New PFAS Forever Chemicals Breakdown Methods Found

To understand why PFAS resist breakdown, we have to consider bond energy, a topic that is taught in A-level chemistry when looking at covalent bonds. The dissociation energy of a carbon-fluorine bond is about 485 kilojoules per mole, which is notably higher than that of a carbon-hydrogen bond amounting to around 413 kilojoules per mole, and much greater than the carbon-chlorine bonds present in a number of other persistent pollutants. In reality this means that a great deal more energy is required to break a carbon-fluorine bond than most of the energy that environmental microbes, sunlight or ordinary chemical reactions can provide. Since PFAS molecules may have dozens of such bonds arranged along a carbon backbone, they are in effect chemically protected, and conventional water treatment methods such as filtration or standard oxidation have very little effect on them, which is the reason that PFAS has been able to build up in the environment over the past seventy years with little change.

Hydrodynamic cavitation is based on a simple principle of fluid mechanics. When water is forced through a narrow passage at a high speed, the local pressure drops sharply in accordance with the pressure-velocity relationship that A-level physics students learn from Bernoulli’s principle; and if the pressure falls low enough, the dissolved gases and vapour form small bubbles. These bubbles are naturally unstable, and as the water moves past the constriction and the pressure rises once again, the bubbles collapse in a fraction of a microsecond. This collapse is by no means gentle: the temperature inside a collapsing bubble can momentarily rise to several thousand degrees, and the pressure can reach hundreds of atmospheres, all of this taking place in a volume smaller than that of a grain of sand. It is this intense and highly localised release of energy which is capable of breaking carbon-fluorine bonds that would otherwise remain unaffected, even though the bulk of the surrounding water remains at or near room temperature.

Cold atmospheric plasma functions in a different manner: when a gas is passed through a strong electric field, electrons are removed from some of the atoms, resulting in a mixture of ions, electrons and highly reactive species such as hydroxyl radicals, all while the gas remains at a temperature close to room temperature, which is what distinguishes cold plasma from the much hotter plasmas employed in industrial cutting or welding. These reactive species are strong enough to attack carbon-fluorine bonds directly, but only in those places where they can reach the PFAS molecules, which is the reason why producing plasma above a body of water achieves considerably less than might be expected - since most of the reactive particles never come into contact with the pollutants dissolved below the surface. The gas dispersion system overcomes this problem by splitting the plasma-treated gas into a stream of fine bubbles that are pushed through the water itself, greatly increasing the surface area on which the reactive species can interact with the PFAS molecules and thus explaining why this method was able to break down a wider range of compounds than cavitation alone.

No method has so far achieved complete mineralisation, which means converting every fluorine atom in a PFAS molecule into harmless inorganic fluoride, and that difference is significant. Since partially broken down PFAS molecules can in some cases produce smaller fragment compounds that remain persistent, even if to a lesser extent than the original molecule, a treatment process that only partly degrades PFAS must be closely monitored in order not to replace one problem with a more subtle one. That is the reason the HZDR team has set specific targets - above 80 per cent degradation and above 50 per cent fluorine mineralisation - rather than just noting the percentage obtained in the first successful test. The engineering challenge that distinguishes a promising laboratory result from a technology that a water utility could actually put into use lies in reliably reaching those figures over the wide variety of PFAS structures present in real contaminated water rather than on a single test compound.

For A-level students who are studying chemical bonding, reaction kinetics or environmental chemistry, the problem of PFAS - the so-called forever chemicals - provides a clear example of why it is the strength of a bond, and not just its general chemical reactivity, that determines whether a pollutant remains in the environment. It also explains why engineers are now more likely to use physical methods, extremely localised pressure or ionised gas, rather than methods that are purely chemical, when the bond in question is simply too strong for ordinary reagents to break in an economically viable way. Not only does avoiding the use of additional catalysts or chemicals result in cost savings, it also prevents new substances from being introduced into the treated water which would then have to be removed afterwards; this is a major practical benefit for any technology that is intended for actual drinking water or industrial wastewater treatment rather than for use in a closed laboratory reactor.