Optimization of experimental conditions of PFOA defluorination using DMSO/NaOH mixture
Résumé
Per- and polyfluoroalkyl substances, PFAS, are a large group of around 5,000 synthetic chemical
compounds widely used in industrial and consumer applications since the 1950s, most usually where
extremely low surface energy or surface tension and/or durable water- and oil-repellency is needed. Their
persistent nature results in diffuse pollution issues in the environment and adverse health effects.
Trang et al.1 investigated low-temperature thermal treatment (80-120°C) of perfluorooctanoic acid, PFOA
(36 g/L), using a mixture of dimethyl sulfoxide, DMSO (145/1 DMSO/PFCA molar ratio), sodium hydroxide,
NaOH (30/1 NaOH/PFOA molar ratio) and milliQ water (8/1 DMSO/water volume ratio). Based on density
functional theory calculations and by-products analyses, the authors proposed PFOA degradation
mechanism during which the joint action of temperature, aprotic, polar and nucleophile solvent results in
fluoride ions, F- (90% vs total F provided by PFOA in 24 hours), carbonate, formate, oxalate, glycolate,
tartronate and trifluoroacetic acid CF3COOH (TFA). We pursued the optimization of the degradation
process with the ultimate goal of proposing a practical solution for addressing PFOA environmental
contamination and its supposed main degradation by-product, TFA.
PFOA doping concentration (C0 = 893 mg/L) was selected to decrease the dilution factor to analyse PFOA
and its by-products at the μg/L level, while keeping optimum conditions for measuring fluoride by
potentiometric titration, at the mg/L level, in the range from 20 mg/L to 20 μg/L. The degradation kinetics
of PFOA was investigated over a period of six days at 90, 120, and 140°C. The influence of the DMSO/water
volume ratio as well as the NaOH/PFOA molar ratio on the treatment efficiency was studied to minimize
DMSO volume while optimizing PFOA and by-products degradation.
PFOA defluorination process was verified by Ultra-high-Pressure Liquid Chromatograph coupled with Mass
Spectrometer (UPLC-MS), potentiometric titration and SEM-EDS. UPLC-MS analysis was used to quantify
PFOA, perfluoroheptanoic acid (PFHPa), perfluorohexanoic acid (PFHxA), perfluoropentanoic acid (PFPeA),
perfluorobutanoic acid (PFBA) and qualify 325 m/z, 275 m/z, 229 m/z, 225 m/z and 114 m/z (TFA) by-
products. Fluoride selective electrode was used for F- analysis. SEM-EDS allowed us to characterize NaF on
dried surface of a PFOA/DMSO/NaOH mixture (20 μL) after interaction.
The removal reaction follows a first order kinetics that reached 95% at 90°C, after six days. The complete
abatement was observed only after six hours at 120°C and one hour at 140°C. Experiments conducted at
120°C highlighted the necessity of maintaining at least twice larger the DMSO/H2O volume and twenty to
one higher the NaOH/PFOA mole ratio to ensure effective PFOA defluorination whi le minimizing by-
products. PFHpA, PFHxA, PFPeA fell below the limit of quantification after 30 minutes; PFBA after 12 hours,
while peaks surfaces of 325 m/z, 275 m/z, 229 m/z, 225 m/z and 114 m/z (TFA) by-products were declining
versus time without reaching zero after six days. In the same time, the F- content reached its maximum (≤
80% vs total F provided by PFOA) after 18 hours. This 80 % maximum is not exceeded even at 140°C. These
results tend to demonstrate that TFA is not the only persistent degradation by-product that accumulates.
The identification and quantification of unusual by-products are under investigation.
References:
(1) Trang, B.; Li, Y.; Xue, X.S.; Ateia, M.; Houk, K.N.; Dichtel, W.R. Science 2022, 377, 839
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