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Sonochemical degradation for toxic halogenated organic compounds
Ultrasonics Sonochemistry 8 (2001) 241±246
www.elsevier.nl/locate/ultsonch
Sonochemical degradation for toxic halogenated organic compounds Khay Chuan Teo *, Yanrong Xu, Chun Yang Academic Group of Natural Sciences, National Institute of Education, Nanyang Technological University, 1 Nanyang Walk, Singapore 637616, Singapore
Abstract This paper describes the degradation of p-chlorophenol using three dierent ultrasonic devices. The dissipated power in the reaction matrix was measured based on calorimetric method. The study showed that hydrogen peroxide can improve the sonochemical reaction and gases dissolved in reaction matrix can aect the process to a small extent. The reaction mechanism and kinetics of degradation were also investigated. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Sonochemical; Degradation; p-Chlorophenol
1. Introduction Nowadays, millions of organic compounds have been synthesized, and many of these chemicals have been released and appeared as environmental pollutants. Currently the technologies in treatment of organic pollutants include solvent extraction, incineration, chemical dehalogenation and biodegradation, etc. Since 1990, there has been an increase of interest in the use of ultrasound to destroy organic contaminants present in water and/or wastewater, and the scaleup of the corresponding technologies are also under development [1±4]. Promisingly, sonochemical degradation is capable of being developed into a successful technology for environmental clean up. Extensive basic R&D working toward the maturity is required before the process can be called a proven technology. This paper mainly focused on the application of ultrasonic devices in the degradation of p-chlorophenol which often presents as a priority organic pollutant in environment. 2. Experimental Three types of ultrasonic devices including beaker system, cup±horn system and probe system were em-
*
Corresponding author. Tel.: +65-790-3849; fax: +65-896-9432. E-mail address: [email protected] (K.C. Teo).
ployed in this study (Fig. 1). The 140 W, 850 kHz plane ultrasound generator (Fig. 1(A)) manufactured by K.W. Meinhardt-Ultraschalutechnik (Germany). Cup±horn (Fig. 1(B)) and the probe system (Fig. 1(C)) are both available with power output rating of the 475 W, 20 kHz XL2020 ultrasonic processor supplied by Heat system Inc. (USA). The base diameter of beaker system is 9.5 cm, the diameter of cup±horn transducer is 7 cm and the tip diameter of the probe is 1.5 cm. Their emitting surface areas are calculated to be 70.8, 38.5 and 1.8 cm2 , respectively. In the case of beaker system and cup±horn system, the glass reactor was located at 0.5 cm above the base of the ultrasonic generator. The p-chlorophenol aqueous solution was prepared by dissolving the HPLC grade standard into reagent water and diluted to achieve necessary concentrations. A reaction vessel with 100 ml p-chlorophenol aqueous solution was placed in the ultrasonic device and air-equilibrated under atmospheric pressure. The temperature was measured by a multi-channel scanning thermocouple and controlled by the water body in the beaker system, which in turn was maintained by an external cooling ¯uid. The temperature during the reaction process normally ranged from 36°C to 39°C. The sample was collected in a settled minute interval. The analysis was performed on Varian HPLC equipped with Varian 9050Q solvent delivery system, Varian 9100 autosampler and 9012 variable wavelength UV±VIS detector.
1350-4177/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 5 0 - 4 1 7 7 ( 0 1 ) 0 0 0 8 3 - 9
242
K.C. Teo et al. / Ultrasonics Sonochemistry 8 (2001) 241±246
Fig. 1. Schematic diagram of ultrasonic devices used in this study: (A) beaker system; (B) cup±horn system; (C) probe system.
