In-situ stabilization/solidification using adsorbents for treatment of soils impacted by per- and polyfluoroalkyl substances (PFAS) : a bench scale study

Panda, Shradha
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Studies on PFAS (per- and polyfluoroalkyl substances) remediation are of prime importance owing to the carcinogenic effects of these substances on humans. However, the recalcitrance and persistence of PFAS make effective remediation extremely challenging. In-situ soil stabilization/solidification (ISS) techniques based on desorption/partitioning of PFAS from soil, offer great potential in controlling contaminant migration. Though soil mixing has been used in the successful immobilization of contaminants other than PFAS in the past, the focus of this study is to assess if ISS in conjunction with adsorbents, can be extended for the containment of PFAS impacted soils. Hence as a first step, leaching and soil mixing under different environmental factors like pH, time, and pore-fluid composition is simulated on a bench scale. This was facilitated by modifying the existing leach test methods specified by the United States Environmental Protection Agency (USEPA) Leaching Environmental Assessment Framework (LEAF). The broader objective of these experiments in simulating PFAS remediation was to arrive at an optimal mix proportion of a chemical grout that reduced the soil PFAS levels to a safe threshold (USEPA Health Advisory Limit, HAL) and also met a minimum strength recommendation. For doing so, two novel adsorbents: Fluoro-Sorb® Gen-II (foaming variant) and Fluoro-Sorb® Gen-II (non-foaming variant), and a binder: Ordinary Portland Cement (OPC) were employed. Consequently, the kinetics and equilibrium of liquid-solid partitioning in soil mixes prepared with different proportions of adsorbents and binder were studied. The possibility of achieving desirable results by performing soil mixing in the absence of cement was also explored. Furthermore, the structural integrity of soil was tested by characterizing the compressive strength of remediated soils. PFAS concentration in the leachate extracted from different mixes was quantified using High Performance Liquid Chromatography-Mass Spectrometry (HPLC-MS). The untreated soil had an available PFAS leaching load of 1.5×10⁵ ng/L (1500 ng/g). Soils treated with 8% and 10% of cement and 5% (w/w% of dry soil)Fluoro-Sorb® Gen-II (foaming variant) yielded a satisfactory strength exceeding the USEPA recommendation of 50 psi. When sampled at a liquid to solids (L/S) ratio of 10 as per USEPA Method 1316, the available PFAS content in these mixes was reduced to less than the USEPA HAL of 70 ng/L. At a given adsorbent dosage (3% and 5%), mixes treated without OPC yielded a lower PFAS concentration than the ones prepared with OPC indicating that the alkaline nature of cement binders can hinder the PFAS removal efficiency of the treated mix. Thus, the obtained experimental data could be used to determine optimum mix design while accounting for PFAS remediation and strength retention in ISS stabilized soils.