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Reductive dechlorination of CFCS and HCFCS under m

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Reductive dechlorination of CFCs and HCFCs under
methanogenic conditions.
CHRISTIAN BALSIGER*, DAVID WERNER, CHRISTOF HOLLIGER1 AND PATRICK HÖHENER
Swiss Federal Institute of Technology (EPFL), IATE-P, CH-1015 Lausanne, Switzerland
*
corresponding author Phone ++41 21 693 27 23, E-Mail : christian.balsiger@epfl.ch
1
EPFL, Laboratory for Environmental Biotechnology,
Abstract
Investigations were made on the potential for biotransformation of ten chlorofluorocarbons
and hydrochlorofluorocarbons under methanogenic conditions. Transformations were
monitored in batch experiments by analysis of the concentrations in the headspace.
Trichlorofluoromethane (CFC-11) was transformed to dichlorofluoromethane (HCFC-21) and
to chlorofluoromethane (HCFC-31). The methanogenesis inhibitor 2-bromoethanesulfonate
(BES) significantly decreased the CFC-11 transformation rate. 1,1,2-trichloro-1,2,2trifluoroethane (CFC-113) was transformed to two products which were deduced to be 1,2dichloro-1,1,2-trifluoroethane (HCFC-123a) and chlorotrifluoroethene (CTFE). No
transformation was detected for dichlorodifluoromethane (CFC-12), chlorodifluoromethane
(HCFC-22), 1,2-dichloro-1,1,2,2-tetra-fluoroethane (CFC-114), chloropentafluoroethane
(CFC-115), 1,1-dichloro-1-fluoroethane (HCFC-141b), 1-chloro-1,1-difluoroethane (HCFC142b) and 1,1,1,2-tetrafluoroethane (HFC-134a).
Keywords: chlorofluorocarbons, refrigerants, methanogenesis, biotransformation, microcosms
Introduction
Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are stable synthetic
organic chemicals either fully or partially substituted with chlorine and fluorine atoms. They
were first manufactured in the 1930s (Midgley and Henne, 1930) and were used world-wide
in quantities of up to 106 metric tons per year (Fig.1) as aerosol propellants, refrigerants, foam
blowing agents, solvents and intermediates for synthesis of fluorinated polymers. Due to the
release into the environment and to their volatility and excellent chemical stability, CFCs
accumulated in the atmosphere and hydrosphere (Key et al., 1997). It has been recognized that
CFCs are involved in the depletion of the stratospheric ozone and that their infrared
absorption characteristics contribute significantly to the greenhouse warming effect. Many
governments signed thus the "Montreal protocol on substances that deplete the ozone layer"
and decreased the production of CFCs in the 1990's coming to a complete ban in 1996.
HCFC’s are manufactured presently as substitutes for CFC’s (Fig.1), because the presence of
an hydrogen atoms in those molecules allows their tropospheric oxidation, avoiding further
1
Fig.1: World production of 9 CFCs and HCFCs, as reported by
AFEAS (www.afeas.org).
elevation to the troposphere. HCFCs are now in use until a planned ban in 2020 should be
enforced.
As a consequence of their chemical stability, CFC’s have first been expected to be
biologically inert. Since 1989, however, it was reported that CFC-11 and CFC-12 can be
dechlorinated in anaerobic ecosystems such as termite mounds (Khalil and Rasmussen, 1989)
and in rice fields (Khalil and Rasmussen, 1990). CFC-11 biotransformation under anaerobic
conditions has been observed since in methanogenic sediment (Lovley and Woodward, 1992),
anoxic aquifer (Semprini et al., 1992), contaminated groundwater (Sonier et al., 1994), anoxic
marine water (Lee et al., 1999), municipal solid waste (Ejlertsson et al., 1996) and in compost
and marl (Deipser, 1998). The rate of CFC-11 disappearance in various anaerobic
environments was always found to be about ten times faster than CFC-12 disappearance
(Oster et al., 1996). The formation of HCFC-21 and HCFC-31 as products from
biotransformation of CFC-11 has been reported (Eljertson et al, 1996; Deipser, 1998). The
transformation of CFC-11, -12 and -113 also occurs in synthetic solutions containing
corrinoids (Krone and Thauer, 1991) or hematin (Lovley and Woodward, 1992) and in pure
culture of Methanosarcina barkeri strain Fusaro (DSM 804) (Krone and Thauer, 1992). The
proposed transformation pathway corresponds to a stepwise dehalogenation. A mechanism
involving corrinoids has been proposed (Krone and Thauer, 1991). In methanogenic landfill
2
leachates (Lesage et al., 1990; Denovan and Strand, 1992; Lesage et al., 1992) and laboratory
municipal waste digesters (Deipser and Stegmann, 1994; Deipser and Stegmann, 1997;
Deipser, 1998), enzymatic reductive dechlorination of CFC-113 was observed leading to the
formation of HCFC-123a, chlorotrifluoroethene (CTFE), and two isomers of HCFC-133.
