Application of Hydrothermal Alkaline Treatment for Destruction of Per- and Polyfluoroalkyl Substances in Contaminated Groundwater and Soil

Theoretical and experimental insights into the degradation mechanism of PFBS under subcritical hydrothermal conditions

Per- and polyfluoroalkyl substances (PFAS) are known for their strong C-F bonds, which make them highly persistent in the environment and resistant to degradation. Among PFAS, perfluorobutane sulfonate (PFBS), a short-chain PFSA widely used as a replacement for long-chain PFAS like perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS), has garnered attention due to its environmental persistence and toxicity. Although PFBS has lower bioaccumulation potential than its long-chain counterparts, it remains a significant pollutant with limited data available on its degradation mechanisms. To address this, the present study investigates PFBS degradation under subcritical water hydrothermal conditions. Various alkaline substances and additives were tested, and degradation products were analyzed by liquid and gas chromatography-mass spectrometry. Results show that 2 M NaOH at 325°C achieved an ∼99.4 % PFBS removal rate, and the degradation pathway began with the breakdown of the S-C bond, followed by C-F bond cleavage, resulting in the formation of smaller fluorinated compounds, including trifluoroacetic acid. Density functional theory (DFT) calculations provided detailed insights into the degradation mechanism, identifying hydroxide ion attack on the sulfonic acid group as the initial step and elucidating three distinct pathways for subsequent reactions. This study provides key insights into PFBS degradation mechanisms, emphasizing the synergistic effect of alkaline bases and additives. The findings support the optimization of subcritical hydrothermal treatment for PFAS removal, providing a scalable and environmentally sustainable solution.

Review on the sources, distribution and treatment of per- and polyfluoroalkyl substances in global groundwater

Per- and polyfluoroalkyl substances (PFAS) have garnered increasing global attention due to their widespread occurrence in groundwater and the potential health risks to humans. This review aimed to clarify the occurrence and treatment of PFAS in groundwater by summarizing literature published in the Web of Science Core Collection from January 2000 to April 2024. Information on 461 reported PFAS-contaminated groundwater sites was compiled, revealing key characteristics of pollution sources and concentrations. The data indicated that firefighting training activities were a major source of PFAS groundwater contamination, accounting for 41 % of cases, followed by other fluorinated industrial activities, landfill leachate, and wastewater leakage. Non-point sources, such as atmospheric deposition, contributed to a lesser extent. The concentrations distribution of 25 PFAS showed a chain-length dependency, with short-chain PFAS generally exhibiting higher concentrations than long-chain PFAS. Additionally, the review systematically examined the application of separation methods and destructive methods at both laboratory and pilot/field-scales for PFAS-contaminated groundwater. Resins were favored for ex-situ treatment, whereas colloidal activated carbon (CAC) was more commonly used for in-situ treatment. In-situ direct injection of CAC was considered a highly promising approach for remediating PFAS source zones and plumes, offering advantages such as minimal surface disruption, high adsorption capacity and long-term effectiveness. Finally, the research focus and development trends in categories and treatment methods for PFAS in groundwater were noted. Overall, this review identified research gaps in the occurrence and treatment of PFAS in groundwater, and suggested further optimization of CAC-based methods to address the challenges of PFAS-contaminated groundwater.

The influence of surface coating on PFAS filter performance

Novel molecules containing a silane anchor, a hydrocarbon segment, and a polyethylene glycol (PEG) head have been developed to functionalize membranes for capture of per and poly-fluoroalkyl species (PFAS). The membranes have been experimentally tested against PFAS-contaminated water. Based on previous experimental testing and molecular modeling, the length of the hydrophobic PEG segment was identified as a potential feature to improve filter performance, particularly against low molecular-weight, short-chain PFAS. In our previous work, silane molecules featured an eight-carbon hydrocarbon segment and a segment with four consecutive PEG monomers. In this work, versions of the silane molecules with both shorter and longer PEG segments were synthesized and tested. We report results from tests measuring the dynamic filtration ability of aluminum-oxide hydroxide membranes functionalized with amphiphilic molecules against PFAS at part-per trillion and part-per billion concentration levels. Filters utilizing silanes with shorter PEG segments outperformed those with longer PEG segments, particularly against short-chain PFAS. Molecular dynamics simulations using metadynamics sampling methods were employed to construct free-energy interaction profiles, and interaction energies between several different contaminant species and a silane brush layer. Decomposition of the interaction free energy into the molecular interaction contributions to the potential energy (enthalpy) highlights the influence of PFAS length, PEG length, and brush geometry on the overall PFAS-brush interaction. Experimental measurements indicate that membrane performance depends on the length of the PEG segment in the silane brush. Combined experimental and simulation results indicate that both the intermolecular interaction strength and the density of the surface coating depend on the length of the PEG segment. The experiment and simulation results also show that both molecular architecture and membrane processing impact the overall filter performance.

