PFAS Treatment by Electrical Discharge Plasma
Plasma-based water treatment is a technology that, using only electricity, converts water into a mixture of highly reactive species including OH•, O, H•, HO2•, O2•‒, H2, O2, H2O2 and aqueous electrons (e‒aq), called a plasma. These highly reactive species rapidly and non-selectively degrade volatile organic compounds (VOCs), 1,4-Dioxane, and a broad spectrum of per- and polyfluoroalkyl substances (PFAS) including perfluorooctanoic acid (PFOA), perfluorooctane sulfonic acid (PFOS), and short-chain PFAS. A plasma reactor can simultaneously oxidize and reduce organics by producing a mixture of hydroxyl radicals and aqueous electrons, the latter of which act as strong reducing agents and could be the key species in removing PFAS and other non-oxidizable compounds. Additionally, the plasma process produces no residual waste and requires no chemical additions, although adding surfactants or injecting inert gas into the liquid phase can increase interfacial PFAS concentrations, exposing more of the PFAS to the plasma and therefore increasing removal efficiency.
- Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)
- PFAS Ex Situ Water Treatment
- PFAS Soil Remediation Technologies
- PFAS Sources
- PFAS Transport and Fate
- Supercritical Water Oxidation (SCWO)
- Interstate Technology Regulatory Council (ITRC), PFAS – Per- and Polyfluoroalkyl Substances: 12.2 Field-Implemented Liquids Treatment Technologies and 12.5 Limited Application and Developing Liquids Treatment Technologies.
- Physico-Chemical Processes for the Treatment of Per- And Polyfluoroalkyl Substances (PFAS): A review
- Low Temperature Plasma for Biology, Hygiene, and Medicine: Perspective and Roadmap
Plasma processing plays an essential role in various industrial applications such as semiconductor fabrication, polymer functionalization, chemical synthesis, agriculture and food safety, health industry, and hazardous waste management. Plasma is a gaseous state of matter consisting of charged particles, metastable-state molecules or atoms, and free radicals. Depending on the energy or temperature of the electrons, compared with the temperature of the background gas, plasmas can be classified as thermal or non-thermal. In thermal plasma, an example of which is an electrical arc, individual species’ temperatures typically exceed several thousand Kelvins (K). Non-thermal plasmas are formed using less power with temperatures ranging from ambient to approximately 1000 K. An example of a non-thermal plasma is a dielectric barrier discharge used for commercial ozone generation.
Plasma that is applied in water treatment (Figure 1) is typically non-thermal, which offers high-energy process efficiency and selectivity. Since the 1980s when the first plasma reactor was utilized to oxidize a dye, over a hundred different plasma reactors have been developed to treat a range of contaminants of environmental importance including biological species. Examples include treatment of pharmaceuticals, volatile organic compounds (VOCs), 1,4-dioxane, herbicides, pesticides, warfare agents, bacteria, yeasts and viruses using direct-in-liquid discharges with and without bubbles and discharges in a gas over and contacting the surface of a liquid. Different excitation sources including AC, nanosecond pulsed and DC voltages have been utilized to produce pulsed corona, corona-like, spark, arc, and glow discharges, among other discharge types. Many reviews of plasma processing for water treatment applications have recently been published.
Plasma-based water treatment (PWT) owes its strong oxidation and disinfection capabilities to the production of reactive oxidative species (ROS), primarily OH radicals, atomic oxygen, singlet oxygen and hydrogen peroxide. The process also produces reductive species such as solvated electrons and reactive nitrogen species (RNS) when nitrogen and oxygen are present in the discharge. This process has the advantage of synergistic effects of high electric fields, UV/VUV light emissions and in some cases shockwave formation in a liquid. It requires no chemical additions, and can be optimized for batch or continuous processing.
Application of Plasma for the Treatment of PFAS-Contaminated Water
Several research groups have investigated the use of plasma to treat and remove PFAS from contaminated water. Of those studies, the Enhanced Contact (EC) plasma reactor developed by researchers at Clarkson University is one of the most promising in terms of treatment time, cost, the range of PFAS treated and scale up/throughput. Their process has been shown to degrade PFOA, PFOS, and other PFAS in a variety of PFAS-impacted water sources.
