Small, mobile, persistent: Trifluoroacetate in the water cycle – Overlooked sources, pathways, and consequences for drinking water supply
Graphical abstract
Introduction
Trifluoroacetate (TFA) describes the salt of trifluoroacetic acid. It is the perfluorinated analogue of acetate and thus the perfluorinated carboxylate with the shortest possible chain length. Compared to acetic acid, the acidity of the molecule is considerably increased by the substitution of the C-bound hydrogen atoms by fluorine (Chambers, 2004). Due to its low pKa of 0.23 (Solomon et al., 2016), TFA occurs in its anionic form in the aquatic environment and is considered to be very mobile. TFA has been used in biotechnology as a solvent for proteins (Katz, 1954), in peptide synthesis as ion-pairing agent during HPLC purification (Tipps et al., 2012) and for the purification of pharmaceutical products on a preparative scale (Rivier et al., 1984, Kaiser and Rohrer, 2004). According to the European Chemical Agency (ECHA), both the acid and its potassium salt, are manufactured and/or imported in considerable amounts in the European Union with 1000–10,000 t and 100–1000 t per year, respectively (ECHA, 2017a, ECHA, 2017b).
Frank et al. (2002) reported that TFA is evenly distributed in ocean waters with a concentration of about 0.2 μg/L. As water samples from some depths were older than 1000 years, the authors concluded that natural marine sources of TFA must exist. Indeed, similar concentrations of about 0.15 μg/L were measured in the Atlantic Ocean throughout the water column in certain sampling locations by Scott et al. (2005a), but samples from the Pacific Ocean contained considerably lower levels of TFA. The authors assumed that deep-sea hydrothermal vents may cause natural emissions to the marine environment.
TFA was detected in low concentrations in pre-industrial glacier ice and historic Antarctic firn samples (von Sydow et al., 2000) as well as in very low concentrations (below 10 ng/L) in old groundwaters with an age of approx. 700 years (Jordan and Frank, 1999). However, whether TFA is also naturally occurring in freshwaters is controversially discussed. TFA was neither found in samples from older German and Swiss groundwaters (Jordan and Frank, 1999, Berg et al., 2000) nor in pre-industrial samples from Greenland and Denmark (Nielsen et al., 2001).
Direct anthropogenic emission of TFA in freshwater was presumed to be low (Frank et al., 2002) and thus more attention has been paid to potential TFA precursors and their degradation to TFA. Ellis et al. (2001) pointed out that thermolysis of fluoropolymers is a potential TFA source, especially in urban areas. Even more important, TFA occurs as an atmospheric transformation product upon photo-oxidation of fluorinated hydrocarbons containing trifluoromethyl moieties. Such compounds like 1,1,1,2-tetrafluoroethane or 1,1,1,2,3,3,3-heptafluoropropane are used as refrigerants in air conditioning systems, as gaseous fire suppression agents or as propellants and substitutes of chlorofluorocarbons (CFCs) (Frank et al., 2002). However, TFA itself proved to be very stable in photochemical degradation laboratory experiments with a medium-pressure mercury lamp (Qu et al., 2016). Furthermore, TFA can also be released by heating of Teflon® products (Benesch et al., 2002), by the oxidation of fluorinated inhalation anesthetics like isoflurane in the atmosphere (Jordan and Frank, 1999), and possibly by the degradation of several plant protecting agents and pharmaceuticals with trifluoromethyl moieties, which was demonstrated for the lampricide 3-trifluoromethyl-4-nitrophenol (Ellis and Mabury, 2000).