K.C. Teo et al. / Ultrasonics Sonochemistry 8 (2001) 241±246
243
3. Results and discussion 3.1. Comparison of three ultrasonic device by calorimetric power measurement The sonochemical power irradiated into the reaction system is dierent for the individual instrument. It is dicult to compare the sonochemical results from different laboratory without the power output rating value, which is well known as the reproducibility problem in sonochemistry. So far, several methods are available to determine the acoustic power. The most common method is calorimetry, which assumes that all of the energy delivered to the system is dissipated as heat [5]. The power dissipated (Pdiss ) in a reaction mixture can be obtained using Eq. (1) dT Pdiss MCP 1 dt t0 where CP is heat capacity of water, M is mass of water, and dT =dtt0 represents initial slope of the temperature rise of the reaction mixture versus time of exposure to ultrasonic irradiation. The measurement of the acoustic power was performed for the ultrasonic systems used for this study according to the method mentioned above. Assuming 100 g water was used in each system, the values of dT =dt and the power for three devices can be calculated according to Eq. (1) and the experimental results were provided in Table 1. As can be seen from the experimental results, compared with the cup±horn system, beaker system has higher power output due to its higher frequency at 850 kHz. In general, higher frequency leads to higher power output of ultrasonic device. Probe system has the higher value of dT =dt and power output than cup±horn system for the given frequency at 20 kHz, it is well known that probe system is a more ecient device of transmitting ultrasonic energy into a reaction matrix due to its direct sonication. Whereas, the two other ultrasonic devices provide indirect sonication, which will inevitably cause energy loss while the reaction matrix contacts with mechanical vibration indirectly. In order to con®rm these results, the three ultrasonic devices were utilized to degradate p-chlorophenol in water. The results given in Fig. 2 illustrated that within 20 min 50% of p-chlorophenol was degradaded with the probe system and 25% with the beaker system. HowTable 1 Measurement of power dissipated for dierent ultrasonic devices dT =dtt0 Power (W) R2
Probe system
Beaker system
Cup±horn system
0:313 0:011 130:0 4:6 0.9999
0:0570 0:0018 23:8 0:8 0.9969
0:0462 0:0020 19:3 0:8 0.9998
Fig. 2. p-chlorophenol concentration (0.090 mM, 100 ml used) versus time using three ultrasonic devices.
ever, only 2% of p-chlorophenol was degradated with the cup±horn system. These results agree with the previous conclusions derived from calorimetric measurements. It appeared that the probe system is the most appropriate sonochemical system for the treatment of pchlorophenol in water due to its highest degradation eciency within a shorter time. However, the concentration of p-chlorophenol in reaction solution did not decrease any more, which seemed that p-chlorophenol degradation process reaches an equilibrium around 20 min. It is interesting to note that though p-chlorophenol was degradated slowly with beaker type ultrasonic apparatus, degradation eciency higher enough approaching 100% can be achieved by the beaker system. In addition, the sonochemical intermediates can be readily observed in the degradation process with beaker system which is essential to the investigation of the mechanism of sonochemical reactions. The following approach on the degradation of p-chlorophenol was performed using the beaker system. 3.2. Eect of hydrogen peroxide on the degradation of p-chlorophenol According to the theory of hot spot, the temperature and pressure of localized hot spots are formed can excessively reach 5000 K and 500 atm respectively in the ultrasonic cavitation [6]. Under these conditions, hydrogen peroxide readily decomposes into hydroxyl radicals which will accelerate the sonication. Fig. 3 presented the initial rate of sonochemical reaction of 0.4 mM p-chlorophenol aqueous solution in the presence of hydrogen peroxide using beaker system. It was observed that the initial rate increases when the concentration of hydrogen peroxide increases and the value reaches a maximum at approximately 30 mM. In this case, the initial rate of sonication can substantially accelerate by
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K.C. Teo et al. / Ultrasonics Sonochemistry 8 (2001) 241±246
Fig. 3. Eect of hydrogen peroxide on sonochemical degradation (0.395 mM, 100 ml p-chlorophenol used).
twofolds and the degradation time can be reduced by 20 min compared with the corresponding ones in the absence of hydrogen peroxide. Further experiments also demonstrated that no obvious degradation of p-chlorophenol was observed only using hydrogen peroxide. Therefore it can be concluded that sonication controls the overall degradation process and hydrogen peroxide only play an auxiliary role. 3.3. Eect of bubble gases on the degradation of pchlorophenol in water As well known, the propagation of ultrasonic wave in liquid generates cavitation bubbles, the growth and explosion of these bubbles enable the production of certain species of radicals. It is expected that sparging reaction matrix with certain gases probably facilitates the formation of cavitation bubbles and subsequently accelerates the sonication and improves the degradation eciency. In this study, dierent gases such as argon and air were introduced into the reaction matrix by a fritted dispersion glass tube. Fig. 4 shows the ultrasonic degradation of p-chlorophenol in water in the presence of external gases. The initial rate for the ultrasonic reaction with air is higher than the corresponding one with argon gas. This is attributed to oxygen in air which is believed to facilitate the formation of hydroxyl radicals and accelerate the reaction. However, the experimental results also illustrated that the introduction of gases will not absolutely lead to rapid degradation rate and high eciency, because the formed cavitation bubbles can be possibly interrupted or destroyed by the sparging gas.
