Environ. Sci. Technol. 1987, 21, 1208-1213
Nigg, H. N., Eds.; ACS Symposium Series 315; American Chemical Society: Washington, DC, 1986; p 294. Jungclaus, G. A.; Cohen, S. 2. Preprints of Extended Abstracts, 191st National Meeting of the American Chemical Society, Division of Environmental Chemistry, American Chemical Society: Washington,DC, 1986; paper 6, p 12. Vogel, T. M.; Reinhard, M. Environ. Sci. Technol. 1986, 20, 992. Macalady, D. L.; Wolfe, N. L. J . Agric. Food Chem. 1985,
33, 167.
Macalady, D. L.; Wolfe, N. L. In Treatment and Disposal of Pesticide Wastes;Krueger, R. F., Seiber,J. N., Eds.;ACS Symposium Series 259; American Chemical Society: Washington, DC, 1984; pp 221-244.
(22) Crank, J. The Mathematics of Diffusion, 2nd ed.; Glarendon: Oxford, 1975. (23) Cooney, D. 0.;Adesanya, B. A,; Hines, A. L. Chem. Eng.
Sci. 1983, 38, 1535. (24) Burchill, S.; Hayes, M. H. B.; Greenland, D. J. In The
Chemistry of Soil Processes; Greenland,D. J., Hayes, M. H. B., Eds.; Wiley: New York, 1981; p 221. (25) Call, F. J . Sci. Food Agric. 1957, 8, 630. (26) Hayward, D. 0.;Trapnell, B. M. W. Chemisorption; But-
terworths: London, 1964. (27) Alexander, M. Soil Sci. SOC.Am. Proc. 1965, 29, 1. Received for review November 18, 1986. Accepted July 20,1987.
Abiotic and Biotic Transformations of 1,I !I-Trichloroethane under Methanogenic Conditions Timothy M. Vogel" Department of Civil and Environmental Engineering, Michigan State University, East Lansing, Michigan 48824-1 2 12
Perry L. McCarty Environmental Engineering and Science, Department of Civil Engineering, Stanford University, Stanford, California 94305
A common industrial solvent, l,l,l-trichloroethane (TCA), is one of the most frequently found contaminants in groundwater. The fate of TCA in groundwater is complicated by the different possible abiotic and biotic transformations that it may undergo. Abiotic transformation of TCA can result in a mixture of 1,l-dichloroethylene (1,l-DCE) and acetic acid, as shown by others. This study confirms that TCA can be biotransformed by reductive dehalogenation to 1,l-dichloroethane (1,l-DCA) and chloroethane (CA) under methanogenic conditions. Also, reductive dehalogenation of 1,l-DCE to vinyl chloride (VC) is confirmed. This study demonstrates that these transformations can occur stoichiometrically. In addition, [14C]TCA, [14C]-1,1-DCA,[14C]-1,1-DCE, [14C]CA, and [14C]VCwere at least partially mineralized to 14C02under similar methanogenic conditions.
