Detoxification of Trichloroethylene in a Low-Pressure Surface Wave

Research Detoxification of Trichloroethylene in a Low-Pressure Surface Wave Plasma Reactor JOSEP ARNO Ä AND JOHN W. BEVAN* Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255

MICHEL MOISAN Department de Physique, Universite de Montreal, Montreal, Quebec H3C 3J7, Canada

Application of surface wave plasmas as an alternative technology for the destruction and removal of chlorinated hydrocarbon pollutants is demonstrated. The destruction of parts per thousand concentrations of trichloroethylene in air/water or oxygen mixtures has been investigated in a low-pressure reactor at 5 Torr. Effluent analysis included the determination of destruction and removal efficiencies by electron capture gas chromatography and product distribution by Fourier transform infrared spectroscopy, mass spectrometry, and titrimetric analysis. TCE conversions of up to 99.998% were achieved using radio frequency power densities ranging from 1.6 to 7.5 kJ/L and millisecond range residence times within the plasma. Product analysis indicated that trichloroethylene conversion was limited to light gases, primarily CO2, CO, HCl, and Cl2.

Introduction In recent years, much research on chemical detoxification and pollution abatement has focused on the disposal of toxic and hazardous wastes containing halogenated hydrocarbons. These studies have been triggered by concerns over the adverse environmental effects of their emission into the air, soil, and water streams and the hazards associated with their toxicity. Chlorinated hydrocarbons are found in over 10% of all the waste streams at the Department of Energy-Defense Production facilities (1). Specifically, trichloroethylene has shown evidence of being a carcinogen in animals and is suspected of being a human carcinogen and teratogenic agent (2). Moreover, TCE undergoes photochemical reactions that contribute to the production of high-ground level concentrations of ozone * Corresponding author telephone: (409)845-2372; fax: (409)8454719; e-mail address: [email protected].

S0013-936X(95)00343-9 CCC: $12.00

 1996 American Chemical Society

in many urban areas. It was ranked as the 21st largest released and transferred chemical in the United States with a total of over 62 million lb (3) in 1987. The primary technology recommended by the EPA for the destruction of chlorinated hydrocarbons is incineration (4). However, incineration requires higher operating temperatures and residence times than comparable hydrocarbon incinerators. As a result of this and the greater expense associated with environmental controls, the cost of incinerating halogenated organic waste is higher than hazardous waste management designed for non-halogenated organic compounds (5). Thermal combustion of trichloroethylene has been studied in detail using theoretical models and in flames by Senkan and co-workers (6). Unfortunately, the elevated operating temperatures required to comply with environmental waste emission regulations also generate a number of organic radical species leading to the formation of a variety of hazardous byproducts, for example, phosgene, CCl4, and C2Cl4. Recently, a number of alternative technologies have been developed to destroy halogenated organic wastes. Examples of these innovative processes include thermal, chemical, physical (including plasma-based technologies), and biological methods (4, 7). A number of innovative nonthermal and thermal technologies involving plasmas have also been developed (8, 9). Application of highfrequency (including radio and microwave frequency) discharges to the removal of halogenated hydrocarbons has been limited. Barat and Bozzelli (10) and Hertzler (11) have investigated the destruction of halogenated hydrocarbons in low-pressure tubular reactors using microwave discharges. Wakabayashi and co-workers (12) and Krause et al. (13) investigated the plasma destruction of halogenated compounds under atmospheric reactor pressures. The characterization of surface wave plasmas and their application to the detoxification of high concentrations of acetone has also been described (9). In this paper, we have studied the low-pressure surface wave plasma reactions of trichloroethylene with air/water vapor or molecular oxygen in a tubular flow reactor as a function of absorbed radio frequency power. This study includes: determination of destruction and removal efficiencies and effluent analysis.

