PFAS Uncovered: What You Need to Know About These Lingering (Not Forever) Chemicals
Paul Nathanail argues that rather than referring to PFAS as ‘forever chemicals’, the term ‘lingering chemicals’ better captures the challenge of managing their environmental risks once they escape into soil and water
THE FAMILY of per- and polyfluorinated alkyl substances (PFAS) are often, but inaccurately, referred to as forever chemicals.
But contrast the dictionary definitions of “forever” and “lingering”; forever means permanent, lasting for all future time; lingering means lasting for a long time or being slow to end – a far more fitting nickname for PFAS.
Historically manufactured to exploit their thermal, physical, and chemical properties, PFAS are a large and diverse family of synthetic chemicals.
Their usefulness stems from the strength of the carbon-fluorine bond – one of the strongest in organic chemistry. The length and geometry (linear or branched) of the alkyl chain and the functional group, if any, influence the specific properties of a PFAS molecule.
From drinking water to food packaging, firefighting foams to weatherproof fabrics, PFAS are reported to be “silently infiltrating our lives with potentially devastating consequences”.1 Indeed, human exposure to PFAS may lead to a wide range of adverse health effects, including various forms of cancer, decreased fertility, increased blood pressure in pregnant women, low birth weight, accelerated puberty, and reduced vaccine response.3 But what are PFAS and how do they end up in our lives?
A family with many classes, subclasses, groups, and subgroups
The PFAS family forms but one line in the clan of organic fluorine molecules – a line with over 7m molecules that meet the Organisation for Economic Co-operation and Development (OECD) definition of PFAS, of which 764,912 contain an isolated CF2 moiety, 6,092,023 contain an isolated CF3 moiety, and 229,621 contain PFAS parts larger than CF2 or CF3 moieties. Counts are taken from the PubChem open chemistry database at the US’s National Institutes of Health (NIH).2
Among the universe of organofluorines, compounds with a fully fluorinated methyl (CF3) or methylene (CF2) carbon atom are PFAS, including:
- fluorinated aliphatic substances that have a fully fluorinated methyl or methylene carbon atom
- non-fluorinated aromatic ring(s) with fluorinated aliphatic sidechain(s) that have a fully fluorinated methyl or methylene carbon atom
- fluorinated aromatic ring(s) with fluorinated aliphatic side chain(s) that have a fully fluorinated methyl or methylene carbon atom
Compounds with fluorinated aromatics that do not have a fully fluorinated methyl or methylene carbon atom are not considered PFAS.
PFAS comprise a fluorinated alkyl chain and also contain functional groups. Functional groups include hydrogen, other halogens (chlorine, bromine), nitrogen, phosphorus, and terminal functional groups such as sulphonic, carboxylic, or phosphinic acids.
Those PFAS with an acid terminal functional group are likely to be present in the ionised form in environmental conditions. The hydrophobic (nonpolar) and hydrophilic (polar) regions of such amphiphilic molecules give them a surfactant behaviour.
Those compounds that have a fully fluorinated methyl or methylene carbon atom comprise the perfluoroalkyl acids including the perfluoralkyl ether acids, the polyfluoroalkyl acids including the polyfluoralkyl ether acids, PFAS precursors, and other PFAS including the fluoropolymers and the perfluoroalkanes.
PFAS precursors are polyfluorinated compounds that can transform to perfluorinated alkyl acids in the environment or even within the human body.
PFAS – what are they good for?
Individual PFAS or PFAS mixtures have been used for a wide variety of purposes. Aqueous film-forming foams (AFFF) took advantage of the chemical and thermal stability of perfluoralkyl acids coupled with their propensity to accumulate at air-water interfaces. Their water- and fat-repelling properties saw them being used in weatherproof textiles and in food-contact packaging, respectively. A 2020 gold open-access paper identified over 200 uses of over 1,400 individual PFAS,4 with the following industry sectors all represented: aerospace; biotechnology; building and construction; chemical; electroless and electro plating (metal plating); electronics; energy; food production; machinery and equipment; manufacture of metal products; mining; nuclear; oil and gas; pharmaceutical; photographic; production of plastic and rubber; semiconductor; textile production; watchmaking; and wood processing.
