Per- and Polyfluoroalkyl Substances (PFAS)

PFAS substances are a large group of compounds (> 6,000) that have an alkyl chain, typically 2 to 16 carbon atoms in length (Concawe 2016). The perfluoroalkyl compounds have fluorine (F) atoms bonded to all of the carbon (C) atoms in the alkyl chain (also referred to as the backbone). The polyfluoroalkyl compounds have some hydrogen (H) atoms in addition to F atoms bonded to the C atoms of the alkyl chain (Buck et al. 2011).

Manufacturing Processes

Two major manufacturing processes produce nonpolymer PFAS. The type of manufacturing process used has bearing on the environmental behavior of PFAS compounds because the first process produces a molecular structure with significant side chains or branching and the second produces linear molecules with little or no branching.

In the first process, electro-chemical fluorination, organic feedstocks are dispersed in liquid anhydrous hydrogen fluoride and an electric current is passed through the solution, causing the H atoms on the molecule to be replaced with F atoms. Fragmentation and rearrangement of the C skeleton also can occur, potentially forming significant amounts of cleaved, branched, and cyclic structures. It is possible to synthesize fully fluorinated molecules where all of the H atoms of the hydrocarbon feedstock have been replaced by F atoms. Using these molecules as basic building blocks, unique chemistries can be created by further reactions with functionalized hydrocarbon molecules (3M Company 1999).

In the second process, telomerization, a perfluoroalkyl iodide, C_mF_2m+1I (PFAI), most commonly pentafluoroethyl (or perfluoroethyl) iodide, C₂F₅I (PFEI), is reacted with tetrafluoroethylene, CF₂=CF₂ (TFE), to yield a mixture of perfluoroalkyl iodides with longer perfluorinated chains, C_mF_2m+1(CF₂CF₂)_nI. The starting iodide is referred to as the "telogen" and the TFE as the "taxogen." The produced perfluoroalkyl iodide mixture is often then reacted further in a second process step where ethylene is inserted to give C_mF_2m+1(CF₂CF₂)_nCH₂CH₂I. The perfluoroalkyl iodides resulting from telomerization in the first step—C_mF_2m+1(CF₂CF₂)_nI, commonly known as Telomer A—and the "fluorotelomer iodides" formed in the second step—C_mF_2m+1(CF₂CF₂)_nCH₂CH₂I, commonly known as Telomer B—are raw material intermediates used to produce additional building blocks that are further reacted to create a family of fluorotelomer-based surfactant and polymer products. The telomerization process generally produces linear molecules with little or no branching (Buck et al. 2011).

Buck et al. (2011) provides a supplemental data table of nonpolymer PFASs that lists 42 families and subfamilies of PFASs and 268 selected individual compounds with their recommended names and abbreviations, structural formulas, and Chemical Abstracts Service (CAS) registry numbers.

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Behavior

The key to understanding the environmental fate and transport of PFAS compounds is their surface-active behavior. The fluorinated backbone is both hydrophobic (water repelling) and oleophobic/lipophobic (oil/fat repelling) while the terminal functional group is hydrophilic (water loving). This means that PFAS compounds tend to partition to interfaces, such as between air and water with the fluorinated backbone residing in air and the terminal functional group residing in water. The PFAS partitioning behavior also is affected by the alkyl chain length and the charge on the terminal functional group. In general, PFASs with shorter alkyl chain length are more water soluble than those with longer lengths. Adsorption to soil surfaces has been shown to be greater for PFASs with longer alkyl chain length (Anderson et al. 2016).

At environmentally relevant pH, many PFAS compounds have a negatively charged terminal functional group (i.e., anionic), meaning that they will be repelled from soil that tends to have negatively charged surfaces. Some PFAS compounds have a positively charged terminal functional group (i.e., cation), which will strongly bind with soils. And a few PFAS compounds have both positively and negatively charged groups (i.e., zwitterions), which will exhibit partitioning behavior between anionic and cationic compounds. The variability in alkyl chain length and functional group charge means that the behavior of PFAS compounds is not readily generalized.

Two commonly cited PFAS compounds have the following behavior:

• Perfluorooctanoic acid (PFOA) at environmental pH is the anion perfluorooctanoate with estimated water solubility of 9,500 mg/L and negligible vapor pressure (USEPA 2016a).
• Perfluoroooctane sulfonate (PFOS) has an estimated water solubility of 680 mg/L and negligible vapor pressure (USEPA 2016b).

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Databases

Given the primary focus of these pages on PFOS and PFOA, links to the following databases are provided as potential sources of information for other PFAS compounds.

