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Kinetics of ultrasonic degradation of phenol in the presence of TiO2 particles
Ultrasonics Sonochemistry 12 (2005) 263–269 www.elsevier.com/locate/ultsonch
Kinetics of ultrasonic degradation of phenol in the presence of TiO2 particles Masaki Kubo, Kunihiro Matsuoka, Atsushi Takahashi, Naomi Shibasaki-Kitakawa, Toshikuni Yonemoto * Department of Chemical Engineering, Tohoku University, Aoba-yama 07, Aoba-ku, Sendai 980-8579, Japan Received 9 May 2003; received in revised form 16 January 2004; accepted 23 January 2004 Available online 27 February 2004
Abstract The degradation of phenol by ultrasonic irradiation in the presence of TiO2 was investigated in complete darkness. The effects of amount of TiO2 and the combination of TiO2 addition with gas (air or oxygen) supply on the degradation kinetics of phenol and the formation of the reaction products were examined. The degradation rate of phenol increased with the amount of TiO2 . As the dissolved oxygen concentration increased by supplying oxygen, the degradation rate of phenol also increased. A kinetic model for the disappearance of phenol was proposed. The model takes into account the OH radical formation by direct water degradation, indirect degradation by oxygen atom and indirect degradation by TiO2 catalysis. The calculated results explained well the fact that a higher amount of TiO2 and dissolved oxygen concentration gave faster disappearance rate. 2004 Elsevier B.V. All rights reserved. PACS: 82.30.Lp Keywords: Kinetics; Ultrasonic degradation; TiO2 particles; Phenol; OH radical
1. Introduction Ultrasonic irradiation has recently been proposed as one of the techniques for degradation of hazardous organic compounds [1]. Ultrasonic irradiation results in the formation and collapse of micro scale bubbles and generating local high temperature [2,3]. The bubbles are thought to work as the reaction field and to promote the degradation reaction [4,5]. UV light illumination to titania (TiO2 ) photocatalysis is also used as a technique for the degradation of organic compounds [6]. The positive hole generated by UV light irradiation in the vicinity of the surface of catalyst particles reacts with the water at interface between the catalyst and liquid to produce OH radical [7,8]. The OH radical is thought to promote the degradation reaction [9,10].
*
Corresponding author. Tel.: +81-22-217-7255; fax: +81-22-2177258. E-mail address: [email protected] (T. Yonemoto). 1350-4177/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2004.01.039
The catalytic power of titania for formation of OH radical appears [11] by energy supply. The UV light has been normally used as an energy source, however, the light is screened by catalyst particles itself, so that the region exhibiting the catalytic power is spatially limited in the reactor. An ultrasonic irradiation might be used as an alternative energy source for formation of OH radical. The ultrasonic wave propagates throughout the reactor, so that it is possible to increase the catalyst particle concentration. A few studies have been made on the degradation of hazardous organic compounds by combining ultrasonic irradiation with UV light irradiation in the presence of TiO2 [12,13]. Pandit et al. [14] studied the degradation of 2,4,6 trichlorophenol by ultrasonic irradiation in the presence of TiO2 without UV light irradiation. In those studies, however, the amount of TiO2 added was as small as that of photocatalytic degradation so that the degradation rate was not so high. In this work, the degradation of phenol, the model hazardous organic compound, by ultrasonic irradiation
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M. Kubo et al. / Ultrasonics Sonochemistry 12 (2005) 263–269
in the presence of large amount of TiO2 was investigated in complete darkness. The effect of amount of TiO2 on the degradation of phenol and the formation of the reaction products was examined. The effect of combination of TiO2 addition and gas supply was also examined. A novel kinetic model on the disappearance of phenol was proposed.
2. Experimental Anatase type TiO2 particle ()5 lm, 99.9%) was used as catalyst. All other chemicals were purchased from Wako Pure Chemicals Industries, Ltd., Japan. A schematic illustration of the experimental apparatus is shown in Fig. 1. The reaction vessel was made of glass and its inner diameter and height were 32 and 75 mm, respectively. Ultrasonic irradiation was carried out using a Branson 250 Sonifier (USA), which was operated at frequency of 20 kHz. In order to fix the intensity of the ultrasonic irradiation, the vibration tip was fixed at the 13.5 mm high from the bottom of the reactor. An ultrasonic output was fixed at 50 W in all experiments. The temperature of the reaction solution was regulated at 303.7 ± 0.3 K (30.5 ± 0.3 C) with balancing the heat generation by ultrasonic irradiation and heat emission to the temperature controlled water bath. The dissolved oxygen (DO) concentration was measured and recorded by using a monitoring system with a DO electrode (BOG, Able Co., Japan) and a data logger (YODAC8, Yokogawa Electric Co., Japan). The solution was continuously circulated to the DO electrode. Air or O2 was supplied by bubbling through a glass filter (diameter of 10 mm, pore size of 40–50 lm) at the rate of 100 cm3 / min. To avoid the activation of the TiO2 by light, the reaction vessel was blinded by the black out curtain. A specified quantity of TiO2 particles was added to 20 cm3 pure water. To disperse the particles, the ultrasound was irradiated for 30 min. Then, the ultrasonic irradiation was once stopped and 5 cm3 of phenol aqueous
Fig. 1. Schematic illustration of experimental apparatus.
