Farewell, forever chemicals

Per- and polyfluoroalkyl substances first showed up in human blood in the 1960s. In the early 2000s, reports emerged of their presence in Arctic wildlife – including polar bears. Now the race is on to scrub these ‘forever chemicals’ from the environment. Emma Davies reports

Per- and polyfluoroalkyl substances (PFASs) have been used by almost every industry, including in non-stick cookware, water-repellent clothing, stain-resistant fabrics and carpets, cosmetics, firefighting foams, and products that resist grease, water and oil. But the tough carbon-fluorine bonds that make PFASs so useful also make them incredibly persistent in the environment. Contamination is causing alarm because chronic exposure to PFASs is associated with adverse health effects such as thyroid disease, liver damage, high cholesterol, reduced immune responses, low birth weights and several cancers.

Regulators are taking drastic action. For example, the US Environmental Protection Agency (EPA) recently proposed maximum contaminant levels (MCLs) for six PFASs under the Safe Drinking Water Act (SWDA), including perfluorooctanoic acid (PFOA), perfluorooctane sulfonic acid (PFOS) and hexafluoropropylene oxide dimer acid (HFPO-DA, commonly known as GenX chemicals). If finalised in their current form, the MCLs would separately regulate PFOA and PFOS at 4 parts per trillion. The EPA expects that ‘if fully implemented, the rule will prevent thousands of deaths and reduce tens of thousands of serious PFAS-attributable illnesses’.

In February 2023, the European Chemicals Agency (Echa) published a REACH restriction proposal for PFASs, which aims to place limits on all uses of more than 10,000 PFASs. Five European authorities – Denmark, Germany, the Netherlands, Norway and Sweden – have compiled a REACH Annex XV report describing possible options on how to restrict the use of PFASs and a six-month public consultation began on 22 March 2023.

The REACH Annex XV report states persistence is the main concern for PFASs and their degradation products – warning they ‘may persist in the environment longer than any other man-made chemical’. The removal of PFASs from surface water, groundwater, soil, sediment and biota is ‘technically extremely difficult and very costly, if at all possible’, it adds.

Total destruction

ompanies scramble to phase out PFASs, researchers are looking for ways to scrub them from the environment. Treatment technologies such as reverse osmosis and granular activated carbon (GAC) are already used to remove PFASs from water. But the chemicals still need to be destroyed.

‘The big question is what to do with all of the spent GAC and ion exchange resins that are used for the capture of PFASs,’ says Zhouyan Cai, Director of Danish company Aquarden, which has developed technology to ‘fully destroy’ PFASs.[1]

PFAS waste is sometimes dropped in landfill but is most often incinerated. However, the PFASs can escape from landfill in leachates and incineration plants are costly and energy intensive. They are also often not hot enough to fully destroy PFASs and there is a risk that some short-chain substances may be released to the atmosphere. Electrochemical oxidation is another possibility and works reasonably well for long-chain PFAS, but short-chain substances pose more of a problem

People keep talking about water contamination by PFASs but actually this is just the tip of the iceberg. A significant problem is PFAS in solid waste such as sediments and sorbents.
Yang Yang Assistant Professor in civil and environmental engineering, Clarkson University, US.

Aquarden uses supercritical water oxidation – SuperOx – to deal with PFASs. A high pressure of up to 240bar and a temperature of around 600°C forces PFAS-laden wastewater into a supercritical state, while oxygen is pumped in, resulting in rapid and complete oxidation. ‘The oxygen will react very aggressively and rapidly with any organic species and rip it apart into its most basic components within seconds,’ says Cai. ‘We can destroy PFASs to below detection limits.’

The technology runs at quite small volumes compared with other traditional wastewater treatment facilities, handling up to 5m3 of water/day. ‘But we can handle highly concentrated waste so the trick is to try to concentrate as much of the bad stuff as you can and send that into the reactor,’ says Cai. Aquarden takes used ion exchange resins or spent GAC, adds water and blends the solid waste into a ‘smoothie’ that is sent into the reactor where it gets destroyed, he explains. ‘Our primary focus is the destruction of resins and activated carbon because that seems to be the go-to technology. But we’re not married to any particular capture technology. We’re agnostic as to PFAS capture as long as it gets concentrated.’ Other technologies such as foam fractionation, where PFASs stick to bubbles to create a foam also work.

Aquarden needs to analyse what comes out of the reactor to check for PFAS destruction before discharge. But the analysis takes a long time and so it is trialling running treated water through another GAC system to remove any trace PFAS levels. ‘The most important thing is not so much what comes out because we know we can destroy it – it’s more what goes in because if there is any spike in PFAS you create amounts of acid that can destroy the system. It’s very important to have a good way of analysing what goes into the plant as well,’ says Cai.

