Per- and poly-fluorinated alkyl substrates (PFAS) are very resistant to chemical oxidation and biological transformation as they contain multiple C-F bonds. The high reduction potential of fluorine (3.6 V) makes it thermodynamically unfavorable to substitute the C-F bond with a C-H bond with any other one-electron oxidant. No conventional treatment processes are available that can completely mineralize the full range of PFAS compounds effectively. In recent years, degradation of PFAS has been investigated using photolysis, sonolysis, chemical oxidation by persulfate and periodate, plasma, and electrochemical treatment. While direct UV photolysis for the photo-decomposition of PFAS is a slow process, UV-Fenton reactions producing hydroxyl radicals can accelerate PFOA fragmentation and defluorination. It has been proposed that a fast mechanism for degradation of PFOA using OH radicals is followed by slow defluorination with Fe<sup>3+</sup> once the OH concentration is low. Hence, high-valent iron oxidants such as ferrate (Fe<sup>+6</sup>) are appealing due to the high oxidation potentials and the ability to induce multi-electron reactions. Additionally, sulfite has shown to activate Fe(VI) to produce OH and sulfate radicals. We have conducted experiments with 500 mg/l perfluorooctanoic acid (PFOA), 10 mM NaHCO3 (pH 9), 0-500 µM Fe(VI) solution, and 0-500 µM Na2SO3 loaded in a closed-system photoreactor (cooled with 20°C circulating water). Low-pressure mercury lamps (254 nm narrowband irradiation) were used for all reactions.  PFOA could be degraded and the average defluorination efficiency after 40 hr reactions was 26%, 57%, and 12% for UV, UV-Sulfite, UV-Ferrate+Sulfite, respectively. The use of sulfite considerably increased the defluorination efficiency of PFOA when compared to UV alone. However, the addition of Fe(VI) decreases the amount of fluoride concentration in the solution. A scavenging effect is evident as there was no significant increase or decrease in the average defluorination efficiency of PFOA with an increase in concentrations of Fe(VI) and sulfite. Though the error bars were large, the fluoride concentrations in a UV alone, UV with 100 µM Ferrate+Sulfite solution, and a UV treatment with 500 µM Ferrate+Sulfite solution were similar at the end of 40 hrs. The non-stirred reactor produces a considerably lower fluoride concentration over a 40 hr reaction period than a continuously stirred reactor, suggesting a possible co-precipitation effect of the iron oxides.

Defluorination of perfluorooctanoic acid (PFOA) by UV-Fenton process

ACS Spring Meeting 2021

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