Complete Defluorination of PFAS via An Accumulation-and-Destruction Approach

TY - BOOK

T1 - Complete Defluorination of PFAS via An Accumulation-and-Destruction Approach

AU - Xiong, Xingaoyuan

PY - 2024/1

Y1 - 2024/1

N2 - The prevalence of per- and polyfluoroalkyl substances (PFAS) in aquatic environment have been associated with numerous negative health effects on human health due to their exceptional persistence, mobility, and bioaccumulative properties. Nonetheless, their strong C─F bonds and high electronegativity around fluorine atoms render them resistant to conventional treatments including radical-based advanced oxidation processes (AOPs). Degradation methods in achieving complete destruction of PFAS are lacking. Additionally, it remains a practical challenge to removal and degrade the diverse PFAS present at trace-level concentrations in water. In light of these challenges, it is imperative to explore efficient, scalable, and cost-effective approaches to mitigate the impacts of these “forever chemicals”. This Ph.D. project aims to address this critical need. This Ph.D. study delves into the destruction of PFAS using ultrasound (US) with the purpose of leveraging the localized hot spot in cavitation for the accumulation and pyrolysis of PFAS. To further improve defluorination efficiency of PFAS, advanced reduction process (ARP) employing reductive radicals was considered to break the PFAS structure. Specifically, we began by observing the complete defluorination of perfluorooctanoic acid (PFOA), a representative PFAS, through low frequency (20 kHz) US operating at 900 W input power over a 4 h period. Additionally, our research unveiled the competitive partitioning of PFOA and an external radical source (e.g., persulfate (PS)) at the bubble-water interface, and provided theoretical support for pyrolysis occurring within the cavitation bubble. Expanding our scope, we extended our investigation to eight different PFAS compounds, including C8-, C6-, C4-, and C3-perfluoroalkyl carboxylic acids (PFCA), C8-, C6-, and C4-perfluoroalkyl sulfonic acids (PFSA), and GenX, a common alternative to PFOA with reduced US input power to 100 W. The rate of defluorination, dependent on chain length and functional groups, suggests that the migration of PFAS from the bulk solution to the bubble-water interface, followed by vaporization within the cavitation bubble, governs the extent of decomposition. Furthermore, the improved defluorination rate achieved with the addition of methanol underscores the potential for energy-efficient US applications in treating PFAS-laden adsorbent washing solutions. The last research objective concerned using phenol as a chemical source to produce hydrated electrons (𝑒aq−, −2.9 V) to defluorinate PFOA under UV irradiation, assisted by dithionite (DTN), a reductive oxygen scavenger. The efficient defluorination rate under mild conditions indicates the promising scalability of this two-pollutant approach for PFAS treatment. Moreover, we proposed an •H-initiated pathway for PFOA defluorination. In conclusion, this Ph.D. project offers novel perspectives, both academically and practically, for addressing PFAS contamination, contributing significantly to the ultimate goal of achieving zero fluoro-pollution.

AB - The prevalence of per- and polyfluoroalkyl substances (PFAS) in aquatic environment have been associated with numerous negative health effects on human health due to their exceptional persistence, mobility, and bioaccumulative properties. Nonetheless, their strong C─F bonds and high electronegativity around fluorine atoms render them resistant to conventional treatments including radical-based advanced oxidation processes (AOPs). Degradation methods in achieving complete destruction of PFAS are lacking. Additionally, it remains a practical challenge to removal and degrade the diverse PFAS present at trace-level concentrations in water. In light of these challenges, it is imperative to explore efficient, scalable, and cost-effective approaches to mitigate the impacts of these “forever chemicals”. This Ph.D. project aims to address this critical need. This Ph.D. study delves into the destruction of PFAS using ultrasound (US) with the purpose of leveraging the localized hot spot in cavitation for the accumulation and pyrolysis of PFAS. To further improve defluorination efficiency of PFAS, advanced reduction process (ARP) employing reductive radicals was considered to break the PFAS structure. Specifically, we began by observing the complete defluorination of perfluorooctanoic acid (PFOA), a representative PFAS, through low frequency (20 kHz) US operating at 900 W input power over a 4 h period. Additionally, our research unveiled the competitive partitioning of PFOA and an external radical source (e.g., persulfate (PS)) at the bubble-water interface, and provided theoretical support for pyrolysis occurring within the cavitation bubble. Expanding our scope, we extended our investigation to eight different PFAS compounds, including C8-, C6-, C4-, and C3-perfluoroalkyl carboxylic acids (PFCA), C8-, C6-, and C4-perfluoroalkyl sulfonic acids (PFSA), and GenX, a common alternative to PFOA with reduced US input power to 100 W. The rate of defluorination, dependent on chain length and functional groups, suggests that the migration of PFAS from the bulk solution to the bubble-water interface, followed by vaporization within the cavitation bubble, governs the extent of decomposition. Furthermore, the improved defluorination rate achieved with the addition of methanol underscores the potential for energy-efficient US applications in treating PFAS-laden adsorbent washing solutions. The last research objective concerned using phenol as a chemical source to produce hydrated electrons (𝑒aq−, −2.9 V) to defluorinate PFOA under UV irradiation, assisted by dithionite (DTN), a reductive oxygen scavenger. The efficient defluorination rate under mild conditions indicates the promising scalability of this two-pollutant approach for PFAS treatment. Moreover, we proposed an •H-initiated pathway for PFOA defluorination. In conclusion, this Ph.D. project offers novel perspectives, both academically and practically, for addressing PFAS contamination, contributing significantly to the ultimate goal of achieving zero fluoro-pollution.

M3 - Ph.D. thesis

BT - Complete Defluorination of PFAS via An Accumulation-and-Destruction Approach

PB - Aarhus University - Department of Biological and Chemical Engineering

ER -