A review of emerging technologies for remediation of PFASs

The need for remediation of poly‐ and perfluoroalkyl substances (PFASs) is growing as a result of more regulatory attention to this new class of contaminants with diminishing water quality standards being promulgated, commonly in the parts per trillion range. PFASs comprise >3,000 individual compounds, but the focus of analyses and regulations has generally been PFASs termed perfluoroalkyl acids (PFAAs), which are all extremely persistent, can be highly mobile, and are increasingly being reported to bioaccumulate, with understanding of their toxicology evolving. However, there are thousands of polyfluorinated “PFAA precursors”, which can transform in the environment and in higher organisms to create PFAAs as persistent daughter products. Some PFASs can travel miles from their point of release, as they are mobile and persistent, potentially creating large plumes. The use of a conceptual site model (CSM) to define risks posed by specific PFASs to potential receptors is considered essential. Granular activated carbon (GAC) is commonly used as part of interim remedial measures to treat PFASs present in water. Many alternative treatment technologies are being adapted for PFASs or ingenious solutions developed. The diversity of PFASs commonly associated with use of multiple PFASs in commercial products is not commonly assessed. Remedial technologies, which are adsorptive or destructive, are considered for both soils and waters with challenges to their commercial application outlined. Biological approaches to treat PFASs report biotransformation which creates persistent PFAAs, no PFASs can biodegrade. Water treatment technologies applied ex situ could be used in a treatment train approach, for example, to concentrate PFASs and then destroy them on‐site. Dynamic groundwater recirculation can greatly enhance contaminant mass removal via groundwater pumping. This review of technologies for remediation of PFASs describes that:

• GAC may be effective for removal of long‐chain PFAAs, but does not perform well on short‐chain PFAAs and its use for removal of precursors is reported to be less effective;
• Anion‐exchange resins can remove a wider array of long‐ and short‐chain PFAAs, but struggle to treat the shortest chain PFAAs and removal of most PFAA precursors has not been evaluated;
• Ozofractionation has been applied for PFASs at full scale and shown to be effective for removal of total PFASs;
• Chemical oxidation has been demonstrated to be potentially applicable for some PFAAs, but when applied in situ there is concern over the formation of shorter chain PFAAs and ongoing rebound from sorbed precursors;
• Electrochemical oxidation is evolving as a destructive technology for many PFASs, but can create undesirable by‐products such as perchlorate and bromate;
• Sonolysis has been demonstrated as a potential destructive technology in the laboratory but there are significant challenges when considering scale up;
• Soils stabilization approaches are evolving and have been used at full scale but performance need to be assessed using appropriate testing regimes;
• Thermal technologies to treat PFAS‐impacted soils show promise but elevated temperatures (potentially >500 °C) may be required for treatment.

There are a plethora of technologies evolving to manage PFASs but development is in its early stage, so there are opportunities for much ingenuity.     

In Situ treatment of PFAS‐impacted groundwater using colloidal activated Carbon

Poly‐ and perfluoroalkyl substances (PFASs) have been identified by many regulatory agencies as contaminants of concern within the environment. In recent years, regulatory authorities have established a number of health‐based regulatory and evaluation criteria with groundwater PFAS concentrations typically being less than 50 nanograms per liter (ng/L). Subsurface studies suggest that PFAS compounds are recalcitrant and widespread in the environment. Traditionally, impacted groundwater is extracted and treated on the surface using media such as activated carbon and exchange resins. These treatment technologies are generally expensive, inefficient, and can take decades to reach treatment objectives. The application of in situ remedial technologies is common for a wide variety of contaminants of concern such as petroleum hydrocarbons and volatile organic compounds; however, for PFASs, the technology is currently emerging. This study involved the application of colloidal activated carbon at a site in Canada where the PFASs perfluorooctanoate (PFOA) and perfluorooctane sulfonic acid (PFOS) were detected in groundwater at concentrations up to 3,260 ng/L and 1,450 ng/L, respectively. The shallow silty‐sand aquifer was anaerobic with an average linear groundwater velocity of approximately 2.6 meters per day. The colloidal activated carbon was applied using direct‐push technology and PFOA and PFOS concentrations below 30 ng/L were subsequently measured in groundwater samples over an 18‐month period. With the exception of perfluoroundecanoic acid, which was detected at 20 ng/L and perfluorooctanesulfonate which was detected at 40 ng/L after 18 months, all PFASs were below their respective method detection limits in all postinjection samples. Colloidal activated carbon was successfully distributed within the target zone of the impacted aquifer with the activated carbon being measured in cores up to 5 meters from the injection point. This case study suggests that colloidal activated carbon can be successfully applied to address low to moderate concentrations of PFASs within similar shallow anaerobic aquifers.     

