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Thermal Conduction Heating for Treatment of PFAS-Impacted Soil

Removal of Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) compounds from impacted soils is challenging due to the modest volatility and varying properties of most PFAS compounds. Thermal treatment technologies have been developed for treatment of semi-volatile compounds in soils such as dioxins, furans, poly-aromatic hydrocarbons and poly-chlorinated biphenyls at temperatures near 325°C. In controlled bench-scale testing, complete removal of targeted PFAS compounds to concentrations below reporting limits of 0.5 µg/kg was demonstrated at temperatures of 400°C[1]. Three field-scale thermal PFAS treatment projects that have been completed in the US include an in-pile treatment demonstration, an in situ vadose zone treatment demonstration and a larger scale treatment demonstration with excavated PFAS-impacted soil in a constructed pile. Based on the results, thermal treatment temperatures of at least 400°C and a holding time of 7-10 days are recommended for reaching local and federal PFAS soil standards. The energy requirement to treat typical wet soil ranges from 300 to 400 kWh per cubic yard, exclusive of heat losses which are scale dependent. Extracted vapors have been treated using condensation and granular activated charcoal filtration, with thermal and catalytic oxidation as another option which is currently being evaluated for field scale applications. Compared to other options such as soil washing, the ability to treat on site and to treat all soil fractions is an advantage.

Related Article(s):

Contributors: Gorm Heron, Emily Crownover, Patrick Joyce, Ramona Iery

Key Resource:

  • Perfluoroalkyl and polyfluoroalkyl substances thermal desorption evaluation[1]

Introduction

Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) have become prominent emerging contaminants in soil and groundwater. Soil source zones have been identified at locations where the chemicals were produced, handled or used. Few effective options exist for treatments that can meet local and federal soil standards. Over the past 30 plus years, thermal remediation technologies have grown from experimental and innovative prospects to mature and accepted solutions deployed effectively at many sites. More than 600 thermal case studies have been summarized by Horst and colleagues[2]. Thermal Conduction Heating (TCH) has been used for higher temperature applications such as removal of 1,4-Dioxane. This article reports recent experience with TCH treatment of PFAS-impacted soil.

Target Temperature and Duration

PFAS behave differently from most other organics subjected to TCH treatment. While the boiling points of individual PFAS fall in the range of 150-400°C, their chemical and physical behavior creates additional challenges. Some PFAS form ionic species in certain pH ranges and salts under other chemical conditions. This intricate behavior and our limited understanding of what this means for our ability to remove the PFAS from soils means that direct testing of thermal treatment options is warranted. Crownover and colleagues[1] subjected PFAS-laden soil to bench-scale heating to temperatures between 200 and 400°C which showed strong reductions of PFAS concentrations at 350°C and complete removal of many PFAS compounds at 400°C. The soil concentrations of targeted PFAS were reduced to nearly undetectable levels in this study.

Heating Method

For semi-volatile compounds such as dioxins, furans, poly-chlorinated biphenyls (PCBs) and Poly-Aromatic Hydrocarbons (PAH), thermal conduction heating has evolved as the dominant thermal technology because it is capable of achieving soil temperatures higher than the boiling point of water, which are necessary for complete removal of these organic compounds. Temperatures between 200 and 500°C have been required to achieve the desired reduction in contaminant concentrations[3]. TCH has become a popular technology for PFAS treatment because temperatures in the 400°C range are needed.

The energy source for TCH can be electricity (most commonly used), or fossil fuels (typically gas, diesel or fuel oil). Electrically powered TCH offers the largest flexibility for power input which also can be supplied by renewable and sustainable energy sources.

Energy Usage

Treating PFAS-impacted soil with heat requires energy to first bring the soil and porewater to the boiling point of water, then to evaporate the porewater until the soil is dry, and finally to heat the dry soil up to the target treatment temperature. The energy demand for wet soils falls in the 300-400 kWh/cy range, dependent on porosity and water saturation. Additional energy is consumed as heat is lost to the surroundings and by vapor treatment equipment, yielding a typical usage of 400-600 kWh/cy total for larger soil treatment volumes. Wetter soils and small treatment volumes drive the energy usage towards the higher number, whereas larger soil volumes and dry soil can be treated with less energy.

