Thermochemical technologies apply elevated temperature to promote destruction of chemicals of concern. This project seeks to evaluate the effectiveness of applying nascent hydrothermal conversion technologies to destroy per- and polyfluoroalkyl substances (PFAS) in liquid and solid matrices. Subcritical hydrothermal processing refers to a thermochemical process in which reactions are catalyzed in liquid water at temperatures and pressures approaching its critical point (374 °C, 22.1 MPa). These processes are advantageous for treating PFAS-impacted liquid and high moisture content solids; the technology is referred to as hydrothermal alkaline treatment (HALT).

This work is being conducted in two phases. The overall objectives of the Phase I work were 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 in batch systems, and apply the process to treat PFAS-impacted water and soil samples. Specific technical objectives associated with this work included:

1. screen a large number of potential reactive amendments to identify the most promising reagents for promoting defluorination of perfluorooctane sulfonate (PFOS);
2. quantify the kinetics of PFAS degradation and defluorination;
3. evaluate the effectiveness of the most promising reaction conditions for treatment of a wider diversity of PFAS identified in AFFF and AFFF-impacted sites;
4. provide a preliminary analysis of the fate of common AFFF co-occurring chemicals of concern during treatment of PFAS; and
5. assess hydrothermal treatment of AFFF-impacted soil and groundwater collected from impacted sites.

The results of the Phase I effort are available in the Phase I Final Report.

The objectives for the Phase II effort are as follows:

1. improve understanding of the underlying reaction mechanisms and molecular constraints controlling transformation of individual PFAS;
2. expand the application space for HALT-PFAS to treatment of PFAS-impacted concentrate streams and screen heterogeneous catalysts;
3. apply HALT-PFAS to treat PFAS-laden sorbents and PFAS-impacted soils with different properties;
4. construct and apply continuous-flow HALT-PFAS reactors of increasing technology readiness levels (TRLs); and
5. develop a commercialization roadmap for the HALT-PFAS technology and use study results to design a mobile demonstration unit.

During Phase I, 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 PFOS at subcritical hydrothermal conditions (350 °C, 17 MPa). 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 investigation-derived waste (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.

During Phase II, batch and continuous-flow experiments will be conducted to evaluate PFAS destruction. Kinetics experiments, spectroscopic measurements, and density functional theory modeling will be combined to elucidate the underlying reaction mechanisms and identify structure-activity relationships. Destruction of PFAS and co-occurring chemicals will be quantified during treatment of PFAS-impacted concentrate byproducts derived from physical-chemical separation processes (e.g., foam fractionation, ion exchange resin regeneration), PFAS-laden sorbents (e.g., granular activated carbon), and PFAS-impacted soils of varying composition (e.g., clay and organic matter content). PFAS reactions will be monitored using high resolution mass spectrometry methods of analysis. A series of heterogeneous catalysts will also be screened for their potential to further accelerate PFAS destruction. Tubular plug flow reactor systems will be constructed and applied to demonstrate HALT-PFAS treatment under continuous flow conditions that are more representative of treatment systems that will be deployed in the field. Reaction kinetics under continuous-flow conditions will be compared to results obtained in batch reactors. As the project proceeds, reactor systems of increasing TRL will be constructed and optimized to de-risk the technology. Different reactor materials will be examined to assess and mitigate corrosion and scaling of reactor materials during treatment. Experimental and modeling results will be used by commercialization partners to design a mobile treatment unit that could be used for field demonstration work and to inform development of a demonstration and commercialization plan, both critical to commercialization of the HALT-PFAS technology.

Results of Phase I demonstrated that hydrothermal treatment with alkali amendments is highly effective in degrading and defluorinating a wide range of PFAS identified at AFFF-impacted sites. Initial 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. Proof-of-concept experiments also demonstrated that representative chlorinated solvents and aromatic hydrocarbon co-occurring chemicals are transformed during application of HALT-PFAS. Tests with multiple techniques show that HALT-PFAS can achieve nearly complete destruction and defluorination of PFAS present in aqueous and soil matrices.

The HALT-PFAS process has already been demonstrated to degrade and defluorinate the full diversity of PFAS identified in AFFF. Initial 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, the technology has significant potential to overcome limitations identified for other destructive technologies and could be applied as an on-site treatment system. Improved understanding of the underlying mechanisms and molecular constraints can be used to extrapolate findings to emerging classes of PFAS and further optimize reaction conditions. Demonstrating effective treatment of a wide range of PFAS concentrates, impacted soils, and spent sorbent materials will expand the technology’s potential application space. Construction and demonstration of continuous-flow reactors of increasing TRLs will further de-risk the technology and support commercialization and deployment of the HALT-PFAS technology. (Anticipated Phase II Completion - 2025)

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