157
Trifuoroacetic Acid from CFC Replacements
(Lifongo et al., 2010). If the maximum possible reaction rate for TFA at 90°C
in this study was extrapolated to 15°C, then the half-life of TFA at this temperature would be 40,000 years, which means that simple thermal degradation would not be a factor for TFA. Currently, there is extensive research on
mechanisms to degrade PFCAs, including TFA, in wastewater treatment
plants to prevent their release to the environment. However, these processes
are engineering solutions that are not applicable to environmental conditions
(for examples, see Hori et al., 2005; Qian et al., 2016; Singh et al., 2019).
The terrestrial environment also has mechanisms to degrade TFA. Pehkonen
et al. (1995) reported that TFA could be photooxidized by iron oxyhydroxides to
generate CO 2 and a F 3 C• radical that can subsequently be mineralized. However,
the reaction rate for TFA was slower than that for fuoroacetate or difuoroacetate using two iron oxyhydroxides, namely, ferrihydrite ( Fe 5 3 ( ) 9 )
+3 O OH and
maghemite (γ-Fe 2 O 3 ). Maruthamuthu and Huie (1995) also investigated the
photooxidation of iron-carboxyl complexes of haloacetic acid and came to the
conclusion that the reaction rate of TFA was “too slow to measure.” They also
suggest that the mechanism of the reaction is mediated by the generation of
a OH• radical that attacks the haloacetic acid. A review by Boutonnet et al.
(1999) summarized additional experiments that were presented as gray literature reports, but the main conclusion from these reports is that TFA is stable or
degrades very slowly under most environmental conditions.
The other environmentally relevant process by which TFA could be
degraded would be reduction in anaerobic environments. An early experiment suggested that TFA could be reductively degraded by anoxic sediment
samples (Visscher et al., 1994) although subsequent research using samples
from similar areas were unable to replicate the result (Emptage et al., 1997).
TFA was degraded in an anaerobic reactor at 35°C with ethanol added to it,
which proves reductive dehalogenation can occur as a cometabolism process
under certain conditions (Kim et al., 2000). More recently, a 33-week incubation of anaerobic sludge showed no degradation of TFA (Ochoa-Herrera et al.,
2016). Another study did not detect TFA degradation in vernal pool water
sealed in glass jars for 133 days during which the samples became anaerobic
(Cahill et al., 2001). The conficting reports on the reduction of TFA under
anaerobic conditions give rise to the conclusion that the reductive dehalogenation of TFA may be possible, but the circumstances under which it can
occur are probably limited.
9.4.3 Toxicity
The last physicochemical property of TFA that is noteworthy is its toxicity, or
more accurately, the lack thereof. The protonated form of TFA is a strong acid
that can cause burns due to its acidic character in a fashion similar to that of
hydrochloric acid (Blake et al., 1969). However, protonated TFA is effectively
absent in the environment due to its low pK a with the possible exception of
TFA generated in the atmosphere as a vapor that had not yet partitioned into
Trifuoroacetic Acid from CFC Replacements
(Lifongo et al., 2010). If the maximum possible reaction rate for TFA at 90°C
in this study was extrapolated to 15°C, then the half-life of TFA at this temperature would be 40,000 years, which means that simple thermal degradation would not be a factor for TFA. Currently, there is extensive research on
mechanisms to degrade PFCAs, including TFA, in wastewater treatment
plants to prevent their release to the environment. However, these processes
are engineering solutions that are not applicable to environmental conditions
(for examples, see Hori et al., 2005; Qian et al., 2016; Singh et al., 2019).
The terrestrial environment also has mechanisms to degrade TFA. Pehkonen
et al. (1995) reported that TFA could be photooxidized by iron oxyhydroxides to
generate CO 2 and a F 3 C• radical that can subsequently be mineralized. However,
the reaction rate for TFA was slower than that for fuoroacetate or difuoroacetate using two iron oxyhydroxides, namely, ferrihydrite ( Fe 5 3 ( ) 9 )
+3 O OH and
maghemite (γ-Fe 2 O 3 ). Maruthamuthu and Huie (1995) also investigated the
photooxidation of iron-carboxyl complexes of haloacetic acid and came to the
conclusion that the reaction rate of TFA was “too slow to measure.” They also
suggest that the mechanism of the reaction is mediated by the generation of
a OH• radical that attacks the haloacetic acid. A review by Boutonnet et al.
(1999) summarized additional experiments that were presented as gray literature reports, but the main conclusion from these reports is that TFA is stable or
degrades very slowly under most environmental conditions.
The other environmentally relevant process by which TFA could be
degraded would be reduction in anaerobic environments. An early experiment suggested that TFA could be reductively degraded by anoxic sediment
samples (Visscher et al., 1994) although subsequent research using samples
from similar areas were unable to replicate the result (Emptage et al., 1997).
TFA was degraded in an anaerobic reactor at 35°C with ethanol added to it,
which proves reductive dehalogenation can occur as a cometabolism process
under certain conditions (Kim et al., 2000). More recently, a 33-week incubation of anaerobic sludge showed no degradation of TFA (Ochoa-Herrera et al.,
2016). Another study did not detect TFA degradation in vernal pool water
sealed in glass jars for 133 days during which the samples became anaerobic
(Cahill et al., 2001). The conficting reports on the reduction of TFA under
anaerobic conditions give rise to the conclusion that the reductive dehalogenation of TFA may be possible, but the circumstances under which it can
occur are probably limited.
9.4.3 Toxicity
The last physicochemical property of TFA that is noteworthy is its toxicity, or
more accurately, the lack thereof. The protonated form of TFA is a strong acid
that can cause burns due to its acidic character in a fashion similar to that of
hydrochloric acid (Blake et al., 1969). However, protonated TFA is effectively
absent in the environment due to its low pK a with the possible exception of
TFA generated in the atmosphere as a vapor that had not yet partitioned into