Introduction Contamination of groundwater by halogenated aliphatic compounds, including l,l,l-trichloroethane (TCA) ( I ) , has led to investigations to determine their fate in the environment. Previous studies have illustrated that a potential exists for biotransformation of halogenated aliphatic compounds under anaerobic conditions that are conducive to methanogenesis (2-4). The fate of TCA in groundwater is partially governed by both abiotic and biotic transformations. The biotransformation of TCA by a mixed methanogenic culture supported through continuous feed of acetate as the primary source of organic carbon was demonstrated by Bouwer and McCarty (2). The reductive dehalogenation of TCA to 1,l-dichloroethane (1,l-DCA) (5, 6 ) and then to chloroethane (CA) (7) under anaerobic conditions has also been described. In addition, traces of chloroethane (CA) were observed in anaerobic mucks following the disappearance of TCA (5). TCA has also been reported to undergo abiotic transformation to 1,l-dichloroethylene (1,l-DCE) (8-10) and to acetic acid (11-13). Probably both 1,l-DCEand acetic acid are produced simultaneously: 1,l-DCE as a result of 1208
Environ. Sci. Technol., Vol. 21, No. 12, 1987
elimination and acetic acid as a result of hydrolysis (10). The pseudo-first-order rate constant for 1,l-DCE formation from TCA was reported as 0.04 yr-l a t 20 "C (9). While no rate constant for the formation of acetic acid from TCA has yet been documented, Haag et al. (10)have shown that acetic acid is produced 5 times as fast as 1,lDCE at 40 "C. Therefore, the pseudo-first-order rate constant for the disappearance of TCA at 20 "C could be as high as 0.25 yr-l (9). These values are consistent with a TCA hydrolysis (to acetic acid) rate of about 0.2 yr-l. Both acetic acid and 1,l-DCE can be further biotransformed under methanogenic conditions. Acetic acid can be mineralized to COz and CH4 (14). Traces of vinyl chloride (VC) were observed after addition of 1,l-DCE to microcosms (3). On the basis of the above, a possible scheme for the fate of TCA and its abiotic and biotic transformation products, including that of l,l-DCE, l,l-DCA, and CA, is shown in Figure 1. Research to date has not clearly demonstrated whether the indicated bioconversion of TCA to 1,l-DCA and CA and of 1,l-DCE to VC occurs stoichiometrically nor whether any processes can lead to the mineralization of these compounds. Such information was sought in this study.
Materials and Methods Chemicals and Radioisotopes. GC-grade TCA and 1,l-DCA (Supelco Inc., Bellefonte, PA) were used. CA was obtained diluted 0.1 mg/mL of methanol (MeOH) (Supelco Inc., Bellefonte, PA). Other organics used were MeOH, acetone (99.9%; J. T. Baker Chemical Co., Phillipsburg, NJ), and 2-propanol (99+ %; Aldrich Chemical Co., Milwaukee, WI). [14C]TCA, [14C]-l,l-DCA, [14C]l,l-DCE, and [14C]VC (New England Nuclear, Boston, MA) were diluted initially in methanol (absolute; J. T. Baker Chemical Co., Phillipsburg, NJ) to 5.1 X lo6 dpm/pL (74.8 mg of TCA/mL of MeOH), 2.9 X lo6 dpm/pL (57.6 mg of l,l-DCA/mL of MeOH), 4.2 X lo6 dpm/pL (680 mg of l,l-DCE/mL of MeOH), and 1.1 X lo6 dpm/pL (110 mg of VC/mL of MeOH), respectively. [14C]CA(Amersham Corp., Grover Heights, IL) was diluted
0013-936X/87/0921-1208$01.50/0
0 1987 American Chemical Society
e/"g3\
/
J
CH2CC12
1
CH2CHCI
COP
+I B
Table I. Composition of Feed to Large Fixed-Film Column and Large Suspended-Growth Fermentors (Compounds Added to Deionized Water) (mg/L)
\
+I B CH3CH2CI
A
compound
IA
CH3CH20H
4 CH3COOH
lET"OL1
Is
co2
large fixed-film column
inorganics NaOH
lCAl
co2
Figure 1. Probable fate of TCA under methanogenic conditions: biotransformation pathways are denoted by lines marked B and abiotic transformation pathways by lines marked A.
