Environ. Sci. Technol. 2005, 39, 6825-6830
Abiotic Degradation of Trichloroethylene under Thermal Remediation Conditions JED COSTANZA,† EVA L. DAVIS,‡ JAMES A. MULHOLLAND,† AND K U R T D . P E N N E L L * ,† School of Civil and Environmental Engineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, Georgia 30332-0512, and Ground Water and Ecosystems Restoration Division, Office of Research and Development, U.S. Environmental Protection Agency, 919 Kerr Research Drive, Ada, Oklahoma 74820
The degradation of trichloroethylene (TCE) to carbon dioxide (CO2) and chloride (Cl-) has been reported to occur during thermal remediation of subsurface environments. The effects of solid-phase composition and oxygen content on the chemical reactivity of TCE were evaluated in sealed ampules that were incubated at 22 and 120 °C for periods ranging from 4 to 40 days. For all treatments, no more than 15% of the initial amount of TCE was degraded, resulting in the formation of several non-chlorinated products including Cl-, CO2, carbon monoxide, glycolate, and formate. First-order rate coefficients for TCE disappearance ranged from 1.2 to 6.2 × 10-3 day-1 at 120 °C and were not dependent upon oxygen content or the presence of Ottawa sand. However, the rate of TCE disappearance at 120 °C increased by more than 1 order-of-magnitude (1.6 to 5.3 × 10-2 day-1), corresponding to a half-life of 1344 days in ampules containing 1% (wt) goethite and Ottawa sand. These results indicate that the rate of TCE degradation in heated, three-phase systems is relatively insensitive to oxygen content, but may increase substantially in the presence of iron bearing minerals.
Introduction Thermal remediation encompasses a number of technologies designed to deliver thermal energy (heat) to the subsurface, including hot water injection, steam injection, conductive heating, resistive heating, and electromagnetic heating. The resulting increase in subsurface temperature can lead to substantial changes in the distribution of chlorinated ethenes between the solid, liquid, and gas phases, with the majority of the contaminant mass existing in the gas phase at elevated temperature (1). Although thermal treatment technologies are capable of removing large quantities of contaminant mass from the subsurface (2), a fraction of the initial mass is likely to persist after treatment, particularly in highly contaminated source zones. Dissolved-phase contaminant concentrations may also increase during thermal treatment due to enhancements in aqueous solubility, mass transfer, and organic liquid mobility at elevated temperature and pressure. Optimization techniques, such as depressurizing the subsurface during steam flooding, have been employed to * Corresponding author phone: (404)894-9346; fax: (404)894-8266; e-mail:
[email protected]. † Georgia Institute of Technology. ‡ U.S. Environmental Protection Agency. 10.1021/es0502932 CCC: $30.25 Published on Web 07/30/2005
2005 American Chemical Society
reduce the amount of dissolved-phase contaminant mass remaining after thermal treatment (3). In addition, hydrous pyrolysis/oxidation (HPO) is claimed to reduce contaminant levels through chemical reactions that occur during in situ thermal remediation (4, 5). Experimental evidence supporting aqueous phase oxidation of chlorinated ethenes during thermal treatment is based primarily on the results of a single laboratory study in which the concentration of trichloroethylene (TCE) was monitored in a completely water-filled, gold-walled reactor operated at temperatures between 70 and 100 °C (6). At 90 °C, the average first-order half-life for TCE disappearance was 1.8 days, with only carbon dioxide (CO2) and chloride (Cl-) detected as degradation products (6). Earlier studies of dissolved-phase TCE stability yielded a first-order half-life for TCE disappearance of 49 years at 90 °C and neutral pH (7, 8). These contrasting half-life values illustrate the relatively large variability of published TCE degradation rates at elevated temperatures, and the potential sensitivity of thermally induced TCE degradation to experimental conditions. Additional factors that may impact thermal degradation experimental studies include the presence of solids, incubation time, reactor materials, and solution conditions. Improved knowledge of abiotic chlorinated ethene degradation reaction pathways at temperatures and conditions relevant to thermal remediation is needed to assess the potential for chlorinated ethene degradation and to evaluate the potential formation of undesirable degradation products during thermal treatment of DNAPL-contaminated sites. Currently, experimental data are not available to assess the long-term stability of chlorinated ethenes in heated, threephase systems. In addition, chlorinated ethene degradation products that could potentially form during thermal treatment of subsurface systems are not necessarily limited to CO2 and Cl-, as some degradation pathways create stable intermediate products. Toxic compounds such as dichloroacetyl chloride (CHCl2COCl) and phosgene (COCl2) have been detected during gas-phase oxidative treatment of TCE by photocatalytic processes (9, 10), whereas cis-DCE and vinyl chloride (VC) have been detected during reductive dechlorination of TCE. The objective of this study was to evaluate the effects of oxygen content and the presence of solids on the abiotic degradation of TCE at elevated temperature. Batch experiments were performed in hermetically sealed ampules that were incubated at 22 and 120 °C for periods of up to 40 days. The ampules were prepared under either anoxic or oxic conditions and were amended with dissolved-phase TCE and either Ottawa sand or Ottawa sand with 1% (wt) goethite (FeOOH). Goethite was selected as a solid-phase additive because it is known to react with halogenated hydrocarbons (11) and is a common mineral in natural soils (12, 13). The ampules were destructively sampled over time to obtain aqueous and gas-phase constituent data, which were used to assess the rate of TCE disappearance and identity of degradation products.
