Perfluoalkyl chemicals are organic chemicals whose hydrogen atoms have been replaced by fluorine. Polyfluoroalkyls have had many of the hydrogens replaced by fluorine. More than 4,000 perfluorinated and polyfluorinated substances (PFAS) have been produced since they were developed in the 1940s. More than 1,000 are listed in the U.S. Environmental Protection Agency’s (EPA) Toxic Substances Inventory of commercial chemicals.
Fluorination introduces many unusual and useful properties. The chemical, or at least part of it, becomes hydrophobic (repelled by water), whereas other parts, such as a carboxylic acid unit, become more ionic and water soluble. The carbon-fluorine bond is resistant to hydrolysis and other chemical processes and biological degradation, so those chemicals are not readily metabolized after ingestion and are stable in the environment and not removed by standard decomposition mechanisms. Thus, they can be detected in water and soils for many years after they have been introduced. They may also have unusual toxicological properties, partly because they will not be metabolized or eliminated rapidly and will accumulate in the person or animal that has ingested or inhaled them. For these reasons, fluorinated chemicals have become objects of major international regulatory interest aimed at preventing their introduction into the environment and their removal when they are detected in media like food or water.
Their chemistry provides many unique properties and applications. Perfluorocarboxylic acids like perfluorooctanic acid (PFOA) are surfactants with uses in firefighting foams, for example; Teflon is polytetrafluoroethylene with well-known water-repellant and nonstick properties; and water-repellent products are widely used as a soil repellant on fabrics. Some PFASs found in the environment result from disposal of production waste byproducts, and some are from degradation from higher molecular weight perfluorocarbons. The half-life of perfluorooctanesulfonic acid (PFOS) in humans is about 5.4 years, PFOA about 2.3 years, and for perfluorobutanesulfonic acid (PFBS) — a substitute — it is about one month. GenX, a substitute for PFOA, is a hexafluoropropylene oxide-dimer acid that was first reported in North Carolina’s Cape Fear River in 2015, and it has been found in the drinking water for 200,000 people downstream from the manufacturing site.
PFAS product and exposure sources
PFOS is or has been used as a water or stain-proofing agent in carpets, paper packaging, aqueous film forming foam and coating additives. Presence in food can be substantial; studies of commercial fresh fish in Italy reported PFOS at up to 1896 ng/kg (mean value of 627 ng/kg) and PFOA at 487 ng/kg (mean value of 75 ng/kg). Note: ng/kg = nanograms/kg = parts per trillion (ppt).
There are numerous examples of PFAS chemicals found in groundwater drinking water sources. Most of these are near parts-per-trillion levels, but some have been at parts-per-billion levels. PFAS chemicals are or were widely used in food packaging, even popcorn bags; the shorter chain chemicals are still approved by FDA. EPA’s 3rd Unregulated Contaminant Monitoring Rule report in 2018 collected data from almost 4,800 public water supplies that included all large, mostly surface water supplies and about 800 smaller, mostly groundwater supplies and reported 91 with PFOS above 20 ppt and 107 with PFOA above 40 ppt.
Studies of U.S. human blood in 1999 and 2004 detected PFOS, PFOA or other perfluoros in virtually 100 percent of the population. Mean values were 5.21 µg/L (micrograms/liter) PFOA and 30.4 µg/L PFOS. By 2012, these were reduced to 2.08 µg/L and 6.31 µg/L respectively, reflecting stewardship phase-outs and controls instituted by EPA in 2010. Workers at a manufacturing plant had mean serum levels of 1760 µg/L PFOA and 1320 µg/L PFOS. People in the vicinity had 83.6 µg/L mean values of PFOA.
EPA issued revised health advisories for PFOA and PFOS combined at 70 ppt and concluded that adverse health effects would not occur over lifetime exposure at those levels. However, several states have been producing their own standards or guidance as low as 10 ppt.
Water treatment technologies for PFAS compounds
Because of their properties, diversity, numbers and usually low ppb and ppt environmental presence, the PFASs present a significant analytical and treatment technology challenge. Water treatment technologies for low-level concentrations include: granular and powdered activated carbon, membrane filtration, anion exchange and possibly advanced oxidation.
There are several conventional technologies that have been applied to removing PFAS compounds from water. They may be more or less effective, but they are challenged because of the low concentrations found in ambient waters and the low concentration targets that have been evolving for wastewater and drinking water. As usual, benchtop or pilot studies are necessary to determine the appropriate design considerations including pretreatments for any particular water.
Polymeric coagulant aids, particularly cationic polymers, should have some variable efficacy as part of a pretreatment process, at least for the acidic PFAS compounds like PFOA and PFOS. There would be some level of cation/anion complexing, as well as hydrophobic absorption of the nonpolar part of the molecules and the coagulation polymer.