initially in MeOH to 2.2 X lo7dpm/pL (10.8 mg of CA/mL of MeOH) Analytical Methods. The chlorinated aliphatic compounds, both reactants and products, were determined by gas chromatography (GC) using three methods. In the first method, samples were sealed without headspace in 10- or 60-mL vials and extracted with 1 or 2 mL, respectively, of isooctane by vigorous shaking for 30 min [modified after Henderson et al. (15)].Bromochloropropane (10 pg/L) was used as an internal standard. A 2-pL sample of the extract was injected into a packed column GC (10% squalane on Chromosorb W/AW) equipped with a linearized Ni-63 detector (16). Quantitation was achieved by injecting standards, treated like samples, and comparing relative areas by a Spectra-Physics 4000 calculating integrator (Spectra-Physics, Sunnyvale, CA). In the second method (modified after EPA method 601), samples (5 mL) were analyzed with similar column and GC conditions as in the first method but with a Hall electrolytic conductivity detector and a Tekmar purge-and-trap system. In the third method (In,samples were analyzed for organic intermediates by GC/mass spectrometry [GC/MS; Finnigan 4000 with INCOS data system (Finnigan Corp., Sunnyvale, CA)]. Five or 60 mL of sample was purged by closed-loop stripping, trapping the halogenated aliphatic compounds on activated carbon inside a GC injection liner. The liner was then inserted into the GC/MS injection port, and the volatile organics were desorbed ( - 5 min) at 250 OC and immediately trapped on the front loop of a 30 m thick film capillary column (Durabond 5, J&W, Inc.) by placing the fiist loop of the column in a liquid nitrogen bath for 5 min. The GC oven temperature was then increased from 30 to 250 "C at 4 deg/min. Identification by Radioactivity Assay. Two 14C tracer counting procedures were used to identify products from TCA transformation. The first procedure involved the first GC method described above. Fractions of GC effluent gas based upon the retention times of a compound were bubbled through 10 mL of liquid scintillation counting solution in order to trap and measure the 14Cactivity associated with each compound [modified after Gossett (SI]. The second procedure involved a presumptive test for 14C02and has been described previously (4). BaN03 was added to some samples to confirm 14C02formation as described previously. 14Cactivity remaining in acidified samples after being purged with nitrogen gas was quantified and is taken to represent nonvolatile compounds. I4C activity was assayed on a Tricarb Model 4530 liquid scintillation spectrometer (Packard Instrument Co.,
.
suspendedgrowth fermentors 1 2
CH3CHClp
MgGO4.7Hz0 KZHPO4 CaClz KCl FeC1, COClZ NiClz organics acetic acid MeOH acetone 2-propanol l,l,l-trichloroethane tetrachloroethylene trichloroethylene
4 000 215 150
215 150
200 215 150
60
60
60
25 25 5.0 0.5 0.25
25 25 5.0 0.5 0.25
25 25 5.0 0.5
0.25
16 000
50 50
800
0.1
0.4 4.0 3.8
500
16 000 800 500 0.4 4.0 3.8
Downers Grove, IL) also as described previously.
Experimental Systems Since the relative rates of individual transformation steps varied considerably, different reactor types and different liquid detention times were used in order to determine the overall fate of TCA under methanogenic conditions. A large (12-L) continuous-flow fixed-film column was used to confirm the quantitative biotransformation of TCA to 1,l-DCA. Two semicontinuouslyfed suspended-growth fermentors (15 L) were used to investigate the biotransformation of TCA to CA. A small continuous-flow fLued-filmcolumn (40 mL) and small batch fermentors (14 and 60 mL) were used to measure mineralization of TCA, l,l-DCA, l,l-DCE, VC, and CA. These are described in the following paragraphs. Large Fixed-Film Column. An upflow Plexiglas column (20-cm i.d. by 200 cm) filled with smooth 6 cm diameter quartzite rocks ( I @ , supporting methanogenic bacteria, and operated at 35 "C was fed a solution containing trace nutrients, phosphate, ammonia, and bicarbonate (Table I) from a 20-L glass bottle stored at 10 "C. Acetone and 2-propanol were added in order to more closely resemble certain contaminated groundwaters. Feed rate was 2 L/day (superficial loading velocity of 33 cm/ day), resulting in a liquid detention time of 6 days. Organics (acetone, 2-propanol, and TCA) were injected with a syringe pump into the feed line and were mixed prior to entering the column in a small headspace-free Erlenmeyer flask with a stirring bar. Effluent was collected in a 20-L glass bottle, but analyses were conducted only with samples removed directly from the column through ports at varying heights (10, 50, 110, and 180 cm above the influent port). Steady-state operation (no further change in product formation) was attained after about 2 months. After 1year, 2 L of interstitial liquid containing biomass was removed from the 10-cm port to provide seed for batch fermentor experiments. All other conditions remained the same. Six months after the removal of 2 L of interstitial fluid, [14C]TCAwas added to the feed. Small Fixed-Film Column. Two upflow glass columns (2.5-cm i.d. by 22 cm) filled with 3-mm glass beads were connected in series and operated to achieve anaerobic Environ. Sci. Technol., Vol. 21, No. 12, 1987