In the EC plasma reactor (Figure 2), argon gas is continuously pumped through the solution to form a layer of foam and thus concentrate PFAS at the gas-liquid interface where plasma is formed. The process is able to lower the concentrations of PFOA and PFOS in groundwater obtained from multiple DoD sites to below Environmental Protection Agency’s (EPA’s) lifetime health advisory level (HAL) of 70 parts per trillion (70 nanogram per liter, ng/L) within 1 minute of treatment (Figure 3) with energy requirements much lower than those of alternative technologies (~2-6 kWh/m3 for plasma vs. 5000 kWh/m3 for persulfate, photochemical oxidation and sonolytic processes and 132 kWh/m3 for electrochemical oxidation). The EC plasma reactor owes its high efficacy to the plasma reactor design, in particular to the gas bubbling through submerged diffusers to transport PFAS to the plasma-liquid interface and thus minimize bulk liquid limitations.
In 2019, a mobile plasma treatment system (Figure 4) was successfully demonstrated for the treatment of PFAS-contaminated groundwater at the fire-training area at Wright-Patterson Air Force Base.
Over 300 gallons of PFAS-impacted groundwater were treated at a maximum flowrate of 1.1 gallons per minute (gpm) resulting in ≥90% reduction (mean percent removal of 99.7%) of long-chain PFAAs (fluorocarbon chain ≥ 6) and PFAS precursors in a single pass through the reactor (Figure 5) at a treatment cost of $7.30/1000 gallons. As expected, the removal of short-chain PFAS was slower due to their lower potential for interfacial adsorption compared to long-chain PFAS. However, post-field laboratory studies revealed that the addition of a cationic surfactant such as CTAB (cetrimonium bromide) minimizes bulk liquid transport limitations for short-chain PFAS by electrostatically interacting with these compounds and transporting them to the plasma-liquid interface where they are degraded. Both bench and pilot-scale EC plasma-based process have been extended for the treatment of PFAS in membrane concentrate, ion exchange brine, and landfill leachate.
As a part of a currently-funded ESTCP project (ESTCP ER20-5535), the Clarkson University team with the support of GSI Environmental Inc. is evaluating the effectiveness of their plasma process in treating diluted aqueous film-forming foams (AFFFs) as well as the benefits of pre-oxidation of PFAS precursors in high concentration AFFF solutions in terms of post-oxidation plasma treatment time, destruction efficiency and cost.
Advantages and Limitations of the Technology for PFAS Treatment
- High removal rates of long-chain PFAS (C5-C8) due to the production of versatile reactive species
- Requires no chemical additions and produces no residual waste
- Total organic carbon (TOC) concentration and other non-surfactant co-contaminants do not influence the process efficiency
- The process is mobile and scalable
- Versatile: can be used in batch and continuous systems
- Limited removal of short-chain PFAS due to their inability to concentrate at plasma-liquid interfaces. Addition of surfactants such as CTAB improves their removal and degradation rates.
- Excessive foaming caused by bubbling argon gas through a solution containing high (>10 mg/L) concentrations of long-chain (surfactant) PFAS may interfere with the formation of plasma.
PFAS are susceptible to plasma treatment because the hydrophobic PFAS accumulates at the gas-liquid interface, exposing more of the PFAS to the plasma. Plasma-based treatment of PFAS contaminated water successfully degrades PFOA and PFOS to below the EPA health advisory level of 70 ppt and accomplishes the near complete destruction of other PFAS within a short treatment time. PFAS concentration reductions of ≥90% and post-treatment concentrations below laboratory detection levels are common for long chain PFAS and precursors. The lack of sensitivity of plasma to co-contaminants, coupled with high PFAS removal and defluorination efficiencies, makes plasma-based water treatment a promising technology for the remediation of PFAS-contaminated water. The plasma treatment process is currently developed for ex situ application and can also be integrated into a treatment train.
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