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| − | ==PFAS Transport and Fate== | + | ==Thermal Conduction Heating for Treatment of PFAS-Impacted Soil== |
| − | The transport and fate of Per- and Polyfluoroalkyl Substances (PFAS) in the environment is controlled by the nature of the PFAS source, characteristics of the individual PFAS, and environmental conditions where the PFAS are present. Transport, partitioning, and transformation are the primary processes controlling PFAS fate in the environment. PFAS compounds can also be taken up by both plants and animals, and in some cases, bioaccumulate through the food chain.
| + | 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<ref name="CrownoverEtAl2019"> Crownover, E., Oberle, D., Heron, G., Kluger, M., 2019. Perfluoroalkyl and polyfluoroalkyl substances thermal desorption evaluation. Remediation Journal, 29(4), pp. 77-81. [https://doi.org/10.1002/rem.21623 doi: 10.1002/rem.21623]</ref>. 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. |
| − | Understanding PFAS transport and fate is necessary for evaluating the potential risk from a PFAS release and for predictions about PFAS occurrence, migration, and persistence, and about the potential vectors for exposure. This knowledge is important for site characterization, identification of potential sources of PFAS to the site, development of an appropriate conceptual site model (CSM), and selection and predicted performance of remediation strategies.
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| − | '''Related Article(s): ''' | + | '''Related Article(s):''' |
| − | * [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]
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| − | '''Contributor(s): '''
| + | *[[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]] |
| − | Dr. Hunter Anderson and Dr. Mark L. Brusseau
| + | *[[Thermal Conduction Heating (TCH)]] |
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| − | '''Key Resource(s): ''' | + | '''Contributors:''' Gorm Heron, Emily Crownover, Patrick Joyce, Ramona Iery |
| − | *[https://pfas-1.itrcweb.org/wp-content/uploads/2020/04/ITRC_PFAS_TechReg_April2020.pdf Per- and Polyfluoroalkyl Substances (PFAS), PFAS-1. ITRC 2020.]<ref name="ITRC2020">Interstate Technology and Regulatory Council (ITRC), 2020. Technical/Regulatory Guidance: Per- and Polyfluoroalkyl Substances (PFAS), PFAS-1. ITRC, PFAS Team, Washington DC. [https://pfas-1.itrcweb.org/wp-content/uploads/2020/04/ITRC_PFAS_TechReg_April2020.pdf Free Download from ITRC]. [[Media: ITRC_PFAS-1.pdf | Report.pdf]]</ref>
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| − | *[[Media: Brusseau2018manuscript.pdf | Assessing the Potential Contributions of Additional Retention Processes to PFAS Retardation in the Subsurface. Brusseau 2018 (manuscript).]]<ref name="Brusseau2018">Brusseau, M.L., 2018. Assessing the Potential Contributions of Additional Retention Processes to PFAS Retardation in the Subsurface. Science of the Total Environment, 613-614, pp. 176-185. [https://doi.org/10.1016/j.scitotenv.2017.09.065 DOI: 10.1016/j.scitotenv.2017.09.065] [[Media: Brusseau2018manuscript.pdf | Author’s Manuscript]]</ref> | + | '''Key Resource:''' |
| | + | *Perfluoroalkyl and polyfluoroalkyl substances thermal desorption evaluation<ref name="CrownoverEtAl2019"/> |
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| | ==Introduction== | | ==Introduction== |
| − | The transport and fate of [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]] is a rapidly evolving field of science, with many questions that are not yet resolved. Much of the currently available information is based on a few well-studied PFAS compounds. However, there is a large number and variety of PFAS with a wide range of physical and chemical characteristics that affect their behavior in the environment. The transport and fate of some PFAS could differ significantly from the compounds studied to date. Nevertheless, information about the behavior of some PFAS in the environment can be ascertained from the results of currently available research.
| + | [[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<ref name="HorstEtAl2021">Horst, J., Munholland, J., Hegele, P., Klemmer, M., Gattenby, J., 2021. In Situ Thermal Remediation for Source Areas: Technology Advances and a Review of the Market From 1988–2020. Groundwater Monitoring & Remediation, 41(1), p. 17. [https://doi.org/10.1111/gwmr.12424 doi: 10.1111/gwmr.12424] [[Media: gwmr.12424.pdf | Open Access Manuscript]]</ref>. [[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. |
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| − | PFAS transport and fate in the environment is controlled by the nature of the PFAS source, characteristics of the individual PFAS, and environmental conditions where the PFAS are present. Perfluoroalkyl acids (PFAAs) (see [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) | PFAS]] for nomenclature) are strong acids and are anionic in the environmentally-relevant pH range. They are extremely persistent in the environment and do not degrade or transform under typical environmental conditions. Polyfluoroalkyl substances (see [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) | PFAS]] for nomenclature) include compounds that have the potential to degrade to PFAAs. These compounds are commonly referred to as PFAA precursors or just ‘precursors’. Because some polyfluoroalkyl substances can degrade into PFAA via biotic or abiotic degradation pathways, PFAAs are sometimes referred to as “terminal PFAS” or “terminal degradation products”.
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| − | The most important molecular properties controlling PFAA transport are the carbon chain length and functional moieties of the headgroups (e.g., sulfonate, carboxylate). The molecular properties of PFAA precursors are more varied, with different carbon chain lengths, headgroups and ionic states<ref name="Buck2011">Buck, R.C., Franklin, J., Berger, U., Conder, J.M., Cousins, I.T., de Voogt, P., Jensen, A.A., Kannan, K., Mabury, S.A., and van Leeuwen, S.P.J., 2011. Perfluoroalkyl and Polyfluoroalkyl Substances in the Environment: Terminology, Classification, and Origins. Integrated Environmental Assessment and Management, 7(4): pp. 513-541. [ https://doi.org/10.1002/ieam.258 DOI: 10.1002/ieam.258] [https://setac.onlinelibrary.wiley.com/doi/epdf/10.1002/ieam.258 Open Access Article]</ref><ref name="Wang2017">Wang, Z., DeWitt, J.C., Higgins, C.P., and Cousins, I.T., 2017. A Never-Ending Story of Per- and Polyfluoroalkyl Substances (PFASs)? Environmental Science and Technology, 51(5), pp. 2508-2518. American Chemical Society. [https://doi.org/10.1021/acs.est.6b04806 DOI: 10.1021/acs.est.6b04806] [https://pubs.acs.org/doi/pdf/10.1021/acs.est.6b04806 Free Download from ACS]</ref> (see [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) | PFAS]]). All of these properties can influence transport and fate of PFAA precursors in the environment.