Experimental Section Materials. Reagent grade (99.5%, American Chemical Society) TCE, without anti-oxidant stabilizers, was obtained from Sigma-Aldrich (Milwaukee, WI). Formate and glycolate solutions were prepared from 99% grade solids (ACROS Organics, Morris Plains, NJ), while acetate, oxalate, and sulfate solutions were prepared from ACS certified solids (Fisher Scientific, Fair Lawn, NJ). Approximately 2 kg of ASTM 2030 mesh Ottawa sand, obtained from U.S. Silica (Berkeley VOL. 39, NO. 17, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Springs, WV), was treated by soaking in a 1 N solution of nitric acid for 30 min. Following a second 1 N nitric acid wash, the sand was rinsed with DI water until pH 7 was achieved. The sand was then placed in a drying oven and heated to 130 °C for 3 h to remove excess water, and baked at 200 °C for an additional 2 h. Research grade goethite, collected in Grants County, NM, was obtained from Ward’s Natural Science (Rochester, NY). Goethite chips (∼1 g each) were ground into a fine powder (silt to clay size particles) using a porcelain mortar and pestle. Ground goethite (10 g) was added to 1000 g of acid-treated Ottawa sand to create a 1% (wt) mixture. Prior to use, the Ottawa sand and Ottawa sand with 1% goethite mixture were autoclaved for 25 min at 121 °C. Solution Preparation. Aqueous solutions were prepared with deionized (DI) water that was treated with a Nanopure analytical deionization system (model D4741, Barnstead International, Dubuque, IA) and dispensed through a 0.2 µm pore size filter. Low dissolved-oxygen content water, referred to as anoxic water, was prepared by sparging freshly dispensed DI water with purified argon gas (Airgas-South, Inc., Marietta, GA) for at least 1 h. The dissolved oxygen content of anoxic water was between 0.2 and 0.3 mg/L as indicated by the Rhodazine D method (CHEMetrics, Calverton, VA). Oxygen-saturated water, referred to as oxic water, was prepared by sparging DI water with ultra zero grade air (Airgas-South, Inc., Marietta, GA) for at least 1 h to yield dissolved oxygen concentrations between 8 and 10 mg/L as indicated by the Indigo Carmine method (CHEMetrics, Calverton, VA). Stock TCE solutions were prepared in 2 L flasks that were filled with either anoxic or oxic DI water. Neat TCE was added to the 2 L flasks to create a 100 mg/L solution without the use of a cosolvent. The 2 L flasks were then sealed and covered with aluminum foil to minimize light exposure. The contents of the flasks were mixed with a Teflon-coated magnetic stir bar for a period of 24 h at room temperature (22 °C). Ampule Preparation. Batch experiments were conducted in clear, 50 mL funnel-top borosilicate glass ampules (Wheaton Science Products, Millville, NJ). Prior to use, the ampules were autoclaved for 25 min at 121 °C, rinsed with DI water, and dried in an oven at 200 °C for 2 h. The ampules were removed from the oven and placed in a glass desiccator containing Drierite (W.A. Hammond Drierite Co., Xenia, OH). The desiccator was evacuated under a vacuum of 750 mmHg and flushed with either argon gas or ultra zero grade air to achieve anoxic or oxic conditions, respectively. After being cooled to room temperature, each ampule was removed from the desiccator and filled with approximately 40 mL of 100 mg/L TCE stock solution or TCE-free water. The solutions were dispensed under positive pressure from the appropriate (i.e., anoxic or oxic) 2 L flask through a Teflon tube, and each ampule was temporarily sealed with aluminum foil until flame sealed, similar to the procedure described by Barbash and Reinhard (14). For ampules containing solids, ca. 20 g of autoclaved Ottawa sand or Ottawa sand with 1% (wt) goethite was loaded prior to addition of the aqueous phase. The ampules were flame sealed using a propane-oxygen torch (BernzOMatic, Medina, NY). A batch of 36 ampules was prepared for each experimental system considered (i.e., oxic, anoxic, anoxic with Ottawa sand, and anoxic with Ottawa sand and 1% goethite) to yield a total of 144 ampules. For each batch of ampules, 18 ampules were prepared with 100 mg/L of TCE and 18 ampules contained no TCE. One-half of TCE-amended and TCE-free ampules from each batch were incubated at 120 °C in a convection oven (VWR model 1320, VWR International, West Chester, PA), while the remaining ampules were stored at room temperature (22 °C) in the dark as temperature controls. Oven temperature was monitored daily using a certified 6826
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traceable thermometer (Fisher Scientific, Fair Lawn, NJ). An explosion occurred 4 days after ampules amended with solids were placed in the convection oven, destroying 54 ampules that were in the oven. An explosion-resistant incubation apparatus was constructed from an aluminum block that was milled with 12 holes and placed on a benchtop hot-plate (Fisher Scientific, Fair Lawn, NJ). Two K-type thermocouples connected to a data logger (Campbell Scientific, Logan, UT) were used to monitor the temperature of the heating system. After 1 day of heating, the temperature in the aluminum heat block reached 120 °C, while the temperature in upper portion of the ampule, which was not enclosed within the heat block, was 105 ( 11 °C. Thirty-six of the original temperature control ampules were incubated at 120 °C in the heat block apparatus, while the remaining 18 ampules were stored at 22 °C in the dark. Sample Collection. Immediately prior to flame sealing, 1 mL water samples were collected from three randomly selected ampules in each batch of 18 ampules to determine the initial aqueous phase TCE concentration. To sample the gas phase, the ampule was inverted and the neck was broken by hand. The water within the inverted ampule body did not drain out because air could not flow past the water-filled ampule opening. The opened ampule was then placed in a sampling device, and a 10 mL gas sample was collected using a gastight syringe (Becton Dickinson, Franklin Lakes, NJ) fitted with a 30 cm long needle. A stream of argon gas passed through the sampling device at a rate of 500 mL/min to minimize the introduction of air during sample collection. Following gas sampling, a 1 mL aqueous sample was collected by gastight syringe and transferred to a sealed 20 mL bottle for GC analysis using a headspace method. Analytical Methods. For the headspace method, aqueous phase concentrations of TCE were determined using a Hewlett-Packard (HP) model 6890 GC equipped with a HP 7694 headspace autosampler and a 60 m by 0.32 mm DB-624 column (Agilent Technologies, Palo Alto, CA) connected to a flame ionization detector (FID). The headspace autosampler was programmed to hold each sample at 70 °C for a period of 15 min prior to transferring the headspace gas to the injection port. Calibration standards were prepared by injecting small volumes of a 10 000 mg/L TCE methanol stock solution into 100 mL volumetric flasks that contained DI water chilled to a temperature of 4 °C. The haloacetic acid content of ampule water was determined using procedures based on EPA method 552.2 (15), with methanol as the derivatization agent. Concentrations of formate (CHOO-), glycolate (COH3COO-), acetate (CH3COO-), oxalate (COOHCOO-), sulfate (SO42-), and Clin the aqueous phase were determined using a Dionex DX100 ion chromatograph (IC) equipped with an AS14A IonPac column. A 1 mL aqueous sample from each ampule was injected into the IC at a flow rate of 1 mL/min using an eluent consisting of 8 mM Na2CO3 + 1 mM NaHCO3. Organic acid and sulfate calibration standards were prepared from solids over a concentration range of 0.02-0.50 