1209
conditions at approximately 22 OC as described previously (2). The operation of these columns has been described previously ( 4 ) . These columns were used for biotransformation studies of halogenated organics for over 3 years, generally with acetate as a primary substrate. Initially, a concentrated solution with [14C]TCAand additional acetate as the primary substrate was added to the second column influent feed from a 10-mL gas-tight syringe pump at a rate of 1.0 mL/4.4 mL of medium flow from the first column, resulting in a column influent concentration of 29 pg/L [14C]TCA and 100 mg/L acetate. Initially, the total flow rate to the second column was 10 mL/day (superficial loading velocity of 2 cm/day), resulting in an actual liquid detention time of 4 days. After 3 months of operation, the detention time was increased to 8 days, and then after an additional 3 months, 25 pg/L [14C]-1,1-DCA instead of TCA was fed to the same column at a reduced detention time of 4 days. Then, after 3 additional months, 95 pg/L [14C]-1,1-DCE,without l,l-DCA, was fed at a detention time of 2 days. Finally, [l4C1VCalone was fed at a detention time of 8 days. All influent conditions were maintained until steady state (no chemical change with time) was achieved, and then 14Cactivity was measured. Suspended-Growth Fermentors. Two large anaerobic fermentors (15 L of liquid in 20-L bottles sealed with black rubber stoppers) were inoculated with seed from the large anaerobic filter before any halogenated aliphatic compounds had been fed to the anaerobic filter. One reactor was fed acetic acid as the major primary substrate (4 g/ day) and the other methanol (4 g/day). Trace nutrients, pH buffer, ammonia, and phosphate were added as well as the trace organics being studied: TCA, PCE, and TCE (Table I). With semicontinuous feeding, 250 mL of fermentor supernatant fluid (without mixing) was removed, and 250 mL of feed solution was added each day (60-day liquid detention time). The fermentor contents were mixed for 5 min with a magnetic stirring bar only after feeding. Batch Fermentors. In studies using anaerobic column seed, 20-mL serum bottles were filled from a well-mixed anaerobic jar containing the interstitial liquid (with suspended microorganisms) from the 10-cm port of the large anaerobic column. Similarly prepared controls containing mercuric chloride (10 mg/L) were also used. Either 93 pg/L [14C]TCA(together with 1mg/L MeOH) or 97 pg/L [14C]DCA(together with 1.3 mg/L MeOH) was added to samples and controls. Bottles were sealed with Tefloncoated septa, closed with an aluminum cap, and kept in a glovebag containing N2 gas. The bottles were stored at 20 “C and one by one were sacrificed for analysis over time for the production of 14C02. After 55 days, a primary substrate (MeOH at 100 mg/L) was injected through the septa into the bottles to determine the effect on transformation rates. In a second study, 60-mL serum bottles were filled with completely mixed liquid from the MeOH fed fermentor. Similarly prepared controls containing mercuric chloride (100 mg/L) were also used. Next, either 38 pg/L [14C]1,l-DCE or 34 pg/L [14C]CA was added. Bottles were sealed, stored, and sacrificed for analysis similarly to the first set. Results Large Fixed-Film Column. TCA was transformed into 1,l-DCA after 2 weeks of column operation as indicated by the disappearance of TCA and the formation of nearly an equivalent amount of 1,l-DCA (Figure 2A). Trace amounts of CA (5% of initial TCA) were also formed. After 1 year of operation, similar formation of 1210
Environ. Sci. Technol., Vol. 21, No. 12, 1987
z
gLz loop
A
DISTANCE
ALONG COLUMN (cm)
0
0
i3 Z
““il
I, I-DCA5
B
40
0 IO
50 DISTANCE
-0 10
50 DISTANCE
180
110
COLUMN ( c m )
ALONG
I IO ALONG
I80 COLUMN (cm)
Flgure 2. Percent of influent TCA carbon represented by TCA, 1,lDCA, or CA as a function of dlstance along large anaerobic fixed-film column after different periods of operation: (A) is after 14 days; (E) Is after 1 year; (C)is subsequent to (B) after removal of 2 L of biomass and interstitial water from the 10-cm port.
