Breaking Down Barriers: Innovations in PFAS Destruction

A ‘silver bullet’ technology remains elusive, but Jens Blotevogel and Pradeep Shukla say the development of diverse technologies like electrochemical treatment, thermal and non-thermal plasma destruction, and supercritical water oxidation provides a powerful arsenal for tackling these persistent pollutants

THERE are no known natural mechanisms that can fully degrade per- and polyfluoroalkyl substances (PFAS) in a meaningful timeframe. Even conventional water treatment methods, including advanced oxidation techniques like Fenton chemistry, struggle to break down the perfluorinated tail – the key structural component of PFAS.

This extraordinary stability is due to two main factors. First, the carbon-fluorine bond itself is exceptionally strong. Second, the fluorine atoms surrounding the carbon backbone form a dense electron cloud, effectively shielding the more reactive carbon atoms. This protective shield prevents chemical agents and biological enzymes from accessing and breaking down the molecule, rendering PFAS hard to destroy in the natural environment. Engineers need to develop and deploy innovative approaches to effectively break these bonds and mitigate the growing environmental challenge posed by PFAS contamination.

Chemistry behind destruction tool

Traditionally, advanced oxidation processes (AOP) have been a key tool in the chemist’s arsenal for destruction of recalcitrant chemicals. However, PFAS present a unique challenge, pushing the limits of conventional AOPs. PFAS decomposition can occur through reductive, oxidative, or combined pathways, involving stepwise defluorination that ultimately converts carbon-bonded fluorine atoms to hydrogen fluoride (HF) or fluoride ions, which are typically stabilised as sodium or calcium fluoride in alkaline conditions.

The reduction chemistry is usually facilitated by aqueous (hydrated) electrons (eaq⁻). The aqueous electrons are essentially free electrons enveloped by water molecules, which readily react with electron-deficient sites, such as the fluorinated carbon atoms prevalent in PFAS molecules. Specifically, the aqueous electron attacks the C–F bond, forming an intermediate radical ion (CF*) that subsequently undergoes bond cleavage to release a fluoride ion (F⁻). The aqueous electrons also attacks the C=O bond within the PFAS head group to initiate decomposition. It is then followed by an oxidation step for complete mineralisation.

Oxidation initiated pathway is another route for PFAS destruction, but its success hinges on using potent oxidising agents capable of overcoming the inherent stability of these compounds. Electrochemical advanced oxidation processes (EAOPs), for example, leverage high overpotentials at electrodes to directly transfer (or abstract) electrons from PFAS molecules, initiating sequential breakdown. Similarly, high-energy UV-catalytic processes produce reactive oxidising species (ROS) that trigger PFAS degradation.

These active radicals for oxidation are typically generated through a combination of oxidising agents (eg peroxide, ozone, persulfate) and their activation methods (eg UV  irradiation, ultrasound, microwave radiation, pH adjustment, heat, and ionising radiation).

A third pathway involves direct bond cleavage through brute force thermal processes, leading to the near-instantaneous thermal decompostion or atomisation of PFAS into their constituent elements. Subsequently, these elements recombine into thermodynamically stable compounds such as carbon monoxide (CO), CO2, HF, and sulfur oxides (SOx). Such molecular dissociation is characteristic of processes with high energy density, such as arc plasma, radio frequency plasma, and laser-induced thermal decomposition. Some thermal treatment can also result in partial disintegration, progressively breaking down PFAS into smaller fragments until complete decomposition is achieved, as observed in high-temperature incinerators. In this latter scenario, the potential formation of toxic intermediates necessitates the addition of catalysts and base agents, such as caustic or lime, to prevent recombination of radical intermediates, and thus to ensure complete and safe destruction.

Innovations in PFAS destruction

The pressure to solve the escalating PFAS challenge has fundamentally shifted the economic landscape and has spurred the innovations in new destructive technologies. Stringent regulatory limits, often measured in parts per trillion, have fundamentally altered the cost-benefit analysis, making previously cost-prohibitive solutions not only feasible but increasingly necessary. This focus has yielded a diverse range of promising next-generation techniques, including plasma excitation and UV photolytic reductive processes, both of which rely on the generation and use of aqueous electrons. Beyond these, sonolytic processes and electrochemical oxidations have also emerged as a viable pathway.

