Formatted Title
Immobilization of PFAS in AFFF-Contaminated Soil: Impact on Ecological and Human Exposure
Background/Objectives
A cost-effective strategy for minimizing PFAS leachability in contaminated soil is through the use of immobilization strategies. Several soil amendments (e.g., activated carbon, biochar and proprietary products) have been shown to decrease PFAS leachability through an enhancement in hydrophobic and electrostatic interactions. Studies utilizing various leaching methodologies (e.g., TCLP, SPLP, ASLP, LEAF) have shown that ∑28 PFAS leachability may be reduced by >95% due to electrostatic interactions with inorganic constituents in addition to hydrophobic and Van der Waal interactions with activated carbon and other organic components. While a considerable amount of leaching data are available for PFAS immobilization strategies, limited studies have assessed the impact of soil amendments on biological receptors to assess exposure minimization for ecological and human health. In this study, earthworm and rat bioassays (surrogate human assay) were utilized to determine PFAS immobilization efficacy following treatment of AFFF-contaminated soil using a carbon-based soil amendment.
Approach/Activities
AFFF-contaminated surface soils were collected from eight sites across southern and eastern Australia. Soils were characterized using standard wet chemistry methodologies while ∑28 PFAS concentrations were determined using LC-MS/MS. Bench-scale immobilization studies involved the addition of a composite soil amendment (aluminium hydroxide, activated carbon, kaolin clay) to AFFF-contaminated soil to achieve an application rate of 5% w/w. After thorough mixing and addition of water (to ~30% water holding capacity), soils were cured at room temperature for 5 days before the assessment of PFAS availability in amended and unamended soil. PFAS leachability was determined using the Australian Standard leaching Procedure (ASLP) while PFAS bioaccumulation in earthworms was determined using OECD method 222. An in vivo rat bioassay was utilized to determine the relative bioavailability of target PFAS via oral exposure following the establishment of PFAS toxicokinetics.
Results/Lessons Learned
The concentration of PFAS (∑28) in AFFF-contaminated soil ranged from 1,280-8,130 ng g-1 with PFSA comprising 86.0-99.3% of the ∑28 PFAS concentration. Of the 28 compounds quantified, PFOS was present at the highest concentration in all soils (740-7,000 ng g-1, comprising 57.8 to 96.4% of the ∑28 PFAS concentration) while the concentration of PFHxS was approximately an order of magnitude lower (45-290 ng g-1). In unamended soil, ∑28 PFAS in ASLP leachates ranged from 26.0-235 µg l-1 with PFOS (21.5-185 µg l-1) and PFHxS (0.93-18.5 µg l-1) being the major constituents, driven by their soil concentration. However, in amended soil, ∑28 PFAS in ASLP leachates was reduced to £ 0.62 µg l-1 while PFOS and PFHxS leachability was reduced to 0.01-0.57 µg l-1 and £ 0.02 µg l-1 respectively (³ 99.1% reduction in leachability). Earthworm bioassays identified that PFAS in unamended AFFF-contaminated soil were highly bioavailable with biota-soil accumulation factors ranging from 12-44 (PFOS) and 45-83 (PFHxS). However, following exposure of earthworms to amended soil, biota-soil accumulation factors were reduced significantly to 0.2-0.6 (PFOS) and 0.2-0.9 (PFHxS). The incidental soil ingestion pathway was assessed using a rat bioassay with PFOS accumulation in the liver used as the bioavailability endpoint. PFOS relative bioavailability in unamended soil ranged from 69-95% while significantly lower values (<17%) were observed in amended soil. These results highlight that an immobilization strategy utilizing a composite soil amendment was able to significantly reduce PFOS availability through multiple lines of evidence.