Table 11. Distribution of 14CActivity in Large Anaerobic Column Aqueovs Samples after 18 Months of Operation
percent of initial I4C activity 1,lnonTCA DCA volatile* COz influent 95 ndc 2.0 110-cm port 0.5 i 0.1 84 f 4 2.1 f 0.8 180-cm port 0.4 i 0.1 79 f 2 3.2 0.4
*
3.0 10 i 2.8 9.3 & 0.1
total 100 98.6 94.2
Mean f standard deviation. bNonvolatile refers to I4C activity that was not stripped from aqueous samples under acidic conditions. ‘nd = not detected.
1,l-DCA was still found, although no CA was then observed (Figure 2B). Subsequently, when 2 L of interstitial fluid was removed from the 10-cm port, decreasing the bacterial concentration in the column, the rate of TCA transformation decreased such that TCA could be measured in samples from the 10-cm port (Figure 2C). Yet, 1,l-DCAwas still the major product, and CA concentration was low. The lack of mass balance might be related to unmeasured intermediates. The disappearanceof CA from the columns might have been the result of CA mineralization to C02. When [14C]TCAwas added to the influent, mass balance for 14C activity in the column (Table 11) confirmed [14C]-1,1-DCAas the major product but also showed about 6% conversion of [14C]TCAto 14C02.These results confirm the near-equivalent transformation of TCA to 1,l-DCA under these methanogenic conditions and suggest the possibility of some conversion to CA and C02. Suspended-GrowthFermentors. In both fermentors, TCA was transformed to CA even though they were fed
Table 111. Liquid Scintillation and GC Analysis of Small Anaerobic Column Aqueous Samples
halogenated substrate conditions TCA, 4-day detention time influent effluent TCA, %day detention time influent effluent l,l-DCA, 4-day detection time influent effluent l,l-DCE, 2-day detention time influent effluent VC, 8-day detention time influent effluent
concn," pg/L
percent of initial 14Cactivityb nonvolatile COZ
volatile
total
20.9 f 1.1 ndd
99 f 5 83 f 8
0.1 f 0.1 0.8 f 0.2
0.5 f 0.2 8.7 f 1.0
92.9
21.2 f 0.3 nd
99 f 1 68 f 3.5
0.1 f 0.1 2.5 f 0.4
0.5 f 0.3 13 f 3
100 83.1
22.7 f 1.3 19 f 3
99 f 1 86 f 2
0.1 f 0.1 0.7 f 0.1
0.1 f 0.1 5.5 f 0.2
100 92.2
95 f 14 55 f 3
99 f 2 61.4 f 2.6
1f1 0.7 f 0.1
Of0 7.8 f 1.0
100 69.9
NAe NA
97 f 5 44 f 4
3f2 If1
Of0
100 72
27 f 6.4
100
Gas chromatographic (GC) analysis. Mean f standard deviation. Nonvolatile is 14Cactivity not stripped from aqueous samples under acidic conditions. dnd = not detected. 'NA = not determined. Table IV. Distribution of 14C Activity in Batch Fermentors
percent of initial 14Cactivity' nonvolatile*
halogenated substrate
time, day
1,l-DCE
vc
1,l-DCE 1,l-DCE
0 107
91.4 1.4 f 0.5
ndc 54 & 2
halogenated substrate
time, day
CA
CA CA CA
0 107 107 (control)
90.6 76.1 f 0.5 76.1
1.4 4f3
COZ
total
12.2 f 0.7
100 71.6
7.2
percent of initial 14Cactivity' nonvolatileb COZ 9.4 f 0.2
2.2 15.8 f 0.4
17.6
1.3
7.2
total 100 101.3 95
Mean f standard deviation. Nonvolatile refers to 14Cactivity that was not stripped from aqueous samples under acidic conditions. = not detected. (I
