Environ. Sci. Technol. 2000, 34, 274-281
Detection of Chlorodifluoroacetic Acid in Precipitation: A Possible Product of Fluorocarbon Degradation J O N A T H A N W . M A R T I N , * ,† JAMES FRANKLIN,‡ MARK L. HANSON,† KEITH R. SOLOMON,† SCOTT A. MABURY,§ DAVID A. ELLIS,§ BRIAN F. SCOTT,| AND DEREK C. G. MUIR| Department of Environmental Biology, University of Guelph, Guelph, Ontario, N1G 2W1 Canada, Solvay S.A., rue de Ransbeek 310, Brussels, Belgium B-1120, Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, M53 3H6 Canada, and National Water Research Institute, Environment Canada, 867 Lakeshore Road, Burlington, Ontario, L7R 4A6 Canada
Chlorodifluoroacetic acid (CDFA) was detected in rain and snow samples from various regions of Canada. Routine quantitative analysis was performed using an in-situ derivatization technique that allowed for the determination of CDFA by GC-MS of the anilide derivative. Validation of environmental CDFA was provided by strong anionic exchange chromatography and detection by 19F NMR. CDFA concentrations ranged from 97% purity; Fluka Chemical Co., Milwaukee, WI) concentrations were used (0.01, 0.1, 1, 10, 50, 100, 500, and 1000 mg L-1); each replicated (n ) 2; one plant per replicate) with either 2 (M. sibiricum) or 4 (M. spicatum) controls. The CDFA stock solution was neutralized to pH 7 with 1 M NaOH (Sigma, ACS grade) before addition to the plant media. The end points examined were shoot length, wet mass, node number, root number, root length, chlorophyll a/b, and carotenoid concentration. For all end points, the EC25 and EC50 were calculated when possible using the statistical program Linear Interpolation Method for Sublethal Toxicity: The ICp Approach (Version 2.0) (26). This program handles nonmonotonically decreasing data, as was observed for some end points of M. sibiricum, by smoothing the data through pooling of adjacent means. CDFA Degradation. A CDFA degradation study was conducted under controlled environmental conditions to assess the degradative capacity of a typical freshwater ecosystem. Field water and field sediment (20.4% organic matter, dry weight) containing an uncharacterized community of phytoplankton, zooplankton and benthic species was collected from the University of Guelph Microcosm Facility (UGMF). A growth chamber (Controlled Environments Limited Canada, model E7H) was programmed to produce a night air temperature of 18 °C, a day temperature of 22 °C, and a light intensity of 2200 kJ h-1 with a 12-h day. Field water was spiked with either CDFA or DCA (>99% purity; Fisher Scientific, New Jersey) at a concentration of 30 mg L-1 and brought back to pH 8 using 1 M NaOH (Sigma, ACS). A 200-mL aliquot of the CDFA spiked field water was then added to 3 separate 250-mL bottles containing 25 g of wet sediment. The bottles were sampled at day 0, 1, 2, 4, 7, 14, 21, and 28 by removing a 3-mL aliquot and storing it frozen in a 5-mL amber vial. CDFA and DCA analysis was performed by ion chromatography. A Perkin-Elmer model 200 binary pump, an Alltech model 550 conductivity detector, an ERIS 1000Hp autosuppressor, and a Waters WISP 712 autosampler were used in the analysis. The column was a Dionex AS14 (4 × 250 mm) with a Dionex guard column AG14 (4 × 50 mm). The mobile phase was 3.5 mM carbonate/1.8 mM bicarbonate at a flow rate of 1.8 mL min-1 for 18 min. DCA and CDFA eluted at 9.9 and 12.4 min, respectively, and the LOD for both compounds was 5.0 mg L-1. A four-point standard curve was used to quantify the HAAs.