Fungal biotransformation of 6:2 fluorotelomer alcohol

Fungal degradation of 6:2 fluorotelomer alcohol (6:2 FTOH, C₆F₁₃CH₂CH₂OH) by two wood‐decaying fungal strains and six fungal isolates from a site contaminated with per‐ and polyfluoroalkyl substances (PFASs) was investigated. 6:2 FTOH is increasingly being used in FTOH‐based products, and previous reports on the microbial fate of 6:2 FTOH have focused on bacteria and environmental microbial consortia. Prior to this study, one report demonstrated that the 6:2 FTOH biotransformation by the wood‐decaying fungus, Phanerochaete chrysosporium, generated more polyfluoroalkyl substances, such as 5:3 acid (F(CF₂)₅CH₂CH₂COOH), and diverted away from producing the highly stable perfluorocarboxylic acids (PFCAs). Most of the fungi (Gloeophyllum trabeum and isolates TW4‐2, TW4‐1, B79, and B76) examined in this study showed similar degradation patterns, further demonstrating that fungi yield more 5:3 acid (up to 51 mol% of initial 6:2 FTOH dosed) relative to other metabolites (up to 12 mol% total PFCAs). However, medium amendments can potentially improve 6:2 FTOH biotransformation rates and product profiles. The six fungal isolates tolerated up to 100 or 1,000 milligrams per liter of perfluorooctanoic acid and perfluorooctane sulfonic acid, and some isolates experienced increased growth with increasing concentrations. This study proposes that fungal pathways must be considered for the biotransformation of potential PFAS precursors, such as 6:2 FTOH, and suggests the basis for selecting proper microorganisms for remediation of fluoroalkyl‐contaminated sites.     

PFAS Experts Symposium: Statements on regulatory policy,chemistry and analytics,toxicology, transport/fate,and remediation for per‐ and polyfluoroalkyl substances (PFAS) contamination issues

Sixty leading members of the scientific, engineering, regulatory, and legal communities assembled for the PFAS Experts Symposium in Arlington, Virginia on May 20 and 21, 2019 to discuss issues related to per‐ and polyfluoroalkyl substances (PFAS) based on the quickly evolving developments of PFAS regulations, chemistry and analytics, transport and fate concepts, toxicology, and remediation technologies.  The Symposium created a venue for experts with various specialized skills to provide opinions and trade perspectives on existing and new approaches to PFAS assessment and remediation in light of lessons learned managing other contaminants encountered over the past four decades. The following summarizes several consensus points developed as an outcome of the Symposium:

• Regulatory and policy issues: The response by many states and the US Environmental Protection Agency (USEPA) to media exposure and public pressure related to PFAS contamination is to relatively quickly initiate programs to regulate PFAS sites. This includes the USEPA establishing relatively low lifetime health advisory levels for PFAS in drinking water and even more stringent guidance and standards in several states. In addition, if PFAS are designated as hazardous substances at the federal level, as proposed by several Congressional bills, there could be wide‐reaching effects including listing of new Superfund sites solely for PFAS, application of stringent state standards, additional characterization and remediation at existing sites, reopening of closed sites, and cost renegotiation among PRPs.
• Chemistry and analytics: PFAS analysis is confounded by the lack of regulatory‐approved methods for most PFAS in water and all PFAS in solid media and air, interference with current water‐based analytical methods if samples contain high levels of suspended solids, and sample collection and analytical interference due to the presence of PFAS in common consumer products, sampling equipment, and laboratory materials.
• Toxicology and risk: Uncertainties remain related to human health and ecological effects for most PFAS; however, regulatory standards and guidance are being established incorporating safety factors that result in part per trillion (ppt) cleanup objectives. Given the thousands of PFAS that may be present in the environment, a more appropriate paradigm may be to develop toxicity criteria for groups of PFAS rather than individual PFAS.
• Transport and fate: The recalcitrance of many perfluoroalkyl compounds and the capability of some fluorotelomers to transform into perfluoroalkyl compounds complicate conceptual site models at many PFAS sites, particularly those involving complex mixtures, such as firefighting foams. Research is warranted to better understand the physicochemical properties and corresponding transport and fate of most PFAS, of branched and linear isomers of the same compounds, and of the interactions of PFAS with other co‐contaminants such as nonaqueous phase liquids. Many PFAS exhibit complex transport mechanisms, particularly at the air/water interface, and it is uncertain whether traditional transport principles apply to the ppt levels important to PFAS projects. Existing analytical methods are sufficient when combined with the many advances in site characterization techniques to move rapidly forward at selected sites to develop and test process‐based conceptual site models.
• Existing remediation technologies and research: Current technologies largely focus on separation (sorption, ion exchange, or sequestration). Due to diversity in PFAS properties, effective treatment will likely require treatment trains. Monitored natural attenuation will not likely involve destructive reactions, but be driven by processes such as matrix diffusion, sorption, dispersion, and dilution.

