@the-no-dont-do-its very good question! firstly, it's important to point out that on their own, they don't. we have to actively apply methods to remove them from the environment. these methods are LARGELY based on adsorption, which is sort of like filtering except it involves the chemical getting stuck to something else (the adsorbing material).
you can think of this sort of like how water wicks into a paper towel. the water gets stuck to the paper because it's attracted to it via capillary forces, even though there's no chemical reaction going on.
the two main methods used are granular activated carbon (GAC) adsorption and ion exchange (IX).
activated carbon is already pretty familiar to a lot of us; it's the stuff in a lot of replaceable water filters. the activated carbon has a huge internal surface area, and that allows for the fairly weak intermolecular forces to add up and allow contaminants to get "stuck" onto the surface of the activated carbon. over time, the activated carbon gets filled with junk, and you have to replace it.
GAC is essentially this, except that the activated carbon is granularized and produced in specific ways to maximize how much it attracts certain chemicals. this can be tuned because activated carbon gets its massive surface area from internal "pores", and various processes will change how large and frequent those pores are.
It's essentially a Russian nesting doll of pores, and controlling the size of the larger pores influences the permeability of the activated carbon and controlling the size of the smaller pores (micropores) influences what exactly is most attracted to the activated carbon.
However, GAC has a few major downsides:
It is not specific to PFAS. This is more of a mixed blessing because it was already frequently used and well understood, and the infrastructure for producing and distributing it already existed. However,
It loses effectiveness over time and must be replaced. This is a continued cost, albeit a low one, but this has one final major issue
As time goes on, the PFAS previously adsorbed to the activated carbon is desorbed and replaced by other things that have a higher affinity for the activated carbon.
As such, ion exchange (IX) was always very compelling. The whole point of it relies on the fact that PFAS molecules are predominantly made of two parts: An acid head group (either a carboxylic or sulfonic acid group) and a perfluorinated tail.
The head groups on the right are what become ionized—or specifically, deprotonated. A hydrogen leaves and is replaced with a metal cation (usually sodium), forming a PFAS salt (chemical meaning of salt!). These are much more soluble in water because of polarity reasons, and so the mobile PFAS molecules are almost always in that salt form.
By passing through these PFAS salts through a permeable polymer matrix that has (1) numerous positively charged groups like quaternary amines and (2) highly mobile negative ions loosely attached to those stationary positive groups (most often chlorides), you can actually get the PFAS to be "stuck" inside the polymer matrix and what comes out is just good ol' sodium chloride, or salt (culinary meaning of salt!).
This shows a version with hydroxide (OH-) ions as the mobile anion, but it's the same idea. The +NR3 in yellow are stuck to the polymer matrix, but the OH- can freely move around. However, without another anion to replace the OH-, the ionic attraction prevents the hydroxides from leaving.
In comes the PFAS. Despite being slightly soluble in water, the anionic PFAS aren't really that mobile, and when they pass through, it's much easier for the hydroxide ions to leave. Another very important effect is that the long perfluorinated tail of the PFAS is attracted to the polymer matrix, whilst the counterions are ONLY attracted via the ionic force. Thus, PFAS would much rather hang out in the polymer matrix.
Of course, IX has its own downsides
These resins are much more expensive, both to manufacture and to transport.
While they can be "regenerated", it's a tricky process that currently requires the use of nearly anhydrous methanol, which is both poisonous and extremely flammable, increasing the operating costs.
As the hydrophobic tail is a key part of allowing the PFAS to stick to the matrix, short-chain PFAS are very poorly dealt with by this system. This is exacerbated by competition between different PFAS molecules, as long-chain ones will cause short-chain ones to desorb.
Overall, the best method appears to be using a series of ion exchange resins followed by an activated carbon filter. The ion exchange will capture the bulk of the PFAS molecules, and the activated carbon will grab any stragglers. Effective filtering of other contaminants prior to the PFAS removal system will also ensure minimal competition in the activated carbon.
And a SIGNIFICANT amount of this understanding has come in the last fifteen years. In particular, the idea of ion exchange is very new! Twenty years ago, it was seen as WAY too expensive, fragile, and ineffective to ever be a useful technology. Nowadays, it's widely implemented in problem areas and we've built up the infrastructure to support it.
