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TFA in the Biosphere

Environmental Distribution

Trifluoroacetic acid, CF3C(O)OH or TFA, is a strong organic acid with pKa of 0.23 and it is miscible with water (solubility over 10000 g/l). The vapor pressure is relatively high, 105.7 hPa at 20°C, the partition coefficient n-octanol/water (log) is -0.2. Laboratory studies have been performed to assess the strength of binding of TFA to a variety of soil types (van Dijk, 1992; Richey et al., 1997). Some studies showed that TFA did not adsorb to soil, others showed that TFA generally interacted weakly with most soils but was strongly adsorbed by some soils containing high levels of organic matter. The data are not necessarily contradictory but may reflect the heterogeneity of soils tested.

Degradation

TFA is a stable ion in the aqueous phase and no significant loss process such as hydrolysis, photolysis, or formation of insoluble salts has been identified. There have been two reports of TFA degradation under anaerobic conditions. In the first study, natural sediments reduced TFA (Visscher et al., 1994). However, even though this work was done in replicate, the experimenters and others were unable to reproduce it in subsequent studies (Matheson et al., 1996; Emptage et al., 1997). In the second study (Chauhan et al., 1995), labelled TFA was removed from a mixed anaerobic in vitro microcosm. Limited evidence of decarboxylation has also been reported for two strains of bacteria grown under highly specific conditions (Chauhan et al., 1995). A field study indicated that TFA was retained in vegetation and soil of a temperate North American forest, especially in the case of wetlands with organic soils (Richey et al., 1997; Likens et al., 1997).

Bioaccumulation in Animals and Plants

Potential for bioaccumulation in animals is highly unlikely due to the extremely low log Kow (-0.2). Due to its high solubility TFA can accumulate in plants via roots uptake of water. This phenomenon is supported by experimental data which demonstrate that TFA is a xylem mobile herbicide, transported through the stem and accumulated in leaves (Rollins et al., 1989). The bioaccumulation in vegetation calculated according the following equation: log CF= 5.943 2.385 log MW gives a value of 10.89 as concentration factor (CF), of the same order of magnitude of those experimentally observed.

    TFA is not concentrated in lower aquatic-life forms such as bacteria, small invertebrates, oligochaete worms and some aquatic plants including Lemna gibba (duckweed). In terrestrial higher plants such as sunflower and wheat some bioaccumulation was seen (Thompson et al., 1994). This appeared to be related to uptake with water and then concentration due to transpiration water loss. When transferred to clean hydroponic media, some elimination of TFA was seen. Also, over 80% of the TFA in leaves was found to be water extractable, suggesting that no significant metabolism of TFA had occurred.

Ecotoxicology

Effect of TFA on Activated Sludge: The semi-continuous activated sludge test indicated that TFA had no discernible effect on the performance of the sludge for the biodegradation of organic carbon (van Ginkel and Kroon, 1992).

    Effect of TFA on metabolism of microbial communities: The mineralization of acetate to carbon dioxide is a key link in the biogeochemical carbon cycle. Therefore it is essential to know whether TFA, which is structurally close to acetate, could interfere with acetate metabolism. Experimental results (Bott and Standley, 1998) suggest that TFA at concentrations several orders of magnitude higher than those anticipated in the environment did not impact acetate mineralization to carbon dioxide nor did it affect incorporation of acetate into cellular material. The effect of TFA has been investigated in free-living nitrogen-fixing bacteria (Nagel and Odom, 1997). The experiments were designed to determine whether TFA was specifically toxic to nitrogen fixation. No effect of TFA on growth or N2 fixation with ammonium ion as nitrogen source was noted at concentrations as high as 100 ppm TFA. It has been shown that TFA has no effect on methanogenic environments where acetate is an important intermediate (Emptage et al., 1997).

    Aquatic toxicity: In acute toxicity tests no effects of NaTFA on water fleas (Daphnia magna) and zebra fish (Brachydanio rerio) were found at a concentration of 1200 mg/l (Rhone-Poulenc, 1995). A 7-day study with duckweed (Lemna gibba) revealed a Predicted No Effect Concentration (NOEC) of 300 mg/l (Smyth et al., 1993). Based on the results of five toxicity tests with Selenastrum capricornutum a NOEC of 0.12 mg/l was found. However, algal toxicity tests with NaTFA and Chlorella vulgaris, Scenedesmus subspicatus, Chlamydomonas reinhardtii, Dunaliella tertiolecta, Euglena gracilis, Phaeodactylum tricornutum, Navicula pelliculosa, Skeletonema costatum, Anabaena flos-aquae and Microcystis aeruginosa resulted in NOEC values which were all higher than 100 mg/l (Smyth et al., 1994a,b,c). Recovery of the growth of S. capricornutum was found when TFA was removed from the test solutions and therefore TFA should be considered algistatic and not algicidal for S. capricornutum. The reason for the unique sensitivity of this strain is unknown, but a recovery of the growth rate was seen when citric acid was added, suggesting a competitive inhibition of the citric acid cycle.

    One semi-field study (Bott and Standley, 1995) with mesocosm streams has been conducted with NaTFA to study the potential effects of TFA on freshwater algal communities and primary productivity. Long-term exposure to a mean NaTFA concentration of 31-32 µg/l had no effect on the primary productivity of the diatom-dominated algal flora. Effects on organic carbon excretion which were related to high levels of TFA were noted in some experiments. TFA did not alter the algal species composition in the stream mesocosm.