The consensus message from the Symposium participants is that PFAS present far more complex challenges to the environmental community than prior contaminants. This is because, in contrast to chlorinated solvents, PFAS are severely complicated by their mobility, persistence, toxicological uncertainties, and technical obstacles to remediation—all under the backdrop of stringent regulatory and policy developments that vary by state and will be further driven by USEPA. Concern was expressed about the time, expense, and complexity required to remediate PFAS sites and whether the challenges of PFAS warrant alternative approaches to site cleanups, including the notion that adaptive management and technical impracticability waivers may be warranted at sites with expansive PFAS plumes. A paradigm shift towards receptor protection rather than broad scale groundwater/aquifer remediation may be appropriate.     

Bench-scale testing of a novel soil PFAS treatment train for informed remedial planning and decision-making

Bench-scale batch tests were conducted to assess the potential applicability of a combined separation/concentration/destruction treatment train to address soils and sediments impacted by per- and polyfluoroalkyl substances (PFAS) contamination at Schriever Space Force Base with historic aqueous film-forming foam (AFFF). Specifically, a novel treatment train coupling soil washing (for treatment of impacted soil/sediment) with foam fractionation (for treatment of the wash water [WW] generated during soil washing) and electrochemical oxidation (ECO, for treatment of the foam fractionate generated during foam fractionation) was evaluated at the bench scale using site-specific materials. Results presented herein show that the AFFF-impacted sandy soils with low organic content were amenable to treatment via soil washing. However, the removal of hydrophobic PFAS, such as perfluorooctanesulfonic acid (PFOS), from the organic-rich sediments was challenging. Results from batch desorption experiments were within a factor of 2 of those generated by soil washing bench studies, suggesting that simple batch tests can potentially be used to reasonably predict the treatment efficacy of soil washing. Long-chained perfluoroalkyl acids (PFAAs) within the WW were removed more effectively in the foam fractionation studies as compared to short-chain PFAAs. Addition of a surfactant, such as cetrimonium bromide (CTAB), enhanced foaming but only marginally improved the treatment of short-chained PFAAs and in some cases inhibited PFOS removal. ECO reduced PFAS concentrations in the foam fractionate generated during foam fractionation by several orders of magnitude. However, generation of unwanted byproducts may warrant further treatment and/or disposal. Overall, results from this study provide a novel data set highlighting the site-dependent nature of these PFAS remedial technologies and how simple, low-cost bench tests can be reliably leveraged for informed decision-making during PFAS remedial planning.     

Technology review and evaluation of different chemical oxidation conditions on treatability of PFAS

Per‐ and polyfluoroalkyl substances (PFAS) are a class of stable compounds widely used in diverse applications. These emerging contaminants have unique properties due to carbon–fluorine (C–F) bonds, which are some of the strongest bonds in chemistry. High energy is required to break C–F bonds, which results in this class of compounds being recalcitrant to many degradation processes. Many technologies studied that have shown treatment effectiveness for PFAS cannot be implemented in situ. Chemical oxidation is a demonstrated remediation technology for in situ treatment of a wide range of organic environmental contaminants. An overview of relevant literature is presented, summarizing the use of single or combined reagent chemical oxidation processes that offer insight into oxidation–reduction chemistries potentially capable of PFAS degradation. Based on the observations and results of these studies, bench‐scale treatability tests were designed and performed to establish optimal conditions for the formation of specific free radical species, including superoxide and sulfate radicals, via various combinations of oxidants, catalysts, pH buffers, and heat to assess PFAS treatment by chemical oxidants. The study also suggests the possible abiotic transformations of some PFAS when chemical oxidation is or was used for treatment of primary organic contaminants (e.g., petroleum or chlorinated organic compounds) at a site. The bench‐scale tests utilized field‐collected samples from a firefighter training area. Much of the available data related to chemical oxidation of PFAS has only been reported for one or both of the two more commonly discussed PFAS (perfluorooctane sulfonic acid and/or perfluorooctanoic acid). In contrast, this treatability study evaluates oxidation of a diverse list of PFAS analytes. The results of this study and published literature conclude that heat‐activated persulfate is the oxidation method with the best degradation of PFAS. Limited reduction of reported PFAS concentrations in this study was observed in many oxidation reactors; however, unknown mass of PFAS (such as precursors of perfluoroalkyl acids) that cannot be identified in a field collected sample complicated quantification of how much oxidative destruction of PFAS actually occurred.     

