Environ. Sci. Technol. 1994, 28, 1481-1486
Sonochemical Destruction of Chlorinated C1 and C2 Volatile Organic Compounds in Dilute Aqueous Solution Ashlsh Bhatnagart and H. Michael Cheung’
Chemical Engineering Department, The University of Akron, Akron, Ohio 44325-3906 The results of an investigationon the ultrasonic destruction of chlorinated C2 volatile organic compounds (VOCs) in water and preliminary studies of (1)the effect of steadystate temperature on the destruction efficiency and (2) the mixture effects are also reported. Aqueous solutions of methylene chloride, chloroform, carbon tetrachloride, 1,2-dichloroethane,l,l,l-trichloroethane, trichloroethylene, and perchloroethylene in concentrationsranging from 50 to 350 mg/L were irradiated with 20-kHz ultrasonic waves in a sonochemical reactor (0.1 kW/L) under ambient temperature and pressure conditions. The concentration of solutes decreased rapidly in all cases. The pH of the solutions decreased rapidly during sonication, indicating that a significant fraction of the C1liberated was converted into HC1 or other acid species. GUMS analysis indicated that no other chlorinated products were formed as a result of sonication. Initial pH of the solution and the presence of other chlorinated VOC’s (mixture effects) were found to have little influence upon the destruction of the compound. Percent destruction varied from 72 to 99.9 % .
and implosive collapse of cavitation bubbles in the solvent. Cavities are formed at the weak spots in the liquid and rapidly grow to two or three times their original size due to rectified diffusion. Once a cavity reaches the critic1 size, it can no longer sustain itself, and the surrounding liquid rushes in. The implosion of cavities establishes an unusual environment for chemical reactions in the liquid. Compression of gas and vapor inside the cavity generates intense heat and created a local hot spoot (3). Thus, the diffuse energy of sound is concentrated to produce hot spots in the liquid. By observing the rate of familiar reactions due to ultrasound, Suslick and his collaborator have determined the temperature of the imploding cavity to be 5500 OC and the pressure to be around 500 atm (2,3). This short-lived hot spot with heating and cooling rates greater than lo9 K/s is the source of sonochemistry (2). The reactions that take place at the gas-liquid interface of the bubble are similar to combustion (41,though strong reduction as well as oxidation reactions have also been observed ( 5 ) due to the generation of H and OH radicals.
Introduction
Experimental Procedures
The presence of human-derived or naturally occurring volatile organic compounds (VOCs) in drinking water has become a major national and international concern. The presence of many VOCs in a few parts per billion is enough to render a water supply unpotable for reasons of longterm health risks, even though the water may have no discerible taste and odor. The pervasiveness of VOC’s is due to their use in a wide variety of products such as paints, solvents, dyes, pesticides, and ink that are used commonly in daily life. Under the SafeDrinking Water Act (SDWA), the Environmental Protection Agency (EPA) has promulgated maximum contaminant levels (MCLs) for 83 specificdrinking water contaminants including 22 volatile organic compounds (1). Power ultrasound has been used for medical imaging, plastic welding, emulsification, cleaning, polymerization, and depolymerization. Ultrasound is a unique means of interacting energy and matter and differs from traditional energy sources such as light, heat, or ionizing radiation in duration, pressure, and energy per molecule (2). The vast majority of published work in the field of ultrasound has dealt with the synthetic applications of sonochemistry;its use as a tool for environmental remediation has been relatively an untouched area. The chemical effects of ultrasound derive from acoustic cavitation. Cavitation is the fundamental nonlinear acoustic process that serves as a means of concentration the diffuse energy of sound in liquids. The process of cavitation consists of three steps: formation, rapid growth,
Aqueous solutions of methylene chloride (CH2C12), chloroform (CHCL), carbon tetrachloride (Cc&), 1,2dichloroethane (CZH~C~Z), trichloroethylene (CzHCls),and perchloroethylene (c&14)were exposed to 20-kHz ultrasound in a batch reactor to determine the efficacy of ultrasonic process in the destruction of undesirable compounds in water. Aqueous mixtures of two or more volatile organic compounds were also sonicated to investigate the potential of sonication in more realistic situations and to study the kinetics of individual halomethanes in presence of other halomethanes. A brief experimental study was conducted on a specially constructed version of the sonication appartus which permits headspace sampling. The purpose of the study was to evaluate whether there was significant volatization of the VOC undergoing sonication. Thus far, only methylene chloride and carbon tetrachloride have been studie with this system (which will become the standard for our batch sonication studies henceforth), and there is very little volatization of these two compounds. Only approximately 5 % of the initial moles of either were volatized while the rest were destroyed. We expect that, due to their lower vapor pressures, even less of the C2 chlorinated hydrocarbons would be “lost” into the vapor headspace. Further studies using the new closed batch system, which incorporates improved temperature control and the ability to control pressur along with headspace sampling, will be forthcoming. Experimental Setup. Figure 1 shows a schematic diagram of the experimetnal setup used to sonicate the aqueous solutions of VOCs. The reactions were carried out in a 2-L glass batch reactor. The temperature inside the reaction vessel was kept relatively constant to maintain
* Author to whom correspondence should be addressed. + Present address: The Department of Chemical Engineering and
Materials Science, The University of Minnesota, Minneapolis, MN 55455. 0013-936X/94/0928-1481$04.50/0
0 1994 American Chemical Society
Environ. Sci. Technol., Vol. 28, No. 8, 1994
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GC - MS System Ultrasonic Generator
pH Meter
Ultrasonic Convertor and Probe
Reachon Vessel
Flgurs 1.
