Electrically Assisted Sorption and Desorption of Per- and Polyfluoroalkyl Substances

Douglass Call | North Carolina State University

ER18-1395

Objective

Activated carbon (AC) is a widely used sorbent to remove per- and polyfluoroalkyl substances (PFASs) from groundwater. Two limitations of AC are 1) the inability to effectively remove some PFASs, especially short-chain species, and 2) the lack of cost-effective regeneration methods. The overall objective of this proof-of-concept project is to electrically enhance adsorption of high priority PFASs onto AC and electrically discharge them as an innovative, chemical-free regeneration technique. The technology that will be investigated is capacitive deionization (CDI), an emerging, yet proven, scalable method that removes charged species from water using a fraction of the energy of pressure-driven membrane processes.

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Technical Approach

There are two fundamental principles of CDI that make it an ideal technological solution to fulfill the objectives of the project. First, it uses electrical fields to enhance electrostatic interactions between ions and AC. As a result, AC adsorption capacities and rates can be increased with inputs as low as one volt of electricity. The preliminary results confirm this enhancement for perfluorobutanoic acid (PFBA), a short-chain PFAS that is poorly adsorbed in conventional AC adsorption processes. Applying a voltage of 1.0 V yielded a more than six-fold increase in PFBA uptake by AC. Second, by short-circuiting the electrodes or reversing the voltage/current, adsorbed ions are “pushed” back into solution, effectively regenerating the AC. For inorganic ions, sustained removals upwards of 95% after months of continuous operation have been reported using this in situ regeneration approach in CDI.

To fulfill the overall objective, the project will be divided into the following two tasks, each of which fulfills a corresponding specific objective:

  • Task 1: Determine the increase in adsorption rates and capacities of individual PFASs and PFAS mixtures using electrically charged AC electrodes in a groundwater treatment context. This task will focus on PFASs that vary in structure and affinity for AC. The removal of individual short- and long-chain PFASs will first be compared in addition to mixtures of both. Subsequently, competitor anion (Cl−), co-contaminants (trichloroethene, toluene), and dissolved organic matter (DOM) will be exampined for their impact on PFAS removals.
  • Task 2: Determine the increase in PFAS desorption rates and AC regeneration efficiencies using electrically assisted desorption. This task will involve the investigation of three mechanistically distinct electrodesorption methods: electrode short-circuiting, reverse voltage, and reverse current. To complete this task, multiple electro-sorption/desorption cycles will be completed with structurally diverse PFASs and results compared to open circuit voltage (OCV) controls.

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Benefits

This project will yield two key benefits for the DOD. First, it will provide a comprehensive understanding of electrically assisted adsorption of individual PFASs and PFAS mixtures. Previous research focused only on PFASs that are known to adsorb well to AC (PFOA and PFOS). This systematic approach, focusing on a range of structurally distinct PFASs, will provide new insight into removal behaviors of many PFASs that are currently difficult to remove at DOD sites and expand applications of electrosorption beyond common inorganic ions. Second, this project will reveal the efficacy of electrically regenerating AC. This approach could provide DOD sites where AC is currently being used with a low-cost and chemical-free method to extend the lifetime of AC-based treatment. (Anticipated Project Completion - 12/2019)

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Points of Contact

Principal Investigator

Douglas Call

North Carolina State University

Phone: 919-515-6455

Program Manager

Environmental Restoration

SERDP and ESTCP

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