Trifluoroacetic Acid in the Environment: Consensus, Gaps, and Next Steps

Abstract

There is ongoing debate about the sources, fate, toxicity, and, ultimately, the ecological risk posed by trifluoroacetic acid (TFA; Brunn et al., 2023; Joudan et al., 2021; Madronich et al., 2023; Scheringer et al., 2024). The debate is sparked in part by TFA's persistence; ubiquity in the environment, especially aquatic ecosystems; and increasing concentrations globally. This Point of Reference provides an overview of the current science, including a distillation of which topics have significant uncertainty or ongoing debate, and suggests the next steps to move our collective understanding of the potential ecological impact of TFA forward.

There is broad scientific consensus on the following: TFA is a short-chain perfluoroalkyl carboxylic acid that contains a single −CF₃ moiety bound to a carboxyl functional group, is a strong acid with a negative base-10 logarithm of the acid dissociation constant (pKa) of 0.3, and is completely miscible with water. It is an atmospheric degradation product of some ozone-depleting chlorofluorocarbon (CFC) replacements, including several hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), and hydrofluoroolefins (HFOs). These compounds produce TFA through hydrolysis of acyl halides, for example, CF₃CFO (trifluoroacetyl fluoride; Wallington et al., 1994), or via secondary photochemistry of trifluoroacetaldehyde (CF₃CHO; Sulbaek Andersen et al., 2004). Once in the environment, TFA has no obvious or significant pathway of degradation and will be deprotonated as its freely dissolved salt that will move with flowing water and accumulate in terminal (endorheic) water bodies, especially marine systems (Boutonnet et al., 1999). The Environmental Effects Assessment Panel of the United Nations Environment Programme has routinely assessed global contributions of TFA from replacement CFC gases under the purview of the Montreal Protocol. It is estimated that between 2020 and the year 2100, 31.5 to 51.9 Tg of TFA (acid equivalent) will be produced from the atmospheric degradation of CFC replacement gases. Simplified models show that deposition to the ocean would increase the concentration of TFA from a nominal value of 200 ng L⁻¹ (acid equivalent) in 2020 to 736 to 1058 ng L⁻¹ (as Na salt) if uptake is limited to the epipelagic zone (top 200 m of the ocean) or 266 to 284 ng L⁻¹ (as Na salt) if distributed throughout the ocean (Madronich et al., 2023). The salts of TFA are not toxic to aquatic and terrestrial organisms at these environmental concentrations (Berends et al., 1999; Boutonnet et al., 1999; Figure 1). Because of its physicochemical properties such as high water solubility and low log octanol–water partition coefficient, TFA is unlikely to accumulate in biota (Boutonnet et al., 1999; Madronich et al., 2024). Current and predicted concentrations (to year 2100) of TFA in the oceans are orders of magnitude lower than thresholds of toxicity, and the risks to environmental health have been assessed to be de minimis (Figure 1; Boutonnet et al., 1999; Madronich et al., 2024).

Debate centers on the sources of TFA, specifically the contribution of natural sources. Natural sources were suggested in the 1990s, and a recent inventory of the use of fluorine-containing minerals in industry from the 1930s to 1999 determined that these uses could not account for the concentrations of TFA in the oceans by the end of the 20th century and concluded that there must be geogenic sources that are not yet fully understood (Lindley, 2023). The possibility of geogenic sources has recently been contested, primarily on the grounds that the detection of TFA in the deep ocean is not itself evidence of a natural source, as well as concerns about the analytical methods used and the limited scope of sampling at the time of reporting, which was >20 years ago, with few new data since (Joudan et al., 2021). To address these knowledge gaps and to test the hypothesis of geogenic production, systematic monitoring of TFA in the oceans generally, and along gradients from putative natural sources (e.g., volcanoes and hydrothermal vents) to no known sources, are needed, as well as elucidation of a mechanism of formation.

Another source of uncertainty is the contribution of TFA from anthropogenic sources other than CFC replacements, such as manufacturing of fluorinated chemicals and the degradation of pharmaceuticals and pesticides that contain –CF₃ moieties. The addition of –CF₃ moieties provides useful properties such as enhanced stability; still, these compounds can undergo transformation in the environment (e.g., via photolysis and/or metabolism), producing TFA. However, their relative contributions to the global mass balance of TFA are uncertain because manufacturing inventories and data on use are not readily available and degradation rates have not been characterized for most compounds (Madronich et al., 2024). Updated production inventories and release of compounds that contain the –CF₃ moiety are needed to assess their contribution of TFA to the environment.

Regional gradients in the distribution of TFA are also highly uncertain. Atmospheric models (David et al., 2021; Luecken et al., 2010) indicate that, downwind of large source regions, some CFC replacements could lead to peak deposition rates of several kilograms per square kilometer per year, with concentrations in rain reaching several micrograms per liter. Although these are well below the no-observed-effect concentrations (NOECs) shown in Figure 1, the extent of localized accumulation in surface waters (e.g., lakes) depends on many factors including drainage area and water residence times. Advances in coupled atmospheric–hydrological modeling are needed to better understand the local and regional dispersal of TFA.

As noted, the bulk of TFA will ultimately be transported to oceans. We acknowledge that evaporation in terminal (endorheic) basins leads to greater concentrations of TFA relative to marine environments, but this process also results in other minerals at much greater concentrations than TFA. These minerals render the water unsuitable for consumption by wildlife and exclude aquatic organisms other than halophilic species from these habitats. Therefore, exposure to TFA as the salt will be greatest and most ubiquitous to marine organisms, as well as most significant ecologically from a scale perspective, relative to other ecosystems. Currently, only two laboratory toxicity tests for marine algae (Skeletonema costatum and Phaeodactylum tricornutum) are available, with a reported 96-h NOEC for biomass of 2400 mg L⁻¹ and a 72-h NOEC of 117 mg L⁻¹, respectively (Berends et al., 1999; Solomon et al., 2016). To address this knowledge gap, a comprehensive suite of acute and chronic tests for marine organisms to better inform ecological risk assessment should be undertaken.

In conclusion, core uncertainties remain in our understanding of the sources, fate, and ecotoxicity of TFA in the environment. Priority recommendations to address these are (1) development of inventories for production of chemicals with –CF₃ moieties, (2) atmospheric and hydrological modeling to characterize TFA transport from source regions to the oceans, (3) additional measurements of TFA levels in the oceans, and (4) characterization of the toxicology of TFA for marine organisms. Closing these knowledge gaps will significantly advance our collective understanding of the environmental toxicology and chemistry of TFA.

The authors are current members of the United Nations Environment Programme's (UNEP) Environmental Effects Assessment Panel (EEAP). The opinions expressed in this Points of Reference are entirely their own.

Mark L. Hanson, Sasha Madronich, Keith Solomon, Mads P. Sulbaek Andersen, Timothy J. Wallington: Conceptualization; Writing–original draft; Writing—review & editing.