Quantification of fluoride ions using ion-specific electrode: application for PFOA defluorination
Résumé
Due to their large commercial and industrial applications based on their chemical properties, per-and polyfluoroalkyl substances (PFAS) are ubiquitous and persistent in the environment. Due to their proven or potential adverse effects on environmental and human health, several remediation techniques are being developed for their degradation, which are expected to result in the formation of fluorinated by-products and fluoride (F-) ions. However, due to the large number of known/unknown PFAS and the lack of standards for proper quantification of all PFAS, the mass balance is often incomplete when using only liquid chromatography coupled to mass spectrometry measurements. In this context, the development of analytical method for determining free fluoride ions to evaluate quantitatively PFAS defluorination is required.
Here, we present a robust method for F- measurement based on a fluoride-ion selective electrode (F-ISE) and Tisab IV as a metal ion complexing and ionic concentration adjustment agent. The method consists in the calibration of the F-ISE with solutions of known F- concentration, the preparation of samples with known F- concentration to verify the accuracy of the calibration, and the preparation and measurement of the samples with unknown F- concentration. The calibration of the F-ISE showed good electrode slopes, close to −59 mV, and the quantification of samples of known F- concentration was accurate, with less than 3 % of difference between the theoretical and the measured values in different matrixes (pure water and water with traces of DMSO) for F- content higher than 1 mg L-1.
The method was applied to F- quantification during PFOA mineralization using a modified protocol of the one developed by Trang et al. (2022). It consists in using DMSO as aprotic polar solvent for PFOA decarboxylation, in absence or in presence of NaOH for the substitution of F- by OH-, at a temperature of 90 and 120 °C (Tur et al. 2024a). The preparation of the samples for F- measurements consists first in a complete washing and dilution in pure water (i) to limit DMSO content and (ii) to recover all precipitated NaF – whose formation was qualitatively confirmed by SEM/EDS analysis. Then, the pH of this diluted sample was measured and adjusted between 5 and 6.5 with nitric acid. Finally, after the addition of Tisab IV at a 1:1 volume ratio, the F- content was measured. Even if PFOA disappearance was observed in both the absence and the presence of NaOH, without significant accumulation of known by-products, F- ions were only produced and quantified in the presence of NaOH (up to 80-85% of defluorination). Unusual by-products must then have been formed, confirming the necessity to quantify F- ions and not only the typical PFAS when assessing PFAS degradation.
This method was also applied to PFOA removal using zero-valent iron particles (Tur et al. 2024b). The results indicated PFOA disappearance but without any production of F- ions, suggesting that PFOA removal can be attributed to adsorption rather than defluorination. Overall, the method provides a robust tool for evaluating the defluorination efficiency of current and future PFAS degradation technologies.
Acknowledgements: This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101036449. S. Borensztajn done SEM/EDS analyses.
References:
Trang, B.; Li, Y.; Xue, X.S.; Ateia, M.; Houk, K.N.; Dichtel, W.R. Science 2022, 377, 839
Tur, R.; et al., 2024a and 2024b. Oral presentations in this 18th ISPTS 2024
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