|
Research Activities
The AFEAS research program focused on the key questions related to the ultimate physico-chemical
and biological fate of TFA. A summary of the individual studies and available results
follows.
Phase I: Preliminary Studies (1992)
|
Physical / Chemical Properties
In a study of the physical and chemical properties of TFA and sodium trifluoroacetate (NaTFA),
Solvay's environmental research laboratory determined that TFA is completely miscible with
water. With a pKa close to zero, TFA is entirely dissociated at all environmentally relevant pH
levels. The extremely low octanol/water partition coefficient of TFA indicates that it has little
or no potential for passive accumulation in fatty tissues and is likely to partition into aqueous
compartments of the environment. Chemical degradation (i.e., decarboxylation, defluorination)
was undetectable under environmental conditions and very slow under extreme conditions of heat
and pH. The photolysis is unlikely because TFA does not absorb tropospheric radiation. TFA
also demonstrated little or no potential for complexation and sequestration. No insoluble salts
were formed in artificial seawater. In a screening test, no adsorption to three different standard
soils was observed.
Preliminary Biodegradation Studies
Biodegradation of TFA was not demonstrated under aerobic conditions using two standard OECD
guideline tests: an extended closed bottle test, and a modified semi-continuous activated sludge
(SCAS) test. In a study by Stroud Water Research Center, no evidence could be obtained for co-
metabolic utilization of TFA by organisms growing on natural growth compounds. In a study
of a TFA-containing industrial wastestream, there was no evidence for the presence of indigenous
microorganisms adapted to growth on TFA as a sole carbon source.
Ecotoxicological Profile of TFA
After the completion of these initial studies, a preliminary assessment of the available information
on the environmental fate of TFA was conducted by Prof. Davide Calamari (University of Milan).
He concluded that the compound is mobile and persistent with a mild herbicide potential and
possible uptake by plants.
|
Phase II: Research and Assessment (1993-1997)
|
Program Area A --- Biodegradation and Biotransformation
|
Microbial Degradation
Despite early results indicating anaerobic reductive dehalogenation (Visscher et al. 1994), the bulk
of the results from several laboratories --- Michigan State University, U.S. Geological Survey,
DuPont, Stroud Water Research, and Akzo --- suggests that TFA is generally recalcitrant to
microbial degradation (Emptage et al. 1997). A broad range of microbial types have been
investigated in these studies including aerobes, strict anaerobes, photosynthetics, and denitrifying
bacteria. No reproducible evidence for TFA breakdown products has been obtained from studies
of microbial activity inherent in any natural environmental sample. However, strong evidence
was obtained for decarboxylation of TFA by particular bacterial strains under controlled
laboratory conditions at Michigan State University. The conditions under which this transformation
occurred are not likely to be reproduced in nature and therefore this mechanism would
probably not represent a natural sink for TFA. The fate of the fluorine atoms in the Michigan
State process is unknown.
Incorporation into Biomass
A study conducted by Stroud Water Research Center focused on the incorporation of TFA into
cellular components of aquatic microorganisms, aquatic invertebrates, and plants. Bacteria were
shown to take up very low levels of TFA. Oligochete worms and one plant species contained
low levels of radioactivity after exposure to 2-14C-TFA. No fluorinated biotransformation
products were identified in the organisms studied.
|
Program Area B --- Potential Environmental Sinks for TFA
|
AFEAS has sponsored work to examine potential environmental sinks for TFA. As indicated
elsewhere in this review, no significant chemical or biological degradation or other loss processes
have been identified so far. A summary of the studies on the environmental occurrence of TFA
follows.
Environmental Occurrence of TFA
The determination of background levels of TFA in the environment was conducted by two
research groups. Prof. Hartmut Frank (University of Bayreuth, Germany) examined the presence
of TFA in urban and remote areas. Prof. Anders Grimvall (Linköping University, Sweden)
investigated the occurrence of TFA in precipitation at remote sites and in historic samples.
Air, rain and surface waters in Europe and Israel were extensively sampled and analyzed for
trifluoroacetate by Prof. Frank during 1995 1996 and were reported in a letter to Nature (Frank
et al. 1996). The results showed that trifluoroacetate is a ubiquitous contaminant of surface
waters at the locations sampled, ranging in concentration from 6.4 µg/l in the Dead Sea to 0.25
µg/l in the North Atlantic and 0.04 µg/l in the Baltic Sea; concentrations in rivers ranged from
0.63 µg/l in the Rhine at Coblenz to 0.06 µg/l in the Roter Main near Bayreuth. TFA
concentrations were near or below the detection limit of about 0.01 µg/l in centuries-old samples
of ground water from Germany, and in samples of river water from remote areas of Siberia and
South America.
