Photodegradation of phenol catalyzed by TiO2 coated on acrylic sheets: Kinetics and factorial design analysis

Desalination 274 (2011) 192–199

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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l

Photodegradation of phenol catalyzed by TiO2 coated on acrylic sheets: Kinetics and factorial design analysis Tanisata Luenloi a,c, Benjapon Chalermsinsuwan a,c, Thammanoon Sreethawong b,c, Napida Hinchiranan a,c,⁎ a b c

Department of Chemical Technology, Faculty of Science, Chulalongkorn University, 254 PhayaThai Road, Bangkok, 10330, Thailand The Petroleum and Petrochemical College, Chulalongkorn University, 254 PhayaThai Road, Bangkok, 10330, Thailand Center for Petroleum, Petrochemicals, and Advanced Materials, Chulalongkorn University, 254 PhayaThai Road, Bangkok, 10330, Thailand

a r t i c l e

i n f o

Article history: Received 14 December 2010 Received in revised form 3 February 2011 Accepted 6 February 2011 Available online 25 February 2011 Keywords: Titanium dioxide Photocatalyst Phenol Acrylic Degussa P-25

a b s t r a c t The separation of nanosized titanium dioxide (TiO2) is generally required for conventional photodegradation carried out in a stirred reactor. To eliminate this step for reduction of the operating cost, this research aimed to prepare TiO2 coated on acrylic sheets via dip-coating for photodegradation of phenol. The three cycles of TiO2 coating by using acetylacetone (ACA) to TiO2 (ACA/TiO2) molar ratio of 3 gave a smooth and uniform thin TiO2 film on the surface of acrylic sheets. The effects of the number of TiO2-coated acrylic sheets, initial phenol concentration, hydrogen peroxide (H2O2) concentration and UV light power on the apparent reaction rate constant (kapp) of phenol photodegradation were also studied. Under the optimum operation, the highest kapp value of 7.2 × 10−3 min−1 was achieved. The relationship of these parameters on the phenol removal efficiency was also statistically evaluated using a two-level factorial design. The significance of reaction parameters was shown as followed: UV light power N the number of TiO2-coated acrylic sheets N initial phenol concentration N H2O2 concentration. The reduced degradation efficiency of the TiO2-coated acrylic sheets, compared to that of free suspensions, could be overcome by increasing the number of photodegradation stages, with 97.1% efficiency being attained with three photodegradation stages. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Phenol and phenolic compounds synthesized from the Hock processes are widely applied in industry and daily life, such as disinfectants, veterinary medicine, extracting solvents in refinery, and lubricant production, including phenolic resins (phenol-formaldehyde resins or Bakelite), which are used in plywood adhesive and construction. Moreover, the reaction of phenol with acetone produces bisphenol A, a raw material for the production of epoxy resins and polycarbonate resins [1]. However, phenol and phenolic compounds have been reported to have a high stability and high environmental toxicity with additionally carcinogenic properties and so can damage human health [1,2]. Thus, the Environmental Protection Agency (EPA) has limited the phenol concentration in standard surface waters to less than 1 part per billion (ppb) [1]. There are several conventional methods to eliminate phenolic compounds in wastewater, such as adsorption using activated carbon, chemical oxidation and biological treatment. Unfortunately, the main drawback of these techniques

⁎ Corresponding author at: Department of Chemical Technology, Faculty of Science, Chulalongkorn University, 254 PhayaThai Road, Bangkok, 10330, Thailand. Tel.: +66 2218 7518; fax: +66 2255 5831. E-mail address: [email protected] (N. Hinchiranan). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.02.011

relates to the disposal of the spent contaminated activated sludges, the control of the appropriate reaction conditions and the slow reaction rates, respectively [3]. Moreover, these conventional methods still require additional steps after the wastewater treatment, resulting in a higher operating cost [4]. Recently, photocatalytic oxidation catalyzed by titanium dioxide (TiO2) has been proposed as an effective and economical method to convert organic pollutants in contaminated water and air to carbon dioxide, water and mineral acids, which are less toxic substances [5]. Commercial TiO2 (Degussa P-25) is generally applied as the active photocatalyst for photooxidation due to the rapidity of its electron transfer to molecular oxygen for decomposition of pollutants [6]. Although TiO2 has many advantages, including those of nontoxicity, high chemical stability, powerful oxidation strength and relative inexpensiveness, nanosized TiO2 powder should be avoided in the slurry system due to the requirement of solid-liquid separation and the inefficient illumination of the particles, resulting in an actually higher operating cost and lower reactivity, respectively [5,6]. To overcome these drawbacks, many attempts to develop supported TiO2 via immobilization techniques onto solid materials, such as dip-coating, coating, sputtering, sol–gel and electrophoretic deposition, have been reported [7]. The selection of a suitable substrate as a support of TiO2 for the photocatalytic reaction is important. Any such selected support requires a high resistance to the oxidizing environment, excellent