Activated carbon, particularly certain granular activated carbons (GAC), are capable of removing longer chain PFOS compounds. There are differences between various types of GAC for efficacy and capacity. Removal rates above 90 percent with end water concentrations at very low ppt and no detectable levels have been reported, with usually longer-than-typical empty bed contact times and fairly frequent bed replacements. As usual, the water composition and presence of competitive adsorbers will have an impact on the outcomes and economics of particular waters. Certain powdered activated carbons (PAC) may also have applicability for short-term use for spills or in combination with coagulation processes. PACs may have higher adsorption capacities than equivalent GACs. All of the nondestructive technologies generate problems for disposal of the spent materials and concentrates that are generated.
Anion exchange processes
Anion exchange resins are capable of removing several perfluorocarbon anions at high efficiencies and with small footprints and space requirements. The resins can be regenerated in place, which offers an advantage to their use. The brine produced by the regeneration process requires containment and proper disposal or decomposition. Some waters will require pretreatment because there are competitive water composition factors, including other anions that will affect the removal efficiencies.
Reverse osmosis (RO) and nanofiltration (NF) are generally effective for removing PFAS compounds. Reverse osmosis has achieved better than 99 percent removals of PFOS and PFOA from drinking water and 90 percent removal from some wastewater. Nanofiltration has higher throughputs but would likely be less effective than RO for lower molecular weight chemicals. Both require pretreatment in many applications but perhaps not much when applied to otherwise high-quality groundwater. Both will generate reject water with accumulated PFAS chemicals that require proper disposal. Reject water volumes may range from about 5 to 25 percent of the input, depending upon the water composition, specific membrane and design and operating pressures.
Advanced oxidation processes
Oxidation by ozone or chlorine would not be efficacious for decomposing PFAS compounds because of the high stability of the C-F chemical bond. Advanced oxidation processes (AOP) are used in some common wastewater treatment processes to remove trace organic chemicals. They use processes that generate hydroxyl radicals that are much more aggressive than other oxidizing chemicals. However, they are still not highly effective when dealing with C-F chemical bonds. Removal rates as low as 10 percent have been reported, but other studies have achieved up to 50 percent in some conditions. However, AOP is indiscriminate and requires water that has been highly purified to remove competing (usually natural) chemicals that are much more susceptible to oxidation than PFAS chemicals and are usually present in much greater concentrations. So, AOP is not likely to be cost-effective for removal of most of the highly stable fluorinated organic chemicals.
EPA Action Plan for PFAS
EPA recently issued an Action Plan to address these fluorinated compounds with all of the legislative authorities and resources that it can access. Most of the current activity has focused on PFOA and PFOS because they have a long history of use in numerous products, and they have long elimination half-lives. However, numerous additional fluorinated compounds have also been in use and are being detected in water and food, so the plan will also address the broader spectrum.
The plan includes: making a regulatory determination for a drinking water standard by the end of 2019, along expanded analytical methods, an interim groundwater cleanup standard, toxicity assessments for GenX and PFBS and collecting additional monitoring and product composition data. In 2020, additional risk assessments and an Unregulated Monitoring Rule (UCMR) will be proposed. Ambient Water Quality Criteria will be considered in 2021 under the Clean Water Act.
Perfluorinated chemicals will be subject to studies, risk assessments, possible regulations and data collection under several statutes, so product- and waste-reduction practices should be considered immediately to reduce any potential consequences. Many states have adopted the EPA Health Advisory or lower values. It is undecided whether the PFAS issue requires a national drinking water maximum contaminant level under the Safe Drinking Water Act specifications.
There are EPA Health Advisories in place, and they are essentially Maximum Contaminant Level Goals (MCLG). However, the critical element for understanding drinking water health and economic impacts of a potential national standard should generate an immediate shortcut effort to obtain drinking water data for PFAS chemicals from all groundwater supplies. Issuing a proposal for a UCMR in 2020 will not generate essential information for at least three to five more years.
- EPA Action Plan for Per- and Polyfluoro Substances. https://www.epa.gov/pfas/epas-pfas-action-plan. February 2019.
- Assessment of Perfluorooctane Sulfonate and Perfluorooctanoic Acid Exposure Through Fish Consumption in Italy. Ital J Food Saf. 2016 Sep 20; 5(4): 6055. Doi:10.4081/ijfs.2016,6055.
- PFOA & PFOS in Drinking Water. July 1, 2016. https://www.watertechonline.com/professor-poupoe-pfoa-pfos-drinking-water.
- ATSDR Toxicological Profile for Perfluoroalkyls. https://www.atsdr.cdc.gov/toxprofiles/tp.asp?id=1117&tid=237.
- Water Research Foundation (2014). Advances in Water Research 24(2):19-23.
- Appleman, T, et al. Treatment of poly-and perfluoroalkyl substances in US full-scale water treatment systems. Water Research, 51, 246-255.
- Rahman, M, et al. Behavior and fate of perfluoroalkyl and polyfluoroalkyl substances in drinking water treatment: A review. Water Research, 50, 318-340.