Realizing Direct Hot-Electron Transfer from Metal Nanoparticles to Per- and Polyfluoroalkyl Substances

Per- and polyfluoroalkyl substances (PFAS) are a group of forever synthetic chemicals. They are widely utilized in industries and household appliances because of their remarkable stability and distinctive oil- and water-repellent properties. Despite their broad applications, unfortunately, PFAS are hazardous to all forms of life, including humans. In recent years, the environmental persistence of PFAS has raised significant interest in degrading these substances. However, the strong C-F bonds in these chemicals pose several challenges to their degradation. Plasmons of noble metal nanoparticles (NPs) offer many exciting applications, including photocatalytic reactions. However, an atomistic understanding of plasmon-driven processes remains elusive. In this work, using the real-time time-dependent density functional theory, we have studied the real-time formation of plasmons, hot-carrier generation, and subsequent direct hot-carrier transfer from metal NPs to the PFAS. Our simulations show that there is an apparent direct hot-electron transfer from NPs to PFAS. Moreover, using Ehrenfest dynamics simulations, we demonstrated that the transferred hot electrons can efficiently degrade PFAS without requiring any external thermal bath. Thus, our work provides an atomistic picture of plasmon-induced direct hot-carrier transfer from NPs to PFAS and the efficient degradation of PFAS. We strongly believe that this work generates the impetus to utilize plasmonic NPs to mitigate PFAS.

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.

Iron Doping of hBN Enhances the Photocatalytic Oxidative Defluorination of Perfluorooctanoic Acid

There is a growing need to effectively eliminate perfluorooctanoic acid (PFOA) from contaminated water, which requires extensive defluorination. Photocatalysis offers potential for PFOA degradation under ambient conditions without the need for treatment chemicals. However, photocatalytic treatment generally results in limited defluorination and, thus, incomplete elimination of potential toxicity and liability. This underscores the need to advance mechanistic understanding of the factors limiting PFOA oxidative defluorination. Here, we tested the hypothesis that direct electron transfer from PFOA to transition metals enhances photocatalytic defluorination. We developed a novel, facile approach to simultaneously functionalize and dope hexagonal boron nitride (hBN) (which is known to effectively catalyze photocatalytic PFOA oxidation) with Fe(III), using deep-eutectic solvents (DES). Addition of Fe(III) to synthesize Fe-hBN created new active sites for PFOA oxidation and doubled the defluorination extent (>40% fluoride release from initial 50 mg L-1 PFOA) compared to undoped hBN in 4 h reactions under 254 nm irradiation (64.4 W m-2). The mechanism of defluorination was elucidated through scavenger experiments that show the importance of photocatalytically generated electron holes for initiating PFOA degradation. Experiments also suggest that Fe(III) played a key role in PFOA removal, contributing to the improved extent of defluorination over undoped hBN. Density functional theory indicates that Fe(III) sites enable electrostatic adsorption of PFOA to the catalyst surface, enhance charge transfer, and promote hole localization to improve charge carrier separation, which is essential for oxidative defluorination of PFOA. This mechanistic insight informs catalytic material design to enhance oxidative defluorination processes.

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.

New Insights into Thermal Degradation Products of Long-Chain Per- and Polyfluoroalkyl Substances (PFAS) and Their Mineralization Enhancement Using Additives