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ultraviolent (UV) transparency and good adhesion between the support material surface and the photocatalyst. Typically, borosilicate glass or quartz is normally used as a support for TiO2 [7,8]. However, this material is fragile and has a high density. These characteristics limit its application in some reactors, such as the stirred tank and fluidized bed reactor. Thus, polymeric substrates, which are lightweight and easy to form in desired shapes, have been extensively used as a support for TiO2 photocatalysts. The transparent polymers, such as poly(ethylene terephthalate) (PET) [8], cellulose acetate [8], and poly(methyl methacrylate) (PMMA) [9–11], have been reported to be effective supports for photocatalysis. Although polymeric materials are normally degraded under sunlight or UV irradiation during photochemical reactions, inorganic-organic composite materials containing nanosized TiO2 powder provide improved mechanical properties, including thermal and UV stability, since TiO2 acts as strong UV light absorber instead of the polymer matrix [12]. For engineering instruments, baffles are an important part in a mechanically stirred vessel to increase the mixing efficiency, to stabilize the power drawn by the impeller, and to decrease the vortex phenomenon for liquid mixing [13,14]. Therefore, the aim of this work was to prepare the baffles made from PMMA or acrylic sheets coated with nanosized TiO2 powder (Degussa P-25) via dip-coating for using in the stirred reactor. The performance and morphology of the TiO2-coated acrylic sheets on the kinetics of the photocatalytic degradation of phenol were investigated as functions of the TiO2 coating cycle (n), molar ratio of acetylacetone (ACA) to TiO2 (ACA/TiO2), number of TiO2-coated acrylic sheets, initial phenol concentration, UV light power and hydrogen peroxide (H2O2) concentration. A two-level factorial design experiment was applied to statistically evaluate the significance of each studied parameter, allowing for any possible interaction between the parameters. An interaction is the deviation of the parameter to deliver the similar effect on the response at different levels of other parameters. The important parameters were then related and constructed as a regression model via the least square method. Moreover, the reusability of TiO2coated acrylic sheets and the number of photocatalytic degradation stage to give the highest performance of phenol removal at a given reaction time were investigated and are reported. 2. Experimental 2.1. Materials Phenol (99.5% purity) was purchased from Fisher Scientific (The New Zealand). Commercial nanosized TiO2 powder (Degussa P-25) was provided from J.J. Degussa Hüls Co, Ltd. (Singapore), and contained 74 and 26 wt.% of anatase and rutile forms; respectively, with a BET specific surface area of 65 m2/g. ACA, used as binder between the TiO2 and the acrylic sheet, was obtained from S D FineChem Limited (India). Isopropanol (PrOH) (Lab Supplies, The New Zealand) and nitric acid (HNO3) (QRëc Qatar, The New Zealand) were analytical reagent grade quality. The 99.7% purity oxygen gas was obtained from Praxair (Thailand). 2.2. Preparation and characterization of TiO2-coated acrylic sheets TiO2 slurry was prepared by dispersing of the TiO2 powder (1 g) in 10 mL of water/PrOH solution (3/7 (v/v)) under vigorous stirring at room temperature. Then, the desired amount of ACA was added to the TiO2 slurry under agitation for 24 h. The resulting TiO2 slurry was then deposited onto the acrylic sheets (20 × 130 × 2 mm) via dipcoating at room temperature for 10 min. After withdrawing the acrylic sheets from TiO2 slurry, the TiO2-coated acrylic sheets were dried at 80 °C for 1 h. Prior to deposition of TiO2 film, the acrylic substrates were treated in 2 M HNO3 followed by rinsing with ethanol and then water and then dried at 80 °C for 2 h. The number of coating cycles, which is related to the amount of TiO2 content on