Evaluation of the adsorption behavior of mixed perfluoroalkyl and polyfluoroalkyl substances onto granular activated carbon and styrene-divinylbenzene resins

Because of the remarkable chemical structure of perfluoroalkyl and polyfluoroalkyl substances (PFAS), as well as the complex conditions of water, selecting an appropriate adsorbent for treating PFAS is critical. Adsorption needs to be environmentally friendly, low cost, and consider the types of adsorbents that work well in mixed PFAS solutions. In the present study, we used mixed PFAS to estimate the PFAS activity. This research aimed to evaluate and compare the efficacy of the adsorption of PFAS from water using different adsorbents: granular activated carbon (GAC), IRA 910 (strong anion resin), and DOWEX MB-50 (mixed exchange resin). Batch adsorption isotherms and kinetic studies were performed for perfluorooctanoic acid (PFOA), perfluorooctane sulfonic acid (PFOS), and perfluorohexane sulfonic acid (PFHxS). Freundlich models consistently described the kinetic behavior with a high correlation coefficient (R² > 0.98). PFAS adsorption capacities on GAC and IRA910 were dependent on the chain length (PFOS > PFOA > PFHxS). The adsorption capacity of DOWEX MB-50 decreased because of the sulfonate effects (PFOS > PFHxS > PFOA). The rate constants (k₂) that represented the adsorption of PFAS on different adsorbents observed within 96 h were accurately determined by the pseudo-second-order (PSO) model. GAC achieved followed the relationship k₂(PFOS) > k₂(PFOA) > k₂(PFHxS). Furthermore, k₂ of IRA910 decreased in the order of k₂(PFOA) > k₂(PFOS) > k₂(PFHxS), implying that IRA910 promoted hydrophobicity more significantly on the adsorption of PFCAs than perfluoroalkane (-alkyl) sulfonic acids. The kinetics of DOWEX MB-50 revealed k₂(PFHxS) > k₂(PFOS) > k₂(PFOA) because gel-type resins like DOWEX MB-50 are more suitable for shorter-chain PFAS. Further investigation is needed to determine the effect of organic matter under natural conditions and evaluate adsorptive selection caused by operational complexities.     

Residual organic fluorinated compounds from thermal treatment of PFOA,PFHxA and PFOS adsorbed onto granular activated carbon (GAC)

Perfluorooctanoic acid (PFOA), perfluorohexanoic acid (PFHxA) and perfluorooctane sulfonate (PFOS) adsorbed onto granular activated carbon (GAC) were thermally treated in N₂ gas stream. The purpose was to assess the fate of perfluoroalkyl and polyfluoroalkyl substances (PFASs) during thermal regeneration of GAC, which had been used for water treatment. Mineralized F, residual PFASs including short-chained species, and volatile organic fluorine (VOF) were determined. In a temperature condition of 700 °C, VOF were 13.2, 4.8, and 5.9 % as for PFOA, PFHxA, and PFOS. However, the VOF decreased to 0.1 %, if the GAC and off-gas were kept at 1000 °C. No PFASs remained in GAC at 700–1000 °C; at the same time, short-chained PFASs were slightly detected in the aqueous trapping of off-gas at 800 and 900 °C conditions. The destruction of PFASs on GAC could be perfect if the temperature is higher than 700 °C; however, the process is competitive against volatile escape from GAC. Destruction in gaseous phase needs a temperature as high as 1000 °C. Destruction of PFASs on the surface of GAC, volatile escape from the site, and thermolysis in gas phase should be considered, as to thermal regeneration of GAC.     

Evaluating the longevity of a PFAS in situ colloidal activated carbon remedy

The remediation of per‐ and polyfluoroalkyl substances by injection of colloidal activated carbon (CAC) at a contaminated site in Central Canada was evaluated using various visualization and modeling methods. Radial diagrams were used to illustrate spatial and temporal trends in perfluoroalkyl acid (PFAA) concentrations, as well as various redox indicators. To assess the CAC adsorption capacity for perfluorooctane sulfonate (PFOS), laboratory Freundlich isotherms were derived for PFOS mixed with CAC in two solutions: (1) PFOS in a pH 7.5 synthetic water that was buffered by 1 millimolar NaHCO₃ (K_f = 142,800 mg^1‐a L^a/kg and a = 0.59); and (2) a groundwater sample (pH = 7.4) containing PFOS among other PFAS from a former fire‐training area in the United States (K_f = 4,900 mg^1‐a L^a/kg and a = 0.24). A mass balance approach was derived to facilitate the numerical modeling of mass redistribution after CAC injection, when mass transitions from a two‐phase system (aqueous and sorbed to organic matter) to a three‐phase system that also includes mass sorbed to CAC. An equilibrium mixing model of mass accumulation over time was developed using a finite‐difference solution and was verified by intermodel comparison for prediction of CAC longevity in the center of a source area. A three‐dimensional reactive transport model (ISR‐MT3DMS) was used to indicate that the CAC remedy implemented at the site is likely to be effective for PFOS remediation for decades. Model results are used to recommend remedial design and monitoring alternatives that account for the uncertainty in long‐term performance predictions.     