Furthermore, high-pressure hydrothermal or supercritical processes, along with ultra-high-temperature thermal and RF plasma technologies, are demonstrating significant potential. Each of these approaches offers unique advantages and challenges, but they all share the common goal of effectively and safely breaking down these persistent and harmful chemicals.  

Electrochemical oxidation process

This technology has undergone numerous trials and has been demonstrated via bench studies and pilot-scale reactor to be very effective for treatment of long-chain PFAS and precursors.¹

What makes electrochemical methods so appealing is their ability to directly transfer electrons and to generate powerful oxidising and reducing agents in situ, minimising the need for adding extra chemicals to the water. At the anode, direct electron transfer with support from strong oxidising radicals trigger the decomposition of PFAS molecules that are either nearby or directly adsorbed on to the electrode. Critical to the efficacy of this approach is choosing the right electrode material – one that can exhibit a high oxygen evolution potential to maximise the destruction of organic contaminants like PFAS, while simultaneously minimising the number of electrons wasted on splitting water.

With sufficiently high applied potentials to generate strong radicals, PFAS can be completely broken down into simpler and manageable substances like hydrofluoric acid and CO2. Boron-doped diamond is a material often used in research for this purpose, but it can be quite expensive. Other promising materials have emerged. Magnéli-phase titanium suboxides, for example, have proven highly effective for PFAS oxidation, and mixed-metal oxide and carbon-based electrodes may offer significant cost savings. However, it’s important to acknowledge the challenges. Breaking that stubborn carbon-fluorine bond requires high potentials, which also trigger side reactions leading to the formation of unwanted oxidation byproducts, such as chlorates and perchlorate. Issues with electrode longevity have also been a commercialisation hurdle which needs addressing.

Advancements: While most research focuses on using the anode for direct PFAS destruction, new studies are exploring the potential of cathode reactions with aqueous electrons. Electrochemical reduction could significantly reduce unwanted byproducts, but it faces challenges due to excessive hydrogen evolution. Overcoming this will require more efficient cathode materials or catalysts, making it a promising area of research.

UV-catalytic PFAS destruction

Using UV light to boost oxidation processes with semiconductors as a catalyst is a well-established technique in advanced water treatment. The idea is simple: UV or visible light activates catalysts, driving chemical reactions that break down pollutants. However, PFAS present a unique challenge. PFAS molecules lack chromophores, ie don’t readily absorb UV light – and are thus not very susceptible to direct photolysis. The conventional approach of using UV light to generate active radicals, like hydroxyl radicals, which then attack the pollutants, also proves largely ineffective against PFAS.

Therefore, in the quest to destroy PFAS photochemically, most successful strategies have turned to reduction mechanism, specifically harnessing the power of aqueous (hydrated) electrons (eaq-). Generation of hydrated electrons can be achieved through various methods, including the introduction of chemical species such as sulfite (SO₃²⁻) and iodide (I⁻) into the aqueous matrix. In UV/sulfite systems, for instance, vacuum UV irradiation induces photoionisation of sulfite and water molecules, yielding hydrated electrons.² While this technology boasts remarkable energy efficiency, it appears to achieve only partial decomposition of long-chain and precursor PFAS molecules, leaving the issue of short-chain PFAS and hence may need additional treatment steps. A further operational constraint is the requirement for highly alkaline conditions (pH ≈ 12) during treatment. This necessitates pre- and post-treatment pH adjustments, increasing the chemical demand and cost of the process. The technology has been pilot-tested for treatment of PFAS concentrates at lab scale with commercial pilot trial conducted for fluoropolymes and other PFAS waste with destruction efficiency ranging from 64 to 99%.

Non-thermal plasma based PFAS destruction

A plasma is a partially or fully ionised state of matter comprising of a mixture of electrons, free radicals and ions, which is an ideal reactive environment needed to facilitate dissociation of pollutants. There are two main types of plasma: thermal and non-thermal (or cold plasma). The former is characterised by high temperatures (eg an arc plasma and laser-induced plasma), while the latter maintain low average temperatures while exhibiting high electron excitation (eg pulsed plasma discharge (PPD), dielectric barrier discharges and corona discharge).