Results and Discussion CDFA Detection and Confirmation. As this was the first report, to our knowledge, of CDFA in precipitation and surface waters, care was taken to confirm the compound by two independent methods. Validation of CDFA detection by GCVOL. 34, NO. 2, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. (A) 19F NMR spectra for a concentrated rainwater sample and (B) a CDFA standard. The chemical shift of CDFA in the rainwater sample occurred at -62.169, corresponding to the standard. The internal standard and TFA are also shown. MS was demonstrated by comparing sample chromatograms and mass spectra to those obtained by injection of the pure acid anilide. Retention time was always highly conserved between standard and sample injections and between major ions within a sample injection. Two molecular ions (m/z 241 and 243) were detected in all samples, which are consistent with the expected masses of the CDFA anilide. Furthermore, the ratio of parent ions from a sample injection was invariably the same as for the pure CDFA anilide. Confirmation of CDFA detection was provided by analysis of rainwater by SAX extraction and 19F NMR. Figure 1 shows the 19F NMR spectra for a concentrated rainwater sample and a standard. The chemical shift of CDFA in rainwater is observed at -62.169 ppm, corresponding to the standard; trifluoroacetic acid is also shown at a chemical shift of -76.592 ppm (Figure 1). 276
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Environmental CDFA. CDFA was originally detected in Guelph rainwater samples and later detected in Guelph’s stormwater retention ponds, Toronto rain, and snowpack samples from Alberta (Figure 2). The range of CDFA concentrations in all precipitation samples ranged from 1000 >1000
43.5 ( 21.5 59.7 ( 22.4 109 ( 23.3 13.7 ( 18.7 16.2 ( 21.5 112 ( 31.0 104 ( 37.1 109 ( 29.2
125 ( 35.2 491 ( 217 782 ( 212 49.8 ( 25.4 60.3 ( 22.2 161 ( 22.8 168 ( 17.7 162 ( 17.1
hazard quotient approach (28, 29). This method takes the measured environmental concentration (MEC), divides it by a toxicological benchmark concentration (TBC), and then multiples it by an uncertainty factor. In this case, the most sensitive EC50 (49.8 mg L-1) for M. sibiricum was used as the TBC, and the highest CDFA concentration from stormwater retention ponds (120 ng L-1) was used as the MEC. An uncertainty factor of 0.001 was used, as it is the U.S. EPA standard (30). A hazard quotient of 0.002 is obtained following this method. Using this estimate of risk, CDFA does not represent a risk of acute toxicity to these aquatic macrophytes. Further investigations of CDFA toxicity may be warranted on the basis that phytoplankton are generally more sensitive to TFA than aquatic macrophytes (23). The degradation study revealed no decrease in the concentration of CDFA over a 28-d period. The initial concentration was 34.0 mg L-1, while at day 28 the concentration had increased slightly to 39.3 (( 1.4) mg L-1; presumably a result of evapoconcentration. In an identical study with DCA, at an intitial concentration of 31.5 mg L-1, DCA degraded rapidly and was undetectable after only 4 d. CDFA does not appear to be broken down microbially or abiotically in any timely manner relative to DCA under these simulated field conditions. In spiked river water, a 30-d period was sufficiently long to allow the degradation of most chlorinated and brominated HAAs, with the exception of TCA which remained stable (11). A more extensive degradation study is warranted based on these initial findings, which suggest that CDFA is relatively stable in the freshwater environment. Only on the basis of its concentration in precipitation and the limited risk to aquatic macrophytes, CDFA may well be an inconsequential component of precipitation. However, when we consider that CDFA could be a highly stable molecule with respect to biotic and abiotic degradation processes, as has been concluded for TFA (30), its presence in precipitation becomes a significant issue. CDFA concentrations may increase over time in the aquatic environment and could reach appreciable concentrations under conditions of high evapoconcentration and limited seepage, as predicted for TFA (31). Possible Sources of CDFA. The presence of CDFA in air (1) and precipitation is suggestive of an atmospheric source. Possible pathways leading to the formation of CDFA in the atmosphere are the degradation of 1-chloro-1,1-difluoroethane (HCFC-142b) and 1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113); however, other sources, including combustion of plastics (17), cannot be ruled out. The concentration of HCFC-142b (CClF2CH3) in the troposphere in 1997-1998 was close to 10 ppt by volume (pptv) (32, 33). The atmospheric lifetime of HCFC-142b being 18.4 yr (33), the maximum rate of CDFA formation from HCFC-142b, assuming a yield of 100%, would be 0.54 pptv yr-1. For the whole atmosphere (1044 molecules), this would represent 12 Gg yr-1. Global annual precipitation being 5 × 1011 Gg yr-1 would lead to an upper limit of 24 ng L-1 CDFA in average global precipitation. 278