Water quality impacts on sorbent efficacy for per- and polyfluoroalkyl substances treatment of groundwater

Per- and polyfluoroalkyl substances (PFAS) are a large group of synthetic compounds that have emerged as chemicals of concern in drinking water and groundwater. Typically, such waters are treated to remove PFAS by passing the water through a bed of sorbent material (e.g., activated carbon and anion exchange resins [AIX]). However, the efficacy of these sorbents varies depending on the types and concentrations of PFAS, in addition to water quality conditions such as organic matter content and conductivity (ionic strength). The choice of sorbent material to effectively treat PFAS in complex natural waters will, therefore, depend upon site water quality and PFAS conditions. To help inform these decisions, a series of evaluations using a rapid small-scale column test approach was conducted with two sorbent materials (a granulated activated carbon [GAC] and an AIX), individually and combined, under conditions where conductivity, pH, and organic carbon concentrations were varied in a semifactorial approach. Artificial groundwater batches were prepared to meet these test conditions and spiked with six PFAS compounds (perfluorobutane sulfonic acid [PFBS], perfluorobutanoic acid [PFBA], perfluorohexane sulfonic acid [PFHxS], perfluorohexanoic acid [PFHxA], perfluorooctane sulfonic acid [PFOS], and perfluorooctanoic acid [PFOA]), passed through small columns packed with ground sorbent material for ∼30,000 bed volumes of water for single sorbent treatments and ∼20,000 bed volumes for combined sorbent treatments, during which samples of effluent were captured and analyzed to quantify breakthrough of PFAS from the sorbent materials over time. AIX was found to be more effective than GAC at removing the tested perfluoroalkyl sulfonic acids (PFBS, PFHxS, and PFOS), but GAC was similarly or more effective than AIX at removing perfluorocarboxylic acids (PFBA, PFHxA, and PFOA) under high conductivity conditions. Overall, the efficacy of AIX at removing PFAS was more strongly impacted by organic carbon and conductivity than GAC, while pH had less of an effect on either sorbent's efficacy compared to the other test conditions.     

Ambient levels of PFOS and PFOA in multiple environmental media

Making remediation and risk management decisions for widely‐distributed chemicals is a challenging aspect of contaminated site management. The objective of this study is to present an initial evaluation of the ubiquitous, ambient environmental distribution of poly‐ and perfluoroalkyl substances (PFAS) within the context of environmental decision‐making at contaminated sites. PFAS are anthropogenic contaminants of emerging concern with a wide variety of consumer and industrial sources and uses that result in multiple exposure routes for humans. The combination of widespread prevalence and low screening levels introduces considerable uncertainty and potential costs in the environmental management of PFAS. PFAS are not naturally‐occurring, but are frequently detected in environmental media independent of site‐specific (i.e., point source) contamination. Information was collected on background and ambient levels of two predominant PFAS, perfluorooctane sulfonate and perfluorooctanoate, in North America in both abiotic media (soil, sediment, surface water, and public drinking water supplies) and selected biotic media (human tissues, fish, and shellfish). The background or ambient information was compiled from multiple published sources, organized by medium and concentration ranges, and evaluated for geographical trends and, when available, also compared to health‐based screening levels. Data coverage and quality varied from wide‐ranging and well‐documented for soil, surface water, and serum data to more localized and less well‐documented for sediment and fish and shellfish tissues and some uncertainties in the data were noted. Widespread ambient soil and sediment concentrations were noted but were well below human health‐protective thresholds for direct contact exposures. Surface water, drinking water supply waters (representing a combination of groundwater and surface water), fish and shellfish tissue, and human serum levels ranged from less than to greater than available health‐based threshold values. This evaluation highlights the need for incorporating literature‐based or site‐specific background into PFAS site evaluation and decision‐making, so that source identification, risk management, and remediation goals are properly focused and to also inform general policy development for PFAS management.     

Occurrence and behavior of per‐ and polyfluoroalkyl substances from aqueous film‐forming foam in groundwater systems

Per‐ and polyfluoroalkyl substances (PFAS) are fluorinated compounds and the active ingredient in aqueous film‐forming foam (AFFF). AFFF has been identified as a significant source of PFAS contamination in groundwater. PFAS are also present in many other industrial and consumer products and their manufacture and use has led to numerous contaminated sites. Human health risks have been identified with studies linking firefighter cancers to training facilities where AFFF was used. Given the widespread release of these compounds to the environment and their potential health risks, understanding their mobility characteristics is important. This article details the occurrence and behavior of these substances in groundwater systems to help guide the emerging fields of PFAS investigation and remediation. Background is presented on AFFF and PFAS source characteristics, including common industrial and consumer PFAS sources. In addition, chemical properties, sorption and retention parameters, and observed transformation properties of PFAS and related compounds are discussed. Finally, knowledge gaps are identified for future laboratory and field studies.     