Expertmental apparatus.
approximately isothermal conditions by means of a stainless steel cooling coil. The temperature was monitored by means of a thermometer and ranged from 20 to 25 "C for all the runs under amhient conditions. A ColeParmer Digi-sense LCD pH meter (Model 5994-10) was used to detect the changes in pH. A Heat Systems W-385 ultrasonicator was used to produce ultrasonic waves of 20-kHz frequency. All the runs were conducted in a continuousmodeat50%duty cycle. The W-385aonicator is capable of delivery 475 W of ultrasonic energy. The sonicator was operated at its maximum output setting (10) and was typically delivering 40% (approximately200 W) of its rated power to the reactor. Titanium tips with a flat radiating surface of 1/2 in. diameter were used for all the reactions. Buffer solutions of pH 5,7, and 9 (Fisher Scientific) were used to standardize the pH meter before the runs. Though the temperature and pH of the reactor contentswere continuously monitored, they were recorded at regular intervals with the sonicator on standby as the sonicator interfered with the stability of the pH reading. Periodically, 10-mL samples were withdrawn with the sonicator on the standby for GC/MS analysis. Materials. Methylene chloride, chloroform, carbon tetrachloride, l,l,l-trichloroethane, and perchloroethylene were obtained from Aldrich Chemical Co. Inc. Trichloroethylene and hexanes were obtained from Fisher Scientific. The 1,2-dichloroethane used was obtained from Fluka. All chemicals were of at least 99% purity and were used as received. Aqueous solutions of VOCs were prepared by stirring the neat liquid with deionized and distilled water in a closed flask. Extreme care was taken to prevent the contamination of solutions. The solutions were left overnight with slow stirring to equilibrate. Care was taken to minimize the effects of evaporative losses hy using nearly full Erlenmeyer flasks with tight-fitting ground-glass stoppers that were further sealed with a wrapping of Teflon tape. Analysis. The initial concentration of VOCs ranged from 350 to 50 mg/L in water and decreased with time as a result of sonication. To perform an efficient qualitative and quantitative analysis at such low concentrations, a Hewlett-Packard GC/MS (Model 5890 GC, Model 5970 mass sensitivity detector, and Model 9133 ChemStation) 1482 E m n . Sol. Tebnol.. Vol. 28. No. 8. 1994
with a 12 m X 0.2 mm X 0.33 pm film thickness HP Ultra 1 capillary column coated withcrosa-linked methylsilicone gum was used. Calibration curves for each compound were produced prior to the experiments. The analytical protocol to calibrate the instrument for a particular compound involved the preparation of a stock solution of that compound in water. From the stock solution, various calibration standards (samples of known concentration) were prepared. In all the cases, stock solution concentration served as the highest calibration standard. Aliquid-liquid extraction technique was usedto extract the chlorinated hydrocmhons from water. A 10-mLsample of calihration standards was contacted with 1mL of hexane or pentane (in the case of chloroform) in screw cap, conical hottomtesttubesfollowed by shaking for 2-5 min hy hand. The samples were then allowed to equilibrate for at least 10 min. Then 1 pL of the hexane phase was carefully injected into the GC/MS. A Hamilton syringe equipped with the Chaney adapter of 1-pL capacity was used to inject a constant volume every time for reproducibility of results. The syringe was always cleaned after every injection by the application of heat and vacuum. At least three injections in succession were done for every concentration. The calibration curves were linear from 20 to 500 mg/L for all the chlorinated hydrocarbons. To study the kinetics of destruction, 10-mL samples were periodically withdrawn from the reactor for analysis during the course of experiments. The sampleswere then contacted with 1mL of hexane or pentane (in the case of chloroform) and were analyzed in the same manner as described above for Calibration. Allthe sampleswereimmediatelyanalyzed following the sonication and were never stored for future analysis.
