Soil Washing_Sum2022_V32-3_P151.pdf

DOI: 10.1002/rem.21727

R E S E A R CH AR T I C L E

Ex situ soil washing to remove PFAS adsorbed to soils from source zones

Joseph Quinnan1 | Colin Morrell2 | Nathan Nagle3 | Ken G. Maynard2

1Arcadis, Novi, Michigan, USA

2CleanEarth Technologies, Goffs, Nova Scotia, Canada

3Arcadis, Newtown, Pennsylvania, USA

Correspondence

Joseph Quinnan, Arcadis, 28550 Cabot Dr, Suite 500, Novi, MI 48377, USA.

Email: joseph.quinnan@arcadis.com

Funding information

ESCTP ER20‐5258

Abstract

This article presents the results of an Environmental Security Technology

Certification Program (ESTCP) demonstration conducted at Eielson Air Force Base in Fairbanks, Alaska: Ex situ soil washing to remove per‐ and polyfluoroalkyl substances (PFAS) adsorbed to soils from source zones. The ESTCP project information can be found here: https://www.serdp-estcp.org/Program-Areas/

Environmental-Restoration/Risk-Assessment/ER20-5258. The final report is ex-pected to be published in September 2022. The objective of the project was to demonstrate and validate soil washing as a cost‐effective mass removal technology that can be applied to treat soil in source zones impacted by PFAS. The approach focused on the treatment of coarse soil fractions (sand and gravel) and separation of fines (silt/clay) for potential treatment by other means such as landfilling, thermal desorption, or stabilization. The goal was to optimize the treatment process to minimize wastes requiring more expensive treatment so that the cost would be competitive with landfilling in the continental United State. The treatment process was designed and optimized at the bench‐scale and demonstrated at the field‐scale by conducting a treatment trial at the Eielson Air Force Base using a mobile soil washing plant. Approximately 180 tons of PFAS‐contaminated soil was treated with untreated perfluorooctane sulfonate (PFOS) concentrations ranging from 4.5 to

560 μg/kg. The technology was validated by measuring PFAS mass removal efficiencies based on soil concentrations of PFAS compounds measured via

Department of Defense Quality Standards Manual 5.3 Table B‐14 and PFAS leachate concentrations as measured by synthetic precipitation leaching procedure testing. PFAS concentrations in treated soils were compared to Alaska Department of Environmental Conservation soil screening levels for the migration to ground-water pathway through leaching for perfluorooctanoic acid (PFOA), PFOS, and perfluorobutane sulfonate (PFBS). PFAS concentrations in soil leachate were compared to US Environmental Protection Agency groundwater screening criteria for PFOA, PFOS, and PFBS. The results of the treatability testing and pilot tests confirmed that coarse grained sand and gravel were successfully treated to meet performance goals and the fines were segregated for alternative treatment. The benefit of the waste minimization approach is that more cost‐effective means can be applied to treat most of the impacted soil volume while limiting expensive residuals

Remediation. 2022;32:151–166. wileyonlinelibrary.com/journal/rem © 2022 Wiley Periodicals LLC. | 151 mailto:joseph.quinnan@arcadis.com https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Risk-Assessment/ER20-5258 https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Risk-Assessment/ER20-5258 https://wileyonlinelibrary.com/journal/rem http://crossmark.crossref.org/dialog/?doi=10.1002%2Frem.21727&domain=pdf&date_stamp=2022-06-30 disposal or more expensive means of treatment such as incineration or thermal desorption. Treated soils can be beneficially reused on‐site and fines could also be stabilized to meet leaching‐based standards, as an alternative to off‐site disposal or treatment. This flexible approach allows one to tailor the combined remedy strategy to meet clients' objectives and risk tolerance through a sustainable, least‐cost remedial approach.

1 | INTRODUCTION

The discharge of aqueous film‐forming foam (AFFF) for fire training and fighting activities since the 1970s has resulted in PFAS‐impacted soil source zones throughout the world (Adamson et al., 2022;

Bräunig et al., 2019; Høisæter et al., 2019). At these source zones, PFAS migration through the vadose zone can result in widespread groundwater contamination, causing risk for human and environ-mental receptors (Guelfo & Adamson, 2018). Retention of PFAS in soil can result in long‐term leaching and PFAS migration through the vadose zone long after initial AFFF releases (Brusseau et al., 2020).

The current state of very low soil (µg/kg) and groundwater standards

(ng/L), combined with recalcitrance of the carbon−fluorine bond creates an exceptional challenge for practitioners and stakeholders when evaluating options for PFAS‐impacted soil treatment. Evolving regulations including the likelihood of hazardous substance designa-tion under the Comprehensive Environmental Response, Compensa-tion, and Liability Act (CERCLA) and potentially decreasing standards due to emerging toxicological data increase the need for cost‐ effective methods to remediate PFAS contaminated soil.

Eielson Air Force Base is an active military installation located near Fairbanks, Alaska and has been used for military operations since its establishment in 1944. PFAS specific environmental investigations have been ongoing at Eielson Air Force Base since

2014 and, in 2016, the Alaska Department of Environmental

Conservation (ADEC) established soil screening levels for the migration to groundwater pathway through leaching for perfluor-obutane sulfonate (PFBS) (1900 µg/kg), perfluorooctane sulfonate

(PFOS) (3 µg/kg) and perfluorooctanoic acid (PFOA) (1.7 µg/kg)

(ADEC, 2016). Characterization of soil stockpiles generated from construction projects at Eielson Air Force Base resulted in large volumes of soil with PFAS concentrations exceeding ADEC screening levels (US Air Force, 2021). Based on the access and need for future management of PFAS‐impacted soil, Eielson Air Force Base was selected as the site to conduct this demonstration.

