Influence of Residual Nonaqueous-Phase Liquids (NAPLs) on the Transport and Retention of Perfluoroalkyl Substances

Abstract

Per-and polyfluoralkyl substances (PFAS) are known to accumulate at interfaces, and the presence of nonaqueous-phase liquids (NAPLs) could influence the PFAS fate in the subsurface. Experimental and mathematical modeling studies were conducted to investigate the effect of a representative NAPL, tetrachloroethene (PCE), on the transport behavior of PFAS in a quartz sand. Perfluorooctanesulfonate (PFOS), perfluorononanoic acid (PFNA), a 1:1 mixture of PFOS and PFNA, and a mixture of six PFAS (PFOS, PFNA, perfluorooctanoic acid (PFOA), perfluoroheptanoic acid (PFHpA), perfluorohexanesulfonate (PFHxS), and perfluorobutanesulfonate (PFBS)) were used to assess PFAS interactions with PCE-NAPL. Batch studies indicated that PFAS partitioning into PCE-NAPL (K nw < 0.1) and adsorption on 60−80 mesh Ottawa sand (K d < 6 × 10 −5 L/g) were minimal. Column studies demonstrated that the presence of residual PCE-NAPL (∼16% saturation) delayed the breakthrough of PFOS and PFNA, with minimal effects on the mobility of PFBS, PFHpA, PFHxS, and PFOA. Breakthrough curves (BTCs) obtained for PFNA and PFOS alone and in mixtures were nearly identical, indicating the absence of competitive adsorption effects. A mathematical model that accounts for NAPL−water interfacial sorption accurately reproduced PFAS BTCs, providing a tool to predict PFAS fate and transport in cocontaminated subsurface environments.

Predicting Interfacial Tension and Adsorption at Fluid–Fluid Interfaces for Mixtures of PFAS and/or Hydrocarbon Surfactants

“…PFAS are widespread and have contaminated surface water, soils, sediments, groundwater, and the atmosphere. In particular, vadose zones serve as significant PFAS reservoirs that pose long-term threats for contaminating groundwater. − The amphiphilic properties of PFAS distinguish their vadose zone transport behaviors from that of traditional non-surface-active contaminants. , Adsorption at fluid–fluid interfaces was shown to contribute to PFAS retention in soils by laboratory experiments, − field porewater sampling, − and mathematical modeling studies. ,− Air–water interfacial adsorption also affects the retention of PFAS by aerosols and the subsequent atmospheric transport − and the operation of multiple remediation methods such as foam fractionation , and carbon adsorption. , …”

“…Most PFAS-impacted sites comprise mixtures of PFAS and hydrocarbon surfactants. ,− The multicomponent PFAS and hydrocarbon surfactants may interact with each other, such as competing for adsorption sites at the fluid–fluid interfaces, which will subsequently influence the reduction of IFT. Mixtures of hydrocarbon surfactants have been widely studied for potential synergistic effects for reducing IFT. , IFT data of multicomponent PFAS or mixtures of PFAS and hydrocarbon surfactants have also been reported, ,,− some of which have demonstrated the presence of competitive adsorption among PFAS and hydrocarbon surfactants.…”

“…Several studies applied a direct extension of the single-component Langmuir–Szyszkowski isotherm to model the fluid–fluid interfacial adsorption of multicomponent PFAS. ,,,, However, the multicomponent Langmuir isotherm is not thermodynamically consistent unless all components have equal maximum adsorption capacity, − which is not fulfilled for most PFAS and hydrocarbon surfactants. More advanced models have been previously developed for predicting IFT and fluid–fluid interfacial adsorption of hydrocarbon surfactant mixtures, − but these advanced models are less practically useful due to the large number of required model parameters.…”

Polyfluoroalkyl substances requiring a renewed focus on groundwater‐surface water interactions

“…Recently, USEPA issued lifetime drinking water health advisory levels (HALs) of 4 and 20 picograms per liter, or parts per quadrillion (ppq), for PFOA and PFOS. d. The behavior of PFASs in the environment is very complex; for example, most PFAS compounds are resistant to biological degradation processes (e.g., Liou et al 2010), sorb to sediment (e.g., Schaefer et al 2021) and microplastics (e.g., Pramanik et al 2020;Cheng et al 2021;Scott et al 2021), exhibit self-assembly behavior (e.g., Dong et al 2021), partition into non-aqueous phase liquid (NAPL) (e.g., Liao et al 2022), and concentrate at air-water interfaces (e.g., Li et al 2020;Brusseau and Guo 2022). e. Many scenarios where PFASs are released to the environment are notably different and can be more complex than common release scenarios for other groundwater contaminants (e.g., petroleum hydrocarbons, chlorinated solvents, metals).…”

“…There are thousands of individual PFAS compounds, a wide range of materials that contain PFASs, and a very large number of potential sites (e.g., Salvatore et al 2022 report a presumptive estimate of more than 57,000 PFAS‐impacted sites in the USA alone). Very low regulatory standards have been established and/or are proposed in many jurisdictions (typically low parts per trillion [ppt], or nanograms per liter [ng/L]). Recently, USEPA issued lifetime drinking water health advisory levels (HALs) of 4 and 20 picograms per liter, or parts per quadrillion (ppq), for PFOA and PFOS. The behavior of PFASs in the environment is very complex; for example, most PFAS compounds are resistant to biological degradation processes (e.g., Liou et al 2010), sorb to sediment (e.g., Schaefer et al 2021) and microplastics (e.g., Pramanik et al 2020; Cheng et al 2021; Scott et al 2021), exhibit self‐assembly behavior (e.g., Dong et al 2021), partition into non‐aqueous phase liquid (NAPL) (e.g., Liao et al 2022), and concentrate at air‐water interfaces (e.g., Li et al 2020; Brusseau and Guo 2022). Many scenarios where PFASs are released to the environment are notably different and can be more complex than common release scenarios for other groundwater contaminants (e.g., petroleum hydrocarbons, chlorinated solvents, metals).…”