Experimental Section The experimental apparatus used in this study has been described in detail elsewhere (9) and will only be considered briefly here. The surface wave high-frequency discharge was produced using a radio frequency wave generator, a power amplifier, a matching box, and a surface wave launcher. Whenever oxygen was used, its flow was metered using a Matheson 603 flow meter and introduced into a 13 mm o.d., 10.5 mm i.d. quartz tubular reactor. Air was saturated with water by forcing ambient air through a fritted glass tube submerged in room temperature distilled water. The resultant water vapor/air mixture was delivered into the reactor at a fixed rate of 3.2 L/min using a Matheson

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603 flow meter. A wide bottom vacuum-sealed glass container was filled with trichloroethylene, and its vapor was introduced into the reactor via a calibrated 602 Matheson flow meter. The volumes of TCE and water delivered into the reactor were determined by accurately measuring the weight differences before and after each experiment. The TCE concentration was fixed throughout the study to approximately 20 ppt. Such high initial concentrations were chosen to allow the detection of the minute TCE concentrations remaining in the low-pressure effluent mixture after considering the detection limits of the analytical instrument. The overall reactor system was leak checked and kept under low pressure utilizing a twostage E2M-175 Edwards pump.

Product Analysis Trichloroethylene Analysis. Quantitative analysis of TCE was performed using an in-line 14-A Shimadzu gas chromatograph equiped with an electron capture detector (GC/ ECD). Aliquots from the reactor effluent gas mixture were drawn and pressurized to slightly over 1 atm using N2 gas, and 10-mL volumes were injected into the GC via a 10-port Valco injection valve. Before sample gas collection, all transport lines, loops, and volumes were purged and evacuated to a few pascals using a Welch dual seal vacuum pump. In addition, all surfaces were heated to minimize condensation and to avoid adsorption of analytes onto the confining walls. Before each run, background volumes were systematically collected, injected, and analyzed to confirm low TCE contamination levels. The electron capture/gas chromatograph used nitrogen (99.995% purity) as the carrier gas. The sample constituents were separated using a 60/ 80 Carbopack B/1% SP-1000, 3 m long, 3.2 mm diameter stainless steel column. The oven was programed to maintain 313 K for 3 min and to ramp at 32 K/min until it reached 493 K. In certain selected runs, the plasma product species were preconcentrated by an on-line liquid nitrogen trap before entering the vacuum pump. After cryotrapping for a fixed amount of time (20 min), the trap was isolated and allowed to reach room temperature. One half of the volume of liquid remaining at the bottom of the trap was dissolved in acetone, and 1-µL samples were injected into the previously described GC/ECD to provide a second independent quantitative analysis of the remaining trichloroethylene. In order to compensate for trap losses, we have experimentally determined the efficiency of the trap in collecting TCE (60%). The actual trichloroethylene destruction and removal efficiencies (DREs) were determined by averaging independent gas and liquid analysis. A calibration curve was generated in order to characterize the electron capture detector response to the analyte concentration. This calibration curve was carefully chosen to cover the concentration range of the samples injected and to ensure the relatively short linear dynamic range of the electron capture detector (14). We have compared the trichloroethylene mass density (g/mL) before and after entering the plasma reactor to compute the correspondent DRE from its standard definition (15):

% DRE )

(

)

Win - Wout × 100 Win

(1)

where W corresponds to the weight of TCE in the waste

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and the in and out subscripts denote the quantity before and after being processed. Organic Liquid Analysis (C3 or Larger). Samples of the cryotrapped liquid (1 µL) were injected into a Hewlett Packard 5890 Series II GC/MS equipped with a 30 m, 0.25 mm i.d. Alltech SE-54 (cross-linked 5% phenylmethyl silicone) for qualitative analysis of C3 or larger organic species. In addition, 1-mL volumes were extracted directly from the reactor using a 10-mL Dynatech Pressure Lock Series A-2 gas syringe and introduced into the previously described GC/MS for analysis. VOC and Gas Analysis. Once the liquid nitrogen trap was isolated, a closed system was devised to allow captured gas molecules to expand into a previously evacuated 4-L glass bulb as the system reached room temperature. We have used a Bomem DA8.002 spectrophotometer equipped with an MCT detector and a 18-cm path length with KBr windows gas cell to characterize these gas mixtures. The spectrophotometer operated at 0.1 cm-1 resolution, averaged 25 scans, and covered the spectral region between 1800 and 4000 wave numbers. Identification of HCl, NO2, N2O, CO2, and ClNO was accomplished on the basis of rotational constants and band origins and by characterizing rotationally resolved vibrational bands with the substructure frequency transitions of standard reference molecules chosen for calibration purposes. The absorbances of selected rovibrational lines of different concentrations of pure CO2, HCl, NO2, and N2O gases were measured to generate calibration lines for each species. The concentrations of these gases in the unknown mixture were determined by matching the absorbances of the selected lines with the calibrated ones. Because of the unavailability of standard ClNO gas, calibration of nitrosyl chloride was performed indirectly by measuring the concentration of HCl and nitrogenated species in the multi-step reaction:

2NO2 + 4HCl a 2ClNO + 2H2O + Cl2

(2)

Equal concentrations of the reactants were introduced in the previously described gas cell, and the equilibrium concentrations of the HCl and nitrogenated species were monitored via FTIR spectroscopy. The absorbance measurements were carried out for the reaction at 296, 305, and 317 K, and the absorbances of the selected lines were temperature corrected accordingly. The kinetics of reaction 2 are explained in detail elswhere (16). The concentration of carbon monoxide was estimated from the overall carbon mass balance on the basis of total conversion of parent carbon into CO2 and CO. This assumption is based on the results from mass spectrometric analysis, which failed to detect any carbon-containing compound other than TCE. In addition, previous work involving direct FTIR observation of the effluent gases resulting from a low-pressure oxygen/argon TCE reaction (17) corroborated this conjecture. That reaction resulted in the conversion of all carbon atoms contained in TCE into CO2 and CO without producing detectable concentrations of CH4, C2H2, or any other light hydrocarbons. Similarly, chlorine gas yields were approximately determined from chlorine mass balance assuming a complete conversion of original chlorine into HCl, NOCl, and Cl2. Any water remaining at the bottom of the trap was titrated with a 0.3 M NaOH solution to determine the total HCl concentration. The presence of HCl was confirmed by precipitation of AgCl upon the addition of a characterized

TABLE 1

Plasma Reactor Conditions and Trichloroethylene Conversions feed composition plasma power (molar ratios) power length density residence av. pressure (W) (cm) (W/mL) time (ms)

% TCE conversion

5.3 Torr

100 150 200 300 400

Air/H2O/TCE (200/3.5/1) 17.4 2.6 1.0 20.0 3.5 1.1 22.5 4.1 2.8 28.1 4.9 3.1 32.7 5.7 3.6

99.96 99.990 99.994 ( 0.002a 99.995 ( 0.002b 99.997 ( 0.004c

6.3 Torr

200 300 400

O2/TCE (200/1) 25.0 3.7 2.9 29.3 4.7 3.5 32.7 5.6 3.6

99.990 ( 0.002d 99.998 ( 0.002e 99.9947

a Average from three independent samples using two quantitative analytical methods. b Average from five independent samples using two quantitative analytical methods. c Average from two independent samples using two quantitative analytical methods. d Average from two independent samples. e Average from three independent samples using two quantitative analytical methods.

solution of AgNO3. Due to the reactive nature of some of the products (especially HCl and Cl2), the stability of the final gas mixture was not guaranteed, and potential further reactions of these chlorinated gases (especially with NO and NO2) could induce uncertainties in the product distribution.

Results and Discussion The conversion of trichloroethylene was studied using different applied rf powers in a low-pressure, air/water or oxygen, surface wave plasma reactor. The plasma column length was found to increase with applied power ranging between 17 to 33 cm in length for applied rf powers of 100-400 W used in our experiments. Residence times were computed as a function of plasma length and gas velocity. The latter was determined using the following expression (18):

(QT Ap )

vf (m/s) ) 5.31 × 10-9

(3)

where Q is the total gas flow rate (in L/s), A is the cross sectional area of the reactor (in m2), and T (K) and p (Pa) are the average reactor temperature and pressure. In this study, the latter were approximated to be the ones measured 1 m from the wave launcher and ranged between 330 and 410 K depending on specific reactor conditions. A summary of the plasma reactor conditions and trichloroethylene conversions are included in Table 1. The determined percent TCE destruction for the air/water/TCE as a function of applied rf power is illustrated in Figure 1. We have used three criteria to approximate the efficiency of the trap to capture selected light gases and volatile organic compounds. Foremost, only the compounds with freezing and boiling points above the temperature of the walls of the liquid nitrogen trap (77 K) had a chance to be captured and later detected. Second, the vapor pressure of the substances at the trap temperatures had to be orders of magnitude lower than the partial pressure of the gas in the vacuum system to avoid significant losses from the trap. In order to quantitate this effect, we have used the expression for the rate at which a cold trap collects a given compound