solution was added to the particle suspension. After the DO concentration reached to the steady state, the ultrasonic irradiation was restarted to proceed the degradation reaction. The initial concentration of the phenol was set at 1 mol/m3 in all experiments. The amount of the TiO2 catalyst was varied up to 10 g/(25 cm3 solution). To analyze the sample, the suspension was centrifuged and supernatant was filtered through an ultrafiltration filter with the molecular weight permeation limit of 10 000 Da to remove TiO2 from reaction solution. The concentrations of phenol and products were measured with an HPLC system (D-6100, Hitachi, Japan) equipped with a Gelpack column (GL-C610-S, Hitachi, Japan) and a UV detector (L-3000 Photodiode Array, Hitachi, Japan) at 210 nm. The mobile phase was the phosphate buffer of 100 mol/m3 and pH 2, and its flow rate was set at 1.2 cm3 /min. The concentration of the formic acid was measured with an Ion chromatography system (IC299, YOKOGAWA, Japan) equipped with an anion column (SAX1-205, YOKOGAWA, Japan). The mobile phase was the solution of 4 mol/m3 Na2 CO3 and 4 mol/m3 NaHCO3 , and its flow rate was set at 1.0 cm3 / min. The solids on the filter were photographed by using scanning electron microscope (SEM) (S-3000H, Hitachi, Japan) to determine the diameter distribution of TiO2 particles. The diameters of more than four hundred particles were measured to calculate arithmetic mean particle size, standard deviation, and geometric standard deviation.
3. Results and discussion Fig. 2 shows the particle size distribution of TiO2 at reaction time of 720 min for 1 g/(25 cm3 solution) of TiO2 added. The mean particle size was 95 nm, standard deviation was 87 nm, and geometry standard deviation was 1.96. Fig. 3 shows the time course of the phenol concentration for various amount of TiO2 . Plots denote the experimental data. The lines are the calculated results and are discussed later. The phenol concentration monotonously decreased. In spite of the complete darkness, the degradation rate of phenol in the presence of TiO2 is higher than that in the absence of TiO2 . Without ultrasonic irradiation, the degradation of the phenol did not proceed even if in the presence of TiO2 . These results suggested that the catalytic property of the TiO2 for degradation of organic compound appeared by ultrasonic irradiation acting as an energy source. The degradation rate of phenol increased with the amount of TiO2 , because catalytic surface area working for the radical formation by ultrasonic irradiation was increased.
M. Kubo et al. / Ultrasonics Sonochemistry 12 (2005) 263–269
265
Fig. 2. Particle size distribution of TiO2 . Amount of TiO2 added is 1 g/ (25 cm3 ), and reaction time is 720 min.
Fig. 4. HPLC chromatograms at reaction time of 720 min. (a) Without TiO2 addition, and (b) 1 g/(25 cm3 ) of TiO2 added.
Fig. 3. Time courses of phenol concentration for various amount of TiO2 . Initial phenol concentration is 1 mol/m3 . Symbol denotes the case without ultrasonic irradiation.
Fig. 4 shows the HPLC chromatograms at reaction time of 720 min for the absence of TiO2 and for 1 g/ (25 cm3 ). The HPLC chromatogram for the absence of TiO2 (Fig. 4a) showed that the main products were hydroquinone, catechol, and formic acid. The peaks for unidentified compounds were also detected near the retention time of 10 and 25 min. Hydroquinone and catechol, which are OH adducts of phenol, are also toxic as well as phenol and are desirable to reduce. When 1 g/(25 cm3 ) of TiO2 was added (the Fig. 4b), the amount of hydroquinone was smaller than that of the Fig. 4a and catechol was disappeared, so that these toxic compounds were reduced by addition of TiO2 . The peak height of formic acid was also smaller. Table 1 shows the carbon balance at 720 min for various amount of TiO2 . The phenolic compounds are phenol, catechol, and hydroquinone. The initial phenol concentration of 1 mol/m3 corresponds to 6 mol/m3 of carbon. The sums of percentage of detected compounds
(phenolic compounds + formic acid) were less than 100% for any condition. Subtracting the concentration of detected compounds from initial mole concentration of reactant in carbon base was considered to give the concentration of low molecular weight compounds, such as CO2 , not detected by HPLC. At the large amount of TiO2 , the percentage of the phenolic compounds was small and that of the low molecular weight compounds was high. Thus, the TiO2 addition is considered to be effective for the elimination of toxic phenolic compounds. Fig. 5 shows the time course of the phenol concentration at 1 g/(25 cm3 ) of TiO2 with gas supply. The degradation rate of phenol was promoted by gas supply, and the rate was fastest in case the oxygen was supplied. The DO concentration was 0.24, 0.27 and 1.05 mol/m3 for no gas supply, air supply, and oxygen supply, respectively. When the DO concentration was higher, the degradation rate of phenol was faster. In the case without ultrasonic irradiation, even with oxygen supply and in the presence of TiO2 , the degradation of the phenol did not proceed, same as the case without gas supply. These results support the idea introduced in the discussion of Fig. 3; the catalytic property of TiO2 for degradation of organic compound appeared by ultrasonic irradiation. Fig. 6 shows the time course of the phenol concentration with oxygen supply for various amount of TiO2 in case the oxygen was supplied. As well as the result of Fig. 3, the degradation rate of phenol increased with the amount of TiO2 .