Three soil stockpiles at Eielson Air Force Base were selected for this project to validate the soil washing technology across a range of

PFAS concentrations and soil types: EIE382‐5 (40 tons), EIE385‐4

(100 tons), and EIE‐393‐1 (40 tons). The first two stockpiles were selected to be representative of the range of impacts encountered in construction‐related stockpiles with PFOS concentrations in the

10 and 100 s of µg/kg. The third stockpile, EIE393‐1 was derived from an AFFF source area with PFOS concentrations in the low

1000 s of µg/kg. Because the volume of approximately 10 tons was insufficient to fill the treatment system, soil cuttings generated from

AFFF source area soil borings during a PFAS Remedial Investigation at Eielson Air Force Base were obtained to supplement EIE393‐1 and increase the volume to 40 tons. Soil was excavated from the stockpiles and stored on an untreated stockpile storage pad adjacent to the front end of the soil washing process.

A modular 1‐ to 10‐ton per hour pilot‐scale soil washing system was designed to duplicate the unit treatment processes and sequence of treatment evaluated at the laboratory scale. The pilot‐scale process was shipped to Eielson Air Force Base, Alaska, with equipment arriving on‐site between August 13 and 20, 2021.

Commissioning activities consisting of site preparation, liner installa-tion, placement of equipment, and connection of piping and electrical were completed by August 24th. On future applications of the pilot‐ scale system, if equipment delivery could be coordinated to be completed within two consecutive days, it is estimated that commissioning durations can be reduced to 7 days. The system was designed to accommodate operation in remote locations and can be powered by a 185 kVA generator. Water was not available at the processing area and needed to be transported by water truck from a nearby pond, which was tested and confirmed PFAS‐free. The system was filled with 32,000 gallons of water, leak tested and wet‐ commissioned by running the process water through the system in the absence of soil to balance water flows. Processing of soil was completed from August 25th through September 23rd when the onset of freezing temperatures brought a premature end to the soil processing phase of the pilot.

1.1 | PFAS‐Impacted soil disposal and treatment options

Soil treatment options are limited and expensive for PFAS‐impacted soils. Current soil treatment options include excavation and landfill disposal, destructive technologies such as incineration and thermal desorption, and stabilization methods designed to limit leaching.

Landfilling is becoming more expensive as regulations and restrictions limit disposal to specialized Resource Conservation and Recovery Act

(RCRA) Subtitle D landfills with leachate collection and treatment systems, RCRA Subtitle C landfills, and zero‐net‐leachate landfills in arid environments. Disposal costs have increased in anticipation of hazardous substance designation of PFOS and PFOA under CERCLA, 152 | QUINNAN ET AL.

but also because transportation costs are significant due to geography of these specialized landfills. Transportation and disposal costs in the continental United States are on the order of $200/ton but can increase to more than $400/ton depending on the location and PFAS concentration in the waste. While there are no national standards, our experience is that the threshold for the most expensive landfill disposal is at PFAS concentrations of approximately

1mg/kg. Landfill disposal options could be more costly in the future after Federal regulations contemplating hazardous substance desig-nation are enacted.

Incineration has long been considered the gold standard for

PFAS‐containing waste destruction because the high temperatures

(650 to more than 1650°C) effectively desorb PFAS compounds from the soil and are thought to treat the PFAS under appropriate residence time and temperature. Recent public concern about incomplete treatment of off‐gas emissions has led to a US Department of Defense (DoD) moratorium on incineration of PFAS‐containing wastes until the US Environmental Protection Agency (USEPA) establishes emissions criteria and certifies technology providers

(U.S Congress, 2021). The USEPA indicated additional research is needed to address fluorinated products of incomplete combustion

(USEPA, 2020). Incineration costs typically start at $350/ton but can be much higher due to transportation costs associated with centralized fixed treatment facilities. Thermal desorption processes are advancing

PFAS‐containing soil treatment through optimization of the desorption process to reduce energy requirements and off‐gas emissions treatment methods using thermal oxidizers with sufficient residence time and temperature to destroy PFAS compounds. Emerging approaches include smoldering combustion to desorb and treat PFAS

(Duchesne et al., 2020), or thermal desorption and treatment of

PFAS‐laden vapors through condensation and treatment in the liquid state (Stallings, 2022). Thermal desorption has the advantage that the equipment can be set up for on‐site treatment, creating a cost off‐set for large scale treatment projects, where the mobilization would be much less than transportation of large volumes of soil. Commercial costs for thermal desorption are evolving with more experience and technology providers; however, the energy requirements to heat the entire volume of soil to 350 to 400°C for desorption will result in costs on the order of $300/ton for large volumes, and substantially higher for small volumes. Thermal desorption processes that use thermal oxidation treatment will also require air permits, which are also difficult to obtain given the evolving regulatory climate.

Adsorption and stabilization have been demonstrated to be an effective tool for managing PFAS‐impacted soil (McDonough et al., 2021). The general approach is to chemically “fix” the PFAS through amendments that bind the PFAS and reduce their mobility or creating a low‐permeability monolith that limits leaching. This approach could be used for on‐site treatment or pretreatment for off‐site disposal. However, further studies are needed to evaluate the longevity of this approach. Soil stabilization has the advantage that the mobilization of equipment and addition of amendments can be applied to smaller volumes more cost effectively than thermal desorption. Because adsorption and stabilization do not remove

PFAS from soil, on‐going long‐term management and monitoring are required. Commercial costs for stabilization are not widely available and depend on the soil characteristics, PFAS concentrations, and long‐term management and monitoring requirements. Capital costs for mixing and amendments will likely range between $150 and more than $300/ton, depending on the soil characteristics and PFAS concentrations.

1.2 | Soil washing historical review

Soil washing does not refer to a singular process or approach. It is a

“catch‐all” phrase that describes a range of physical/chemical techniques for soil remediation that involve processing soil through physical and chemical separation equipment connected in parallel or in series. While most of the equipment being used in soil washing circuits is commercially available through mining or aggregate equipment suppliers, the soil washing processes that have been applied at contaminated sites all differ in the equipment used, the sequence the equipment is applied, and in the use of chemical amendments to enhance contaminant recovery.

Historically, soil washing processes have taken on one of three general forms:

(1) Size separation processes that segregate the coarse‐grained soil particles from the fine‐grained to reduce the volume of contaminated soil. This approach is used on sites where the contaminant concentrates in the fine‐grained fraction of the soil and the fine‐ grained fraction is a small percentage of the bulk soil.

