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 : [email protected] 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. 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