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Effect of irradiation distance on degradation of phenol using indirect ultrasonic irradiation method
Ultrasonics Sonochemistry 18 (2011) 1205–1210
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultsonch
Effect of irradiation distance on degradation of phenol using indirect ultrasonic irradiation method Daisuke Kobayashi a,⇑, Kazuki Sano b, Yusuke Takeuchi b, Koichi Terasaka a a b
Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan School of Science for Open and Environmental Systems, Graduate School of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan
a r t i c l e
i n f o
Article history: Received 10 May 2010 Received in revised form 18 January 2011 Accepted 25 January 2011 Available online 20 February 2011 Keywords: Degradation Phenol Indirect ultrasonic irradiation Irradiation distance Kinetics analysis
a b s t r a c t Ultrasound is used as degradation of hazardous organic compounds. In this study, indirect ultrasonic irradiation method was applied to the degradation process of phenol, the model hazardous organic compound, and the effects of irradiation distance on radical generation and ultrasonic power were investigated. The chemical effect estimated by KI oxidation dosimetry and ultrasonic power measured by calorimetry fluctuated for the irradiation distance, and there was a relationship between the period of the fluctuation of ultrasonic effect and the wavelength of ultrasound. The degradation of phenol was considered to progress in the zero-order kinetics, before the decomposition conversion was less than 25%. Therefore, the simple kinetic model on degradation of phenol was proposed, and there was a linear relation in the degradation rate constant of phenol and the ultrasonic power inside the reactor. In addition, the kinetic model proposed in this study was applied to the former study. There was a linear relation in the degradation rate constant of phenol and ultrasonic energy in the range of frequency of 20–30 kHz in spite of the difference of equipment and sample volume. On the other hand, the degradation rate constant in the range of frequency of 200–800 kHz was much larger than that of 20–30 kHz in the same ultrasonic energy, and this behaviour was agreed with the former investigation about the dependence of ultrasonic frequency on chemical effect. Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved.
1. Introduction Recently, techniques for the degradation of hazardous organic compounds have been investigated. UV light irradiation to TiO2 photocatalysis is one of the favorite techniques [1]. The positive holes generated by UV light irradiation in the vicinity of the surface of TiO2 particles react with water to produce OH radicals. Devlln and Harris have proposed the detailed reaction scheme of the oxidation of aqueous phenol by oxygen [2], and the photocatalytic degradation of phenol is considered to progress similar reaction scheme. However, UV light is screened by TiO2 particles, so that there is a limit amount of TiO2 particle addition in which degradation rate increases. Ultrasound has been investigated as one of the techniques for degradation of hazardous organic compounds [3,4]. Especially, degradation of phenol and some of its derivatives such as chlorophenol and nitrophenol using ultrasound has been investigated by many researchers [5]. Berlan et al. have investigated the effects of pressure, dissolved gas and ultrasonic frequency on the degrada⇑ Corresponding author. Address: Department of Industrial Chemistry, Faculty of Engineering, Tokyo University of Science, Tokyo 162-8601, Japan. Tel.: +81 3 5228 8310; fax: +81 3 5261 4361. E-mail address: [email protected] (D. Kobayashi).
tion of phenol, and the reaction scheme of degradation of phenol has been proposed [6]. Serpone et al. have reported that the degradation of phenol progresses in the zero-order kinetics at 20 kHz [7]. Pétrier and Francony have investigated the effect of ultrasonic frequency in the range of 20 kHz–800 kHz on the degradation of phenol, and it has been reported that the most effective frequency for degradation of phenol is 200 kHz [8]. In these studies, the effects of ultrasonic frequency, dissolved gas, and pH of solution on degradation efficiency have been investigated. However, there have been few investigations compared with other results reported from different laboratories. And, the ultrasonic horn or transducer is immersed directly in the sample solution. From the viewpoint of realizing the degradation process using ultrasound, indirect ultrasonic irradiation method is more advantageous than direct irradiation method. However, there have been few investigations on the effects of the type of irradiation method on degradation process. In addition, the rate of sonochemical reaction is influenced not only by frequency and ultrasonic intensity but also by the shape of a reactor, sample volume, etc. Especially, using indirect ultrasonic irradiation method, geometric relationship between reactor and ultrasonic transducer is an important factor of the rate of reaction. Several methods are available to estimate the amount of ultrasonic power entered into a solution. Calorimetry is one of the most
1350-4177/$ - see front matter Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2011.01.010
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common methods to estimate the mean ultrasonic power dissipated into the solution [9]. This method is based on the assumption that almost all the mechanical energy into heat. The ultrasonic output power, W is calculated by Eq. (1).