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| − | Important environmental characteristics include the nature of the source (mode of input into the environment), the length of time that the source was active, and the magnitude of the input, as well as precipitation and infiltration rates, depth to groundwater, surface water and groundwater flow rates and interactions, prevailing atmospheric conditions, the properties of the porous-media (e.g., soil and sediment) and aqueous solution, microbiological factors, and the presence of additional fluid phases such as air and non-aqueous phase liquids [[Wikipedia: Non-aqueous phase liquid | (NAPLs)]] in the vadose zone and water-saturated source. In the subsurface, soil characteristics (texture, organic carbon content, clay mineralogy, metal-oxide content, solid surface area, surface charge, and exchange capacity) and solution characteristics (pH, redox potential, major ion chemistry, and co-contaminants) can influence PFAS transport and fate.
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| − | ==PFAS Transport and Fate Processes==
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| − | [[File:AndersonBrusseau1w2Fig1.png | thumb | 600px | Figure 1. Illustration of PFAS partitioning and transformation processes. Source: D. Adamson, GSI, used with permission.]]
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| − | Transport, partitioning, and transformation are the primary processes controlling PFAS fate in the environment (Figure 1). PFAS compounds can also be taken up by both plants and animals, and in some cases, bioaccumulate through the food chain. However, PFAS uptake and bioaccumulation is not discussed in this article (see “Environmental Concern” section of [[Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)]]).
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| − | * '''Transport:''' PFAS can be transported substantial distances in the atmosphere<ref name="Ahrens2012">Ahrens, L., Harner, T., Shoeib, M., Lane, D.A. and Murphy, J.G., 2012. Improved Characterization of Gas–Particle Partitioning for Per- and Polyfluoroalkyl Substances in the Atmosphere Using Annular Diffusion Denuder Samplers. Environmental Science and Technology, 46(13), pp. 7199-7206. [https://doi.org/10.1021/es300898s DOI: 10.1021/es300898s] Free download available from [https://www.researchgate.net/profile/Tom_Harner/publication/225046057_Improved_Characterization_of_Gas-Particle_Partitioning_for_Per-_and_Polyfluoroalkyl_Substances_in_the_Atmosphere_Using_Annular_Diffusion_Denuder_Samplers/links/5cc730c4299bf12097893fdc/Improved-Characterization-of-Gas-Particle-Partitioning-for-Per-and-Polyfluoroalkyl-Substances-in-the-Atmosphere-Using-Annular-Diffusion-Denuder-Samplers.pdf ResearchGate].</ref>, surface water<ref name="Taniyasu2013">Taniyasu, S., Yamashita, N., Moon, H.B., Kwok, K.Y., Lam, P.K., Horii, Y., Petrick, G. and Kannan, K., 2013. Does wet precipitation represent local and regional atmospheric transportation by perfluorinated alkyl substances? Environment International, 55, pp. 25-32. [https://doi.org/10.1016/j.envint.2013.02.005 DOI: 10.1016/j.envint.2013.02.005]</ref>, soil<ref name="Braunig2017">Bräunig, J., Baduel, C., Heffernan, A., Rotander, A., Donaldson, E. and Mueller, J.F., 2017. Fate and redistribution of perfluoroalkyl acids through AFFF-impacted groundwater. Science of the Total Environment, 596, pp. 360-368. [https://doi.org/10.1016/j.scitotenv.2017.04.095 DOI: 10.1016/j.scitotenv.2017.04.095]</ref>, and groundwater<ref name="Weber2017">Weber, A.K., Barber, L.B., LeBlanc, D.R., Sunderland, E.M. and Vecitis, C.D., 2017. Geochemical and Hydrologic Factors Controlling Subsurface Transport of Poly- and Perfluoroalkyl Substances, Cape Cod, Massachusetts. Environmental Science and Technology, 51(8), pp. 4269-4279. [https://doi.org/10.1021/acs.est.6b05573 DOI: 10.1021/acs.est.6b05573] [https://bgc.seas.harvard.edu/assets/weber2017_final.pdf Free Download]</ref>. The primary mechanisms controlling PFAS transport are [[Wikipedia:Advection | advection]] and [[Wikipedia:Dispersive_mass_transfer | dispersion]], similar to other dissolved compounds. For additional information on transport in groundwater, see [[Advection and Groundwater Flow]] and [[Dispersion and Diffusion]].
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| − | * '''Partitioning:''' Partitioning of PFAS between the mobile and immobile phases is one of the most important processes controlling the rate of migration in the environment. The primary mobile phases are typically air and water. Relatively immobile phases include stream sediments, soils, aquifer material, NAPLs, and interfaces between different phases (air-water, NAPL-water). Partitioning of a significant portion of the PFAS mass into an immobile phase increases the amount of material stored in the system and slows the apparent rate of migration in the mobile phase – a phenomenon that has been observed in field metadata<ref name="Anderson2019">Anderson, R.H., Adamson, D.T. and Stroo, H.F., 2019. Partitioning of poly-and perfluoroalkyl substances from soil to groundwater within aqueous film-forming foam source zones. Journal of Contaminant Hydrology, 220, pp. 59-65. [https://doi.org/10.1016/j.jconhyd.2018.11.011 DOI: 10.1016/j.jconhyd.2018.11.011] Manuscript available from [https://www.researchgate.net/profile/Hans_Stroo3/publication/329227107_Partitioning_of_poly-_and_perfluoroalkyl_substances_from_soil_to_groundwater_WITHIN_aqueous_film-forming_foam_source_zones/links/5e56996b299bf1bdb83e2f69/Partitioning-of-poly-and-perfluoroalkyl-substances-from-soil-to-groundwater-WITHIN-aqueous-film-forming-foam-source-zones.pdf ResearchGate]</ref>.