mM. Chloride concentrations were also determined using the titration method of Bergmann and Sanik (16). Chloride calibration standards, ranging in concentration from 0.02 to 1 mM, were prepared by serial dilution of a certified 1000 mg/L chloride solution (SPEX CertiPrep, Metuchen, NJ). The dissolved oxygen content of water held in the ampule neck was measured immediately after opening using either the Rhodazine D or the Indigo Carmen method (CHEMetrics, Calverton, VA). Concentrations of carbon monoxide (CO) and CO2 in gasphase samples were determined using a HP6890 GC equipped with a heated gas sampling valve with a 250 uL sample loop and a Carboxen-1010 column (Supleco, Bellefonte, PA) connected to a thermal conductivity detector (TCD). This
FIGURE 1. Total amount of TCE, expressed as carbon, in anoxic ampules stored at 22 °C (b) and incubated at 120 °C (O). Amount of chloride ([) and organic acids (3) in the aqueous phase, and amount of CO and CO2 (0) in the gas phase of ampules incubated at 120 °C. Total carbon (×) is sum of TCE, organic acids, CO, and CO2 in ampules incubated at 120 °C. technique achieved a detection limit of ca. 300 uL/L (ppmv) for CO, and 500 uL/L (ppmv) CO2. The GC/TCD was calibrated using a serial dilution of certified gas mixture (Scotty Specialty Gases, Plumsteadville, PA) that contained CO2 (15%), CO (7%), oxygen (5%), and nitrogen (73%). After analyzing 6 mL of the gas sample using the GC/ TCD, approximately 1 mL of the gas sample was injected into the inlet of a Varian Star 3400 GC equipped with a Varian CP-Sil 8ms column connected to a Saturn 2000 mass spectrometer (MS). Unknown compounds were identified using software (SaturnView ver. 5.41, Varian, Palo Alto, CA) that matched mass spectra to compounds in the NIST/EPA/ NIH Mass Spectral Library (NIST98). The mass spectrometer was automatically tuned by adjusting the electron multiplier voltage to achieve a gain of 1 × 105 electrons per ion and by mass axis calibration to an internal reference compound (perfluorotributylamine) prior to each use.
Results and Discussion Change in TCE Content and Formation of Degradation Products. The total amount of TCE in each ampule was calculated as the sum of the mass detected in the aqueous phase and the mass estimated to be present in the gas phase using a dimensionless Henry’s Law constant of 0.351 at 22 °C (17). The inclusion of TCE in the gas phase accounts for differences in gas-phase volume between ampules, which typically represented less than 15% of the total TCE content. Although less TCE was detected in anoxic ampules incubated at 120 °C as compared to those stored at 22 °C (Figure 1), the observed differences in TCE content were not significant based on the results of a paired t-test (P-values > 0.05, see Supporting Information). While there was no measurable change in the amount of TCE in the anoxic ampules incubated at 120 °C, there was evidence that a small amount of TCE had been degraded. The first indication was that the amount of chloride measured in the aqueous phase of anoxic ampules incubated at 120 °C was 7-8 times greater than that in the anoxic ampules stored at 22 °C. The second indication of TCE degradation was the detection of two organic acids, glycolate (COH3COO-) and formate (CHOO-) in the aqueous phase of ampules incubated at 120 °C, which exhibited a steady increase in combined mass over the 41-day experiment (Figure 1). The third indication of TCE degradation was that both CO and CO2 were observed in the gas phase of anoxic ampules incubated at 120 °C, whereas no organic acids, CO, or CO2 were detected in anoxic ampules stored at 22 °C or in TCE-free ampules. No pH buffers were used in these experiments, and consequently the pH of the ampule solutions decreased from 7.0 to 6.4 after 41 days at 120 °C (see Supporting Information).
