Plasma-based water treatment: an environmentally safe approach for complete PFAS destruction*

Per- and polyfluoroalkyl substances (PFAS) are toxic and recalcitrant human-made contaminants with widespread use in military, aerospace, automotive, construction, and electronic industries. Drinking water is typically considered the major source of human exposure to these chemicals; however, elevated PFAS levels have also been detected in food, as well as personal care and consumer products. Advanced oxidation processes, such as ultraviolet light or ozone with peroxide, are ineffective in PFAS treatment because of the high stability of the carbon-fluorine bond comprising these compounds. We have shown that electrical discharge plasma generated in the gas phase is extremely effective in degrading long-chain PFAS, in particular perfluorooctanoic acid (PFOA). The process is capable of reducing a range of starting PFOA concentrations to non-detect limits within minutes although the exact mechanism(s) and processess responsible for the degradation are largely unknown. Our recent work suggests that both plasma species and bulk liquid mass transport of the contaminant to the plasma-liquid interface are equally important in overall removal and process scaleup.

This work investigates the effects of process gas type, operational parameters, and bulk liquid mixing on the removal of PFOA. The effectiveness of argon, helium, air, and nitrogen in degrading and defluorinating PFOA was compared in a pin-to-ring plasma reactor. State-of-the-art diagnostics techniques were applied to characterize the plasma and correlate plasma properties with PFOA kinetic profiles. Additionally, a square channel laminar flow reactor was utilized to investigate the influence of mixing (e.g., residence time) and size of the plasma-liquid contact area on PFOA removal. This presentation will reveal the key process and operational parameters controlling PFAS removal in electrical discharge plasma reactors.