Chemical Book
As a private sector site sponsored by chemical manufacturers, Chemical Book is most readily searched by CAS registry number to disclose chemical properties and MSDS sheets for the targeted chemical.

Hazardous Substances Data Bank (HSDB)
The HSDB database focuses on potentially hazardous chemicals, providing information on human exposure, industrial hygiene, emergency handling procedures, environmental fate, regulatory requirements, nanomaterials, and related areas, including chemical properties. The information in HSDB has been assessed by a Scientific Review Panel.

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Overviews

Fate and Transport of Per- and Polyfluoroalkyl Substances (PFAS)
Interstate Technology and Regulatory Council (ITRC), 4 pp, 2022

Processes that influence the environmental fate and transport of PFASs from major sources—fire training/fire response sites, industrial sites, landfills, and wastewater treatment plants/biosolids—include partitioning, transport, and abiotic and biotic transformation. These processes affect PFAS concentrations in air, surface water, groundwater, soil and sediment, and biota.

Occurrence and Behavior of Per- and Polyfluoroalkyl Substances from Aqueous Film-Forming Foam in Groundwater Systems
Hatton, J., C. Holton, and B. DiGuiseppi.
Remediation Journal 28(2):89-99(2018) [Open Access]

Background is presented on AFFF and PFAS source characteristics in Part 1 of this paper, including common industrial and consumer PFAS sources. Part 2 discusses chemical properties, sorption and retention parameters, observed transformation properties of PFAS and related compounds, and knowledge gaps.

Survey of PFOS, PFOA and Other Perfluoroalkyl and Polyfluoroalkyl Substances
Danish Environmental Protection Agency, Environmental Project No. 1475, 196 pp, 2013

While this overview report focuses on PFOS and PFOA it also touches on PFASs in general in sections on chemistry, regulatory framework, manufacture and uses, waste management, environmental effects and fate, human health, and monitoring data and exposure.

Synthesis Paper on Per- and Polyfluorinated Chemicals (PFCs)
OECD/UNEP Global PFC Group, Environment, Health and Safety, Environment Directorate, Organisation for Economic Co-operation and Development (OECD). 60 pp, 2013

Separate chapters in this report provide overviews of (1) historical and current major uses of PFASs; (2) scientific evidence on sources to the environment, environmental fate, human exposure, and potential adverse effects; (3) recent developments on alternatives to long-chain PFASs; and (4) European regulatory approaches with respect to PFASs.

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Chemistry

Estimation of the Acid Dissociation Constant of Perfluoroalkyl Carboxylic Acids Through an Experimental Investigation of their Water-to-Air Transport
Vierke, L., U. Berger, and I.T. Cousins.
Environmental Science & Technology 47(19):11032-9(2013) doi: 10.1021/es402691z

A method is suggested for estimating acid dissociation constants of perfluoroalkyl carboxylic acids (PFCAs), which when combined with the water pH, allows for estimating the extent of volatilization of PFCAs in the environment. Perfluoroalkane sulfonic acids were also evaluated.

Lists of PFOS, PFAS, PFOA, PFCA Related Compounds and Chemicals That May Degrade to PFCA
Organisation for Economic Co-operation and Development (OECD). Environment, Health and Safety Publications Series on Risk Management No. 21, ENV/JM/MONO(2006)15, 157 pp, 2007

This document contains the following lists of compounds that might degrade to PFCA:

• PFOS and Related Compounds.
• Perfluoroalkyl Sulfonate and Related Compounds.
• PFOA and Related Compounds.
• Fluorinated Chemicals that Potentially Degrade to PFCA.

Perfluoroalkyl and Polyfluoroalkyl Substances in the Environment: Terminology, Classification, and Origins
Buck, R.C., J. Franklin, U. Berger, J.M. Conder, I.T. Cousins, P. de Voogt, A. Astrup, et al.
Integrated Environmental Assessment & Management 7(4):513-541(2011)

This paper provides an overview of PFASs and recommends clear, specific, and descriptive terminology, names, and abbreviations for the compounds. Particular emphasis is placed on long-chain perfluoroalkyl acids, substances related to the long-chain perfluoroalkyl acids, and substances intended as alternatives to the use of the long-chain perfluoroalkyl acids or their precursors. The text contains a brief description of the two main PFAS production processes—electrochemical fluorination and telomerization (important for compound branching and chemical analysis)—and shows how the principal families of PFASs are interrelated as industrial, environmental, or metabolic precursors or transformation products. The Supplemental Data Table lists 42 families and subfamilies of PFASs and 268 selected individual compounds, providing recommended names and abbreviations, structural formulas, and Chemical Abstracts Service registry numbers.