FIGURE 2. Total amount of TCE, expressed as carbon, in oxic ampules stored at 22 °C (b) and incubated at 120 °C (O). Amount of chloride ([) and organic acids (3) in the aqueous phase, and amount of CO and CO2 (0) in the gas phase of ampules incubated at 120 °C. Total carbon (×) is sum of TCE, organic acids, CO, and CO2 in ampules incubated at 120 °C. Gas samples from each ampule were screened for additional degradation products by GC/MS analysis, and aqueous samples were processed to enable detection of haloacetic acids by GC/ECD analysis. The amount of carbon detected as TCE, formate, glycolate, CO, and CO2 in ampules incubated at 120 °C was 4% less than the amount of carbon detected as TCE in the ampules stored at 22 °C (Figure 1), and the amount of chlorine detected as chloride and TCE in the 120 °C ampules was 1.6% less than the amount as TCE in the 22 °C ampules. The products detected represent the oxidation of TCE as opposed to the reduced products (i.e., cis-DCE, vinyl chloride, and ethene) commonly observed under reducing conditions. The fraction of TCE degraded in these ampules was 9.1 ( 8.4% based on the amount of carbon degradation products detected, and 6.9 ( 5.5% based on the amount of chloride detected. Minor differences in the degradation product distribution were detected in anoxic ampules incubated for 10 days in the convection oven as compared to those incubated for 30 and 41 days in the heating block apparatus, which is indicative of the sensitivity of these reactions to the experimental conditions. The amounts of TCE, organic acids, chloride, CO, and CO2 in oxic ampules incubated at 120 °C for up to 30 days are shown in Figure 2. No organic acids, CO, or CO2 were detected in oxic ampules stored at 22 °C or in TCE-free ampules. The compounds detected in the oxic ampules (i.e., formate, glycolate, chloride, CO, and CO2) were the same as those observed in anoxic ampules incubated at 120 °C. The fraction of TCE degraded after 30 days at 120 °C was 5.8 ( 3.0% based on the carbon degradation products detected, and 5.1 ( 9.3% based on the amount of chloride detected. The initial oxygen content of each ampule was ca. 96 µmol, with 10 µmol dissolved in water (8 mg/L) and 86 µmol in the gas phase. The initial amount of TCE was 39.0 µmol; therefore, oxygen was in excess of TCE by a factor of 2.5. Based on correlation for dissolved oxygen in water presented by Ji et al. (18), the dissolved oxygen concentration in the oxic ampules would decrease from 8.5 to 7.4 mg/L at 120 °C. Likewise, the concentration of TCE in the aqueous phase would decrease from 100 mg/L at 25 °C to 23.3 mg/L at 120 °C using the temperature-dependent Henry’s coefficient reported by Staudinger and Roberts (17). Thus, the initial aqueous phase TCE/O2 molar ratio for the oxic ampules at 120 °C was 0.77, within 15% of the 0.67 stoichiometric ratio suggested by Knauss et al. (12) as necessary for aqueous phase oxidation of TCE. Solids-free oxic ampules stored at 22 °C and incubated at 120 °C exhibited significant decreases (9.5% and 19.7%, respectively) in TCE content (P-values < 0.05) over the course VOL. 39, NO. 17, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Total amount of TCE, expressed as carbon, in anoxic ampules containing Ottawa sand that were stored at 22 °C (b) and incubated at 120 °C (O). Amount of chloride ([), organic acids (3), and sulfate (2) in the aqueous phase, and amount of CO and CO2 (0) in the gas phase of ampules incubated at 120 °C. Total carbon (×) is sum of TCE, organic acids, CO, and CO2 in ampules incubated at 120 °C.
FIGURE 4. Total amount of TCE, expressed as carbon, in anoxic ampules containing Ottawa sand and 1% (wt) goethite stored at 22 °C (b) and incubated at 120 °C (O). Amount of chloride ([), formate (3), and sulfate (2) in the aqueous phase, and amount of CO and CO2 (0) in the gas phase of ampules incubated at 120 °C. Total carbon (×) is sum of TCE, organic acids, CO, and CO2 in ampules incubated at 120 °C.