W ¼ ðdT=dtÞcp M
ð1Þ
Here, cp is the heat capacity of water, M is the mass of water, T is temperature of sample solution and t is ultrasonic irradiation time. Several chemical dosimetrys have also been proposed. KI oxidation dosimetry has been used as a popular chemical dosimeter [10]. When ultrasound is irradiated into an aqueous KI solution, I ions are oxidized to give I2. When excess I ions are present in solutions, I2 reacts with the excess I ion to form I3 ion, and the concentration of generated I3 ion is analyzed by a UV spectrometer at 355 nm. Kobayashi et al. have investigated the effects of the irradiation distance on polymerization of styrene and KI oxidation dosimetry using indirect ultrasonic irradiation method [11]. In emulsion polymerization process using indirect ultrasonic irradiation, the characteristics of the polymer are influenced by irradiation distance, and the indices of chemical and physical effects of ultrasound have been proposed [12–14]. In this study, indirect ultrasonic irradiation method was applied to degradation process of phenol, the model hazardous organic compound. We focused on the effects of the relationship between ultrasonic transducer and reactor on degradation of phenol, radical generation and ultrasonic power inside the reactor. Firstly, the effects of irradiation distance on radical generation and ultrasonic power were investigated by KI oxidation dosimetry and calorimetry. And, the effects of irradiation distance on degradation of phenol were compared with the abovementioned dosimetry. Finally, the zero-order kinetic model on the degradation of phenol was proposed. And, the effect of ultrasonic energy inside the reactor on degradation rate constant for various study was investigated. The purpose of this study was to examine the effective degradation process using indirect ultrasonic irradiation.
2. Experimental 2.1. Degradation of phenol Fig. 1 shows the entire experimental setup. Indirect ultrasonic irradiation was carried out with an ultrasonic cleaner (SC – 300D, SMT), and the ultrasonic frequency was operated at 28 kHz. In this equipment, seven ultrasonic transducers were set up, and the box with these transducers was installed in a water bath. A glass reactor with variable vertical position was located in the center of the transducers. The diameter of the glass reactor was 30 mm, and the volume of the glass reactor was about 50 106 m3. The temperature of the water bath was kept constant by a thermostat, and level of water bath was equalized with the level of the sample solution. Table 1 shows the experimental condition for degradation of phenol. Process variables were defined as follows: distance between the ultrasonic transducer and bottom of the reactor, l; distance between the ultrasonic transducer and level of the sample solution, l + h; and ultrasonic irradiation time, t. The output power of the ultrasonic generator, W; ultrasonic frequency, f; volume of the sample solution, V; temperature of water bath, T; and initial concentration of phenol, C Phenol;0 were kept constant. Before ultrasonic irradiation, the sample solution and the rest of the space in the reactor were deoxygenated by flowing nitrogen gas (99%, Toyoko Kagaku) for 30 min at 303 K. After deoxygenation, the sample was irradiated with ultrasound under a continuous flow of nitrogen gas.
Fig. 1. The entire experimental setup.
2.2. Analysis After ultrasonic irradiation, the concentration of phenol, CPhenol; the concentration of catechol, CCatechol; and the concentration of hydroquinone, CHydroquinone, were analyzed by HPLC system (Prominence, Shimadzu) equipped with a Luna column (Luna C18, Phenomenex) and a UV detector (SPD–20A, Shimadzu) at 280 nm. The mobile phase was acetonitrile (HPLC grade, Wako) and flow rate was 1.0 mL/min. The ultrasonic power in the reactor was measured by calorimetry, and ion exchanged water was used as a sample solution. The difference of an ultrasonic power in the reactor between phenol solution and ion exchanged water was considered to be small, because dilute phenol solution was used in this study. The chemical effect was estimated by KI oxidation dosimetry. In this study, we observed OH radical generation as a function of irradiation distance, and changes in OH radical generation caused by changing irradiation distance were analyzed by the value of the absorbance of I3 ion generated by KI oxidation. The absorbance at 355 nm was measured by UV spectrometer (U–1800, Hitachi). The concentration of KI was 0.10 mol/L and the KI solution volume was 10 106 m3.
3. Results and discussions 3.1. Effect of irradiation distance on ultrasonic power and OH radical generation Fig. 2 shows the effect of irradiation distance on absorbance of I3 ion and ultrasonic power in the reactor. Absorbance represented the amount of generated I3 ion by 15 min ultrasonic irradiation, and power represented the ultrasonic power measured Table 1 Experimental condition for degradation of phenol. W
[W] 300
f [kHz]
l [mm]
l+h [mm]
V [mL]
t [min]
T [K]
CPhenol, 0 [mol/m3]
28
10–60
25–75
10
0–360
303
1.06
D. Kobayashi et al. / Ultrasonics Sonochemistry 18 (2011) 1205–1210
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Fig. 2. Effect of irradiation distance on absorbance of I3 ion and ultrasonic power in the reactor.
by calorimetry. The absorbance and ultrasonic power fluctuated for the irradiation distance, and the length between peaks was about 30 mm. The wavelength of ultrasound using this study was about 53 mm. Therefore, there was a relationship between the period of the fluctuation of ultrasonic effect and the wavelength of ultrasound. Kobayashi et al. have reported that chemical effect evaluated by KI oxidation dosimetry fluctuates for the irradiation distance, and there is the relationship between the length between peaks and wavelength of ultrasound at f = 28 kHz and 45 kHz [11]. In addition, Soudagar and Samant have investigated the pressure intensity distribution in an ultrasonic cleaner, and the period of the position of maximum pressure intensity was related with wavelength of ultrasound [15]. Then, the fluctuation of ultrasonic effect in this study was agreed with the previous results. On the other hand, there was an error between half wavelength and period of the fluctuation of ultrasonic effect. The reason is found that the formation of standing wave changes with irradiation distance, because level of water bath was equalized with the level of the sample solution. 3.2. Degradation of phenol Fig. 3 shows the effect of irradiation distance on temporal change of concentration of phenol. The degradation efficiency of phenol is considered to be influenced by irradiation distance.