Vapor Treatment

During the TCH process a significant fraction of the PFAS compounds are volatilized by the heat and then removed from the soil by vacuum extraction. The vapors must be treated and eventually discharged while meeting local and/or federal standards. Two types of vapor treatment have been used in past TCH applications for organics: (1) thermal and catalytic oxidation and (2) condensation followed by granular activated charcoal (GAC) filtration. Due to uncertainties related to thermal destruction of fluorinated compounds and future requirements for treatment temperature and residence time, condensation and GAC filtration have been used in the first three PFAS treatment field demonstrations. It should be noted that PFAS compounds will stick to surfaces and that decontamination of the equipment is important. This could generate additional waste as GAC vessels, pipes and other wetted equipment need careful cleaning with solvents or rinsing agents such as PerfluorAdTM.

PFAS Reactivity and Fate

While evaluating initial soil treatment results, Crownover et al[1] noted the lack of complete data sets when the soils were analyzed for non-targeted compounds or extractable precursors. Attempts to establish the fluorine balance suggest that the final fate of the fluorine in the PFAS is not yet fully understood. Transformations are likely occurring in the heated soil as demonstrated in laboratory experiments with and without calcium hydroxide (Ca(OH)2) amendment[4]. Amendments such as Ca(OH)2 may be useful in reducing the required treatment temperature by catalyzing PFAS degradation. With thousands of PFAS potentially present, the interactions are complex and may never be fully understood. Therefore, successful thermal treatment may require a higher target temperature than for other organics with similar boiling points – simply to provide a buffer against the uncertainty.

Case Studies

Stock-pile Treatment, Eielson AFB, Alaska (ESTCP project ER20-5198[5])

Figure 1. TCH treatment of a PFAS-laden stockpile at Eielson AFB, Alaska[5]

Since there has been no approved or widely accepted method for treating soils impacted by PFAS, a common practice has been to excavate PFAS-impacted soil and place it in lined stockpiles. Eielson AFB in Alaska is an example where approximately 50 stockpiles were constructed to temporarily store 150,000 cubic yards of soil. One of the stockpiles containing 134 cubic yards of PFAS-impacted soil was heated to 350-450°C over 90 days (Figure 1). Volatilized PFAS was extracted from the soil using vacuum extraction and treated via condensation and filtration by granular activated charcoal. Under field conditions, PFAS concentration reductions from 230 µg/kg to below 0.5 µg/kg were demonstrated for soils that reached 400°C or higher for 7 days. These soils achieved the Alaska soil standards of 3 µg/kg for PFOS and 1.7 µg/kg for PFOA. Cooler soils near the top of the stockpile had remaining PFOS in the range of 0.5-20 µg/kg with an overall average of 4.1 µg/kg. Sampling of all soils heated to 400°C or higher demonstrated that the soils achieved undetectable levels of targeted PFAS (typical reporting limit was 0.5 µg/kg).

In situ Vadose Zone Treatment, Beale AFB, California (ESTCP project ER20-5250[6])

Figure 2. In situ TCH treatment of a PFAS-rich vadose zone hotspot at Beale AFB, California

A former fire-training area at Beale AFB had PFAS concentrations as high as 1,970 µg/kg in shallow soils. In situ treatment of a PFAS-rich soil was demonstrated using 16 TCH borings installed in the source area to a depth of 18 ft (Figure 2). Soils which reached the target temperatures were reduced to PFAS concentrations below 1 µg/kg. Perched water which entered in one side of the area delayed heating in that area, and soils which were affected had more modest PFAS concentration reductions. As a lesson learned, future in situ TCH treatments will include provisions for minimizing water entering the treated volume[6]. It was demonstrated that with proper water management, even highly impacted soils can be treated to near non-detect concentrations (greater than 99% reduction).