The reductive dehalogenation of the HCFCs has received less attention. HCFC-21 and HCFC123 were reported to be enzymatically dechlorinated under anaerobic conditions in freshwater
and salt marsh sediments (Oremland et al., 1996). A disruption of fluorine-carbon bonds
under environmental conditions has not been observed so far neither in CFCs nor in HCFCs
(Key et al., 1997). No data on dehalogenation of HFC-134a, HCFC-141b, HCFC-142b which
are currently in use as replacement products for CFCs, are reported. Also, no comparative
study has been reported so far on the dehalogenation of CFCs relative to HCFCs.
Two major processes determine the fate of (H)CFCs in groundwater: volatilization and
biodegradation. A parallel study showed the importance of water table fluctuations on the
volatilization of these compounds from the saturated zone (Werner and Höhener, 2002). This
process can represent a non negligible contribution to the removal of (H)CFC’s from
groundwater.
The aim of the present study was to investigate the potential reductive dehalogenation of the
nine most widely used CFCs and HCFCs in a methanogenic enrichment culture from sewage
sludge. The compounds were chosen from the production data (see Fig.1) published by the
Alternative Fluorocarbons Environmental Acceptability Study (AFEAS), an association of the
leading manufacturers of CFCs and HCFCs.
Materials and methods
Chemicals
(Hydro)chlorofluorocarbons CFC-11, CFC-12, CFC-114, HCFC-21, HCFC-22, HCFC-142b
and HFC-134a were obtained from Fluka (Buchs, Switzerland) in the highest available
purity. CFC-113 was obtained from Merck (Dietikon, Switzerland). HCFC-141b was
obtained from Prochimac SA (Neuchâtel, Switzerland). CFC-115 was obtained from a
refrigeration system (unknown manufacturer). The relevant physical-chemical properties of
the compounds are given in Table 1. All water used was distilled and was 18 megaohm
resistance or greater.
Table 1: Chemical properties, industrial use and experimental parameters of the CFCs and
HCFCs used in this study.
Short
name
Name
Use1)
Henry
GC
Initial
coefficient
retention
concentration
[mol/l*atm] time [min] in solution [nM]
CFC-11
Trichlorofluoromethane
R, I, A
0.01042)
4.18
365
CFC-12
Dichlorodifluoromethane
R, I, A
0.002912)
2.19
371
HCFC-21
Dichlorofluoromethane
0.08443)
4.09
4540
HCFC-22
Chlorodifluoromethane
R
0.03194)
2.13
2540
CFC-113
1,1,2-Trichloro-1,2,2-trifluoroethane
S
0.0024)
8.40
99
CFC-114
1,2-Dichloro-1,1,2,2-tetrafluoroethane A
0.000834)
3.23
80
CFC-115
Chloropentafluoroethane
R
0.000384)
1.93
89
HFC-134a 1,1,1,2-Tetrafluoroethane
R
0.025)
2.42
1780
HCFC-141b 1,1-Dichloro-1-fluoroethane
I, A, S
0.00796)
7.80
2257
HCFC-142b 1-Chloro-1,1-difluoroethane
I
0.0146)
3.40
2500
1) Refrigerant; I: Insulation foam; A: Aerosol; S: Solvent; 2) Warner and Weiss, 1985; 3) VP/WSOL;
4) Yaws et al., 1991; 5) Chang and Criddle, 1995; 6) Kanakidou et al., 1995.
