Ball milling breaks PFAS down into industrially useful fluoride source

Per- and polyfluoroalkyl substances (PFAS) – the persistent, bio-accumulative anthropogenic pollutants colloquially known as ‘forever chemicals’ – can be mechanochemically broken down into fluoride sources for industrially important molecules, researchers in the UK and US have shown.¹ The process, which followed a serendipitous discovery, could potentially reduce demand for fluorspar (calcium fluoride), which is considered a critical mineral in many parts of the world.

PFAS, which are used for everything from non-stick coatings to water-repellant clothing and lithium-ion batteries, are classified as persistent organic pollutants because they do not degrade in the environment. They have been associated with cancer, immunotoxicity and other diseases, so multiple research groups are developing methods to destroy them.

The traditional method of producing fluorochemicals from fluorspar requires the use of sulfuric acid to convert the calcium fluoride to hydrogen fluoride gas, which is highly toxic and corrosive. In 2023, researchers in Véronique Gouverneur’s group at the University of Oxford in the UK and colleagues developed a safer method by ball milling the calcium fluoride with a potassium phosphate salt, producing calcium phosphate and solid potassium salts that could be used to build carbon–fluorine bonds.²

The new work arose from a chance observation when the researchers explored the effect of different jar size for this process. ‘When the jar had a seal that was made from PTFE [polytetrafluoroethylene – commonly known as Teflon] instead of rubber, the fluoride recovery was more than what we introduced as fluorspar,’ explains Gouverneur. Together with computational chemist Robert Paton at Colorado State University in the US, the researchers worked out that the phosphate acts as a nucleophilic oxyanion, causing cleavage of the carbon–fluorine bond in the PTFE. ‘The collaboration with Rob enables us to study the feasibility of such a nucleophilic substitution process; there are not many reactions invented by chemists that use a phosphate ion as a nucleophile,’ Gouverneur notes.

The researchers then tested the reaction using other PFAS, including perfluorooctanoic acid, which is classified as carcinogenic by the World Health Organization. They recorded yields of over 50% in all PFAS tested and 100% in some. ‘Many processes invented to date are PFAS-specific,’ says Gouverneur. ‘Very often they work with a PFAS that has a functional group like a carboxylic acid or a sulfonic acid motif that they can use to trigger the decomposition. But if you have a fluoroplastic like PTFE or PVDF [polyvinylidene fluoride] – which is a component of lithium-ion batteries and has many other applications – there is no weak bond. So it was really exciting that we could mineralise PFAS in a more generic manner.’

The researchers successfully recovered the fluorine content of PFAS as potassium fluoride and potassium monofluorophosphate, the latter amenable to conversion into commonly used fluorinating reagents to produce a variety of valuable compounds, including pharmaceuticals, herbicides and electrolytes.

Michael Wong at Rice University in Texas, whose own group tackles the breakdown of aqueous PFAS using UV photocatalysis, is intrigued. ‘They’ve discovered a way whereby combining mechanochemistry with this phosphate salt it somehow changes the chemistry in such a way that it breaks down the liquid versions, the solution versions and the solid versions of PFAS unexpectedly fast. I think that was the most surprising thing,’ he says. He is unsure, however, whether fluorspar is sufficiently scarce to provide an economic incentive to use PFAS waste as an alternative fluorinating reagent. Gouverneur believes this is likely to vary from country to country and may be subject to geopolitical tension. Nevertheless, Wong believes that treating the breakdown products as intermediates and showing that they can be used to do potentially valuable chemistry is ‘interesting’.