of the 30-day experiment (Figure 2). In addition, the TCE content of ampules incubated at 120 °C was significantly less (P-values 0.05) collected from the oxic ampules. The amount of carbon in the oxic ampules incubated at 120 °C was 5% less than the amount of carbon detected as TCE in the ampules stored at 22 °C (Figure 2), and the amount of chlorine detected in the 120 °C ampules was 5.6% less than of the amount of chlorine as TCE in the 22 °C ampules. Both formate and glycolate were detected after 10 and 20 days of heating at 120 °C in the convection oven, while only glycolate was detected in oxic ampules incubated for 30 days in the heating block apparatus. The amounts of TCE, organic acids, chloride, CO, and CO2 in anoxic ampules that contained Ottawa sand incubated at 120 °C for up to 40 days are shown in Figure 3. There was a significant decrease (up to 36% after 40 days) in the TCE content (P-values < 0.05) of anoxic ampules containing Ottawa sand that were incubated at 120 °C as compared to those stored at 22 °C. At 120 °C, the loss of TCE was significantly greater than was observed in anoxic and oxic ampules that contained only water and gas phases. With the exception of sulfate (SO42-), the degradation products observed in ampules that contained Ottawa sand incubated at 120 °C were identical to those detected in the absence of solids. The fraction of TCE degraded was 8.7 ( 4.3% on a carbon basis, and 13.7 ( 3.8% on a chloride basis. Sulfate was hypothesized to have formed as a result of the thermal dissolution of pyrite (FeS2), which was present in the Ottawa sand (19). Ampules that contained Ottawa sand, but no TCE, were incubated at 120 °C along with TCE-containing ampules to determine the baseline amounts of Cl- and CO2 generated in the presence of Ottawa sand alone. Although chloride was detected in the TCE-free ampules, greater than 10 times more chloride was found in ampules that contained TCE under the same conditions. While CO2 was detected in the TCEfree ampules (1.32 ( 0.08 µmol after 41 days at 120 °C), the amount of CO2 detected in the ampules with TCE was at least 1.7 times greater (see Supporting Information). The net amount of CO2 reported in Figure 3 was determined by subtracting the amount of CO2 detected in the TCE-free ampules to correct for the amount of CO2 formed during the
incubation of Ottawa sand alone. After 40 days, the amount of carbon detected in anoxic ampules containing Ottawa sand and TCE incubated at 120 °C was 22% less than the amount detected as TCE in ampules stored at 22 °C, while the amount of chloride was 15% less than of the amount detected as TCE at 22 °C (Figure 3). The amounts of TCE, organic acids, chloride, CO, and CO2 in anoxic ampules containing Ottawa sand and 1% goethite that were incubated at 120 °C for up to 4 days are shown in Figure 4. The incubation time for this experiment was limited to 4 days because the oven explosion occurred 4 days after placing the first set of ampules that contained Ottawa sand and goethite in the convection oven. A 35% decrease in the amount of TCE was observed after only 4 days of incubation at 120 °C, clearly demonstrating that the presence of 1% (wt) goethite had a dramatic effect on the rate of TCE disappearance. With the exception of glycolate, the predominant degradation products detected were the same as those found in the ampules containing Ottawa sand, and represented TCE degradation of 6.4 ( 9.5% (carbon basis) and 6.4 ( 4.0% (chloride basis) after 4 days at 120 °C. For ampules that contained Ottawa sand and goethite at 120 °C, the amount of carbon and chloride was 13% less than the amounts detected as TCE in ampules stored at 22 °C. Under the experimental conditions employed here, the primary TCE degradation products included glycolate, formate, CO, and CO2. These products are generally considered to be less toxic than TCE, and the organic acids could potentially serve a beneficial role in posttreatment microbial reductive dechlorination processes. Of particular concern, however, was the explosion that occurred soon after the solids-containing ampules were placed in the convection oven at 120 °C. The explosion was attributed to the formation of dichloroacetylene (C2Cl2), which was detected in gas samples collected from all ampules incubated at 120 °C using GC/MS (see Supporting Information). The amount of dichloroacetylene (DCA) detected by GC/MS was less than 1% of the amount of TCE present based on the ratio of chromatogram area. Dichloroacetylene is a heat-sensitive explosive gas, which is reported to ignite on contact with air (20). However, DCA has been shown to be stable in air when excess TCE was present (21, 22) and is also reported to be stable in water at concentrations up to 63.3 mg/L, and at vapor concentrations up to 16 000 ppmv after removing excess dissolved oxygen (23). Dichloroacetylene was hypothesized to have formed by elimination of the lone hydrogen atom from TCE (24). During thermal remediation in natural systems, any DCA that might form could react with soil constituents such as soil organic matter and alkali earth
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metals to form other chlorinated byproducts or be removed from the subsurface by gas-phase extraction. TCE Degradation Rates. In previous studies, rates of TCE disappearance in heated systems have been described with a first-order reaction model (6-8):