The products of incomplete destruction (PIDs) of per- and polyfluoroalkyl substances (PFAS) represent a substantial ambiguity when employing thermal treatments to remediate PFAS-contaminated materials. In this study, we present new information on PIDs produced in both inert and oxidative environments from five long-chain PFAS, including three now regulated under the U.S. Safe Drinking Water Act, one cationic precursor compound, and one C10 PFAS. The data did not support the generation of tetrafluoromethane from any of the studied PFAS, and carbonyl fluoride was found only from potassium perfluorooctanesulfonate (K-PFOS) when heated in air in a narrow temperature range. Oxidative conditions (air) were observed to facilitate PFAS thermal degradation and accelerate the mineralization of K-PFOS. Spectroscopic data suggest that PFAS thermal degradation is initiated by the cleavage of bonds that form perfluoroalkyl radicals, leading to organofluorine PIDs (e.g., perfluoroalkenes). In air, perfluoroalkyl radicals react with oxygen to form oxygen-containing PIDs. The mineralization of PFAS was enhanced by adding solid additives, which were categorized as highly effective (e.g., granular activated carbon (GAC) and certain noble metals), moderately effective, and noneffective. Remarkably, simply by adding GAC, we achieved >90% mineralization of perfluorooctanoic acid at 300 °C and ∼1.9 atm within just 60 min without using water or solvents.

Speciation and biogeochemical behavior of perfluoroalkyl acids in soils and their environmental implications: A review

Perfluoroalkyl acids (PFAAs) are emerging organic pollutants that have attracted significant attention in the fields of environmental chemistry and toxicology. Although PFAAs are pervasive in soils and sediments, there is a paucity of research regarding their environmental forms and driving mechanisms. This review provides an overview of the classification and biotoxicity of per- and polyfluoroalkyl substances (PFAS), organic pollutant forms, PFAS extraction and analytical methods, the prediction of PFAS distribution in soils, and current PFAS remediation strategies. Four predominant PFAA forms have been proposed in soils: (i) aqueous-extracted PFAAs, (ii) organic-solvent extracted PFAAs, (iii) embedded or sequestered PFAAs, and (iv) covalently bound PFAAs. Furthermore, it suggests suitable extraction methods and predictive models for different PFAA forms, which are instrumental in the research on PFAA speciation and prediction in soils. Simultaneously, it was proposed that elemental cycling and microbial activity may affect the speciation of PFAS. Additionally, the categorization of PFAA forms facilitated the analysis of pollution remediation. Understanding the interplay between PFAA speciation, element cycling, and bacterial activity during soil remediation is essential for understanding remediation mechanisms and assessing the long-term stability of remediation methods. Future studies should expand the investigation of varying PFAA forms in different media, consider the potential binding forms of PFAAs to minerals, organic matter, and microbes, and evaluate the possible mechanisms of PFAA speciation variation.

Analysis of perfluorinated compounds in sewage sludge and hydrochar by UHPLC LTQ/Orbitrap MS and removal assessment during hydrothermal carbonization treatment

Wastewater treatment plants have been recognized as important sinks for per- and polyfluoroalkyl substances (PFAS) because of their ineffectiveness in removing them reflecting both water and sewage sludge discharge routes. Hydrothermal treatment represents an alternative technology for treating sludge to recover energy and other valuable products. In this study, 15 PFAS were determined in sludge and hydrochar substrates using sonication-solid phase extraction procedure and analyzed using LC-Orbitrap-High Resolution-MS/MS. The method was fully validated, exhibiting very good linearity, recoveries in the range of 48 to 126 %, low detection and quantification limits with expanded uncertainty and precision below 32 % and 21.9 %, respectively. The method was applied to sludge samples from the WWTP of Ioannina city (Greece), as well as to hydrothermally treated samples under various conditions. The most abundant PFAS were PFHxA (0.5-38.3 ng g-1) and PFOS (4.4-22.1 ng g-1). Finally, the hydrothermally treated sludge samples spiked with PFAS presented removal efficiencies for total PFAS of 86.9 %, 91.8 % and 95.7 % at three spiking levels namely 10, 50 and 200 ng g-1, respectively. Results indicated that PFCAs were almost completely removed, except for PFOA, while the concentrations of PFSAs increased in the produced hydrochar with the formation of several intermediates, as detected by HR-LC-MS/MS. The results of this study demonstrate the effect of hydrothermal treatment to the fate of PFAS in sewage sludge and contribute for further studies on design and scale up of hydrothermal carbonization technology as a management option for safer disposal of municipal wastewater sludge.

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.

