Rapid Destruction and Defluorination of Perfluorooctanesulfonate by Alkaline Hydrothermal Reaction

Abstract

Here, we report a promising new strategy for achieving rapid and complete destruction of perfluorooctanesulfonate (PFOS) through the application of hydrothermal conditions (condensed water, 200−350 °C, 2−16.5 MPa) to solutions amended with NaOH. Initial screening experiments with a wide range of solution amendments (e.g., acids, alkalis, oxidants, reductants) revealed highly variable extents of PFOS defluorination, ranging from 0% to 80% after 90 min of reaction at 350 °C. The most reactive amendments, regardless of type, shifted solution pH to highly alkaline conditions (pH ≥ 9), suggesting a base-promoted mechanism. For NaOHamended solutions, rates of PFOS degradation increased with temperature and followed a second-order rate law, −d-, with k 2 = 0.052 ± 0.004 M −1 min −1 at 350 °C, and 19 F-NMR measurements show complete conversion of C−F bonds to F − (2.5 g/L PFOS) within 40 min for the reaction with 1 M NaOH. Small quantities of shortchain perfluorocarboxylic acids (≤1.5% [PFOS] init ) were detected as transient intermediates, indicating that an initial OH −catalyzed cleavage of the sulfonate headgroup is followed by rapid sequential decarboxylation reactions, eventually leading to complete mineralization. These findings suggest a promising technology for destruction of PFOS-containing wet concentrates (e.g., aqueous film-forming foam (AFFF) stockpiles, industrial waste, sorbent regenerate, and membrane reject waste streams).

US Department of Defense–Funded Research on Treatment of Per‐ and Polyfluoroalkyl Substance–Laden Materials

“…Hydrothermal and SCWO processes use a combination of heat and pressure for destruction of PFAS constituents, without the need to drive off moisture, thereby allowing for destruction to occur at lower temperatures. Data from a recent SERDP project indicate that destruction temperatures for hydrothermal treatment can be further reduced by elevating the pH (i.e., alkaline hydrothermal treatment; Wu et al 2019 ).…”

Transitioning Per‐ and Polyfluoroalkyl Substance Containing Fire Fighting Foams to New Alternatives: Evolving Methods and Best Practices to Protect the Environment

“…Relevant destructive mechanisms for PFAS associated with liquid wastes are limited to molecular decomposition or chemically mediated sequential defluorination (Vecitis et al 2009). Practically speaking, that limits PFAS destruction technologies for liquids to various forms of extreme thermal treatment (SCWO and/or high‐pressure afterburner chambers following gasification), sonochemical degradation, electrochemical oxidation, or advanced reducing processes (e.g., plasma, electron beam, and/or ultraviolet activated catalysts and/or reductants; Hori et al 2004; Mader et al 2008; Schaefer et al 2015; Wang et al 2016; Stratton et al 2017; Wu et al 2019; Tenorio et al 2020).…”

Understanding and Managing the Potential By‐Products of PFAS Destruction

“…While these example detections are extremely low, based on the volumetric emission rate of the hazardous waste incinerator, they amount to milligrams of PFOS and PFOA on a yearly basis. Furthermore, to achieve the reported thermal destruction temperatures for PFOS in an aqueous matrix, the critical point must be considered, and a recent evaluation of an alkaline hydrothermal reaction, conducted at 350 °C and approximately 2400 pounds per square inch of pressure, demonstrated the importance of pressure application to achieve mineralization of PFOS (Wu et al ). Lastly, even in studies claiming effective thermal destruction of PFOS in the literature, residuals of PFOS were observed in laboratory combustion trials (Yamada and Taylor ) and within combusted municipal solid waste ash (Solo‐Gabriele et al ).…”