FIGURE 1. Trichloroethylene percent destruction versus absorbed power in an air/water TCE surface wave plasma reactor.

defined as (19)

(

)

Pvp A S ) 62.5 1(L/s) Ppp xMW

(4)

where MW, Ppp, and Pvp are the molecular weight, the partial pressure, and the vapor pressure at the trap temperature of the gas, and A is the full cold area (in cm2). Equation 4 was used to discriminate between common effluent gases, ruled out the possibility to significantly collect carbon monoxide, methane, and nitrogen oxide, and warranted quantization of CO2, N2O, NO2, HCl, NOCl, certain light hydrocarbons, and other nitrogenated and chlorinated organic compounds. It is important to note that eq 4 neglects the effects of possible molecular diffusion of condensable compounds through a noncondensable gas. The third criteria was based on an investigation by Brown and Wang (20) involving the capture coefficients of a 77 K cryosurface for several common gases. They observed a relationship between capture coefficients and both the molecular weight and the heat of condensation of the gas. These physical parameters have been used in our study to estimate the capture coefficients of the compounds observed, ranging from 0.6 for HCl, CO2, and N2O to 0.9 for phosgene. Gas chromatographic/mass spectrometric analysis did not reveal the presence of any high molecular weight (C > 3) organic species produced in the plasma reactor. In particular, the absence of phosgene in wet oxidation plasma processes has been explained (8) using reaction pathways involving OH radicals. This inability to generate COCl2 has also been reported by Newhouse and co-workers in their silent discharge plasma wet oxidation of TCE (21). All sample injections from the acetone-dissolved trapped liquid contained high concentrations of hexachloroacetone, confirming the presence of HCl in the highly acidic solution. Air/Water/TCE Product Analysis. We have investigated the surface wave plasma reaction of trichloroethylene in water vapor and air. The reaction conditions (see Table 1) included fixed pollutant concentration at 23 ppt, average pressure of 5.3 Torr, and applied rf powers in the range between 100 and 400 Watts. Although no effort was made

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TABLE 2

Product Distribution from Plasma Reaction of Trichloroethylene composition and molar ratios

power (W)

plasma length (cm)

res. time (ms)

HCl/Cl2 molar ratio

CO2/CO molar ratio

[N2O] ppm (air/N2O)

[NO2] ppm (air/NO2)

[NOCl] ppm (air/NOCl)

air/water/TCE (195/3.5/1) air/water/TCE (205/3.5/1) air/water/TCE (196/3.5/1) oxygen/TCE (193/1)a

200 300 400 300

21.2 27.6 33.9 30.4

2.7 3.6 3.8 3.6

1.7 5.3 1.0 0.36

0.9 1.36 32.3 1.7

232 258 204 n/a

37 97 764 n/a

104 324 177 n/a

a

Trace amounts of phosgene detected.

to measure the temperature within the plasma, the temperature of the effluent gases measured 1 m from the wave launcher increased from 340 K at the lowest power to 360 K at its highest. The plasma column length was observed to increase linearly with applied input power, resulting in longer interaction between reactants and ionizing radiation. Figure 1 illustrates the determined averaged TCE destruction and removal efficiencies at different applied powers. As expected, the efficiency of the reactor increased with increasing applied rf power and resulted in relatively large pollutant removals (99.96%) when utilizing 100 W of rf power. The approximate product distributions from selected 200-, 300-, and 400-W trial run analysis are summarized in Table 2. Between 1.1 and 1.2 mL of highly acidic liquid was collected at the bottom of the trap with a density of 1.17 g/mL. Mass spectrometric analysis did not identify any chlorinated, nitrogenated, or hydrogenated nonparent organic species in these liquid solutions other than chlorinated derivatives of the solvent used to extract any potential organic products. Titrimetric analysis determined H2O/HCl molar ratios of 4.6, 2.4, and 6.2 for the 200-, 300-, and 400-W trial runs, respectively. Carbon dioxide formation was favored by higher delivered powers reaching a CO2/CO molar ratio of 32 at 400 W. The nitrous oxide (N2O) concentration, on the other hand, remained relatively constant (200-250 ppm) regardless of the reactor conditions. The generation of nitrogen dioxide (NO2) experienced an exponential growth with delivered power, while nitrosyl chloride (ClNO) concentration reached its maximum at 300 W. Hydrogen chloride to chlorine gas molar ratios reached a maximum of 5.3:1 at 300 W of applied rf power. The striking simplicity of the species produced by the plasma reaction (diatomic and triatomic molecules) is in contrast to previously reported non-equilibrium plasma studies. Hollahan (22) described the reactions of saturated and unsaturated organic substances with hydrogen and oxygen leading to a wide product distribution of nonparent hydrocarbon species. This has been corroborated by Bozzelli et al. (10) in their low-pressure reactions of chlorinated hydrocarbons under reducing and oxidizing conditions. More surprising is the absence of other nitrogenated organic species such as hydrogen cyanide or ammonia as they have been found (22, 23) in all nitrogenorganic nonthermal plasma reactions. Oxygen/TCE Product Analysis. We have also studied the conversion of trichloroethylene in an oxygen surface wave plasma reactor at average pressures of 6.3 Torr. Pollutant concentrations were kept at 20 ppt, and the forward radio power varied between 200 and 400 W. Both the temperature of the effluent gases 1 m away from the wave launcher and the plasma column length increased linearly from 350 K and 25 cm at 200 W to 410 K and 33 cm