Fig. 4. Eect of gas on sonochemical degradation. The initial rate and rate constant are 2:90 0:14 lM/min, 0:00646 0:00032 min 1 for argon gas and 4:04 0:28 lM/min, 0:00899 0:00062 min 1 air gas, respectively (0.449 mM, 100 ml p-chlorophenol used).
photometer. The UV spectra of reaction matrix at different time were used to monitor the progress of the degradation. p-Chlorophenol aqueous solution has three strong absorbance at 204, 230 and 280 nm respectively. These peaks shifted when the sonochemical reaction proceeded and only one obvious UV absorbance was observed at 60 min. It can be concluded that p-chlorophenol was thoroughly degradaded, which has been veri®ed by HPLC analysis. In order to investigate the kinetic of sonochemical reaction of p-chlorophenol in water, a plot of the logarithmic value of C=C0 and ultrasonic time yielded a straight line with an equation ln C=C0 0:190 0:050t (Fig. 5). The correlation coecient of 0.9888 indicated the reasonableness of the straight line ®t. The
3.4. Kinetic of sonochemical reaction of p-chlorophenol in water The qualitative determination of the analytes was carried out during the experiments using UV spectro-
Fig. 5. Kinetic of ultrasonic transformation of p-chlorophenol (0.395 mM, 30 ml used). The initial rate and rate constant are 19:75 1:18 lM/min and 0:0500 0:003 min 1 .
K.C. Teo et al. / Ultrasonics Sonochemistry 8 (2001) 241±246
245
linear relationship between ln C=C0 and time demonstrated that the overall degradation follows a pseudo®rst-order kinetic rate law. The rate constant was obtained from the slope of this straight line and the initial rate can be calculated to be 19:75 1:18 lM/min by the equation vi kC0 . 3.5. Mechanism of the ultrasonic degradation of pchlorophenol in water The acidity of reaction matrix was monitored by a pH meter. Fig. 6 showed the pH evolution with the ultrasonication time of p-chlorophenol aqueous solution. The decreasing value of pH revealed that hydrogen ion was produced in the sonochemical reaction. As stated in the previous sections, the acidity of the reaction matrix increases, theoretically, some anions must have been produced to neutralize the production of H in the aqueous solution. One of the anions was determined to be the chloride ion by means of silver nitrate titration and chloride ion selective electrode (Fig. 7). Ion selective electrode was used to quantitatively measure the produced chlorides. Fig. 7 showed a plot of potential versus the logarithmic chloride concentration. The curve is perfectly linear between 1 and 1000 ppm with a correlation coecient of 0.9993. The results clearly indicated the presence of Cl from the cleavage of C±Cl bond of the chlorophenol during the ultrasonication process. It is interesting to note that when pchlorophenol was totally degraded in this study, it was found that the determined chloride ions in the reaction matrix was less (approximately 30%) than that calculated from the input of p-chlorophenol. It can be deduced that the contribution of the loss of chloride come from some chlorinated intermediates and products derived from the sonication of p-chlorophenol. The HPLC analysis disclosed that there are several intermediates produced in the degradation process and
Fig. 6. pH versus ultrasonication time for the degradation of pchlorophenol.
Fig. 7. Plot of potential versus the logarithmic chloride concentration chloride.
some of them are preliminarily identi®ed to be hydroquinone, 1,4-benzoquinone, 4-chlorocatechol and 4chlororesorcinol by external standards. Fig. 8 showed the evolution of p-chlorophenol and the accumulation of intermediates produced during the sonochemical destruction. At ®rst, the concentrations of intermediates increase with the degradation of p-chlorophenol. Then, their concentrations approach their maximum value and slowly decrease as the sonochemical reactions continue. It is due to the degradation of these intermediates and the consumption rate of these intermediates is faster than their formation rate. Further studies have been performed on the ultrasonic degradation of the individual intermediate spiked in water. Experimental results in Fig. 9 showed the degradation of these intermediates all follow a ®rst-order kinetic rate law.