different primary substrates (acetic acid or MeOH). Influent TCA (400 pg/L) resulted in 40 pg/L CA (equivalent to 83 pg/L TCA) in the liquid phase of the fermentors. CA was also measured in the gas phase (10.6 pg/L) due to gas stripping of the liquid by the methane and C02produced. Since the liquid exchange was 0.25 L/day and the gas production was 2.4 L/day, 100 hg of TCA was added each day, and a total of 35 pug of CA (equivalent to 73 pg of TCA) was removed each day. Steady-state TCA and 1,l-DCA concentrations were 1 pg/L or lower (normally below quantification level) and 2.5 f 2 pg/L, respectively. The acetic acid fed fermentor had acetone and 2propanol steady-state concentrations of 5.6 (99% removal) and 2.3 (99% removal) mg/L, respectively. The MeOH fed fermentor had acetone and 2-propanol concentrations of 150 (82% removal) and 1.2 (99% removal) mg/L, respectively. The 2.4 L/day gas produced had a methane content of 65 f 3% and carbon dioxide content of 35 f 5% during steady-state operation, corresponding stoichiometrically to the amount of primary substrate added. Primary substrate utilization itself was not measured. When at one point the feed to the fermentors was halted in order to observe the fermentors with reduced gas production, the CA decreased by more than half over 30 days. These results indicate TCA is transformed to CA and that CA then disappears under methanogenic conditions. Small Anaerobic Fixed-Film Column. All of the chlorinated aliphatic compounds (TCA, l,l-DCA, l,l-DCE, and VC) fed to the small fixed-film column were partially mineralized to C02 (Table 111). On the basis of GC analysis and 14Cactivity, TCA was mainly transformed to volatile product(s). However, 1,l-DCA and 1,l-DCE were
not transformed as readily, and their GC analyses approximately match their 14Cactivity measurements. For example, 76% of 1,l-DCA in comparison with 86% of the initial volatile 14Cactivity remained in the effluent. Thus, only the difference of 10% represents volatile components not accounted for by 1,l-DCA. VC was not directly measured by GC, although significant mineralization to COz occurred (Table 111). Low 14C activity recoveries, especially for 1,l-DCE and VC, may be due to losses from the system or from conversion to methane, or perhaps ethylene as noted by Belay and Daniels (22) for 1,2-dichloroethane. Methane and ethylene may not have been measured as volatile components by the procedures used. Batch Fermentors. Both [14C]TCAand [14C]-1,1-DCA were partially mineralized to 14C02during incubation over an 84-day period (Figure 3). After the primary substrate (MeOH) was added at day 55 to all samples, some increase in the rate of 14C02production, at least from l,l-DCA, was evident. The overall rates of mineralization for both TCA and 1,l-DCA were roughly equivalent (about 20% over 84 days). After 107 days in the subsequent study with 60-mL serum bottles, 56% of [14C]-1,1-DCEwas transformed to [14C]VC(Table IV). Some mineralization (about 6%) to 14C02also occurred. Some mineralization of [14C]CAto 14C02was also observed (about 13%) over this time period. Considerable transformation of [ 14C]CAto nonvolatile product(s) (10.4%) occurred in the controls, although no significant mineralization to 14C02was observed (Table IV). Since the total [14C]CA conversion to nonvolatile product plus 14C02was essentially equivalent for both sample and control, the initial CA transformation appears Environ. Sci. Technol., Vol. 21,
No. 12, 1987 1211
25
1
I,I-DCA
SAMPLE
I Ndy,TCA
CONTROL
5 0 0
1
TIME ( D A Y S )
Flgure 3. Percent conversion of [I4C]TCA and ['4C]-l,l-DCA to 14C02 as a function of time in small anaerobic batch fermentors.
to have been an abiotic one. A pseudo-first-order rate constant for the abiotic hydrolysis of CA can be calculated by use of the controls with HgC12,assuming the nonvolatile product (10.4%) is the sole product of hydrolysis [It = 1.18 exp(-8) s-l], yielding an approximate half-life at 20 "C of 680 days.