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the acrylic sheets, was also considered as a potentially important parameter for the photocatalytic efficiency of phenol degradation, and so evaluated. The surface morphology of the thin TiO2 film coated on acrylic sheets was investigated by scanning electron microscopy (SEM) (JEOL model JSM-6480 LV). The TiO2-coated acrylic sheets were cut and mounted on a SEM stub using double-sided tape. The samples were then sputter-coated with gold and examined using the SEM operated at 15 kV. 2.3. Photoreactor design and photocatalytic performance testing All experiments on the photocatalytic degradation of phenol were conducted in a batch photoreactor as shown in Fig. 1a. The one-liter cylindrical photoreactor (with a height of 20 cm and an inner diameter of 10 cm) was made from borosilicate glass and placed on a hot plate magnetic stirrer in the opaque box (50 × 60 × 70 cm3). This photoreactor was equipped with a cylindrical stainless steel rack to hold the TiO2-coated acrylic sheets, which acted as the baffles of the reactor (Fig. 1b). The reaction temperature was maintained at 30 °C using a glass water-cooling coil. The outside of photoreactor was surrounded by 15 low pressure mercury lamps (9 W/lamp, UV-A, λmax = 400 nm, manufactured by Philips, Poland). The 500 mL of solution containing various phenol concentrations (10–100 ppm) was transferred into the photoreactor in the presence of a 200 mL/min flow rate of oxygen and left under agitation for desired reaction time (0–5 h). A 10 mL sample of the reaction solution was collected every 1 h during the photo-oxidation to monitor the reduction in the phenol concentration by using a UV–visible spectrophotometer (Jasco, model: V-530, USA) performed under a scanning wavelength in the range of 190–700 nm. The phenol concentrations before and after photodegradation were calculated from the calibration curve equation (Eq. 1), firstly obtained from the plot of the height of the phenol absorbance peak at 270 nm (H) versus phenol concentration. The degree of phenol removal as a function of time is then given by Eq. (2): Phenol concentration (C, ppm) = 0.0154 H

(1)

  C × 100 %Phenol removal = 1− C0

ð2Þ

where C0 and C are the phenol concentrations at the initial and any subsequent reaction time. 3. Results and discussion 3.1. Surface morphology of TiO2-coated acrylic sheets The effects of the ACA content, represented as the molar ratio of ACA/TiO2 and the number of coating cycles (n), on the surface morphology of the TiO2-coated acrylic sheets were observed by SEM (Fig. 2). With three TiO2 coating cycles, increasing the ACA/TiO2 molar ratio from 1.0 (Fig. 2a) to 3.0 (Fig. 2b) revealed a higher surface homogeneity of the TiO2-covered acrylic sheets with less cracking flaws. Considering the TiO2 content on the acrylic sheet, increasing the ACA content also significantly (3.37-fold) increased the amount of TiO2 (Table 1), and accordingly increased the phenol removal efficiency (1.5-fold). This suggested that ACA acted as an adhesion improver between the TiO2 particles and the surface of the polymeric substrates. Similar results have been reported for the preparation of colloidal TiO2 nanoparticles via a modified sol–gel process [15]. The ACA probably modified some proportion of the TiO2 surface from hydrophilic to hydrophobic, resulting in the enhancement of the interfacial adherence between TiO2 and polymer surface.

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Fig. 1. (a) Schematic drawing of the photoreactor for phenol degradation and (b) TiO2-coated acrylic sheets in a stainless steel rack.

When the ACA/TiO2 molar ratio was kept constant at 3.0, a decrease in the number of TiO2 coating cycles from 3 (Fig. 2b) to 1 (Fig. 2c) decreased the amount of TiO2 (1.86-fold) (Table 1), and resulted in a higher surface homogeneity of the thin TiO2 film on the surface of the acrylic sheets, but the phenol removal efficiency was only slightly affected (1.05-fold reduction). Although increasing the number of TiO2 coating cycles to 10 resulted in a 1.59-fold higher amount of TiO2 particles deposited on the acrylic surface compared to that with three coating cycles (Table 1 and Fig. 2d), a 1.15-fold reduction in the phenol removal efficiency was observed along with many surface cracking flaws and pores on the TiO2-coated acrylic sheet. The latter is possibly

due to the effect of volatilization of the solvent and ACA during the drying step of the coated acrylic sheet [15]. In addition, the hydrophobic property of the acrylic sheet might lead to the cracking flaws on the covering surface of TiO2 thin film as the number of TiO2-coating cycles was increased, as has been reported before for 12 coating cycles [11]. 3.2. Kinetics of phenol degradation catalyzed by TiO2-coated acrylic sheets In general, the kinetics of photocatalytic reactions of organic water impurities, including phenol and its derivatives, follows the Langmuir–

Fig. 2. SEM micrographs of thin TiO2 film coated on acrylic sheets at various molar ratios of ACA/TiO2 and number of coating (n): (a) ACA/TiO2 = 1, n = 3, (b) ACA/TiO2 = 3, n = 3; (c) ACA/TiO2 = 3, n = 1; and (d) ACA/TiO2 = 3, n = 10.

T. Luenloi et al. / Desalination 274 (2011) 192–199 Table 1 Effect of ACA content and the number of TiO2-coating cycle on photocatalytic degradation of phenol. ACA/TiO2 molar ratio

Number of TiO2 coating cycle (n)

TiO2 content on acrylic sheeta (wt.%)

%Phenol removal at 5 h

1.0 3.0 3.0 3.0

3 3 1 10

0.038 ± 0.003 0.128 ± 0.004 0.069 ± 0.006 0.203 ± 0.014

41.6 62.5 59.5 54.4

a

TiO2 content (wt.%) = [(wt of TiO2-coated acrylic sheet/wt of acrylic sheet) − 1] × 100.

