Seven years of laboratory research at Krüger, a subsidiary of Veolia, have revealed promising results in the treatment of PFAS contaminated soil. Through careful analysis and repeated testing, we've developed a deeper understanding of how these so-called 'forever chemicals' respond to thermal treatment. In this article, we'll take you through our research journey and share how PFAS in soil responds to thermal treatment, what works in practice, and how waste streams from the remediation process can be minimised.
Why are PFAS commonly referred to as “forever chemicals”?
PFAS (per- and polyfluoroalkyl substances) are found everywhere in our environment. They have been used widely and have revolutionized countless consumer products since their introduction over 75 years ago. The compound's properties such as thermal stability and resistance to both water and grease have made them very popular. From non-stick cookware and water-resistant fabrics to cosmetics, food packaging, and firefighting foams, PFAS have become widespread in our daily lives.
However, the same extraordinary durability that makes PFAS so valuable in industry has become their most problematic feature in nature. The persistence of these organic pollutants have earned them the name “forever chemicals” — they simply refuse to break down naturally. Instead, studies suggest they accumulate in our environment, which may cause severe pollution of our water and soil resources.
Which technologies can treat PFAS in soil?
Significant progress has been made over the last few years in cleaning PFAS compounds from water and quite a suite of technologies is available today. For example, Veolia’s new end-to-end Beyond PFAS offer provides a comprehensive approach to PFAS management in water and soil, from sampling and analysis to responsible disposal of contaminants in Veolia’s hazardous waste treatment facilities.
When it comes to remediation of PFAS contaminated soil, few technologies are available and it is crucial to distinguish between two approaches:
- remediation that actually reduce PFAS concentrations in soil and
- technologies that bind PFAS through stabilization.
Several commercial reagents claim to bind the PFAS, creating a “controlled chemical landfill”. However, the long-term effectiveness of these binding agents is uncertain, requiring continuous monitoring over the life span of these products.
When looking at technologies that reduce PFAS in soil to below regulatory criteria, heat treatment appears to be one of the most effective technologies available today. The traditional approach involves high-temperature desorption and incineration, including the treatment of flue gases. Due to the thermal stability of the PFAS compounds and the typical short residence time, this approach requires temperatures above 900°C.
Can PFAS contaminated soil be treated effectively at lower temperatures? And what happens to PFAS compounds during thermal soil remediation? In the following sections, we'll share our discoveries from the past seven years of research and their implications for soil remediation.
A game-changer in PFAS treatment
At Krüger, a subsidiary of Veolia, we have been working with a soil remediation technology called Thermal Conductive Heating (TCH) for 20 years. This technology heats contaminated soil in-situ (in place) to typically 100°C, allowing us to vaporize and capture contaminants such as chlorinated solvents or other volatile or semivolatile compounds. We have successfully implemented this proven technology on numerous remediation projects worldwide.
One of the key advantages of TCH is that the technology relies on thermal conduction rather than soil properties like water content, hydraulic or electrical conductivity. This enables treatment temperatures well above 100°C - a capability we have demonstrated by treating soil contaminated with substances like dioxin, PCB and PCP (polychlorinated biphenyls and phenols) at temperatures reaching 350°C.
In the world of thermal soil remediation, this application is known as high temperature TCH. This name can be a bit misleading, since we apply temperatures significantly below 900°C required for thermal incineration.
What's the secret behind achieving such high reduction rates at the "lower" temperatures?
The answer lies in treatment time. While traditional incineration exposes soil to high heat for just seconds or minutes, our process maintains target temperatures for days or weeks, allowing for more complete contaminant treatment.
What have laboratory tests revealed about PFAS treatment?
In 2018, Krüger, along with Veolia Australia and New Zealand, conducted the first lab treatability test using TCH on Australian soil samples containing sum of targeted PFAS concentrations of up to 200 mg/kg dry matter. The tests demonstrated that it is possible to reduce the measured PFAS concentrations to low levels by treating the soil at temperatures between 350-500°C for one week. Even PFOS, typically the most persistent PFAS compound, was reduced to less than 1-5 micrograms per kg dry matter. Another learning was that we barely saw any reductions in analyzed PFAS compounds at 250°C.
