Bacteria that break down some types of “forever chemicals” can be found in sludge from wastewater treatment plants.
Perfluoroalkyl and polyfluoroalkyl substances (PFAS) are a class of synthetic chemicals widely used in coatings and foams that resist oil, heat and water. There are thousands of types of PFAS, several of which have been shown to cause harmful health effects. They are also long-lasting environmental contaminants thanks to the tough carbon-fluorine bonds they contain.
One way to deal with this contamination would be to identify microbes that degrade those carbon-fluorine bonds, says Yujie Men at the University of California, Riverside. But fluorine bonds are rare in nature, and microbes that can break the bonds also appear to be rare.
In search of such microbes, Men and her colleagues collected sludge from a nearby municipal wastewater treatment plant. They then spiked samples of the sludge with three types of chlorinated PFAS that had a low, medium and high number of carbon-chlorine bonds, which are more vulnerable to biodegradation than fluorine bonds are. They also added methanol to feed any microbes present.
After 84 days in low-oxygen conditions, 10 per cent of the fluorine bonds in the low group had degraded, as did 20 per cent in the medium group and around 80 per cent in the high group. When the sludge was then exposed to oxygen, activating any aerobic bacteria present, the remaining bonds across all groups were degraded a further 12 per cent.
The researchers isolated the bacteria responsible for breaking down the molecules in anaerobic conditions. Their genomes were most similar to Desulfovibrio aminophilus and Sporomusa sphaeroides, bacterial species commonly found in water environments. “They are not unique,” says Men. Similar microbes could already be breaking down chlorinated PFAS contamination, she says.
The bacteria don’t break the tough carbon-fluorine bond directly, says Men. Instead, they cleave the weaker bonds between carbon and chlorine. They then replace the chlorine with an oxygen and hydrogen group, which destabilises the molecule and makes it more likely for the fluorine bond to break.
Breaking down chlorinated PFAS wouldn’t do anything to address the contamination from many other types of PFAS that don’t contain chlorine. “We’re not going to solve every problem with one magic bacterium,” says Lawrence Wackett at the University of Minnesota.
But understanding how these molecules break down could help researchers design alternatives to PFAS that biodegrade more readily by incorporating more of these chlorine “weak points”, he says. However, those molecules would also have to be tested to make sure they aren’t also toxic.