It is also found that there is a relationship between the variation in chemical effect along the vertical axis above the transducer and the degradation rate of phenol. In addition, before the decomposition conversion was less than 25%, the degradation of phenol was considered to progress in the zero-order kinetics. Fig. 4 shows the effect of irradiation distance on decomposition conversion of phenol at 180 min ultrasonic irradiation, x. The decomposition conversion fluctuated for the irradiation distance, and the decomposition conversion was large at l = 20 and 50 mm. In this study, the wavelength of the ultrasound in the water was about 53 mm. Thus, it was thought that there was a relationship between the variation in decomposition conversion along the vertical axis above the transducer and the wavelength of ultrasound, and the decomposition conversion was considered to be related to ultrasonic power and OH radical generation. On the other hand, from the results of the HPLC chromatogram, there were peaks except for phenol such as catechol and hydroquione. Fig. 5 shows the temporal change of concentration of generated catechol and hydroquione at l = 50 mm. Phenol was decomposed by oxidation reaction with OH radical, and catechol and hydroquione were considered to be generated firstly. However, considering the carbon balance, the quantity of generated catechol and hydroquione did not agree with the quantity of decomposed phenol.
Fig. 3. Effect of irradiation distance on temporal change of concentration of phenol.
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Fig. 4. Effect of irradiation distance on decomposition conversion of phenol at 180 min ultrasonic irradiation.
Fig. 5. Temporal change of concentration of catechol and hydroquione at l = 50 mm.
Fig. 6 shows the effect of decomposition conversion on selectivity of catechol and hydroquione for various irradiation distances. The selectivity of catechol and hydroquione was in the range of 15–45%. Then, the part of generated catechol and hydroquione were decomposed, and low molecular weight compounds such as formic acid and carbon dioxide were considered to be generated. On the other hand, it is found that the selectivity of catechol and hydroquione at l = 60 mm is larger than that of other irradiation distance. Then, generated catechol and hydroquione were difficult to be decomposed at l = 60 mm. From the results of KI oxidation dosimetry along the vertical axis above the transducer, the chemical effect at l = 60 mm was almost same at l = 20 mm. On the other hand, from the results of ultrasonic power along the vertical axis above the transducer, the ultrasonic power at l = 60 mm was smaller than other irradiation distance. It is suggested that the variation in chemical effect and physical effect along the vertical axis above the transducer are different. Then, it is found that irradiation distance influences not only decomposition conversion but also decomposition product.
3.3. Kinetic model on degradation of phenol The simple kinetic model on degradation of phenol was examined. Water was directly decomposed leading to the formation of OH radical and H radical.
H2 O ! OH þ H
ð2Þ
Phenol was decomposed by reaction with OH radical to form phenoxy radical.
OH þ PhOH ! PhO þ H2 O
ð3Þ
The reaction of phenoxy radical with OH radical was termination reaction.
PhO þ OH ! nonradical product
ð4Þ
Here, k1, k2 and k3 represented the rate constants of Eqs. (2)–(4), respectively. Assuming the pseudo steady state all radicals gives the rate of change in phenol concentration as Eq. (5).
dC Phenol ¼ k dt
ð5Þ
Here, k was defined as Eq. (6) and C Phenol represented the concentration of phenol.
k¼
k1 CH O 2 2
ð6Þ
Here, C H2 O represented the concentration of water. From the results of temporal change of concentration of phenol, the degradation of phenol was considered to progress in the zeroorder kinetics, before the decomposition conversion was less than 25%. The kinetic model proposed in this study was agreed with the experimental results of degradation. Therefore, the degradation
D. Kobayashi et al. / Ultrasonics Sonochemistry 18 (2011) 1205–1210
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Fig. 6. Effect of decomposition conversion on selectivity of catechol and hydroquione for various irradiation distances.
rate constant, k was estimated by fitting Eq. (5) to experimental data obtained under different irradiation distance. Fig. 7 shows the effect of ultrasonic power inside the reactor on the degradation rate constant. There was a linear relation in the degradation rate constant of phenol and the ultrasonic power inside the reactor. Therefore, the kinetic model proposed in this study was applied to the former study. Fig. 8 shows the effect of ultrasonic energy inside the reactor on degradation rate constant for various study. Here, ultrasonic energy inside the reactor, P was defined as the ultrasonic power inside the reactor per unit volume, and initial concentration of phenol was around 1 mol/m3. The symbols painted out in the white were the results in the range of frequency of 20–30 kHz, and the symbols painted out in the black were the results in the range of frequency of 200–800 kHz. In the results of 20 kHz, Kubo et al. [16] used a Branson Sonifier 250 (P was calculated using electric power), Pétrier and Francony [8] used a Branson Sonifier 450, and Pétrier et al. (CPhenol, 0 was 0.5 mol/m3) [17] used Branson Sonifier 450, and a titanium horn was immersed in the sample solution. In the results over 200 kHz, Pétrier and Francony [8], and Pétrier et al. (CPhenol, 0 was 0.5 mol/m3) [17] used a homemade high-frequency power supply, and a transducer was set at the bottom of the reactor.
