Environ. Sci. Technol. 1994, 28, 1619-1622
Sonochemical Destruction of CFC 11 and CFC 113 in Dilute Aqueous Solution H. Michael Cheung' and Shreekumar Kurup Department of Chemical Engineering, The University of Akron, Akron, Ohio 44325-3906
The results of a preliminary, proof-of-principle investigation of the sonochemical destruction of the chlorofluorocarbons CFC 11(fluorotrichloromethane) and CFC 113 (trifluorotrichloroethane) in dilute aqueous solution are reported. The chlorofluorocarbonsolutions with an initial concentration of approximately 50 mg/L of the CFC were exposed to 20-kHz ultrasound with a power per unit volume of either 4.6 W/mL in a batch reactor or 0.64 W/mL in a circulating reactor. Destruction was fairly rapid with very little, less than 5 % ,of the CFCs undergoing volatilization. Destruction rates were slightly higher at 5 "C than at 10 "C. This decrease in reaction rate with an increase in solution temperature is common in sonochemistry. The solution pH decreased upon sonication, indicating the acidic species as a final halogen acceptor for at least a portion of the C1 and F. Introduction There has been considerable interest lately in means for the destruction of chlorofluorocarbons. This is due mainly to their implication as a factor in stratospheric ozone depletion ( I ) . Much of the reported work on CFC destruction has focused on oxidation of the compounds in air over various catalysts including zeolites (2),aluminasupported gold (3), metal oxides ( 4 ) ,and a wide variety of others. We report here preliminary results on the use of ultrasonically driven chemistry, commonly referred to as sonochemistry, for the destruction of fluorotrichloromethane (CFC 11) and trifluorotrichloroethane (CFC 113) in dilute aqueous solution. The sonochemical effect is due to the formation of cavitation bubbles in the fluid which grow through several cycles of the ultrasonic wave and then collapse precipitously. The collapse is virtually adiabatic and leads to surprisingly high local pressures and temperatures. In water (51, the local pressure may exceed 500 atm, and the local temperature may exceed 5000 K. The localized hot spot is very short lived, which leads to extremely high heating and cooling rates, in the neighborhood of lo9 K/s in water (6). The reactions which take place in this environment,specifically near the bubble/liquid interface, are similar to combustion (7), though reduction as well as oxidation has been observed (8). In water, the chemistry is dominated by the reactions of hydrogen and hydroxyl radicals with each other, other water molecules, and dissolved materials such as the CFC 11 and CFC 113 studied in this work. Prior work by our group and others with chlorinated compounds, including the chloromethanes (9-11) and chloroethanes and chloroethylenes (12),indicated that sonochemistry may provide a route for the destruction of CFCs. Experimental Section Two reactor systems were utilized in the work reported here: a batch reactor with a 35-mL working volume and
* Author to whom correspondence should be addressed. 0013-938X/94/0928-1819$04.50/0
0 1994 American Chemical Society
Ultrasonic Generator
Flgure 1. Batch system schematic.
Ultrasonic Generator
1 Temperature 1
\
D Valve
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Pressure Gauge
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Temperature and pH Indicator
Pump
Flgure 2. Circulating system schematic.
a circulating reactor with a 250-mL workingvolume. Both used a Heat Systems Inc. Model XL-2020 ultrasonicator. This unit produces 20-kHz ultrasound with up to 475 W maximum power. The batch system (Figure 1)consists of a closed reactor body into which the ultrasonicator horn is threaded with provision for temperature control via a coolingjacket and for headspace analysis. The circulating system (Figure 2) utilized the same reactor body, but with Environ. Sci. Technoi., Vol. 28, No. 9, 1994
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8 Circulating System, 5 psig, 5 deg C
Batch System, 5 deg C
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Flgure 3. CFC 11 batch results, 5 "C.
provision for on-line sampling, pH measurement, temperature measurement, and pressure control. Sample analysis was accomplished using a HewlettPackard G U M S system (5890 GC, 5971 MSD, and ChemStation) fitted with aTekmar ModelLSC 2000 purge and trap. Periodic calibrations were performed for CFC 11using solutions in which the CFC concentrations were set via careful volumetric measurements and relatively large volumes to ensure accuracy. No calibration was performed for the CFC 113, instead the area ratios which are equivalent to the dimensionless concentration C(t)/ C(t=O) are reported here. The solutions used were prepared by adding the requisite amount of the CFC to 2 L of deionized, distilled water. The stock solution was stirred overnight in a tightly closed flask. Care was taken to choose a flask that was very nearly full to prevent significant volatilization losses during the stirring. In a typical batch run, the reactor is loaded with the solution to be sonicated. The reactor is closed up by threading in the ultrasonic horn and run for the specified time. If a headspace analysis is to be done, the reactor was fitted with a custom adapter which permits purging of the headspace directly onto the trap of the Tekmar purge and trap. After the headspace analysis, the reactor is opened, and a 5-mL sample is introduced into the purge chamber of the purge and trap. Only one data point is obtained,and a concentration versus time curve (required for the determination of the apparent first-order rate constant) requires a number of separate experimental runs to complete. The circulating reactor, on the other hand, has a provision for sampling during the course of the run, but does not have a provision for headspace analysis. Also since less power per unit volume is delivered to the sample, longer runs are required to obtain high conversion. For both reactor setups, the temperature is controlled via circulation of water from a controlled-temperature bath (Cole-Parmer Model 01238-00) through the jacket surrounding the reactor vessel. The temperature and pH are monitored on-line for the circulating reactor. 1620 Environ. Sci. Technol., VoI. 28, No. 9, 1994
10
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Sonication Time, minutes
Sonication Time, minutes
Flgure 4. pH for CFC 11 circulating system run, 5 psig and 5 OC.
