Effective Destruction of Per- and Polyfluoroalkyl Substances in Water by Modified SiC-Based Photocatalysts
Dr. Zachary Hendren | Research Triangle Institute
Per- and polyfluoroalkyl substances (PFASs) were used extensively as aqueous film-forming foams (AFFF) for firefighting since the 1970s and may have impacted groundwater in as many as 400 Department of Defense (DoD) bases throughout the country. These substances are persistent in the environment, recalcitrant to chemical and biological degradation, have been detected in human blood and tissues, and are classified as carcinogens and endocrine disruptors. There are adsorption or membrane separation-based mechanisms for removing PFAS from water, but there is no reliable method for disintegrating the molecules. The main objective of this project is to assess the ability of silicon carbon (SiC)-based photocatalysts to destroy PFAS compounds. The development of a process that leads to complete molecule destruction will facilitate achieving target water quality and reducing the time required for site cleanup and closure.
The team suggests to destroy PFAS molecules using a combination of ultraviolet (UV) light and a SiC and graphene quantum dot (GQD)-based catalyst. It is expected that the catalyst UV combination can achieve degradation via stepwise defluorination of PFAS molecules. This effort will assess the reaction rate and explore methods to immobilize the catalyst material so its performance can be assessed in a flow-through reactor. UV-based processes are promising because they are one of the few methods that can break the carbon-fluorine (C-F) bond and break down perfluoronated molecules, which allows the reuse of the treated water without producing brine or sludge that may have higher PFAS concentrations. Both 185nm and 254nm UV will be tested in series to achieve complete destruction of perfluorooctanoic acid (PFOA)/perfluorooctanesulfonic (PFOS) compounds. The stepwise degradation of PFAS molecules will be monitored using ultra-high performance liquid chromatography with high resolution time-of-flight mass spectrometry (UHPLC-TOF MS). The research team will further test methods to enhance the reaction rate by incorporating a single atom catalyst, such as platinum. The development of a reactor with faster kinetics will allow for modular design with flexible operating conditions. In addition, the suggested advanced catalytic process is expected to be effective for a wide range of PFAS class molecules.
It is expected that a near complete degradation can be achieved in this process and that the fast reaction time will allow for a compact system. The modular design with flexible operating condition allows facile integration with existing pre- and post-treatment systems to achieve target water quality. The advantages of pump-and-treat as opposed to in situ biological treatment will significantly reduce the time required for site cleanup and closure. In addition, the suggested advanced catalytic process is expected to be applicable in other areas of environmental cleanup for the U.S. military where recalcitrant contaminants need to be decomposed.
Because of the toxic and carcinogenic concerns with PFOA/PFOS, in 2009, DuPont introduced 2,3,3,3- Tetrafluoro-2-(heptafluoropropoxy) propanoic acid (known as GenX) to replace PFOA/PFOS. However, recent studies have shown that the GenX replacement is likely similar to its PFOA/PFOS predecessors in toxicity, bioaccumulation, and environmental impacts. There are an estimated 3,000 to 6,000 different species of polyfluoralkyl and perfluorinated compounds in the environment. In addition to GenX, in 2017, the Environmental Protection Agency (EPA) identified two additional unregulated compounds being released near Wilmington, North Carolina, known as Nafion Byproducts 1 and 2. Although their chemical formulas are not yet public, they are likely perfluorinated compounds that will pose significant health and environmental risks. It is very likely that the technology developed for PFOA/PFOS can also treat these fluorinated chemicals. (Anticipated Completion - February 2019)