(2) Size separation and density separation process that both concentrate the contaminant in the fine‐grained soil and remove it by density separation from the coarser soil fractions. This approach is effective on sites where there is a significant density difference between the contaminant and host soil, especially for sites impacted by lead bullets or sites contaminated with elemental mercury.

(3) Size and/or density separation combined with a chemically amended process water. The chemical amendments enhance the solubilization or desorption of the contaminant into the aqueous phase and can be used to treat both the coarse‐grained and fine‐ grained soil fractions. Chemical amendments to enhance removal efficiency are contaminant specific, but typically include acids, bases, alcohols, surfactants, and chelating agents.

Soil washing has been utilized on a commercial scale in Europe since the mid‐1980s (Nunno & Hyman, 1988). Fixed facilities located in the Netherlands and Germany have treated a range of contami-nants from cyanide, petroleum hydrocarbons, polycyclic aromatic hydrocarbons, and metals. The processes differed, but in general were designed to reduce the volume of contaminated soil by combining physical separation of the coarse‐grained from the fine‐ grained soil fraction with the treatment of the separated coarser grained soil fractions using chemically amended process water.

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In North America one of the first tests of a soil washing system was conducted by the USEPA. The “Mobile Soil Washing System” was developed in the early 1980s (Griffiths, 1995; Scholz &

Milanowski, 1984) to demonstrate treatment through scrubbing combined with a chemically amended process water. The system consisted of a wet trommel with scrubber and screening sections to separate the +2mm soil fraction, with the −2mm soil fraction undergoing additional treatment by contact with chemically amended process water in a series of counter current reactors.

In the late 1980s, the EPA developed the “Mobile Volume

Reduction Unit,” which was designed as a small‐scale mobile research system that could process soils at a throughput rate of 100 pounds per hour (USEPA, 1993). The process reduced the volume of contaminated soil requiring disposal through the separation of the coarse‐grained fraction from the contaminant laden fine‐grained fraction. The system consisted of a heated screw feeder, wet trommel, wet screens, clarifier, and water treatment. The process had a flexible design and could be run as a water only process at neutral pH and ambient temperature or could be operated with the addition of surfactants and adjustment of pH and temperature to further reduce the contaminant concentrations in the coarse‐grained soils.

During the 1990s a few commercial vendors demonstrated soil washing in the United States. BESCORP/COGINIS processed over

20,000 tons of metal‐contaminated soil at the Twin Cities Army

Ammunition Plant in New Brighton, Minnesota (Fristad, 1995; USEPA, 1999). The process combined mechanical screening with gravity separation to treat the coarse‐grained soil and acid leaching to treat the fines. The process consisted of a trommel screening out the

+6.35mm fraction with the oversize handpicked for munitions and cartridges. The −6.35mm soil was processed through a hydraulic classifier to separate the sand from the fines. The sand stream was furthered processed by gravity separation via spirals and jigs to remove metal fragments before transferring to a counter current reactor for leaching. The separated fines were sent to a series of leaching clarifiers with the soil fractions dewatered, recombined, and neutralized.

In a similar application, ContraCon Northwest and BESCORP were contracted to perform a pilot‐scale demonstration of soil washing at a small arms range in Fort Polk, Louisiana. The demonstration was designed to evaluate the treatment of the lead‐ contaminated soil using physical separation combined with acid leaching (Sams et al., 1998). The two processes used similar equipment of soil feed conveyors, classification/attrition scrubbing, gravity separation of lead from the sand fraction by jigs, and leaching of the lead from the fine‐grained soil in leach tanks. As part of the demonstration each vendor was assigned a different acid to evaluate the effect of using a strong (hydrochloric) or weak (acetic) acid as the chemical amendment added to the leaching circuit.

RMI Environmental Services and Alternative Remedial Technol-ogies, Inc. (ART) completed a pilot‐scale demonstration on 64 tons of uranium‐contaminated soil at the RMI Titanium Company Extrusion

Plant in Ashtabula, Ohio. The pilot‐scale chemical leaching system was operated in early 1997 and consisted of a heated batch reactor, mechanical screens, and dewatering equipment (USEPA, 1999). The leaching circuit contacted a sodium bicarbonate solution in the heated batch reactor for 1−2 h at 30% solids content (USEPA, 1999).

Following processing both the coarse and fine‐grained soils met the treatment standard of 30 pCi/g.

ART also completed a full‐scale soil washing project at the King of Prussia Technical Corporation site in Winslow Township, New

Jersey. A 25‐ton per hour soil washing process was constructed on‐ site and operated from June to October 1993. The process consisted of wet screening, attrition cells, hydrocyclones, a lamella clarifier, and a belt press. The system was operated as a volume reduction process producing a treated coarse‐grained soil and a contaminant enriched fine‐grained soil that was shipped off‐site for disposal.

The treatment of PFAS‐impacted soil using soil washing is a relatively new application with a limited number of trials beyond the laboratory scale. In addition to the project described in this paper, another pilot‐scale soil washing project has been completed by

Ventia at the Royal Australian Air Force Base Edinburgh, in Australia.

Over 2000 tons of PFAS‐contaminated soil from the source zone of the Fire Fighting Training Area were processed over the course of the pilot test using a soil washing system that incorporated treatment elements of soil size separation and density separation of organics

(Grimison et al., 2020).

Despite the limited pilot‐scale trials on PFAS‐impacted soils, the chemical properties of PFAS make it a good candidate contaminant for treatment by soil washing. Organic matter content, mineralogy, fine‐ grained soil percentage, and pH are all soil properties that interact with the chemical characteristics of PFAS to influence their retention on a given soil (Liu et al., 2020). In a soil washing process, the high aqueous solubility of PFAS along with ability to control the pH of the process water, segregate the fine‐grained fraction of the soil from the coarse‐ grained, and target the separation of organics or other soil minerals from the bulk soil offer the ability to reduce the volume of contaminated soil requiring further treatment or disposal. If treated fines concentrations exceed remediation standards, they can be managed using secondary treatment, including incineration, thermal desorption, stabilization and adsorption, or specialized landfilling.

