New ‘forever chemical’ elimination method ‘might be the start of something practically interesting’

A TEAM of multinational researchers has developed a novel method for breaking down environmentally persistent and toxic per- and polyfluoroalkyl substances (PFAS). Chemical engineer Paul Stevenson says while the method is promising, further work is needed to enable implementation.

PFAS are a large group of synthesised chemicals used in a variety of industries, such as aerospace, automotive, and construction, and products including non-stick cookware. In a recent feature discussing practical methods for remediating these forever chemicals, Stevenson, who engineers separation technologies, noted that PFAS continue to be manufactured in some jurisdictions despite the huge environmental contamination caused by their environmental persistence, and their toxicity to humans. Research into PFAS alternatives and environmental elimination methods has increased as authorities seek to deal with the hazards they pose.

The multinational research team, which includes chemists from Northwestern University, US, focussed on breaking down perfluoroalkyl carboxylic acids (PFCAs), a class of PFAS compounds. They used PFCAs of various lengths which were efficiently mineralised in the presence of a NaOH reagent in mixtures of water and dimethyl sulfoxide (DMSO) as solvent, at mild temperatures (80–120°C) and ambient pressure.

In a particular reaction, 0.089 M perfluorooctanoic acid (PFOA) was degraded in an excess of NaOH, in 8:1 DMSO:H2O solvent, at 120°C. This resulted in a mixture of fluoride, trifluoroacetate ions, and carbon-containing products. The researchers found that about 90% of fluoride atoms from the PFOA were recovered as ions after 24 hours.

The study has been published in the journal Science and has received considerable media attention. Stevenson says that the research “has some worth in that it demonstrates a chemical mechanism, but this is all a very long way away from a ‘breakthrough’. Highlighting that the process takes 24 hours at 120°C, he says “the kinetics are far too slow to be of any practical value at scale. 120°C may be a low temperature for a micro-reactor on a university bench, but it is a very high temperature on a water treatment plant.”

Also, he says the 0.089 M concentration used in the experiments is 80m times higher than would be found in the feed concentration of PFOA to an Australian water treatment plant which he previously described in an article reviewing the processing options for remediating PFAS contamination.

“If [the researchers] used the Australian concentrations then the batch time would approach infinity. Of course, if you take the concentrate of a foam fractionator as feed to the…process then the PFOA concentration is much bigger, but we are still looking at kinetics such that the batch times may be in the order of months.”

Foam fractionation can be used to capture PFAS by adsorbing them to the gas-liquid interface of bubbles rising through a liquid.

Stevenson also notes that trifluoroacetic acid (TFA) produced by the reaction is not a “harmless product”, though it is “probably not as toxic as PFAS, and it is not known to be bioaccumulative”.

Shedding light on the potential impact he adds that the research does not demonstrate efficacy with PFAS classes outside of PFCAs. While the researchers state that PFCAs are “one of the largest classes of PFAS compounds”, Stevenson highlights that the group only accounts for a “minority of generally-proscribed PFAS species”.

There are also issues to consider around the use of DMSO, which Stevenson said should be recycled, requiring efficient downstream separation. He also highlights the very small scale of the batch process (24 ml total) compared to a medium-sized water treatment plant, which could have throughput of 1m L/d.

William Dichtel, Professor of Chemistry at Northwestern, says: “We are interested in exploring the scale-up of a PFAS destruction process based on our discovery, but there are several more aspects to work out at small scale first, including modifying the conditions to degrade perfluoroalkyl sulfonates, if possible. After that, I would envision that this method would be best applied to the back end of a PFAS removal technology for drinking water, such as a regenerable adsorbent system or a reverse osmosis waste stream. But there might be other contexts where our method might also make sense.

“For now, I think the immediate and hopefully lasting impact of our work is that it points to a previously unknown reactivity of many PFAS compounds at a more fundamental level. Many creative chemical engineers might find unique ways to access these pathways that we do not envision within the existing approaches to PFAS degradation.”

Stevenson concludes: “There is absolutely nothing wrong with the…research. It might be the start of something practically interesting, but it is currently a universe away from plant implementation.”

Relevant bodies such as the US Environmental Protection Agency and the European Commission have been recognising the harm of PFAS through regulations requiring companies to report releases and targets to phase them out. Earlier this year, the UK said its REACH chemicals regulations would consider restricting PFAS.