There was a linear relation in the degradation rate constant of phenol and the ultrasonic energy in the range of frequency of 20–30 kHz in spite of the difference of equipment and sample volume. On the other hand, the degradation rate constant in the range of frequency of 200–800 kHz was much larger than that of 20– 30 kHz in the same ultrasonic energy. The effect of ultrasonic frequency on chemical efficiency has been investigated in various reactions [8,10,18–23]. In many studies, the maximum sonochemical effects were observed around 300 kHz. And, it has been reported that the sonochemical effects in the range of frequency of 200–500 kHz are 10 times larger than those in the low and high frequency regions [10]. In degradation of phenol, the degradation rate constant at 200 kHz was 10 times larger than that at 20 kHz in the condition of same ultrasonic energy. Therefore, the degradation rate constant calculated by kinetic model on degradation of phenol proposed this study was able to estimate using the sonochemical effect. On the other hand, multibubble sonoluminescence intensity and chemical effects rapidly quench around 60 W [24,25], and a linear relation in the degradation rate constant of phenol and the ultrasonic energy is considered to be saturated with increasing ultrasonic energy.
Fig. 7. Effect of ultrasonic power inside the reactor on degradation rate constant.
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Fig. 8. Effect of ultrasonic energy on degradation rate constant for various study.
4. Conclusions
References
The degradation process of phenol using indirect ultrasonic irradiation system was constructed. Phenol was oxidized with OH radical, and catechol and hydroquione were generated in an ultrasonic field. And, the part of generated catechol and hydroquione were decomposed by oxidation reaction, and low molecular weight compounds were generated. The decomposition conversion of phenol was influenced by irradiation distance. The effects of irradiation distance on the degradation rate overlapped with its effects on OH radical generation and ultrasonic power. The simple kinetic model on degradation of phenol was proposed, and the order of reaction was zero-order. There was a linear relation on the degradation rate constant and ultrasonic energy inside the reactor, and there were no effects of ultrasonic generator and shape of reactor in the range of frequency of 20–30 kHz. It is important to increase the ultrasonic power introduced to the reactor for effective degradation process using indirect ultrasonic irradiation.
[1] C. Kormann, D.W. Bahnemann, M.R. Hoffmann, Environ. Sci. Tech. 25 (1991) 494. [2] H.R. Devlln, I.J. Harris, Ind. Eng. Chem. Fundam. 23 (1984) 387. [3] Y.G. Adewuri, Ind. Eng. Chem. Res. 40 (2001) 4681. [4] P. Chowdhury, T. Viraraghavan, Sci. Total Environ. 407 (2009) 2474. [5] R. Kidak, N.H. Ince, Ultrason. Sonochem. 13 (2006) 195. [6] J. Berlan, F. Trabelsi, H. Delmas, A.M. Wilhelm, J.F. Petrignani, Ultrason. Sonochem. 1 (1994) S97. [7] N. Serpone, R. Terzian, P. Colarusso, Res. Chem. Inter. 18 (1992) 183. [8] C. Pétrier, A. Francony, Ultrason. Sonochem. 4 (1997) 295. [9] R.F. Contamine, A.M. Wilhelm, J. Berlan, H. Delmas, Ultrason. Sonochem. 2 (1995) S43. [10] S. Koda, T. Kimura, T. Kondo, H. Mitome, Ultrason. Sonochem. 10 (2003) 149. [11] D. Kobayashi, H. Matsumoto, C. Kuroda, Kagaku Kogaku Ronbunshu (Japanese) 32 (2006) 88. [12] D. Kobayashi, H. Matsumoto, C. Kuroda, Ultrason. Sonochem. 15 (2008) 251. [13] D. Kobayashi, H. Matsumoto, C. Kuroda, Chem. Eng. J. 135 (2008) 43. [14] D. Kobayashi, H. Matsumoto, C. Kuroda, Kagaku Kogaku Ronbunshu (Japanese) 34 (2008) 254. [15] S.R. Soudagar, S.D. Samant, Ultrason. Sonochem. 2 (1995) S49. [16] M. Kubo, K. Matsuoka, A. Takahashi, N.-S. Kitakawa, T. Yonemoto, Ultrason. Sonochem. 12 (2005) 263. [17] C. Pétrier, M.-F. Lamy, A. Francony, A. Benahcene, B. David, J. Phys. Chem. 98 (1994) 10514. [18] C. Pétrier, A. Jeunet, J.-L. Luche, G. Reverdy, J. Am. Chem. Soc. 114 (1992) 3148. [19] G. Mark, A. Tauber, R. Laupert, H.-P. Schuchmann, D. Schulz, A. Mues, C. von Sonntag, Ultrason. Sonochem. 5 (1998) 41. [20] H.-M. Hung, M.R. Hoffmann, J. Phys. Chem. A 103 (1999) 2734. [21] Y. Kojima, S. Koda, H. Nomura, Jpn. J. Appl. Phys. 37 (1998) 2992. [22] Y. Kojima, S. Koda, H. Nomura, Ultrason. Sonochem. 8 (2001) 75. [23] M.A. Beckett, I. Hua, J. Phys. Chem. A 105 (2001) 3796. [24] S. Hatanaka, K. Yasui, T. Kozuka, T. Tuziuti, H. Mitome, Ultrasonics 40 (2002) 655. [25] A. Henglein, M. Gutierrez, J. Phys. Chem. 94 (1990) 5169.
Acknowledgements This research was supported in part by a Grant – in – Aid for Scientific Research (No. 20760524) from the Ministry of Education, Culture, Sports, Science and Technology.