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| − | * '''Transformation:''' Transformation of PFAS is controlled by the molecular structure of the individual compounds. Perfluorinated compounds, including PFAAs, are resistant to abiotic and biotic transformation reactions under typical conditions and highly persistent in the environment. In contrast, precursors can be transformed by both abiotic and biotic processes, often resulting in the production of so-called “terminal” PFAA daughter products.
| + | ==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<ref name="CrownoverEtAl2019"/> 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. |
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| − | ==Transport and Partitioning in the Atmosphere== | + | ==Heating Method== |
| − | Air serves as a transport media for PFAS, particularly for uncharged polyfluorinated PFAS. Airborne PFAS transport contributes to global distribution and can lead to localized deposition to soils and surface water in the vicinity of emission sources<ref name="Simcik2005">Simcik, M.F. and Dorweiler, K.J., 2005. Ratio of Perfluorochemical Concentrations as a Tracer of Atmospheric Deposition to Surface Waters. Environmental Science and Technology, 39(22), pp. 8678-8683. [https://doi.org/10.1021/es0511218 DOI: 10.1021/es0511218] Free download available from [https://www.researchgate.net/profile/Matt_Simcik/publication/7444956_Ratio_of_Perfluorochemical_Concentrations_as_a_Tracer_of_Atmospheric_Deposition_to_Surface_Waters/links/5f035861299bf1881603c3be/Ratio-of-Perfluorochemical-Concentrations-as-a-Tracer-of-Atmospheric-Deposition-to-Surface-Waters.pdf ResearchGate]</ref><ref name="Prevedouros2006">Prevedouros, K., Cousins, I.T., Buck, R.C. and Korzeniowski, S.H., 2006. Sources, Fate and Transport of Perfluorocarboxylates. Environmental Science and Technology, 40(1), pp. 32-44. [https://doi.org/10.1021/es0512475 DOI: 10.1021/es0512475] Free download available from [https://d1wqtxts1xzle7.cloudfront.net/39945519/Sources_Fate_and_Transport_of_Perfluoroc20151112-1647-19vcvbf.pdf?1447365456=&response-content-disposition=inline%3B+filename%3DSources_Fate_and_Transport_of_Perfluoroc.pdf&Expires=1605023809&Signature=Z6KqgaDN6lKdAazoe6qoASoCtVystG5i~5EnrTcb~qMg3xZPz4O49Kghh62WmMzqEKE788~6EwrnlBVo9o6cM0hjf2vymFYxg4mx-eSIOEonfFjk6RonSaWp5gRbA6m~SNjwsjaKXID3OQyWIlLVpUd2LzAdI5rLGFA~gIXXtNPyCArLuGn-kbPYUIcBUg5TIkTZ6TDLXF~ujmzK9tNv~55UYabsJL4pmwIGC2sNGkEyJrYMfU577fbactdrmQXTJH7XbgpfDSfd4-xWkDZTdvVf~TypDDqUCZdtCkY8wINdpqtfe1KEzLrAj7rxxALAHUYxlVbPB45XTkLAGe5qww__&Key-Pair-Id=APKAJLOHF5GGSLRBV4ZA Academia]</ref><ref name="Ahrens2011">Ahrens, L., Shoeib, M., Harner, T., Lane, D.A., Guo, R. and Reiner, E.J., 2011. Comparison of Annular Diffusion Denuder and High Volume Air Samplers for Measuring Per- and Polyfluoroalkyl Substances in the Atmosphere." Analytical Chemistry, 83(24), pp. 9622-9628. [https://doi.org/10.1021/ac202414w DOI: 10.1021/ac202414w] Free download available from [https://www.informea.org/sites/default/files/imported-documents/UNEP-POPS-POPRC11FU-SUBM-PFOA-Canada-2-20151211.En.pdf Informea].</ref><ref name="Rauert2018">Rauert, C., Shoieb, M., Schuster, J.K., Eng, A. and Harner, T., 2018. Atmospheric concentrations and trends of poly-and perfluoroalkyl substances (PFAS) and volatile methyl siloxanes (VMS) over 7 years of sampling in the Global Atmospheric Passive Sampling (GAPS) network. Environmental Pollution, 238, pp. 94-102. [https://doi.org/10.1016/j.envpol.2018.03.017 DOI: 10.1016/j.envpol.2018.03.017] Open access article available from [https://reader.elsevier.com/reader/sd/pii/S0269749117352521?token=4C770E6E8AEDB0B3BA6A1D5B2C20ED5385F81823612551FA3380AAA1DA7A978F9CB36834AF6B7F91F35FF57E32013252 ScienceDirect] [[Media:Rauert2018.pdf | Report.pdf]]</ref>.
| + | 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<ref name="StegemeierVinegar2001">Stegemeier, G.L., Vinegar, H.J., 2001. Thermal Conduction Heating for In-Situ Thermal Desorption of Soils. Ch. 4.6, pp. 1-37. In: Chang H. Oh (ed.), Hazardous and Radioactive Waste Treatment Technologies Handbook, CRC Press, Boca Raton, FL. ISBN 9780849395864 [[Media: StegemeierVinegar2001.pdf | Open Access Article]]</ref>. TCH has become a popular technology for PFAS treatment because temperatures in the 400°C range are needed. |
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| − | PFAAs, which are ionic and possess a negative charge under ambient environmental conditions, are far less volatile than many other groundwater contaminants. An online database of vapor pressures and Henry’s Law constants for different PFAS, including PFAAs, is maintained by the Interstate Technology Regulatory Council<ref name="ITRC2020"/>. In general, vapor pressures of PFAS are low and water solubilities are high, limiting partitioning from water to air<ref name="ITRC2020"/>. However, under certain conditions, particularly within industrial stack emissions, PFAS can be transported through the atmosphere in both the gas phase and associated with fugitive particulates. In particular, volatile compounds including fluorotelomer alcohols (FTOHs) may be present in the gas phase, whereas, PFAAs can aerosolize and be transported as particulates<ref name="Ahrens2012"/>. In addition, precursors can be transformed to PFAAs in the atmosphere, which can result in PFAA deposition.