TFA was also present consistently in air and rain samples at Bayreuth --- averaging 50 pg/m3 (0.01
parts per trillion by volume) in air and 0.1 µg/l in rain --- over the period March to December
1995. Since the current potential source of TFA from fluorochemicals in the atmosphere would
provide about 0.0016 µg/l in rainwater in this region, a substantial additional --- but as yet
unknown --- source of TFA must exist to account for the observations.
In the other study, Prof. Grimvall analyzed samples of rain and of glacier ice and snowpack
collected at remote sites in the Northern and Southern hemispheres. Information about TFA
concentrations in precipitation in both hemispheres will provide valuable input to global budget
calculations which will help determine the nature of the other sources of TFA in the environment.
Work on TFA in the environment has also been conducted outside of the AFEAS-sponsored
program under Dr. James Seiber (University of Nevada --- Reno). Rainwater collected in Nevada
and California was found to contain 0.03 0.09 µg/l, while TFA levels in surface waters ranged
up to 40 µg/l in surface samples taken from a land-locked lake in Nevada (Zehavi and Seiber,
1996).
|
Program Area C --- Toxicity, Metabolism and Bioaccumulation
|
The results of the AFEAS-sponsored tests of toxicity, metabolism and bioaccumulation are
discussed below. Table 1 contains a summary of the toxicity of sodium trifluoroacetic acid to
the aquatic and terrestrial organisms studied, along with definitions of common terms (e.g., EC50,
LOEC, NOEC).
Toxicity to Aquatic Organisms
The effects of sodium trifluoroacetate (NaTFA) on the growth of duckweed (Lemna gibba) was
studied by Brixham Environmental Laboratory (Zeneca, Ltd). The EC50 for frond increase was
1100 mg/l and for weight increase was 1200 mg/l. The tissues showed only a slight bioconcentration
of the compound after 7 days.
The acute toxicity of NaTFA was evaluated in different aquatic organisms by Solvay's environmental
research laboratory. The LC50 in fish and crustacea was greater than 1200 mg/l. The
EC50 for a freshwater green alga, Selenastrum capricornutum (new name: Raphidocelis subcapitata),
was 4.8 mg/l for biomass and 160 mg/l for growth rate; the NOEC was 0.12 mg/l. Elf
Atochem's Laboratory of Ecotoxicology obtained similar results on the same species: EC50 was
1.5 mg/l for biomass and 7.7 mg/l for growth rate; EC10 was 0.15 and 1.2 mg/l, respectively, for
biomass and growth rate. (Note: EC10 for biomass can be considered a surrogate for NOEC.)
Brixham studied the effects of NaTFA on the growth of three other species of algae: a
freshwater diatom (Navicula pelliculosa), a marine diatom (Skeletonema costatum), and a fresh-water
blue-green alga (Anabaena flos-aquae). The EC50 value for biomass for all three species
was calculated to be 1200 mg/l to >2400 mg/l, with a NOEC in the range of 600 2400 mg/l.
All of the species studied are standard test organisms.
Following these experiments, Solvay compared the toxicity of sodium trifluoroacetate and its
potential dehalogenation products --- difluoroacetic acid, sodium monofluoroacetate, and sodium
fluoride --- with two different species of algae: Selenastrum capricornutum and Scenedesmus
subspicatus. Sodium monofluoroacetate was the most toxic compound for both Selenastrum and
Scenedesmus. Sodium fluoride showed a small inhibitory effect only on Selenastrum. Sodium
trifluoroacetate and difluoroacetic acid showed an intermediate degree of toxicity to Selenastrum,
while they were only slightly toxic to Scenedesmus.
Further research on the mechanism of the toxicity of TFA to the most sensitive species of alga
studied, Selenastrum capricornutum, was inconclusive. The biotransformation of trifluoroacetate
was less than 4% during the 3-day incubation period; the bioaccumulation factor was less than
10.
Multi-Species Algal Study
Further toxicity tests with sodium trifluoroacetate in algae were conducted by Solvay on the
following species: Chlorella vulgaris, Chlamydomonas reinhardtii, Dunaliella tertiolecta,
Euglena gracilis, Phaeodactylum tricornutum, and Microcystis aeruginosa. None of these species
showed any effect when exposed to about 120 mg/l sodium trifluoroacetate.