Fig. 8. Concentrations of p-chlorophenol (0.198 mM, 30 ml used) and intermediates versus degradation time.
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K.C. Teo et al. / Ultrasonics Sonochemistry 8 (2001) 241±246
Experimental results clearly illustrated that chloride ion and hydrogen ion were produced in the sonochemical degradation of p-chlorophenol in water. The possible intermediate compounds were preliminarily identi®ed with HPLC chromatography to be hydroquinone, 1,4-benzoquinone, 4-chlorocatechol and 4-chlororesorcinol. Ultrasonication can further the degradation of intermediates into smaller molecules even into carbon dioxide and water. Even though the laboratory study has shown that ultrasonic process has an excellent eect on the destruction of organic pollutants such as p-chlorophenol in aquatic system, more work should be done before this technology can be applied for practical wastewater treatment. Fig. 9. Degradations of standard materials of four intermediates.
4. Conclusion Dierent degradation eciencies were achieved for p-chlorophenol in water using three ultrasonic devices including probe, beaker and cup±horn system. Probe system has the highest initial rate for the ultrasonic treatment of p-chlorophenol. However, beaker type ultrasonic device is suitable for the study of sonochemical mechanism. The degradation of p-chlorophenol can be aected in the presence of hydrogen peroxide and external sparging gas. The former at a proper concentration can accelerate the sonochemical reaction by means of the production hydroxyl radical; however, the latter will not de®nitely enhance the sonication.
Acknowledgements The author would like to thank the kind support from Professor L.S. Chia, the scholarship supported by the Nanyang Technological University and the generous support from Lee Foundation of Singapore for the conference trip to Biarritz, France. References [1] P. Colarusso, N. Serpone, Res. Chem. Intermed. 22 (1996) 61. [2] M.R. Homann, I. Hua, M. Hochemer, Ultrason. Sonochem. 3 (1996) S163. [3] J.N. Jenson, Hazard. Ind. Wastes. 28 (1996) 265. [4] C. Petrier, Y. Jiang, M.F. Lamy, Environ. Sci. Technol. 9 (1998) 1316. [5] T.J. Mason, J.P. Lorimer, D.M. Bates, Ultrasonics 1 (1992) 40. [6] M.A. Margulis, Ultrasonics 23 (1985) 157.
www.elsevier.nl/locate/ultsonch
Sonochemical degradation for toxic halogenated organic compounds Khay Chuan Teo *, Yanrong Xu, Chun Yang Academic Group of Natural Sciences, National Institute of Education, Nanyang Technological University, 1 Nanyang Walk, Singapore 637616, Singapore
Abstract This paper describes the degradation of p-chlorophenol using three dierent ultrasonic devices. The dissipated power in the reaction matrix was measured based on calorimetric method. The study showed that hydrogen peroxide can improve the sonochemical reaction and gases dissolved in reaction matrix can aect the process to a small extent. The reaction mechanism and kinetics of degradation were also investigated. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Sonochemical; Degradation; p-Chlorophenol
1. Introduction Nowadays, millions of organic compounds have been synthesized, and many of these chemicals have been released and appeared as environmental pollutants. Currently the technologies in treatment of organic pollutants include solvent extraction, incineration, chemical dehalogenation and biodegradation, etc. Since 1990, there has been an increase of interest in the use of ultrasound to destroy organic contaminants present in water and/or wastewater, and the scaleup of the corresponding technologies are also under development [1±4]. Promisingly, sonochemical degradation is capable of being developed into a successful technology for environmental clean up. Extensive basic R&D working toward the maturity is required before the process can be called a proven technology. This paper mainly focused on the application of ultrasonic devices in the degradation of p-chlorophenol which often presents as a priority organic pollutant in environment. 2. Experimental Three types of ultrasonic devices including beaker system, cup±horn system and probe system were em-
*
Corresponding author. Tel.: +65-790-3849; fax: +65-896-9432. E-mail address: [email protected] (K.C. Teo).