Circulating System, 5 psig, 5 deg C k, = 0.30 +/- 0.02 ml/watt rnin.
0
20
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Sonication Time, minutes
Figure 5. CFC 11 concentration versus sonication time, 5 psig and 5 "C.
Results and Discussion The destruction of CFC 11and CFC 113was fairlyrapid. Figure 3 shows the concentration versus time data for the batch destruction of CFC 11at 5 "C with 160-Wultrasonic energy. Virtually all of the CFC 11is destroyed within 6 min. While we have not yet developed the means for making a complete C, F, and C1 balance, it is apparent from the pH data that much of the F and C1 is forming HC1 and H F or other acidic species. Figure 4 shows a typical pH versus sonication time curve for CFC 11in the circulatingreactor a t 5 psig and 5 "C. The pH falls rapidly from its initial value of 7.4 to about 5.4 by the end of the run, indicating the formation of acidic species. The corresponding concentration versus sonication data is shown in Figure 5. The kl values reported in the figures are the first-order rate constants for the rate of chlorofluorocarbon destruction scaled by the power per unit volume delivered by the ultrasonicator. The concentration versus time data are fitted to an equation of the form:
C ( t ) = C,
+ C',
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Circulating System, 5 psig, 10 deg C
Batch System, 5 deg. C.
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Sonication Time, minutes
Flgure 6. Headspace analysis for CFC 11, batch system, 5 OC.
30
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Flgure 8. CFC 11 destruction in the circulating system, 5 psig and 10 OC.
1.o
1.o
Circulating System, 5 psig, 5 deg C 08
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._6
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Sonication Time, minutes
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Circulating System, 5 psig, 10 deg C
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Flgure 7. CFC 113 destruction in the circulating system, 5 psig and 5 OC.
Flgure 9. CFC 113 destruction in the circulating system, 5 psig and 10 OC.
The baseline term is needed because in an experiment without pH control the reaction rate decreases rapidly as the pH decreases. Work to further elucidate this effect is presently in progress in our laboratory. The kl values reported in the figures are the regressed kl' values divided by the power per unit volume delivered to the reactor. To ascertain whether there was significant volatilization of the CFC, headspace analysis runs were conducted for CFC 11 sonication. The results are shown in Figure 6 where the fraction of the original CFC 11in both the liquid and the vapor headspace is plotted. The data show that less than 5 % of the original CFC 11is volatilized, with the bulk of the material destroyed in the liquid phase. This is consistent with similar measurements made on other chlorinated hydrocarbons, most notably carbon tetrachloride. Volatilization of the target molecule does not seem to be an important contribution to its rate of removal from the aqueous phase. Similar studies were conducted on CFC 113. Figure 7 shows the dimensionless concentration versus time data for the sonochemical destruction of CFC 113 at 5 "C and 5 psig in the circulating reactor. Runs at 10 OC for both
CFC 11 and CFC 113 were conducted in the circulating reactor system to examine temperature effects. The data for CFC 11 are plotted in Figure 8, and the data for CFC 113 are plotted in Figure 9. In both cases, the apparent first-order rate constant declined very slightly. The decline was somewhat lower for the CFC 113than for the CFC 11 and, in fact, given the error bars on the fitted rate constants, may have remained unchanged. This result is similar to temperature effects noted for other chlorinated hydrocarbons (12)and is the result of adecrease in the cavitation intensity due to an increase in the solvent vapor pressure. Often sonochemical reaction rates decline with increasing temperature due to this effect (5, 6).
Conclusions The preliminary results reported here indicate the possibility of using sonochemistry as a means of destroying CFCs, especially if they are present as a dilute aqueous solution. A sonochemical approach has the advantage of not requiring any transference of the target molecule from an aqueous phase as would be required in combustion, Envlron. Sci. Technol., Vol. 28, No. 9, 1884
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catalytic or otherwise. Much progress needs to be made in the ultrasonic energy delivery system before sonochemistry can be deployed on a large scale, but the promise of the method is considerable. Future work will examine other CFCs and optimization of the pH, pressure, and temperature for the most efficient destruction of the CFCs. Recent studies in this laboratory of the pH, pressure, and temperature effects on the sonochemicaldestruction of carbon tetrachloride indicate that around 1 order of magnitude improvement in the destruction rate may be possible by optimizingthe reaction conditions from those used in this preliminary work. Reports of that work will be forthcoming.
(3) Aida, T.; Higuchi, R.; Niyama, H. Chem. Lett. 1990, 2247. (4) Okazaki, S.; Kurosaki, A. Chem. Lett. 1989, 1901. (5) Suslick, K. S. Sci. Am. 1989, 260 (2), 80. (6) Suslick, K. S. Science 1990, 247, 1439. (7) Henglein, A. Ultrasonics 1987, 25, 6. (8) Margulis, M. A. Ultrasonics 1985, 23, 157. (9) Cheung, H. M.; Bhatnagar, A.; Jansen, G. Environ. Sci. Technol. 1991, 25 (8), 1510. (10) Wu, J. M.; Huang,H. S.; Livengood, C. D.Environ. Prog. 1992, I 1 (3), 195. (11) Alippi, A.; Cataldo, F.; Galbato, A. Ultrasonics 1992,30 (3), 148. (12) Bhatnagar, A.; Cheung, H. M. Environ. Sci. Technol. 1994, 28, 1481-1486.
Literature Cited
Received for review November 9, 1993. Reuised manuscript received May 11, 1994. Accepted May 23, 1994."
(1) Molina, M. J.; Rowland, F. S. Nature 1974, 249, 810. (2) Karmaker, S.; Greene, H. L. J. Catal. 1992, 138, 364.
1622 Environ. Sci. Technol., Vol. 28, No. 9, 1994
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Abstract published in Advance ACS Abstracts, July 1, 1994.