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Table 1 Carbon balance for phenolic compounds, formic acid and low molecule compounds at 720 min for various amounts of TiO2 added TiO2 [g/(25 cm3 )]
Phenolic compounds [%]
( ¼ Phenol + catechol + hydroquinone [%])
Formic acid [%]
Phenolic compounds + formic acid [%]
Low molecular weight compounds (calculated) [%]
0 1 5 10
40.8 22.9 13.4 11.3
( ¼ 17.1 + 13.8 + 9.9) ( ¼ 15.2 + 0.0 + 7.7) ( ¼ 13.2 + 0.0 + 0.2) ( ¼ 11.3 + 0.0 + 0.0)
1.8 0.3 0.2 0.5
42.6 23.2 13.6 11.8
57.4 76.8 86.4 88.2
Fig. 5. Effect of gas supply on time courses of phenol concentration. Amount of TiO2 is 1 g/(25 cm3 ) and flow rate of supplied gas is 100 cm3 /min. Symbol denotes the case of O2 supply without ultrasonic irradiation.
Fig. 7. Schematic illustration of reaction scheme under ultrasonic irradiation with TiO2 . (h: positive hole.)
k1
H2 O!OH þ H
ð1Þ
Oxygen is decomposed to form oxygen atom [16], which reacts with water to form OH radical [17]. k2
O2 !2O
ð2Þ k3
H2 O þ O !2OH
ð3Þ
According to the idea introduced in the discussion of Fig. 3, the thermal energy by ultrasonic irradiation is assumed to generate positive hole in the vicinity of TiO2 surface. The positive hole reacts with water to produce OH radical. k4
Fig. 6. Effect of amount of TiO2 on time courses of phenol concentration under oxygen supply. Flow rate of oxygen is 100 cm3 /min.
H2 O þ h!OH þ Hþ
ð4Þ
Symbol h in Eq. (4) denotes the positive hole. Phenol is disappeared by reaction with OH radical to form phenoxy radical, PhO [18].
4. Kinetic model for phenol disappearance
OH þ PhOH!PhO þ H2 O
Fig. 7 shows a schematic illustration of hydroxyl radical formation and disappearance of phenol under the ultrasonic irradiation in the presence of TiO2 . Water is directly decomposed leading to the formation of OH and H radicals [15].
Two termination reactions are taken into account; the reaction of PhO with OH radical [18] and recombination of OH radical to form H2 O2 [19,20].
k5
k6
PhO þ OH !nonradical product
ð5Þ
ð6Þ
M. Kubo et al. / Ultrasonics Sonochemistry 12 (2005) 263–269 k7
2OH !H2 O2
ð7Þ
The rate of change in concentration of phenol, hydroxyl radical, phenoxy radical, and oxygen atoms are dCPhOH ¼ k5 CPhOH COH dt dCOH ¼ k1 CH2 O þ 2k3 CO CH2 O þ k4 CH2 O Ch dt 2 k5 CPhOH COH k6 CPhO COH 2k7 COH dCPhO ¼ k5 CPhOH COH k6 CPhO COH dt
ð8Þ
ð9Þ ð10Þ
dCO ¼ 2k2 CO2 k3 CO CH2 O ð11Þ dt Assuming the pseudo steady state for all radicals and atomic oxygen concentration gives the rate of change in phenol concentration as dCPhOH 2 ¼ ka CPhOH dt CPhOH
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 ka2 CPhOH þ kb þ kc CO2 þ kd0 Ch
ð12Þ
where ka , kb , kc , and kd0 in Eq. (12) are given by ka ¼
k52 2k7
ð13Þ
kb ¼
k1 k52 C H2 O 2k7
ð14Þ
kc ¼
2k2 k52 k7
ð15Þ
kd0 ¼
k4 k52 C H2 O 2k7
ð16Þ
267
Eq. (18) to the eight sets of experimental data obtained under different reaction conditions; four different amount of TiO2 without gas supply, 1 g/(25 cm3 ) of TiO2 with air supply, and three different amount of TiO2 with oxygen supply. The state of the cavitation was considered not to be affected by the dissolved oxygen concentration, so that the reaction rate constants were estimated as the same values in all experimental conditions. The fitting procedure is as follows. Using a certain set of constants, Eq. (18) was numerically solved by means of the Runge–Kutta method. The best fit values of the constants were determined using the simplex parameter fitting method [21] to minimize the sum of the absolute value of the error between experimental data and calculated value of the phenol concentration. X S¼ jCPhOH;calc: CPhOH;exp: j ð20Þ The fitting results are shown in Figs. 3, 5 and 6 by lines. All lines well described the effect of the amount of TiO2 and the DO concentration so that the validity of the mathematical model was verified. The estimated values are shown in Table 2. As shown in Eq. (5), phenol is disappeared by reaction with OH radical, that is, the formation rate of OH radical affects to the disappearance rate of phenol. There are three routes for formation of OH radical, direct degradation of water, indirect degradation of water by oxygen atom, and indirect degradation of water by TiO2 catalysis, and the formation rates are given as Eqs. (21)– (23), respectively. rOH ð¼ k1 CH2 O Þ ¼
kb ka
rOH ;O ð¼ 4k2 CO2 Þ ¼
ð21Þ
kc CO ka 2
ð22Þ
The surface area of the TiO2 particles is proportional to the TiO2 concentration, so that the positive hole concentration, Ch is given as
rOH ;TiO2 ð¼ ak4 CH2 O CTiO2 Þ ¼
Ch ¼ aCTiO2
The total formation rate of OH radical is written as
ð17Þ
Here, a represents the proportional constant. Combining Eq. (17) with Eq. (12) gives the rate of change in phenol concentration as dCPhOH 2 ¼ ka CPhOH dt CPhOH
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 ka2 CPhOH þ kb þ kc CO2 þ kd CTiO2 ð18Þ
Here, kd is kd ¼
ak4 k52 C H2 O 2k7
rOH ;total ¼
There exists four unknown constants, ka , kb , kc , and kd in Eq. (18). These constants were estimated by fitting
ð23Þ
kb kc kd þ CO2 þ CTiO2 ka ka ka
ð24Þ
The values of formation rate of OH radical by ultrasonic irradiation in the absence of TiO2 are summarized in Table 3. The formation rate in our work was much larger than literature value. However,
Table 2 Estimated values of rate constants Constant
ð19Þ
kd CTiO2 ka
ka kb kc kd
Value
Unit 3
5.05 · 10 1.76 · 105 1.34 · 105 4.60 · 109
[mol1 m3 min1 ] [min2 ] [mol1 m3 min2 ] [mol1 m3 min2 ]
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Table 3 Formation rate of OH radical by ultrasonic irradiation in the absence of TiO2 Authors
Output [W]
Volume [cm3 ]
Rate [lmol/(dm3 min)]
Rate/(output/volume) [109 mol/W min]
This work Hua and Hoffmann
50 15.8
25 200
6.28 0.0699
3.14 0.885
Actual values and those normalized by ultrasonic output per unit volume.