ployed in this study (Fig. 1). The 140 W, 850 kHz plane ultrasound generator (Fig. 1(A)) manufactured by K.W. Meinhardt-Ultraschalutechnik (Germany). Cup±horn (Fig. 1(B)) and the probe system (Fig. 1(C)) are both available with power output rating of the 475 W, 20 kHz XL2020 ultrasonic processor supplied by Heat system Inc. (USA). The base diameter of beaker system is 9.5 cm, the diameter of cup±horn transducer is 7 cm and the tip diameter of the probe is 1.5 cm. Their emitting surface areas are calculated to be 70.8, 38.5 and 1.8 cm2 , respectively. In the case of beaker system and cup±horn system, the glass reactor was located at 0.5 cm above the base of the ultrasonic generator. The p-chlorophenol aqueous solution was prepared by dissolving the HPLC grade standard into reagent water and diluted to achieve necessary concentrations. A reaction vessel with 100 ml p-chlorophenol aqueous solution was placed in the ultrasonic device and air-equilibrated under atmospheric pressure. The temperature was measured by a multi-channel scanning thermocouple and controlled by the water body in the beaker system, which in turn was maintained by an external cooling ¯uid. The temperature during the reaction process normally ranged from 36°C to 39°C. The sample was collected in a settled minute interval. The analysis was performed on Varian HPLC equipped with Varian 9050Q solvent delivery system, Varian 9100 autosampler and 9012 variable wavelength UV±VIS detector.
1350-4177/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 5 0 - 4 1 7 7 ( 0 1 ) 0 0 0 8 3 - 9
242
K.C. Teo et al. / Ultrasonics Sonochemistry 8 (2001) 241±246
Fig. 1. Schematic diagram of ultrasonic devices used in this study: (A) beaker system; (B) cup±horn system; (C) probe system.
K.C. Teo et al. / Ultrasonics Sonochemistry 8 (2001) 241±246
243
3. Results and discussion 3.1. Comparison of three ultrasonic device by calorimetric power measurement The sonochemical power irradiated into the reaction system is dierent for the individual instrument. It is dicult to compare the sonochemical results from different laboratory without the power output rating value, which is well known as the reproducibility problem in sonochemistry. So far, several methods are available to determine the acoustic power. The most common method is calorimetry, which assumes that all of the energy delivered to the system is dissipated as heat [5]. The power dissipated (Pdiss ) in a reaction mixture can be obtained using Eq. (1) dT Pdiss MCP 1 dt t0 where CP is heat capacity of water, M is mass of water, and dT =dtt0 represents initial slope of the temperature rise of the reaction mixture versus time of exposure to ultrasonic irradiation. The measurement of the acoustic power was performed for the ultrasonic systems used for this study according to the method mentioned above. Assuming 100 g water was used in each system, the values of dT =dt and the power for three devices can be calculated according to Eq. (1) and the experimental results were provided in Table 1. As can be seen from the experimental results, compared with the cup±horn system, beaker system has higher power output due to its higher frequency at 850 kHz. In general, higher frequency leads to higher power output of ultrasonic device. Probe system has the higher value of dT =dt and power output than cup±horn system for the given frequency at 20 kHz, it is well known that probe system is a more ecient device of transmitting ultrasonic energy into a reaction matrix due to its direct sonication. Whereas, the two other ultrasonic devices provide indirect sonication, which will inevitably cause energy loss while the reaction matrix contacts with mechanical vibration indirectly. In order to con®rm these results, the three ultrasonic devices were utilized to degradate p-chlorophenol in water. The results given in Fig. 2 illustrated that within 20 min 50% of p-chlorophenol was degradaded with the probe system and 25% with the beaker system. HowTable 1 Measurement of power dissipated for dierent ultrasonic devices dT =dtt0 Power (W) R2
Probe system
Beaker system
Cup±horn system
0:313 0:011 130:0 4:6 0.9999
0:0570 0:0018 23:8 0:8 0.9969
0:0462 0:0020 19:3 0:8 0.9998
Fig. 2. p-chlorophenol concentration (0.090 mM, 100 ml used) versus time using three ultrasonic devices.
