Environ. Sci. Technol. 1999, 33, 4102-4106
Heterogeneous Degradation of Carbon Tetrachloride: Breaking the Carbon-Chlorine Bond with Activated Carbon Surfaces GAYLE NICOLL AND JOSEPH S. FRANCISCO* Department of Chemistry and Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907-1393
Destruction of carbon tetrachloride was found to be 94% ( 1% at 300 °C for a single pass through activated carbon. Product analysis indicates that conversion was limited to low molecular weight gases. No formation of toxic halogenated organic compounds, such as phosgene, dioxins, dibenzo-p-dioxins, or dibenzofurans, were observed in the degradation process. Isotopic labeling experiments suggest that the activated carbon acts as a reagent and that carbon from the activated carbon may initiate the breaking of the carbon-chlorine bond in carbon tetrachloride.
Introduction Chlorofluorocarbons (CFCs) have been widely used as refrigerants, aerosol propellants, foam blowing agents, and chemical solvents in the microelectronic industry. Because these materials are chemically stable, nonflammable, and relatively nontoxic, CFCs are desirable materials for practical use. Because of their inertness and their unique stability, however, CFCs have accumulated in the stratosphere. The only sink for their destruction in the stratosphere is by photolysis, whereby chlorine is released. This causes destruction of stratospheric ozone (1-4). For this reason, production of CFCs has been reduced in accordance with the London amendment of the Montreal Protocol. Since the advent of the Montreal Protocol, which has limited further production of CFCs, there has become a need for environmentally friendly and efficient means of degrading these materials. There exists a massive stockpile of CFCs on the Earth’s surface (5). Disposing of this excess poses a difficult problem. While several methods have been proposed, including incineration (6), catalytic decomposition (7-9), photodestruction techniques (10), photocatalytic methods (11), biological degradation in anaerobic environments (12), and aerosol mineralization (13). Sonochemical destruction has also recently been explored as an alternative method (14-19). Burdeniuc and Crabtree (7) introduced a method by which the carbon fluorine bond could be broken, thereby initiating the oxidation process. There have been several recent reports on the use of activated carbon to decompose or reductively decompose organohalogen compounds (20, 21). These studies have also achieved destruction of these materials by reaction with alcohols or water over modified and unmodified activated carbon surfaces (21, 22). Here we report an alternative method * Corresponding author phone: (765)494-7851; fax: (765)494-0239; e-mail:
[email protected]. 4102
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by which the carbon-chlorine bond from CCl4 is broken using activated carbon as a reagent, followed by oxidation of the material into carbon dioxide using oxygen as a stoichiometric oxidant.
Experimental Methods Carbon tetrachloride (12CCl4 and 13CCl4) was obtained in high purity. The 12CCl4 (99.9%) was obtained from Aldrich. The 13 CCl4 (99.9%) was obtained from Cambridge Isotope Laboratory. The carbon tetrachloride samples were subjected to several freeze-pump-thaw cycles prior to use. No impurities were detected using FTIR and gas chromatographic/mass spectroscopic analysis (GC/MS). The method consists of passing equal quantities of oxygen and carbon tetrachloride through a reaction tube packed with activated carbon. The activated carbon (BPL 4-6 mesh) was obtained from Calgon Carbon Co. and had an average surface area of 1050 cm2/g. This activated carbon source was used consistently throughout all studies. However, activated carbon sources vary from source to source, so that this activated carbon source may or may not be unique. The activated carbon was first heated under vacuum (10-3 Torr) at 700 °C for 12 h to desorb water and contaminants (23). Prior to being passed through the activated carbon, the gases were passed through a desiccant cell to remove water from the reactants. Nitrogen was bubbled through the liquid carbon tetrachloride. The tube was heated to various temperatures while the gases were passed over the activated carbon once. Finally, the gases were passed through the activated carbon at a flow rate of 150 standard cm3min-1. Gas pressures were monitored using a MKS baritron capacitance manometer which is accurate to (0.01 Torr. Results were taken after a 15-min reaction time. This reaction interval corresponded to the time between the background scan and the sample scans, in which period the reactant gases were allowed to interact with the activated carbon surface. To minimize the reactions within the cell walls, reactions were carried out in Monel tubing, which is resistant to attack by halogenated compounds. Studies were carried out using on-line FTIR to monitor the concentrations of reactants and products using a Matteson Instruments Galaxy 7020 series spectrometer. Initially, pressure calibrations were done on carbon tetrachloride and carbon dioxide in order to monitor the concentrations of reactants and products in the reaction chamber. In addition, a background spectrum was recorded and subtracted from the data collected by the FTIR. Sixteen scans were averaged for each reaction, with a spectral resolution of 4 cm-1. By performing control reactions with the individual reactants and correcting the data as necessary, we have corrected all results for any side reactions attributable to activated carbon reacting with either oxygen or carbon tetrachloride separately. In particular, control experiments in which O2 was allowed to react with the activated carbon over the temperature range from 200 to 500 °C were performed. The CO and CO2 provided from this side reaction was corrected for in the data. To construct a calibration curve of the FTIR absorbances with reactant and product concentrations, individual gases were placed in the cell, and their absorbances were monitored as a function of pressure. Reproducibility, deviation, and accuracy of the sample concentrations were found to be approximately (2%.