Evaluation of the effects of PFAS soil adsorption and transformation in the presence of divalent cations under ambient conditions

A bench‐scale study was conducted to evaluate the effect of divalent cations on the adsorption of perfluoroalkyl and polyfluoroalkyl substances (PFAS) onto soil particles. The study entailed batch testing of a synthetic soil mixture under a range of Epsom salt (soluble magnesium sulfate heptahydrate) concentrations. The synthetic soil was produced using a mix of sand, silt, clay, and organic matter that then was mixed and saturated with water collected from a PFAS‐impacted water source. The results indicate that variable concentrations of magnesium (divalent cation) had a minor effect on the sorption of perfluorooctane sulfonate with the highest sorption occurring in the strongest solution of Epsom salt. An unanticipated result of the test involved apparent biomediated transformation of polyfluorinated alkylated sulfonates (fluorotelomers or FTS) to perfluorooctanoic acid, perfluoroheptanoic acid (PFHpA), and perfluorononanoic acid. We believe this is the first time the complete transformation of 6:2 FTS to PFHpA has been observed and reported under ambient surface water‐like conditions within 6 months, a relatively short timeframe.     

The in situ treatment of TCE and PFAS in groundwater within a silty sand aquifer

Chlorinated ethenes such as trichloroethene (TCE), cis‐1,2‐dichloroethene (cis‐1,2‐DCE), and vinyl chloride along with per‐ and polyfluoroalkyl substances (PFAS) have been identified as chemicals of concern in groundwater; with many of the compounds being confirmed as being carcinogens or suspected carcinogens. While there are a variety of demonstrated in‐situ technologies for the treatment of chlorinated ethenes, there are limited technologies available to treat PFAS in groundwater. At a former industrial site shallow groundwater was impacted with TCE, cis‐1,2‐DCE, and vinyl chloride at concentrations up to 985, 258, and 54 µg/L, respectively. The groundwater also contained maximum concentrations of the following PFAS: 12,800 ng/L of perfluoropentanoic acid, 3,240 ng/L of perfluorohexanoic acid, 795 ng/L of perfluorobutanoic acid, 950 ng/L of perfluorooctanoic acid, and 2,140 ng/L of perfluorooctanesulfonic acid. Using a combination of adsorption, biotic, and abiotic degradation in situ remedial approaches, the chemicals of concern were targeted for removal from the groundwater with adsorption being utilized for PFAS whereas adsorption, chemical reduction, and anaerobic biodegradation were used for the chlorinated ethenes. Sampling of the groundwater over a 24‐month period indicated that the detected PFAS were treated to either their detection, or below the analytical detection limit over the monitoring period. Postinjection results for TCE, cis‐1,2‐DCE, and vinyl chloride indicated that the concentrations of the three compounds decreased by an order of magnitude within 4 months of injection, with TCE decreasing to below the analytical detection limit over the 24‐month monitoring period. Cis‐1,2‐DCE, and vinyl chloride concentrations decreased by over 99% within 8 months of injections, remaining at or below these concentrations during the 24‐month monitoring period. Analyses of Dehalococcoides, ethene, and acetylene over time suggest that microbiological and reductive dechlorination were occurring in conjunction with adsorption to attenuate the chlorinated ethenes and PFAS within the aquifer. Analysis of soil cores collected pre‐ and post‐injection, indicated that the distribution of the colloidal activated carbon was influenced by small scale heterogeneities within the aquifer. However, all aquifer samples collected within the targeted injection zone contained total organic carbon at concentrations at least one order of magnitude greater than the preinjection total organic carbon concentrations.     

