Effective Destruction of Per- and Polyfluoroalkyl Substances in Water by Modified SiC-Based Photocatalysts
Dr. Zachary Hendren | Research Triangle Institute
Per- and polyfluoroalkyl substances (PFAS) were used extensively as aqueous film-forming foam (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 study was to develop and demonstrate a novel catalyst material’s ability to enhance the ultraviolet (UV) based carbon-fluorine bond cleavage using a silicon carbide (SiC) catalyst composited with single atom platinum catalyst (SAC Pt).
The project team investigated both perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) as well as byproducts of the reaction. The research sought to answer specific technical questions on catalyst performance that are necessary for scale-up and deployment of this technology:
- Evaluation of catalyst performance as measured by reduced reaction time relative to titanium dioxide (TiO2),
- Measurement of the percent destruction of perfluorooctanoic acid molecules and monitoring of daughter products, and
- Understand catalyst degradation pathways and ultimate lifecycle of the material in order to assess potential for field deployment.
All technology components were evaluated at laboratory scale to demonstrate proof of concept for this novel photocatalytic defluorination approach. Demonstration of this approach and laboratory scale data obtained provided crucial understanding necessary for scale-up reactor design.
The platinum silicon carbide (Pt/SiC) catalyst showed excellent destruction efficiency for PFOA. Analytical results demonstrated sequential stepwise destruction of PFOA as well as all daughter products. The initial degradation rate for Pt/SiC was approximately 30% faster than that for TiO2 and equivalent reaction conditions. In addition, while PFOA destruction via TiO2 decreased by more than 70% upon repeating dosing cycles, the PFOA destruction rate was maintained. This suggest the material has promise as a long-term option for site treatment. However, these results were not observed for PFOS, with only approximately 50% degradation observed. Additionally, catalyst surface characterization identified -F atoms on the surface, which supported a degradation mechanism showing that the catalyst is the final sink, but the degradation capacity was not reached under test conditions and remains a key question for this material. Finally, results showed that UV185 nanometers (nm) showed significantly higher degradation performance relative to UV254 nm, with more than 90% reduction in energy per order (EEO). This is a new finding and has implications for UV-based photocatalytic PFAS destruction.
Based on the experimental results and challenges identified, two future research pathways are envisioned:
Extended Testing. What is the ultimate milligram (mg) PFOA destroyed/g Pt/SiC? Is the Pt/SiC regenerable? Does material lifecycle offset energy savings realized?
Project results showed that continued dosing of PFOA demonstrated a degradation capacity is maintained over the timeframe investigated, but F on surface suggests the material may be capacity limited. The key research questions the research team seeks to answer next is the ultimate mg PFOA destroyed/g Pt/SiC.
PFOS Destruction Potential. Can Pt/SiC be combined with additional oxidative processes/pH control or other measures to achieve PFOS degradation?
Although excellent PFOA destruction was observed, only ~50% PFOS destruction was achieved. Results from PFOA/PFOS co-testing indicate that the catalyst was not poisoned, suggesting there may still be a pathway to achieving efficient PFOS degradation via this catalyst material.
No additional testing under SERDP is planned at this time.