Theme: 07. Per- and Polyfluorinated Alkyl Substances (PFAS)

Topic Area: 7d. Ex Situ PFAS Water Treatment Technologies

Formatted Title:

Optimization of Experimental Conditions of PFOA Defluorination Using DMSO/NaOH Mixture

Background/Objectives:

Per- and polyfluoroalkyl substances (PFAS) are a large group of 5,000 synthetic chemical compounds widely used in industrial and consumer applications since the 1950s¹, most usually where extremely low surface energy or surface tension and/or durable water- and oil-repellency is needed, i.e., fire-fighting foams, surface treatment of textiles, carpets and papers, shampoos, paints, etc. Their persistent nature results in diffuse pollution issues in the environment² and adverse health effects³.

Trang et al.⁴ 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 byproduct analyses, the authors proposed PFOA degradation mechanism during which the joint action of temperature, aprotic, polar and nucleophile solvent results in fluorides, F^- (90% versus 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 byproduct, TFA.

Approach/Activities:

Ultra-high-pressure liquid chromatograph coupled with a mass spectrometer (UPLC-MS) 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) byproducts. Fluoride selective electrode was used for F^-analysis. SEM-EDS allowed for NaF characterization of dried surface of 20 µL PFOA/DMSO/NaOH mixture after interaction.

The degradation kinetics of PFOA (≈ 900 mg/L) was investigated over a period of six days at 90°C, 120°C, and 140°C. This PFOA concentration was selected to reduce the limit of quantification for PFOA and its byproducts from 20 mg/L to 20 µg/L while enabling F^- measurements. 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 byproducts degradation.

Results/Lessons Learned:

PFOA defluorination process was verified by UPLC-MS, selective F^- and SEM-EDS analysis. The removal reaction follows 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 20 to one higher the NaOH/PFOA mole ratio to ensure effective PFOA defluorination while minimizing byproducts. PFHpA, PFHxA, and 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) byproducts were declining versus time without reaching zero after six days. In the same time, the F^- content reached its maximum (£ 80% versus 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 byproduct that accumulates. The identification and quantification of unusual byproducts are under investigation.

In the same time, studies are devoted to propose a greener alternative to DMSO as aprotic polar solvent and to NaOH as OH^- source.

Acknowledgements. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101036449.

Project coordinator (BRGM): Julie Lions; UPLC-MS analysis team (BRGM): Anne Togola, Anne Berrehouc, Sébastien Bristeau, Emeline Coisy.

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Sébastien Bristeau