Ex situ soil washing to remove PFAS adsorbed to soils from source zones

Abstract

AbstractThis article presents the results of an Environmental Security Technology Certification Program (ESTCP) demonstration conducted at Eielson Air Force Base in Fairbanks, Alaska: Ex situ soil washing to remove per‐ and polyfluoroalkyl substances (PFAS) adsorbed to soils from source zones. The ESTCP project information can be found here: https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Risk-Assessment/ER20-5258. The final report is expected to be published in September 2022. The objective of the project was to demonstrate and validate soil washing as a cost‐effective mass removal technology that can be applied to treat soil in source zones impacted by PFAS. The approach focused on the treatment of coarse soil fractions (sand and gravel) and separation of fines (silt/clay) for potential treatment by other means such as landfilling, thermal desorption, or stabilization. The goal was to optimize the treatment process to minimize wastes requiring more expensive treatment so that the cost would be competitive with landfilling in the continental United State. The treatment process was designed and optimized at the bench‐scale and demonstrated at the field‐scale by conducting a treatment trial at the Eielson Air Force Base using a mobile soil washing plant. Approximately 180 tons of PFAS‐contaminated soil was treated with untreated perfluorooctane sulfonate (PFOS) concentrations ranging from 4.5 to 560 μg/kg. The technology was validated by measuring PFAS mass removal efficiencies based on soil concentrations of PFAS compounds measured via Department of Defense Quality Standards Manual 5.3 Table B‐14 and PFAS leachate concentrations as measured by synthetic precipitation leaching procedure testing. PFAS concentrations in treated soils were compared to Alaska Department of Environmental Conservation soil screening levels for the migration to groundwater pathway through leaching for perfluorooctanoic acid (PFOA), PFOS, and perfluorobutane sulfonate (PFBS). PFAS concentrations in soil leachate were compared to US Environmental Protection Agency groundwater screening criteria for PFOA, PFOS, and PFBS. The results of the treatability testing and pilot tests confirmed that coarse grained sand and gravel were successfully treated to meet performance goals and the fines were segregated for alternative treatment. The benefit of the waste minimization approach is that more cost‐effective means can be applied to treat most of the impacted soil volume while limiting expensive residuals disposal or more expensive means of treatment such as incineration or thermal desorption. Treated soils can be beneficially reused on‐site and fines could also be stabilized to meet leaching‐based standards, as an alternative to off‐site disposal or treatment. This flexible approach allows one to tailor the combined remedy strategy to meet clients' objectives and risk tolerance through a sustainable, least‐cost remedial approach.