PFAS degradation: nothing lasts forever, even "forever chemicals"

The invention of Teflon, the very first PFAS compound, happened by accident in 1938. Scientists at DuPont were trying to make a different molecule entirely, but they unintentionally synthesized PFTE, known commercially as Teflon, instead. This nonstick, water repellant, heat resistant compound was initially used in manufacturing applications, but quickly found use in consumer products as well.

That original compound, PTFE, has expanded to an entire class of chemicals known as per- and polyfluoroalkyl substances, or PFAS. The compounds have been used for decades in a wide variety of consumer products, from non-stick cookware to waterproof clothing to food packaging to electronics. Nobody could have predicted that these compounds would eventually become ubiquitous in the environment and pose significant health concerns. Well, that's not quite true. One group was able to predict it: the very company that invented Teflon in the first place.

A brief dive into PFAS chemistry

PFAS, or per- and poly-fluoroalkyl substances, get their characteristic properties such as heat- and water-resistance, from their chemical structure. Breaking the name down, alkyl means long chains of carbon molecules, while fluoro- indicates that fluorine molecules are attached to the carbon chains. The length of the chain, the number of fluorines, and the presence of functional groups that convey additional properties determine the name of the compound. There are between 4000 and 7 million compounds classified as PFAS, depending on which definition is used. Some of the most widely studied are PFOS and PFOA.

PFOS, one of the most well-studied PFAS compounds

The characteristic fluorine molecules are the reason that PFAS are difficult to break down, enough so that they have been dubbed "forever chemicals" in the environment. The carbon-fluorine (C-F) bond is very strong, and fluorine molecules released by breakdown are toxic to the cell. As a result, there are only a few microorganisms that break down PFAS. This has resulted in an accumulation of the compounds in the environment, with a variety of human and environmental health consequences.

PFAS distribution and detrimental effects

In the environment, PFAS are found in water, air, and soil, and are widespread even in areas where PFAS manufacture has never taken place, such as the arctic and the amazon. Industrial sources include manufacturing, landfills, and the release of firefighting chemicals. In addition, everyday products release small quantities of PFAS, which add up over time due to the sheer number of items that contain the compounds. They accumulate in plant and animal tissues, travel through the air, and reach groundwater via leaching through the soil. These act as entry points into the human body.

PFAS have a wide variety of negative impacts when ingested and inhaled. These include cancers, endocrine disruption, damage to the liver and other organs, and pregnancy complications. The impact that PFAS exposure will have on any individual depends on quantity, types of PFAS, and individual biological variation.

DuPont, the creators of Teflon and other PFAS, actually knew of these issues as early as the 1970s. They suppressed the information so that they could continue to make money from their inventions. The full story of this coverup, which is both fascinating and horrifying, is detailed in the 2019 film Dark Waters.

Environmental degradation of PFAS

Despite their reputation as "forever chemicals," degradation of PFAS in the environment does occur. Microbes can break down PFAS alone or in communities. Most degradation occurs aerobically by microbial communities found in soil and sludge environments. Anaerobic degradation has been studied in two specific microbes, Acidimicrobuim and Dehalococcoides, although both perform better in mixed microbial communities than in pure cultures. Notably, most PFAS studies are conducted over periods of several months, during which even low concentrations of PFAS are only degraded 10-50%. This may reflect the non-specific nature of the enzymes responsible for the degradation.

Microbial enzymes are highly specific and only act on one compound, but some act on a variety of additional compounds beyond the one they evolved to interact with. This non-specific activity is called enzyme promiscuity, and explains the way that most PFAS degradation occurs in the environment. The classes of enzymes that act on PFAS include laccases and peroxidases. Both of these enzyme groups work by generating free radicals that then attack and degrade complex compounds.

A few well-studied enzymes for PFAS degradation are lignin peroxidases produced by fungi, which originally evolved to break down complex plant polymers called lignin. The enzymes react with natural compounds in the environment  to generate radical cations, which attack lignin. When PFAS is present, it undergoes the attack as well. Another mechanism may involve breaking down long PFAS molecules into smaller units, which other enzymes then convert to CO2 and fluoride ions.

There may also be enzymes that have evolved, or will evolve, to break down PFAS in a more targeted manner. I discuss the details of microbial enzyme evolution in the context of 6PPD, another emerging contaminant, here. To summarize, microbes copy their DNA, including genes for enzymes, before dividing. Sometimes there are mistakes, called mutations. These mutations are usually neutral and have no impact on enzyme activity, or detrimental because the prevent the enzyme from working properly. But sometimes, mutations change enzyme structure so that it can bind and break down non-target compounds, opening up a new food source and providing a competitive advantage to the organism.

The process of enzyme evolution raises an interesting question about PFAS. If these compounds have been around since the 1930s, why have a plethora of specific enzymes to degrade them not emerged? Why are PFAS so resistant that they have been classified as "forever chemicals?" The answer may lie in the characteristic C-F bond. There are genes in nature that cleave the C-F bond, but they are fairly rare because it takes a lot of energy to break it, and because of the toxicity of fluoride ions. Thus, there is a negative selective pressure against enzymes with this activity. Still, they do exist, and they provide a potential starting point for the evolution of PFAS-specific enzymes.

The future of forever chemicals

There is still much to be discovered about PFAS degradation in the environment. The best treatments may require pairing microbial activities with non-biological processes, like activated charcoal sorption, ultrasonication, and thermal treatments. These physical methods are effective, but carry their own issues related to energy efficiency and cost.

And microbial PFAS degradation is also varied - each compound is unique, and thus microbes have different degradation efficiencies. This obstacle may be overcome with techniques such as enzyme engineering and targeted evolution. There is no one best way to remediate PFAS, and instead we might have better luck applying specific combinations of physical, chemical, and biological treatment to each individual pollution scenario.

If you want to learn more about the microbial treatment of forever chemicals and other environmental contaminants, AppliedMicrobio can help. Reach out today!