Hinshelwood model, and the first-order reaction depends on the initial substrate concentration, as expressed in Eq. (3): r=−

dC kKad C = dt 1 + Kad C

ð3Þ

where r is the reaction rate, k is the rate constant, Kad is the adsorption equilibrium constant and C is the concentration of substrate remaining in the solution at time (t) [16–19]. When the effect of adsorption is not significant, and/or the substrate concentration is low, Eq. (3) can be simplified to pseudo-first order kinetics, as shown in Eq. (4): ln

  C0 = kKad t = kapp t C

ð4Þ

where kapp is the apparent reaction rate constant, which can be calculated from the slope of the ln plot, and C0 is the initial substrate concentration [18,19]. The effects of various reaction parameters on the photocatalytic performance of TiO2-coated acrylic sheets for phenol degradation were expressed in terms of kapp. The TiO2-coated acrylic sheets used for all experiments in this section were prepared by using three TiO2 coating cycles with an ACA/TiO2 molar ratio of 3.

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proportionally increased the kapp, presumably from the enhanced TiO2 content and so an increased number of active sites and the amount of OH• and O2•− radicals in the system [20,21]. Thus, eight plates of TiO2coated acrylic sheets were used for all further experiments. With respect to the stability of the TiO2 coated on the acrylic sheet, evaluation of the sheet weight loss during photocatalytic degradation of phenol for 5 h revealed that only 8.64 wt.% of TiO2 was lost, which implies that the TiO2 coated on the acrylic sheets had high stability. 3.2.2. Effect of the initial phenol concentration on the kapp value The effect of the initial phenol concentration on the kapp value of photocatalytic degradation of phenol was investigated over the range of 10 to 100 ppm as shown in Fig. 4. The kapp significantly decreased as the initial phenol concentration was increased, with the kapp falling 2.8-fold from 4.5 × 10−3 to 1.6 × 10−3 min− 1 as the phenol concentration was increased from 10 to 100 ppm. This can be explained in terms of either saturation of the limited number of accessible active sites on the photocatalyst surface, or poisoning (deactivation) of the active sites of the catalyst. Either way, this would define it as the limiting parameter for controlling the catalytic performance of photocatalysts in the presence of a high substrate concentration. Several reports have commented that high organic substrate loadings induce the formation of intermediates that could be adsorbed onto the catalyst surface and deactivate the active sites [4,21]. Moreover, the significant absorption of light by the substrate at high concentrations might decrease the level of UV light reaching the photocatalyst and thus its efficiency by reduction of the amount of OH• and O•− 2 free radical production [22].

3.2.1. Effect of the number of TiO2-coated acrylic sheets on the kapp value The influence of the number of TiO2-coated acrylic sheets on the kapp of photocatalytic degradation of phenol was shown in Fig. 3. Under UV light power at 135 W without the use of TiO2-coated acrylic sheets, the kapp of the photodegradation of phenol was only 0.2 × 10−3 min− 1. Thus, it is necessary to apply the photocatalyst into the system to enhance the rate of reaction. In this section, the number of TiO2-coated acrylic sheets directly related to the TiO2 content in the photocatalytic system was varied in the range of 2–8 plates. It was found that the increase in the number of TiO2-coated acrylic sheets

3.2.3. Effect of the UV light power on the kapp value Fig. 5 shows the effect of UV light power on the kapp value of photocatalytic degradation of phenol. The degree of UV light power could be adjusted by variation of the number of UV lamps (9 W/lamp) to obtain a range of UV light power from 45–135 W. As such the photodegradation of phenol with TiO2-coated acrylic sheets was clearly related, and potentially proportional to the UV light power with pseudo-first order kinetics. The kapp for the photocatalytic degradation of phenol was increased 3.3-fold (from 0.9 × 10−3 to 3 × 10−3 min− 1) as the UV light power was increased from 45 to 135 W. The UV illumination level applied here was in the weak UV light intensity range [20,23]. However, the use of high UV light intensity is not applied for photodegradation of organic substances, since previous work has reported that high UV light intensity does not improve the photocatalytic degradation rate due to the recombination effect of electron (e−) and hole (h+) of TiO2 [20,23].

Fig. 3. Effect of the number of TiO2-coated acrylic sheets on the apparent reaction rate constant of photocatalytic degradation of phenol (initial phenol concentration = 50 ppm and UV light power = 135 W).