In some sense, we have come full circle back to the late 1980s, where the focus was on volume reduction when remediating difficult to treat contaminants. The promise for PFAS‐impacted soil washing treatment is that future optimization of fines and organic treatment will minimize the volumes requiring secondary treatment, resulting in more cost‐effective and sustainable solutions. This article focuses on key aspects of an Environmental Security Technology Certification

Program (ESTCP) demonstration project (ER20‐B2‐5258) conducted by Arcadis and CleanEarth Technologies to demonstrate and validate soil washing as a cost‐effective mass removal technology that can be applied to treat soil in source zones impacted by PFAS. The treatment process was optimized at the bench‐scale and ex‐situ soil washing was demonstrated at pilot‐scale at the Eielson Air Force Base using a mobile soil washing plant. Approximately 180 tons of PFAS‐ contaminated soil was treated; untreated PFOS concentrations ranged from 4.5 to 560μg/kg. The technology was validated by measuring PFAS mass removal efficiencies (RE) and screening total

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and leachable PFAS concentrations in coarse‐grained treated soil against ADEC soil migration to groundwater standards.

2 | MATERIALS AND METHODS

The performance objectives were to optimize the soil washing process by separating the fines and treating the coarse‐grained fraction to achieve posttreatment concentrations in soil and leachate including:

(1) soil concentrations of PFOA, PFOS, and PFBS below regional screening levels—ADEC (2016) soil migration to groundwater values for

PFOA, PFOS, and PFBS at 1.7, 3.0, and 1900µg/kg, respectively.

(2) Greater than 99% removal of PFAS (sum of 24 PFAS compounds on

Table B‐15, Quality Standards Manual [QSM] 5.3 plus N‐Methyl perfluorooctane sulfonamide [N‐MeFOSA]). And, (3) leachate concen-trations via synthetic precipitation leaching procedure (SPLP) testing below the USEPA screening levels for groundwater of 40 ng/L for PFOA and PFOS, and 600 ng/L for PFBS (USEPA, 2019, 2021). The ADEC

(2016) soil values were used to benchmark beneficial reuse of the soils based on posttreatment concentrations.

Soil characteristics of stockpiles tested in the bench‐scale and field demonstration are presented in Table 1 and were obtained from

US Air Force (2021) and particle size analysis samples collected during the field demonstration.

2.1 | Treatability trial

Three, 20 L samples of PFAS‐contaminated soil were obtained from three stockpiles at Eielson Air Force Base: EIE393‐1, EIE377‐1, and

EIE379‐2. The bench‐scale treatability was completed at the

CleanEarth Technologies Laboratory in Halifax, Nova Scotia. The objectives of the treatability trial were to evaluate the processes required to treat the soils and verify that the soil washing process could reduce PFAS concentrations in coarse‐grained fraction of the soil to concentrations below ADEC screening levels and meet USEPA groundwater screening levels for leachate.

Figure 1 shows the series of treatment stages that comprise the experimental design of the treatability test. The design for the test consisted of three main stages; (1) size separation; (2) attrition scrubbing; and (3) chemical extraction. Both dry‐ and wet‐screening were conducted using an 18‐in. round vibratory screener. Three separate fractions were created using 2.5, 4, and 200 Tyler mesh screens placed in sequence.

Attrition scrubbing of the plus and minus 4.76mm fractions of the soil sample were placed in a Denver attrition mill and processed in a 5:1 liquid/solid slurry. At the end of the attrition scrubbing, the slurry from each run was passed through the SWECO screen to separate into the final soil fractions of +4.76mm (gravel), −4.76 + 0.074mm (sand), and −0.074mm (fines).

If the remedial objectives were not achieved through the screening and attrition scrubbing stages, chemical extraction was applied to the samples. Proprietary extraction solutions were tested on the different soil fractions using a modified Phipps and Bird six‐paddle stirrer with a variable speed mixer. Approximately 200ml samples of the soil were reacted with the extracting solution at a 5:1 (v:v) ratio of solution to soil for 30min at the specified mixing intensity. Upon completion of the extraction period, the coarser soil fractions are allowed to settle, decanted, and screened on a 200‐mesh screen. The finer grained soil fractions were mixed with a conventional polymer to create a flocculated slurry, allowed to settle for 10min, and decanted before final dewatering using a laboratory‐scale filter press. Soil samples were collected at the points indicated in Figure 1.

Process water produced during the bench scale soil washing test was treated through a laboratory scale wastewater treatment process shown in Figure 1. The PFAS impacted process water was pumped through a series of 5‐cm columns consisting of a 0.35‐ to 0.45‐mm filter sand, 8 × 30 mesh granular activated carbon (GAC), and a strong base anionic ion exchange resin (IXR). Empty bed contact time for each of the columns was 3, 15, and 5min, respectively.

2.2 | Pilot‐Scale mobile system treatment process

The complete modular pilot‐scale soil washing process consisted of two soil modules and a water treatment circuit. Each soil treatment module was built within a rectangular frame measuring 40 feet long by 8 feet wide and certified for shipment by road, rail, and ocean to create flexibility for mobilization to remote project sites. The process area footprint consisted of a 40‐foot by 100‐foot bermed lined area that allowed space for equipment set up and stockpiling of untreated and treated soil.

Figure 2 shows different stages of the process commissioning and set up.

The modular pilot‐process was designed to allow the tuning of key process variables such as soil throughput, retention time, liquid to solid ratios, and the incorporation of the chemical extraction and the optional physical optimization equipment. The intent was to evaluate the different processing conditions from the treatability testing versus soil type and contaminant concentration to create a data set that would support design of a full‐scale soil washing process.

