Scientists and engineers are developing new ways to destroy per- and poly-fluoroalkyl substances (PFAS) efficiently and sustainably.
This class of chemicals is known as “forever chemicals” because PFAS persist and accumulate in the environment, animals and our bodies.
PFAS have been used for decades to make everything from firefighting foam, packaging, waterproof clothes and non-stick frying pan coatings.
The chemistry that makes these compounds so useful makes them extremely difficult to destroy or fully remove from the environment.
PFAS are associated with numerous illnesses including cancers and infertility. The annual cost of inaction on resultant health issues is around €84 billion (£70 billion) for European countries.
Getting rid of the more than 14,000 different PFAS poses a huge and costly challenge. In England alone, remediation costs are predicted at up to £121 billion.
There are many ways to remove PFAS from contaminated soil, groundwater or drinking water, but the key challenge is to destroy PFAS without contributing to pollution elsewhere.
Existing removal tech
Recommendations for safe PFAS concentrations in drinking water are in the range of nanograms per liter or parts per trillion—this is like counting grains of sand in an Olympic-sized swimming pool.
To treat contaminated water, PFAS need to be removed, usually by some kind of separation or concentration technique, before then being destroyed.
Separation techniques traditionally use tiny solid particles of porous activated carbon—this is like small grains of charcoal with tiny holes. The carbon is packed into a column that the PFAS-contaminated water flows through.
PFAS stick to the particles in the column and when no more PFAS can be collected the carbon solid is treated at high temperatures (900°C-950˚C) to remove the PFAS. Any PFAS not destroyed in this process must be exposed to even higher temperatures for complete destruction. Then the activated carbon can be reused to collect more PFAS.
Another way to collect PFAS could involve 3D-printed materials. Some researchers have added catalysts to activated carbon to collect and degrade PFAS simultaneously.
PFAS contamination can be separated from water using techniques such as filtration to create a PFAS concentrate or foam fractionation which bubbles air through contaminated water. Since PFAS act like soap (or a surfactant), they are attracted and stick to the surfaces of those bubbles which then rise to the top of the tank and can be removed.
Strong bonds, high heat
PFAS are fluorinated chemicals, which means the carbon atoms within their structure are bonded to fluorine atoms—those strong chemical bonds are hard to break, hence needing very high incineration temperatures.
Most PFAS destruction occurs via creation (and slower destruction) of smaller PFAS. The carbon-fluorine bonds in shorter chain PFAS are the most difficult to break down, so the creation of smaller, more persistent PFAS should be avoided.
Unsurprisingly, PFAS are resistant to methods that destroy other pollutants such as exposure to ozone (a powerful oxidizing agent), bacteria or high temperatures.
Only temperatures above 1,400˚C will completely destroy PFAS but the UK only has four high-temperature incinerators, not all of which will accept PFAS-contaminated waste.
New solutions
More widely available, cost-effective and sustainable technologies to degrade PFAS are urgently needed. Many new solutions aim to work in ambient conditions (at room temperatures and pressures) to save energy. Innovations include microbial degradation whereby bacteria feed on PFAS pollution and degrade it. Energy input is low but microbial processes tend to produce different PFAS.
Our team uses high-frequency sound waves to destroy PFAS through what’s known as “ultrasonic degradation” or “sonolysis.” This completely degrades PFAS at room temperature so high energy inputs and pressures are not needed. Sonolysis can break down a range of contaminated materials, including used firefighting foam and liquid leachate from landfills.
When a contaminated liquid is treated with high-pitched sound waves (ultrasound), gas bubbles compress and expand rapidly, up to millions of times per second. The bubbles grow and then violently collapse in these pressure cycles, momentarily reaching temperatures hotter than the sun and pressures around a thousand times higher than our atmosphere, breaking down PFAS.
Another method called “hydrodynamic cavitation” uses fast-moving water to create bubble cavities that works in a similar way.
Three other chemical destruction techniques that don’t use extreme heat or high pressures include electrolysis, photolysis and plasmas. Electrolysis can destroy PFAS using an electric current that travels via specialist electrodes.
Photolysis uses a catalyst that is powered by sunlight or other light sources, but the contaminated liquid has to be clear for the light to activate the catalyst.
Plasmas are like a soup of charged particles that drive difficult-to-achieve reactions such as PFAS destruction. Plasmas on or just beneath the surface of contaminated water are formed using high voltage electricity or electromagnetic energy. However, this technology is largely limited to laboratory research.
Whatever technology is used for the destruction of PFAS, the challenge is to ensure effective treatment for all PFAS types. Ideally, PFAS pollution should be prevented altogether.
But even if PFAS production was stopped now, the legacy of more than 70 years of PFAS manufacturing and release has created a long-lasting challenge.
Written by Madeleine Bussemaker, Mehrdad Zare and Patrick Sears, The Conversation.