| + | 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. |
| − | Short-range atmospheric transport and deposition can result in PFAS contamination in terrestrial and aquatic systems near points of significant emissions, impacting soil, groundwater, and other media of concern<ref name="Fang2018">Fang, X., Wang, Q., Zhao, Z., Tang, J., Tian, C., Yao, Y., Yu, J. and Sun, H., 2018. Distribution and dry deposition of alternative and legacy perfluoroalkyl and polyfluoroalkyl substances in the air above the Bohai and Yellow Seas, China. Atmospheric Environment, 192, pp. 128-135. [https://doi.org/10.1016/j.atmosenv.2018.08.052 DOI: 10.1016/j.atmosenv.2018.08.052]</ref><ref name="Brandsma2019">Brandsma, S.H., Koekkoek, J.C., van Velzen, M.J.M. and de Boer, J., 2019. The PFOA substitute GenX detected in the environment near a fluoropolymer manufacturing plant in the Netherlands. Chemosphere, 220, pp. 493-500. [https://doi.org/10.1016/j.chemosphere.2018.12.135 DOI: 10.1016/j.chemosphere.2018.12.135] Open access article available from [https://reader.elsevier.com/reader/sd/pii/S0045653518324706?token=E541D5C4B200C8626A86F41049FE9DCA92652BC9A8BA7D9E47832C08070AB5AF256F4872474C50B5C4908F5CA4C24947 ScienceDirect]. [[Media: Brandsma2019.pdf | Report.pdf]]</ref>. Releases of ionic PFAS from factories are likely tied to particulate matter, which settle to the ground in dry weather and are also wet-scavenged by precipitation<ref name="Barton2006">Barton, C.A., Butler, L.E., Zarzecki, C.J., Flaherty, J. and Kaiser, M., 2006. Characterizing Perfluorooctanoate in Ambient Air near the Fence Line of a Manufacturing Facility: Comparing Modeled and Monitored Values. Journal of the Air and Waste Management Association, 56(1), pp. 48-55. [https://doi.org/10.1080/10473289.2006.10464429 DOI: 10.1080/10473289.2006.10464429] Free access article available from [https://www.tandfonline.com/doi/pdf/10.1080/10473289.2006.10464429?needAccess=true Taylor and Francis Online] [[Media: Barton2006.pdf | Report.pdf]]</ref>. The impact of other potential sources, such as combustion emissions or wind-blown fire-fighting foam from fire training and fire response sites, on the fate and transport of PFAS in air may need to be assessed.
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| − | Long-range transport processes are responsible for the wide distribution of neutral and ionic PFAS across the Earth as evidenced by their occurrence in biota, surface snow, ice cores, seawater, and other environmental media in regions as remote as the Arctic and Antarctic<ref name="Bossi2016">Bossi, R., Vorkamp, K. and Skov, H., 2016. Concentrations of organochlorine pesticides, polybrominated diphenyl ethers and perfluorinated compounds in the atmosphere of North Greenland. Environmental Pollution, 217, pp. 4-10. [https://doi.org/10.1016/j.envpol.2015.12.026 DOI: 10.1016/j.envpol.2015.12.026]</ref><ref name="Ahrens2010">Ahrens, L., Gerwinski, W., Theobald, N. and Ebinghaus, R., 2010. Sources of polyfluoroalkyl compounds in the North Sea, Baltic Sea and Norwegian Sea: Evidence from their spatial distribution in surface water. Marine Pollution Bulletin, 60(2), pp. 255-260. [https://doi.org/10.1016/j.marpolbul.2009.09.013 DOI: 10.1016/j.marpolbul.2009.09.013]</ref>.
| + | ==Energy Usage== |
| − | Distribution of PFAS to remote regions far removed from direct industrial input is believed to occur from both: a) long-range atmospheric transport and subsequent degradation of volatile precursors; and b) transport via ocean currents and release into the air as marine aerosols (sea spray)<ref name="DeSilva2009">De Silva, A.O., Muir, D.C. and Mabury, S.A., 2009. Distribution of perfluorocarboxylate isomers in select samples from the North American environment. Environmental Toxicology and Chemistry: An International Journal 28(9), pp. 1801-1814. [https://doi.org/10.1897/08-500.1 DOI: 10.1897/08-500.1]</ref><ref name="Armitage2009">Armitage, J.M., 2009. Modeling the global fate and transport of perfluoroalkylated substances (PFAS). Doctoral Dissertation, Institutionen för tillämpad miljövetenskap (ITM), Stockholm University. [[Media: Armitage2009.pdf | Report.pdf]]</ref>.
| + | 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. |
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| − | ==Transport and Partitioning in Aqueous Systems== | + | ==Vapor Treatment== |
| − | PFAS adsorb from water to a variety of solid materials including organic materials, clay minerals, metal oxides, and granular activated carbon<ref name="Du2014">Du, Z., Deng, S., Bei, Y., Huang, Q., Wang, B., Huang, J. and Yu, G., 2014. Adsorption behavior and mechanism of perfluorinated compounds on various adsorbents – A review. Journal of Hazardous Materials, 274, pp. 443-454. [https://doi.org/10.1016/j.jhazmat.2014.04.038 DOI: 10.1016/j.jhazmat.2014.04.038]</ref>. This process is thought to occur through two primary mechanisms: 1) sorption to organic-carbon components of the solids; and 2) electrostatic (and other) interactions with inorganic constituents of the solids, including clay minerals and metal-oxides<ref name="Guelfo2013">Guelfo, J.L. and Higgins, C.P., 2013. Subsurface Transport Potential of Perfluoroalkyl Acids at Aqueous Film-Forming Foam (AFFF)-Impacted Sites. Environmental Science and Technology, 47(9), pp. 4164-4171. [https://doi.org/10.1021/es3048043 DOI: 10.1021/es3048043] [https://mountainscholar.org/bitstream/handle/11124/80055/Guelfo_mines_0052E_10298.pdf?sequence=1#page=64 Doctoral Dissertation]</ref><ref name="Zhao2014">Zhao, L., Bian, J., Zhang, Y., Zhu, L. and Liu, Z., 2014. Comparison of the sorption behaviors and mechanisms of perfluorosulfonates and perfluorocarboxylic acids on three kinds of clay minerals. Chemosphere, 114, pp. 51-58. [https://doi.org/10.1016/j.chemosphere.2014.03.098 DOI: 10.1016/j.chemosphere.2014.03.098] Free download available from [https://www.researchgate.net/profile/Lixia_Zhao8/publication/262148355_Comparison_of_the_sorption_behaviors_and_mechanisms_of_perfluorosulfonates_and_perfluorocarboxylic_acids_on_three_kinds_of_clay_minerals/links/5b1be5dca6fdcca67b681a4f/Comparison-of-the-sorption-behaviors-and-mechanisms-of-perfluorosulfonates-and-perfluorocarboxylic-acids-on-three-kinds-of-clay-minerals.pdf ResearchGate].</ref>. The relative contribution of each mechanism varies depending on surface chemistry and other geochemical factors, as well as the molecular properties of the PFAS. In general, the impact of electrostatic interactions with charged soil constituents is more important for PFAS than non-polar, hydrophobic organic contaminants (e.g. hydrocarbons, chlorinated solvents). Adsorption of PFAS by solids is often nonlinear, with greater sorption at lower solute concentrations. The impacts of adsorption kinetics and their potential reversibility on PFAS transport have not yet been examined for most PFAS compounds.
