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

Topic Area: 7f. In Situ PFAS Treatment Approaches

Formatted Title:

PRB Enhanced Electrokinetic Process for the Removal of PFAS

Background/Objectives:

Per- and polyfluoroalkyl substances (PFAS) are known for their resistance to thermal, chemical, and biological degradation. Consequently, the successful removal of PFOA from soil necessitates the implementation of high-energy intensity and highly destructive techniques. Electrokinetic (EK) soil remediation is an alternative green and sustainable remediation technology for the removal of PFAS that can be applied in situ or ex situ, eliminating the need for excavation of the hazardous soil. The efficacy of the standalone EK process in soil PFAS removal is negligible, primarily due to the intersecting mechanisms of electromigration and electroosmosis transportation. Consequently, the redistribution of PFAS across the soil matrix occurs, hampering effective remediation efforts. Permeable reactive barriers (PRB) have been used to capture contaminants and extract them at the end of the EK process. The primary aim of this study was to evaluate the viability of employing an enhanced electrokinetic process in conjunction with a high-capacity, PRB to effectively eliminate PFAS contaminants from polluted soil.

Approach/Activities:

A laboratory-scale EK process in conjunction with iron Slag/AC PRB has been evaluated for the removal of PFOA in kaolinite soil. Kaolin soil was used as a model soil in the EK process due to its low permeability, low carbon content and low cation exchange capacity to examine the transport mechanism, PRB interaction with PFAS. A laboratory scale reactor cell was set up with spiked soil and a PRB was placed in the middle of the reactor cell to capture the PFOA.

The reactor comprises two electrode compartments at either end, a soil compartment, a PRB compartment, and an electrolyte reservoir. In the middle of the soil compartment, a 2 cm PRB inserted between two filter papers was employed, and contaminated soil was loaded onto both sides. Filter paper (5-13 µm) was supported by a perforated plexiglass plate placed between the electrode chamber and the soil compartment to prevent soil from entering the electrolyte chambers.

Results/Lessons Learned:

The soil's electric conductivity (EC) plays a vital role in the EK treatment, which involves the movement of the contaminants and other charged particles under direct electric current. During the EK process, the transportation of charged particles to the anode and cathode create a concentration gradient that can result in soil resistivity and changes in pH. The soil has a remarkable ability to transmit electrical charges when the EC of the soil is high, therefore removing ionic species during the electrokinetic treatment can cause a decrease in electric conductivity. Experimental results indicated that the EC is lower in soil regions with high PFOA removal. Post EK treatment, the soil pH in sections close to the anode compartment decreased to lower than the initial soil pH and gradually increased in the soil sections towards the cathodic region. The generation of hydrogen ions (H⁺) associated with electrolysis reactions caused a decreased soil pH while the production of hydroxide ions (OH^-) at the cathode region increased pH. PFOA removal was superior in sections close to the cathode area which as attributed by an increased pH and a decreased PFOA adsorption on the soil surface. The soil’s pH was inversely proportional to the EC of the soil. At the end of the EK process 87% of the PFOA was adsorbed onto the PRB and overall, 94% removal of PFOA removal was achieved.

Building on this success, parallel reactor and pilot-scale investigations are underway with Photon Water and the University of Technology Sydney to evaluate the applicability of nZVI and other alternative amendments as a permeable reactor barrier for EK PFAS remediation. This body of research will include the feasibility of different types of sustainable PRBs in EK process with examination of reaction mechanisms and removal efficiency to support process scale up. The use of PRB enhanced EK remediation offers a unique opportunity for more efficient and comprehensive PFAS removal in or ex situ, supporting a sustainable solution to this pervasive environmental challenge.

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Ali Altaee