PFAS Porewater concentrations in unsaturated soil: Field and laboratory comparisons inform on PFAS accumulation at air-water interfaces

Poly- and perfluoroalkyl substance (PFAS) leaching from unsaturated soils impacted with aqueous film-forming foams (AFFFs) is an environmental challenge that remains difficult to measure and predict. Complicating measurements and predictions of this process is a lack of understanding between the PFAS concentrations measured in a collected environmental unsaturated soil sample, and the PFAS concentrations measured in the corresponding porewater using field-deployed lysimeters. The applicability of bench-scale batch testing to assess this relationship also remains uncertain. In this study, field-deployed porous cup suction lysimeters were used to measure PFAS porewater concentrations in unsaturated soils at 5 AFFF-impacted sites. Field-measured PFAS porewater concentrations were compared to those measured in porewater extracted in the laboratory from collected unsaturated soil cores, and from PFAS concentrations measured in the laboratory using batch soil slurries. Results showed that, despite several years since the last AFFF release at most of the test sites, precursors were abundant in 3 out of the 5 sites. Comparison of field lysimeter results to laboratory testing suggested that the local equilibrium assumption was valid for at least 3 of the sites and conditions of this study. Surprisingly, PFAS accumulation at the air-water interface was orders of magnitude less than expected at two of the test sites, suggesting potential gaps in the understanding of PFAS accumulation at the air-water interface at AFFF-impacted sites. Finally, results herein suggest that bench-scale testing on unsaturated soils can in some cases be used to inform on PFAS in situ porewater concentrations.

Cutting-edge technologies and relevant reaction mechanism difference in treatment of long- and short-chain per- and polyfluoroalkyl substances: A review

Per- and polyfluoroalkyl substances (PFAS) are emerging contaminants. Compared with short-chain PFAS, long-chain PFAS are more hazardous. Currently, little attention has been paid to the differences in reaction mechanisms between long-chain and short-chain PFAS. This pressing concern has prompted studies about eliminating PFAS and revealing the mechanism difference. The reaction rate and reaction mechanism of each technology was focused on, including (1) adsorption, (2) ion exchange (IX), (3) membrane filtration, (4) advanced oxidation, (5) biotransformation, (6) novel functional material, and (7) other technologies (e.g. ecological remediation, hydrothermal treatment (HT), mechanochemical (MC) technology, micro/nanobubbles enhanced technology, and integrated technologies). The greatest reaction rate k of photocatalysis for long- and short-chain PFAS high up to 63.0 h-1 and 19.7 h-1, respectively. However, adsorption, membrane filtration, and novel functional material remediation were found less suitable or need higher operation demand for treating short-chain PFAS. Ecological remediation is more suitable for treating natural waterbody for its environmentally friendly and fair reaction rate. The other technologies all showed good application potential for both short- and long-chain PFAS, and it was more excellent for long-chain PFAS. The long-chain PFAS can be cleavaged into short-chain PFAS by C-chain broken, -CF2 elimination, nucleophilic substitution of F-, and HF elimination. Furthermore, the application of each type of technology was novelly designed; and suggestions for the future development of PFAS remediation technologies were proposed.

Occurrence, spatiotemporal trends, fate, and treatment technologies for microplastics and organic contaminants in biosolids: A review

This review provides a comprehensive overview of the occurrence, fate, treatment and multi-criteria analysis of microplastics (MPs) and organic contaminants (OCs) in biosolids. A meta-analysis was complementarily analysed through the literature to map out the occurrence and fate of MPs and 10 different groups of OCs. The data demonstrate that MPs (54.7% occurrence rate) and linear alkylbenzene sulfonate surfactants (44.2% occurrence rate) account for the highest prevalence of contaminants in biosolids. In turn, dioxin, polychlorinated biphenyls (PCBs) and phosphorus flame retardants (PFRs) have the lowest rates (<0.01%). The occurrence of several OCs (e.g., dioxin, per- and polyfluoroalkyl substances, polycyclic aromatic hydrocarbons, pharmaceutical and personal care products, ultraviolet filters, phosphate flame retardants) in Europe appear at higher rates than in Asia and the Americas. However, MP concentrations in biosolids from Australia are reported to be 10 times higher than in America and Europe, which required more measurement data for in-depth analysis. Amongst the OC groups, brominated flame retardants exhibited exceptional sorption to biosolids with partitioning coefficients (log Kd) higher than 4. To remove these contaminants from biosolids, a wide range of technologies have been developed. Our multicriteria analysis shows that anaerobic digestion is the most mature and practical. Thermal treatment is a viable option; however, it still requires additional improvements in infrastructure, legislation, and public acceptance.

