Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) have been used in coatings for textiles, paper products, and cookware; in some firefighting foams; and have a range of applications in the aerospace, photographic imaging, semiconductor, automotive, construction, electronics, and aviation industries. Although PFAS and PFAS-containing products have been manufactured since the 1950s, PFAS were not widely documented in environmental samples until the early 2000s. Understanding PFAS manufacturing history, past and current uses, and waste management over the last six to seven decades is necessary for the identification of potential environmental sources of PFAS, possible release mechanisms, and associated pathway-receptor relationships.
- Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)
- PFAS Ex Situ Water Treatment
- PFAS Soil Remediation Technologies
- PFAS Transport and Fate
- PFAS Treatment by Electrical Discharge Plasma
- Supercritical Water Oxidation (SCWO)
Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS) are a complex family of more than 3,000 manmade fluorinated organic chemicals although not all of these are currently in use or production. PFAS are produced using several different processes. Fluorosurfactants, which include perfluoroalkyl acids (PFAAs) (see PFAS article for nomenclature) and side-chain fluorinated polymers, have been manufactured using two major processes: electrochemical fluorination (ECF) and telomerization. ECF was licensed by 3M in the 1940s and used by 3M until 2001. ECF produces a mixture of even and odd numbered carbon chain lengths of approximately 70% linear and 30% branched substances. Telomerization was developed in the 1970s, and yields mainly even numbered, straight carbon chain isomers. PFAS manufacturers have provided PFAS to secondary manufacturers for production of a vast array of industrial and consumer products.
During manufacturing, PFAS may be released into the atmosphere then redeposited on land where they can also affect surface water and groundwater, or PFAS may be discharged without treatment to wastewater treatment plants or landfills, and eventually be released into the environment by treatment systems that are not designed to mitigate PFAS (see also PFAS Transport and Fate). Industrial discharges of PFAS were unregulated for many years, but that has begun to change. In January 2016, New York became the first state in the nation to regulate PFOA as a hazardous substance followed by the regulation of PFOS in April 2016. Consumer and industrial uses of PFAS-containing products can also end up releasing PFAS into landfills and into municipal wastewater, where it may accumulate undetected in biosolids which are typically treated by land application.
PFAS are used in many industrial and consumer applications, which may have released PFAS into the environment and impacted drinking water supplies in many areas of the United States. Both in the United States (US) and abroad, primary manufacturing facilities produce PFAS and secondary manufacturing facilities use PFAS to produce goods. Environmental release mechanisms associated with these facilities include air emission and dispersion, spills, and disposal of manufacturing wastes and wastewater. Potential impacts to air, soil, sediment, surface water, stormwater, and groundwater are present not only at primary release points but potentially over the surrounding area, as illustrated in Figure 1. Some of the potential primary and secondary sources of PFAS releases to the environment are listed here:
- Textiles and leather: Factory or consumer applied coating to repel water, oil, and stains. Applications include protective clothing and outerwear, umbrellas, tents, sails, architectural materials, carpets, and upholstery.
- Paper products: Surface coatings to repel grease and moisture. Uses include non-food paper packaging (for example, cardboard, carbonless forms, masking papers) and food-contact materials (for example, pizza boxes, fast food wrappers, microwave popcorn bags, baking papers, pet food bags)
- Metal Plating & Etching: Corrosion prevention, mechanical wear reduction, aesthetic enhancement, surfactant, wetting agent/fume suppressant for chrome, copper, nickel and tin electroplating, and post-plating cleaner
- Industrial Surfactants, Resins, Molds, Plastics: Manufacture of plastics and fluoropolymers, rubber, and compression mold release coatings; plumbing fluxing agents; fluoroplastic coatings, composite resins, and flame retardant for polycarbonate
- Photolithography, Semiconductor Industry: Photoresists, top anti-reflective coatings, bottom anti-reflective coatings, and etchants, with other uses including surfactants, wetting agents, and photo-acid generation
Class B Firefighting Foams
Aqueous film forming foam (AFFF) and other fluorinated Class B firefighting foams are another important source of PFAS to the environment, especially in military and aviation settings. Class B firefighting foams have been used since the 1960s to extinguish flammable liquid hydrocarbon fires and for vapor suppression. These foams contain complex and variable mixtures of PFAS that act as surfactants. Fluorinated surfactants are both hydrophobic and oleophobic (oil-repelling), as well as thermally stable, chemically stable, and highly surface active. These properties make them uniquely suited to fighting hydrocarbon fuel fires. Use of fluorinated Class B foams is prevalent and is a major source of PFAS release to the environment, as shown in Figure 2. Release to the environment typically occurs during firefighting operations, firefighter training, apparatus testing, or leakage during storage. Research into fluorine-free alternatives is underway and Congressional pressure is leading towards banning fluorinated Class B firefighting foams in the United States.