PPD uses high-voltage pulses (5–20 kV) to create an electrohydraulic discharge in water. This process generates aqueous electrons (eaq-) and other reactive oxygen species (ROS), such as hydrogen peroxide, hydroxyl, and hydroperoxyl radicals. While these aqueous electrons are short-lived, they effectively react with pollutants in their immediate vicinity. Furthermore, PPD generates shockwaves and cavitation bubbles, which create plasma channels that contribute to pollutant breakdown throughout the water.

Several field trials have been conducted using various types of non-thermal plasma systems for PFAS remediation in groundwater, with destruction efficiencies of long-chain PFAS reported to be greater than 90% in a single pass flow through reactor.³ However, the incomplete decomposition of long-chain and precursor PFAS molecules can result in the formation of short-chain PFAS byproducts.

Advancements: Recent advancements have focused on combining non-thermal plasma with gas bubbling, driven by the fact that plasma-generated radicals are concentrated at the water’s surface where the discharge occurs. This technique introduces gas into the water, effectively transporting amphoteric PFAS molecules to the surface, where they are brought into direct contact with the plasma-generated radicals. By concentrating PFAS at the surface, this approach significantly enhances the degradation process compared to traditional plasma treatment alone, which is limited by the restricted range of radical activity in bulk water.

Sonolysis

Sonolysis uses sound waves to break down organic chemicals. Ultrasonic frequencies in the range of 100 kHz–1 MHz are particularly effective for water treatment. These frequencies break hydrogen bonds between water molecules and generate tiny bubbles. Because PFAS are surface-active, they tend to accumulate at the gas-water interfaces of these so-called cavitation bubbles. Subsequent bubble collapse is violent, generating highly localised temperatures in excess of 5,000°C and tearing up or ionising molecules in their vicinity. The exact mechanism of sonochemical PFAS destruction is not fully known or agreed on by scientists. However, high temperatures well above those needed for PFAS incineration suggest thermal destruction processes, potentially supported by products from torn-up water molecules. Correctly tuned, sonolysis is very effective in completely mineralising PFAS. It is also extremely easy to operate, practically just having an on/off switch. However, sonolysis has high energy demand. Its niche is therefore the treatment of smaller waste streams, such as concentrates from separation processes or possibly even residential under-the-sink treatment prior to discharge into the sewer system.⁴

Thermal steam plasma destruction

Unlike non-thermal plasma process, thermal plasma is one of the highest energy density methods used for toxic waste destruction. It’s a direct, high-temperature treatment where pollutants are heated to extreme temperatures, causing them to decompose. The core of the thermal plasma plume can reach nearly 11,000K, while the average temperature is around 2,000–2,500K. The plasma is generated by striking an electric arc across an electrode and passing a gas (like argon, steam, helium, or air) through the arc zone, where it absorbs energy to create a high-temperature plasma plume. The pollutant can either be mixed with the gas and passed through together into the arc, or it can be injected as a fine mist directly into the plasma.

As PFAS is exposed to high-temperature plasma, the molecule undergoes thermal dissociation into constituent atoms and radicals. As the energy of species is dissipitated they recombine to form thermodynamically stable end products such as CO, CO2, SOx, and HF. The acidic gaseous byproducts are readily removed from the effluent gas stream via lime or caustic scrubbing. This process converts HF into benign mineral products like CaF2, while SOx is converted to gypsum, and CO2 to limestone (CaCO3). Gas scrubbing therefore constitutes an essential downstream process in thermal plasma technology, typically accounting for approximately 60% of the total process footprint.

Advancements: Conventional thermal plasma is produced by passing argon and air across the arc, resulting in an oxidising plasma. In contrast, advanced thermal steam plasma is an innovative process that generates plasma using steam, resulting in an excess of hydrogen ions. This allows the formation of HF as the most favourable fluorine-carrying product.