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at 400 W, respectively. The dependence of percent TCE conversion on delivered high frequency power is included in Table 1. Almost 100% destruction of the contaminant was attained utilizing as low as 200 W of applied power. These results are similar to those observed for the air/water/ TCE mixture investigation. Thus, other than the formation of ppm concentrations of NOx species, there is no inherent disadvantage in using air as the carrier gas using this technology. Product distribution analysis was performed only for the 300-W experiment, and the results are summarized in Table 2. Mass spectrometric analysis failed to identify any nonparent species (C > 3) in the trap or within the gas mixture. Spectrophotometric analysis indicated a 1:3 ratio between plasma-generated molecular HCl and chlorine gas. This is not surprising considering the Cl to H atomic ratio of the reactants introduced into the reactor. The process favored the production of CO2 over CO with a determined molar ratio of 1.7. In addition, small concentrations of phosgene were also detected. The concentration of COCl2 has been determined using the IR absorbtion cross section of phosgene relative to the already calibrated carbon dioxide. Using this indirect method, the concentration of phosgene was determined to be 25 ppm. The product distributions resulting from the surface wave plasma reactions that we have investigated are likely to stem from both thermal and nonthermal processes. During the nonthermal stage, the species travel throughout the plasma zone and are subjected to a number of electron collisions in a time range of 1-5 ms. Unlike plasma processes occurring in the few pascal pressure range, the neutral gas temperatures within the plasma at the reactor pressures used in our investigation are estimated to be between 1000 and 1700 K, depending on the power applied, reactor pressure, and gas composition (24). Thus, in the second stage, the stable and intermediate species generated in the first stage experience a fast cooling (ca. 100 K/ms) as they exit the plasma volume. The resulting short residence times are likely to limit the number of slow multistep mechanisms essential in the formation of more complex nonparent chlorohydrocarbon species. In conclusion, this study demonstrates the feasibility of utilizing low-pressure surface wave plasmas to detoxify volatile halocarbons in gaseous mixtures. The reactor has been shown to efficiently destroy TCE under conditions of low molecular residence times and pressures. Moreover, the process did not require the addition of expensive rare gases or preheating the reactant gas mixture. In addition, as expected in these oxygen-rich conditions, we did not observe any soot formation within the reactor walls that would hinder continuous operation. These aforementioned characteristics make surface wave discharges an attractive alternative for the remediation of airstreams from industrial

processes operating under low pressures like semiconductor processing or pump-and-treat systems.

Acknowledgments The Robert A. Welch Foundation of Houston, TX, is thanked for partial financial support of this research in the form of a pre-doctoral fellowship for J.A. We want to acknowledge the Texas Advanced Technology Research Program (TATRP), the Office of the Vice President for Research and Associate Provost for graduate studies, through the Center for Energy and Mineral Resources, Texas A&M University, and Rf Environmental Systems for providing the financial support for this research. The authors are grateful to Dr. B. A. Wofford for his technical advice and to Laura Roessler from ATMI EcoSys Corp. for her help in the calibration of phosgene.

