Theme: 07. Per- and Polyfluorinated Alkyl Substances (PFAS)

Topic Area: 7f. In Situ PFAS Treatment Approaches

Formatted Title:

Insights into the Fate of PFAS during Thermal Treatment Using TCH and a Novel Approach for High Temperature Catalytic Treatment of Vapors

Background/Objectives:

Per- and polyfluoroalkyl substances (PFAS) are a family of high molecular weight fluorinated polar organic compounds that are under increasing regulatory scrutiny and focus in the environmental community. Used for decades in various industrial and commercial applications as temperature resistant coatings and surfactants, these compounds are termed “forever chemicals” because of their extremely recalcitrant nature. Heating soils containing PFAS to temperatures in the range of 350-400°C has been shown to effectively “remove” target PFAS compounds, supporting full scale thermal applications for PFAS remediation either in situ or in a In Pile Thermal Desorption (IPTD®) application. However, the degree of mineralization and partial degradation to non-target PFAS, that could be volatile, non-polar and potentially mobile in the extracted vapor stream, was not well understood.

Approach/Activities:

A bench-scale study was conducted to simulate high temperature thermal conduction heating (TCH) of PFAS impacted soil, coupled with a novel high temperature catalytic vapor treatment step to remove any mobilized PFAS, precursors, or other fluorinated compounds in the vapor phase. Batches of perfluorooctanesulfonic acid (PFOS) spiked sand were heated to 350°C for 7 days while the off gases were directed through different catalyst materials heated to 600°C. Total fluorine, extractable fluoride, and PFAS mass balance calculations were performed for each thermal test run, to better understand the amount of mineralization as well as the fate of target and non-target PFAS species throughout the bench scale simulation (i.e. soil, catalyst media, condensed liquid, and vapor). In addition, surface chemistries and elemental composition of the PFOS spiked sand and catalytic material before and after use were investigated using X-ray photoelectron spectroscopy (XPS).

Results/Lessons Learned:

The study provided important insights into the destruction and fate of target and non-target PFAS during thermal treatment and indicated that thermal treatment at 350-400°C, combined with high-temperature catalytic treatment can be effective at destroying and reducing target PFAS concentrations in soil to very low levels, and ensuring that target and non-target PFAS are not emitted in the discharged vapors. The results indicated that ~45% of the PFOS was mineralized in the soil with the remaining ~55% removed by the catalyst, while standard target PFAS in the vapor stream were below nanogram reporting limits. Importantly, the results indicate that the PFOS was degraded in the soil at 350°C to volatile and semi volatile fluorinated compounds which were removed from the soil, transferred to the catalyst, and eventually further degraded and/or bound to the catalyst materials at high temperatures (600°C). One hypothesis is that the fluorinated compounds volatized from the soil at 350°C are non-polar as the hydrophilic surfactant moiety would be removed first during degradation. This has significant impacts with regards to selecting proper vapor treatment approaches, as designs relying on condensing and wet scrubbing may not effectively remove the non-polar fluorinated compounds and more robust systems such as thermal catalysts/oxidation may be required to ensure complete treatment. The study results will be compared with other similar recent studies; recommendations for future research and considerations for full scale in situ and IPTD® thermal treatment of PFAS-impacted materials will be discussed.

Xiao Xiao

Affiliation

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(Group 1, Poster Board #178) Insights into the Fate of PFAS during Thermal Treatment Using TCH and a Novel Approach for High Temperature Catalytic Treatment of Vapors