| + | 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 PerfluorAd<sup><small>TM</small></sup>. |
| | | | |
| − | Sorption of hydrocarbons, chlorinated solvents and other hydrophobic organics is often controlled the by organic-carbon components of the solid phase (see [[Sorption of Organic Contaminants]]). However, studies of PFAS sorption to solid phase organic carbon have reported conflicting results. In a study of field sites with aqueous film-forming foam (AFFF, a type of fire-fighting foam) releases, solid phase organic carbon content was found to significantly influence PFAS soil-to-groundwater concentration ratios. Statistical modeling was then used to derive apparent organic carbon partition coefficients for 18 different PFAS<ref name="Anderson2019"/>. A recent compilation of published organic carbon partition coefficients found a good correspondence to PFAS molecular structure<ref name="Brusseau2019a">Brusseau, M.L., 2019. Estimating the relative magnitudes of adsorption to solid-water and air/oil-water interfaces for per-and poly-fluoroalkyl substances. Environmental Pollution, 254B, p. 113102. [https://doi.org/10.1016/j.envpol.2019.113102 DOI: 10.1016/j.envpol.2019.113102]</ref>. However, other studies have shown a general lack of correlation between solid phase partition coefficients and organic carbon<ref name="Li2018">Li, Y., Oliver, D.P. and Kookana, R.S., 2018. A critical analysis of published data to discern the role of soil and sediment properties in determining sorption of per and polyfluoroalkyl substances (PFASs). Science of the Total Environment, 628, pp. 110-120. [https://doi.org/10.1016/j.scitotenv.2018.01.167 DOI: 10.1016/j.scitotenv.2018.01.167]</ref>. It is possible that greater variability may be observed for broader data sets that incorporate different ranges of PFAS concentrations, different solution conditions, different measurement methods, and field-based data which often have less well-defined conditions and may also be influenced by other retention processes<ref name="Anderson2019"/><ref name="Brusseau2019a"/>.
| + | ==PFAS Reactivity and Fate== |
| | + | While evaluating initial soil treatment results, Crownover ''et al''<ref name="CrownoverEtAl2019"/> 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)<small><sub>2</sub></small>) amendment<ref>Koster van Groos, P.G., 2021. Small-Scale Thermal Treatment of Investigation-Derived Wastes Containing PFAS. [https://serdp-estcp.mil/ Strategic Environmental Research and Development Program (SERDP) - Environmental Security Technology Certification Program (ESTCP)], [https://serdp-estcp.mil/projects/details/2f1577ac-c8ea-4ae8-804e-c9f97a12edb3/small-scale-thermal-treatment-of-investigation-derived-wastes-idw-containing-pfas Project ER18-1556 Website], [[Media: ER18-1556_Final_Report.pdf | Final Report.pdf]]</ref>. Amendments such as Ca(OH)<sub><small>2</small></sub> 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. |
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| − | [[File:AndersonBrusseau1w2Fig2.png | thumb | 500px | Figure 2. Example of expected orientation and accumulation of PFAS at air-water interface. Source: D. Adamson, GSI, used with permission.]] | + | ==Case Studies== |
| − | Most solids present in the environment contain both fixed-charged (negative) and variably charged surfaces. At neutral to high pH, variably charged clay minerals have a net-negative charge. As a result, negatively charged PFAAs do not strongly interact electrostatically in most soils, although as the soil pH decreases electrostatic sorption would be expected to increase in soils with variably charged clay minerals. Cationic and zwitterionic precursors are expected to be more strongly sorbed than anionic PFAAs in most environments due to well-established cation exchange reactions. Other factors, including ionic strength, composition, and the presence of co-solutes, can affect adsorption of PFAS<ref name="Higgins2006">Higgins, C.P. and Luthy, R.G., 2006. Sorption of Perfluorinated Surfactants on Sediments. Environmental Science and Technology, 40(23), pp. 7251-7256. [https://doi.org/10.1021/es061000n DOI: 10.1021/es061000n]</ref><ref name="Chen2009">Chen, H., Chen, S., Quan, X., Zhao, Y. and Zhao, H., 2009. Sorption of perfluorooctane sulfonate (PFOS) on oil and oil-derived black carbon: Influence of solution pH and [Ca2+]. Chemosphere, 77(10), pp. 1406-1411. [https://doi.org/10.1016/j.chemosphere.2009.09.008 DOI: 10.1016/j.chemosphere.2009.09.008]</ref><ref name="Pan2009">Pan, G., Jia, C., Zhao, D., You, C., Chen, H. and Jiang, G., 2009. Effect of cationic and anionic surfactants on the sorption and desorption of perfluorooctane sulfonate (PFOS) on natural sediments. Environmental Pollution, 157(1), pp.325-330. [https://doi.org/10.1016/j.envpol.2008.06.035 DOI: 10.1016/j.envpol.2008.06.035] Free download available from [https://www.researchgate.net/profile/Gang_Pan2/publication/23189567_Effect_of_cationic_and_anionic_surfactants_on_the_sorption_and_desorption_of_perfluorooctane_sulfonate_PFOS_on_natural_sediments/links/5be19d23a6fdcc3a8dc2550d/Effect-of-cationic-and-anionic-surfactants-on-the-sorption-and-desorption-of-perfluorooctane-sulfonate-PFOS-on-natural-sediments.pdf ResearchGate]</ref><ref name="Guelfo2013"/><ref name="Zhao2014"/>.