Discussion This study has provided further evidence to help confirm the overall pathways for transformation of TCA under methanogenic conditions as outlined in Figure 1. This includes both abiotic and biotic processes. First considering the middle route of TCA transformation to CA, such biological reductive dehalogenation under anaerobic conditions was suggested previously, although not shown to be stoichiometric (2, 5-7, 19). This consists of the replacement of a halogen by a hydrogen atom. The equivalent transformation of TCA to 1,l-DCA observed with the large anaerobic fixed-film column at 6-day detention time supports this hypothesis. Transformation occurred rapidly and was more than 90% complete at the 10-cm port, thus indicating a half-life of less than 1day. However, 1,l-DCA appeared to be more recalcitrant than TCA and did not decrease significantly within the column, indicating a half-life much greater than 6 days. Reductive dehalogenation of 1,l-DCA would result in the formation of CA. The trace amount of CA found in the large anaerobic fixed-film column supports the possibility of such transformation. More complete transformation of TCA to CA (presumably with 1,l-DCA as an intermediate) was observed in the anaerobic fermentors where the liquid detention time was 60 days. Reductive dehalogenation of CA would result in the formation of ethane, but analyses for ethane were not conducted. Other evidence for CA transformation was obtained with the batch fermentors; [14C]CAwas transformed to nonvolatile product(s) in controls at rates similar to 14C02 formation in biologically active samples. This is consistent with the reported abiotic hydrolysis of CA to ethanol (20) as the initial step in CA anaerobic transformation. Ethanol might then be transformed in biologically active samples to COP. Since the decrease in CA and the increase in the sum of the nonvolatile plus C02fractions were similar for controls and biologically active samples, abiotic hydrolysis appears to be the major mechanism for CA transformation found here. The data for CA decrease permit the rough estimate of a pseudo-first-order rate constant for CA hydrolysis (k = 0.37 yr-l), which indicates that CA would have a half-life of about 1.9 yr. This half-life is considerably longer than the hydrolysis 30-day half-life at 25 OC extrapolated from laboratory work performed at 100 OC (21). Confirmation of the side routes for abiotic transformation of TCA shown in Figure 1, which can occur under anaerobic and aerobic conditions, is provided in part from 1212
Environ. Sci. Technol., Vol. 21, No. 12, 1987
the literature. Acetic acid abiotically formed as a result of TCA hydrolysis (13), illustrated in the right route in Figure 1, should be quickly mineralized under methanogenic conditions to C02 and methane (14). For the left route, 1,l-DCE is abiotically formed as a result of TCA elimination (9). The results of this study show that 1,lDCE can then undergo complete biological reductive dehalogenation to form vinyl chloride, as suggested by Barrio-Lage et al. (3). This transformation of 1,l-DCE is similar with that of 1,2-DCE, which can be formed from TCE transformation under similar conditions ( 4 ) . Further evidence was provided for the slow mineralization of VC to carbon dioxide. The mechanism for this transformation is open to speculation; possibly water is first added to the double bond to form chloroethanol. The possibility also exists that VC can be reduced to ethylene, similar to 1,2dichloroethane (22),although this has not yet been demonstrated. Considering the relative rates of the processes noted in Figure 1, the relatively slow rates of production of COz from TCA and 1,l-DCA by the middle route were about the same, which is consistent with the observed relatively rapid reductive dehalogenation of TCA to 1,l-DCA and much slower conversion of 1,l-DCA to CA. Whichever transformation step limits the rate of mineralization to C02,it appears to be the same for both TCA and 1,l-DCA. The relatively rapid transformation of TCA to 1,l-DCA reported here (greater than 90% with 6-day detention time in the large anaerobic fixed-film column) compared with more than 9 yr for an equivalent degree of abiotic transformation (9) indicates that TCA abiotic transformation would probably not be a significant process in the presence of highly active methanogenic conditions. However, groundwater conditions vary considerably, and abiotic transformation of TCA might predominate in areas where little or no methanogenic activity occurs. Under such conditions, the hydrolysis of TCA to acetic acid would be expected to occur about 5 times more rapidly than the elimination of TCA to 1,l-DCE (IO). However, 1,l-DCE production presents a more significant health hazard than acetic acid production. The schematic diagram (Figure 1)of TCA transformation pathways under anaerobic conditions when combined with a qualitative understanding of the relative transformation rates of TCA and its intermediates should aid in determining which transformation products are likely in a given situation. Detectable concentrations of 1,l-DCE (5 pg/L) should be observed where significant concentrations of TCA (about 120 pg/L or more) have been present in water for a period approaching 1year or longer. If the microorganisms capable of mediating the reductive dehalogenation of TCA are active, 1,l-DCA should also be observed following TCA contamination. CA might not be detected if its abiotic transformation rate is faster than the reductive dehalogenation of 1,l-DCA to CA. These relative rates would depend upon the extent and activity of methanogenic conditions. Where 1,l-DCE is formed, traces of vinyl chloride might also be observed if proper anaerobic microbial conditions prevail (reductive dehalogenation). However, if TCA is about 120 pg/L and the subsequent concentration of 1,l-DCE after 1 year is about 5 pg/L, then expected concentrations of vinyl chloride would be difficult to detect. In addition, intermediates formed would be subject to partitioning onto aquifer solids, further reducing their concentrations and perhaps their activities. This would complicate interpretations of measurements made for these contaminants in aquifer interstitial fluid.