Together with the species tested previously, a total of 11 different species of algae have now been
evaluated. The species belong to four different classes: Chlorophyceae (4 freshwater and
1 marine species), Euglenophyceae (1 freshwater species), Cyanophyceae (2 freshwater species),
and Bacillariophyceae (1 freshwater and 2 marine species).
| Table 1. Summary of the Toxicity of Sodium Trifluoroacetate |
|---|
| Aquatic Organisms | EC50 / LC50 | LOEC | NOEC |
|---|
Selenastrum capricornutum
(freshwater green alga) (new name: Raphidocelis subcapitata) | 4.8 mg/l
1.5 mg/l | 0.36 mg/l | 0.12 mg/l
0.15 mg/l |
Anabaena flos-aquae
(blue-green alga) | 2400 mg/l | 1200 mg/l | 600 mg/l |
Navicula pelliculosa
(freshwater diatom) | 1200 mg/l | 1200 mg/l | 600 mg/l |
Skeletonema costatum
(marine diatom) | >2400 mg/l | --- | 2400 mg/l |
Chlorella vulgaris
(freshwater green alga) | >1200 mg/l | --- | 1200 mg/l |
Scenedesmus subspicatus
(freshwater green alga) | >120 mg/l | --- | --- |
| Chlamydomonas reinhardtii
| >120 mg/l | | >120 mg/l |
| Microcystis aeruginosa
| >117 mg/l | | >117 mg/l |
Phaeodactylum tricornutum
(marine alga) | >117 mg/l | | >117 mg/l |
| Dunaliella tertiolecta
| >124 mg/l | | >124 mg/l |
Euglena gracilis
(freshwater alga) | >112 mg/l | | >112 mg/l |
Daphnia magna
(crustacea) | >1200 mg/l | --- | 1200 mg/l |
Brachydanio rerio
(Zebra fish) | >1200 mg/l | --- | 1200 mg/l |
Lemna gibba
(duckweed) --- vegetative growth | 1100 mg/l | 600 mg/l | 300 mg/l |
| Terrestrial Plants |
|---|
Multiple species
(monocotyledons and dicotyledons)
--- seed germination |
>1000 mg/l |
--- |
1000 mg/l |
Mung Bean
--- soil application |
5.7 mg/kg |
10 mg/kg |
1 mg/kg |
Sunflower (Helianthus annuus)
--- soil application
--- foliar application |
12 mg/kg
--- |
1 mg/kg
--- |
<1 mg/kg
100 mg/l |
Wheat (Triticum aestivum)
--- soil application
--- root exposure
--- foliar application |
12 mg/kg
---
--- |
10 mg/kg
5 mg/l
100 mg/l |
1 mg/kg
1 mg/l
50 mg/l |
Plantain (Plantago major)
--- root exposure
--- foliar application |
---
--- |
100 mg/l
--- |
32 mg/l
100 mg/l |
Soya
--- root exposure
--- foliar application |
---
--- |
10 mg/l
100 mg/l |
1 mg/l
10 mg/l |
Maize, oilseed rape, rice
--- foliar application |
--- |
--- |
100 mg/l |
NOTE: For comparison, the anticipated environmental concentration is in the region of 0.00016 mg/l.
| Units: | |
| mg/l --- for tests with NaTFA in aqueous solution
mg/kg (dry weight of soil) --- for tests with NaTFA applied to soil
|
| Terms: | |
| EC50 = test concentration resulting in a 50% effect; the table lists EC50 for biomass
LC50 = concentration at which 50% of the organisms show an effect (mortality)
LOEC = lowest observed effect concentration
NOEC = no observed effect concentration
|
Mesocosm Study
One semi-field stream study has been conducted by the Stroud Water Research Center with
sodium trifluoroacetate to investigate the potential effects of trifluoroacetate on freshwater algal
communities and primary productivity. The long-term exposure to a mean sodium trifluoroacetate
concentration of ~30 µg/l had no effect on the algal primary production in the stream mesocosm.
In addition, no severe effects or consistent trends were observed in the algal species composition.
