Hydrothermal Alkaline Treatment (HALT) of Foam Fractionation Concentrate Derived from PFAS-Contaminated Groundwater

PFAS removal through foam harvesting during wastewater aeration

Aeration in wastewater treatment plants (WWTPs) is used for removal of organic matter and nutrients. Here we show that aeration can also lead to removal of per- and polyfluoroalkyl substances (PFAS), by foam fractionation. Rising air bubbles facilitate air-liquid interfacial adsorption of PFAS and spontaneous foaming occurrence. This suggests that some modifications to conventional treatment processes that enable foam removal may be sufficient to achieve PFAS removal at WWTPs. However, high suspended solids concentrations in the mixed liquor suspension within the aerated bioreactors may complicate PFAS removal in foam fractionation, as both air bubbles and suspended biomass retain PFAS. This study explored the feasibility of foam fractionation for PFAS removal and enrichment using actual mixed liquor suspensions with typical total suspended solids concentrations and WWTP-relevant PFAS concentrations. The mechanisms involved in PFAS removal and enrichment in both aqueous and solid phases were suggested, and a mass balance analysis was performed to show PFAS distribution between the two phases. Overall, PFAS removal from the aqueous phase ranged from 70 % to 100 % for PFAS with perfluorinated carbon numbers ≥ 6, while PFAS with perfluorinated carbon numbers < 6 showed low removal of < 20 %. PFAS removal from the solid phase ranged from 20 % to 60 %, depending on the PFAS species. This study represents an ongoing effort to advance the potential implementation of foam fractionation in aerated bioreactors at WWTPs.

Reuse of spent granular activated carbon for PFAS removal following hydrothermal alkaline treatment

Per- and polyfluoroalkyl substances (PFAS) pose a significant challenge for water treatment facilities facing strict regulatory standards. Granular activated carbon (GAC) adsorption is effective for PFAS removal, but media exhaustion and replacement can be costly, highlighting the need for innovative GAC regeneration methods. While thermal reactivation of GAC can eliminate adsorbed PFAS, it requires high temperatures and is mainly feasible for large-scale media users. This study investigates spent GAC regeneration by hydrothermal alkaline treatment (HALT), which applies subcritical water (e.g., 350 °C, 16.5 MPa) amended with strong base (e.g., NaOH) to destroy PFAS. Previous research indicates that HALT successfully degraded and defluorinated PFAS while maintaining GAC surface area and equilibrium adsorption capacity. This study presents data from rapid small-scale column tests (RSSCTs) demonstrating effective removal of long-chain PFAS by a HALT-treated spent GAC sample collected from a long-term PFAS treatment field pilot study (BV50 > 50,000 for PFOS, PFHxS, and PFNA). HALT-treated virgin GAC and untreated virgin GAC evaluated using RSSCTs exhibited similar PFAS breakthrough behavior, with comparable overall PFAS removal to the HALT-treated spent GAC. Physisorption measurements revealed that HALT recovers GAC pore surface area lost during field-use. Surface chemical characterization techniques indicated mostly similar surface composition and functional groups in virgin and HALT-treated GAC, with limited change in the carbon structure following HALT and differences between virgin and field-spent samples. Analyses of reactor liquid products, media mass loss, and NaOH neutralization by GAC also provided evidence for removal of adsorbed non-target organic matter and possible GAC surface renewal by carbon gasification reactions occurring in parallel with PFAS destruction, analogous to surface carbon burn-off that occurs during high-temperature thermal reactivation. Retention of adsorbed metal ions that accumulated on the spent GAC during field testing may be responsible for enhanced adsorption behavior observed for some PFAS following HALT regeneration. Results indicate that HALT can enable reuse of spent GAC, potentially alleviating the high demand for virgin media in PFAS treatment processes.

PFAS removal via adsorption: A synergistic review on advances of experimental and computational approaches

Per- and polyfluoroalkyl substances (PFAS), commonly known as "forever chemicals", have become a major focus of current research due to their toxicity and persistence in the environment. These synthetic compounds are notoriously difficult to degrade, accumulating in water systems and posing long-term health and environmental risks. Adsorption is one of the most investigated technologies for PFAS removal. This review comprehensively reviewed the PFAS adsorption process, focusing not only on the adsorption itself, but also on the behavior of PFAS in the aquatic environment prior to adsorption because these behaviors directly affect PFAS adsorption. Significantly, this review summarized in detail the advances made in PFAS adsorption from the computational approach and emphasized the importance of integrated experimental and computational studies in gaining molecular-level understanding on the adsorption mechanisms of PFAS. Toward the end, the review identified several critical research gaps and suggested key interdisciplinary research needs for further advancing our understanding on PFAS adsorption.