Krüger has performed more treatability studies on other PFAS contaminated soils from different locations with similar outcomes. Results consistently show that heating the soil to 350-450°C for 14 days is sufficient to bring the measured PFAS concentrations in soil below the Danish national guidelines.
In 2020, we concentrated on analyzing the gases released during thermal soil treatment. Using a mass spectrometer with different ionization methods, it was possible to identify several compounds, decomposition products and fragments in the outgassing.
Mass spectrum recorded on extracted vapors from a soil sample heated to 500°C. Fluorinated compounds are marked with arrows. Selected ions are listed in the table giving the ion mass (g/mol). The ionization method is selective reactive ion (SRI) using O2+-ions which is prone to significant fragmentation.
What is the fate of PFAS compounds during thermal treatment ?
The next step of the research was focused on establishing a fluorine mass balance in order to look at the fate of PFAS compounds during treatment. Key questions needed answering:
- Do these compounds evaporate entirely?
- Does thermal degradation occur?
- What is the pathway of the reductions that we see?
In 2022, Krüger together with our long-term partner in the US, TerraTherm Inc., performed yet another set of lab treatability studies. This time to simplify the chemistry, we used clean sand spiked with a known amount of PFOS. The test also included a patent-pending DeFlourinator catalyst on the outgassing from the soil treatment.
The treatability test again demonstrated very convincing reductions in PFOS soil concentrations at 350°C. The comprehensive mass balance analysis reveals the fate of fluorine compounds:
- No conversion of PFOS to other of the 30 analyzed PFAS compounds
- Only 0.05% of original PFOS is found unconverted in condensate
- ≈ 45% of PFOS-fluorine retained in soil as inorganic and metal fluorides
- ≈ 55% of PFOS-fluorine released from the soil, probably as a mixture of hydrogen fluoride (HF) and volatile PFAS originating from PFOS degradation
How to minimise waste streams from the remediation process?
When passing the outgassing through the DeFlourinator, approximately 45% of the total amount of fluorine was retained in the catalyst, while the remaining 10% was captured as inorganic fluoride in the alkaline scrubber. Within the accuracy of the analysis method the results suggest a high degree of mineralization.
This means that 99.95% of the total fluorine is converted to inorganic forms found in soil ( ~45%), catalyst (~45%) and scrubber fluid (~10%) while the remaining 0.05% of total fluorine in the is unconverted PFOS.
Diagram showing the lab test setup during high temperature soil treatment. Vapors from the soil are fed through the DeFluorinator (DeF) to degrade PFAS vapors. HF formed is absorbed in the alkaline impinger and PFAS vapors are trapped on a XAD2 sorbent tube for analysis.
The laboratory treatability tests demonstrate two important findings:
Effective reductions of the targeted PFAS compounds in soil can be achieved at temperatures significantly lower than traditional incineration.
Mineralisation observed during treatment makes it possible to greatly reduce the waste streams from this kind of treatment.
What's next for PFAS treatment?
The next logical step is to validate these laboratory findings through pilot-scale testing using larger soil volumes under full-scale treatment conditions. In 2025, Krüger was awarded a pilot scale testing on roughly 30 tons of PFAS contaminated soil at Denmark's first PFAS Test Center in Kørsør. Stay tuned for the next blog post that will take you through the pilot area, the setup and current project status.
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The information contained in this statement is based on the Veolia group's understanding and know-how of the scientific, regulatory and technical fields discussed herein as of the time of publication. No contractual undertaking or offer is made on the basis hereof and no representation or warranty is given as to the accuracy, completeness or suitability for the purpose of the relevant information. Descriptions contained herein apply exclusively to those examples and/or to the general situations specifically referenced, and in no event should they be considered to apply to specific scenarios without prior review and validation. Statements that may be interpreted as predictive of future outcomes or performance should not be considered guarantees of such, but rather reasoned assessments of the possible evolution of the technologies described.
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