Table 4 Calculated values of rOH , rOH ;O , rOH ;TiO2 Amount of TiO2 [g/(25 cm3 )]
DO conc. [mol m3 ]
Supplied gas
rOH [103 mol m3 min1 ]
rOH ;O [103 mol m3 min1 ]
rOH ;TiO2 [103 mol m3 min1 ]
rOH ;Total [103 mol m3 min1 ]
0 1 1 1 5 10
0.24 0.24 0.27 1.05 1.05 1.05
None None Air O2 O2 O2
3.49 3.49 3.49 3.49 3.49 3.49
0.64 0.64 0.72 2.79 2.79 2.79
0 0.46 0.46 0.46 2.28 4.56
4.13 4.59 4.67 6.74 8.56 10.84
experimental conditions such as ultrasonic output and solution volume were also different. The formation rate of OH radical was considered to depend on ultrasonic output per unit volume [22], so that the comparison in the rate normalized by ultrasonic output per unit volume was desirable. In this case, both results were in same order of magnitude. Thus, the value estimated in this study was reasonable. Table 4 shows the calculated results of the formation rate of OH radical for various conditions. The rate by oxygen atom, rOH ;O , the fifth column of the table, increased in order of no supplied gas (the second row), air supply (the third row), and oxygen supply (the fourth row), so that the total rate (the right end column) for oxygen supply at 1 g/(25 cm3 ) of TiO2 was higher than that for the case without both TiO2 and gas supply. The rate by TiO2 catalysis, rOH ;TiO2 , the sixth column, increased with the amount of TiO2 (the fourth to sixth row), so that the total rate for 10 g/(25 cm3 ) of TiO2 with oxygen supply was much higher than that for the case without both TiO2 and gas supply.
5. Conclusions The degradation of phenol by ultrasonic irradiation in the presence of TiO2 was investigated. The effects of amount of TiO2 on the degradation kinetics of phenol and the formation of the reaction products were examined. The degradation rate of phenol increased with the amount of TiO2 . As the amount of TiO2 increased, the concentration of the low molecular weight compounds increased. The combination of TiO2 addition and gas supply on the degradation kinetics of phenol was examined. As the dissolved oxygen concentration increased by supplying oxygen, the degradation rate of phenol increased.
A kinetic model for the disappearance of phenol was proposed. The OH radical formation by direct water degradation, the indirect degradation by oxygen atom and indirect degradation by TiO2 catalysis are taken into account in the model. The calculated results explained well the fact that a higher amount of TiO2 and the DO concentration gave a faster disappearance rate.
References [1] K.S. Suslick, S.J. Doktycz, E.B. Flint, Ultrasonics 28 (1990) 280. [2] K.S. Suslick, D.A. Hammerton, R.E. Cline Jr., J. Am. Chem. Soc. 108 (1986) 5641. [3] S. Sochard, A.M. Wilhelm, H. Delmas, Chem. Eng. Sci. 53 (1998) 239. [4] Y.Y. Lur’e, P.F. Kandzas, A.A. Mokina, Russ. J. Phys. Chem. 36 (1962) 1422. [5] N. Serpone, R. Terzian, P. Colarusso, C. Minero, E. Pelizzetti, H. Hidaka, Res. Chem. Intermed. 18 (1992) 183. [6] D.S. Bhatkhande, V.G. Pangarkar, A.A.C.M. Beenackers, J. Chem. Technol. Biotechnol. 77 (2001) 102. [7] C.D. Jaeger, A.J. Bard, J. Phys. Chem. 83 (1979) 3146. [8] R.W. Matthews, J. Chem. Soc. Faraday Trans. 80 (1984) 457. [9] K. Okamoto, Y. Yamamoto, H. Tanaka, M. Tanaka, Bull. Chem. Soc. Jpn. 58 (7) (1985) 2015. [10] K.H. Wang, Y.H. Hsieh, M.Y. Chou, C.Y. Chang, Appl. Catal. B: Environ. 21 (1999) 1. [11] A.R. West, Basic Solid State Chemistry, Wiley, New York, 1988. [12] I.Z. Shirgaonkar, A.B. Pandit, Ultrason. Sonochem. 5 (1998) 53. [13] P. Theron, P. Pichat, C. Guillard, C. Petrier, T. Chopin, Phys. Chem. Chem. Phys. 1 (1999) 4663. [14] A.B. Pandit, P.R. Gogate, S. Mujumdar, Ultrason. Sonochem. 8 (2001) 227. [15] K. Makino, M.M. Mossoba, P. Riesz, J. Phys. Chem. 87 (1983) 1369. [16] C. Petrier, A. Jeunet, J.L. Luche, G. Reverdy, J. Am. Chem. Soc. 114 (1992) 3148. [17] A.J. Colussi, L.K. Weavers, M.R. Hoffmann, J. Phys. Chem. A. 102 (1998) 6927. [18] S. Gopalan, P.E. Savage, J. Phys. Chem. 98 (1994) 12646.