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultsonch
Effect of irradiation distance on degradation of phenol using indirect ultrasonic irradiation method Daisuke Kobayashi a,⇑, Kazuki Sano b, Yusuke Takeuchi b, Koichi Terasaka a a b
Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan School of Science for Open and Environmental Systems, Graduate School of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan
a r t i c l e
i n f o
Article history: Received 10 May 2010 Received in revised form 18 January 2011 Accepted 25 January 2011 Available online 20 February 2011 Keywords: Degradation Phenol Indirect ultrasonic irradiation Irradiation distance Kinetics analysis
a b s t r a c t Ultrasound is used as degradation of hazardous organic compounds. In this study, indirect ultrasonic irradiation method was applied to the degradation process of phenol, the model hazardous organic compound, and the effects of irradiation distance on radical generation and ultrasonic power were investigated. The chemical effect estimated by KI oxidation dosimetry and ultrasonic power measured by calorimetry fluctuated for the irradiation distance, and there was a relationship between the period of the fluctuation of ultrasonic effect and the wavelength of ultrasound. The degradation of phenol was considered to progress in the zero-order kinetics, before the decomposition conversion was less than 25%. Therefore, the simple kinetic model on degradation of phenol was proposed, and there was a linear relation in the degradation rate constant of phenol and the ultrasonic power inside the reactor. In addition, the kinetic model proposed in this study was applied to the former study. There was a linear relation in the degradation rate constant of phenol and ultrasonic energy in the range of frequency of 20–30 kHz in spite of the difference of equipment and sample volume. On the other hand, the degradation rate constant in the range of frequency of 200–800 kHz was much larger than that of 20–30 kHz in the same ultrasonic energy, and this behaviour was agreed with the former investigation about the dependence of ultrasonic frequency on chemical effect. Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved.
1. Introduction Recently, techniques for the degradation of hazardous organic compounds have been investigated. UV light irradiation to TiO2 photocatalysis is one of the favorite techniques [1]. The positive holes generated by UV light irradiation in the vicinity of the surface of TiO2 particles react with water to produce OH radicals. Devlln and Harris have proposed the detailed reaction scheme of the oxidation of aqueous phenol by oxygen [2], and the photocatalytic degradation of phenol is considered to progress similar reaction scheme. However, UV light is screened by TiO2 particles, so that there is a limit amount of TiO2 particle addition in which degradation rate increases. Ultrasound has been investigated as one of the techniques for degradation of hazardous organic compounds [3,4]. Especially, degradation of phenol and some of its derivatives such as chlorophenol and nitrophenol using ultrasound has been investigated by many researchers [5]. Berlan et al. have investigated the effects of pressure, dissolved gas and ultrasonic frequency on the degrada⇑ Corresponding author. Address: Department of Industrial Chemistry, Faculty of Engineering, Tokyo University of Science, Tokyo 162-8601, Japan. Tel.: +81 3 5228 8310; fax: +81 3 5261 4361. E-mail address: [email protected] (D. Kobayashi).
tion of phenol, and the reaction scheme of degradation of phenol has been proposed [6]. Serpone et al. have reported that the degradation of phenol progresses in the zero-order kinetics at 20 kHz [7]. Pétrier and Francony have investigated the effect of ultrasonic frequency in the range of 20 kHz–800 kHz on the degradation of phenol, and it has been reported that the most effective frequency for degradation of phenol is 200 kHz [8]. In these studies, the effects of ultrasonic frequency, dissolved gas, and pH of solution on degradation efficiency have been investigated. However, there have been few investigations compared with other results reported from different laboratories. And, the ultrasonic horn or transducer is immersed directly in the sample solution. From the viewpoint of realizing the degradation process using ultrasound, indirect ultrasonic irradiation method is more advantageous than direct irradiation method. However, there have been few investigations on the effects of the type of irradiation method on degradation process. In addition, the rate of sonochemical reaction is influenced not only by frequency and ultrasonic intensity but also by the shape of a reactor, sample volume, etc. Especially, using indirect ultrasonic irradiation method, geometric relationship between reactor and ultrasonic transducer is an important factor of the rate of reaction. Several methods are available to estimate the amount of ultrasonic power entered into a solution. Calorimetry is one of the most
1350-4177/$ - see front matter Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2011.01.010
1206
D. Kobayashi et al. / Ultrasonics Sonochemistry 18 (2011) 1205–1210
common methods to estimate the mean ultrasonic power dissipated into the solution [9]. This method is based on the assumption that almost all the mechanical energy into heat. The ultrasonic output power, W is calculated by Eq. (1).