When discussing the relationship between firefighting foams and sources of PFAS to the environment, the emphasis is typically on AFFF; however, many different types of Class B firefighting foams exist. These may or may not be fluorinated (contain PFAS). Class B foams are used to extinguish Class B fires, that is, those involving flammable liquids. Fluorinated Class B foams spread across the surface of the flammable liquid forming a thin film and extinguish fires by (1) excluding air from the flammable vapors, (2) suppressing vapor release, (3) physically separating the flames from the fuel source, and (4) cooling the fuel surface and surrounding metal surfaces. From a PFAS perspective, Class B firefighting foams can be divided into two broad categories: fluorinated foams (that contain PFAS) and fluorine-free foams (that do not contain PFAS). This distinction and examples of each type are shown in Figure 3.
AFFF was developed by the US Navy in the 1960s and in 1969, the US Department of Defense (DoD) issued military specification MIL-F-24385 listing firefighting performance requirements for all AFFF used within the US DoD. These performance standards are often referred to as “Mil-Spec.” Products that meet the Mil-Spec have been added to the US DoD Qualified Product Listing (QPL). In 2006 the US Federal Aviation Administration (FAA) also began requiring that 14-CFR-139-certified commercial airports purchase Mil-Spec compliant AFFF only. Because the US DoD and FAA have been the primary purchasers of AFFF, development of AFFF product mixtures has historically been performance-driven (to comply with the Mil-Spec) rather than formula-driven (the specific PFAS mixtures utilized have varied over time and by manufacturer). Multiple manufacturers in the US and throughout the world produce or have produced AFFF concentrate. AFFF concentrate is or has been available in 1%, 3%, or 6% formulations, where the percentage designates the recommended percentage of concentrate to be mixed into water during application.
The specific mixtures of PFAS found in AFFF have varied by manufacturer and over time due to differences in production processes and voluntary formula changes. AFFF formulations can generally be grouped into three categories:
- Legacy Perfluorooctane Sulfonate (PFOS) AFFF This type of AFFF was manufactured exclusively by 3M under the brand name “Lightwater” from the late 1960s until 2002 using the ECF production process. They contain PFOS and perflouroalkane sulfonates (PFSAs) such as perfluorohexane sulfonate (PFHxS). Legacy PFOS AFFF produced by ECF were voluntarily phased out in 2002, however, use of stockpiled product was permitted after that date.
- Legacy fluorotelomer AFFF This group consists of AFFF manufactured and sold in the U.S. from the 1970s until 2016 and includes all brands that were produced using a process known as fluorotelomerization (FT). The FT manufacturing process produces polyfluorinated substances that can degrade in the environment to perfluoroalkyl substances (specifically PFAAs) including Perfluorooctanoic Acid (PFOA). Polyfluoroalkyl substances that degrade to create terminal PFAAs are referred to as “precursors” .
- Modern fluorotelomer AFFF This group consists of AFFF developed in response to the USEPA 2010-2015 voluntary PFOA Stewardship Program, which asked companies to commit to first reducing and then eliminating the following: PFOA, precursors that can break down to PFOA, and related chemicals from facility emissions and products. In response, manufacturers began producing only short-chain fluorosurfactants targeting fluorotelomer PFAS with 6 carbons per chain (C6), rather than the traditional long-chain fluorosurfactants (8 or more carbons per chain). These short-chain PFAS do not breakdown in the environment to PFOS or PFOA. Their toxicity in comparison to long-chain fluorosurfactants is a topic of current research.
In the US, AFFF users including the US DoD (predominantly the Navy and Air Force), civilian airports, oil refineries, other petrochemical industries, and municipal fire departments. AFFF is used, for example, in fire fighting vehicles, in fixed fire suppression systems (including sprinklers and fixed spray systems in or at aircraft hangars, flammable liquid storage areas, engine hush houses, and fuel farms), and onboard military and commercial ships. Fluorinated Class B foams may be introduced to the environment through the following practices:
- low volume releases of foam concentrate during storage, transfer or operational requirements that mandate periodic equipment calibration
- moderate volume discharge of foam solution for apparatus testing and episodic discharge of AFFF-containing fire suppression systems within large aircraft hangars and buildings
- occasional, high-volume, broadcast discharge of foam solution for firefighting and fire suppression/prevention for emergency response
- periodic, high volume, broadcast discharge for fire training
- accidental leaks from foam distribution piping between storage and pumping locations, and from storage tanks and railcars
The DoD is currently replacing legacy, long-chain AFFF with modern, short-chain fluorotelomer AFFF and disposing of the legacy foams through incineration. While the PFAS included in modern fluorotelomer AFFF formulations are currently understood to be less toxic and less bioaccumulative than those used in legacy formulations, they are also environmentally persistent and can degrade to produce other PFAS that may pose environmental concerns. While fluorine free alternatives exist, they do not meet the current Mil-Spec which requires that fluorine-based compounds be used. The US DoD is working to revise the Mil-Spec to allow fluorine-free foams, and several states have passed laws prohibiting the use of fluorinated Class B foams for training and prohibiting future manufacture, sale or distribution of fluorinated foams, with limited exceptions (e.g., WA Rev Code § 70.75A.005 (2019); VA § 9.1-207.1 (2019)). Additionally, a bill passed in the US Congress in 2018 directs the FAA to allow fluorine-free foams for use at commercial airports. Research into the development of Mil-Spec compliant fluorine-free foams that will be compatible with existing AFFF and supporting equipment is ongoing and includes the following:
- Novel Fluorine-Free Replacement for Aqueous Film Forming Foam (Lead investigator: Dr. Joseph Tsang, Naval Air Warfare Center Weapons Divisions) SERDP/ESTCP Project WP-2737
- Fluorine-Free Aqueous Film Forming Foam (Lead investigator: Dr. John Payne, National Foam) SERDP/ESTCP Project WP-2738
- Fluorine-Free Foams with Oleophobic Surfactants and Additives for Effective Pool fire Suppression (Lead investigator: Dr. Ramagopal Ananth, U.S. Naval Research Laboratory) SERDP/ESTCP Project WP-2739
Wastewater Treatment Plants
Consumer and/or industrial uses of PFAS-containing materials results in the discharge of PFAS to industrial and municipal wastewater treatment plants (WWTPs). Conventional WWTP treatment processes remove less than 5% of PFAAs. WWTPs, particularly those that receive industrial wastewater, are possible sources of PFAS release, as shown in Figure 4.
