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

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

Formatted Title:

Soil Washing 2.0: Sustainable Cost-Effective Treatment for Per- and Polyfluoroalkyl Substances

Background/Objectives:

Since the 1980s soil washing has been used as a cost-effective volume reduction soil treatment technology for metals and hydrocarbons by treating the coarse-grained soil fraction and segregating the fines and organics for secondary treatment (e.g., landfilling or incineration). Soil washing is increasingly being recognized as a viable technology for PFAS, however achieving evolving regulatory standards, which can be in the sub-part per billion range in soil and part per trillion range in leachate, can be difficult using traditional soil washing circuits. Arcadis and TUQ4 have built on results from a PFAS soil washing demonstration at Eielson Air Force Base (ESTCP ER20-5258) to develop a more effective and sustainable PFAS soil washing circuit that can achieve evolving regulatory standards and reduce, and potentially eliminate the need for secondary treatment of the fine-grained soil fraction.

Approach/Activities:

The ESTCP demonstration focused on treating the coarse-grained soil fraction and segregating the fine-grained fraction for secondary treatment to achieve sub-part part per billion soil concentrations. This approach achieved total PFAS reductions in the coarse-grained soil of greater than 99.9% and soil leachate PFAS reductions greater than 99.8%. Building on these results we applied advanced treatment steps to the soil washing circuit (“Soil Washing 2.0”) at the bench scale to achieve high percentages of PFAS reduction in the coarse and fine-grained fractions in soil from a Canadian Department of National Defence site. These advanced treatment elements included intensive attrition scrubbing and chemical polishing of the fines post-segregation to improve desorption during the treatment process and to reduce leachability post-treatment. We also used the SiteWise^TM tool to evaluate the environmental footprint of traditional soil washing and Soil Washing 2.0, and compared them to other PFAS soil treatment methods, including landfilling, incineration and stabilization.

Results/Lessons Learned:

The untreated sum of PFAS soil concentration was 77.7 μg/kg, which is indicative of military construction soil across North America. PFAS reduction in the treated coarse-grained soil was consistent with the ESTCP demonstration (>99% for Sum PFAS) and concentrations ranged from non-detect to sub-part per billion range. PFAS reduction in the treated fine-grained soil was also >99% for sum of PFAS and PFAS concentrations were in the sub-part per billion range. The untreated sum of PFAS leachate concentration was 1,047 ng/L. Leachate in the treated coarse-grained soil was non-detect and in the single digit part per trillion range, representing >99.9% removal. United States Environmental Protection Agency (USEPA) Regional Screening Levels (RSLs) for soil-to-groundwater (soil concentrations protective of groundwater) are extremely low and can be an order of magnitude lower than laboratory reporting limits. A reasonable proxy for soil concentration-based standards for soil-to-groundwater pathway is to compare post-treatment soil leachate results to groundwater RSLs. Results indicated that Soil Washing 2.0 can achieve sub-part per billion soil concentrations in the coarse- and fine-grained soil fractions and post-treatment leachate results below USEPA groundwater RSLs. The sustainability analysis indicates that soil washing has a low environmental footprint compared to other soil treatment alternatives, and the potential to reduce or eliminate the need for secondary management of the fines will further reduce its footprint.

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Colin Morrell