W ¼ ðdT=dtÞcp M
ð1Þ
Here, cp is the heat capacity of water, M is the mass of water, T is temperature of sample solution and t is ultrasonic irradiation time. Several chemical dosimetrys have also been proposed. KI oxidation dosimetry has been used as a popular chemical dosimeter [10]. When ultrasound is irradiated into an aqueous KI solution, I ions are oxidized to give I2. When excess I ions are present in solutions, I2 reacts with the excess I ion to form I3 ion, and the concentration of generated I3 ion is analyzed by a UV spectrometer at 355 nm. Kobayashi et al. have investigated the effects of the irradiation distance on polymerization of styrene and KI oxidation dosimetry using indirect ultrasonic irradiation method [11]. In emulsion polymerization process using indirect ultrasonic irradiation, the characteristics of the polymer are influenced by irradiation distance, and the indices of chemical and physical effects of ultrasound have been proposed [12–14]. In this study, indirect ultrasonic irradiation method was applied to degradation process of phenol, the model hazardous organic compound. We focused on the effects of the relationship between ultrasonic transducer and reactor on degradation of phenol, radical generation and ultrasonic power inside the reactor. Firstly, the effects of irradiation distance on radical generation and ultrasonic power were investigated by KI oxidation dosimetry and calorimetry. And, the effects of irradiation distance on degradation of phenol were compared with the abovementioned dosimetry. Finally, the zero-order kinetic model on the degradation of phenol was proposed. And, the effect of ultrasonic energy inside the reactor on degradation rate constant for various study was investigated. The purpose of this study was to examine the effective degradation process using indirect ultrasonic irradiation.
2. Experimental 2.1. Degradation of phenol Fig. 1 shows the entire experimental setup. Indirect ultrasonic irradiation was carried out with an ultrasonic cleaner (SC – 300D, SMT), and the ultrasonic frequency was operated at 28 kHz. In this equipment, seven ultrasonic transducers were set up, and the box with these transducers was installed in a water bath. A glass reactor with variable vertical position was located in the center of the transducers. The diameter of the glass reactor was 30 mm, and the volume of the glass reactor was about 50 106 m3. The temperature of the water bath was kept constant by a thermostat, and level of water bath was equalized with the level of the sample solution. Table 1 shows the experimental condition for degradation of phenol. Process variables were defined as follows: distance between the ultrasonic transducer and bottom of the reactor, l; distance between the ultrasonic transducer and level of the sample solution, l + h; and ultrasonic irradiation time, t. The output power of the ultrasonic generator, W; ultrasonic frequency, f; volume of the sample solution, V; temperature of water bath, T; and initial concentration of phenol, C Phenol;0 were kept constant. Before ultrasonic irradiation, the sample solution and the rest of the space in the reactor were deoxygenated by flowing nitrogen gas (99%, Toyoko Kagaku) for 30 min at 303 K. After deoxygenation, the sample was irradiated with ultrasound under a continuous flow of nitrogen gas.
Fig. 1. The entire experimental setup.
2.2. Analysis After ultrasonic irradiation, the concentration of phenol, CPhenol; the concentration of catechol, CCatechol; and the concentration of hydroquinone, CHydroquinone, were analyzed by HPLC system (Prominence, Shimadzu) equipped with a Luna column (Luna C18, Phenomenex) and a UV detector (SPD–20A, Shimadzu) at 280 nm. The mobile phase was acetonitrile (HPLC grade, Wako) and flow rate was 1.0 mL/min. The ultrasonic power in the reactor was measured by calorimetry, and ion exchanged water was used as a sample solution. The difference of an ultrasonic power in the reactor between phenol solution and ion exchanged water was considered to be small, because dilute phenol solution was used in this study. The chemical effect was estimated by KI oxidation dosimetry. In this study, we observed OH radical generation as a function of irradiation distance, and changes in OH radical generation caused by changing irradiation distance were analyzed by the value of the absorbance of I3 ion generated by KI oxidation. The absorbance at 355 nm was measured by UV spectrometer (U–1800, Hitachi). The concentration of KI was 0.10 mol/L and the KI solution volume was 10 106 m3.
3. Results and discussions 3.1. Effect of irradiation distance on ultrasonic power and OH radical generation Fig. 2 shows the effect of irradiation distance on absorbance of I3 ion and ultrasonic power in the reactor. Absorbance represented the amount of generated I3 ion by 15 min ultrasonic irradiation, and power represented the ultrasonic power measured Table 1 Experimental condition for degradation of phenol. W
[W] 300
f [kHz]
l [mm]
l+h [mm]
V [mL]
t [min]
T [K]
CPhenol, 0 [mol/m3]
28
10–60
25–75
10
0–360
303
1.06
D. Kobayashi et al. / Ultrasonics Sonochemistry 18 (2011) 1205–1210
1207
Fig. 2. Effect of irradiation distance on absorbance of I3 ion and ultrasonic power in the reactor.
by calorimetry. The absorbance and ultrasonic power fluctuated for the irradiation distance, and the length between peaks was about 30 mm. The wavelength of ultrasound using this study was about 53 mm. Therefore, there was a relationship between the period of the fluctuation of ultrasonic effect and the wavelength of ultrasound. Kobayashi et al. have reported that chemical effect evaluated by KI oxidation dosimetry fluctuates for the irradiation distance, and there is the relationship between the length between peaks and wavelength of ultrasound at f = 28 kHz and 45 kHz [11]. In addition, Soudagar and Samant have investigated the pressure intensity distribution in an ultrasonic cleaner, and the period of the position of maximum pressure intensity was related with wavelength of ultrasound [15]. Then, the fluctuation of ultrasonic effect in this study was agreed with the previous results. On the other hand, there was an error between half wavelength and period of the fluctuation of ultrasonic effect. The reason is found that the formation of standing wave changes with irradiation distance, because level of water bath was equalized with the level of the sample solution. 3.2. Degradation of phenol Fig. 3 shows the effect of irradiation distance on temporal change of concentration of phenol. The degradation efficiency of phenol is considered to be influenced by irradiation distance.