Constructed Pile Treatment, JBER, Alaska (ESTCP Project ER23-8369[7])

Figure 3. Treatment of a 2,000 cubic yard soil pile at JBER, Alaska

In 2024, a stockpile of 2,000 cubic yards of PFAS-impacted soil was thermally treated at Joint Base Elmendorf-Richardson (JBER) in Anchorage, Alaska[7]. This ESTCP project was implemented in partnership with DOD’s Defense Innovation Unit (DIU). Three technology demonstrations were conducted at the site where approximately 6,000 cy of PFAS-impacted soil was treated (TCH, smoldering and kiln-style thermal desorption). Figure 3 shows the fully constructed pile used for the TCH demonstration. In August 2024 the soil temperature for the TCH treatment exceeded 400°C in all monitoring locations. At an energy density of 355 kWh/cy, Alaska Department of Environmental Conservation (ADEC) standards and EPA Residential Regional Screening Levels (RSLs) for PFAS in soil were achieved. At JBER, all 30 post-treatment soil samples were near or below detection limits for all targeted PFAS compounds using EPA Method 1633. The composite of all 30 soil samples was below all detection limits for EPA Method 1633. Detection limits ranged from 0.0052 µg/kg to 0.19 µg/kg.

Advantages and Disadvantages

Thermal treatment of PFAS in soils is energy intensive, and the cost of that energy may be prohibitive for some clients. Also, while it often is the least costly option for complete PFAS removal when compared to excavation followed by offsite disposal or destruction, heating soil to treatment temperatures on site or in situ typically takes longer than excavation. Major advantages include:

  • On site or in situ treatment eliminates the need to transport and dispose of the contaminated soil
  • Site liabilities are removed once and for all
  • Treatment costs are competitive with excavation, transportation and off-site treatment or disposal.

Recommendations

Range Runoff Treatment Technology Components

Based on the conceptual foundation of previous research into surface water runoff treatment for other contaminants, with a goal to “trap and treat” the target compounds, the following components were selected for inclusion in the technology developed to address range runoff contaminated with energetic compounds.

Peat

Previous research demonstrated that a peat-based system provided a natural and sustainable sorptive medium for organic explosives such as HMX, RDX, and TNT, allowing much longer residence times than predicted from hydraulic loading alone[8][9][10][11][12]. Peat moss represents a bioactive environment for treatment of the target contaminants. While the majority of the microbial reactions are aerobic due to the presence of measurable dissolved oxygen in the bulk solution, anaerobic reactions (including methanogenesis) can occur in microsites within the peat. The peat-based substrate acts not only as a long term electron donor as it degrades but also acts as a strong sorbent. This is important in intermittently loaded systems in which a large initial pulse of MC can be temporarily retarded on the peat matrix and then slowly degraded as they desorb[10][12]. This increased residence time enhances the biotransformation of energetics and promotes the immobilization and further degradation of breakdown products. Abiotic degradation reactions are also likely enhanced by association with the organic-rich peat (e.g., via electron shuttling reactions of humics)[13].

Soybean Oil

Modeling has indicated that peat moss amended with crude soybean oil would significantly reduce the flux of dissolved TNT, RDX, and HMX through the vadose zone to groundwater compared to a non-treated soil (see ESTCP ER-200434). The technology was validated in field soil plots, showing a greater than 500-fold reduction in the flux of dissolved RDX from macroscale Composition B detonation residues compared to a non-treated control plot[10]. Laboratory testing and modeling indicated that the addition of soybean oil increased the biotransformation rates of RDX and HMX at least 10-fold compared to rates observed with peat moss alone[12]. Subsequent experiments also demonstrated the effectiveness of the amended peat moss material for stimulating perchlorate transformation when added to a highly contaminated soil (Fuller et al., unpublished data). These previous findings clearly demonstrate the effectiveness of peat-based materials for mitigating transport of both organic and inorganic energetic compounds through soil to groundwater.

Biochar

Recent reports have highlighted additional materials that, either alone, or in combination with electron donors such as peat moss and soybean oil, may further enhance the sorption and degradation of surface runoff contaminants, including both legacy energetics and insensitive high explosives (IHE). For instance, biochar, a type of black carbon, has been shown to not only sorb a wide range of organic and inorganic contaminants including MCs[14][15][16][17], but also to facilitate their degradation[18][19][20][21][22][23]. Depending on the source biomass and pyrolysis conditions, biochar can possess a high specific surface area (on the order of several hundred m2/g)[24][25] and hence a high sorption capacity. Biochar and other black carbon also exhibit especially high affinity for nitroaromatic compounds (NACs) including TNT and 2,4-dinitrotoluene (DNT)[26][27][28]. This is due to the strong π-π electron donor-acceptor interactions between electron-rich graphitic domains in black carbon and the electron-deficient aromatic ring of the NAC[27][28]. These characteristics make biochar a potentially effective, low cost, and sustainable sorbent for removing MC and other contaminants from surface runoff and retaining them for subsequent degradation in situ.