| + | ===Stockpile Treatment, Eielson AFB, Alaska ([https://serdp-estcp.mil/projects/details/62098505-de86-43b2-bead-ae8018854141 ESTCP project ER20-5198]<ref name="CrownoverEtAl2023">Crownover, E., Heron, G., Pennell, K., Ramsey, B., Rickabaugh, T., Stallings, P., Stauch, L., Woodcock, M., 2023. Ex Situ Thermal Treatment of PFAS-Impacted Soils, [[Media: ER20-5198 Final Report.pdf | Final Report.]] Eielson Air Force Base, Alaska. [https://serdp-estcp.mil/ Strategic Environmental Research and Development Program (SERDP) - Environmental Security Technology Certification Program (ESTCP)], [https://serdp-estcp.mil/projects/details/62098505-de86-43b2-bead-ae8018854141 Project ER20-5198 Website]</ref>)=== |
| | + | [[File: HeronFig1.png | thumb | 400 px | Figure 1. TCH treatment of a PFAS-laden stockpile at Eielson AFB, Alaska<ref name="CrownoverEtAl2023"/>]] |
| | + | 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). |
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| − | Most PFAS compounds act as surface-active agents (or [[Wikipedia:Surfactant | surfactants]]) due to the presence of a hydrophilic headgroup and a hydrophobic tail. The hydrophilic headgroup will preferentially partition to the aqueous phase and the hydrophobic tail will preferentially partition to the non-aqueous phase (air or organic material). As a result, PFAS tend to accumulate at interfaces (air-water, water-NAPL, water-solid) (Figure 2). This tendency to accumulate at interfaces can influence transport in the atmosphere (on water droplets and hydrated aerosols), in the vadose or unsaturated zone at air-water interfaces, in the presence of NAPLs, and in wastewater treatment systems<ref name="Brusseau2018"/><ref name="Brusseau2019b">Brusseau, M.L., 2019. The Influence of Molecular Structure on the Adsorption of PFAS to Fluid-Fluid Interfaces: Using QSPR to Predict Interfacial Adsorption Coefficients. Water Research, 152, pp. 148-158. [https://doi.org/10.1016/j.watres.2018.12.057 DOI: 10.1016/j.watres.2018.12.057] [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6374777/ Author’s Manuscript]</ref>.
| + | ===''In situ'' Vadose Zone Treatment, Beale AFB, California ([https://serdp-estcp.mil/projects/details/94949542-f9f7-419d-8028-8ba318495641/er20-5250-project-overview ESTCP project ER20-5250]<ref name="Iery2024">Iery, R. 2024. In Situ Thermal Treatment of PFAS in the Vadose Zone. [https://serdp-estcp.mil/ Strategic Environmental Research and Development Program (SERDP) - Environmental Security Technology Certification Program (ESTCP)], [https://serdp-estcp.mil/projects/details/94949542-f9f7-419d-8028-8ba318495641 Project ER20-5250 Website]. [[Media: ER20-5250 Fact Sheet.pdf | Fact Sheet.pdf]]</ref>)=== |
| − |
| + | [[File: HeronFig2.png | thumb | 600 px | Figure 2. ''In situ'' TCH treatment of a PFAS-rich vadose zone hotspot at Beale AFB, California]] |
| − | In theoretical and experimental studies of transport in unsaturated porous media, adsorption at the air-water interface increased PFOS and PFOA retention<ref name="Brusseau2018"/><ref name="Lyu2018">Lyu, Y., Brusseau, M.L., Chen, W., Yan, N., Fu, X., and Lin, X., 2018. Adsorption of PFOA at the Air-Water Interface during Transport in Unsaturated Porous Media. Environmental Science and Technology, 52(14), pp. 7745-7753. [https://doi.org/10.1021/acs.est.8b02348 DOI: 10.1021/acs.est.8b02348] [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6312111/ Author’s Manuscript]</ref><ref name="BrusseauEtAl2019">Brusseau, M.L., Yan, N., Van Glubt, S., Wang, Y., Chen, W., Lyu, Y., Dungan, B., Carroll, K.C., and Holguin, F.O., 2019. Comprehensive Retention Model for PFAS Transport in Subsurface Systems. Water Research, 148, pp. 41-50. [https://doi.org/10.1016/j.watres.2018.10.035 DOI: 10.1016/j.watres.2018.10.035] [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6294326/ Author’s Manuscript]</ref>, contributing approximately 20% to 80% of total retention in sands and soil. The impact of oil-water interfacial adsorption on PFAS transport was also quantitatively characterized in recent studies and shown to contribute to total retention on a similar scale as air-water interfacial adsorption<ref name="Brusseau2018"/><ref name="BrusseauEtAl2019"/>. These processes may result in increased PFAS mass retained in NAPL source zones, increased PFAS sorption with the resulting retardation of transport, and greater persistence of dissolved PFAS in the environment.
