Theme: 7. Per- and Polyfluorinated Alkyl Substances (PFAS)

Topic Area: 7i. PFAS Fate and Transport Properties

Formatted Title:

Unsaturated Transport of PFAS Mixtures: Column Experiments and Model Validation

Background/Objectives:

Background/Objectives: Per- and polyfluoroalkyl substances (PFAS) are a class of several thousand compounds that have been manufactured for the better portion of a century and were widely used in food packaging, waterproofing, fire suppression and many other commercial products. PFAS have become widespread in environmental media including groundwater, freshwater, and soil. An important behavior of PFAS is their tendency to accumulate to the air-water interface due to their amphiphilic properties. This behavior affects the transport of PFAS in the vadose zone as soil pores may be partially saturated with both air and water. In agricultural settings, where biosolids and/or wastewater are often applied as nutrient amendments and for irrigation, it is important to understand the migration of PFAS in variably saturated soils due to the potential for crop uptake and leaching to shallow aquifers that may serve as drinking water sources in rural areas. The objective of this study was to investigate the role of accumulation at the air-water interface on the migration and persistence of PFAS mixtures in unsaturated soils as a function of water content.

Approach/Activities:

Approach/Activities: A combination of soil-column experiments and mathematical modeling was used to investigate effects of the air-water interface on transport and retention of PFAS mixtures as a function of soil water content. Columns were packed with 40-50 mesh size Ottawa sand, which exhibits minimal solid-phase sorption of PFAS. The columns were initially saturated with synthetic rainwater, and then slowly desaturated to the desired soil water content: low saturation (~25%), medium saturation (~55%) and full saturation (100%). For each saturation level, a PFAS mixture (~0.04 uM PFOS, PFOA, PFBS) and a single PFAS (~0.04 uM PFOA) were employed to investigate competitive adsorption and quantify the air-water interfacial area, respectively. Prior to PFAS transport, a nonreactive tracer test was performed to obtain water flow characteristics in the column. A pulse of ~50 pore volumes of the PFAS or PFAS mixture was injected into the column followed by ~50 pore volumes of background solution. Concentrations of PFAS in effluent and solid samples were quantified using a Waters liquid chromatography quadrupole mass spectrometer (LC-MS/MS). Surface tension measurements for individual PFAS and the PFAS mixture were determined using the Wilhelmy plate method. A modified version of HYDRUS 1-D was used to simulate the transport and retention of FPAS mixtures in the unsaturated soil columns.

Results/Lessons Learned:

Results/Lessons Learned: At the lowest water saturation (25%), PFBS exhibited minimal retardation, with breakthrough occurring at approximately 1.1 pore volumes, while PFOA exhibited breakthrough after 2.5 pore volumes. In contrast, PFOS transport was strongly retarded, appearing in the column effluent at 25 pore volumes with 50% of the PFOS mass remaining in the column after 100 pore volumes. The retained PFOS mass was successfully recovered via an extraction procedure to yield ~100% mass balance. The behavior of PFOS suggests that a fraction of the PFOS mass was irreversibly accumulated to the air-water interface. The breakthrough curves in the three-component mixture study of PFOA and PFBS exhibit concentration overshooting in the first few pore volumes, indicative of competition between species at the air-water interface.

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Shana Xia