Figure 3 and Figure 4 present the process flow and the pilot‐scale process as assembled at Eielson, respectively. The flow of soil through the system begins by feeding soil into the oversize feed screen labeled as (1). This is a wet grizzly screen that can remove rock in the range of

TABLE 1 Soil characteristics

Stockpile Gravel Sand Fines

EIE377‐1 37%a 35%a 28%a

EIE379‐2 32%a 40%a 28%a

EIE382‐5 32%a−40%b 40%b−42%a 21%b−26%a

EIE385‐4 24%b−43%a 34%a−40%b 23%a−36%b

EIE393‐1 1%b−53%a 34%a−39%b 8%a−65%b aSoil characteristics obtained from table 6 of US Air Force (2021).

bSoil characteristics from particle size analysis sampling conducted during the bench‐scale testing and the pilot demonstration.

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2−36 in. in size. The screen is equipped with spray bars that apply a spray of the process wash water. The oversize material (boulders, sticks, steel, and plastic) is retained and discharged from the screen while the bulk of the soil passes into a second screening Section (2) that splits the feed into +4.76 and –4.76‐mm particle sizes. The oversize is retained on the screen and can be stockpiled directly (3). The attritted +4.76mm material is dewatered and stockpiled (4). Both stockpiles are segregated and can be further processed through the optional physical processing equipment, if required. This step could not be conducted during the demonstration due to timing constraints.

The undersize from screen (2) flows as a slurry (5) into two aerated mixing tanks (6) and (8) to allow for desorption of the

PFAS to the aqueous phase and contact with the process water in a turbulent mixing environment. The slurry in the aerated mix tank (6) is pumped though a dewatering cyclone (7) to separate the fine‐grained soil from the sand range. The dewatered sand is

F IGURE 1 Soil Washing Treatability Test Design: Process flow from the bench‐scale treatability test illustrates the treatment elements, sample collection points, and water treatment circuit. GAC, granular activated carbon; IXR, ion exchange resin.

F IGURE 2 Soil washing process commissioning. (1) Bermed liner being placed down. (2) Equipment was transported to site using trucks. (3) and (4) Soil modules were lifted off trucks and placed onto the liner.

156 | QUINNAN ET AL.

transferred to the aerated mix tank (8) and pumped into a hydraulic classifier (9) where the sand is dewatered (10) before stockpiling (11).

The fine‐grained soil separated at various points of the process is conveyed to a thickener (12) where the partially dewatered fines are sent to dewatering (13). The dewatered fines are stockpiled (14) while the filtrate from this process and the overflow from the thickener are pumped to the water treatment process (15) to remove the PFAS from the process water before the water is recycled to the various points in the process.

In the two‐stage water treatment process, the GAC removed potential dissolved natural organics, cocontaminants, and the bulk of the PFAS mass from the water; the IXR removed any potential PFAS that passed through the GAC, ensuring high removal efficiency to enable recycling of the water through the soil treatment process.

F IGURE 3 Soil washing process flow of pilot plant. Flow of soil, process water, and slurry through the treatment elements of the pilot scale soil washing system.

F IGURE 4 Eielson pilot‐scale soil washing process elements and equipment

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At the end of the process, three separate sized soil fractions are generated and deposited in separate material bays: gravel (−50.8 to

+4.76mm), sand (−4.76 to +0.074mm) and fines (−0.074mm).

During the bench‐scale treatability testing the organics were successfully treated without segregation from the fines. The pilot process therefore did not segregate the organics.

2.3 | Sampling

To evaluate the performance of the treatment process with different starting PFAS concentrations and soil types, soil and process water samples were collected pre‐ and posttreatment. In the bench‐scale treatability trial, one soil sample was collected from the untreated soil and one soil sample was collected from the three soil fractions

(gravel, sand, and fines) for each of the three stockpiles. Samples were collected in 250‐ml sample bottles. For stockpile EIE393‐1, multiple treatment processes were tested, and a new sample was collected from soil fractions after each test.

In the field demonstration, samples were collected using the incremental sampling method (ISM) to evaluate overall treatment performance and compliance with the performance goals. ISM sampling was based on guidance from the Interstate Technology &

Regulatory Council (ITRC, 2020). A sample was collected from the untreated soil and from each of the treated fractions. Replicate ISM samples (triplicates) were collected for each sample to quantify uncertainty in the estimate of the mean concentration for each sample. A minimum of 30 increments of approximately 50 g per increment was collected for each ISM sample in a 2‐L sample bottle.

Increments were collected progressively during treatment to enable equal access to the soil from all depths.

One composite sample was also collected from the untreated soil and the treated soil fractions for every 20 tons of soil treated/ generated and placed in 250‐ml sample bottles. These samples were collected to evaluate variability in treatment performance over time and will be presented in the final report for the ESCTP project.

In the bench‐scale treatability test and the field demonstration, samples of the untreated and treated process water were collected to confirm the effectiveness of the water treatment circuit. In the field demonstration the untreated sample was collected from the thickener and the treated sample was collected after IXR treatment.

2.4 | Analytical testing

Soil and process water samples were sent to Pace Analytical in West

Columbus, South Carolina for analysis of 25 target PFAS compounds via the USEPAMethod 537 (modified)/US DoD QSM 5.3 (DoD, 2019)

(Table B‐15) plus N‐MeFOSA (list of 25 PFAS). Bench‐scale treatability samples were analyzed for leaching via the Australian

Standard Leaching Procedure by method AS 4439.3. Field demon-stration samples were analyzed for leaching via the SPLP method

SW‐846 Test Method 1312.

Extractable organofluorine (EOF) analysis and adsorbable orga-nofluorine (AOF) analysis were also conducted on soil and water samples to evaluate the presence of nontarget PFAS. EOF and AOF analysis was conducted by SGS in Belgium using their commercial

Combustion Ion Chromatography Method. Total oxidizable precursor

(TOP) assay analysis (Houtz & Sedlak, 2012) was also conducted by

SGS on select samples for comparison to EOF.

3 | RESULTS

3.1 | Treatability trials

Table 2 and 3 present a summary of the soil and leachate results from the bench‐scale treatability testing. If the results were below the detection limit, the concentration is presented as less than the detection limit, and the result was not included in the sum of PFAS calculations.

The treatability testing was designed with sequential treatment processes to enable understanding of the mechanisms of treatment and to facilitate optimization. Three rounds of testing were required to meet the treatment objectives. Round 1 evaluated different process conditions such as basic chemistry of the wash solution, temperature, retention time, liquid to solid ratios and agitation.