Results and Discussion Figure 1 shows a sample FTIR spectrum obtained from the reaction of CCl4 with oxygen and activated carbon, showing 10.1021/es990316a CCC: $18.00
1999 American Chemical Society Published on Web 10/12/1999
FIGURE 1. (a) FTIR scan at 300 °C of the unreacted CCl4. (b) FTIR scan of the reaction between oxygen, activated carbon, and CCl4 at 300 °C showing the small amount of unreacted CCl4 and the production of carbon dioxide. (c) FTIR difference spectrum showing the decrease in CCl4 and corresponding increase in carbon dioxide. (d) FTIR scan of the reaction between oxygen, activated carbon, hydrogen, and CCl4 at 300 °C showing the small amount of unreacted CCl4 and the production of carbon dioxide and HCl. that carbon dioxide is the major product of the reaction. Table 1 is a summary of the product distributions obtained from the reaction of CCl4 with oxygen and activated carbon. The only species present in the FTIR spectrum are unreacted carbon tetrachloride, carbon dioxide, and carbon monoxide.
Note that 94% ( 1% of the CCl4 has been consumed in the reaction at 300 °C. The carbon monoxide produced is the result of the side reaction between activated carbon and oxygen (24). These results were also verified by using on-line GC/MS. VOL. 33, NO. 22, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Plot of CCl4 degradation versus temperature. The y-axis is the ratio of the CCl4 present at the end of the experiment to the CCl4 initially added to the reaction vessel.
TABLE 1. Product Distributions Obtained at Different Temperatures When 50 Torr of Carbon Tetrachloride in Nitrogen Is Reacted over Activated Carbon in the Presence of 50 Torr of Oxygen concentration (Torr) temperature (°C)
compound
initial
final
500 CO2 300 CO2
CCl4 0 CCl4 0
50
1.3 14.0 3.0 7.4
50
Solid samples of the activated carbon, which was used in a 300 °C reaction of 50 Torr of CCl4 with 50 Torr of oxygen, were subjected to thermal desorption studies in which the sample of activated carbon was placed in a Dynatherm Analytical model 850 thermal desorber. The temperature was ramped from 25 °C to 320 °C at a heating rate of approximately 80 °C/min. The temperature was then held at this final temperature of 320 °C for 2 min. The gases were fed into a GC/MS system to separate and detect the individual gases. The thermal desorption studies revealed that the only species present on the surface of the activated carbon were adsorbed carbon dioxide and relatively small quantities of adsorbed CCl4. This not only confirms the results found by FTIR and GC/MS but is also consistent with solid-state 13C NMR and microanalysis of portions of the same activated carbon sample. This also indicates that the unreacted carbon tetrachloride could be desorbed and collected for multiple passes through the reaction chamber. Because CCl4 has chlorine and the reaction is promoted by oxygen, there is the possibility of organic chlorinated compounds being produced. It has been reported in incineration studies of CFCs (2, 5) that dioxins and other halogenated hydrocarbons suspected to act as precursors for polychlorinated dibenzo-p-dioxins (PCDD) and dibenzofurans (PCDF) are produced. From the FTIR spectrum in Figure 1, which shows the chemical components in the exhaust gas from a CCl4/O2 mixture passed over activated carbon, the presence of phosgene or dioxin was not observed. At the higher temperature, 500 °C, neither phosgene nor dioxin was observed in the gas-phase exhaust. Moreover, a GC/MS scan taken during the reaction time period confirmed the observations from the FTIR. These substances were not detected using our system, which has a lower limit of detection of less than 1 × 10-3 Torr for these compounds using the FTIR. GC/MS scans taken during the reaction time 4104
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period have confirmed the observations from the FTIR that no phosgene or dioxin were produced to the detection limits of our experiments (roughly 0.2 nmol). Moreover, thermal desorption and solid-state NMR results did not indicate the presence of such compounds on the surface of the activated carbon. Temperature studies performed on the reaction indicated that significant degradation began at 150 °C, leveling off above 300 °C, as shown in Figure 2. The concentrations of reactant and the carbon dioxide product were also monitored as a function of time for the single-pass reaction. In all cases, the carbon dioxide production was directly proportional to the decline in carbon tetrachloride concentration, as shown in