Objective

The overall goal of this project was to evaluate the effectiveness of applying nascent hydrothermal conversion technologies to destroy per- and polyfluoroalkyl substances (PFAS) and co-occurring chemicals present in liquid and soil wastes. Specifically, the project aimed to identify reaction conditions (e.g., temperature, time) and low-cost amendments that promote rapid degradation and defluorination of PFAS associated with aqueous film-forming foam (AFFF), assess the reaction kinetics and mechanisms, and apply the process to treat PFAS-impacted water and soil samples, including investigation-derive waste (IDW).

Technical Approach

A series of research tasks were performed to address the project objectives and test associated hypotheses. An initial screening experiment tested the effectiveness of several solution amendments, including acids, bases, salts, oxidants, reductants, and metal nanoparticles, in promoting degradation and defluorination of perfluorooctane sulfonate (PFOS) at subcritical hydrothermal conditions (350°C, 17 megapascal).

Detailed studies were then conducted with the most promising amendments to evaluate reaction kinetics and assess the applicability for treatment of a wider diversity of PFAS identified in AFFF mixtures. Experiments were then undertaken to evaluate the stability and degradation of co-solvents (e.g., diethylene glycol monobutyl ether) and co-occurring chemicals (e.g., hydrocarbon fuels) often associated with AFFF use. Finally, proof-of-concept experiments were performed to apply the optimal reaction conditions identified to treat PFAS-impacted soil and water samples obtained from Department of Defense (DoD) sites, including IDW. These results were used to inform an analysis of the heat requirements for hydrothermal destruction of PFAS-impacted water in comparison to incineration of the same materials.

Results

Results of screening experiments showed that amendments that raise pH conditions, including low-cost alkalis like sodium hydroxide (NaOH), are effective in promoting rapid degradation and defluorination of PFOS. Reaction rates are proportional to the added NaOH concentration, and the observed kinetics and products support a reaction mechanism involving nucleophilic attack at the polar head group in the PFAS structure. The same reaction conditions were then found to be effective for degrading and defluorinating the full suite of PFAS detected by high resolution mass spectrometry methods in AFFF mixtures and AFFF-impacted water and soil samples collected from DoD sites. Tests with multiple techniques show that alkaline hydrothermal treatment can achieve ~100% destruction and defluorination of PFAS present in aqueous and soil matrices.

Benefits

Analysis of process heat requirements indicate that hydrothermal processing of PFAS-impacted water requires significantly less energy inputs than incineration of the same materials. Thus, findings from this proof-of-concept project support further development of this technology pathway for use in managing PFAS impacts at DoD facilities. Translation of the technology into mobile units that can be used on-site to treat IDW or heavily impacted source zones is particularly promising. Additional research aimed at strengthening the underlying science, expanding the application space, and developing continuous processing reactors is recommended.

Publications

Hao, S., Y.J. Choi, B. Wu, C. Higgins, R. Deeb, and T.J. Strathmann. 2021. Hydrothermal Alkaline Treatment for Destruction of Per- and Polyfluoroalkyl Substances (PFASs) in Aqueous Film-Forming Foam (AFFF). Environmental Science & Technology. 55: 3283-3295. https://doi.org/10.1021/acs.est.0c06906

Wu, B., S. Hao, Y. Choi, C.P. Higgins, R. Deeb, and T.J. Strathmann. 2019. Rapid Destruction and Defluorination of Perfluorooctanesulfonate by Alkaline Hydrothermal Reaction. Environmental Science & Technology Letters, 6:630-636. DOI: 10.1021/acs.estlett.9b00506

Yu, J., A. Nickerson, Y. Li, Y. Fang, and T.J. Strathmann. 2020. Fate of Per- and Polyfluoroalkyl Substances (PFASs) during Hydrothermal Liquefaction of Municipal Wastewater Treatment Sludge.Environmental Science: Water Research & Technology, 6:1388-1399. DOI: 10.1039/c9ew01139k