Fig. 4. Effect of the initial phenol concentration on the apparent reaction rate constant of photocatalytic degradation of phenol (the number of TiO2-coated acrylic sheets = 8 plates and UV light power = 135 W).

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the reaction system was higher than the optimum point (0.62 M). This is due to the decreased numbers of the valence bond holes (Eq. (8)) [27] and the amount of hydroxyl radicals produced via the reaction of H2O2 to form HO•2 (Eq. (9)) or dimerization reaction (Eq. (10)) [28]. þ

þ

2hVB + H2 O2 →O2 + 2H

ð8Þ

•

•

ð9Þ

OH + OH →H2 O2

ð10Þ

H2 O2 + OH →HO2 + H2 O •

•

3.3. Factorial design experiment and data analysis

Fig. 5. Effect of the UV light power on the apparent reaction rate constant of photocatalytic degradation of phenol (the number of TiO2-coated acrylic sheets = 8 plates and initial phenol concentration = 50 ppm).

3.2.4. Effect of H2O2 concentration on the kapp value The H2O2 concentration is a key factor that can significantly influence the photocatalytic degradation of phenol because the H2O2 concentration is directly related to the number of OH• radicals generated in the photo-assisted reaction [21]. When the H2O2 concentration was increased within the range from 0–6.26 M, the kapp increased to a maximum value (2.4-fold increase) at 0.62 M H2O2, and then steadily decreased thereafter with increasing H2O2 concentrations from 0.62 M to 6.26 M (Fig. 6). This increase in the kapp as the H2O2 concentration increased to 0.62 M was possibly due to the increases in the amount of hydroxyl radical (OH•), via H2O2 photodissociation (Eq. (5)) [24,25] and the reaction between H2O2 and photogenerated electrons on TiO2 surface (e− cb) (Eq. (6)) [26] or peroxyl ion (O•2−) (Eq. (7)), which is produced from the reaction of dissolved O2 and e− cb [27]. hν

•

H2 O2 → 2 OH

ð5Þ −

H2 O2 + TiO2 ðe cb Þ→OH •−

−

H2 O2 + O2 →OH

•

+ OH •

+ OH + O2

ð6Þ ð7Þ

Similar to the powder TiO2 dispersion system, the photodegradation of phenol catalyzed by TiO2-coated acrylic sheets also exhibited a dramatic reduction in the kapp value when the H2O2 concentration in

Factorial designs are generally used for experimental systems involving several factors in order to study the main parameter effects and their interaction on the system response, and to evaluate the significance of the studied parameters [29]. In this work, two levels for each of the four related factors, i.e. the number of TiO2-coated acrylic sheets, initial phenol concentration, UV light power and H2O2 concentration, which affected the %phenol removal, were arbitrarily assigned as “low (− 1)” and “high (+ 1)”, as shown in Table 2. The response of design matrix for 24 factorial design experiments with a constant reaction time of 5 h is presented in Table 3. In the normal probability plot (Fig. 7), the effects that deviated from the normal probability straight line are considered to be the significant factors for this system. Thus, all the four evaluated variables above, including the interactions between the number of TiO2-coated acrylic sheets × UV light power and the initial phenol concentration × UV light power, had a considerable influence on the %phenol removal. The analysis of variance (ANOVA) of the %phenol removal is summarized in Table 4. The “Percent contribution”, calculated from the sum of square of each parameter to the total sum of square, validates the importance of each selected factors obtained from the normal probability plot (Fig. 7). The significance of the main reaction parameters was (highest to lowest): UV light power N the number of TiO2-coated acrylic sheetsN initial phenol concentration N H2O2 concentration. For the F-test statistic analysis of the data using the table of percentage points of the F distribution [29], the F-value for each selected factor (F0.05, 1, 9) was 5.12. This reveals that all the selected factors were significant. To express the relationship between the main and interaction factors and the response, the general regression model could be applied [29], as shown in Eq. (11): k

y = β0 + ∑ βj xj + ∑∑βij xi xj + ε j=1

ð11Þ

ibj

where β's are the regression coefficients, xi and xj are the variables representing factor i and j, respectively, and ε is a random error term. The xi and xj variables are defined based on a code scale ranging from −1 and +1, and xixj is the interaction between xi and xj [29]. The regression model employed to explain the experimental phenomena is shown in Eq. (12): yˆ = 32:03 + 13:14x1 −9:81x2 + 14:21x3 + 5:94x4

ð12Þ

+ 4:32x1 x3 −4:26x2 x3 Table 2 Actual factors and their levels used for two-level factorial design experiment.

Fig. 6. Effect of the H2O2 concentration on the apparent reaction rate constant of photocatalytic degradation of phenol (the number of TiO2-coated acrylic sheets = 8 plates, initial phenol concentration = 50 ppm, and UV light power = 135 W).