Evaluation of full-scale WWTPs has indicated that conventional primary (sedimentation and clarification) and secondary (aerobic biodegradation of organic matter) treatment processes can result in changes in PFAS concentrations and classes. For example, higher concentrations of PFAAs have been observed in effluent than in influent, presumably due to transformation of precursor PFAS. Some data has indicated that the terminal PFAS compounds PFOS and PFOA were among the most frequently detected PFAS in wastewater. A state-wide study in Michigan indicated that PFAS were detected in all of the samples from 42 WWTPs, including influent, effluent, and biosolids/sludge samples, and that the short-chain PFAS were more frequently detected in the liquid process flow (influent and effluent), while long-chain PFAS were more common in biosolids.
Multiple studies have found PFAS in municipal sewage sludge. The US EPA states that more than half of the sludge produced in the United States is applied to agricultural land as biosolids, therefore there are concerns that biosolids applications may become a potential source of PFAS to the environment. Application of biosolids as a soil amendment can potentially result in transfer of PFAS to soil, surface water and groundwater and can possibly allow PFAS to enter the food chain. Limited studies have shown that PFAS concentrations can be elevated in surface and groundwater in the vicinity of agricultural fields that received PFAS contaminated biosolids for an extended period. The most abundant PFAS found in biosolids are the long-chain PFAS. Based on the persistence and stability of long-chain PFAS and their interaction with biosolids, research is ongoing to determine PFAS leachability from biosolids and their bioavailability for uptake by plants, soil organisms, and the consumers of potentially PFAS-impacted plants and soil organisms.
Solid Waste Management Facilities
Industrial, commercial, and consumer products containing PFAS that have been disposed in municipal solid waste (MSW) landfills or other legacy disposal areas since the 1950s are potential sources of PFAS release to the environment. Environmental and drinking water impacts from disposal of legacy PFAS-containing industrial and consumer wastes have been documented.
Several studies have identified a wide variety of PFAS in MSW landfill leachates. PFAS composition and concentration in leachates vary depending on waste age, climate, and waste composition. The relative concentrations of various PFAS in leachate and groundwater from landfill sites is different from those found at WWTPs and AFFF-contaminated sites. In particular, 5:3 fluorotelomer carboxylic acid (FTCA) is a common and often dominant PFAS found in landfills, and has been released from carpet in model anaerobic landfill reactors. This compound could prove to be an indicator that PFAS in the environment originated from a landfill. PFAS may also be released to the air from landfills, predominantly as fluorotelomer alcohols (FTOHs) and perfluorobutanoate (PFBA). In one study, total airborne PFAS concentrations were 5 to 30 times greater at landfills than at background reference sites. PFAS release rates within landfills vary over time for a given waste mass, with climate (for example, rainfall) serving as the apparent driving factor for the variations.
Commercial and Consumer Products
PFAS are widely used in consumer products and household applications, with a diverse mixture of PFAS found in varying concentrations depending on the product. Environmental releases associated with the commercial and consumer products are primarily related to their production. To a much lower extent, the environmental releases may be associated with the management of solid waste (for example, disposal of used items in a MSW landfill) and wastewater disposal (for example, discharge to WWTPs, private septic systems, or other subsurface disposal systems).
Studies have shown that physical degradation of some consumer products (such as PFAS-treated paper, textiles, and carpets) may release PFAS in house dust. Additionally, studies have also shown that professional ski wax technicians may have significant inhalation exposures to PFAS and snowmelt and surface waters near ski areas could have measurable PFAS impacts.
As increased environmental sampling for PFAS occurs, additional information will become available to further our understanding of the major and minor PFAS contributors to the environment.
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