Toxicity to Terrestrial Plants
The effects of sodium trifluoroacetate in soil on seed germination and early plant growth of
wheat, sunflower and mung bean were studied by Brixham Environmental Laboratory. The EC50
values for wheat, sunflower and mung bean were calculated to be 12, 12 and 5.7 mg/kg dry soil,
respectively. The NOEC's were determined to be 1 mg/kg dry soil for wheat and mung bean and
< 1mg/kg dry soil for sunflower. In another study, the seeds of 10 species of plants
(monocotyledons and dicotyledons) were exposed to a range of aqueous concentrations of NaTFA
for 5 to 7 days. There was no significant inhibition of germination of any seeds up to a
concentration of 1000 mg/l of NaTFA (maximum tested concentration).
Brixham Environmental Laboratory conducted a toxicity study with higher plants by aqueous
exposure of the roots. Sodium trifluoroacetate showed toxicity to wheat and to Plantago major
(plantain). In the preliminary study with wheat, a significant effect in both leaf and root weight
was observed at all three concentrations (32, 100, 320 mg/l). In the definitive study, wheat
seedlings were exposed to 1 and 10 mg/l concentrations of 14C radiolabelled NaTFA. The growth
of plants exposed to 10 mg/l of NaTFA was significantly inhibited; there was no significant effect
on growth at 1 mg/l exposure concentration and no other symptoms of toxicity were observed.
Growth of Plantago major seedlings was inhibited at 100 mg/l of NaTFA. No effects on the
seedlings were observed at or below 32 mg/l.
AFEAS also funded a study at the University of Missouri to investigate the effects of NaTFA-treated
soils at concentrations of 1, 10 and 100 mg/kg soil on soybean germination and seedling
growth. The results show no effect of 1 mg/kg NaTFA on germination or growth, or on acetylene
reduction activity of the soybean nodules. Toxic effects on plant growth were observed at
the 10 and 100 mg/kg levels.
Prof. Alan Davison at the University of Newcastle (U.K.) investigated the effects of foliar
application of solutions of NaTFA. Seedlings of seven species of terrestrial plant --- sunflower,
soya, wheat, maize, oilseed rape, rice and plantain --- were field-grown to ensure that the leaves
were fully hardened, and exposed to sprayed solutions of NaTFA for 24 hours. The soil and
plant roots were protected from the application. After three weeks, there was no effect of TFA
on height, leaf number, stomatal conductance, final harvest weight, or chlorophyll and carotenoid
concentrations of any of the species at the maximum concentration tested which was 100 mg
NaTFA/l. There was a significant effect on specific leaf area for one species (wheat) at 100 mg/l,
but no effect at 50 mg/l.
A subsequent study, at only 100 mg NaTFA/l, used a similar system but with laboratory-grown
plants --- wheat, maize, sunflower and soya --- both with and without protection of the soil and
roots from the spray. Only soya showed any symptoms of toxicity. These were apparent by both
routes of exposure, suggesting that laboratory-grown plants were more sensitive to foliar
application than those that had been field-hardened; however, the symptoms were more severe
when the TFA solution was able to reach the roots. Soya seedlings, with soil and roots exposed
to the foliar spraying, were treated with a range of concentrations of NaTFA to simulate 10 mm
of rainfall. There was no effect at 1, 5 and 10 mg/l on dry weight, leaf size or stomatal conductance
(transpiration). At 100 mg/l, symptoms of toxicity were apparent and leaf size --- but not
dry weight --- was reduced, suggesting TFA affected leaf expansion.
Soya seedlings were exposed to 5 and 10 mg NaTFA/l for 6 weeks by root exposure using a
hydroponic system. Symptoms of toxicity were observed after 20 days at 10 mg/l. Less severe
and sporadic symptoms were observed at 5 mg/l by the end of the study. In a similar study with
wheat (Hanno variety) no effects were seen at either concentration after 52 days, at which stage
flowering was beginning. The absence of effects at 10 mg/l suggested that this variety of wheat
might be less sensitive than the Katepwa variety used in Brixham's earlier study. Therefore, the
experiment was repeated using seed from the same batch used by Brixham, testing at 1, 5, 10 and
100 mg NaTFA/l. At 100 mg/l, all the plants died and at 10 mg/l growth stopped. At 5 mg/l
symptoms of toxicity were evident after 30 to 35 days. There was no significant effect on final
harvest weight at 1 mg NaTFA/l after 43 days, which was in agreement with the Brixham study.
Further hydroponic exposures with soya have also shown no effect on shoot weight at 1 mg
NaTFA/l after 43 days.