Furthermore, black carbon such as biochar can promote abiotic and microbial transformation reactions by facilitating electron transfer. That is, biochar is not merely a passive sorbent for contaminants, but also a redox mediator for their degradation. Biochar can promote contaminant degradation through two different mechanisms: electron conduction and electron storage[29].

First, the microscopic graphitic regions in biochar can adsorb contaminants like NACs strongly, as noted above, and also conduct reducing equivalents such as electrons and atomic hydrogen to the sorbed contaminants, thus promoting their reductive degradation. This catalytic process has been demonstrated for TNT, DNT, RDX, HMX, and nitroglycerin[30][31][32][20][22] and is expected to occur also for IHE including DNAN and NTO.

Second, biochar contains in its structure abundant redox-facile functional groups such as quinones and hydroquinones, which are known to accept and donate electrons reversibly. Depending on the biomass and pyrolysis temperature, certain biochar can possess a rechargeable electron storage capacity (i.e., reversible electron accepting and donating capacity) on the order of several millimoles e–/g[33][34][35]. This means that when "charged", biochar can provide electrons for either abiotic or biotic degradation of reducible compounds such as MC. The abiotic reduction of DNT and RDX mediated by biochar has been demonstrated[21] and similar reactions are expected to occur for DNAN and NTO as well. Recent studies have shown that the electron storage capacity of biochar is also accessible to microbes. For example, soil bacteria such as Geobacter and Shewanella species can utilize oxidized (or "discharged") biochar as an electron acceptor for the oxidation of organic substrates such as lactate and acetate[36][37] and reduced (or "charged") biochar as an electron donor for the reduction of nitrate[37]. This is significant because, through microbial access of stored electrons in biochar, contaminants that do not sorb strongly to biochar can still be degraded.

Similar to nitrate, perchlorate and other relatively water-soluble energetic compounds (e.g., NTO and NQ) may also be similarly transformed using reduced biochar as an electron donor. Unlike other electron donors, biochar can be recharged through biodegradation of organic substrates[37] and thus can serve as a long-lasting sorbent and electron repository in soil. Similar to peat moss, the high porosity and surface area of biochar not only facilitate contaminant sorption but also create anaerobic reducing microenvironments in its inner pores, where reductive degradation of energetic compounds can take place.

Other Sorbents

Chitin and unmodified cellulose were predicted by Density Functional Theory methods to be favorable for absorption of NTO and NQ, as well as the legacy explosives[38]. Cationized cellulosic materials (e.g., cotton, wood shavings) have been shown to effectively remove negatively charged energetics like perchlorate and NTO from solution[39]. A substantial body of work has shown that modified cellulosic biopolymers can also be effective sorbents for removing metals from solution[40][41][42][43] and therefore will also likely be applicable for some of the metals that may be found in surface runoff at firing ranges.

Technology Evaluation

Based on the properties of the target munition constituents, a combination of materials was expected to yield the best results to facilitate the sorption and subsequent biotic and abiotic degradation of the contaminants.