| + | 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<ref name="Iery2024"/>. It was demonstrated that with proper water management, even highly impacted soils can be treated to near non-detect concentrations (greater than 99% reduction). |
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| − | ==Transformation== | + | ===Constructed Pile Treatment, JBER, Alaska ([https://serdp-estcp.mil/projects/details/eb7311db-6233-4c7f-b23a-e003ac1926c5/pfas-treatment-in-soil-using-thermal-conduction-heating ESTCP Project ER23-8369]<ref name="CrownoverHeron2024">Crownover, E., Heron, G., 2024. PFAS Treatment in Soil Using Thermal Conduction Heating. Defense Innovation Unit (DIU) and [https://serdp-estcp.mil/ Strategic Environmental Research and Development Program (SERDP) - Environmental Security Technology Certification Program (ESTCP)], [https://serdp-estcp.mil/projects/details/eb7311db-6233-4c7f-b23a-e003ac1926c5/pfas-treatment-in-soil-using-thermal-conduction-heating Project ER23-8369 Website]</ref>)=== |
| − | [[File:AndersonBrusseau1w2Fig3.png | thumb | 600px | Figure 3. Conceptual model of precursor transformation resulting in the formation of PFAAs. Source L. Trozzolo, TRC and C. Higgins, Colorado School of Mines, used with permission.]] | + | [[File: HeronFig3.png | thumb | 600 px | Figure 3. Treatment of a 2,000 cubic yard soil pile at JBER, Alaska]] |
| − | Certain polyfluorinated substances have the potential to transform to other PFAS, with PFAAs as the typical terminal daughter products. These polyfluorinated substances are often referred to as “precursors”. The transformation potential of polyfluorinated precursors is influenced by the presence, location, and number of carbon-hydrogen (C-H) bonds and potentially carbon-oxygen (C-O) bonds throughout the carbon chain. Specifically, PFAS with C-H bonds are subject to a variety of biotic and abiotic reactions that ultimately result in the formation of PFAAs with perfluorinated carbon chains of the same length or shorter than the initial polyfluorinated precursor<ref name="Houtz2013">Houtz, E.F., Higgins, C.P., Field, J.A. and Sedlak, D.L., 2013. Persistence of perfluoroalkyl acid precursors in AFFF-impacted groundwater and soil. Environmental Science and Technology, 47(15), pp. 8187-8195. [https://doi.org/10.1021/es4018877 DOI: 10.1021/es4018877] Free download from [https://www.researchgate.net/profile/Erika_Houtz/publication/252323955_Persistence_of_Perfluoroalkyl_Acid_Precursors_in_AFFF-Impacted_Groundwater_and_Soil/links/59dbddeeaca2728e2018336d/Persistence-of-Perfluoroalkyl-Acid-Precursors-in-AFFF-Impacted-Groundwater-and-Soil.pdf ReseqarchGate]</ref><ref name="McGuire2014">McGuire, M.E., Schaefer, C., Richards, T., Backe, W.J., Field, J.A., Houtz, E., Sedlak, D.L., Guelfo, J.L., Wunsch, A., and Higgins, C.P., 2014. Evidence of Remediation-Induced Alteration of Subsurface Poly- and Perfluoroalkyl Substance Distribution at a Former Firefighter Training Area. Environmental Science and Technology, 48(12) pp. 6644-6652. [https://doi.org/10.1021/es5006187 DOI: 10.1021/es5006187] Manuscript available from [https://ir.library.oregonstate.edu/downloads/td96k706f Oregon State University]</ref><ref name="Anderson2016">Anderson, R.H., Long, G.C., Porter, R.C. and Anderson, J.K., 2016. Occurrence of select perfluoroalkyl substances at US Air Force aqueous film-forming foam release sites other than fire-training areas: Field-validation of critical fate and transport properties. Chemosphere, 150, pp. 678-685. [https://doi.org/10.1016/j.chemosphere.2016.01.014 DOI: 10.1016/j.chemosphere.2016.01.014]</ref><ref name="Weber2017"/>.
| + | 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<ref name="CrownoverHeron2024"/>. 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. |
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| − | Transformation studies published to date have tested only a small subsample of possible precursors and, therefore, much uncertainty exists regarding 1) the extent to which precursor transformation occurs on a global scale, 2) which environmental compartments represent the majority of transformation, 3) relevant environmental conditions that affect transformation processes, and 4) transformation rates and pathways. Nevertheless, a portion of the precursors are expected to transform to PFAAs over time as shown in Figure 3.
| + | ==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. |
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| − | Precursors can be transformed by a variety of abiotic processes including hydrolysis, photolysis, and oxidation. Hydrolysis of some precursors, followed by subsequent biotransformation, can produce perfluoroalkyl sulfonates (PFSAs). An important example is the production of PFOS from perfluorooctane sulfonyl fluoride (POSF)<ref name="Martin2010">Martin, J.W., Asher, B.J., Beesoon, S., Benskin, J.P. and Ross, M.S., 2010. PFOS or PreFOS? Are perfluorooctane sulfonate precursors (PreFOS) important determinants of human and environmental perfluorooctane sulfonate (PFOS) exposure? Journal of Environmental Monitoring, 12(11), pp.1979-2004. [https://doi.org/10.1039/C0EM00295J DOI: 10.1039/C0EM00295J] Free download from [https://www.researchgate.net/profile/Matthew_Ross3/publication/47415684_PFOS_or_PreFOS_Are_perfluorooctane_sulfonate_precursors_PreFOS_important_determinants_of_human_and_environmental_perfluorooctane_sulfonate_PFOS_exposure/links/00b7d520a6132da945000000.pdf ResearchGate]</ref>. Other hydrolysis reactions produce perfluoroalkyl carboxylates (PFCAs). At neutral pH, the hydrolysis of fluorotelomer-derived polymeric precursors results in the formation of monomeric precursors of PFOA and other PFAAs with half-lives of 50 to 90 years)<ref name="Washington2010">Washington, J.W., Ellington, J.J., Jenkins, T.M. and Yoo, H., 2010. Response to Comments on “Degradability of an Acrylate-Linked, Fluorotelomer Polymer in Soil”. Environmental Science and Technology, 44(2), pp. 849-850. [https://doi.org/10.1021/es902672q DOI: 10.1021/es902672q] [https://pubs.acs.org/doi/pdf/10.1021/es902672q Free Download from ACS].</ref>. Oxidation of precursors by hydroxyl radicals can occur in natural waters, with the fluorotelomer-derived precursors being oxidized relatively rapidly<ref name="Gauthier2005">Gauthier, S.A. and Mabury, S.A., 2005. Aqueous photolysis of 8: 2 fluorotelomer alcohol. Environmental Toxicology and Chemistry, 24(8), pp.1837-1846. [https://doi.org/10.1897/04-591R.1 DOI: 10.1897/04-591R.1] Free download from [https://www.researchgate.net/profile/Suzanne_Gauthier/publication/7609648_Aqueous_photolysis_of_8_2_fluorotelomer_alcohol/links/5ec16c4792851c11a86d9438/Aqueous-photolysis-of-8-2-fluorotelomer-alcohol.pdf ResearchGate].</ref><ref name="Plumlee2009">Plumlee, M.H., McNeill, K. and Reinhard, M., 2009. Indirect Photolysis of Perfluorochemicals: Hydroxyl Radical-Initiated Oxidation of N-Ethyl Perfluorooctane Sulfonamido Acetate (N-EtFOSAA) and Other Perfluoroalkanesulfonamides. Environmental Science and Technology, 43(10), pp.3662-3668. [https://doi.org/10.1021/es803411w DOI: 10.1021/es803411w] Free download from [https://www.researchgate.net/profile/Megan_Plumlee/publication/26309488_Indirect_Photolysis_of_Perfluorochemicals_Hydroxyl_Radical-Initiated_Oxidation_of_N-Ethyl_Perfluorooctane_Sulfonamido_Acetate_N-EtFOSAA_and_Other_Perfluoroalkanesulfonamides/links/5aac0437a6fdcc1bc0b8d002/Indirect-Photolysis-of-Perfluorochemicals-Hydroxyl-Radical-Initiated-Oxidation-of-N-Ethyl-Perfluorooctane-Sulfonamido-Acetate-N-EtFOSAA-and-Other-Perfluoroalkanesulfonamides.pdf ResearchGate].</ref>.