ever, only 2% of p-chlorophenol was degradated with the cup±horn system. These results agree with the previous conclusions derived from calorimetric measurements. It appeared that the probe system is the most appropriate sonochemical system for the treatment of pchlorophenol in water due to its highest degradation eciency within a shorter time. However, the concentration of p-chlorophenol in reaction solution did not decrease any more, which seemed that p-chlorophenol degradation process reaches an equilibrium around 20 min. It is interesting to note that though p-chlorophenol was degradated slowly with beaker type ultrasonic apparatus, degradation eciency higher enough approaching 100% can be achieved by the beaker system. In addition, the sonochemical intermediates can be readily observed in the degradation process with beaker system which is essential to the investigation of the mechanism of sonochemical reactions. The following approach on the degradation of p-chlorophenol was performed using the beaker system. 3.2. Eect of hydrogen peroxide on the degradation of p-chlorophenol According to the theory of hot spot, the temperature and pressure of localized hot spots are formed can excessively reach 5000 K and 500 atm respectively in the ultrasonic cavitation [6]. Under these conditions, hydrogen peroxide readily decomposes into hydroxyl radicals which will accelerate the sonication. Fig. 3 presented the initial rate of sonochemical reaction of 0.4 mM p-chlorophenol aqueous solution in the presence of hydrogen peroxide using beaker system. It was observed that the initial rate increases when the concentration of hydrogen peroxide increases and the value reaches a maximum at approximately 30 mM. In this case, the initial rate of sonication can substantially accelerate by
244
K.C. Teo et al. / Ultrasonics Sonochemistry 8 (2001) 241±246
Fig. 3. Eect of hydrogen peroxide on sonochemical degradation (0.395 mM, 100 ml p-chlorophenol used).
twofolds and the degradation time can be reduced by 20 min compared with the corresponding ones in the absence of hydrogen peroxide. Further experiments also demonstrated that no obvious degradation of p-chlorophenol was observed only using hydrogen peroxide. Therefore it can be concluded that sonication controls the overall degradation process and hydrogen peroxide only play an auxiliary role. 3.3. Eect of bubble gases on the degradation of pchlorophenol in water As well known, the propagation of ultrasonic wave in liquid generates cavitation bubbles, the growth and explosion of these bubbles enable the production of certain species of radicals. It is expected that sparging reaction matrix with certain gases probably facilitates the formation of cavitation bubbles and subsequently accelerates the sonication and improves the degradation eciency. In this study, dierent gases such as argon and air were introduced into the reaction matrix by a fritted dispersion glass tube. Fig. 4 shows the ultrasonic degradation of p-chlorophenol in water in the presence of external gases. The initial rate for the ultrasonic reaction with air is higher than the corresponding one with argon gas. This is attributed to oxygen in air which is believed to facilitate the formation of hydroxyl radicals and accelerate the reaction. However, the experimental results also illustrated that the introduction of gases will not absolutely lead to rapid degradation rate and high eciency, because the formed cavitation bubbles can be possibly interrupted or destroyed by the sparging gas.
Fig. 4. Eect of gas on sonochemical degradation. The initial rate and rate constant are 2:90 0:14 lM/min, 0:00646 0:00032 min 1 for argon gas and 4:04 0:28 lM/min, 0:00899 0:00062 min 1 air gas, respectively (0.449 mM, 100 ml p-chlorophenol used).
photometer. The UV spectra of reaction matrix at different time were used to monitor the progress of the degradation. p-Chlorophenol aqueous solution has three strong absorbance at 204, 230 and 280 nm respectively. These peaks shifted when the sonochemical reaction proceeded and only one obvious UV absorbance was observed at 60 min. It can be concluded that p-chlorophenol was thoroughly degradaded, which has been veri®ed by HPLC analysis. In order to investigate the kinetic of sonochemical reaction of p-chlorophenol in water, a plot of the logarithmic value of C=C0 and ultrasonic time yielded a straight line with an equation ln C=C0 0:190 0:050t (Fig. 5). The correlation coecient of 0.9888 indicated the reasonableness of the straight line ®t. The
3.4. Kinetic of sonochemical reaction of p-chlorophenol in water The qualitative determination of the analytes was carried out during the experiments using UV spectro-
Fig. 5. Kinetic of ultrasonic transformation of p-chlorophenol (0.395 mM, 30 ml used). The initial rate and rate constant are 19:75 1:18 lM/min and 0:0500 0:003 min 1 .