Results and Discussion Dilute aqueous solutions of three C1 compounds (methylene chloride, chloroform, and carbon tetrachloride) and four C2 compounds (1,2-dichloroethane, 1.1,l-trichloroethane, trichloroethylene, and perchloroethylene) were exposed to ultrasound under amhient conditions. Inevery case, the concentration decreased exponentially with
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Flgure 2. Methylene chloride concentration and solution pH versus sonication time.
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Time, minutes
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200
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Flgure 4. Carbon tetrachloride concentration and solution pH versus sonication time.
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Time (minutes)
sonication time, strongly indicating first-order kinetics. Figure 2-4 show the concentration and pH versus sonication time results for the C1 compounds: methylene chloride, chloroform, and carbon tetrachloride, respectively. Figure 5 shows the effects of the addition a small aliquot of NaOH solution (to increase the pH) during the course of a sonication run on carbon tetrachloride. The sharp discontinuity in the pH curve denotes the time at which the NaOH was added. the pH is increased, as expected, by the addition of NaOH but decreases rapidly upon continued sonciation. Figure 6 depicts the results of using the pH data for methylene chloride sonication (with repeated episodic NaOH additions to increase the pH; three cycles are shown) to estimate the rate of HC1 formation. The rate peaks at pH ofjust over 4. This indicates that pH control may be significant for determining the final form of C1 liberated during sonication. Ongoing work in our laboratory also indicates that pH control may be significant for optimizing the rate of sonochemical destruction for chlorinated compounds. Figures 7-10 show the concentration and pH versus sonication time results for the C2 compounds: 1,2-DCA, l,l,l-TCA, TCE, and PCE, respectively.
Figure 5. Solution pH versus sonication time for the homogeneous sonochemical destruction of carbon tetrachloridein water. The vertlcal discontlnuity at 2 1 min is due to the addition of sodium hydroxkie solution to raise the system pH.
The first-order rate constants determined by fitting the experimental data are listed in Table 1 (as well as being shown on the respective data plots). Preliminary experiments on the sonochemical destruction of C1 compounds have been reported earlier (6-9). It appeared that the vapor pressure of the chloromethane can affect the rate of destruction. As the vapor pressure of the C1 compounds increased, the first-order rate constant seemed to go down (Table 1). Unlike C1 compounds, there seems to be no relation between the vapor pressure of the chlorinated C2 compound and its destruction rate. It was observed that the reaction rate for the sonochemical polymerization of nitrobenzene was lowered by the presence of liquid solutes with high vapor pressure (IO). As compared to C1 compounds, C2 compounds have a relatively low vapor pressue (Table 1). The microbubble in this case mostly encloses a vapor of the solvent (water). So for the compounds having low vapor pressure (C2 compounds), there seems to be no correlation between the destruction Envlron. Sci. Technol., Vol. 28, No. 8, 1994
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Figure 8. l,l,l-TCA concentration and solution pH versus sonication time.
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Figure 6. Rate of HCi formation versussolution pHfor the sonochemical destruction of methylene chloride in water. The plot is derived from four 'cycles' of sonlcation with pH adjustment via addition of sodium hydroxide solution in between.
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Figure 9, TCE concentration and solution pH versus sonication time. 20
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1 50
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Envlron. Scl. Technol., Vol. 28, No. 8, 1994
0
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Figure 7. 1,P-DCA concentration and solution pH versus sonication time.