    Terrestrial plants: Application of NaTFA at 1000 mg/l to seeds of sunflower, cabbage, lettuce, tomato, mungbean, soybean, wheat, corn oats and rice did not affect germination (Thompson and Windeatt, 1994; Emerich, 1997). Foliar application of a solution of 100 mg/l of NaTFA to field grown plants did not affect growth of sunflower, soya, wheat, maize, oilseed rape, rice and plantain (Davison and Pearson, 1997). When plantain, wheat and soya were grown in hydroponic systems containing NaTFA, no effects were seen on plantain at 32 mg/l, on Triticum and soya at 1 mg/l, and on wheat at 10 mg/l (Thompson, 1995; Thompson et al., 1995; Davison and Pearson, 1997).

Mammalian Toxicity

TFA is not metabolised in mammalian systems. The half-life of TFA in humans is 16 hours. As expected, the free acid is more acutely toxic than the sodium salt. In one study, 2 of 5 mice died from a dose of 150 mg/kg of HTFA, an effect comparable to that seen with an equimolar dose of HCl (Blake et al., 1969). In contrast, no deaths were seen when mice were given an oral dose of 5.000 g/kg of NaTFA. In studies involving single intraperitoneal injections of doses up to 2 g/kg, only mild liver toxicity was seen (Rosenberg, 1971; Rosenberg and Wahlstrom, 1971). There was only one report of an acute inhalation toxicity study (Kheilo and Kremneva, 1966). This study involved single two-hour exposures of both rats and mice. The LC 50 for mice was 13.5 mg/l (2,900 ppm) and for rats it was 10 mg/l (2,140 ppm). This would classify TFA as having low inhalation toxicity. These same authors reported that the irritation threshold for humans was 54 ppm.

    As one would expect of a strong acid, HTFA is a severe irritant to the skin (Patty, 1963; Kheilo and Kremneva, 1966). Concentrations as low as 2% were moderate skin irritants. It would be expected to be a severe eye irritant. When conjugated with protein, it has been shown to elicit an immunological reaction (Mathieu et al., 1974; Reves and McCracken, 1976; Ford et al., 1984; Satoh et al., 1985;1989), however it is unlikely that TFA itself would elicit a sensitisation response (Waldon and Ratra, 1972; Reves and McCracken, 1976; Ford et al., 1984). Repeated administrations of aqueous solutions have shown that TFA can cause increased liver weight and induction of peroxisomes (Just et al., 1989; Warheit, 1993). Relative to the doses (0.5% in diet or 150 mg/kg/day) the effects are mild.

    In a series of Ames assays, TFA was reported to be non-mutagenetic (Blake et al., 1981). Its carcinogenic potential has not been evaluated. Although TFA was shown to accumulate in amniotic fluid following exposure of pregnant animals to high levels of halothane (1,200 ppm), no foetal effects were seen. Given the high levels of halothane exposure, it is unlikely that environmental TFA is a reproductive or developmental hazard.

TFA Risk Assessment

The environmental risk from TFA as degradation product of CFCs substitutes can be deduced from the value of the ratio of exposure to effect, or of Predicted Environmental Concentrations to Predicted No Effect Concentrations (PEC/PNEC). To derive a PEC, production and releases of parent compounds, along with rates of transformation into TFA, have been modeled. A range of concentrations in rain water has been calculated, taking into account geographical variations (OH radical concentrations, amount of rain, regional releases of parent compound). A rough average concentration of 0.1 µg/l in rainwater, by the year 2020, is taken as a global PEC. But an important question remains concerning the origin of the large current levels of TFA which have been measured in the environment (fresh and marine surface waters, rain and air) and which cannot be explained by the known industrial sources.

    The physico-chemical properties of TFA allow a prediction of no bioaccumulation in animals. In terrestrial plants, accumulation can take place but is transient and effects on vegetation can be observed only in experiments using non environmentally relevant concentrations (102-106 mg/l).

    TFA is not retained in soils, with the exception of those having high organic content; in this case, it is uncertain if it can still be bioavailable. Generally speaking, TFA remains associated with water and has been shown to be not effective on several types of metabolism related to basic biogeochemical cycles.

    Given the fact that deposited TFA would remain in water, a number of studies have been done aiming to derive a PNEC in the aquatic compartment. Standard acute tests on fish and daphnia carried out with NaTFA show no effects on animals at large concentrations (up to 1 g/l TFA). This is reinforced by toxicological data on mammals. On the other hand, the NOEC for the standard algal species Selenastrum capricornutum is around 0.10 mg/l (as TFA). To see if this sensitivity was general among algae, 10 other species belonging to 4 different classes have been tested: no one was sensitive to NaTFA (NOEC> 100 mg/l). It is proposed to use as a PNECaqua (the NOEC of this species of algae) as a worst case, without any further safety factor. So, PNECaqua =0.1 mg/l.

    A large amount of research has been devoted to effects on higher plants, as they could be exposed to TFA in rain water through leaves and stems and TFA in pore water through roots. A few tested species (selected among those having an important role in feeding people and cattle) show a significant sensitivity to TFA (as NaTFA). The threshold for effects on wheat and soya is around 5 mg/l with a NOEC=1 mg/l. Other studies show that nitrogen fixation is not affected up to 1 mg/kg soil. Therefore, a conservative figure could be PNEC soil = 0.1 mg/l.

    No significant risk is anticipated from TFA produced by atmospheric degradation of the present and future production of HCFCs and HFCs, as there is a 1000-fold difference between the PNEC and the PECs.


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