| + | ==Recommendations== |
| − | Evidence of aerobic biotransformation is provided from studies of PFAS composition throughout the continuum of wastewater treatments<ref name="Arvaniti2015">Arvaniti, O.S. and Stasinakis, A.S., 2015. Review on the occurrence, fate and removal of perfluorinated compounds during wastewater treatment. Science of the Total Environment, 524, pp. 81-92. [https://doi.org/10.1016/j.scitotenv.2015.04.023 DOI: 10.1016/j.scitotenv.2015.04.023]</ref>, from field studies at AFFF-impacted sites<ref name="Houtz2013"/><ref name="McGuire2014"/><ref name="Anderson2016"/><ref name="Weber2017"/>, and from microcosm experiments. In general, the literature on aerobic biotransformation collectively demonstrates or indirectly supports the following conclusions as summarized in ITRC 2020<ref name="ITRC2020"/>:
| + | Recent research suggests: |
| − | * Numerous aerobic biotransformation pathways exist with relatively rapid kinetics
| + | *Successful thermal treatment of PFAS may require a higher target temperature than for other organics with similar boiling points |
| − | * All polyfluorinated precursors studied to date have the potential to aerobically biotransform to PFAAs
| + | *Prevention of influx of water into treatment zone may be necessary. |
| − | * Aerobic biotransformation of various fluorotelomer-derived precursors exclusively results in the formation of PFCAs, including PFOA, without necessarily the conservation of chain-length
| + | Future studies should examine the potential for enhanced degradation during the thermal process by using soil amendments and/or manipulation of the local geochemistry to reduce the required treatment temperatures and therefore also reduce energy demand. |
| − | * Aerobic biotransformation of various electrochemical fluorination-derived precursors primarily results in the formation of PFAAs, including PFOS, with the conservation of chain-length
| |
| − | Precursor transformation can complicate CSMs (and risk assessments) and should be considered during comprehensive site investigations. For example, atmospheric emissions of volatile precursors can result in long-range transport where subsequent transformation and deposition can result in detectable levels of PFAAs in environmental media independent of obvious point-sources<ref name="Vedagiri2018">Vedagiri, U.K., Anderson, R.H., Loso, H.M. and Schwach, C.M., 2018. Ambient levels of PFOS and PFOA in multiple environmental media. Remediation Journal, 28(2), pp. 9-51. [https://doi.org/10.1002/rem.21548 DOI: 10.1002/rem.21548]</ref>. With respect to site-related precursors, transformation of otherwise unmeasured PFAS into detectable PFAAs is obviously relevant to site investigations to the extent transformation occurs after initial site characterization efforts or if past remedial efforts have accelerated ''in situ'' transformation rates<ref name="McGuire2014"/>. Additionally, differential transport rates between precursor PFAS and the corresponding terminal PFAA could also confound CSMs if transformation rates are slower than transport rates as has been suggested<ref name="Weber2017"/>.
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| − | To account for otherwise unmeasurable precursors, several surrogate analytical methods have been developed. See [[PFAS Sampling and Analytical Methods]] for additional detail.
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| | ==References== | | ==References== |
| − | <references/> | + | <references /> |
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| − | ==See Also:== | + | ==See Also== |
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
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).
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
Recent research suggests:
- Successful thermal treatment of PFAS may require a higher target temperature than for other organics with similar boiling points
- Prevention of influx of water into treatment zone may be necessary.
Future studies should examine the potential for enhanced degradation during the thermal process by using soil amendments and/or manipulation of the local geochemistry to reduce the required treatment temperatures and therefore also reduce energy demand.
References
- ^ 1.0 1.1 1.2 1.3 Crownover, E., Oberle, D., Heron, G., Kluger, M., 2019. Perfluoroalkyl and polyfluoroalkyl substances thermal desorption evaluation. Remediation Journal, 29(4), pp. 77-81. doi: 10.1002/rem.21623
- ^ Horst, J., Munholland, J., Hegele, P., Klemmer, M., Gattenby, J., 2021. In Situ Thermal Remediation for Source Areas: Technology Advances and a Review of the Market From 1988–2020. Groundwater Monitoring & Remediation, 41(1), p. 17. doi: 10.1111/gwmr.12424 Open Access Manuscript
- ^ Stegemeier, G.L., Vinegar, H.J., 2001. Thermal Conduction Heating for In-Situ Thermal Desorption of Soils. Ch. 4.6, pp. 1-37. In: Chang H. Oh (ed.), Hazardous and Radioactive Waste Treatment Technologies Handbook, CRC Press, Boca Raton, FL. ISBN 9780849395864 Open Access Article
- ^ Koster van Groos, P.G., 2021. Small-Scale Thermal Treatment of Investigation-Derived Wastes Containing PFAS. Strategic Environmental Research and Development Program (SERDP) - Environmental Security Technology Certification Program (ESTCP), Project ER18-1556 Website, Final Report.pdf
- ^ 5.0 5.1 Crownover, E., Heron, G., Pennell, K., Ramsey, B., Rickabaugh, T., Stallings, P., Stauch, L., Woodcock, M., 2023. Ex Situ Thermal Treatment of PFAS-Impacted Soils, Final Report. Eielson Air Force Base, Alaska. Strategic Environmental Research and Development Program (SERDP) - Environmental Security Technology Certification Program (ESTCP), Project ER20-5198 Website
- ^ 6.0 6.1 Iery, R. 2024. In Situ Thermal Treatment of PFAS in the Vadose Zone. Strategic Environmental Research and Development Program (SERDP) - Environmental Security Technology Certification Program (ESTCP), Project ER20-5250 Website. Fact Sheet.pdf
- ^ 7.0 7.1 Crownover, E., Heron, G., 2024. PFAS Treatment in Soil Using Thermal Conduction Heating. Defense Innovation Unit (DIU) and Strategic Environmental Research and Development Program (SERDP) - Environmental Security Technology Certification Program (ESTCP), Project ER23-8369 Website
See Also
