In order to overcome the challenges of breaking the strong C-F bonds associated with PFAS compounds, as well as determining methods that will be effective at taking PFAS out of the environment, researchers are looking into novel ways to advance “off-the-shelf” treatment technologies. These treatment innovations have revolved around using advanced oxidation processes to generate free radicals to break bonds, polymers to selectively remove PFAS, catalysts to speed up reactions and produce more energy, and a combination of these methods.
To date, researchers have developed powerful chemical oxidation methods (e.g., hydrogen peroxide, Fenton’s Reagent, ozone) that have been successful at removing organic and inorganic contaminants. This treatment method works by producing free radicals that react with the contaminant to break its bonds. Early studies have shown that these conventional chemical oxidation methods are not powerful enough to break the C-F bonds of PFAS. Therefore, being able to increase in the power of these free radicals has been a novelty of current research in the area of applying oxidation treatment to remove PFAS.
Researchers have found ways around this challenge by evaluating electrochemical oxidation and siupercritical waste oxidation methods as opposed to chemical oxidation. Electrochemical oxidation is similar to chemical oxidation, but uses electricity instead of chemicals to generate free radicals. This is done by using an anode and cathode where electricity runs from the anode to the cathode. This principle is the same process that occurs in the battery of our cell phones, which use anodes and cathodes to make sure we can all stay up to date on our emails and social media. The novelty of electrochemical oxidation is that the use of electricity allows research to manipulate the energy required to overcome the C-F bond strength. It has been demonstrated that when you combine electrochemical oxidation with a separation process, such as filtration, you can remove shorter chain PFAS compounds. These are the same PFAS compounds that we are starting to see replace the legacy compounds. This approach actually uses PFAS free radicals against other PFAS compounds during treatment to break these strong C-F bonds.
Another application of electrochemical oxidation that can specifically target the longer-chain PFAS compounds is a process called electrical discharge plasma. Electrical discharge plasma is a process that combines the use of electricity in water to produce plasma. What is plasma? Other than something you would probably hear about in a sci-fi movie? Researchers discovered that when electricity is applied to water you can generate what they call aqueous electrons which acts like fire in solution to remove PFAS. Initial studies have shown the longer-chain PFAS compounds can be removed to levels below detection limits (9 ng/L).
Beyond the use of electricity in water or the application running it through a battery set up, we can use heat and pressure to change the state of water to remove PFAS. You might remember that matter can exist as either a liquid, solid, or vapor and this will depend on temperature and pressure. Water is unique because at a certain pressure and temperature, water can exist in a special state where it isn’t a solid, liquid, or a vapor. This state is called supercritical. This state is reached when the temperature reaches above approximately 704°F and a pressure above 218 atm. This temperature is approaching the lower range of what we would see at waste-to-energy facilities. In terms of pressure, this would be equal to diving to a depth of 1.36 miles in the ocean. Once a supercritical state is achieved, there is enough energy to completely breakdown compounds to clean water, energy, and CO2. Preliminary removals of almost 100% have been demonstrated for various PFAS compounds.
Sunlight has been shown to be a powerful oxidant that is not always thought about as an effective treatment method for contaminants. Using sunlight is a common treatment method for wetlands and is called photolysis. This treatment method has been shown to be effective at removing organics from leachate as well as other wastewaters. The direct application of sunlight (UV254) alone is not powerful enough to overcome the C-F bond. Researchers at Rice University have found that the addition of boron nitride to water containing PFOA exposed to UV light was able to remove 99.9% of PFOA from water. Although this method has not been tested on leachate, there is potential for similar approaches to be applied to leachate with adjustments to account for leachate composition.
These advancements have been able to take conventional chemical oxidation methods and improve their efficiency against C-F bonds through the use of electricity, heat, pressure, and sunlight.