Theme: 07. Per- and Polyfluorinated Alkyl Substances (PFAS)

Topic Area: 7p. PFAS: Groundwater Treatment Case Studies

Formatted Title:

Regenerable Ion Exchange Treatment with Hydrothermal Alkaline Treatment (HALT): Full Spectrum PFAS Capture and Destruction

Background/Objectives:

At a former United States Air Force Base (US AFB), an ECT2 regenerable ion exchange (IX) PFAS treatment system has operated over a 5-year period. When breakthrough is observed through the lead media vessel, the resin is regenerated to desorb the retained PFAS before return to service. The regenerant solution is distilled, leaving an IX still bottom, which is ultra-concentrated in the desorbed PFAS and a prime candidate for a high concentration (e.g., high milligrams per liter [mg/L]), low volume PFAS destruction application. The hydrothermal alkaline treatment (HALT) process, which utilizes high pH, subcritical water (T~350 ˚C, P~25 MPa), efficiently destroys a wide range of PFAS and is compatible with high ionic strength solutions such as IX still bottoms. The objectives of this project were to demonstrate the technical feasibility of destroying PFAS in IX still bottoms from an operating full-scale system in a flow-through Aquagga HALT treatment system and hone in the operating conditions that would achieve optimal process efficiency.

Approach/Activities:

Still bottoms containing elevated PFOS and PFHxS were obtained from the former US AFB treatment system. Because regenerable IX systems can be tuned to treat virtually any PFAS, including ultrashort PFAS, a range of additional recalcitrant PFAS (e.g., trifluoromethane sulfonate [TFMS], perfluroethoxyethane sulfonic acid) were amended to the still bottoms. Multiple batches with additional salt amendments were also prepared to evaluate the impact of higher salinity and nitrate on the HALT process. Three to ten- gallon batches were delivered to Aquagga and processed through their flow through HALT treatment system (Pilot Series) at multiple temperatures, hydroxide loadings, and treatment durations. Samples were analyzed after treatment for speciated PFAS, including ultrashort PFAS like trifluoracetic acid (TFA) and TFMS, total organofluorine, and dissolved inorganic fluoride.

Results/Lessons Learned:

In the forward flow system, media lifetime and robustness to regeneration was demonstrated over five years and one hundred million gallons of water treated. PFAS are able to be desorbed from the media recurrently into a small volume liquid concentrate.
The HALT treatment was extremely successful at breaking down all PFAS tested in a true still bottoms matrix, including PFOS and PFOA, perfluoroalkyl and perfluoroalkyl ether sulfonates, and PFAS that are very recalcitrant to other destruction processes - C1 to C4 PFAS. Treatment was more robust for the perfluoroalkyl carboxylates (>99.9% destruction) than the perfluoroalkyl sulfonates (~95% destruction). Interestingly, the perfluoroalkyl sulfonate chain length did not impact the observed destruction efficiency, which is unusual among PFAS destruction applications.
Increasing the chloride to 60,000 and nitrate to 20,000 mg/L did not impact HALT performance, suggesting that extreme salinity and nitrate will not negatively impact PFAS destruction. This is also a unique feature of HALT among other destruction approaches.
Operational conditions were optimized for the still bottoms matrix, pointing to a path forward for coupled full-scale operations. In such a scenario, the treated still bottoms would be re-blended with system influent.
The combined system of regenerable IX and HALT solves one of the most challenging problems in the PFAS treatment space: treatment of short chain PFAS. The combined approach can be tuned to capture, highly concentrate, and destroy the full range of PFAS to meet evolving regulations that target more mobile and less sorptive PFAS.

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Augusto Millevolte