It is also found that there is a relationship between the variation in chemical effect along the vertical axis above the transducer and the degradation rate of phenol. In addition, before the decomposition conversion was less than 25%, the degradation of phenol was considered to progress in the zero-order kinetics. Fig. 4 shows the effect of irradiation distance on decomposition conversion of phenol at 180 min ultrasonic irradiation, x. The decomposition conversion fluctuated for the irradiation distance, and the decomposition conversion was large at l = 20 and 50 mm. In this study, the wavelength of the ultrasound in the water was about 53 mm. Thus, it was thought that there was a relationship between the variation in decomposition conversion along the vertical axis above the transducer and the wavelength of ultrasound, and the decomposition conversion was considered to be related to ultrasonic power and OH radical generation. On the other hand, from the results of the HPLC chromatogram, there were peaks except for phenol such as catechol and hydroquione. Fig. 5 shows the temporal change of concentration of generated catechol and hydroquione at l = 50 mm. Phenol was decomposed by oxidation reaction with OH radical, and catechol and hydroquione were considered to be generated firstly. However, considering the carbon balance, the quantity of generated catechol and hydroquione did not agree with the quantity of decomposed phenol.
Fig. 3. Effect of irradiation distance on temporal change of concentration of phenol.
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Fig. 4. Effect of irradiation distance on decomposition conversion of phenol at 180 min ultrasonic irradiation.
Fig. 5. Temporal change of concentration of catechol and hydroquione at l = 50 mm.
Fig. 6 shows the effect of decomposition conversion on selectivity of catechol and hydroquione for various irradiation distances. The selectivity of catechol and hydroquione was in the range of 15–45%. Then, the part of generated catechol and hydroquione were decomposed, and low molecular weight compounds such as formic acid and carbon dioxide were considered to be generated. On the other hand, it is found that the selectivity of catechol and hydroquione at l = 60 mm is larger than that of other irradiation distance. Then, generated catechol and hydroquione were difficult to be decomposed at l = 60 mm. From the results of KI oxidation dosimetry along the vertical axis above the transducer, the chemical effect at l = 60 mm was almost same at l = 20 mm. On the other hand, from the results of ultrasonic power along the vertical axis above the transducer, the ultrasonic power at l = 60 mm was smaller than other irradiation distance. It is suggested that the variation in chemical effect and physical effect along the vertical axis above the transducer are different. Then, it is found that irradiation distance influences not only decomposition conversion but also decomposition product.
3.3. Kinetic model on degradation of phenol The simple kinetic model on degradation of phenol was examined. Water was directly decomposed leading to the formation of OH radical and H radical.
H2 O ! OH þ H
ð2Þ
Phenol was decomposed by reaction with OH radical to form phenoxy radical.
OH þ PhOH ! PhO þ H2 O
ð3Þ
The reaction of phenoxy radical with OH radical was termination reaction.
PhO þ OH ! nonradical product
ð4Þ
Here, k1, k2 and k3 represented the rate constants of Eqs. (2)–(4), respectively. Assuming the pseudo steady state all radicals gives the rate of change in phenol concentration as Eq. (5).
dC Phenol ¼ k dt
ð5Þ
Here, k was defined as Eq. (6) and C Phenol represented the concentration of phenol.
k¼
k1 CH O 2 2
ð6Þ
Here, C H2 O represented the concentration of water. From the results of temporal change of concentration of phenol, the degradation of phenol was considered to progress in the zeroorder kinetics, before the decomposition conversion was less than 25%. The kinetic model proposed in this study was agreed with the experimental results of degradation. Therefore, the degradation
D. Kobayashi et al. / Ultrasonics Sonochemistry 18 (2011) 1205–1210
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Fig. 6. Effect of decomposition conversion on selectivity of catechol and hydroquione for various irradiation distances.
rate constant, k was estimated by fitting Eq. (5) to experimental data obtained under different irradiation distance. Fig. 7 shows the effect of ultrasonic power inside the reactor on the degradation rate constant. There was a linear relation in the degradation rate constant of phenol and the ultrasonic power inside the reactor. Therefore, the kinetic model proposed in this study was applied to the former study. Fig. 8 shows the effect of ultrasonic energy inside the reactor on degradation rate constant for various study. Here, ultrasonic energy inside the reactor, P was defined as the ultrasonic power inside the reactor per unit volume, and initial concentration of phenol was around 1 mol/m3. The symbols painted out in the white were the results in the range of frequency of 20–30 kHz, and the symbols painted out in the black were the results in the range of frequency of 200–800 kHz. In the results of 20 kHz, Kubo et al. [16] used a Branson Sonifier 250 (P was calculated using electric power), Pétrier and Francony [8] used a Branson Sonifier 450, and Pétrier et al. (CPhenol, 0 was 0.5 mol/m3) [17] used Branson Sonifier 450, and a titanium horn was immersed in the sample solution. In the results over 200 kHz, Pétrier and Francony [8], and Pétrier et al. (CPhenol, 0 was 0.5 mol/m3) [17] used a homemade high-frequency power supply, and a transducer was set at the bottom of the reactor.
