Fluorinated waste, like that found in firefighting aqueous film-forming foam, can act as a significant environmental pollutant. Recently, James Tour's lab at Rice University developed a method to capture the fluoride from these waste streams and recycle it into a useful reagent, silver fluoride, that could be used in other manufacturing processes. This process was recently published in Nature Chemical Engineering.
"The concerning elements in such waste streams are perfluoroalkyl and polyfluoroalkyl substances, or PFAS," said Tour, the T.T. and W.F. Chao Professor of Chemistry and corresponding author. "We have previously been able to convert the fluorine in these molecules into stable inorganic fluorides, which is environmentally friendly waste — but it is still waste. Here, we found a way to move the fluoride from a waste stream to an incredibly efficient recycling stream."
In a previous work, Tour's team used activated carbon to absorb PFAS from water. They then converted the carbon into graphene while capturing the fluoride as calcium fluoride, the natural and nontoxic mineralized form of fluoride in the environment. However, the captured fluoride wasn't available for any other use at that point.
"We saw this as an opportunity to turn this from a waste-destruction problem into a resource-utilization problem," said Yi Chen, a former Rice Academy Fellow and co-first author who is now an assistant professor at Fudan University. "Fluorine is a valuable element in pharmaceuticals, agrochemicals and advanced materials. Our flash encapsulated fluorination process recovers fluorine from PFAS waste in a form that can be put directly back into useful chemistry."
Flash encapsulated fluorination involves treating the activated carbon-adsorbed PFAS, in the presence of silver nitrate, with short electrical pulses to rapidly heat them to several hundred degrees Celcius. Once the material is hot enough, the fluorine atoms are released from the PFAS and can be quickly captured by nearby silver atoms, resulting in silver fluoride, a molecule that can be easily used to fluorinate organic substrates.
"The biggest challenge was that the hot carbon needed to release fluorine also creates a reducing environment that can convert newly formed silver fluoride back to metallic silver ," said Bowen Li, a co-first author and former Tour lab postdoctoral fellow who is now a professor at Soochow University. "We solved this by physically separating the carbon and silver with a porous quartz-fiber barrier. Fluorine-containing gaseous species can pass through the barrier, while the solid carbon and silver remain separated."
When the team tested it in their lab, the silver fluoride produced was equivalent to the commercially available version used to create a variety of useful compounds in everything from pharmaceuticals to agrochemicals. And rather satisfyingly, the silver itself could also be reused. Once the silver delivered its fluoride to the intended organic compound, it could be collected and used again in the flash encapsulated fluorination. This process removes over 99.9% of the fluoride available in the PFAS with 90% of that collected as silver fluoride.
"This takes PFAS waste treatment from a process focused on neutralizing and removing waste into circular materials chemistry," Tour said. "We can neutralize the environmentally harmful PFAS, put the fluorides in a usable format and then return them to organic chemists to use for production elsewhere."
This work was supported by the Air Force Office of Scientific Research (FA9550-22-1-0526), the U.S. Army Corps of Engineers' Engineer Research and Development Center (W912HZ-21-2-0050 and W912HZ-24-2-0027) and the Rice Academy Fellowship.