Electrochemical destruction of PFAS at low oxidation potential enabled by CeO2 electrodes utilizing adsorption and activation strategies

The persistence and ecological impact of per- and poly-fluoroalkyl substances (PFAS) in water sources necessitate effective and energy-efficient treatment solutions. This study introduces a novel approach using cerium dioxide (CeO2) electrodes enhanced with oxygen vacancy (Ov) to catalyze the defluorination of PFAS. By leveraging the unique affinity between cerium and fluorine-containing species, our approach enables adsorptive preconcentration and catalytic degradation at low oxidation potentials (1.37 V vs. SHE). Demonstrating high removal and defluorination efficiencies of perfluorooctanoic acid (PFOA) at 94.0 % and 73.0 %, respectively, our approach also proves effective in the environmental matrix. It minimizes the impacts of co-existing natural organic matter and chloride ions, crucial benefits of operating at lower oxidation potentials. The role of Ov in CeO2 is validated by both experimental results and density functional theory modeling, demonstrating that these sites can activate the C-F bond and substantially reduce the energy barriers for defluorination. Consequently, our CeO2-based method not only achieves defluorination efficiencies comparable to more energy-intensive techniques but does so while requiring less than 0.62 kWh/m3 per order. This positions our approach as a promising, cost-effective alternative for the remediation of PFAS-contaminated waters, emphasizing its relevance and effectiveness in environmental remediation scenarios.

Regeneration of PFAS-laden granular activated carbon by modified supercritical CO2 extraction

Granular activated carbon (GAC) is widely used to treat contaminated per- and polyfluoroalkyl substances (PFAS) waste streams, resulting in the accumulation of large quantities of spent GAC that need to be landfilled or regenerated. A novel modified supercritical CO2 (scCO2) extraction for regeneration of spent GAC is developed. With the addition of organic solvents and acid modifiers, the procedure yielded >99% perfluorooctanoic acid (PFOA) desorption after a 60-min treatment in a continuous flow reactor. The mild extraction conditions at T ∼100 °C do not trigger the formation of volatile organic fluorine or changes in GAC sorbent properties. Mechanistically, the high miscibility of co-solvent/scCO2 eliminates diffusion transport limitations, enabling rapid reagent and PFAS transport in a single-phase (gas-like) medium. The introduction of organic co-solvent and the absence of water reverses hydrophobic interactions between GAC and the PFAS. The acid modifier minimizes the electrostatic PFOA/GAC interactions by protonating the perfluorooctanoate ion and providing competition for active GAC sites. The approach offers an economically effective regeneration scheme, enabling the reuse of sorbents and yielding effluent with a high loading of PFAS that is amenable to subsequent end-of-life treatment technologies.

PFAS in landfill leachate: Practical considerations for treatment and characterization

Per- and polyfluoroalkyl substances (PFAS) are widely used in consumer products and are particularly high in landfill leachate. The practice of sending leachate to wastewater treatment plants (WWTPs) is an issue for utilities that have biosolids land application limits based on PFAS concentrations. Moreover, landfills may face their own effluent limit guidelines for PFAS. The purpose of this review is to understand the most appropriate treatment technology combinations for mitigating PFAS in landfill leachate. The first objective is to understand the unique chemical characteristics of landfill leachate. The second objective is to establish the role and importance of known and emerging analytical techniques for PFAS characterization in leachate, including quantification of precursor compounds. Next, an overview of technologies that concentrate PFAS and technologies that destroy PFAS is provided, including fundamental background content and key operating parameters. Finally, practical considerations for PFAS treatment technologies are reviewed, and recommendations for PFAS treatment trains are described. Both pros and cons of treatment trains are noted. In summary, the complex matrix of leachate requires a separation treatment step first, such as foam fractionation, for example, to concentrate the PFAS into a lower-volume stream. Then, a degradation treatment step can be applied to the concentrated PFAS stream.

Hydrothermal Destruction and Defluorination of Trifluoroacetic Acid (TFA)

Per- and polyfluoroalkyl substances (PFAS) have received increased attention due to their environmental prevalence and threat to public health. Trifluoroacetic acid (TFA) is an ultrashort-chain PFAS and the simplest perfluorocarboxylic acid (PFCA). While the US EPA does not currently regulate TFA, its chemical similarity to other PFCAs and its simple molecular structure make it a suitable model compound for studying the transformation of PFAS. We show that hydrothermal processing in compressed liquid water transforms TFA at relatively mild conditions (T = 150-250 °C, P < 30 MPa), initially yielding gaseous products, such as CHF3 and CO2, that naturally aspirate from the solution. Alkali amendment (e.g., NaOH) promotes the mineralization of CHF3, yielding dissolved fluoride, formate, and carbonate species as final products. Fluorine and carbon balances are closed using Raman spectroscopy and fluoride ion selective electrode measurements for experiments performed at alkaline conditions, where gas yields are negligible. Qualitative FTIR gas analysis allows for establishing the transformation pathways; however, the F-balance could not be quantitatively closed for experiments without NaOH amendment. The kinetics of TFA transformation under hydrothermal conditions are measured, showing little to no dependency on NaOH concentration, indicating that the thermal decarboxylation is a rate-limiting step. A proposed TFA transformation mechanism motivates additional work to generalize the hydrothermal reaction pathways to other PFCAs.