M. Kubo et al. / Ultrasonics Sonochemistry 12 (2005) 263–269 [19] K. Vinodgopal, M. Ashokkumar, F. Grieser, J. Phys. Chem. B 105 (2001) 3338. [20] C. Petrier, A. Francony, Ultrason. Sonochem. 4 (1997) 295.
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[21] J.A. Nelder, R. Mead, Comput. J. 7 (1964) 308. [22] I. Hua, M.R. Hoffmann, Environ. Sci. Technol. 31 (1997) 2237.
Kinetics of ultrasonic degradation of phenol in the presence of TiO2 particles Masaki Kubo, Kunihiro Matsuoka, Atsushi Takahashi, Naomi Shibasaki-Kitakawa, Toshikuni Yonemoto * Department of Chemical Engineering, Tohoku University, Aoba-yama 07, Aoba-ku, Sendai 980-8579, Japan Received 9 May 2003; received in revised form 16 January 2004; accepted 23 January 2004 Available online 27 February 2004
Abstract The degradation of phenol by ultrasonic irradiation in the presence of TiO2 was investigated in complete darkness. The effects of amount of TiO2 and the combination of TiO2 addition with gas (air or oxygen) supply on the degradation kinetics of phenol and the formation of the reaction products were examined. The degradation rate of phenol increased with the amount of TiO2 . As the dissolved oxygen concentration increased by supplying oxygen, the degradation rate of phenol also increased. A kinetic model for the disappearance of phenol was proposed. The model takes into account the OH radical formation by direct water degradation, indirect degradation by oxygen atom and indirect degradation by TiO2 catalysis. The calculated results explained well the fact that a higher amount of TiO2 and dissolved oxygen concentration gave faster disappearance rate. 2004 Elsevier B.V. All rights reserved. PACS: 82.30.Lp Keywords: Kinetics; Ultrasonic degradation; TiO2 particles; Phenol; OH radical
1. Introduction Ultrasonic irradiation has recently been proposed as one of the techniques for degradation of hazardous organic compounds [1]. Ultrasonic irradiation results in the formation and collapse of micro scale bubbles and generating local high temperature [2,3]. The bubbles are thought to work as the reaction field and to promote the degradation reaction [4,5]. UV light illumination to titania (TiO2 ) photocatalysis is also used as a technique for the degradation of organic compounds [6]. The positive hole generated by UV light irradiation in the vicinity of the surface of catalyst particles reacts with the water at interface between the catalyst and liquid to produce OH radical [7,8]. The OH radical is thought to promote the degradation reaction [9,10].
*
Corresponding author. Tel.: +81-22-217-7255; fax: +81-22-2177258. E-mail address: [email protected] (T. Yonemoto). 1350-4177/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2004.01.039
The catalytic power of titania for formation of OH radical appears [11] by energy supply. The UV light has been normally used as an energy source, however, the light is screened by catalyst particles itself, so that the region exhibiting the catalytic power is spatially limited in the reactor. An ultrasonic irradiation might be used as an alternative energy source for formation of OH radical. The ultrasonic wave propagates throughout the reactor, so that it is possible to increase the catalyst particle concentration. A few studies have been made on the degradation of hazardous organic compounds by combining ultrasonic irradiation with UV light irradiation in the presence of TiO2 [12,13]. Pandit et al. [14] studied the degradation of 2,4,6 trichlorophenol by ultrasonic irradiation in the presence of TiO2 without UV light irradiation. In those studies, however, the amount of TiO2 added was as small as that of photocatalytic degradation so that the degradation rate was not so high. In this work, the degradation of phenol, the model hazardous organic compound, by ultrasonic irradiation
264
M. Kubo et al. / Ultrasonics Sonochemistry 12 (2005) 263–269
in the presence of large amount of TiO2 was investigated in complete darkness. The effect of amount of TiO2 on the degradation of phenol and the formation of the reaction products was examined. The effect of combination of TiO2 addition and gas supply was also examined. A novel kinetic model on the disappearance of phenol was proposed.