K.C. Teo et al. / Ultrasonics Sonochemistry 8 (2001) 241±246
245
linear relationship between ln C=C0 and time demonstrated that the overall degradation follows a pseudo®rst-order kinetic rate law. The rate constant was obtained from the slope of this straight line and the initial rate can be calculated to be 19:75 1:18 lM/min by the equation vi kC0 . 3.5. Mechanism of the ultrasonic degradation of pchlorophenol in water The acidity of reaction matrix was monitored by a pH meter. Fig. 6 showed the pH evolution with the ultrasonication time of p-chlorophenol aqueous solution. The decreasing value of pH revealed that hydrogen ion was produced in the sonochemical reaction. As stated in the previous sections, the acidity of the reaction matrix increases, theoretically, some anions must have been produced to neutralize the production of H in the aqueous solution. One of the anions was determined to be the chloride ion by means of silver nitrate titration and chloride ion selective electrode (Fig. 7). Ion selective electrode was used to quantitatively measure the produced chlorides. Fig. 7 showed a plot of potential versus the logarithmic chloride concentration. The curve is perfectly linear between 1 and 1000 ppm with a correlation coecient of 0.9993. The results clearly indicated the presence of Cl from the cleavage of C±Cl bond of the chlorophenol during the ultrasonication process. It is interesting to note that when pchlorophenol was totally degraded in this study, it was found that the determined chloride ions in the reaction matrix was less (approximately 30%) than that calculated from the input of p-chlorophenol. It can be deduced that the contribution of the loss of chloride come from some chlorinated intermediates and products derived from the sonication of p-chlorophenol. The HPLC analysis disclosed that there are several intermediates produced in the degradation process and
Fig. 6. pH versus ultrasonication time for the degradation of pchlorophenol.
Fig. 7. Plot of potential versus the logarithmic chloride concentration chloride.
some of them are preliminarily identi®ed to be hydroquinone, 1,4-benzoquinone, 4-chlorocatechol and 4chlororesorcinol by external standards. Fig. 8 showed the evolution of p-chlorophenol and the accumulation of intermediates produced during the sonochemical destruction. At ®rst, the concentrations of intermediates increase with the degradation of p-chlorophenol. Then, their concentrations approach their maximum value and slowly decrease as the sonochemical reactions continue. It is due to the degradation of these intermediates and the consumption rate of these intermediates is faster than their formation rate. Further studies have been performed on the ultrasonic degradation of the individual intermediate spiked in water. Experimental results in Fig. 9 showed the degradation of these intermediates all follow a ®rst-order kinetic rate law.
Fig. 8. Concentrations of p-chlorophenol (0.198 mM, 30 ml used) and intermediates versus degradation time.
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K.C. Teo et al. / Ultrasonics Sonochemistry 8 (2001) 241±246
Experimental results clearly illustrated that chloride ion and hydrogen ion were produced in the sonochemical degradation of p-chlorophenol in water. The possible intermediate compounds were preliminarily identi®ed with HPLC chromatography to be hydroquinone, 1,4-benzoquinone, 4-chlorocatechol and 4-chlororesorcinol. Ultrasonication can further the degradation of intermediates into smaller molecules even into carbon dioxide and water. Even though the laboratory study has shown that ultrasonic process has an excellent eect on the destruction of organic pollutants such as p-chlorophenol in aquatic system, more work should be done before this technology can be applied for practical wastewater treatment. Fig. 9. Degradations of standard materials of four intermediates.
4. Conclusion Dierent degradation eciencies were achieved for p-chlorophenol in water using three ultrasonic devices including probe, beaker and cup±horn system. Probe system has the highest initial rate for the ultrasonic treatment of p-chlorophenol. However, beaker type ultrasonic device is suitable for the study of sonochemical mechanism. The degradation of p-chlorophenol can be aected in the presence of hydrogen peroxide and external sparging gas. The former at a proper concentration can accelerate the sonochemical reaction by means of the production hydroxyl radical; however, the latter will not de®nitely enhance the sonication.
Acknowledgements The author would like to thank the kind support from Professor L.S. Chia, the scholarship supported by the Nanyang Technological University and the generous support from Lee Foundation of Singapore for the conference trip to Biarritz, France. References [1] P. Colarusso, N. Serpone, Res. Chem. Intermed. 22 (1996) 61. [2] M.R. Homann, I. Hua, M. Hochemer, Ultrason. Sonochem. 3 (1996) S163. [3] J.N. Jenson, Hazard. Ind. Wastes. 28 (1996) 265. [4] C. Petrier, Y. Jiang, M.F. Lamy, Environ. Sci. Technol. 9 (1998) 1316. [5] T.J. Mason, J.P. Lorimer, D.M. Bates, Ultrasonics 1 (1992) 40. [6] M.A. Margulis, Ultrasonics 23 (1985) 157.