Bioaccumulation via Plant Root
Brixham Environmental Laboratory conducted experiments on the potential for bioaccumulation
of TFA via plant roots. Sunflower seedlings were exposed to a single concentration of radiolabelled
TFA of 2 µg/l (0.002 mg/l) in the aqueous medium surrounding the roots. The bioconcentration factor
(BCF) in the leaves was calculated to be 22 after 12 days; the BCF for the whole plant was ~10.
The accumulation rate was somewhat less than would be expected from
passive influx in the transpiration stream without efflux. A significant quantity of radiolabel was
lost to the medium surrounding the roots. It was concluded that the plants showed some
excretion via the roots. Further studies would be required to quantify the depuration rate.
At the end of the sunflower study, more than 80% of the 14C-residues in the leaves were found
to be extractable in water after tissue maceration. Fractionation of the extract using ion
chromatography showed the residues co-eluted with a trifluoroacetate standard spiked into leaf
extract, suggesting that no significant metabolism of TFA had occurred.
The accumulation of 14C-residues of trifluoroacetate was also monitored as part of Zeneca's
hydroponic toxicity study with wheat (described previously). At 1 mg/l, the bioconcentration
factor, based on fresh weight, increased continuously over the exposure period to a final value
of 27. The concentrations in the tips of the leaves were approximately 4 times greater than in
the remaining leaf tissues. At 10 mg/l, a similar BCF was observed and tissue necrosis in the
tips of the leaves appeared to occur as tissue residues reached approximately 1000 mg/kg (fresh
weight).
Accumulation of trifluoroacetate by soya and wheat was investigated by the University of
Newcastle, by determination of fluoride ion, after fusion of the tissue with sodium carbonate to
cleave CF bonds. For soya exposed to 5 mg/l, the tissue concentration in the oldest leaves
reached a plateau after about 20 days. Each successive set of leaves, if fully developed, attained
higher tissue concentrations. First symptoms of toxicity were associated with average leaf tissue
levels of approximately 150 mg/kg (dry weight). For wheat exposed to 1 mg/l, tissue concentrations
also reached a plateau after approximately 25 days. At 5 mg/l, the average leaf tissue concentration
was approximately 200 mg/kg (dry weight) when growth inhibition was first evident.
|
Program Area D --- Effects of TFA on Key Biogeochemical Processes
|
Effects on Carbon Cycling
The effects of a range of concentrations of TFA on carbon cycling was determined in freshwater
environments by Stroud Water Research Center. No evidence of acute toxicity of TFA to the
freshwater incubated communities was observed, as described previously. Overall, the data
showed evidence of a very weak competitive interaction between TFA and acetate when TFA
concentrations were at several hundred fold higher concentrations than those anticipated in the
environment. There was no evidence of a TFA effect on photosynthetic carbon dioxide fixation
at or even near the concentration which could be anticipated in the environment.
Soil Adsorption Studies
Preliminary studies in Phase I indicated that TFA did not adsorb to three different soils.
However, in a much more extensive investigation of 54 soils by researchers at Syracuse
University, TFA adsorption was found to correlate with the content of oxidizable organic matter.
Soils from wetlands, peat bogs and a boreal forest showed the highest TFA retention. Soils low
in organics were found to be non-interactive with TFA. TFA retention was influenced by pH and
other anions. The indications are that some soils could act to retard the transmission of TFA with
water and that the TFA could accumulate to some degree in these soils.
Effects on Nitrogen Fixation
DuPont's environmental biotechnology group initiated laboratory experiments to determine
whether there was any impact of TFA on the ability of pure cultures of certain eubacterial strains
to grow by nitrogen fixation. Two parameters were measured as a function of TFA
concentration: nitrogen-dependent growth and nitrogenase activity.
The effect of TFA has been investigated in free-living nitrogen-fixing bacteria as well as
nitrogen-fixing bacteria which live in symbiosis with leguminous plants. Three species of free-living
nitrogen-fixing bacteria have been investigated: an aerobic organism (Azotobacter
vinelandii), a photosynthetic bacterium (Rhodobacter capsulatus), and an anaerobe (Clostridium
pasteurianum). These species were tested for their ability to grow by nitrogen fixation. Tests
for any direct effects of TFA on nitrogen fixation were also performed. The investigators found
no effects of TFA on growth in these species at concentrations up to 100 mg/kg.