If formed in the atmosphere, a rapid partitioning of TFA into droplets of clouds, rain and fog can be expected (Solomon et al., 2016). Precipitation is assumed to be the major source of TFA in the biosphere, e. g. for Switzerland, Berg et al. (2000) calculated that wet deposition accounts for 96% of the annual mass flux. About one third of the overall TFA is dislocated by rivers, which results in a considerable amount introduced in terrestrial environments where TFA is prone to be leached into the groundwater. TFA was already measured in rain and snow samples from Switzerland (Berg et al., 2000), Japan (Taniyasu et al., 2008), China (Wang et al., 2014), the United States (Wujcik et al., 1998), Sweden, Canada, New Zealand, Ireland, Poland (von Sydow et al., 2000), Malawi, Chile (Scott et al., 2005b) and Germany (Frank et al., 1996) up to concentrations in the μg/L-range. It was clearly demonstrated by Berg et al. (2000) that a “first-flush” effect exists, as concentrations measured in the first few mm of a rain event were considerably higher. Some authors postulated that high concentrations of TFA in rain positively correlate with the degree of urbanization/industrialization in the catchment area (Scott et al., 2005b, Wang et al., 2014). However, Berg et al. (2000) found no difference between precipitation samples collected in a densely populated catchment close to the city of Zurich and a remote alpine sampling site.
Due to the variety of TFA sources discussed above, it is not surprising that detections of TFA in surface waters were reported. TFA concentrations in Swiss rivers were between 0.01 and 0.33 μg/L (Berg et al., 2000) and an average concentration of 0.14 μg/L was observed in major rivers in Germany (Jordan and Frank, 1999) with a maximum concentration in the Rhine River with 0.63 μg/L in the Lower Rhine region (Frank et al., 1996).
The here presented study was initiated when during screening of potable and surface waters in South-West Germany, maximum TFA concentrations of more than 100 μg/L were detected. As the German Federal Environment Agency specified a health-related indication value (HRIV (Dieter, 2014)) of 1 μg/L (now 3 µg/L) for TFA in tap water (based on the classification of TFA as non-relevant metabolite of the herbicide flurtamone), an extended monitoring program at rivers and streams was initiated to identify potential TFA dischargers and emission routes responsible for the high concentrations. Furthermore, the present study provides information regarding options for removal of TFA from drinking water if contaminated raw water resources are used for drinking water production. Despite sporadically reported detections of TFA in tap water (0.04 μg/L to 0.15 μg/L; (Boutonnet et al., 1999)), treatment options were surprisingly not addressed in previous publications. Furthermore, monitoring of wastewater treatment plants (WWTPs) is complemented by systematic laboratory batch tests in order to assess if potential precursors can be biologically and chemically degraded to TFA.
Section snippets
Chemicals
Methanol (MeOH, Optigrade) was obtained from LGC Standards (Wesel, Germany). Sodium trifluoroacetate, fluoxetine hydrochloride, tembotrione, fluopyram, flurtamone, flufenacet, ammonium bicarbonate, sodium thiosulphate, potassium indigotrisulfonate, sodium hydroxide (NaOH, p.a., ≥98.0%) and ammonium hydroxide solution (NH4OH, ≥25%, puriss.) were purchased from Sigma Aldrich (Steinheim, Germany). The isotopically labeled internal standard sodium trifluoroacetate-13C2 was obtained from TRC
Occurrence of TFA in major streams and identification of industrial dischargers
Based on spatially resolved monitoring of the longitudinal profile of the Neckar River, a point source of TFA close to the city of Bad Wimpfen was identified. Downstream of Bad Wimpfen, concentrations of TFA between 5.4 μg/L and 140 μg/L were measured. Upstream the point of discharge the concentrations were much lower but TFA was still present with concentrations around 1 μg/L. The discharger was identified as a producer of inorganic and organic fluorinated chemicals. Besides the production of
Conclusions
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TFA occurs ubiquitously in surface water.
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Routes of discharge for TFA are diverse. The release of TFA into the aquatic environment is not limited to photochemical degradation of precursors in the atmosphere and subsequent entry of TFA by precipitation. Further sources are industrial emissions, municipal WWTPs, and possibly the biological degradation of micropollutants like plant protecting agents or pharmaceuticals in general.
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(Industrial) point sources of TFA can have substantial impact on
Acknowledgement
The study was partly financed by the German Federal Ministry of Education and Research (BMBF, Project OPTI, grant number 02WIL138). The authors are very grateful to Peter Knaus from the water supply company Hermentingen and the representatives of the anonymized water utilities for providing the samples from the full-scale waterworks. We also thank Dominic Armbruster for preliminary measurements.
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