Parameter name

Parameter code

Low level (− 1)

High level (+ 1)

Number of TiO2-coated acrylic sheet (plates) Initial phenol concentration (ppm) UV light power (W) H2O2 concentration (M)

A B C D

2 50 45 0

8 100 135 0.63

Conditions: reaction temperature = 30 °C and reaction time = 5 h.

T. Luenloi et al. / Desalination 274 (2011) 192–199 Table 3 Design matrix and response of 24 factorial design experiment for photocatalytic degradation of phenol. Run

A

B

C

D

Response (%phenol removal at 5 h)

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

−1 +1 −1 +1 −1 +1 −1 +1 −1 +1 −1 +1 −1 +1 −1 +1

−1 −1 +1 +1 −1 −1 +1 +1 −1 −1 +1 +1 −1 −1 +1 +1

−1 −1 −1 −1 +1 +1 +1 +1 −1 −1 −1 −1 +1 +1 +1 +1

−1 −1 −1 −1 −1 −1 −1 −1 +1 +1 +1 +1 +1 +1 +1 +1

9.71 24.2 4.21 16.2 35.9 69.0 10.2 39.4 14.2 45.4 7.86 20.8 44.9 91.4 24.2 54.9

where x1, x2, x3 and x4 are the coded variables representing the number of TiO2-coated acrylic sheets, initial phenol concentration, UV light power and initial H2O2 concentration, respectively. To validate the adequacy of the regression model (Eq. (12)), the difference in the response value obtained from the experimental data and the model was evaluated and is expressed as plots of the normal probability versus residuals (Fig. 8a), and the residuals versus the predicted values (Fig. 8b). The reasonably close proximity of all the residual plots to the straight line (Fig. 8a) supports the normal independent distribution assumption of the residuals, whilst the plot of residuals versus the predicted values (Fig. 8b) exhibits a scattered or structureless pattern, suggesting that all the information are extracted and the residuals are unrelated to any other variables. Taken together, the results indicate that the four selected factors and two paired interactions (the number of TiO2-coated acrylic sheets × the UV light power and the initial phenol concentration × the UV light power) were significant factors determining the %phenol removal. In addition, the regression model described above appears to be satisfactory to explain the results.

197

Table 4 Analysis of variance for photocatalytic degradation of phenol. Source of variation

Sum of square

Percent contribution

Degree of freedom

Mean square

F-value

A B C D AC BC Error Total

2764.13 1538.60 3229.65 563.83 298.43 289.85 302.84 8987.32

30.75 17.12 35.93 6.27 3.32 3.22 3.36

1 1 1 1 1 1 9 15

2764.13 1538.60 3229.65 563.83 298.43 289.85 33.65

82.15 45.73 95.98 16.76 8.87 8.61

dation of phenol was investigated without any further treatment steps. The photocatalytic activity, in terms of the obtained %phenol removal, of the TiO2-coated acrylic sheets decreased steadily with each use by about 4.3% per cycle over the five cycles (Fig. 9). Thus, the five-cycle used TiO2-coated acrylic sheets still had a reasonably good catalytic reactivity for photodegradation. This is also similar to the photocatalytic degradation of X-3B using TiO2 sol [30]. Due to the limited mobility of the TiO2 when coated on the acrylic sheets, which results in a lower reactivity compared to TiO2 powder, increasing the number of photocatalytic degradation stages could significantly enhance the %phenol removal from 61.6% to 85.6% and 97.1% as the number of stages increased from 1 to 2 and to 3, respectively (Fig. 10). 4. Conclusions This research investigated the kinetics and the relationship between several reaction parameters using initially a set of single-factor

3.4. Effects of reuse and the number of photodegradation stage One advantage of immobilized heterogeneous catalysts is the ease of their separation from the reactants or products, which results in the possibility to reuse or recycle them without additional steps. The reuse of the TiO2-coated acrylic sheets for the photocatalytic degra-

Fig. 7. Normal probability plot of various effects for photocatalytic degradation of phenol.

Fig. 8. Residual plots for the photocatalytic degradation of phenol: (a) normal probability plot and (b) residuals versus predicted value.

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efficiency of the limited mobility of TiO2-coated acrylic sheets (compared to TiO2 powder), an increase in the number of stages of the photodegradation process could be applied.