Comparing PFAS to other groundwater contaminants: Implications for remediation

Established groundwater contaminants such as chlorinated solvents and hydrocarbons have impacted groundwater at hundreds of thousands of sites around the United States and have been responsible for multibillion dollar remediation expenditures. An important question is whether groundwater remediation for the emerging contaminant class comprised of per‐ and polyfluoroalkyl substances (PFAS) will be a smaller, similar, or a larger‐scale problem than the established groundwater contaminants. A two‐pronged approach was used to evaluate this question in this paper. First, nine quantitative scale‐of‐remediation metrics were used to compare PFAS to four established contaminants: chlorinated solvents, benzene, 1,4‐dioxane, and methyl tert‐butyl ether. These metrics reflected the prevalence of the contaminants in the U.S., attenuation potential, remediation difficulty, and research intensity. Second, several key challenges identified with PFAS remediation were evaluated to see similar situations (qualitative analogs) that have been addressed by the remediation field in the past. The results of the analysis show that four out of nine of the evaluated quantitative metrics (production, number of potential sites, detection frequency, required destruction/removal efficiency) indicate that the scale of PFAS groundwater remediation may be smaller compared to the current scale of remediation for conventional groundwater contaminants. One attenuation metric, median plume length, suggests that overall PFAS remediation could pose a greater challenge compared to hydrocarbon sites, but only slightly larger than chlorinated volatile organic compounds sites. The second attenuation metric, hydrophobic sorption, was not definitive regarding the potential scale of PFAS remediation. The final three metrics (regulatory criteria, in‐situ remediation capability, and research intensity) all indicate that PFAS remediation might end up being a larger scale problem than the established contaminants. An assessment of the evolution of groundwater remediation capabilities for established contaminants identified five qualitative analogs for key PFAS groundwater remediation issues: (a) low‐level detection analytical capabilities; (b) methods to assess the risk of complex chemical mixtures; (c) nonaqueous phase dissolution as an analog for partitioning, precursors, and back diffusion at PFAS sites; (d) predictions of long plume lengths for emerging contaminants; and (e) monitored natural attenuation protocols for other non‐degrading groundwater contaminants. Overall the evaluation of these five analogs provided some comfort that, while remediating the potential universe of PFAS sites will be extremely challenging, the groundwater community has relevant past experience that may prove useful. The quantitative metrics and the qualitative analogs suggest a different combination of remediation approaches may be needed to deal with PFAS sites and may include source control, natural attenuation, in‐situ sequestration, containment, and point‐of‐use treatment. However, as with many chlorinated solvent sites, while complete restoration of PFAS sites may be uncommon, it should be possible to prevent excessive exposure of PFAS to human and ecological receptors.     

Perfluoroalkyl and polyfluoroalkyl substances thermal desorption evaluation

Per‐ and polyfluoroalkyl substances (PFAS) are highly resistant to biotic and abiotic degradation and can withstand very high temperatures before breaking down. The storage of PFAS‐impacted water and sediments in a holding pond or stockpiled investigation or remedial action‐derived waste is occurring on an increasing number of sites. The most common PFAS water treatment options include granular‐activated carbon and resins and the most common soil treatment options have been primarily limited to excavation, offsite incineration, and, in some cases, soil stabilization. An increasing number of states across the United States are establishing part per trillion PFAS guidance levels for drinking water. Removing PFAS from soils removes PFAS source impacts to groundwater. In this study, volatilization of PFAS from soil treated using in situ thermal heating is evaluated as a treatment method to achieve a high degree of PFAS removal from soils. The evaluation of temperatures needed to achieve removal is described. To minimize vapor treatment required for PFAS thermal remediation, a scrubber was incorporated into the treatment train to transfer PFAS to the liquid phase in a concentrated, low‐volume solution. Vapor‐liquid equilibrium behavior and the extent of PFAS volatilization from impacted soil over a range of temperatures were evaluated. Results showed that heating soil to 350°C and 400°C reduces PFAS soil concentrations by 99.91% and 99.998%, respectively. It was also confirmed that sulfonate‐based PFAS generally required higher temperatures for volatilization to occur than carboxylate‐based PFAS.     

Fate and transport of phenol in a packed bed reactor containing simulated solid waste

An assessment of the risk to human health and the environment associated with the presence of organic contaminants (OCs) in landfills necessitates reliable predictive models. The overall objectives of this study were to (1) conduct column experiments to measure the fate and transport of an OC in a simulated solid waste mixture, (2) compare the results of column experiments to model predictions using HYDRUS-1D (version 4.13), a contaminant fate and transport model that can be parameterized to simulate the laboratory experimental system, and (3) determine model input parameters from independently conducted batch experiments. Experiments were conducted in which sorption only and sorption plus biodegradation influenced OC transport. HYDRUS-1D can reasonably simulate the fate and transport of phenol in an anaerobic and fully saturated waste column in which biodegradation and sorption are the prevailing fate processes. The agreement between model predictions and column data was imperfect (i.e., within a factor of two) for the sorption plus biodegradation test and the error almost certainly lies in the difficulty of measuring a biodegradation rate that is applicable to the column conditions. Nevertheless, a biodegradation rate estimate that is within a factor of two or even five may be adequate in the context of a landfill, given the extended retention time and the fact that leachate release will be controlled by the infiltration rate which can be minimized by engineering controls.     