Different retardation factors governing the movement of this group of compounds as a result of their different partition coefficients, their analytical detection limits, the presence of suitable bacteria, the environmentalconditions, and the time can all affect the actual observed cooccurrences of contaminants. However, the group of halogenated organics associated with the abiotic and biotic transformation of TCA are commonly found together in groundwater. For example, in a recent cooccurrence study by the Environmental Protection Agency, 1,l-DCE and 1,l-DCA were observed in many (21% and 36%, respectively) of the same well waters where TCA was detected (23). Understanding the pathways and products of transformation can be useful in evaluating contamination patterns and in selecting the most appropriate remediation procedures.
Acknowledgments The assistance of David Kurz with the laboratory studies described is appreciated. Registry No. TCA, 71-55-6; l,l-DCA, 75-34-3; CA, 75-00-3; VC, 75-01-4.
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(8) Pearson, C. R.; McConnell, G. h o c . R. SOC.London, B 1975, 189, 305. (9) Vogel, T. M.; McCarty, P. L. J. Contam. Hydrol. 1987, I , 299. (10) Haag, W. R.; Mill, T.; Richardson, A. Extended Abstracts, 192nd National Meeting of the American Chemical Society, Anaheim, CA; American Chemical Society: Washington, DC, 1986; p 248. (11) Britton, E. C.; Reed, W. R. Chem. Abstr. 1932, 26, 5578. (12) Walraevens, R.; Trouillet, P.; Devos, A. Int. J. Chem. Kinet. 1974, 7, 777. (13) Mabey, W. R.; Barich, V.; Mill, T. Extended Abstracts, 186th National Meeting of the American Chemical Society, Washington, DC; American Chemical Society: Washington, DC, 1983; p 359. (14) Buswell, A. M.; Sollo, F. W. J. Am. Chem. SOC.1948, 70, 1778. (15) Henderson, J. E.; Peyton, G. R.; Glaze, W. H. In Zdentification and Analysis of Organic Pollutants in Water; Keith, L. H., Ed.; Ann Arbor Science: Ann Arbor, MI, 1976; pp 105-112. (16) Reinhard, M.; Everhart, E. T.; Schreiner, J. E.; Graydon, J. W. Abstracts, NATO/CCMS Conference on Adsorption Techniques; NATO/CCMS: Washington, DC, 1979. (17) Graydon, J. W.; Grob, K.; Zuercher, F.; Giger, W. J. Chromatogr. 1983,285, 307. (18) Young, J. C.; McCarty, P. L. J.-Water Pollut. Control Fed. 1969, 41, R160. (19) Parsons, F.; Lage, G. B. J-Am. Water Works Assoc. 1985, 77(5), 52. (20) Laughton, P. M.; Robertson, R. E. Can. J. Chem. 1959,37, 1491. (21) Mabey, W.; Mill, T. J. Phys. Chem. Ref. Data 1978,7,383. (22) Belay, N.; Daniels, L. Appl. Environ. Micrbiol. 1987, 53, 1604. (23) Price, P. S. Memo of U.S. Environmental Protection Agency, Office of Water, Washington DC, Aug 21, 1985.
Received for review December 2,1986. Accepted August 24,1987. This research was funded in part by the Fairchild Semiconductor Corp. and the U S . Environmental Protection Agency (EPA), the latter under Cooperative Agreement CR 8088510. This paper has not been subjected to the EPAs required peer and administrative review and therefore does not necessarily reflect the views of the Agency, and no official endorsement should be inferred.
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