12 Mar PFAS: Forever Chemicals – Forever a Challenge?
Nebojša Ilić (ESR13)
Greetings fellow water and science enthusiasts. Today I want to discuss with you a challenge we are facing globally, but are not all aware of. This is one of the biggest challenges that we are facing in water treatment and a problem that has many different fields of science scrambling to join forces in search of a solution.
What I am talking about, which you might already know from my previous posts on the topic, is the removal of per- and polyfluoroalkyl substances (PFAS) from water. I wrote a brief introduction on what PFAS are in my first blog post on the topic. Briefly, they are a class of man-made organic chemicals that are significantly drawing public attention in recent years due to their toxicity and bioaccumulation potential.
Why do these compounds present such a big treatment issue? In order for this to be tangible, I will explain it using the example of perfluorooctanoic acid (PFOA), one of the most known compounds from the group. As you can see in figure 1, PFOA is an organic molecule with a 7-carbon chain and a carboxylic headgroup.
Furthermore, all hydrogen atoms on the carbon chain have been substituted for fluorine. The widespread use of these compounds is due to their desirable properties, which originate from the high electronegativity and small size of the fluorine atom, making the carbon-fluorine bond one of the strongest in organic chemistry. This makes PFAS thermally stable, resistant towards acids, bases and most oxidizing agents! As you can imagine, they are very hard to degrade into less harmful compounds since they practically have a fluorine shield around them.
PFAS are classified as trace organic contaminants, “trace” being the key word here as the concentrations of PFAS found in water are usually very low, in the low ng/L range. This number is higher (usually in the lower µg/L range) in high PFAS activity areas such as PFAS-related industries or airports.
Such low concentrations give the challenge of removing PFAS from water a whole new level of difficulty. Even the processes that can degrade PFAS are quickly disregarded, considering that operational costs are often highly dependent on the volume of treated water. We are therefore left with very limited options for treatment as a consequence.
To date, granular activated carbon (GAC) is the favored solution on the market. Water flows through beds filled with GAC, which serves as an adsorbent removing many different contaminants from water. Among these removed contaminants are long-chain PFAS (note that short-chain PFAS are not effectively removed with this process). However, GAC is a non-selective adsorbent, which means that the “available spots” for adsorption quickly get occupied by many different compounds.
This is why the current GAC filters that are in operation to remove PFAS from drinking water need to be exchanged every 6-9 months. And even then, the environmental issue remains, as the removed GAC is saturated with toxic chemicals that still need to be disposed of. This is again why many simple but effective separation processes such as reverse osmosis do not present an effective solution on their own – their concentrate is rich in PFAS and still an issue to dispose of.
In order to address these issues, a process is needed that either selectively removes PFAS from water or has a destructive effect with no regard for selectivity. This is why our group is conducting research in these two directions, one technology for each of the conditions mentioned. You can read more about our research in the second post I have written on the topic of PFAS.
Ultimately, I highly doubt that a single miracle process will tackle this treatment challenge with success. Engineering multiple processes into a treatment train in order to use their strengths in synergy is the most likely approach to succeed in effectively reducing PFAS concentration to levels deemed safe for drinking water.
In the meantime, if you’re interested in how current water treatment facilities cope with the PFAS issue, this publication is a good study to start with.
In the end, with everything bad comes some good. This environmental crisis has made us look long and hard at our current water protection practices and regulations. The times are changing, but the laws take time to catch up. The new European Commission drinking water directive is proof of that, finally being put in place as of December 2020.
This directive addresses many emerging and known issues. It is the first regulatory decision to put a strict limit on the amount of PFAS present in drinking water provided to the residents of the European Union (as opposed to advisory regulations) – 100 ng/L for a sum of 20 selected PFAS and 500 ng/L for PFAS total.
As other countries are sure to follow this example, more interest in the topic and more allocated funding will drive more people into this research field. So, keep your fingers crossed and your eyes peeled; this is just the beginning!