Figure 3. In the absence of activated carbon, no degradation was observed at 300 °C for CCl4/O2 mixtures at 50 Torr of each gas through the reactor. Single passes of the CCl4/O2 mixtures through the reactor at 300 °C using 150 g of activated carbon in the reactor resulted in degradation yield of 94%, while at 500 °C this increased to 97%. Air could be used instead of oxygen without affecting the efficiency of the process. The reaction times were not impacted with the use of air. However, because air does contain water, the water can react with the chlorine adsorbed on the surface to yield HCl. Studies were done in which hydrogen gas was added as a reagent. In contrast to previous research (25), in which halogenated methanes were degraded using hydrogen gas alone, this study employed carbon tetrachloride, oxygen, and varying concentrations of hydrogen as reagents to help determine the fate of the chlorine in the reaction vessel in the CCl4 reaction. With increasing hydrogen gas, the amount of HCl generated increased but started to level off when the ratio of hydrogen to carbon tetrachloride approaches about 1.5, as can be seen in Figure 4. There was also a corresponding decrease in the concentration of unreacted CCl4. The HCl peaks were observed to increase with temperature. These data suggest that the HCl product may form from the reaction of hydrogen gas with Cl2 or Cl atoms adsorbed at the surface of the activated carbon. An attempt was made to determine the form of the chlorine on the activated carbon. The reaction between 12CCl , oxygen, and activated carbon was carried out at 500 4 °C and pumped out. Nitrogen was passed over the activated carbon to desorb any species that was still adsorbed on the surface of the activated carbon. This gas was bubbled through water, which was placed downstream of the reaction tube, and the results were monitored by FTIR and GC/MS. These
FIGURE 3. Plot of the ratio of the concentration of carbon dioxide generated to the concentration of CCl4 remaining in the chamber versus time and the subsequent production of CO2 versus time.
TABLE 2. Ratio of 13CO2 to 12CO2 Produced at Varying Temperatures from Degradation of 13CCl4 over Activated Carbon and Oxygen When 50 Torr of 13CCl4 and 50 Torr of O2 Are Useda temperature (°C)
[13CO2]/[12CO2]
250 300 350 400 450 500 550 600
0.91 0.86 1.03 1.24 0.78 0.84 0.72 0.96
avg
0.92 ( 0.15
a
The results are consistent with a ratio of 1 within the error of the experiments.
FIGURE 4. Plot of the relative concentration of HCl produced when increased amounts of hydrogen gas are added to the reaction chamber with carbon tetrachloride and oxygen over activated carbon at 300 °C. studies did not indicate the presence of HCl as one of the products. Had Cl2 been produced as part of the reaction, this should have resulted in the production of HCl. In the absence of HCl being produced, this suggests that chlorine in the reaction may be bonded to the surface of the activated carbon. To determine some of the intermediate mechanistic steps of the oxidation process, experiments were performed using 13CCl as a reactant. In these experiments, 50 Torr of oxygen 4 and 50 Torr of only 13CCl4 were used. No 12CCl4 was present in the initial reactant mixture. Results from the FTIR show a significant increase in 13CO2 levels as well as the presence of 12CO2. In fact, FTIR results from the temperature studies of 13CCl4 from 200 to 600 °C (see Table 2) show that the 12CO /13CO ratio was consistently 1:1, as expected from the 2 2 proposed reaction scheme. This further indicates that activated carbon is playing a role in the process. In addition, 13CO was produced in direct proportion to the amount of 2 13CCl that decomposed, and this proportion increased with 4 increasing temperature. GC/MS results indicate the presence of both 12CO2 and 13CO2 as well as small quantities of unreacted 13CCl4, which corroborates the FTIR data. In addition, thermal desorption studies were performed on the activated carbon from the reaction, showing an increased
12CO
2/
13CO