2. Experimental Anatase type TiO2 particle ()5 lm, 99.9%) was used as catalyst. All other chemicals were purchased from Wako Pure Chemicals Industries, Ltd., Japan. A schematic illustration of the experimental apparatus is shown in Fig. 1. The reaction vessel was made of glass and its inner diameter and height were 32 and 75 mm, respectively. Ultrasonic irradiation was carried out using a Branson 250 Sonifier (USA), which was operated at frequency of 20 kHz. In order to fix the intensity of the ultrasonic irradiation, the vibration tip was fixed at the 13.5 mm high from the bottom of the reactor. An ultrasonic output was fixed at 50 W in all experiments. The temperature of the reaction solution was regulated at 303.7 ± 0.3 K (30.5 ± 0.3 C) with balancing the heat generation by ultrasonic irradiation and heat emission to the temperature controlled water bath. The dissolved oxygen (DO) concentration was measured and recorded by using a monitoring system with a DO electrode (BOG, Able Co., Japan) and a data logger (YODAC8, Yokogawa Electric Co., Japan). The solution was continuously circulated to the DO electrode. Air or O2 was supplied by bubbling through a glass filter (diameter of 10 mm, pore size of 40–50 lm) at the rate of 100 cm3 / min. To avoid the activation of the TiO2 by light, the reaction vessel was blinded by the black out curtain. A specified quantity of TiO2 particles was added to 20 cm3 pure water. To disperse the particles, the ultrasound was irradiated for 30 min. Then, the ultrasonic irradiation was once stopped and 5 cm3 of phenol aqueous
Fig. 1. Schematic illustration of experimental apparatus.
solution was added to the particle suspension. After the DO concentration reached to the steady state, the ultrasonic irradiation was restarted to proceed the degradation reaction. The initial concentration of the phenol was set at 1 mol/m3 in all experiments. The amount of the TiO2 catalyst was varied up to 10 g/(25 cm3 solution). To analyze the sample, the suspension was centrifuged and supernatant was filtered through an ultrafiltration filter with the molecular weight permeation limit of 10 000 Da to remove TiO2 from reaction solution. The concentrations of phenol and products were measured with an HPLC system (D-6100, Hitachi, Japan) equipped with a Gelpack column (GL-C610-S, Hitachi, Japan) and a UV detector (L-3000 Photodiode Array, Hitachi, Japan) at 210 nm. The mobile phase was the phosphate buffer of 100 mol/m3 and pH 2, and its flow rate was set at 1.2 cm3 /min. The concentration of the formic acid was measured with an Ion chromatography system (IC299, YOKOGAWA, Japan) equipped with an anion column (SAX1-205, YOKOGAWA, Japan). The mobile phase was the solution of 4 mol/m3 Na2 CO3 and 4 mol/m3 NaHCO3 , and its flow rate was set at 1.0 cm3 / min. The solids on the filter were photographed by using scanning electron microscope (SEM) (S-3000H, Hitachi, Japan) to determine the diameter distribution of TiO2 particles. The diameters of more than four hundred particles were measured to calculate arithmetic mean particle size, standard deviation, and geometric standard deviation.
3. Results and discussion Fig. 2 shows the particle size distribution of TiO2 at reaction time of 720 min for 1 g/(25 cm3 solution) of TiO2 added. The mean particle size was 95 nm, standard deviation was 87 nm, and geometry standard deviation was 1.96. Fig. 3 shows the time course of the phenol concentration for various amount of TiO2 . Plots denote the experimental data. The lines are the calculated results and are discussed later. The phenol concentration monotonously decreased. In spite of the complete darkness, the degradation rate of phenol in the presence of TiO2 is higher than that in the absence of TiO2 . Without ultrasonic irradiation, the degradation of the phenol did not proceed even if in the presence of TiO2 . These results suggested that the catalytic property of the TiO2 for degradation of organic compound appeared by ultrasonic irradiation acting as an energy source. The degradation rate of phenol increased with the amount of TiO2 , because catalytic surface area working for the radical formation by ultrasonic irradiation was increased.
M. Kubo et al. / Ultrasonics Sonochemistry 12 (2005) 263–269
265
Fig. 2. Particle size distribution of TiO2 . Amount of TiO2 added is 1 g/ (25 cm3 ), and reaction time is 720 min.
Fig. 4. HPLC chromatograms at reaction time of 720 min. (a) Without TiO2 addition, and (b) 1 g/(25 cm3 ) of TiO2 added.
Fig. 3. Time courses of phenol concentration for various amount of TiO2 . Initial phenol concentration is 1 mol/m3 . Symbol denotes the case without ultrasonic irradiation.
Fig. 4 shows the HPLC chromatograms at reaction time of 720 min for the absence of TiO2 and for 1 g/ (25 cm3 ). The HPLC chromatogram for the absence of TiO2 (Fig. 4a) showed that the main products were hydroquinone, catechol, and formic acid. The peaks for unidentified compounds were also detected near the retention time of 10 and 25 min. Hydroquinone and catechol, which are OH adducts of phenol, are also toxic as well as phenol and are desirable to reduce. When 1 g/(25 cm3 ) of TiO2 was added (the Fig. 4b), the amount of hydroquinone was smaller than that of the Fig. 4a and catechol was disappeared, so that these toxic compounds were reduced by addition of TiO2 . The peak height of formic acid was also smaller. Table 1 shows the carbon balance at 720 min for various amount of TiO2 . The phenolic compounds are phenol, catechol, and hydroquinone. The initial phenol concentration of 1 mol/m3 corresponds to 6 mol/m3 of carbon. The sums of percentage of detected compounds
(phenolic compounds + formic acid) were less than 100% for any condition. Subtracting the concentration of detected compounds from initial mole concentration of reactant in carbon base was considered to give the concentration of low molecular weight compounds, such as CO2 , not detected by HPLC. At the large amount of TiO2 , the percentage of the phenolic compounds was small and that of the low molecular weight compounds was high. Thus, the TiO2 addition is considered to be effective for the elimination of toxic phenolic compounds. Fig. 5 shows the time course of the phenol concentration at 1 g/(25 cm3 ) of TiO2 with gas supply. The degradation rate of phenol was promoted by gas supply, and the rate was fastest in case the oxygen was supplied. The DO concentration was 0.24, 0.27 and 1.05 mol/m3 for no gas supply, air supply, and oxygen supply, respectively. When the DO concentration was higher, the degradation rate of phenol was faster. In the case without ultrasonic irradiation, even with oxygen supply and in the presence of TiO2 , the degradation of the phenol did not proceed, same as the case without gas supply. These results support the idea introduced in the discussion of Fig. 3; the catalytic property of TiO2 for degradation of organic compound appeared by ultrasonic irradiation. Fig. 6 shows the time course of the phenol concentration with oxygen supply for various amount of TiO2 in case the oxygen was supplied. As well as the result of Fig. 3, the degradation rate of phenol increased with the amount of TiO2 .