Sorbents

Table 1. Freundlich and Langmuir adsorption parameters for insensitive and legacy explosives
Compound Freundlich Langmuir
Parameter Peat CAT Pine CAT Burlap CAT Cotton Parameter Peat CAT Pine CAT Burlap CAT Cotton
HMX Kf 0.08 +/- 0.00 -- -- -- qm (mg/g) 0.29 +/- 0.04 -- -- --
n 1.70 +/- 0.18 -- -- -- b (L/mg) 0.39 +/- 0.09 -- -- --
r2 0.91 -- -- -- r2 0.93 -- -- --
RDX Kf 0.11 +/- 0.02 -- -- -- qm (mg/g) 0.38 +/- 0.05 -- -- --
n 2.75 +/- 0.63 -- -- -- b (L/mg) 0.23 +/- 0.08 -- -- --
r2 0.69 -- -- -- r2 0.69 -- -- --
TNT Kf 1.21 +/- 0.15 1.02 +/- 0.04 0.36 +/- 0.02 -- qm (mg/g) 3.63 +/- 0.18 1.26 +/- 0.06 -- --
n 2.78 +/- 0.67 4.01 +/- 0.44 1.59 +/- 0.09 -- b (L/mg) 0.89 +/- 0.13 0.76 +/- 0.10 -- --
r2 0.81 0.93 0.98 -- r2 0.97 0.97 -- --
NTO Kf -- 0.94 +/- 0.05 0.41 +/- 0.05 0.26 +/- 0.06 qm (mg/g) -- 4.07 +/- 0.26 1.29 +/- 0.12 0.83 +/- .015
n -- 1.61 +/- 0.11 2.43 +/- 0.41 2.53 +/- 0.76 b (L/mg) -- 0.30 +/- 0.04 0.36 +/- 0.08 0.30 +/- 0.15
r2 -- 0.97 0.82 0.57 r2 -- 0.99 0.89 0.58
DNAN Kf 0.38 +/- 0.05 0.01 +/- 0.01 -- -- qm (mg/g) 2.57 +/- 0.33 -- -- --
n 1.71 +/- 0.20 0.70 +/- 0.13 -- -- b (L/mg) 0.13 +/- 0.03 -- -- --
r2 0.89 0.76 -- -- r2 0.92 -- -- --
ClO4 Kf -- 1.54 +/- 0.06 0.53 +/- 0.03 -- qm (mg/g) -- 3.63 +/- 0.18 1.26 +/- 0.06 --
n -- 2.42 +/- 0.16 2.42 +/- 0.26 -- b (L/mg) -- 0.89 +/- 0.13 0.76 +/- 0.10 --
r2 -- 0.97 0.92 -- r2 -- 0.97 0.97 --
Notes:
-- Indicates the algorithm failed to converge on the model fitting parameters, therefore there was no successful model fit.
CAT Indicates cationized material.

The materials screened included Sphagnum peat moss, primarily for sorption of HMX, RDX, TNT, and DNAN, as well as cationized cellulosics for removal of perchlorate and NTO. The cationized cellulosics that were examined included: pine sawdust, pine shavings, aspen shavings, cotton linters (fine, silky fibers which adhere to cotton seeds after ginning), chitin, chitosan, burlap (landscaping grade), coconut coir, raw cotton, raw organic cotton, cleaned raw cotton, cotton fabric, and commercially cationized fabrics.

As shown in Table 1[39], batch sorption testing indicated that a combination of Sphagnum peat moss and cationized pine shavings provided good removal of both the neutral organic energetics (HMX, RDX, TNT, DNAN) as well as the negatively charged energetics (perchlorate, NTO).

Slow Release Carbon Sources

Table 2. Slow-release Carbon Sources
Material Abbreviation Commercial Source Notes
polylactic acid PLA6 Goodfellow high molecular weight thermoplastic polyester
polylactic acid PLA80 Goodfellow low molecular weight thermoplastic polyester
polyhydroxybutyrate PHB Goodfellow bacterial polyester
polycaprolactone PCL Sarchem Labs biodegradable polyester
polybutylene succinate BioPBS Mitsubishi Chemical Performance Polymers compostable bio-based product
sucrose ester of fatty acids SEFA SP10 Sisterna food and cosmetics additive
sucrose ester of fatty acids SEFA SP70 Sisterna food and cosmetics additive

A range of biopolymers widely used in the production of biodegradable plastics were screened for their ability to support aerobic and anoxic biodegradation of the target munition constituents. These compounds and their sources are listed in Table 2.

