153
Trifuoroacetic Acid from CFC Replacements
plastic is essential. The mechanism appears to be the breakdown of the fuoropolymer into difuorocarbene units (•CF 2 •) that can then condense to form
hexafuoropropene (CF 3 −CF=CF 2 ). The double bond in this molecule can
then be epoxidized, which then rearranges to form trifuoroacetyl fuoride
(CF 3 −C(O)−F). This acid halide then hydrolyzes in water to form TFA (Ellis
et al., 2001b). In addition to TFA, some longer chain perfuorinated carboxylic
acid species may also be generated. Additionally, fuoropolymers destroyed
in incinerators might create the volatile precursor gases that could transform
into TFA. This could explain the abnormally high concentrations of TFA in
urban areas that could not be attributed to HFC degradation when HFCs
were starting to be utilized.
The degradation of long-chain perfuorinated surfactants and fuorotelomer alcohols (Ellis et al., 2004) can also give rise to TFA. Compounds such as
perfuorooctanesulfonate (PFOS) and perfuorooctanoic acid (PFOA) are persistent, toxic, and bioaccumulate in the environment. Considerable research
has been dedicated to devise mechanisms to degrade these compounds in
wastewater treatment plants, so they are not released to the environment.
Most of the strategies for degrading these chemicals involves the stepwise
removal of carbons starting at the acid end of the molecule. This creates a
series of shorter chain perfuorinated acids as intermediates which can result
in the production of some TFA (Hori et al., 2005; Singh et al., 2019) although
many studies do not measure TFA so the exact contribution of the perfuorinated acids in making TFA is unclear.
An unexpected source of TFA was the degradation of certain pesticides
containing a trifuoro functional group. 3-Trifuoromethyl-4-nitrophenol
(TFM), used to control lamprey in the Great Lakes of the United States and
Canada, has been shown to degrade into TFA (Ellis and Mabury, 2000). This
compound is used at a rate of 50 metric tons per year in the Great Lakes.
Trifuralin, a preemergent herbicide, has also been suggested as a potential
source of TFA, but it has not been defnitively demonstrated (Jordan and
Frank, 1999). The pesticides furtamone (herbicide), fuopyram (fungicide),
tembotrione (herbicide), and fufenacet (herbicide) have been shown to generate TFA during ozonation treatment of wastewater (Scheurer et al., 2017).
Pharmaceuticals represent another potential source of TFA. In this case,
the pharmaceuticals are used and end up in the wastewater where they
can be degraded, either naturally or by wastewater treatment programs,
to form TFA. Fluoxetine (trade name Prozac) and sitagliptin (trade name
Januvia) are known to form TFA during the ozonation of wastewater
(Scheurer et al., 2017).
The last, and somewhat debated, source of TFA is nature. Very early
research showed TFA present in rivers and seawater in Europe at high concentrations early in the adoption of the HFC replacement transition, so they
could not be explained by HFC oxidation (Frank et al., 1996). Further analysis
of old well water showed that TFA was largely undetectable, giving rise to
the assessment that “TFA seems to be predominantly if not exclusively of