N
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Sonication Time, minutes
rate and the vapor pressure. It was found that the complete destruction of l,l,l-TCA could be achieved in about 40 min while PCE, TCE, and 1,2-DCArequired about 60 min of sonication for 100% destruction. GUMS analyses of the samples showed no other chlorinated products in the sonolysis of C1 or C2 compounds. To explore the application of sonication in more realistic situations and to study the kinetic behavior of individual VOCs in the presence of other VOCs, mixtures of chlorinated C1 and C2 compounds were sonicated using the same procedure as described earlier. The first mixture consisted of two chloromethanes-methylene chloride and carbon tetrachloride-while the second mixture was made of CHzC12, CCld (C1 compounds), and l,l,l-TCA (C2 compound). Figure 11and 12 show the percent destruction and pH versus sonication time data for the compounds in binary and ternary mixtures. The values of the first-order rate constants for the compounds were essentially unchanged by the presence of other reacting species. The
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1,P-DCA TCE PCE 1.1.1-TCA 60
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ki (rnin-l)
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(Torr)
0.033 0.043 0.043 0.021 0.021 0.026 0.046
0.002 0.005 0.002 0.003 0.001 0.004 0.003
2.080 3.500 0.380 2.480 0.632 1.086 1.067
348.900 160.000 90.000 64.000 57.800 20.000 100.000
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15
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25
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Sonication Time, minutes
Flgure 12. Concentration and pH versus sonication time for a ternary mixture of methylene chloride, carbon tetrachloride, and l,l, 1-TCA.
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Flgure 11. Concentration versus sonication time for binary mixtures of methylene chloride and carbon tetrachloride
sonication period, more destruction of CCld was observed than of CHzClz in agreement with earlier observations (6-9). The pH decay seemed to follow the same pattern as for individual VOCs. The maximum overall concentration of the VOCs in any case was around 150 mg/L, and it seems that sufficient cavitation bubbles and free radicals were produced to oxidize all the volatile organic components of the mixture. The mechanism of sonolysis seems to be so intense for the concentrations examined that the decomposition of one compound was independent of the other. Conventionally, increased raction rates are produced by raising the reaction temperature (Arrhenius equation). In sonochemistry, increased reaction rates are usually observed by lowering the reaction temperature. In sonication, increasing the bulk temperature raises the vapor pressure of the solvent. The vapor of the solvent instantaneously fills any cavitation bubbles formed. Col-
lapse of these cavities is 'cushioned', which reduces the extremes of temperature and pressure generated (11).So, raising the reaction temperature results ion lower intensity of bubble collapse, which is the fundamental process for the chemical reactivity of the compounds exposed to ultrasound. Hence, raising the bulk temperature results in lower reaction rates. Proposed Mechanism of Destruction. The scheme was developed by considering all the major elementary reactions in the field of ultrasonics (5,13). The reaction could take place inside the cavitation bubble where the temperature and pressure are so high that the solute molecule breaks down or in the bulk liquid phase, where the free radicals generated as a result of high-intensity ultrasonic waves oxidize the target molecule. Since highintensity ultrasonic waves break down nitrogen and oxygen (14-16) gas molecules dissolved in water, the target compound is probably decomposed in the cavity. Pyrolysis-type reactions in the cavity usually take place at a high solute concentration. It has been shown by many experimental studies that irradiation of water generates H and OH radicals (17). In the absence of any solute, these radicals lead to the generation of H202. It has been mentioned before that the cavity might contain the vapors of the solute (18). Henglein ( 4 ) discovered additional products in the sonolysis of sodium acetate under argon which would have come from direct thermal action only. If the reaction takes place in the cavitation bubbles and all the experimental conditions such as reaction vessel size, steady-state temperature, power supplied, and pressure remain unchanged, the destruction of the compound can be expressed by a first-order rate equation: -d [CXHyC1Zl /dt = k [ CXH $lZ] where CxHyClz is the target compound. A fit to the experimental data for all the C1 compounds indicated a first order of destruction (Table 1). It seems that the cavity encloses a vapor of the solute because of the high vapor pressure of these compounds. The primary reaction pathway for these compounds appears to be the thermal dissociation in the cavities. The activation energy required to cleave the bond is provided by the high temperature and pressure in the cavitation bubbles. This Environ. Scl. Technol., Vol. 28, No. 8, 1994
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leads to the generation of radicals of water and the target compound. These radicals then travel to the bulk liquid phase where they initiate secondary oxidation reactions. The products of sonication of water (H, OH, HO2, etc.) are highly reactive and are capable of oxidizing the target compound. The solute molecule then breaks down as a result of free radical attack generated as a result of highintensity ultrasonic waves. Many oxidation-reduction type of reactions have been observed in the field of ultrasonics (19). The oxidation of target molecules by free radicals in the bulk liquid phase under normal operating pressures and temperatures can be represented by a second-order rate equation: -d[c]dt = ~,[C][OH] + k2[cI[Hl
+ k,[cl[l + ...