Lighting up

In the US, researchers at Clarkson University are looking at how to deal with PFAS-laden solid waste as well as soils and sediments in the environment.[2]

‘People keep talking about water contamination by PFASs but this is just the tip of the iceberg,’ says Yang Yang, Assistant Professor in Clarkson’s civil and environmental engineering department. ‘A significant problem is PFAS in solid waste such as sediments and sorbents.’

Incineration is a reasonable short-term solution for disposing of PFAS waste. ‘But the problem is that we have so little understanding about what happens when we burn PFASs,’ he says. Instead of combustion, his team sought a technique that can be used at room temperature and atmospheric pressure. They turned to ball milling, which has already been used to break down PFASs in the presence of potassium hydroxide. ‘The idea is really simple and the alkaline hydrolysis of halogenated compounds is available in any chemistry textbook,’ says Yang.

However, the process requires large volumes of potassium hydroxide and creates caustic waste. While looking for alternatives, Yang and his colleagues were inspired by cigarette lighter technology. Lighters use a spring-loaded hammer to hit a small piezoelectric ceramic crystal. The resulting crystal deformation produces a high voltage and electrical discharge to ignite the gas in the lighter. The team selected inexpensive boron nitride and used a ball mill instead of a hammer to create a technique they call piezoelectric material-assisted ball milling (PZM-BM). In tests, they mixed PFAS powders with boron nitride and spun them with stainless steel or zirconium balls at 580rpm. Mass spectrometry analysis revealed the PFOS and PFOA had been destroyed.

The team also used PZM-BM on samples of PFAS-rich sediment from the Schriever Air Force Base in Colorado Springs, US. A six-hour treatment removed about 80% of the 21 measured PFASs although extending the milling to ten hours achieved higher levels of destruction. The researchers have found that ball milling can generate kiloVolt PZ potentials upon collision, causing electrochemical oxidation. ‘It turns out it’s a universal strategy, non-selective to structure,’ says Yang.

The researchers suggest that PFASs lose electrons to boron nitride charged with high PZ potentials to form fluoroalkyl radicals, which then oxidise to form short chain carboxylates. The steps repeat until complete mineralisation, leaving ammonia and borates which can be easily handled.

‘It seems very exciting, very promising,’ says Yang. The team recently received another award from the National Science Foundation, which it will use to test other piezoelectric materials. Once they have selected one, they plan to optimise it by tuning the crystal structure.

Fire fighting

At the University of Queensland’s Australian Institute for Bioengineering and Nanotechnology, researchers are using magnetic fluorinated polymer sorbents to pull PFASs from water using magnets. In tests, the technique removes over 99% of GenX within 30 seconds.[3] Andrew Whittaker’s team grafted magnetic iron oxide nanoparticles onto a poly(ethylene glycol)-perfluoropolyether (PFPE) block polymer using phosphate chemistry. The phosphate group provides robust coupling to the polymer. ‘We don’t want to introduce fluorinated molecules back into the environment. That was very, very important to us,’ says Whittaker.

The PFPE provides fluorine-fluorine hydrophobic interactions between the sorbent and the perfluoroalkyl segment of PFASs while cationic groups in the sorbents increase PFAS capture efficiency through electrostatic interactions. Once the polymers have trapped the environmental PFASs they can be collected using a magnet. They are fully recoverable and can be reused multiple times. In tests, the sorbents worked efficiently in ground water matrices at parts per billion PFAS concentrations, highlighting their promise for PFAS remediation.

The team is now working with US chemical company and PFAS producer Chemours to scale up polymer synthesis. It has designed the chemistry to be ‘readily adoptable’ by the Chemours manufacturing plant, says Queensland’s Cheng Zhang. In the lab, they can currently make up to 1kg of polymer per batch, which will increase to 10kg in the next couple of years. ‘But on an industry scale we will need to scale up to 100 or 1000kg/batch,’ he says.

The plan is to use the technology to concentrate PFASs from a waste stream so they can be broken down using electrochemical oxidation. ‘We’re hoping that will help electrochemical oxidation become commercially viable,’ says Whittaker. ‘Our technology is very good at concentrating up PFASs so it’s also very good if you want to analyse very low concentrations. We can take waste streams, concentrate them up and allow more conventional analysis of what’s in groundwater, for example,’ he says.

The team is investigating different configurations of the base technology, including membrane systems so that it can be used in conventional cartridges similar to those used to purify water at home. Regulatory changes and public awareness are driving rapid changes in PFAS design, manufacture and remediation, says Whittaker, who has worked on fluorinated polymers for 20 years. ‘It’s pretty overwhelming for us, what’s happening at the moment’. He is optimistic that ‘innovative chemistry’ will save the day. ‘We are a pretty innovative species.’

References
1 https://aquarden.com/applications/pfas-pfos-pfoa/
2 Environ. Sci. Technol. Lett., doi: 10.1021/acs.estlett.2c00902
3 Angew. Chem. Int. Ed., doi: 10.1002/anie.202213071
4 Nature Commun., doi: 10.1038/s41467-022-31881-5