Figure 3. Schematic of interactions between biochar and munitions constituents

Multiple pure bacterial strains and mixed cultures were screened for their ability to utilize the solid biopolymers as a carbon source to support energetic compound transformation and degradation. Pure strains included the aerobic RDX degrader Rhodococcus species DN22 (DN22 henceforth)[44] and Gordonia species KTR9 (KTR9 henceforth)[44], the anoxic RDX degrader Pseudomonas fluorencens species I-C (I-C henceforth)[45][46], and the aerobic NQ degrader Pseudomonas extremaustralis species NQ5 (NQ5 henceforth)[47]. Anaerobic mixed cultures were obtained from a membrane bioreactor (MBR) degrading a mixture of six explosives (HMX, RDX, TNT, NTO, NQ, DNAN), as well as perchlorate and nitrate[48]. The results indicated that the slow-release carbon sources polyhydroxybutyrate (PHB), polycaprolactone (PCL), and polybutylene succinate (BioPBS) were effective for supporting the biodegradation of the mixture of energetics.

Biochar

Figure 4. Composition of the columns during the sorption-biodegradation experiments
Figure 5. Representative breakthrough curves of energetics during the second replication of the column sorption-biodegradation experiment

The ability of biochar to sorb and abiotically reduce legacy and insensitive munition constituents, as well as biochar’s use as an electron donor for microbial biodegradation of energetic compounds was examined. Batch experiments indicated that biochar was a reasonable sorbent for some of the energetics (RDX, DNAN), but could also serve as both an electron acceptor and an electron donor to facilitate abiotic (RDX, DNAN, NTO) and biotic (perchlorate) degradation (Figure 3)[49].

Sorption-Biodegradation Column Experiments

The selected materials and cultures discussed above, along with a small amount of range soil and crushed oyster shell as a slow-release pH buffering agent, were packed into columns, and a steady flow of dissolved energetics was passed through the columns. The composition of the four columns is presented in Figure 4. The influent and effluent concentrations of the energetics was monitored over time. The column experiment was performed twice. As seen in Figure 5, there was sustained almost complete removal of RDX and ClO4-, and more removal of the other energetics in the bioactive columns compared to the sorption only columns, over the course of the experiments. For reference, 100 PV is approximately equivalent to three months of operation. The higher effectiveness of sorption with biodegradation compared to sorption only is further illustrated in Figure 6, where the energetics mass removal in the bioactive columns was shown to be 2-fold (TNT) to 20-fold (RDX) higher relative to that observed in the sorption only column. The mass removal of HMX and NQ were both over 40% higher with biochar added to the sorption with biodegradation treatment, although biochar showed little added benefit for removal of other energetics tested.

Trap and Treat Technology

Figure 6. Energetic mass removal relative to the sorption only removal during the column sorption-biodegradation experiments. Dashed line given for reference to C1 removal = 1.

These results provide a proof-of-concept for the further development of a passive and sustainable “trap-and-treat” technology for remediation of energetic compounds in stormwater runoff at military testing and training ranges. At a given site, the stormwater runoff would need to be fully characterized with respect to key parameters (e.g., pH, major anions), and site specific treatability testing would be recommended to assure there was nothing present in the runoff that would reduce performance. Effluent monitoring on a regular basis would also be needed (and would be likely be expected by state and local regulators) to assess performance decline over time.

The components of the technology would be predominantly peat moss and cationized pine shavings, supplemented with biochar, ground oyster shell, the biopolymer carbon sources, and the bioaugmentation cultures. The entire mix would likely be emplaced in a concrete vault at the outflow end of the stormwater runoff retention basin at the contaminated site. The deployed treatment system would have further design elements, such as a system to trap and retain suspended solids in the runoff in order to minimize clogging the matrix. the inside of the vault would be baffled to maximize the hydraulic retention time of the contaminated runoff. The biopolymer carbon sources and oyster shell may need be refreshed periodically (perhaps yearly) to maintain performance. However, a complete removal and replacement of the base media (peat moss, CAT pine) would not be advised, as that would lead to a loss of the acclimated biomass.

Summary

Novel sorbents and slow-release carbon sources can be an effective way to promote the sorption and biodegradation of a range of legacy and insensitive munition constituents from surface runoff, and the added benefits of biochar for both sorption and biotic and abiotic degradation of these compounds was demonstrated. These results establish a foundation for a passive, sustainable surface runoff treatment technology for both active and inactive military ranges.

References

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