Effectiveness of Photocatalysis, Radiolysis, and Ultrasonic Irradiation in the Remediation of GenX: Computational Study of the Ultrasonically Induced Mineralization

Perfluoro-2-propoxypropanoic acid (PFPrOPrA), a free acid form of GenX, is a problematic perfluorinated alkyl substance (PFAS). Standard and advanced wastewater treatment methods are unable to effectively degrade PFAS due to its strong and unreactive C-F bonds. ${\mathrm{TiO}}_2$ photocatalytic, radiolytic, and ultrasonic irradiation (USI) methods were applied in an attempt to degrade PFPrOPrA. A bimolecular rate constant for the reaction of ${\mathrm{e}}_{\mathrm{aq}}^{-}$ and GenX of $(3.09\pm 0.03)\times {10}^7{\mathrm{M}}^{-1}·{\mathrm{s}}^{-1}$ was measured in a buffered aqueous solution by monitoring the transient signal of the hydrated electron as a function of GenX concentration. Corroborating this relatively slow rate constant, less than 2% GenX degradation was observed after 8 h of continuous ${}^{60}\mathrm{Co}$ gamma radiolysis under a variety of conditions. ${\mathrm{TiO}}_2$ photocatalysis at 350 nm under alkali conditions showed minimal destruction of GenX without detectable levels of defluorination as measured by the production of fluoride ions. However, upon ultrasonic irradiation at 640 kHz and 396 W in an argon-saturated aqueous solution, greater than 80% of GenX was degraded within 60 min, yielding fluoride ions as the major product. We propose that the ultrasonic-induced degradation of GenX occurs primarily by pyrolysis. Computational methods were used to probe the energetics of the completing degradation pathways and possible pyrolytic products. The results demonstrate ultrasonic-induced pyrolysis is a promising process to mineralize GenX. The process can be accurately monitored and likely extended to mineralize a variety of perfluorinated and polyfluorinated substances.

This study probed the application of radiolysis, ${\mathrm{TiO}}_2$ photocatalysis, and USI to degrade an emerging problematic perfluorinated compound, GenX. Under the experimental conditions employed, the degradation of GenX was slow by ${\mathrm{TiO}}_2$ photocatalysis, and radiolysis. Although reductive transformation of PFAS by hydrated electrons generated during radiolysis has been demonstrated, the reaction did not lead to rapid mineralization and is highly dependent upon reaction conditions. USI, however, resulted in the rapid degradation of GenX significant mineralization as measured by the formation of fluoride ions. Complementary computational studies of the pyrolytic pathways provide insight about mineralization, which may be extrapolated to different classes of PFAS and thermal-induced degradation. The results of this research illustrate that USI is a powerful method to mineralize GenX and suggest it may be applicable for treatment of an array of PFAS-contaminated waters and wastewaters. Although the energy demand for USI-based water treatments can be high and thus costly, USI is a promising alternative for mineralization of PFAS in preconcentrated or concentrated waste streams.