Simulating behavior of perfluorooctane sulfonate (PFOS) in the mainstream of a river system with sluice regulations

Perfluorooctane sulfonate (PFOS) is a persistent, anionic and ubiquitous contaminant that undergoes long-range transport within the environment. Its behavior has attracted wide-range academic and regulatory attention. In this article, a mass balance model was employed to simulate PFOS concentrations in the mainstream of Haihe River water system, encompassing sluices and artificial rivers. The dynamic simulation of PFOS concentrations in both sediment and freshwater took into account fluctuations in PFOS emissions, water levels and water discharge. Furthermore, the study delved into exploring the impacts of sluices and artificial rivers on the behavior of PFOS. The simulated concentrations of PFOS in steady state agreed with the measured concentrations in surveys carried out in Nov. 2019, July 2020, Oct. 2020, and June 2021. Every year, approximately 24 kg PFOS was discharged into the Bohai Sea with Chaobai New River being the largest contributor for 44 %. Moreover, the transport of PFOS in the original rivers is likely to be restricted by sluices and replaced by artificial rivers. Monte Carlo analysis showed that model predictions of PFOS concentrations in sediment were subject to greater uncertainty than those in freshwater as the former is impacted by more parameters, such as density of sediment. This study provides a scientific basis for the local government to manage and control PFOS.

Remediation of PFAS-impacted soils using magnetic activated carbon (MAC) and hydrothermal alkaline treatment (HALT)

Per- and polyfluoroalkyl substances (PFAS) are a group of synthetic pollutants that are bioaccumulative, toxic, and persistent. One long-term source for PFAS release is PFAS-contaminated soil. Addition of activated carbon (AC) to soil has shown the potential to immobilize PFAS and reduce PFAS bioavailability, but PFAS-loaded spent AC remaining in the treated soil could lead to remobilization. Here we report a novel approach to address this challenge. By applying magnetic activated carbon (MAC) to remediate PFAS-impacted soil, the PFAS-loaded MAC can be retrieved from the treated soil and sorbed PFAS in the spent MAC can be destroyed using hydrothermal alkaline treatment (HALT). Effective MAC recovery was observed when water/soil ratios (w/w) were either <0.07 or > 1. Soil organic content and pH affected PFAS adsorption by the MAC added to soil. After three months of incubation with MAC, high PFAS removals [PFOS (87.6 %), PFOA (83.8 %), and 6:2 FTSA (81.5 %)] were observed for acidic environmental sandy soils with low organic content. In contrast, PFAS removal by MAC was poor for garden soils with high organic matter content. MAC was also used to remediate aqueous film-forming foam (AFFF)-impacted and PFAS-contaminated aged soils with varying PFAS removal performance. HALT technology was able to destroy and defluorinate PFAS adsorbed to the spent MAC. Additionally, the HALT-treated MAC retained its magnetic properties and PFOS sorption capacity, suggesting the potential reusability of HALT-treated MAC. Considering the low energy footprint of HALT compared to conventional PFAS thermal destruction techniques, the combination of MAC and HALT could be a promising treatment train for PFAS-contaminated soils.

A review of innovative approaches for onsite management of PFAS-impacted investigation derived waste

The historic use of aqueous film-forming foam (AFFF) has led to widespread detection of per- and polyfluoroalkyl substance (PFAS) in groundwater, soils, sediments, drinking water, wastewater, and receiving aquatic systems throughout the United States (U.S.). Prior to any remediation activities, in order to identify the PFAS-impacted source zones and select the optimum management approach, extensive site investigations need to be conducted. These site investigations have resulted in the generation of considerable amount of investigation-derived waste (IDW) which predominantly consists of well purging water and drill fluid, equipment washing residue, soil, drill cuttings, and residues from the destruction of asphalt and concrete surfaces. IDW is often impacted by varying levels of PFAS which poses a substantial challenge concerning disposal to prevent potential mobilization of PFAS, logistical complexities, and increasing requirement for storage as a result of accumulation of the associated wastes. The distinct features of IDW involve the intermittent generation of waste, substantial volume of waste produced, and the critical demand for onsite management. This article critically focuses on innovative technologies and approaches employed for onsite treatment and management of PFAS-impacted IDW. The overall objective of this study centers on developing and deploying end-of-life treatment technology systems capable of facilitating unrestricted disposal, discharge, and/or IDW reuse on-site, thereby reducing spatial footprints and mobilization time.

