Theme: 7. Per- and Polyfluorinated Alkyl Substances (PFAS)

Topic Area: 7f2. In Situ PFAS Soil Treatment Approaches

Formatted Title:

Systematic Monitoring of PFAS Immobilization in Soil

Background/Objectives:

Per- and poly-fluorinated alkyl substances (PFAS) are compounds of critical concern due to their widespread distribution coupled with their persistence and bioaccumulation in the environment. The global problem of ground/surface waters being contaminated with PFAS originating from highly contaminated soils has raised a need to understand their leaching from these soils and to remediate these locations. One of the cost-effective and preferred methods is in situ immobilization of PFAS in soil by applying sorbents such as activated carbon (AC), but the long-term efficacy of sorbents has not yet been investigated.

One of the key questions often posed by stakeholders is the durability of the immobilization and under what conditions the compounds might become released again. Changes in soil physicochemical properties such as pH, redox or salinity can occur over time, and it is important that any sorbent used for in situ remediation is durable enough to withstand these environmental perturbations. The changes in the sorbent's physicochemical properties may also affect the PFAS immobilization in soil and choosing the right sorbent is critical for long-term immobilization of PFAS in soil. This study aims to evaluate the potential release of PFAS from contaminated soils after remediation with some of the common PFAS sorbents and to investigate which sorbents properties can affect PFAS immobilization in soil.

Approach/Activities:

Fourteen different PFAS-contaminated soils were used over a few studies to understand PFAS leaching and the efficiency of in situ immobilization using different sorbents. Sorbents such as powdered AC (PAC), granular AC (GAC), graphene-based materials and biochar were characterized by various physical and chemical properties and used for the immobilization studies. The PFAS-contaminated soil and treated soils with sorbents were then exposed to a series of laboratory leaching tests such as the Australian standard Leaching procedure (ASLP), multiple extractions (MEP), Leaching Environmental Assessment Framework (LEAF) LEAF1313 and LEAF 1314, and rainfall simulation study to determine the potential leaching and runoff of PFAS from remediated soils, in comparison to control (untreated) soils. The long-term efficiency of PFAS immobilization was also tested in some of the treated soil 4 years after immobilization.

Results/Lessons Learned:

Treatment of the soils using the sorbents reduced concentrations of leachable PFAS significantly for most of the PACs and GACs using ASLP, MEP, LEAF 1313 and 1314. However, the leachability of PFAS was related to PFAS chemistry and the physical and chemical properties of the sorbents. Sorbent surface area determined by the methylene blue method and surface charge were correlated with the efficiency of sorbents for PFAS immobilization in soil. PFAS runoff also decreased significantly from treated soil using a simulated storm event. The efficiency of some sorbents for PFAS immobilization decreased after 4 years. Temperature extremes, ionic strength and competing ions did not affect PFAS immobilization in soil while PFAS desorption was slightly greater in alkaline pH than acidic in treated soil. The data show that in situ remediation of PFAS-contaminated soils with AC-based sorbents could be considered robust and durable with little risk of subsequent PFAS desorption, and systematic monitoring of their immobilization should be performed over time depending on sorbent type and site condition.

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