Round 2 included the most successful conditions from Round 1 and a chemical extraction step.

Concentrations of PFBS and PFOA were initially low in untreated samples and often treated to concentrations below detection limits, meeting all performance goals at the end of the treatability process.

Discussion of results for the bench‐scale treatability trial will focus on

PFOS and the sum of PFAS.

For EIE377‐1 and EIE379‐2, the soil performance goals were achieved using the Round 1 process. For EIE393‐1, the soil concentra-tion performance goals were achieved after using the Round 3 processes. The bench‐scale treatability trial indicated that, during the field demonstration, soil with lower PFAS concentrations could be successfully treated using the simpler Round 1 process conditions.

However, soil with higher PFAS concentrations would require the physical optimization step included in the Round 2 and 3 processes.

Table 4 and 5 present the RE for PFOS and sum of PFAS from the bench‐scale treatability test. RE were calculated by comparing the concentration of the treated soil fraction to the concentration of the untreated sample. If the treated sample exhibited a concentration less than the detection limit, the removal efficiency was not calculated. PFOS and sum of PFAS REs of >99% or PFAS removal to concentrations below detection limits were achieved for EIE379‐2 and EIE393‐1. EIE377‐1 did not achieve the >99% RE, however, soil concentrations of PFBS, PFOA, and PFOS were below performance goals.

3.2 | Field demonstration results

Table 6 and 7 present the triplicate results from ISM sampling for the three stockpiles. The arithmetic mean of the three ISM results is

158 | QUINNAN ET AL.

presented in Table 8 and these were used to evaluate overall performance of the demonstration and compliance with the performance goals. If the primary and replicate results were below the detection limit, the concentration is presented as less than the detection limit “<DL,” and the result was not included in the sum of

PFAS calculations. If there was at least one detection, the arithmetic mean was calculated by substituting one half the detection limit and the result was included in sum of PFAS calculations. The PFAS REs presented in Table 9 were calculated by comparing the soil and leachable concentrations in treated soil to the untreated concentra-tions presented in Table 8.

Untreated soil concentrations from samples collected in this demonstration were lower than the 95% upper confidence limit (UCL) concentrations reported in the Stockpile Characterization Report (US

Air Force, 2021). EIE382‐5 reported the lowest PFOS concentration among the three stockpiles (4.5μg/kg). This was an order of magnitude lower than the PFOS 95% UCL range of 67−111 μg/kg (US Air

Force, 2021) and was likely due to heterogeneity in the stockpile.

Stockpile EIE393‐1 reported the highest untreated PFOS concentra-tion of 560 μg/kg which was lower than the 95% UCL of 1900 μg/kg

(US Air Force, 2021); however, this was likely due to supplementing the stockpile with additional soil with unknown PFAS concentrations.

TABLE 3 Stockpile EIE393‐1 bench‐scale treatability results

Treatment round Soil fraction

PFBS PFOA PFOS Sum PFAS Soil μg/kg Leachate ng/L

Soil μg/kg Leachate ng/L

Soil μg/kg Leachate ng/L

Soil μg/kg Leachate ng/L

Untreated 6.6 J 230 <4.2 350 2700 32,000 3155 39,795

Round 1 Rock <0.2 <10 <0.2 <10 88 200 97 270

Gravel <0.2 <10 <0.2 <10 27 620 32 803

Sand 0.26 <10 0.55 J 26 J 150 900 170 1005

Fines 3.1 J 22 J <3 53 2400 530 2822 1123

Round 2 Rock/gravel <0.23 <10 <0.22 <10 8.8 120 9.4 194

Sand <0.19 <10 <0.19 <10 12 75 14 181

Round 3 Rock/gravel/ Sand

<0.24 <20 <0.24 <20 0.34 J 36 J 0.34 J 36 J

Performance goals 1900 40 1.7 40 3 40 ‐ ‐

Note: Bold values exceeded the performance goals.

Abbreviations: J, estimated result, less than limit of quantitation and greater than detection limit; PFAS, per‐ and polyfluoroalkyl substances;

PFBS, perfluorobutane sulfonate; PFOA, perfluorooctanoic acid; PFOS, perfluorooctane sulfonate; Sum of PFAS, sum of 24 PFAS compounds on Table B‐15, QSM 5.3 plus N‐MeFOSA.

TABLE 2 Stockpiles EIE377‐1 and EIE379‐2 bench‐scale treatability results

Stockpile Soil fraction PFBS PFOA PFOS Sum PFAS Soil μg/kg Leachate ng/L Soil μg/kg Leachate ng/L Soil μg/kg Leachate ng/L Soil μg/kg Leachate ng/L

EIE377‐1 Untreated <0.22 3.4 J 0.78 J 20 15 450 20.24 687.5

Rock <0.18 <0.95 <0.18 <0.95 0.32 J <0.95 0.32 J <DL

Sand <0.24 <1 <0.24 <1 0.50 J <1 0.5 J <DL

Fines <0.29 1.2 J <0.29 <1 5.4 47 5.4 151.3

EIE379‐2 Untreated 0.28 J 13 0.45 J 29 59 3300 62.85 4045.9

Rock <0.19 <1 <0.19 1.1 J <0.19 29 <DL 147.8

Sand <0.22 <1 <0.22 <1 0.42 J 2.2 J 0.42 J 64

Fines <0.29 <0.95 <0.29 <0.95 9.6 7.8 11.9 116.7

Performance Goal 1900 40 1.7 40 3 40 ‐ ‐

Note: Bold values exceeded the performance goals.

Abbreviations: <DL, No compounds were detected above the detection limit so Sum of PFAS could not be calculated; J, estimated result, less than limit of quantitation and greater than detection limit; PFAS, per‐ and polyfluoroalkyl substances; PFBS, perfluorobutane sulfonate; PFOA, perfluorooctanoic acid;

PFOS, perfluorooctane sulfonate; Sum of PFAS, sum of 24 PFAS compounds on Table B‐15, QSM 5.3 plus N‐MeFOSA.