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

While foam fractionation (FF) process has emerged as a promising technology for removal of per- and polyfluoroalkyl substances (PFASs) from contaminated groundwater, management of the resulting foam concentrates with elevated concentrations of PFASs (e.g., >1 g/L) remains a challenge. Here, we applied hydrothermal alkaline treatment (HALT) to two foam concentrates derived from FF field demonstration projects that treated aqueous film-forming foam (AFFF)-impacted groundwater. Results showed >90% degradation and defluorination within 90 min of treatment (350 °C, 1 M NaOH) of all 62 PFASs (including cations, anions, and zwitterions) identified in foam concentrates. Observed rate constants for degradation of individual perfluoroalkyl sulfonates (PFSAs, CnF2n+1-SO3-), the most recalcitrant class of PFASs, in both foam concentrates were similar to values measured previously in other aqueous matrices, indicating that elevated initial PFAS concentrations (e.g., PFHxSinit = 0.55 g/L), dissolved organic carbon (DOC; up to 4.5 g/L), and salt levels (e.g., up to 325 mg/L chloride) do not significantly affect PFAS reaction kinetics. DOC was partially mineralized by treatment, but a fraction (∼15%) was recalcitrant. Spectroscopic characterization revealed molecular features of the HALT-recalcitrant DOC fraction, and nontarget high-resolution mass spectrometry tentatively identified 129 nonfluorinated HALT-recalcitrant molecules. Analysis of process energy requirements shows that treating PFAS-contaminated foam concentrates with HALT would add minimally (<5%) to the overall energy requirements of an integrated FF-HALT treatment train.

Alcohols radicals can efficiently reduce recalcitrant perfluorooctanoic acid

Alcohols are commonly used as eluents for the regeneration of per/poly-fluoroalkyl substances (PFASs) adsorbents, but their potential effects on the subsequent treatment of these eluates have not been fully explored. This work investigated the effect of alcohols on perfluorooctanoic acid (PFOA) degradation by persulfate (PS) based advanced oxidation processes. The results showed that ethanol significantly promoted PFOA degradation in thermal/PS system. Under anoxic conditions, 25.5±1.4% or 91.2±1.6% of PFOA was degraded within 48 h in the absence or presence of ethanol. Electron paramagnetic resonance (EPR) detection, free radical quenching experiments, and chemical probe studies clearly demonstrated that the sulfate radicals (SO4•-) generated from PS activation would react with ethanol to form alcohol radicals, which could efficiently degrade PFOA. The transformation pathways of PFOA were proposed based on degradation products analysis and density function theory (DFT) calculation. The reaction between SO4•- and other alcohols could also induce the formation of alcohol radicals and facilitate to the degradation of PFOA. This work represents the positive roles of alcohols in the degradation of PFASs, providing new insights into developing simple and efficient treatments for PFASs eluate or PFAS-contaminated water.

Life cycle assessment and life cycle cost analysis of anion exchange and granular activated carbon systems for remediation of groundwater contaminated by per- and polyfluoroalkyl substances (PFASs)

Anion exchange resin (AER) and granular activated carbon (GAC) have emerged as prominent technologies for treatment of waters contaminated with per- and polyfluoroalkyl substances (PFASs). This study compares the life cycle environmental impacts and life cycle costs of remediating PFAS-contaminated groundwater with these competing technologies, using field pilot data to inform model inputs. Comparative analysis indicates that AER systems employing single-use "PFAS-selective" resins have lower environmental impacts and costs than systems using regenerable resins or GAC adsorbents, supporting its use in future remediation efforts. Use of GAC operated as a single-use adsorbent led to the highest emissions as well as the highest treatment costs, with thermally-reactivated GAC proving to be less impactful than regenerable AER treatment. Sensitivity analyses highlighted the dominance of media usage rate (MUR), which is highly dependent on the selected PFAS treatment goals, to determine environmental impacts and costs over a 30-year system life cycle. Selection of very stringent changeout criteria (e.g., detection of any PFASs in effluent) significantly reduces the advantages of single-use resins. For regenerable AER, environmental impacts were dominated by management of the PFAS-contaminated brine/co-solvent waste stream used to regenerate the adsorbent, as well as the cosolvent content of the regenerant mixture and the cosolvent recovery efficiency achieved via on-site distillation. High impacts estimated for GAC adsorption, the result of high MUR relative to ion exchange media, can be significantly reduced if spent adsorbents are reused after thermal reactivation, but impacts are still greater than those predicted for single-use ion exchange systems. Findings are expected to hold across a range of diverse sites, including drinking water systems treating more dilute sources of PFAS contamination, as PFAS breakthrough was not found to be highly sensitive to sourcewater PFAS concentrations.