Supercritical water oxidation

The supercritical water oxidation (SCWO) process is another high-energy, brute-force chemical destruction method. It harnesses the unique properties of fluids at supercritical conditions, where temperature and pressure exceed critical values, creating an environment where both the liquid and gas phases coexist. This extreme state enhances the reactivity and solubility of contaminants, making SCWO an effective method for the destruction of persistent pollutants like PFAS.

SCWO targets PFAS species through the generation of reactive oxidising agents and free radicals. Typically operating at temperatures above 550°C and pressures around 24 MPa, SCWO functions above the critical point of water (374°C and 22.1 MPa). Under these conditions, the fluid exhibits enhanced oxygen solubility, and PFAS compounds undergo rapid degradation into simpler constituents, including water, CO2, and simple mineral acids. Fluoride species are transformed into dissolved HF and subsequently converted into recoverable precipitated mineral salts.

Advancements: Despite the high energy needed to reach the supercritical state, some commercial processes achieve low or negative energy consumption by recovering the exothermic heat from high-pressure oxidation reactions. When the feed pollutant lacks sufficient calorific value, high-calorific fuels like methanol or diesel are added to sustain the oxidation reaction, generating 700–800 kJ of energy per mole of fuel.

Hydrothermal alkaline treatment

This process is yet another high-enery process for PFAS destruction. It has long been known that under high-alkalinity, hydroxide ions (OH⁻) can attack organo-polymer chains, resulting in polymer fragmentation. However, the reaction kinetics are very slow under standard conditions. Hydrothermal alkaline treatment builds on this principal and overcomes the kinetic limitation by introducing high temperature and pressure to accelerate the reaction kinetics.

This technology operates under high-pressure (50 to 200 bar) and high-temperature conditions, ranging from 250°C to 400°C and uses an alkaline environment to facilitate the breakdown of PFAS compounds. The high temperature and pressure, combined with the hydroxide ions (OH-) from the alkali, weaken and break the C–F bonds. The hydroxide ions act as nucleophiles, attacking the partially positive carbon atoms in the C–F bond, leading to the displacement of fluorine. As C–F bonds are broken, the PFAS molecule begins to fragment into smaller, shorter-chain compounds. Fluorine atoms are progressively removed from the PFAS molecule, converting them to fluoride ions (F-).

As the PFAS undergo chemical transformations, it results in the production of simpler organic acids, CO2, and fluoride ions. The end products are signficantly less hazardous compared to the original PFAS compounds, with fluoride tending to form soluble salts in the high alkanine environment. However, the high pH requirement leads to a substantial chemical demand for this technology, in addition to concerns regarding corrosion in the presence of the harsh reaction environment.

What the future holds

PFAS destruction remains a significant challenge, but rapid innovation is bringing hope for effective solutions, despite barriers related to cost and ease of adoption. While many technologies are advancing through large-scale trials, they still face the “commercialisation valley of death” before becoming viable market solutions. The presence of co-contaminants complicates even the most promising destruction methods. These co-contaminants can interfere with the treatment process, requiring additional strategies to ensure effective PFAS remediation. Excitingly, research into innovative approaches, such as advanced plasma techniques, shows great promise for overcoming these challenges. At the same time, ongoing research in green approaches to PFAS destruction, particularly through biological methods, brings hope for more sustainable and cost-effective solutions.

References

1. Current Opinion in Chemical Engineering: Scaling up water treatment technologies for PFAS destruction: current status and potential for fit-for-purpose application: https://bit.ly/4gt8UQV
2. Water 2021: Reductive and Oxidative UV Degradation of PFAS—Status, Needs and Future Perspectives: https://bit.ly/410N6HE
3. ACS ES&T Water: Field Demonstration of a Pilot-Scale Plasma Reactor for the Rapid Removal of Poly- and Perfluoroalkyl Substances in Groundwater: https://bit.ly/4aTYvMT
4. Journal of Environmental Engineering: Field Demonstration of a Sonolysis Reactor for Treatment of PFAS-Contaminated Groundwater: https://bit.ly/40KUd5Z

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We would like to thank Pablo Ledezma from SynergenMet for helping with the figures