There was a linear relation in the degradation rate constant of phenol and the ultrasonic energy in the range of frequency of 20–30 kHz in spite of the difference of equipment and sample volume. On the other hand, the degradation rate constant in the range of frequency of 200–800 kHz was much larger than that of 20– 30 kHz in the same ultrasonic energy. The effect of ultrasonic frequency on chemical efficiency has been investigated in various reactions [8,10,18–23]. In many studies, the maximum sonochemical effects were observed around 300 kHz. And, it has been reported that the sonochemical effects in the range of frequency of 200–500 kHz are 10 times larger than those in the low and high frequency regions [10]. In degradation of phenol, the degradation rate constant at 200 kHz was 10 times larger than that at 20 kHz in the condition of same ultrasonic energy. Therefore, the degradation rate constant calculated by kinetic model on degradation of phenol proposed this study was able to estimate using the sonochemical effect. On the other hand, multibubble sonoluminescence intensity and chemical effects rapidly quench around 60 W [24,25], and a linear relation in the degradation rate constant of phenol and the ultrasonic energy is considered to be saturated with increasing ultrasonic energy.
Fig. 7. Effect of ultrasonic power inside the reactor on degradation rate constant.
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Fig. 8. Effect of ultrasonic energy on degradation rate constant for various study.
4. Conclusions
References
The degradation process of phenol using indirect ultrasonic irradiation system was constructed. Phenol was oxidized with OH radical, and catechol and hydroquione were generated in an ultrasonic field. And, the part of generated catechol and hydroquione were decomposed by oxidation reaction, and low molecular weight compounds were generated. The decomposition conversion of phenol was influenced by irradiation distance. The effects of irradiation distance on the degradation rate overlapped with its effects on OH radical generation and ultrasonic power. The simple kinetic model on degradation of phenol was proposed, and the order of reaction was zero-order. There was a linear relation on the degradation rate constant and ultrasonic energy inside the reactor, and there were no effects of ultrasonic generator and shape of reactor in the range of frequency of 20–30 kHz. It is important to increase the ultrasonic power introduced to the reactor for effective degradation process using indirect ultrasonic irradiation.
[1] C. Kormann, D.W. Bahnemann, M.R. Hoffmann, Environ. Sci. Tech. 25 (1991) 494. [2] H.R. Devlln, I.J. Harris, Ind. Eng. Chem. Fundam. 23 (1984) 387. [3] Y.G. Adewuri, Ind. Eng. Chem. Res. 40 (2001) 4681. [4] P. Chowdhury, T. Viraraghavan, Sci. Total Environ. 407 (2009) 2474. [5] R. Kidak, N.H. Ince, Ultrason. Sonochem. 13 (2006) 195. [6] J. Berlan, F. Trabelsi, H. Delmas, A.M. Wilhelm, J.F. Petrignani, Ultrason. Sonochem. 1 (1994) S97. [7] N. Serpone, R. Terzian, P. Colarusso, Res. Chem. Inter. 18 (1992) 183. [8] C. Pétrier, A. Francony, Ultrason. Sonochem. 4 (1997) 295. [9] R.F. Contamine, A.M. Wilhelm, J. Berlan, H. Delmas, Ultrason. Sonochem. 2 (1995) S43. [10] S. Koda, T. Kimura, T. Kondo, H. Mitome, Ultrason. Sonochem. 10 (2003) 149. [11] D. Kobayashi, H. Matsumoto, C. Kuroda, Kagaku Kogaku Ronbunshu (Japanese) 32 (2006) 88. [12] D. Kobayashi, H. Matsumoto, C. Kuroda, Ultrason. Sonochem. 15 (2008) 251. [13] D. Kobayashi, H. Matsumoto, C. Kuroda, Chem. Eng. J. 135 (2008) 43. [14] D. Kobayashi, H. Matsumoto, C. Kuroda, Kagaku Kogaku Ronbunshu (Japanese) 34 (2008) 254. [15] S.R. Soudagar, S.D. Samant, Ultrason. Sonochem. 2 (1995) S49. [16] M. Kubo, K. Matsuoka, A. Takahashi, N.-S. Kitakawa, T. Yonemoto, Ultrason. Sonochem. 12 (2005) 263. [17] C. Pétrier, M.-F. Lamy, A. Francony, A. Benahcene, B. David, J. Phys. Chem. 98 (1994) 10514. [18] C. Pétrier, A. Jeunet, J.-L. Luche, G. Reverdy, J. Am. Chem. Soc. 114 (1992) 3148. [19] G. Mark, A. Tauber, R. Laupert, H.-P. Schuchmann, D. Schulz, A. Mues, C. von Sonntag, Ultrason. Sonochem. 5 (1998) 41. [20] H.-M. Hung, M.R. Hoffmann, J. Phys. Chem. A 103 (1999) 2734. [21] Y. Kojima, S. Koda, H. Nomura, Jpn. J. Appl. Phys. 37 (1998) 2992. [22] Y. Kojima, S. Koda, H. Nomura, Ultrason. Sonochem. 8 (2001) 75. [23] M.A. Beckett, I. Hua, J. Phys. Chem. A 105 (2001) 3796. [24] S. Hatanaka, K. Yasui, T. Kozuka, T. Tuziuti, H. Mitome, Ultrasonics 40 (2002) 655. [25] A. Henglein, M. Gutierrez, J. Phys. Chem. 94 (1990) 5169.
Acknowledgements This research was supported in part by a Grant – in – Aid for Scientific Research (No. 20760524) from the Ministry of Education, Culture, Sports, Science and Technology.