Breakdown Products from Perfluorinated Alkyl Substances (PFAS) Degradation in a Plasma-Based Water Treatment Process

Abstract

Byproducts produced when treating perfluorooctanoic acid (PFOA) and perfluorooctanesulfonate (PFOS) in water using a plasma treatment process intentionally operated to treat these compounds slowly to allow for byproduct accumulation were quantified. Several linear chain perfluoroalkyl carboxylic acids (PFCAs) (C4 to C7) were identified as byproducts of both PFOA and PFOS treatment. PFOA, perfluorohexanesulfonate (PFHxS), and perfluorobutanesulfonate (PFBS) were also found to be byproducts from PFOS degradation. Significant concentrations of fluoride ions, inorganic carbon, and smaller organic acids (trifluoroacetic acid, acetic acid, and formic acid) were also identified. In addition to PFCAs, PFHxS, and PFBS, trace amounts of 43 PFOA-related and 35 PFOS-related byproducts were also identified using a screening and search-based algorithm. Minor concentrations of gas-phase byproducts were also identified (<2.5% of the F originally associated with the parent molecules) some of which are reported for the first time in perfluoroalkyl substance degradation experiments including cyclic perfluoroalkanes (C4F8, C5F10, C6F12, C7F14, and C8F16). The short chain PFCAs detected suggest the occurrence of a stepwise reduction of the parent perfluoroalkyl substances (PFAS) molecule, followed by oxidation of intermediates, perfluoroalkyl radicals, and perfluoro alcohols/ketones. Using a fluorine mass balance, 77% of the fluorine associated with the parent PFOA and 58% of the fluorine associated with the parent PFOS were identified. The bulk of the remaining fluorine was determined to be sorbed to reactor walls and tubing using sorption experiments in which plasma was not generated.

Understanding and Managing the Potential By‐Products of PFAS Destruction

“…Available forms of ARP (e.g., electron beam [eBeam], photolysis, ultra‐violet [UV]‐radiation, and plasma) use different mechanisms to disproportionate the water molecule (i.e., decomposes into both oxidizing and reducing species) into radicals (e.g., hydroxyl, hydrogen, hydroperoxyl, superoxide, and solvated electrons), which subsequently and sequentially defluorinate PFAS (Figure ). Plasma treatment uses a high voltage electrode to discharge successive electrical current pulses to a grounded electrode within a PFAS‐impacted aqueous solution (Singh et al ). The electrical current reacts with the water molecules, continuously generating oxidizing and reducing radicals.…”

“…The alternative approaches are emerging as more mobile options using small‐scale plants, which could be applied at sites where PFAS waste concentrates are generated. Based on numerous sources, an approximate range of energy demand per volume treated for plasma, electrochemical treatment, and sonolysis is 0.01 to 0.5‐kW h per liter (kW‐h/L; 0.04 to 1.9 kW‐h per gallon [kW‐h/gal]; e.g., Gomez‐Ruiz et al ; Soriano et al ; Kempisty et al ; Nzeribe et al ; Singh et al , ). However, approximating a range of energy demand for these relevant technologies is difficult to accurately summarize due to the variety of operating conditions within the respective studies when the energy demand was determined.…”

“…Residence times associated with PFAS destruction are believed to approximately range from 30 min to 8 h (e.g., Mader et al ; Mitchell et al ; Gomez‐Ruiz et al ; Bentel et al ; Nzeribe et al ; Singh et al , ). The broad range of residence times provided represents the different applications reported in the literature and varies according to the targeted destructive mechanism, the influent PFAS concentration, the geochemistry of the treated matrix, and the targeted degree of defluorination.…”

Research and Regulatory Advancements on Remediation and Degradation of Fluorinated Polymer Compounds

“…Several studies have tested the efficacy of these methods to determine if they reach temperatures high enough for a sufficient duration to allow for complete degradation [45,46]. Alternative methods to incineration such as mineralization of fluorinated compounds during thermal treatment, or a plasma based water treatment, are also being investigated as they may be safer and more efficient to treat waste [1,47,48].…”

“…There are currently several methods being investigated to safely dispose of various PFAS waste, specifically end-of-life fluorinated polymers in materials [1,33,34,37,45,46,48,86]. These methods include open burning (OB), open air detonation (OD), incineration, pyrolysis, and smoldering.…”