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Table 1 Carbon balance for phenolic compounds, formic acid and low molecule compounds at 720 min for various amounts of TiO2 added TiO2 [g/(25 cm3 )]
Phenolic compounds [%]
( ¼ Phenol + catechol + hydroquinone [%])
Formic acid [%]
Phenolic compounds + formic acid [%]
Low molecular weight compounds (calculated) [%]
0 1 5 10
40.8 22.9 13.4 11.3
( ¼ 17.1 + 13.8 + 9.9) ( ¼ 15.2 + 0.0 + 7.7) ( ¼ 13.2 + 0.0 + 0.2) ( ¼ 11.3 + 0.0 + 0.0)
1.8 0.3 0.2 0.5
42.6 23.2 13.6 11.8
57.4 76.8 86.4 88.2
Fig. 5. Effect of gas supply on time courses of phenol concentration. Amount of TiO2 is 1 g/(25 cm3 ) and flow rate of supplied gas is 100 cm3 /min. Symbol denotes the case of O2 supply without ultrasonic irradiation.
Fig. 7. Schematic illustration of reaction scheme under ultrasonic irradiation with TiO2 . (h: positive hole.)
k1
H2 O!OH þ H
ð1Þ
Oxygen is decomposed to form oxygen atom [16], which reacts with water to form OH radical [17]. k2
O2 !2O
ð2Þ k3
H2 O þ O !2OH
ð3Þ
According to the idea introduced in the discussion of Fig. 3, the thermal energy by ultrasonic irradiation is assumed to generate positive hole in the vicinity of TiO2 surface. The positive hole reacts with water to produce OH radical. k4
Fig. 6. Effect of amount of TiO2 on time courses of phenol concentration under oxygen supply. Flow rate of oxygen is 100 cm3 /min.
H2 O þ h!OH þ Hþ
ð4Þ
Symbol h in Eq. (4) denotes the positive hole. Phenol is disappeared by reaction with OH radical to form phenoxy radical, PhO [18].
4. Kinetic model for phenol disappearance
OH þ PhOH!PhO þ H2 O
Fig. 7 shows a schematic illustration of hydroxyl radical formation and disappearance of phenol under the ultrasonic irradiation in the presence of TiO2 . Water is directly decomposed leading to the formation of OH and H radicals [15].
Two termination reactions are taken into account; the reaction of PhO with OH radical [18] and recombination of OH radical to form H2 O2 [19,20].
k5
k6
PhO þ OH !nonradical product
ð5Þ
ð6Þ
M. Kubo et al. / Ultrasonics Sonochemistry 12 (2005) 263–269 k7
2OH !H2 O2
ð7Þ
The rate of change in concentration of phenol, hydroxyl radical, phenoxy radical, and oxygen atoms are dCPhOH ¼ k5 CPhOH COH dt dCOH ¼ k1 CH2 O þ 2k3 CO CH2 O þ k4 CH2 O Ch dt 2 k5 CPhOH COH k6 CPhO COH 2k7 COH dCPhO ¼ k5 CPhOH COH k6 CPhO COH dt
ð8Þ
ð9Þ ð10Þ
dCO ¼ 2k2 CO2 k3 CO CH2 O ð11Þ dt Assuming the pseudo steady state for all radicals and atomic oxygen concentration gives the rate of change in phenol concentration as dCPhOH 2 ¼ ka CPhOH dt CPhOH
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 ka2 CPhOH þ kb þ kc CO2 þ kd0 Ch
ð12Þ
where ka , kb , kc , and kd0 in Eq. (12) are given by ka ¼
k52 2k7
ð13Þ
kb ¼
k1 k52 C H2 O 2k7
ð14Þ
kc ¼
2k2 k52 k7
ð15Þ
kd0 ¼
k4 k52 C H2 O 2k7
ð16Þ
267
Eq. (18) to the eight sets of experimental data obtained under different reaction conditions; four different amount of TiO2 without gas supply, 1 g/(25 cm3 ) of TiO2 with air supply, and three different amount of TiO2 with oxygen supply. The state of the cavitation was considered not to be affected by the dissolved oxygen concentration, so that the reaction rate constants were estimated as the same values in all experimental conditions. The fitting procedure is as follows. Using a certain set of constants, Eq. (18) was numerically solved by means of the Runge–Kutta method. The best fit values of the constants were determined using the simplex parameter fitting method [21] to minimize the sum of the absolute value of the error between experimental data and calculated value of the phenol concentration. X S¼ jCPhOH;calc: CPhOH;exp: j ð20Þ The fitting results are shown in Figs. 3, 5 and 6 by lines. All lines well described the effect of the amount of TiO2 and the DO concentration so that the validity of the mathematical model was verified. The estimated values are shown in Table 2. As shown in Eq. (5), phenol is disappeared by reaction with OH radical, that is, the formation rate of OH radical affects to the disappearance rate of phenol. There are three routes for formation of OH radical, direct degradation of water, indirect degradation of water by oxygen atom, and indirect degradation of water by TiO2 catalysis, and the formation rates are given as Eqs. (21)– (23), respectively. rOH ð¼ k1 CH2 O Þ ¼