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3.3 | Treatment performance

On average, PFOS comprised 69% of the total PFAS in untreated soil, 84% of total PFAS in treated soil, and was the driver for meeting the performance goals. Concentrations of PFBS and PFOA in untreated soil were comparatively low and often already below performance goals. PFBS and PFOA soil performance goals were achieved in all stockpiles for the coarse‐grained fraction. PFBS and PFOA performance goals were also achieved for the fine‐grained fraction in EIE382‐5 and EIE385‐4. Due to the limited presence of PFBS and

PFOA in soil, the remainder of this discussion will focus on PFOS and sum of PFAS results. While we summarize the results for PFOS, PFBS, and PFOA for completeness here; the final ESTCP report will provide detailed discussion on the broader suite of PFAS.

The soil performance goals were achieved for EIE382‐5 and

EIE385‐4 using the treatment process verified in the bench‐scale treatability trial. Mean coarse‐grained soil PFOS concentrations in these stockpiles ranged from below the detection limit to 0.9 μg/kg.

Soil performance goals were also achieved in the fine‐grained fraction for EIE382‐5. This result highlights the potential for soil washing to achieve soil performance goals without requiring secondary treatment of the fines if untreated concentrations are relatively low.

In stockpile EIE393‐1, the soil performance goals were not achieved, but this was expected because the physical optimization steps (Round 2 and 3 in treatability testing), which were required to achieve higher levels of PFAS removal, could not be completed due to freezing conditions. Based on the comparable performance of the

Round 1 process at the bench and pilot‐scale, we expect that the pilot treatment unit or full‐scale treatment would perform to meet soil performance goals. Optimization of performance parameters in the full‐scale and confirmation of treatment performance related to

Round 2 and 3 processes would be part of any future full‐scale remediation effort in this regard.

3.4 | PFAS removal

Concentrations of PFBS and PFOA were initially low in untreated samples and often treated to concentrations below detection limits, meeting all performance goals at the end of the treatability process.

PFOS REs were generally highest in the gravel fraction (98.1%

−94.6%), followed by the sand fraction (96.1%−88.6%), and the fines fraction (61.8% to −7.7%). These results aligned with bench‐scale treatability testing and confirmed that soil washing was effective for the coarse‐grained fraction and that the fine‐grained fraction retains higher PFAS concentrations. RE for sum of PFAS in coarse‐grained soil were generally higher than PFOS for each stockpile, indicating that the soil washing process was effective at removing other target PFAS.

The goal of >99% PFAS removal was not achieved in the stockpiles;

however, soil concentrations of PFBS, PFOA, and PFOS in coarse grained fractions of EIE382‐5 and EIE385‐4 achieved the performance goal, which was the primary driver for evaluating treatment performance.

These results confirmed that the simple treatment process employed for these stockpiles with low concentrations was sufficient to achieve soil concentration targets but Round 2 and Round 3 processes would be required to achieve >99% removal.

PFOS removal in coarse‐grained soil was relatively consistent across the three stockpiles. The sand fraction of EIE385‐4 was the only coarse fraction with a removal efficiency less than 94% (88.6%).

The fines for EIE385‐4 also had poor PFOS removal efficiency

(−7.7%) compared to the other two stockpiles (41.1%−61.8%). This negative removal efficiency was likely attributed to variability of

PFAS concentration in the fines during the washing process.

TABLE 4 EIE377‐1 and EIE379‐2 bench‐scale treatability PFAS removal efficiencies

Stockpile Soil fraction

PFOS Sum PFAS Soil Leachate Soil Leachate

RE% RE% RE% RE%

EIE377‐1 Rock 97.9% <DL 98.4% <DL

Sand 96.7% <DL 97.5% <DL

Fines 64.0% 89.6% 73.3% 78.0%

EIE379‐2 Rock <DL 99.1% <DL 96.3%

Sand 99.3% 99.9% 99.3% 98.4%

Fines 83.7% 99.8% 81.1% 97.1%

Abbreviations. <DL, No compounds were detected above the detection limit so sum of PFAS could not be calculated; PFAS, per‐ and polyfluoroalkyl substances; PFOS, perfluorooctane sulfonate; Sum of PFAS, sum of 24 PFAS compounds on Table B‐15, QSM 5.3 plus

N‐MeFOSA.

TABLE 5 EIE393‐1 bench‐scale treatability PFAS removal efficiencies

Treatment round Soil fraction

PFOS Sum PFAS Soil Leachate Soil Leachate μg/kg ng/L μg/kg ng/L

Round 1 Rock 96.7 99.4 96.9 99.3

Gravel 99.0 98.1 99.0 98.0

Sand 94.4 97.2 94.6 97.5

Fines 11.1 98.3 10.6 97.2

Round 2 Rock/gravel 99.7 99.6 99.7 99.5

Sand 99.6 99.8 99.6 99.5

Round 3 Rock/gravel/ sand

99.9 98.7 99.9 98.7

Abbreviations. <DL, No compounds were detected above the detection limit so Sum of PFAS could not be calculated; PFAS, per‐ and polyfluoroalkyl substances; PFOS, perfluorooctane sulfonate; Sum of PFAS, sum of 24 PFAS compounds on Table B‐15, QSM 5.3 plus

N‐MeFOSA.

160 | QUINNAN ET AL.

