Theme: 07. Per- and Polyfluorinated Alkyl Substances (PFAS)

Topic Area: 7f. In Situ PFAS Treatment Approaches

Formatted Title:

Enhanced PFAS Mass Removal from Soil and Groundwater via Novel Surfactant Flushing Column Study

Background/Objectives:

Removal of PFOA and PFOS (PFOS) from aquifers depends on groundwater pump and treat (P&T). Groundwater is recovered and PFAS is removed, most often by filtration and granular activated carbon (GAC), with ion exchange resins having increasing usage. P&T can take decades to complete as the efficacy of the process is limited by contaminant sorption (i.e., adsorption and absorption) to soils, hydraulic conductivity (K), and the relative low concentrations of PFAS in groundwater, even when the PFAS source zone is relatively close by.

Enhanced recovery of contaminants from source zone areas has been demonstrated to be more cost effective than using P&T alone. Operational time frames are lowered, in some cases, from years to months, or months to weeks, but to date they have not been used for PFAS source zones. This work represents the first attempt to understand the potential of solvents and surfactants to increase the removal rates of PFAS, at a scale using reagent concentrations that are environmentally applicable.

Approach/Activities:

Tests were designed to measure the effectiveness of a specifically-developed surfactant (PFAS-SOL) to increase the concentration of PFAS compounds in water in a simulated enhanced recovery experiment. Methanol and surfactant extraction were compared. The experiments were run at the University of Greenwich, UK.

A series of column experiments were carried out using 75 cm high and 14 cm diameter columns. A mineral sand was used in the first instance, followed by the addition of 10% by mass activated carbon to show the impact of organic material. The columns filled with mineral sand and were saturated from the base and spiked with high concentrations of PFOA and PFOS to mimic a source zone. They were then drained and the effluent sampled. They were filled again, one with methanol at 50% concentration and the other with a novel surfactant formulation, at 4% concentration, drained and the effluent sampled. The columns were then deconstructed and a moisture profile was measured, with soil samples taken to measure the retained PFAS.

Further samples of the same soil were then mixed with activated carbon and the columns were set up again. The same procedures were followed, with an additional drainage carried out. A moisture profile was again taken during column deconstruction with soil samples taken to measure retained PFAS.

Results/Lessons Learned:

The experiments showed the effect of both solvent-enhanced extraction and surfactant-enhanced extraction, as well as the mass of PFOA and PFOS retained following the drainage tests. The surfactant enhanced extraction achieved: 160% to 185% PFOA mass removal, 279% to 732% PFAS mass removal, and 242% to 622% total PFAS mass removal. Furthermore, the moisture profiles at the conclusion of the tests showed the positive effects of surfactant on the unsaturated soils. The addition of surfactant was demonstrated to lower the moisture content of soils, allowing more water to be drained and therefore greater mass removal of any liberated contaminant, including PFAS by greater than 200%, in the drained layers.

These repeatable experiments represented a novel approach for PFAS source zone treatment at an environmentally relevant scale, using around 12 L of specimen and reagent concentrations that are applicable to field application.

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David Holmes