information on the interactions between pH value and dye concentration can be obtained from the plots. There is a decrease in degradation efficiency with an increase in pH. This effect is more obvious at higher levels of initial dye concentration, as observed in the contour plots (not shown here). This can be attributed to the surface charge properties of the photocatalyst. The TiO2 surface is positively charged in acidic media. As the pH of the solution increases, the number of negatively charged sites increases. This reduces the adsorption of dye anions due to electrostatic repulsion. This effect is enhanced with an increase in the dye concentration due to com`petition for adsorption between the parental dye and intermediate molecules. Figure 9b shows the effects of H2O2 concentration and pH on Acid Red 73 degradation. Degradation increases smoothly with an increase in H 2O 2 concentration from 0.3 to 0.53 mg/L and decreases gradually when the H2O2 concentration increases beyond 0.53 mg/L whether pH has a low or high value. The positive effect of H 2O 2 concentration may be attributed to the inhibition of electron−hole recombination at the photocatalyst surface by accepting the electron from the conduction band. 23 However, at higher concentrations, H2O 2 can also act as a scavenger of valence

Table 3. ANOVA Results for the Response Surface Quadratic Model source model residual lack of fit pure error total

sum of squares

degree of freedom

mean squares

7344.78 161.08 137.75 23.33

14 15 19 5

524.63 10.74 13.78 4.67

7505.87

29

F value

p value

48.85