Role of Mineral–Organic Interactions in PFAS Retention by AFFF-Impacted Soil

Abstract

A comprehensive, generalized approach to predict the retention of per- and polyfluoroalkyl substances (PFAS) from aqueous film-forming foam (AFFF) by a soil matrix as a function of PFAS molecular and soil physiochemical properties was developed. An AFFF with 34 major PFAS (12 anions and 22 zwitterions) was added to uncontaminated soil in one-dimensional saturated column experiments and PFAS mass retained was measured. PFAS mass retention was described using an exhaustive statistical approach to generate a poly-parameter quantitative structure–property relationship (ppQSPR). The relevant predictive properties were PFAS molar mass, mass fluorine, number of nitrogens in the PFAS molecule, poorly crystalline Fe oxides, organic carbon, and specific (BET-N2) surface area. The retention of anionic PFAS was nearly independent of soil properties and largely a function of molecular hydrophobicity, with the size of the fluorinated side chain as the main predictor. Retention of nitrogen-containing zwitterionic PFAS was related to poorly crystalline metal oxides and organic carbon content. Knowledge of the extent to which a suite of PFAS may respond to variations in soil matrix properties, as developed here, paves the way for the development of reactive transport algorithms with the ability to capture PFAS dynamics in source zones over extended time frames.

In Situ Insight into the Availability and Desorption Kinetics of Per- and Polyfluoroalkyl Substances in Soils with Diffusive Gradients in Thin Films

“…The concentrations of PFAS in the soil pore water ( C d ) and methanol-associated soil solid phase ( C s , w/w) are listed in Table S6 of the Supporting Information. The non-extractable PFAS determined by methanol extraction ranged from 62% (PFOS in the sandy soil) to 89% (PFBA in the brown soil), similar to the PFAS retention trend in a soil column study . For a given PFAS, the non-extractable fraction was in the order of brown soil > loamy soil > black soil > sandy soil, implying that soil properties are important factors that control their distribution in soils. , …”

Repeated Aqueous Film-Forming Foams Applications: Impacts on Polyfluoroalkyl Substances Retention in Saturated Soil

“…The AFFF used in the column experiments was obtained from the Colorado School of Mines and previously characterized by Hao et al and described in Wanzek et al . A complete list of initial concentrations and PFAS names, acronyms, and representative structures can be found in the SI (Tables S1 and S2, respectively) and are consistent with those reported by Hao et al Synthetic tap water was used for experiments, as potable (tap) water was likely the source of water used to dilute AFFF concentrates in firefighter-training areas to achieve the prescribed application rate (e.g., 3% v/v).…”

“…The soil was previously characterized by Wanzek et al as a C2 horizon from the Rubicon soil series and classified as a sandy, mixed, frigid Entic Haplorthod (Table S10). Although aluminum nor manganese content was not measured, aluminum and manganese trend with iron since they commonly co-occur in the parent primary minerals …”

“…A column flow rate was chosen to replicate the groundwater flow of the area surrounding the Oscoda-Wurtsmith Air Force Base, the origin of the soil, where, under saturated conditions, the natural groundwater flow rate is between 0.09 m/day to 0.24 m/day . About 0.25 pore volumes (PV) of 3% AFFF was applied using downward flow (Figure S1) and followed with synthetic tap water flow for about two PV, which corresponded to the volume needed to reduce PFAS concentrations to below detection limits in the column effluent (Figure and Table S4).…”

“…Adding to this complexity, multiple soil properties can affect the mechanisms controlling PFAS sorption onto uncontaminated, saturated soil including electrostatic interactions with cation and anion exchange sites ,,,, and clays ,− as a function of soil pH, , hydrophobic interactions with quartz sand, , or interactions with soil organic carbon. , Recent research employed an aqueous solution composed of a single PFAS indicating that a combination of soil properties such as void ratio and silt+clay content or a combination of iron oxide content, organic carbon content, and particle surface area , is better able to describe sorption of multiple classes of PFAS to saturated soil than single-parameter models. Wanzek et al demonstrated PFAS adsorption onto saturated soils after only a single application of AFFF at concentrations 1000 times below the mixture’s CMC. Anionic PFAS sorption was attributed to the strong hydrophobicity and exclusion from the aqueous phase and was only poorly correlated with electrostatic attraction to oxalate extractable iron (Fe ox ) and organic carbon (%OC), which is supported in the literature. ,− The sorbent parameters, Fe ox and %OC, were statistically related to the number of nitrogen atoms found in zwitterionic PFAS molecules that possess both positive and negative charges, providing a mechanistic understanding of these structural PFAS features to interactions with charged soil surfaces. ,, Wanzek et al also presented a polyparameter quantitative structure–property relationship (ppQSPR) of PFAS retention in saturated soils of AFFF-impacted source zones for concentrations below CMC.…”