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FIGURE 5. Proposed atmospheric degradation scheme of HCFC142b. Radical intermediates are enclosed in ellipses, and molecular species are in rectangular boxes. The actual yield of CDFA from HCFC-142b, however, is likely to be very much smaller than 100%, as demonstrated below. The atmospheric degradation of HCFC-142b, based on experimental studies carried out on HCFC-142b itself and on the analogous compounds CClxF3-xCH3 (where x ) 0, 2, or 3) (16, 34-39), is thought to yield the aldehyde CClF2CHO as the major degradation intermediate (Figure 5). The lifetime of the alkyl peroxynitrate CClF2CH2O2NO2, with respect to thermal decomposition, is only about 1 s at ground level and reaches a maximum of a few days in the colder atmosphere close to the tropopause (40). Taking into account the additional possibility of degradation by photolysis, it seems reasonable to assume that this unstable intermediate is
completely converted to CClF2CHO according to the reactions shown (Figure 5). The hydroperoxide CClF2CH2O2H, is also believed to be a minor, short-lived intermediate (38) and will undergo almost exclusive transformation to CClF2CHO. Aqueous uptake (and possible subsequent conversion to CDFA in cloud droplets) is likely to make only a rather small contribution to the removal of the hydroperoxide CClF2CH2O2H. This is based on the assumption that the actual lifetime of this hydroperoxide, with respect to photolysis and OH reaction, is only a few days (34). Even for the most soluble species, the lifetime with respect to cloud uptake and rainout is greater than 10-15 d when formed more or less uniformly in the troposphere (41, 42). Three initial processes are envisioned for the degradation of CClF2CHO: (a) photolysis; (b) reaction with OH; and (c) uptake by cloudwater, rainwater, or ocean water (Figure 5). The lifetime for photolysis is reported as being only about 1 h (43, 44), based on the assumption of a unit quantum yield. Even if photolysis does not occur solely according to the process shown in Figure 5, it is likely to produce almost exclusively one-carbon products (36, 43). The lifetime for reaction of CClF2CHO with OH is 270 h (43), so this pathway does not likely compete significantly with photolysis. Even if the reaction with OH occurred to any extent, the principal ensuing degradation sequence is expected to be CClF2CO f CClF2C(O)O2 (+NO) f CClF2C(O)O f CClF2 + CO2 (16, 34-37). Although the acyl peroxynitrate CClF2C(O)O2NO2, if formed from the CClF2C(O)O2 radical, will be very stable with respect to thermal decomposition in the colder upper troposphere (40), it is likely to be degraded by photolysis or transported to the lower troposphere where its thermal lifetime will be only a few hours (36, 40). This peroxynitrate is thus likely to be merely a semi-stable reservoir species, and in any case, no chemistry converting it into CDFA has been described. A minor pathway may lead to CDFA via reaction of the acylperoxy radical CClF2C(O)O2 with HO2. In the case of the atmospheric degradation of CCl3CH3 by a reaction scheme analogous to the one suggested here for HCFC-142b, the yield of formation of CCl3CO2H by the corresponding pathway has been estimated at 0.06%, while the yields of the peroxyacid CCl3CO3H and the peroxynitrate CCl3C(O)O2NO2 are much smaller (45). The uptake of CClF2CHO into liquid water, yielding the hydrate CClF2CH(OH)2, may conceivably lead to the ultimate formation of CDFA, after aqueous-phase oxidation initiated by OH. However, as stated above, the lifetime for cloud uptake of even the most soluble species is at least 10 d (240 h), so it is unlikely that for CClF2CHO this process could represent more than about 4% of the overall removal process, dominated by photolysis (lifetime about 10 h or less). Overall, the yield of CDFA from HCFC-142b is anticipated to be low, probably 5% at most, implying a contribution of HCFC-142b to CDFA in precipitation of e5% of 24 ng L-1 (i.e., 1 ng L-1), which is considerably lower than the observed concentrations reported here. CFC-113 is stable in the troposphere, but photolysis in the stratosphere leads to cleavage of a C-Cl bond (46). Two different radicals could potentially be formed: CClF2CClF and CF2CCl2F. However, studies on the products of CFC-113 photolysis in the condensed phase in the presence of a hydrogen donor indicate that the radical CClF2CClF is formed preferentially (47). The presumed subsequent atmospheric chemistry of this radical is shown in Figure 6. The formation of an alkyl peroxynitrate and a hydroperoxide, analogous to those appearing in Figure 5, is not depicted: indeed, in the stratosphere, where CFC-113 degrades, such compounds are likely to have an even more transient existence than in the troposphere because of photolysis by intense UV radiation. The acid fluoride (CClF2COF) ultimately formed may be
FIGURE 6. Proposed atmospheric degradation scheme of CFC-113. Radical intermediates are enclosed in ellipses, and molecular species are in rectangular boxes. photolyzed to some extent, giving one-carbon fragments. Its UV absorption spectrum is expected to lie between those of CCl3COCl (48) and CF3COF (49) but closer to the latter. If this is the case, then CClF2COF will photolyze in the stratosphere somewhat more slowly than phosgene. It has been demonstrated (42) that a significant proportion of phosgene formed in the lower stratosphere is long-lived enough to be transported down to the troposphere, where it will be removed by clouds. CFC-113 is degraded largely in the lower stratosphere (altitude