Evaluating PFAS cross contamination issues

Avoiding cross contamination from per‐ and polyfluoroalkyl substances (PFAS) that may occur during sampling of environmental media is the key to ensure reliable analytical results during a PFAS sampling program. Due to the ubiquitous nature of PFAS in commonly used sampling materials and personal protective equipment, mitigating the risk of cross contamination is a challenge that requires a conservative approach when planning and executing a PFAS sampling program. This article describes a conservative approach to PFAS sampling and includes a case study that evaluated three insect repellent products to determine their suitability for use during PFAS investigation. The three products were verified to be PFAS‐free for the 17 PFAS included in the analysis and, therefore, these products are suitable for use during PFAS sampling activities without concern for cross contamination.     

Six pilot‐scale studies evaluating the in situ treatment of PFAS in groundwater

Per‐ and polyfluoroalkyl substances (PFAS) have been identified by many regulatory agencies as emerging contaminants of concern in a variety of media including groundwater. Currently, there are limited technologies available to treat PFAS in groundwater with the most frequently applied approach being extraction (i.e., pump and treat). While this approach can be effective in containing PFAS plumes, previous studies of pump and treat programs have met with limited remedial success. In situ treatment studies of PFAS have been limited to laboratory and a few field studies. Six pilot‐scale field studies were conducted in an unconfined sand aquifer coimpacted by petroleum hydrocarbon along with PFAS to determine if a variety of reagents could be used to attenuate dissolved phase PFAS in the presence of petroleum hydrocarbons. The six reagents consisted of two chemical oxidants, hydrogen peroxide (H₂O₂) and sodium persulfate (Na₂S₂O₈), and four adsorbents, powdered activated carbon (PAC), colloidal activated carbon (CAC), ion‐exchange resin (IER), and biochar. The reagents were injected using direct push technology in six permeable reactive zone (PRZ) configurations. Groundwater concentrations of various PFAS entering the PRZs ranged up to 24,000 µg/L perfluoropentanoic acid, up to 6,200 µg/L pentafluorobenzoic acid, up to 16,100 µg/L perfluorohexanoic acid, up to 6,080 µg/L perfluoroheptanoic acid, up to 450 µg/L perfluorooctanoic acid, and up to 140 µg/L perfluorononanoic acid. Performance groundwater sampling within and downgradient of the PRZs occurred for up to 18 months using single and multilevel monitoring wells. Results of groundwater sampling indicated that the PFAS were not treated by either the persulfate nor the peroxide and, in some cases, the PFAS increased in concentration immediately following the injection of peroxide and persulfate. Concentrations of PFAS in groundwater sampled within the PAC, CAC, IER, and biochar PRZs immediately after the injection were determined to be less than the method detection limits. Analyses of groundwater samples over the 18‐month monitoring period, indicated that all the PRZs exhibited partial or complete breakthrough of the PFAS over the 18‐month monitoring period, except for the CAC PRZ which showed no PFAS breakthrough. Analysis of cores for the CAC, PAC, and biochar PRZs suggested that the CAC was uniformly distributed within the target injection zone, whereas the PAC and biochar showed preferential injection into a thin coarse‐sand seam. Similarly, analysis of the sand packs of monitoring wells installed before the injection of the CAC, PAC, and biochar indicated that the sand packs of the PAC and biochar preferentially accumulated the reagents compared with the reagent concentrations within the surrounding aquifer by up to 18 times.     

In-situ application of colloidal activated carbon for PFAS-contaminated soil and groundwater: A Swedish case study

Soil and groundwater contamination by per- and polyfluoroalkyl substances (PFAS) has been a significant concern to human health and environmental quality. Remediation of contaminated sites is crucial to prevent plume expansion but can prove challenging due to the persistent nature of PFAS combined with their high aqueous mobility. In this case study, we investigated the potential of colloidal activated carbon (CAC) for soil stabilization at the pilot scale, aiming to entrap PFAS and prevent their leaching from soil into groundwater. Monitoring of the site revealed the presence of two potential sources of PFAS contamination at concentrations up to 23 μg L⁻¹ for ∑₁₁PFAS in groundwater. After CAC application, initial results indicated a 76% reduction of ∑₁₁PFAS and high removal rates for long-chain PFAS, such as perfluorooctane sulfonic acid and perfluorooctanoic acid. A spike in concentrations was noticed 6 months after injection of CAC, showing a rebound of the plume and a reduction of treatment effectiveness. Based on long-term monitoring data, the treatment effectiveness for ∑₁₁PFAS dropped to 52%. The rebound of concentrations was attributed to the plume bypass of the barrier due to the presence of high conductivity zones, which likely occurred because of seasonal changes in groundwater flow directions or the CAC application at the site. This demonstrates the need for a detailed and accurate hydrogeological understanding of contaminated sites before designing and applying stabilization techniques, especially at sites with high geologic and hydrologic complexity. The results herein can serve as a guideline for treating similar sites and help avoid potential pitfalls of remedial efforts.