Surface-Active Behavior of Select Per- And Polyfluoroalkyl Substances (PFAS) and Their Mixtures
Chen, J., A. Adegbule, J. Huang, and M. Brooks.
ORISE Meets the World Monthly webinar, 2 December, 15 slides, 2021

Surface tension of seven PFAS compounds with different carbon chain lengths (n = 4, 6, and 8) and functional groups (COO^- and SO₃^-) was measured as a function of concentration for individual compounds and their mixtures. Six PFAS showed a sharp decline in surface tension with increasing concentration with no evidence of micelle formation. The PFAS compounds showed different surface activities which may relate to carbon chain length and functional groups. All mixtures showed surface tension measurements intermediate to individual compounds.

Using COSMOtherm to Predict Physicochemical Properties of Poly- and Perfluorinated Alkyl Substances (PFASs)
Wang, Z., M. MacLeod, I.T. Cousins, M. Scheringer, and K. Hungerbuhler.
Environmental Chemistry 8(4):389-398(2011)

The COSMOtherm model was applied to the estimation of physicochemical properties for 130 individual PFASs. The PFASs addressed were perfluoroalkyl acids (including branched isomers for C4-C8 perfluorocarboxylic acids), their precursors, and some intermediates.

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Fate and Transport in Aquifers

Behavior and Fate of PFOA and PFOS in Sandy Aquifer Sediment
Ferrey, M.L., J.T. Wilson, C. Adair, C. Su, D.D. Fine, X. Liu, and J.W. Washington.
Groundwater Monitoring & Remediation 32(4):63-71(2012)

Microcosms were constructed with sediment from beneath a landfill that received waste containing PFOA and PFOS. Based on results, the authors recommend empirical estimates of sorption using affected aquifer sediment for predicting PFOA and PFOS mobility in groundwater.

Integrating Total Oxidizable Precursor Assay Data to Evaluate Fate and Transport of PFASs
Casson, R. and S.-Y.D. Chiang. Remediation Journal 28(2):71-87(2018) [Open Access]

A total oxidizable precursor assay was developed to quantify measurable concentrations of perfluoroalkyl carboxylates (PFCAs) and perfluoroalkyl sulfonates (PFSAs) after aggressive oxidation to convert perfluoroalkyl acid (PFAA) precursors abiotically into PFCAs.

Partitioning of Perfluorooctanoate (PFOA), Perfluorooctane Sulfonate (PFOS) and Perfluorooctane Sulfonamide (PFOSA) Between Water and Sediment
Ahrens, L., L.W. Yeung, S. Taniyasu, P.K. Lam, and N. Yamashita.
Chemosphere 85(5):731-737(2011)

Lab partitioning experiments were conducted to elucidate the sorption behavior and partitioning of PFOA, PFOS, and perfluorooctane sulfonamide (PFOSA) at low environmentally realistic concentrations in three different sediments.

Perfluorooctane Sulfonate (PFOS) and Perfluorooctanoate (PFOA) in Soils and Groundwater of a U.S. Metropolitan Area: Migration and Implications for Human Exposure
Xiao, F., M.F. Simcik, T.R. Halbach, and J.S. Gulliver.
Water Research 72:67-74(2015) [Abstract]

Results of a study to investigate the migration of PFOS and PFOA in soils and groundwater in a U.S. metropolitan area are provided.

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Fate and Transport in Soils

Evidence of Air Dispersion: HFPO-DA and PFOA in Ohio and West Virginia Surface Water and Soil near a Fluoropolymer Production Facility
Galloway, J.E., A.V.P. Moreno, A.B. Lindstrom, et al.
Environmental Science & Technology [Publication 27 May 2020 prior to print]

PFOA used as a fluoropolymer manufacturing aid at a fluoropolymer production facility in Parkersburg, WV, from 1951 to 2013 was replaced in 2013 with the PFAS surfactant hexafluoropropylene oxide dimer acid (HFPO-DA). In a broad-scale assessment of ongoing impacts from the plant, including dispersal of HFPO-DA, samples collected downwind and upstream from the facility showed PFOA and HFPO-DA had dispersed to surface water and soil in multiple locations as far as 30 miles from the plant.

Occurrence and Fate of Perfluorochemicals in Soil Following the Land Application of Municipal Biosolids
Sepulvado, J.G., A.C. Blaine, L.S. Hundal, and C.P. Higgins.
Environmental Science & Technology 45(19):8106-8112(2011)

Investigators evaluated the levels, mass balance, desorption, and transport of PFCs in soil receiving biosolids application at various loading rates. The study is the first to report levels of PFCs in agricultural soils amended with typical municipal biosolids.