Literature Cited (1) Foree, C. S.; Post, R. G. Waste Manage. (Tucson, Ariz.) 1986, 1, 495. (2) Halogenated Solvent Cleaners, Emission Control Technologies and Cost Analysis; Radian Corporation; Noyes Data Corporation: Park Ridge, NJ, 1990. (3) Doa, M. J. Hazard. Waste Hazard. Mater. 1992, 9, 61. (4) Suprenant, N.; Nunno, T.; Kravett, M.; Breton, M. HalogenatedOrganic Containing wastes, Treatment Technologies; Noyes Data Corporation: Park Ridge, NJ, 1988. (5) Santolery, J. J. In Standard Handbook of Hazardous Waste Treatment and Disposal; Freeman, H. M., Ed.; McGraw-Hill: New York, 1988. (6) Yang, M.; Karra, S. B.; Senkan, S. M. Hazard. Waste Hazard. Mater. 1987, 4, 55. (7) Freeman, H. Innovative Thermal Hazardous Organic Waste Treatment Processes; Noyes Data Corporation: Park Ridge, NJ, 1985. (8) Proceedings of the Workshop on the Treatment of Gaseous Emissions Via Plasma Technology; NIST: Gaithersburg, MD, 1995. (9) Arno´, J.; Bevan, J. W.; Moisan, M. Environ. Sci. Technol. 1995, 29, 1961.

(10) Barat, R. B.; Bozzelli, J. W. Environ. Sci. Technol. 1989, 23, 666. (11) Hertzler, B. C. Development of Microwave Plasma Detoxification Process for Hazardous Wastes (Phase II). U.S. EPA Contract 6803-2190, 1979. (12) Wakabayashi, T.; Mizuno, K.; Imagowa, T. I.; Amano, T.; Hirakawa, S.; Komaki, H.; Kobayashi, S.; Kushiyama, S.; Aizawa, R.; Koinuma, Y.; Ohuchi, H. Decomposition of Halogenated OrganicCompounds by R. F. Plasma at Atmospheric Pressure. Presented at the 9th International Symposium on Plasma Chemistry, Pugnochiuso, Italy, September 4-8, 1989. (13) Krause, T. R.; Helt, J. E. In Emerging Technologies in Hazardous Waste Management III; Tedder, D. W., Pohland, F. G., Eds.; ACS Symposium Series 518; American Chemical Society: Washington, DC, 1993. (14) Sevcik, J. Detectors in Gas Chromatography; Elsevier Scientific Publishing Company: New York, 1976. (15) Brunner, C. R. Hazardous Air Emissions from Incinerators; Chapman and Hall: New York, 1985. (16) Ching-Chuan, C. K.; Wilkins, R. A.; Hisatsune, I. C. Ind. Eng. Chem. Fundam. 1976, 15, 236. (17) Arno´, J. I. Ph.D. Dissertation, Texas A&M University, College Station, TX, 1995. (18) Granier, A. In Microwave Discharges: Fundamentals and Applications; Moisan, M., Ferreira, C. M., Eds.; NATO ASI Series B 302; Plenum Press: New York, 1993; p 491. (19) Barrington, A. E. High Vacuum Engineering; Prentice-Hall, Inc.: Englewood Cliffs, NJ, 1963. (20) Brown, R. F.; Wang, E. S. J. Adv. Cryog. Eng. 1964, 10, 283. (21) Newhouse, E. I.; Neely, W. C.; Clothiaux, E. J.; Rogers, J. W. Presented at the I&EC Special Symposium of the American Chemical Society, Atlanta, GA, 1994. (22) Hollahan, J. R. Techniques and Applications of Plasma Chemistry; Hollahan, J. R., Bell, A. T., Eds.; John Wiley & Sons: New York, 1974. (23) Plasma Technology: Fundamentals and Applications; Capitelli, M., Gorse, C., Eds.; Plenum Press: New York, 1992. (24) Brown, L. C.; Bell, A. T. Ind. Eng. Chem. Fundam. 1974, 13, 203.

Received for review May 22, 1995. Revised manuscript received March 29, 1996. Accepted April 5, 1996.X ES950343A X

Abstract published in Advance ACS Abstracts, June 1, 1996.

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