Studies conducted at the University of Missouri have shown that TFA at concentrations of 1
mg/kg of soil had no effect on the germination or growth of soybean seedlings; 1 mg/kg TFA
also had no discernible effect on the nitrogen-fixing capacity of these soybean plants as
determined by plant nodule development as well as acetylene reduction activity of the nodules.
Toxic effects were observed at the level of 10 100 mg/kg TFA.
Projected Distribution of TFA
Using the U.S. Environmental Protection Agency's precursor emission scenario, Atmospheric and
Environmental Research, Inc. (AER) calculated the global average concentration of TFA in
precipitation for the year 2010 to be about 0.16 µg/l (0.00016 mg/l). The results were discussed
at the AFEAS workshop on the environmental fate of trifluoroacetic acid in 1994 and by
Kotamarthi et al. (1997).
A review of the literature was conducted by Dr. Steven E. Schwarzbach and co-workers (U.S.
Fish and Wildlife Service) to assess the extent to which relatively stable solutes deposited with
rain and snow may become more concentrated through evaporative processes. The measurements
reported in the literature, limited to surface water samples, have indicated seasonal evaporative
concentrations factors ranging from 1.2 to 6.5 and from 1.5 to 50 for vernal pools and playa
lakes, respectively.
In a second study, AER conducted a sensitivity analysis addressing possible future concentrations
of TFA in surface waters and soils under a range of various environmental conditions, using the
U.S. EPA precursor emission scenario. The possible outcomes cover a broad range of concentrations.
At the low end, there will be considerable dilution of TFA-containing precipitation in the
oceans and other bodies of water with a long residence time. At the other extreme, TFA concentrations
may conceivably be enhanced in seasonal wetlands and other surface waters undergoing
rapid seasonal evaporation. Depending on meteorological conditions, further enhancement might
occur due to transport of precursors originating in urban areas. A paper on this study was
published by Nature (Tromp et al. 1995).
In the sensitivity study, AER considered several factors that may increase the concentration of
TFA in precipitation in urban areas above the expected global averaged values. These factors
include enhanced concentrations of precursor gases due to local emission sources, trapping and
buildup of precursor gases in a stagnant air mass in a basin during an air inversion, and elevated
concentrations of the OH radical (the principal oxidant initiating atmospheric degradation of
precursors) occurring during air pollution episodes coupled with rainout. Using observed values
of ozone, NOx and hydrocarbons at one location in the Los Angeles basin (Riverside) during June
1990, calculations indicated that maximum OH concentrations would be less than a factor of two
greater than the monthly averaged value. AER also concluded that OH concentrations in typical
urban pollution events will not be significantly elevated over clean air background levels because
OH is suppressed by high concentrations of NOx and hydrocarbons. The study did not include
detailed assessments of the other factors influencing TFA deposition.
|
|
| |
|
Bibliography
| AFEAS. Proceedings of a Workshop on the Decomposition of TFA in the Environment.
(February 1994; Washington D.C.), publ. by Alternative Fluorocarbons Environmental
Acceptability Study, Washington D.C., 1994a. |
| AFEAS. Proceedings of a Workshop on the Environmental Fate of Trifluoroacetic Acid.
(March 1994; Miami Beach, Florida), publ. by Alternative Fluorocarbons Environmental
Acceptability Study, Washington D.C., 1994b.. |
| Berends, A.G., J.C. Boutonnet, C.G. de Rooij, and R.S. Thompson. "The Toxicity of Trifluoroacetate to Aquatic Organisms,"
Environmental Toxicology and Chemistry, in press, 1998. |
| Boutonnet, J.C., et al. "Environmental Risk Assessment of Trifluoroacetic Acid,"
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| Frank, H., A. Klein, and D. Renschen. "Environmental trifluoroacetate,"
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| Kotamarthi, V.R., J.M. Rodriguez, M.K.W. Ko, T.K. Tromp, and N.D. Sze. "Trifluoroacetic acid from the degradation of HCFCs and HFCs: A three-dimensional modeling study,"
J. Geophys. Research, 1997. |
| Tromp, T.K., M.K.W. Ko, J.M. Rodriguez, and N.D. Sze. "Potential accumulation of a CFC-replacement degradation product in seasonal wetlands,"
Nature, 376, 327, 1995. |
| Visscher, P.T., C.W. Culbertson, and R.S. Oremland. "Degradation of trifluoroacetate in oxic and anoxic sediments,"
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| Zehavi, D., and J.N. Seiber. "An analytical method for trifluoroacetic acid in water and air samples
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