Polyfluorinated Chemicals and Transformation Products
Knepper, T.P. and F.T. Lange (eds).
Springer, New York, ISBN: 978-3-642-21871-2, Handbook of Environmental Chemistry, Vol 17, 200 pp, 2012

This text discusses the chemistry, properties, and uses of commercial fluorinated surfactants; sorption and leaching behavior of perfluorinated compounds in soil; and PFASs environmental occurrence and treatment options. Table of contents with abstracts.

Polyfluoroalkyl Compounds in the Aquatic Environment: A Review of Their Occurrence and Fate
Ahrens, L.
Journal of Environmental Monitoring 13:20-31(2011) [Abstract]

Physical transport and multimedia partitioning of PFASs depends on their physicochemical properties, which vary depending on their chain length and functional group. This review covers key loss processes and deposition, the relationship between sources and aqueous environment concentrations, partitioning behavior, and transport mechanisms.

Review of Available Software For PFAS Modeling Within the Vadose Zone
AECOM on behalf of Michigan Department of Environment, Great Lakes, and Energy, 12 pp, 2020

This review identifies the most suitable vadose zone contaminant transport numerical modeling tools (VZMs) to simulate the transport of PFOA and PFOS from municipal biosolid-amended soils through the unsaturated zone to the underlying groundwater, evaluates and summarizes the capabilities and limitations of each VZM in a tabular format, and provides recommendations to select one or more VZMs suitable to simulate critical processes governing fate and transport of PFOS and PFOA in the subsurface.

Sorption Behaviour of Perfluoroalkyl Substances in Soils
Milinovic, J., S. Lacorte, M. Vidal, and A. Rigol. Science of the Total Environment 511:63-71(2015) [Abstract]

The sorption behavior of PFOS, PFOA, and perfluorobutane sulfonic acid (PFBS) was studied in six soils with contrasting characteristics, especially in organic carbon content.

Subsurface Transport Potential of Perfluoroalkyl Acids at Aqueous Film-Forming Foam (AFFF)-Impacted Sites
Guelfo, J.L. and C.P. Higgins.
Environmental Science and Technology 47(9):4164-4171(2013) [Abstract]

This lab study used batch sorption experiments to investigate the subsurface transport potential of a suite of perfluoroalkyl acids (PFAAs) in various soils and in the presence of co-contaminants relevant to AFFF-impacted sites. Additional information: J.L. Guelfo dissertation, Chapter 3

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Fate and Transport in Surface Waters

Concentrations and Patterns of Perfluoroalkyl Acids in Georgia, USA Surface Waters Near and Distant to a Major Use Source
Konwick, B.J., G.T. Tomy, N. Ismail, J.T. Peterson, R.J. Fauver, D. Higginbotham, A.T. Fisk.
Environmental Toxicology and Chemistry 27(10):2011-2018(2008) [Abstract]

Researchers investigated the area of a large carpet manufacturing site to assess the concentrations of PFAAs, including PFOS, PFOA, PFNA, PFDA, PFUA, and PFOSA, in surface waters both near and distant to a wastewater land application system to understand the fate of PFAAs in freshwater.

Evidence of Air Dispersion: HFPO-DA and PFOA in Ohio and West Virginia Surface Water and Soil near a Fluoropolymer Production Facility
Galloway, J.E., A.V.P. Moreno, A.B. Lindstrom, et al.
Environmental Science & Technology [Publication 27 May 2020 prior to print]

PFOA used as a fluoropolymer manufacturing aid at a fluoropolymer production facility in Parkersburg, WV, from 1951 to 2013 was replaced in 2013 with the PFAS surfactant hexafluoropropylene oxide dimer acid (HFPO-DA). In a broad-scale assessment of ongoing impacts from the plant, including dispersal of HFPO-DA, samples collected downwind and upstream from the facility showed PFOA and HFPO-DA had dispersed to surface water and soil in multiple locations as far as 30 miles from the plant.

Environmental-Fate Patterns for Perfluoroalkylates and Their Precursors
Washington, J.W., P.J. Lasier, H. Yoo, J.J. Ellington, and T. Jenkins.
PFAA Days III, Research Triangle Park, NC, 8-11 June 2010. [Abstract]

Two sites with elevated concentrations of perfluoroalkylates and fluorotelomer alcohols were studied: 1) agricultural fields near Decatur, AL, on which sewage sludge had been applied; and 2) the Conasauga River system near Dalton, GA, where treated sewage effluent is sprayed on land abutting the river. The sewage-treatment facilities at both sites received waste streams from industries that used fluorinated compounds. Taking Decatur and Dalton as model terrestrial and aquatic systems, respectively, patterns are compared in PFAS bioaccumulation factors for basal trophic levels, plant from soil for Decatur, and oligochaete from detritus for Dalton.

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