Microbial and thermal treatment techniques for degradation of PFAS in biosolids: A focus on degradation mechanisms and pathways

Per- and polyfluoroalkyl substances (PFAS) are persistent organic chemicals detected in biosolids worldwide, which have become a significant concern for biosolids applications due to their increasing environmental risks. Hence, it is pivotal to understand the magnitude of PFAS contamination in biosolids and implement effective technologies to reduce their contamination and prevent hazardous aftermaths. Thermal techniques such as pyrolysis, incineration and gasification, and biodegradation have been regarded as impactful solutions to degrade PFAS and transform biosolids into value-added products like biochar. These techniques can mineralize PFAS compounds under specific operating parameters, which can lead to unique degradation mechanisms and pathways. Understanding PFAS degradation mechanisms can pave the way to design the technology and to optimize the process conditions. Therefore, in this review, we aim to review and compare PFAS degradation mechanisms in thermal treatment like pyrolysis, incineration, gasification, smouldering combustion, hydrothermal liquefaction (HTL), and biodegradation. For instance, in biodegradation of perfluorooctane sulfonic acid (PFOS), firstly C-S bond cleavage occurs which is followed by hydroxylation, decarboxylation and defluorination reactions to form perfluoroheptanoic acid. In HTL, PFOS degradation is carried through OH^-catalyzed series of nucleophilic substitution and decarboxylation reactions. In contrast, thermal PFOS degradation involves a three-step random-chain scission pathway. The first step includes C-S bond cleavage, followed by defluorination of perfluoroalkyl radical, and radical chain propagation reactions. Finally, the termination of chain propagation reactions produces very short-fluorinated units. We also highlighted important policies and strategies employed worldwide to curb PFAS contamination in biosolids.

Destruction and defluorination of PFAS matrix in continuous-flow supercritical water oxidation reactor: Effect of operating temperature

Cleanup and disposal of stockpiles and waste streams containing per- and polyfluoroalkyl substances (PFAS) require effective end-of-life destruction/mineralization technologies. Two classes of PFAS, perfluoroalkyl carboxylic acids (PFCAs) and perfluoroalkyl sulfonic acids (PFSAs), are commonly found in legacy stockpiles, industrial waste streams, and as environmental pollutants. Continuous flow supercritical water oxidation (SCWO) reactors have been shown to destroy several PFAS and aqueous film-forming foams. However, a direct comparison of the SCWO efficacy for PFSAs and PFCAs has not been reported. We show the effectiveness of continuous flow SCWO treatment for a matrix of model PFCAs and PFSAs as a function of operating temperature. PFSAs appear to be significantly more recalcitrant than PFCAs in the SCWO environment. The SCWO treatment results in a destruction and removal efficiency of 99.999% at a T > 610 °C and at a residence time of ∼30 s. Fluoride recovery lags destruction PFAS at 510 °C and reaches >100% above 610 °C, confirming the formation of liquid and gaseous phase intermediate product during lower temperature oxidation. This paper establishes the threshold for destroying PFAS-containing liquids under SCWO conditions.

Effective PFAS degradation by electrochemical oxidation methods-recent progress and requirement

The presence of per- and poly-fluoroalkyl substances (PFASs) in water is of global concern due to their high stability and toxicity even at very low concentrations. There are several technologies for the remediation of PFASs, but most of them are inadequate either due to limited effectiveness, high cost, or production of a large amount of sludge. Electrochemical oxidation (EO) technology shows great potential for large-scale application in the degradation of PFASs due to its simple procedure, low loading of chemicals, and least amount of waste. Here, we have reviewed the recent progress in EO methods for PFAS degradation, focusing on the last 10 years, to explore an efficient, cost-effective, and environmentally benign remediation technology. The effects of important parameters (e.g., anode material, current density, solution pH, electrolyte, plate distance, and electrical connector type) are summarized and evaluated. Also, the energy consumption, the consequence of different PFASs functional groups, and water matrices are discussed to provide an insight that is pivotal for developing new EO materials and technologies. The proposed degradation pathways of shorter-chain PFAS by-products during EO of PFAS are also discussed.