Acknowledgments

Fig. 9. Effect of the reuse cycle of TiO2-coated acrylic sheets on the %phenol removal efficiency (the number of TiO2-coated acrylic sheets = 8 plates, initial phenol concentration = 50 ppm, and UV light power = 135 W).

variable assays to identify key factors and optimal conditions, and then a two-level factorial design, in the photodegradation of phenol catalyzed by TiO2-coated acrylic sheets prepared by dip-coating. For the preparation of the TiO2-coated acrylic sheets, the optimum molar ratio of ACA/TiO2 and number of coating cycles to achieve a uniform thin TiO2 film on the acrylic sheets were found to be 3.0 and 3, respectively. With these optimal condition for preparation of the TiO2coated acrylic sheets, the optimum conditions for photocatalytic degradation of phenol were obtained as follows: eight plates (as baffles) of TiO2-coated acrylic sheets, an initial phenol and H2O2 concentration of 50 ppm and 0.62 M, respectively, and a UV light power of 135 W. Under these conditions, the highest kapp value of 7.2×10–3 min–1 was attained. From the data derived from the factorial design, the ANOVA analysis revealed that the significant reaction parameters were (most to least significant): UV light power N the number of TiO2-coated acrylic sheets N initial phenol concentration N H2O2 concentration N the interaction between the number of TiO2coated acrylic sheets and the UV light power N the interaction between the initial phenol concentration and the UV light power. The TiO2-coated acrylic sheets can easily be physically reused for successive photocatalytic reactions without any treatment step. However, the photocatalytic efficiency of the used TiO2-coated acrylic sheets decreased slightly with use, at about 4.3% per 5 h cycle stage over five stages. To overcome the reduced phenol degradation

Fig. 10. Effect of the number of photodegradation stage on photocatalytic efficiency of TiO2-coated acrylic sheets for phenol removal (the number of TiO2-coated acrylic sheets = 8 plates, UV light power = 135 W, and reaction time = 5 h).

The authors gratefully acknowledge the co-funding of Thailand Research Fund (TRF)-Master Research Grants (MAG-WII525S017) and the 90th Anniversary of Chulalongkorn University Fund (Ratchadaphiseksomphot Endowment Fund); The National Research University Project of CHE and the Ratchadaphiseksomphot Endowment Fund (AM1024I); Center for Petroleum, Petrochemicals, and Advanced Materials, Chulalongkorn University and the Research Grant for Mid-Career University Faculty (RMU) co-funded by the TRF, the Commission on Higher Education and Chulalongkorn University (for T. Sreethawong) for financial support. The authors thank the technicians of the Department of Chemical Technology, Faculty of Science, Chulalongkorn University, for providing the chemicals and facilities throughout this research. The authors also wish to express their thanks to Dr. Robert Douglas John Butcher (Publication Counseling Unit, Faculty of Science, Chulalongkorn University) for English language editing.

References [1] G. Busca, S. Berardinelli, C. Resini, L. Arrighi, Technologies for the removal of phenol from fluid streams: a short review of recent developments, J. Hazard. Mater. 160 (2008) 265–288. [2] Z. Guo, R. Ma, G. Li, Degradation of phenol by nanomaterial TiO2 in wastewater, Chem. Eng. J. 119 (2006) 55–59. [3] D. Chen, A.K. Ray, Photocatalytic kinetics of phenol and its derivatives over UV irradiated TiO2, Appl. Catal. B Environ. 23 (1999) 143–157. [4] S. Ahmed, M.G. Rasul, W.N. Martens, R. Brown, M.A. Hashib, Heterogeneous photocatalytic degradation of phenols in wastewater: a review on current status and developments, Desalination 261 (2010) 3–18. [5] M. Stylidi, P.L. Kondarides, X.E. Verykios, Visible light-induced photocatalytic degradation of Acid Orange 7 in aqueous TiO2 suspensions, Appl. Catal. B Environ. 47 (2004) 189–201. [6] M.A. Behnajady, N. Modirshahla, N. Daneshvar, M. Rabbani, Photocatalytic degradation of an azo dye in a tubular continuous-flow photoreactor with immobilized TiO2 on glass plates, Chem. Eng. J. 127 (2007) 167–176. [7] S.N. Hosseni, S.M. Borghei, M. Vossoughi, N. Taghavinia, Immobilization of TiO2 on perlite granules for photocatalytic degradation of phenol, Appl. Catal. B Environ. 74 (2007) 53–62. [8] R. Portela, B. Sánchez, J.M. Coronado, R. Candal, S. Suárez, Selection of TiO2support: UV-transparent alternatives and long-term use limitations for H2S removal, Catal. Today 129 (2007) 223–230. [9] L. Rizzo, J. Koch, V. Belgiorno, M.A. Anderson, Removal of methylene blue in a photocatalytic reactor using polymethylmethacrylate supported TiO2 nanofilm, Desalination 211 (2007) 1–9. [10] Y.H. Chen, Y.Y. Liu, R.H. Lin, F.S. Yen, Photocatalytic degradation of pphenylenediamine with TiO2-coated magnetic PMMA microspheres in an aqueous solution, J. Hazard. Mater. 163 (2009) 973–981. [11] Y. Hu, C. Yuan, Low-temperature preparation of photocatalytic TiO2 thin films on polymer substrates by direct deposition from anatase sol, J. Mater. Sci. Technol. 22 (2006) 239–244. [12] S.T. Hwang, Y.B. Hahn, K.S. Nahm, Y.S. Lee, Preparation and characterization of poly(MSMA-co-MMA)-TiO2/SiO2 nanocomposites using the colloidal TiO2/SiO2 particles via blending method, Colloids Surf. A Physicochem. Eng. Aspects 259 (2005) 63–69. [13] R. Deng, D.Y. Arifin, M.Y. Chyn, C.H. Wang, Taylor vortex flow in presence of internal baffles, Chem. Eng. Sci. 65 (2010) 4598–4605. [14] W.M. Lu, H.Z. Wu, M.Y. Ju, Effect of baffle design on the liquid mixing in an aerated stirred tank with standard Rushton turbine impellers, Chem. Eng. Sci. 52 (1997) 3843–3851. [15] J.H. Yang, Y.S. Han, J.H. Choy, TiO2 thin-films on polymer substrates and their photocatalytic activity, Thin Solid Films 495 (2006) 266–271. [16] A. Sobczyński, L. Duczmal, W. Zmudziński, Phenol destruction by photocatalysis on TiO2: an attempt to solve the reaction mechanism, J. Molec. Catal. A Chem. 213 (2004) 225–230. [17] M.H. Priya, G. Madras, Kinetics of photocatalytic degradation of chlorophenol, nitrophenol, and their mixtures, Ind. Eng. Chem. Res. 45 (2006) 482–486. [18] J. Xu, Y. Ao, D. Fu, C. Yuan, Low-temperature preparation of F-doped TiO2 film and its photocatalytic activity under solar light, Appl. Surf. Sci. 254 (2008) 3033–3038. [19] C.H. Chiou, C.Y. Wu, R.S. Juang, Photocatalytic degradation of phenol and mnitrophenol using irradiated TiO2 in aqueous solutions, Sep. Purif. Technol. 62 (2008) 559–564.

