- Steffen Griepke
The need for remediation of poly- and perfluoroalkyl substances (PFAS) is growing substantially as a result of increased regulatory attention to this emerging contaminant. Stockpiles of PFAS contaminated soils are growing and a permanent solution to remediate and clean these piles (and other source zones not yet addressed) is needed.
Fortunately, recent studies have shown that thermal conductive heating (TCH) can be an effective treatment for PFAS. In this blog post, we’ll explain how that can work.
If you’d like a deeper dive (and with the chance to ask questions), register for tomorrow’s webinar, Thermal Treatment for Remediation of PFAS Source Areas. I will be joined by our European partner Krüger’s Søren Eriksen, and we’ll address the literature related to thermal removal of PFAS, describe the conducted lab testing and results, touch on the fate of thermally treated PFAS compounds in the process, and present how a field application can be implemented.
The TCH technology has been used by TerraTherm to remove recalcitrant contaminants for more than 20 years, and we have been involved in using it to remediate dozens of PAH, PCB, pesticide, and dioxin sites. Although PFAS chemicals were designed to resist heat, we know that at high enough temperatures, they become volatile and can be extracted from the subsurface.
Additionally, PFAS compounds have limits to their thermal stability, allowing thermal decomposition and evaporation of the degradation products at temperatures readily achievable with TCH (300 to 350°C)[1]. Several studies have documented the thermal decomposition of PFAS at elevated temperatures, resulting in the production of smaller chain perfluorinated linear and cyclic aliphatic C4-C8 compounds with smaller molecular weights and lower vapor pressures than the parent PFAS compounds, that are easier to remove in the vapor phase. [2] [3] [4] .
Laboratory studies conducted by Krüger have shown that our TCH technology delivers better than 99.99% removal of PFAS contaminants from soil. Soil samples from an aqueous film forming foam (AFFF) handling facility, containing initial PFAS concentrations close to 200 mg/kg, were tested in a laboratory scale treatability study. The soil was heated to temperatures from 250°C to 500°C and soil samples as well as vapor and condensate samples were collected for analysis during and after the study. The studies were designed to simulate the removal of PFAS during TCH full scale treatment either in situ or using the TerraTherm in pile thermal desorption (IPTD®) concept.
Results of the study show that the thermal conductive heating process reduces PFAS concentrations by 99.998% at treatment temperatures of 350 °C within seven days of maintaining the target temperature. Due to their designed-in resistance to elevated temperature, effective remediation of PFAS compounds requires longer thermal treatment residence time at the target temperature—weeks rather than days—as compared with the typical design basis for many other organic compounds. However, the clear conclusion of the study was that PFAS compounds and their precursors are efficiently removed from the soil by heating to temperatures of 350°C for seven days.
The next step, of course, is extending the laboratory studies to full-scale projects—but there is every reason to believe this remedy will be just as effective in the field. Since TerraTherm first implemented TCH technology more than 20 years ago, we have implemented dozens of high temperature TCH applications in exactly the same way the successful PFAS thermal treatment laboratory studies were performed. With Krüger, we have implemented two high temperature IPTD™ projects together, treating more than 60,000 cy per batch. While the projects were targeting different recalcitrant chemicals than PFAS, the in situ thermal treatment concept (target temperature and time at temperature) was no different than what is required for full scale PFAS treatment.
It is exciting to be on the front lines of research regarding effective PFAS treatment, and we look forward to presenting our findings in greater detail in tomorrow’s webinar, Thermal Remediation for Treatment of PFAS Source Areas.
[1] Paul J. Krusic et al.: Journal of Fluorine Chemistry 126 (2005) 1510–1516
[2] J. A. Conesa, R. Font: Polymer Engineering & Science, 41, 12 (2001) p. 2137–2147
[3] Lucia Odochian et al.: Thermochimica Acta 598 (2014) 28–35 (DOI: 10.1016/j.tca.2014.10.023)
[4] R.K. Singh et al.:Environ Sci Technol. 2019 Mar 5;53(5):2731-2738. doi: 10.1021/acs.est.8b07031.