2 ratio from the gases adsorbed on the activated carbon. Thermal desorption results also indicated that the 13 CCl4 had been totally decomposed. To gain additional insight into how the reactants were interacting with the activated carbon, a series of experiments were performed introducing the gases one at a time. Initially, one of the reactants was passed through the activated carbon at 500 °C. The vessel was then cooled to 60 °C and purged with nitrogen for 15 min to desorb any reactant that was physically adsorbed to the activated carbon. The temperature was raised to 500 °C again, and the other reactant was introduced into the cell. FTIR and GC/MS results indicated that when 13CCl4 was initially placed in the cell, followed by oxygen, 12CO2 and 13CO2 were produced in a 1:1 ratio. However, when oxygen was first placed in the cell and followed by 13CCl4, there was no 13CO2 production, indicating that it is necessary for the CCl4 to interact with the surface of the activated carbon in order for a reaction to occur. These data, coupled with the hydrogen studies and the studies without activated carbon in the cell, suggest that the process is heterogeneous. Besides its role as a solid support, the 13C isotope data suggest that the activated carbon is acting as a reagent, initiating the breakdown of carbon tetrachloride. Some recent experiments (26) in which carbon atoms are generated using laser-ablated graphite in the presence of a mixture of carbon tetrachloride and oxygen at room temperature show that carbon atoms can initiate the
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degradation process. It may be the case that dangling carbons on the activated carbon surface provides reactive sites for initiating degradation of carbon tetrachloride. The observation of CO2 suggests that intermediates such as CCl, CCl2, and CCl3 radicals may be involved in subsequent oxidation steps. Such intermediates have been proposed and observed in carbon tetrachloride degradation in sonochemical destruction processes (15, 16). Moreover, similar research has been performed by Hall and Holmes (27) that show that the chlorines can attach to the activated carbon surface from the reaction of phosgene with activated carbon. In summary, the results from these experiments show that carbon tetrachloride may be degraded over activated carbon surfaces. We find that destruction of carbon tetrachloride is quite efficient, i.e., about 94% for a single pass through activated carbon. In experiments in which a mixture of carbon tetrachloride, O2, and H2 are passed over the activated carbon, product analysis indicates that the major products are CO2 and HCl. Results from isotopic labeling experiments and studies with H2 suggest that the activated carbon acts as a reagent in initiating the degradation of carbon tetrachloride.
Literature Cited (1) (2) (3) (4)
Rowland, F. S. Environ. Sci. Technol. 1991, 25, 622. Molina, M. J.; Rowland, F. S. Nature 1974, 249, 810. Rowland, F. S.; Molina, M. J. Geophys. Space Phys. 1975, 13, 1. Anderson, J. G.; Toohey, D. W.; Brune, W. H. Science 1991, 251, 39. (5) Kaye, J. A.; Penkett, S. A.; Ormound, F. M. Report on Concentrations, Lifetimes, and Trends of CFC’s, Halons, and Related Species; NASA Reference Publication 1339; 1994.
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(6) Wakabayash, T.; et al. 9th International Symposium on Plasma Chemistry, September 1989. (7) Burdeniuc, J.; Crabtree, R. Science 1996, 271, 340. (8) Grey, C. P.; Corbin, D. R. J. Phys. Chem. 1995, 99, 12821. (9) Ohnishi, R.; Wang, W.; Ichikawa, M. Appl. Catal. A 1994, 113, 29. (10) Anderson, S. Idaho Business Review 1994, February 28. (11) Tanaka, K.; Hisanaga, T. Solar Energy 1994, 52, 447. (12) Chang, W. K.; Criddle, C. S. Biodegradation 1995, 6, 1. (13) Dufaux, D. P.; Zachariah, M. R. Environ. Sci. Technol. 1997, 31, 2223. (14) Cheung, H. M.; Kurup, Environ. Sci. Technol. 1994, 28, 1619. (15) Hua, I.; Hoffmann, M. R. Environ. Sci. Technol. 1996, 30, 864. (16) Choi, W.; Hoffmann, M. R. J. Phys. Chem. 1996, 100, 2161. (17) Choi, W.; Hoffmann, M. R. Environ. Sci. Technol. 1997, 31, 89. (18) Hung, H. M.; Hoffmann, M. R. Environ. Sci. Technol. 1998, 32, 3011. (19) Hung, H. M.; Hoffmann, M. R. J. Phys. Chem. A 1999, 103, 2734. (20) Tanada, S.; Kawasaki, N.; Nakamura, T.; Abe, I. J. Colloid Interface Sci. 1996, 177, 329. (21) Okajima, S.; Takeda, N. Taiki Kankyo Gakkaishi 1996, 31, 185. (22) Shinoda, K.; Miyatani, D.; Nakamura, T.; Shigaki, Y.; Yasuda, K. Nippon Kaguku Kaishi 1995, 4, 257. (23) (a) Ovcharenko, I. E.; Ya, Kuzyankov, Y.; Tatevaskii, V. M. Opt. Spectrok. 1965, 19, 528. (b) Hildenbrand, D. L. Chem. Phys. Lett. 1975, 32, 523. (24) Fritz, W. W.; Hu ¨ ttinger, K. J. Carbon 1993, 31 (6), 923. (25) Louw, R.; Mulder, P. J. Environ. Sci. Health 1990, A25 (5), 555. (26) Nicoll, G.; Francisco, J. S. Environ. Sci. Technol. 1998, 32, 3200. (27) Hall, C. R.; Holmes, R. J. Carbon 1992, 30 (2), 173.
Received for review March 19, 1999. Revised manuscript received August 30, 1999. Accepted September 14, 1999. ES990316A
