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

Topic Area: 7e. Ex Situ PFAS Treatment: Soils/Solids and Other Waste Streams

Formatted Title:

Smoldering Treatment of PFAS: Part 1. Laboratory Study

Background/Objectives:

Soils and filter media contaminated with per- and polyfluoroalkyl substances (PFAS) present a significant remediation challenge. Under the U.S. Department of Defense (DoD) Strategic Environmental Research Program (SERDP), a series of laboratory column tests was conducted using perfluorooctanesulfonic acid (PFOS)-spiked granular activated carbon (GAC) and silica sand, to evaluate the ability of smoldering combustion to treat PFAS. Tests were performed in a novel smoldering column configuration, and in some cases employed calcium oxide (CaO) as a soil amendment. The project objectives were to characterize the fluorine mass balance of PFAS smoldering and investigate methods of improving treatment.

Approach/Activities:

Smoldering is a flameless form of combustion that has been applied commercially to treat hydrocarbon wastes and impacted soils. Smoldering can be self-sustaining, as high energy fuels such as coal tars can be used as fuel to support their own destruction. PFAS cannot support smoldering and require high temperatures for destruction (>900°C). As such, a sufficiently energetic surrogate fuel must be used. Duchesne et al. (2020) demonstrated small quantities of GAC were able to achieve temperatures needed for PFAS destruction.

Thermal treatment of PFAS may generate transformation and destruction byproducts, such as hydrogen fluoride (HF) and products of incomplete destruction (PIDs) in emissions. Recent research has found CaO can be used as a method of emissions control for PFAS thermal breakdown products by converting HF and PIDs to inert calcium fluoride (CaF₂). Conventional methods of analysis are challenged by the wide array of PFAS and PIDs generated; as such, new total fluorine analytical methods have been developed.

A series of column tests was conducted using a novel smoldering column configuration to assess the overall fluorine mass balance of smoldering combustion and evaluate CaO as a method of emissions control. Four tests were completed with PFOS-spiked GAC and silica sand to evaluate emissions of HF, PFAS and total fluorine under ‘normal’ conditions. A further three tests were completed with CaO amendments to evaluate how HF, PFAS and total fluorine emissions were altered. Particle induced gamma emissions (PIGE) spectrometry was used as a total fluorine detection method to evaluate total fluorine present in the emissions and post-treatment soil.

Results/Lessons Learned:

For all tests, effective removal of PFAS in soil was demonstrated, representing a minimum 99.9% removal following treatment. For tests with no CaO, the majority of fluorine was recovered in the emissions as HF and total fluorine. Where CaO was used, a 90% reduction in HF and fluorine emissions was observed. This fluorine was instead retained within the column post-treatment soil. X-ray diffraction (XRD) measurements confirmed the presence of CaF₂ in the post-treatment soil and no PFAS compounds were detected above the limit of quantification. Overall mass recoveries ranged from 68-128% of the pre-treatment fluorine mass. This study demonstrated CaO is a simple soil amendment to reduce the quantity of breakdown products of PFAS treatment, and formed the basis of a field-deployed pilot study that will be presented by Major et al.

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Gunnar Brown