where c is target compound. The concentration of free radicals in the equation is a function of the power used for sonication. The concentration of free radicals may be controlled by controlling the power delivered to the solution. C2 compounds probably disintegrate by both the mechanisms-pyrolysis type of reaction in the cavitation bubble and free radical attack in the bulk liquid phase. An experimental fit to the data indicated a first-order decay for these compounds where the vapor pressure of the compound had no effect on the rate constant of destruction. Unfortunately, as the radical concentrations were probably constant for the duration of any given run, this is not conclusive evidence that the second-order mechanism is not operative since under these conditions it would behave in a pseudo-firstorder fashion. Conclusions The results of this research indicate that the sonochemical process could serve as an alternative or an adjunct to the advanced oxidation technologies in the field of environmental remediation. The successful destruction of seven selected priority pollutants showed the nonspecificity of the process for at least chlorinated C1 and C2 compounds. The nonspecificity of the process was further verified by treating mixtures of C1 and C2 chlorinated compounds to simulate more realistic situations. All the chlorocarbons in the mixture were destroyed, and the reaction rate constant of a chlorocarbon was not influenced by the presence of another chlorocarbon in the concentration ranges studied. The percent destruction varied from 72 to 99.9%. Physical operating conditions such as steady-state temperature and initial pH of the solution were found to have little effect upon the destruction rate of the compound. Headspace analysis indicated
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that relatively little of the VOC is volatized, though sufficient amounts of the VOC were volatilized that a closed system would be required in any large-scale implementation. The relative efficiencyof this process in terms of the total power delivered per liter of water, 0.1 kW/L, for the destruction of undesirables appears comparable to other advanced oxidation technologies and may be improved by the optimization of oerating conditions. The simplicity and flexibility along with the high efficiencyof destruction indicate the potential of a sonochemical-based process to become a competitive technology with other advanced water treatment technologies. Acknowledgments We gratefully acknowledge financial support by the Westinghouse-Hanford Co.and the Ohio Board of Regents. Literature Cited (1) Ram, N. M.; Christman, R. F.; Cantor, K. P. Significance and Treatment of Volatile Organic Compounds in Water Supplies; Lewis Publishers: Chelsea, MI, 1990. (2) Suslick, K.S.Science, 1990,247, 1439. (3) Suslick, K.S.Sci. Am. 1989,260 (2),80. (4) Henglein, A. Ultrasonics 1987,25,6. (5) Margulis, M. A. Ultrasonics 1985,23, 157. (6)Bhatnagar, A. SonochemicalDestruction of the Chlorinated C1 and C2 Volatile Organic Compounds in Dilute Aqueous Solution. M.S. Thesis, The University of Akron, 1992. (7) Cheung, H. M.; Bhatnagar, A.; Jansen, G. Environ. Sci. Technol. 1991,25 (8),1510. (8) Bhatnagar, A.; Cheung, H. M. Paper 28i,AIChE Summer National Meeting, Pittsburgh, Aug 19,1991. (9) Bhatnagar, A.; Cheung, H. M. Paper 97b,AIChE Annual Meeting, Los Angeles, CA, Nov 19,1991. (10)Lorimer, J. P.;Mason, T. J. Chem. SOC.Rev. 1987,16,239. (11) Mason, T. J. Chemistry with Ultrasound;Elsevier Applied Science: London, 1990. (12) Niemczewski, B. Ultrasonics 1980,18,107. (13) Levenspiel,0.ChemicalReactionEngineering; Wiley: New York, 1982. (14) Hart, E. J.; Henglein, A. J. Phys. Chem. 1986,90,5992. (15) Hart, E.J.; Fisher, H.; Henglein, A. J. Phys. Chem. 1986, 90,5989. (16)Wu, J. M.; Huang, H. S.; Livengood, C. D. Environ. Progr. 1992,11 (3),1951 (17) Reisz, P.; Berdahl, D.; Christman, C. L. Environ. Health Perspect. 1985,64,233. (18) Lindley, J.; Mason, T. J. Chem. SOC.Rev. 1987,16,275. (19) Ley, S.V.; Low, C. M. R. Ultrasound in Synthesis; Springer-Verlag: Berlin, 1989. (20) Montgomery,J. H.;Welkom, L. M. Groundwater Chemicals Desk Reference; Lewis Publishers: Chelsea, MI, 1990. Received for review October 26, 1993.Revised manuscript received April 18, 1994.Accepted April 27, 1994.' e Abstract published in
Advance ACS Abstracts, June 1, 1994.