3
Laboratory microcosm experiments
CFC/HCFC transformation studies were performed using 60 ml glass vials sealed with Viton
rubber septa and aluminium crimp caps. A 33g/l stock solution of Wilkins Chalgren anaerobe
broth (obtained from Oxoid AG, Basel, Switzerland) in water was heated for 1 hour to remove
dissolved oxygen, and nitrogen was bubbled during cooling. 15ml of this solution was then
distributed in each vial. Atmosphere in the vials was changed to N2/CO2 (80/20) by 10
vacuum-fill cycles. The solutions were autoclaved (15 min at 120°C). 5ml of a mixture of
digester sludge collected from several sewage treatment plants in the canton de Vaud
(Switzerland) were then added in each vial under anaerobic conditions, and the atmosphere in
the vial changed again to N2/CO2 (80/20).
The cultures were incubated for one night before addition of the fluorinated compounds. For
each (H)CFC, two live microcosms and one to two controls were prepared. Final
concentrations of (H)CFC and in solution were calculated using the Henry constants (see
Table 2); KH(CH4) = 1.52*10-3 mol/l*atm (Lide, 1999). Vials were incubated at 25°C in
inverted position in order to avoid gas exchange through the septum. On day 87 of the
experiment, 1,5 ml of 330g/l Wilkins Chalgren broth solution has been added to the
microcosms of CFC-11, CFC-12, CFC-113, HCFC-21, HCFC-22 and HFC-134a.
2-bromoethane sulfonic acid was added prior to the CFC-11 addition in one vial to a final
concentration of 20mM as methanogen inhibitor.
In order to relate the inhibitory effects of the (H)CFC’s to each others, one microcosm was
prepared with a mixture of CFC-11, CFC-12 and CFC-113.
Controls consisted of vials with medium solutions only; the fluorinated compounds were
added in the same amount as in batch vials; these controls were made for each gas. An
additional control consisted of a heat-killed microcosm (three times autoclaved at 120°C for
15 min with 2 days interval between each operation), in which all the gases were added,
except the CFC-12.
Analytical methods
Concentrations of (H)CFC’s and methane in the gas phase were analyzed by injecting 100 µl
of gas phase with a gas-tight syringe into a Varian CP-3800 gas chromatograph equipped with
both an ECD and a FID detector, each heated to 300 °C. Before sampling the gas phase was
allowed to equilibrate with atmospheric pressure in order to keep the same sampling
conditions for all analyses. The split ratio at the injector was set to 10. Carrier gas was Helium
at a flow rate of 2 ml/min. A capillary column GS-GasPro (J&W Scientific, 30 m * 0.32 mm)
was used for the separation of the compounds at a temperature of 130°C, and a Y-connection
deviated equal amounts of carrier gas to each detector. The FID detector was alimented by a
H2/O2 mixture generated in situ by water electrolysis (GEMFID system, BON Technologies
SA, Lausanne, Switzerland), improving the sensibility of the detector. Detector responses
were checked with calibration standards with concentrations bracketing those measured in the
microcosms. The transformation product HCFC-31 was confirmed by GC-MS (GC HP 5890
and HP MSD 5971A).
Results and discussion
Biotransformation of CFC-11, HCFC-21 and CFC-113
CFC-11 was transformed to HCFC-21, which was further transformed to HCFC-31 (Fig. 2), a
compound listed as carcinogenic (Forschungsgemeinschaft, 1997). No transformation
occurred in abiotic control. The presence of the methanogenesis inhibitor BES totally
inhibited the production of methane, but transformation of the CFC-11 to HCFC-21 still
4
Fig.2: Degradation of CFC-11, HCFC-21 and CFC-113 in microcosms of anaerobic
digester sludge. The two products of CFC-113 transformation have been deduced
from retention times and literature data (see discussion), and are given in arbitrary
units (a.u.).
occurred although at ten times lower rate. This indicated that methanogens were not the only
microorganisms involved in CFC-11 transformation but that they played a major role in this
reductive dechlorination reaction.