T. Luenloi et al. / Desalination 274 (2011) 192–199 [20] L.B. Reutergardh, M. Langphasuk, Photocatalytic decolourization of reactive azo dye: a comparison between TiO2 and CdS photocatalysis, Chemosphere 35 (1997) 585–596. [21] R. Jain, M. Shrivastava, Photocatalytic removal of hazardous dye cyanosine from industrial waste using titanium dioxide, J. Hazard. Mater. 152 (2008) 216–220. [22] K.M. Parida, S.S. Dash, D.P. Das, Physico-chemical characterization and photocatalytic activity of zinc oxide prepared by various methods, J. Colloid Interface Sci. 298 (2006) 787–793. [23] S. Kaneco, M.A. Rahman, T. Suzuki, H. Katsumata, K. Ohta, Optimization of solar photocatalytic degradation conditions of bisphenol A in water using titanium dioxide, J. Photochem. Photobiol. A 163 (2004) 419–424. [24] C.H. Chiou, C.Y. Wu, R.S. Juang, Influence of operating parameters on photocatalytic degradation of phenol in UV/TiO2 process, Chem. Eng. J. 139 (2008) 322–329.

199

[25] M.A. Barakat, J.M. Tseng, C.P. Huang, Hydrogen peroxide-assisted photocatalytic oxidation of phenolic compounds, Appl. Catal. B Environ. 59 (2005) 99–104. [26] T. Sauer, G. Cesconeto Neto, H.J. José, R.F.P.M. Moreira, Kinetics of photocatalytic degradation of reactive dyes in a TiO2 slurry reactor, J. Photochem. Photobiol. A 149 (2002) 147–154. [27] N.M. Mahmoodi, M. Arami, N.Y. Limaee, N.S. Tabrizi, Kinetics of heterogeneous photocatalytic degradation of reactive dyes in an immobilized TiO2 photocatalytic reactor, J. Colloid Interface Sci. 295 (2006) 159–164. [28] N. Daneshvar, M.A. Behnajady, Y. Zorriyeh Asghar, Photopxidative degradation of 4-nitrophenol (4-NP) in UV/H2O2 process: influence of operational parameters and reaction mechanism, J. Hazard. Mater. B139 (2007) 275–279. [29] D.C. Montgomery, Design and Analysis of Experiments, John Wiley & Sons, New York, 2001. [30] Y. Xie, C. Yuan, Photocatalytic activity and recycle application of titanium dioxide sol for X-3B photodegadation, J. Molec. Catal. A Chem. 206 (2003) 419–428.