The efficacy of soil washing for the remediation of per- and poly-fluoroalkyl substances (PFASs) in the field

This paper aims to describe the performance of a soil washing plant (SWP) for remediating a per- and poly-fluoroalkyl substances (PFASs)-contaminated soil with a high clay content (61%). The SWP used both physical and chemical processes; fractionation of the soil particles by size and partitioning of PFASs into the aqueous phase to remove PFASs from the soil. Contaminated water was treated in series with granulated activated carbon (GAC) and ion-exchange resin and reused within the SWP. Approximately 2200 t (dry weight) of PFAS-contaminated soil was treated in 25 batches of 90 t each, with a throughput of approximately 11 t soil/hr. Efficiency of the SWP was measured by observed decreases in total and leachable concentrations of PFASs in the soil. Average removal efficiencies (RE) were up to 97.1% for perfluorocarboxylic acids and 94.9% for perfluorosulfonic acids. REs varied among different PFASs depending on their chemistry (functional head group, carbon chain length) and were independent of the total PFAS concentrations in each soil batch. Mass balance analysis found approximately 90% of the PFAS mass in the soil was transferred to the wash solution and > 99.9% of the PFAS mass in the wash solution was transferred onto the GAC without any breakthrough.

Uncovering hydrothermal treatment of per- and polyfluoroalkyl substances

Image 1.

Destruction of PFAS in AFFF-impacted fire training pit water, with a continuous hydrothermal alkaline treatment reactor

As regulations are being established to limit the levels of per- and polyfluoroalkyl substances (PFAS) in drinking water and wastewater, effective treatment technologies are needed to remove or destroy PFAS in contaminated liquid matrices. Many military installations and airports have fire training ponds (FTPs) where PFAS-containing firefighting foams are discharged during training drills. FTP water disposal is expensive and challenging due to the high PFAS levels. Hydrothermal alkaline treatment (HALT) has previously been shown to destroy a wide range of PFAS compounds with a high degree of destruction and defluorination. In this study, we investigate the performance of a continuous flow HALT reactor for destroying PFAS in contaminated FTP water samples. Processing with 5 M-NaOH and 1.6 min of continuous processing results in >99% total PFAS destruction, and 10 min processing time yields >99% destruction of every measured PFAS species. Operating with 0.1 M-NaOH or 1 M-NaOH shows little effect on the destruction of measured perfluorosulfonic acids, while all measured perfluorocarboxylic acids and fluorotelomer sulfonates are reduced to levels below the method detection limits. Continuous HALT processing with sufficient NaOH loading appears to destroy parent PFAS compounds significantly faster than batch HALT processing, a positive indicator for scaling up HALT technology for practical applications in environmental site remediation activities.

An Integrated Approach for Determination of Total Per- and Polyfluoroalkyl Substances (PFAS)

Per- and polyfluoroalkyl substances (PFAS) are difficult to analyze in environmental media due challenges such as extraction recovery and lack of analytical standards. The total oxidizable precursor (TOP) assay and suspect screening analysis coupled with semiquantitative (SQ) concentration estimates are two approaches to assess total PFAS in environmental media, but studies are needed to optimize workstreams for total PFAS analysis. This study applied two soil extraction methods, TOP assay, and SQ analysis to three aqueous film-forming foams (AFFFs) and three AFFF-impacted soils. In soils, the total PFAS estimated with results from an extraction method utilizing sequential acidic and basic solvents led to a 35% increase in precursors during TOP assay relative to results from a basic solvent only extraction in one of three soils tested, but concentrations did not increase significantly in remaining soils. Furthermore, sample-specific dilution schemes were required to overcome matrix effects caused by the acidic extraction step that influenced estimates of total PFAS by SQ analysis. The results highlight that there is not an advantage to routine application of an acid extraction step in PFAS-impacted soils. In three AFFFs, suspect screening of post-TOP samples identified eight classes of PFAS present after oxidation. Concentrations of three classes increased, suggesting they are new TOP end points. Concentrations of the remaining five classes either remained constant after TOP or exhibited slight decreases. As a result, combined TOP and SQ workstreams may yield the most representative assessment of total PFAS composition and concentration. The eight classes of PFAS present after TOP did not degrade in harsh conditions. Some are structurally similar to PFCAs and PFSAs and are known to occur in the environment, suggesting a similar degree of persistence and a need for more routine monitoring.