In a mixture of CFC-11, CFC-12 and CFC-113, the degradation of CFC-11 and CFC-113 was
not inhibited by the presence of the other compounds; CFC-12 concentration remained
constant, showing no degradation. This observation is in agreement with the inhibitory effect
of CFC-11 and HCFC-21 on CFC-12 degradation observed by Deipser in compost under
anaerobic conditions (Deipser, 1998). However, the absence of CFC-12 degradation is not due
to inhibitory effect, because CFC-12 degradation did not occur in a CFC-12 only containing
microcosm.
Compared to CFC-11 and CFC-113, the slow decrease of HCFC-21, correlated with a slow
methane production, shows a lower microbial activity in these batch cultures. This indicates a
toxic effect of the high concentration of HCFC-21 for the micro-organisms responsible for
degradation. Because of high solubility and low ECD sensitivity for the HCFC-21, initial
5
concentration of this compound was chosen about ten and hundred times greater than for
CFC-11 and for CFC-113 respectively.
In the microcosm containing CFC-113, two peaks of unknown compounds with similar
evolution profiles appeared in parallel with CFC-113 removal. The retention times of the two
transformation products were 2 and 7,8 min. According to the fact that CFC-113 has a
retention time of 8,4 min and that biotransformation of CFC-113 to HCFC-123a has already
been reported (Lesage et al., 1992), the compound with retention time of 7,8 min is assumed
to be 1,2-dichloro-1,1,2-trifluoroethane (HCFC-123a), because it should be retained in this
column in a similar way as CFC-113. A mechanism proposed by Lesage et al. (Lesage et al.,
1992) involves the abiotic formation of chlorotrifluoroethene (CTFE) either from CFC-113 or
from HCFC-123a. In this mechanism, further biotransformation of HCFC-123a produces
HCFC-133 and HCFC-133b. A hypothetical mechanism proposed by Deipser (Deipser, 1998)
suggests simultaneous production of HCFC-123a and HCFC-123b from CFC-113. From these
four products, only CTFE has a structure that would exhibit such a short retention time as
observed for the second product. Because of the similar curves of the transformation products,
CTFE is assumed to be produced spontaneously from HCFC-123a by an elimination reaction.
Other (H)CFCs
No transformation was observed for the CFC-12, CFC-114, CFC-115, HCFC-22, HCFC141b, HCFC-142b, and HFC-134a. As reported by others (Khalil and Rasmussen, 1989;
Khalil and Rasmussen, 1990; Lovley and Woodward, 1992; Bullister and Lee, 1995;
Ejlertsson et al., 1996; Deipser and Stegmann, 1997), CFC-11 and CFC-12 degradations
occurs generally under the same conditions. However, the degradation of CFC-11 and CFC113 but no degradation of CFC-12 has also been reported earlier for sediments (Denovan and
Strand, 1992).
In contrast to this study, CFC-114 has been reported to transform to 1-chloro,1,1,2,2tetrafluoroethane in municipal solid waste samples (Ejlertsson et al., 1996). However, similar
to this study, HCFC-22 has also not been degraded under these conditions (Ejlertsson et al.,
1996; Deipser and Stegmann, 1997). No information was found in the literature concerning
the CFC-115, HCFC-141b, HCFC-142b and HFC-134a, which were not degraded in our
microcosms.
Methanogenic activity
Methane shows generally an important production in the first 60-80 days followed by a
concentration plateau. The increase in methane concentration does not occur at the same time
for all compounds, but is always related to freon removal, supporting a relationship between
freon removal and methane production.
Conclusions
This study shows the biotransformation of CFC-11, HCFC-21 and CFC-113 in a mixture of
digester sludge. The carcinogenic properties of HCFC-31, a transformation product of the
CFC-11, indicates that the presence of such chlorofluorinated compounds in the environment
where methanogenic conditions occur represents a potential risk for public health. This report
traces back the importance of knowing the fate of such halogenated compounds in the
subsurface.
6
Many studies show the degradation of CFC-11, CFC-12 and CFC-113 under various
anaerobic conditions. Only little was known concerning the fate of other chlorofluorocarbons
and their replacement products. This study indicates that they can be quite stable under
methanogenic conditions.
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