The ‘everywhere’ chemicals
The natural environment operates as a complex and interconnected matrix where chemicals (good or bad) migrate between various compartments, facilitating the rapid spread of PFAS within a relatively short timeframe. These substances, due to their high stability and resistance to degradation, can persist in water, soil, air, and living organisms. Through processes such as runoff, groundwater migration, leaching from landfills, and atmospheric deposition, PFAS can travel vast distances, contaminating ecosystems far from their original sources. Their ability to bioaccumulate and persist in living organisms further amplifies their environmental impact. PFAS have been detected in a wide range of media – soil, sea water, groundwater, rain, polar ice, animals, plants, and human blood - across the world.
Risk management
As analytical capabilities expand to more PFAS, lower limits of detection, and wider range of media, the extent of PFAS pollution is slowly being realised. A hierarchy of responses is emerging: cease use; prevent loss of control; contain spreading; recovery and destruction.
Where environmental PFAS pose risks to human health, ecosystems, and water resources, remediation can reduce those risks by either removing the PFAS or preventing them reaching what we are trying to protect (See PFAS Capture, Structure, and Site Remediation).
Not forever, but lingering chemicals
The second sentence in the Health and Safety Executive’s executive summary of its 2023 PFAS Risk Management Options Appraisal reads: “Due to their chemical bond strength, these “forever chemicals” are slow to degrade and remain in the environment for many decades.” Slow and many decades suggest that, like diamonds, PFAS are not forever.
Over a 100-day period, the aerobic biodegradation of perfluorooctane sulfonic acid (PFOS) (90%), 6:2-fluorotelomer sulfonic acid (6:2 FTS) (21%), and 5:3-fluorotelomer carboxylic acid (5:3 FTCA) (58%) by Labrys portucalensis F11 was demonstrated by a team led from The State University of New York, Buffalo.5 PFOS metabolites with shorter carbon chain lengths that were detected included perfluoroheptane sulfonic acid (PFHpS), perfluorohexane sulfonic acid (PFHxS), perfluorohexanoic acid (PFHxA), perfluoropentanoic acid (PFPeA), perfluorobutanoic acid (PFBA), and perfluoropropanoic acid (PFPrA).
Last year, the University of Minnesota’s Lawrence Wackett6 challenged the accepted thinking that PFAS resistance to microbial degradation has a chemical explanation based on the strength of the C–F bond and suggested instead the as yet underdeveloped enzymatic capabilities of microorganisms as an explanation. If he is right, then the directed evolution of bacteria-degrading PFAS may be an avenue worth exploring.
We already know that energy-intensive chemical processes can cleave the C-F bond (see Breaking Down Barriers), so perhaps it’s time to say goodbye to “forever” chemicals and hello to “lingering” chemicals.
This shift in phrasing changes the narrative. The term forever chemicals might lead society to believe the problem is unsolvable, discouraging action. But we can make a meaningful difference. Through the following series of articles, let’s explore how engineers and technologists are working to break the chains.
References
1. MarketResearch.com: https://bit.ly/3QbkSDX
2. PubChem Classification Browser: https://bit.ly/4hwHsmx
3 US EPA: Our Current Understanding of the Human Health and Environmental Risks of PFAS: https://bit.ly/41arPeE
4. Environmental Sciences: Process & Impacts: An overview of the uses of per- and polyfluoroalkyl substances (PFAS) https://bit.ly/3Qbn8uV
5. Science of The Total Environment: PFAS biodegradation by Labrys portucalensis F11: Evidence of chain shortening and identification of metabolites of PFOS, 6:2 FTS, and 5:3 FTCA: https://bit.ly/42UB1W9
6 Microbial Biotechnology: Evolutionary obstacles and not C–F bond strength make PFAS persistent: https://bit.ly/40YFEMi
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