nt ) noe-kt
(1)
is the number of moles at time t, n0 is the initial number of moles at time zero, and k is the first-order rate coefficient. Here, the rate coefficient for the loss of TCE was determined after correcting for the nondegradative loss of TCE that occurred after cooling the 120 °C ampules to room temperature (22 °C), which was potentially due to sorption to ampule walls or solids, when present. The correction involved subtracting the amount of TCE detected in the ampules stored at 22 °C from the amount measured in ampules incubated 120 °C. A detailed description of the approach used to obtain TCE degradation rates, and the resulting fits to the experimental data, are provided in the Supporting Information (see pages S-5 to S-12). There was no statistically significant change in TCE mass observed in the anoxic ampules that were incubated at either 120 or 22 °C. The first-order TCE degradation rate coefficients based on the rate of carbon degradation product appearance in anoxic, oxic, and anoxic ampules containing Ottawa sand were similar at 2.1 × 10-3 day-1, corresponding to a half-life of 330 days. The TCE degradation rate coefficient based on the rate of chloride formation was also similar for the anoxic, oxic, and anoxic ampules containing Ottawa sand ranging from 1.2 to 3.5 × 10-3 day-1, with corresponding half-lives ranging from 591 to 200 days. These findings suggest that the presence of either oxygen or quartz sand had little influence on TCE degradation. In contrast, TCE disappearance and product formation in ampules containing 1% (wt) goethite occurred rapidly, yielding first-order rate coefficients of 15.7 × 10-3 day-1 (half-life ) 44 days) based on the rate of carbon and chlorine formation and 53.4 × 10-3 day-1 (half-life ) 13 days) based on the rate of TCE disappearance. Therefore, the addition of goethite resulted in more than a 1 orderof-magnitude increase in the rate of TCE degradation as compared to the other systems investigated, which suggests that iron bearing soil minerals have the potential to substantially increase rates of chlorinated ethene degradation during thermal treatment (see Supporting Information, Table S.3). The first-order rate coefficient for TCE disappearance reported by Jeffers et al. (7, 8) was 0.8 × 10-3 day-1 (half-life ) 858 days), which is 4-8 times less than the values obtained for the solids-free anoxic and oxic ampules. In contrast, the coefficient for the rate for TCE disappearance interpolated from data reported by Knauss et al. (6) was 0.61 day-1 (halflife ) 1.1 days), which is 11 times greater than the average value obtained for the ampules containing 1% geothite. The first-order rate coefficients determined for the ampule experiments reported herein represent the rate of thermally induced TCE degradation for a three-phase system containing gas, aqueous, and solid phases. These conditions are anticipated during thermal treatment of subsurface systems. The rate determined by Knauss et al. (6) represents a system with only aqueous phase present at elevated pressure, which may be applicable to confined aquifers where a gas phase is not expected to form at elevated temperatures. It is important to note that the literature rate data are presented for relative comparison purposely only and may not be strictly comparable due to differences in experimental conditions and measurement techniques. The TCE degradation products detected in this study were non-chlorinated carbon gases, organic acids, and Cl-, which represented less than 15% of the initial amount of TCE initially
present when incubated at 22 and 120 °C for periods of up to 40 days. From a practical perspective, these findings suggest that abiotic TCE degradation during thermal treatment may vary considerably depending on site conditions and operational variables, but it is likely that gas-phase recovery of TCE will be the most important process in thermal remediation at temperatures up to 120 °C. Additional laboratoryscale studies and treatability tests with a range of soils and soil constituents are warranted to further elucidate factors controlling abiotic TCE reaction pathways, byproduct formation, and reaction rates. Field samples should also be analyzed to determine if significant concentrations of byproducts are formed during in situ thermal remediation.
Acknowledgments Funding for this research was provided by the U.S. Environmental Protection Agency, Robert S. Kerr Environmental Research Center (GWERD) through Cooperative Agreement No. R-82947401-0, and by a Scholarship Award from the American Petroleum Institute (API) and National Ground Water Association (NGWA). The content of this manuscript does not represent the views of the U.S. EPA, API, or NGWA, and has not been subject to agency review.
Supporting Information Available Ampule TCE content, dissolved oxygen, pH, and amount of CO2 formed in ampules that contained Ottawa sand. Methods used to determine the kinetic rate parameters along with a summary table, and a mass spectrum as evidence for the presence of dichloroacetylene in the ampule gas phase. This material is available free of charge via the Internet at http:// pubs.acs.org.
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Received for review February 12, 2005. Revised manuscript received June 23, 2005. Accepted July 5, 2005. ES0502932
