Date of Thesis

Spring 2025

Description

Due to their toxicity, prevalence, and persistence in the environment, poly- and perfluoroalkyl substances (PFAS) pose a significant threat to human health and ecological systems. When conventional groundwater remediation methods are ineffective for PFAS, in situ containment using engineered vertical barriers (cutoff walls) becomes a potentially viable alternative. This research investigates the efficacy of soil-bentonite (SB) cutoff walls amended with granular activated carbon (GAC) for long-term PFAS containment in groundwater. The focus of this work is characterizing PFAS adsorption onto the mixed matrix for determination of isotherm parameters for modeling. Along the way, considerations for sample handling were investigated to improve future research methods. This research also investigates contaminant transport modeling to demonstrate the breakthrough delay for each compound.

Batch Equilibrium Adsorption Tests (BEATs) were performed using a model sand-bentonite backfill mixture (5% bentonite by dry weight), either unamended or amended with 1% dry wt GAC, and solutions containing either perfluorooctanoic acid (PFOA), perfluorooctanesulfonic acid (PFOS), perfluorohexanesulfonic acid (PFHxS), or perfluorobutanesulfonic acid (PFBS). Centrifugation alone proved inadequate for removing suspended bentonite from the BEAT supernatant due to its fine particle size. Thus, to protect the analytical instrument, centrifuged samples were filtered with 0.45-µm syringe filters. Of the four different filter materials tested, polyethersulfone (PES) was the best material for minimizing PFAS loss from samples due to adsorption onto the filter membrane.

Adsorption kinetics experiments indicated that a BEAT duration of seven days ensured equilibrium for all four PFAS compounds. Results of BEATs demonstrated extensive adsorption of all four compounds to the backfill mixture containing 1% GAC, compared to negligible adsorption without GAC amendment. The adsorption behavior was non-linear and was well described by the Freundlich model. At lower aqueous-phase equilibrium concentrations (Ce), the adsorbed concentration data (qe) showed a gradual downward bend, especially for PFOS, PFOA, and PFHxS, resulting in non-traditional isotherm models being considered as alternatives to the Freundlich model. These alternative models suggest that PFAS may behave as surfactants, exhibiting complex behaviors associated with multilayer adsorption, micelle formation, and other phenomena.

Simulations using a 1-D contaminant transport model showed that adding 1% GAC to an SB matrix can substantially delay PFAS breakthrough by factors ranging from 1,500 to 51,000 times compared to the control scenario without GAC. The compound that was delayed the most was PFOS, as it had the longest breakthrough time, followed by PFHxS, PFOA, and PFBS, which had the shortest breakthrough time.

This research establishes a strong foundation for the application of GAC-amended SB cutoff walls for effective PFAS containment. By combining the established technology of SB cutoff walls with the powerful adsorption capabilities of GAC, this method offers a potential long-term, environmentally sound solution to mitigate the spread of these "forever chemicals."

Keywords

PFAS, soil-bentonite cutoff wall, activated carbon, contaminant transport modeling, groundwater remediation

Access Type

Masters Thesis

Degree Type

Master of Science in Environmental Engineering

Major

Environmental Engineering

First Advisor

Kevin Gilmore

Second Advisor

Michael Malusis

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