kb ka
rOH ;O ð¼ 4k2 CO2 Þ ¼
ð21Þ
kc CO ka 2
ð22Þ
The surface area of the TiO2 particles is proportional to the TiO2 concentration, so that the positive hole concentration, Ch is given as
rOH ;TiO2 ð¼ ak4 CH2 O CTiO2 Þ ¼
Ch ¼ aCTiO2
The total formation rate of OH radical is written as
ð17Þ
Here, a represents the proportional constant. Combining Eq. (17) with Eq. (12) gives the rate of change in phenol concentration as dCPhOH 2 ¼ ka CPhOH dt CPhOH
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 ka2 CPhOH þ kb þ kc CO2 þ kd CTiO2 ð18Þ
Here, kd is kd ¼
ak4 k52 C H2 O 2k7
rOH ;total ¼
There exists four unknown constants, ka , kb , kc , and kd in Eq. (18). These constants were estimated by fitting
ð23Þ
kb kc kd þ CO2 þ CTiO2 ka ka ka
ð24Þ
The values of formation rate of OH radical by ultrasonic irradiation in the absence of TiO2 are summarized in Table 3. The formation rate in our work was much larger than literature value. However,
Table 2 Estimated values of rate constants Constant
ð19Þ
kd CTiO2 ka
ka kb kc kd
Value
Unit 3
5.05 · 10 1.76 · 105 1.34 · 105 4.60 · 109
[mol1 m3 min1 ] [min2 ] [mol1 m3 min2 ] [mol1 m3 min2 ]
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M. Kubo et al. / Ultrasonics Sonochemistry 12 (2005) 263–269
Table 3 Formation rate of OH radical by ultrasonic irradiation in the absence of TiO2 Authors
Output [W]
Volume [cm3 ]
Rate [lmol/(dm3 min)]
Rate/(output/volume) [109 mol/W min]
This work Hua and Hoffmann
50 15.8
25 200
6.28 0.0699
3.14 0.885
Actual values and those normalized by ultrasonic output per unit volume.
Table 4 Calculated values of rOH , rOH ;O , rOH ;TiO2 Amount of TiO2 [g/(25 cm3 )]
DO conc. [mol m3 ]
Supplied gas
rOH [103 mol m3 min1 ]
rOH ;O [103 mol m3 min1 ]
rOH ;TiO2 [103 mol m3 min1 ]
rOH ;Total [103 mol m3 min1 ]
0 1 1 1 5 10
0.24 0.24 0.27 1.05 1.05 1.05
None None Air O2 O2 O2
3.49 3.49 3.49 3.49 3.49 3.49
0.64 0.64 0.72 2.79 2.79 2.79
0 0.46 0.46 0.46 2.28 4.56
4.13 4.59 4.67 6.74 8.56 10.84
experimental conditions such as ultrasonic output and solution volume were also different. The formation rate of OH radical was considered to depend on ultrasonic output per unit volume [22], so that the comparison in the rate normalized by ultrasonic output per unit volume was desirable. In this case, both results were in same order of magnitude. Thus, the value estimated in this study was reasonable. Table 4 shows the calculated results of the formation rate of OH radical for various conditions. The rate by oxygen atom, rOH ;O , the fifth column of the table, increased in order of no supplied gas (the second row), air supply (the third row), and oxygen supply (the fourth row), so that the total rate (the right end column) for oxygen supply at 1 g/(25 cm3 ) of TiO2 was higher than that for the case without both TiO2 and gas supply. The rate by TiO2 catalysis, rOH ;TiO2 , the sixth column, increased with the amount of TiO2 (the fourth to sixth row), so that the total rate for 10 g/(25 cm3 ) of TiO2 with oxygen supply was much higher than that for the case without both TiO2 and gas supply.
5. Conclusions The degradation of phenol by ultrasonic irradiation in the presence of TiO2 was investigated. The effects of amount of TiO2 on the degradation kinetics of phenol and the formation of the reaction products were examined. The degradation rate of phenol increased with the amount of TiO2 . As the amount of TiO2 increased, the concentration of the low molecular weight compounds increased. The combination of TiO2 addition and gas supply on the degradation kinetics of phenol was examined. As the dissolved oxygen concentration increased by supplying oxygen, the degradation rate of phenol increased.
A kinetic model for the disappearance of phenol was proposed. The OH radical formation by direct water degradation, the indirect degradation by oxygen atom and indirect degradation by TiO2 catalysis are taken into account in the model. The calculated results explained well the fact that a higher amount of TiO2 and the DO concentration gave a faster disappearance rate.
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