3.5 | Leachability

PFAS leachate results correlated to the soil PFAS results for the untreated and treated soil fractions, commonly showing approximately one order of magnitude higher soil concentrations for PFOS compared to the leachate concentrations. Leachability was reduced in all coarse‐grained fractions posttreatment with similar levels of PFAS removal to the soil. Untreated leachable PFOS results ranged from 193ng/L in EIE382‐5 to 22,000ng/L in EIE393‐1. The PFOS leachate concentration performance goals were achieved for EIE382‐5 and EIE385‐4. PFOS leachate ranged from less

TABLE 6 Incremental sampling methodology soil PFAS results

Stockpile Soil fraction PFBS (μg/kg) PFOA (μg/kg) PFOS (μg/kg) Sum PFAS (μg/kg)

EIE382‐5 Untreated <0.19 <0.19 <0.19 0.23 <0.19 0.28 J 5.1 4.6 3.9 7.03 6.1 5.38

Gravel <0.23 <0.23 <0.23 <0.23 <0.23 <0.23 <0.23 <0.23 <0.23 0.76 <DL <DL

Sand <0.23 <0.23 <0.23 <0.23 <0.23 <0.23 0.29 <0.23 <0.23 0.29 <DL <DL

Fines <0.24 <0.24 <0.24 <0.24 <0.24 <0.24 1.4 1.9 1.9 1.85 2.35 2.45

EIE385‐4 Untreated <0.20 0.22 J 0.22 J 0.61 J 0.81 J 0.94 J 6.7 8.9 9.1 12.12 16.56 17.305

Gravel <0.20 <0.20 <0.20 <0.20 <0.20 <0.20 <0.20 0.23 J <0.20 <DL 0.23 <DL

Sand <0.20 <0.20 <0.20 <0.20 <0.20 <0.20 1.1 0.85 J 0.87 J 1.47 1.26 1.22

Fines <0.21 <0.21 <0.21 <0.21 0.2 J 0.28 J 7.6 11 8 9.9 13.81 10.48

EIE393‐1 Untreated 1.1 1.4 1.6 4.8 6.1 7.1 420 520 740 514.18 636.29 874.17

Gravel <0.20 <0.20 <0.20 0.2 J 0.22 J 0.74 J 30 29 32 33.59 32.66 37.81

Sand <0.20 <0.20 <0.20 0.4 J 0.44 J 0.4 J 30 32 32 36.04 38.6 39.63

Fines 0.72 J 0.64 J 0.8 J 1.9 1.9 2.7 300 260 430 369.68 325.11 531.18

Performance Goal 1,900 1.7 3 ‐

Note: Bold values exceeded the performance goals.

Abbreviations: <DL, No compounds were detected above the detection limit so Sum of PFAS could not be calculated; J, estimated result, less than limit of quantitation and greater than detection limit; PFAS, per‐ and polyfluoroalkyl substances; PFBS, perfluorobutane sulfonate; PFOA, perfluorooctanoic acid;

PFOS, perfluorooctane sulfonate; Sum of PFAS, sum of 24 PFAS compounds on Table B‐15, QSM 5.3 plus N‐MeFOSA.

TABLE 7 Incremental sampling methodology soil leachate PFAS results

Stockpile Soil fraction PFBS (ng/L) PFOA (ng/L) PFOS (ng/L) Sum PFAS (ng/L)

EIE382‐5 Untreated <10 <10 <10 14 J 11 J <10 200 190 190 327 301 305

Gravel <10 <10 <10 <10 <10 <10 <10 <10 <10 <DL <DL <DL

Sand <10 <10 <10 <10 <10 <10 <10 <10 <10 110 29 <DL

Fines <10 <10 <10 <10 <10 <10 58 66 44 119 138 59

EIE385‐4 Untreated 12 J 11 J 12 J 51 50 48 430 440 420 873 863 739

Gravel <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 32 <10

Sand <10 <10 <10 <10 <10 <10 11 J 15 J 14 J 123 30 25

Fines <10 <10 <10 15 J 16 J 17 J 150 390 190 235 492 273

EIE393‐1 Untreated 52 63 32 J 280 300 150 21,000 28,000 17,000 24,860 32,726 19,608

Gravel ‐ <10 <10 ‐ 14 J 23 J ‐ 1700 1300 ‐ 2020 1567

Sand <10 <10 <10 31 J 34 J 30 J 800 920 800 1007 1186 1082

Fines 13 J 11 J 13 J 84 79 100 12,000 11,000 15,000 14,015 12,883 17,405.5

Performance goal 40 40 40 ‐

Note: Bold values exceeded the performance goals.

Abbreviations. <DL, No compounds were detected above the detection limit so Sum of PFAS could not be calculated; J, estimated result, less than limit of quantitation and greater than detection limit; PFAS, per‐ and polyfluoroalkyl substances; PFBS, perfluorobutane sulfonate; PFOA, perfluorooctanoic acid;

PFOS, perfluorooctane sulfonate; Sum of PFAS, sum of 24 PFAS compounds on Table B‐15, QSM 5.3 plus N‐MeFOSA; −, sample not analyzed due to processing error.

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than the detection limit to 13ng/L in the coarse fractions of these stockpiles. The leachable PFOS results in EIE393‐1 exceeded the performance goals and this result correlated to the higher soil PFAS concentrations in these soil fractions.

In the treated soil fractions, leachable PFOS was generally lowest in the gravel fraction and highest in the fines fraction, except

EIE393‐1, where the sand fraction had the lowest leachable PFOS concentration.

3.6 | Nontarget PFAS

EOF analysis was conducted to assess the potential of additional

PFAS compounds not included in the 24‐compound target list.

Results were intended to compare this to the organic fluorine equivalent detected from target PFAS analysis to determine the relative importance of nontarget PFAS. However, a combination of factors led to limited utility of the EOF analysis. For Stockpiles

TABLE 8 Incremental sampling methodology soil arithmetic mean results

Stockpile Soil fraction PFBS PFOA PFOS Sum PFAS Soil μg/kg Leachate ng/L Soil μg/kg Leachate ng/L Soil μg/kg Leachate ng/L Soil μg/kg Leachate ng/L

EIE382‐5 Untreated <0.19 <10 0.2 10.0 4.5 193 6 316

Gravel <0.23 <10 <0.23 <10 <0.23 <10 0.3 <DL

Sand <0.23 <10 <0.23 <10 0.2 <10 0.2 50

Fines <0.24 <10 <0.24 <10 1.7 56 2 109

EIE385‐4 Untreated 0.2 12 0.8 36 8 430 16 825

Gravel <0.20 <10 <0.20 <10 0.2 <10 0.2 17

Sand <0.20 <10 <0.20 <10 0.9 13 1.3 66

Fines <0.21 <10 0.2 16 9 243 12 345

EIE393‐1 Untreated 1.4 49 6.0 243 560 22,000 675 25,833

Gravel…