Per- and polyfluoroalkyl substances (PFAS) degradation and defluorination involving ultraviolet and ultrasonication activated advanced oxidation and reduction

Abstract

Per- and polyfluoroalkyl substances (PFAS) are an extremely persistent and ubiquitous class of anthropogenic chemicals, used globally in a wide range of industrial and commercial applications. They are toxic to humans and wildlife, and recalcitrant to most of the conventional water treatment methods. PFAS pose a serious challenge to water treatment, considering they are widespread in the environment and difficult to breakdown because of their unique physical and chemical properties, including strong C-F bond structure, hence they are called ‘forever chemicals’. 

Breaking down PFAS into their starting elements, or even naturally degradable products, is extremely difficult with most treatment techniques, thus developing an efficient and cost-effective treatment methodology would be an important breakthrough. 

The objective of this research was to adapt currently available water treatment techniques for micropollutants for improving PFAS breakdown efficiency. We used low frequency ultrasonication (US) in the presence of oxidative and/or reducing agents, and with or without catalysts, to enhance PFAS degradation. 

Two main UV activated defluorination processes exist for PFAS degradation; direct photolysis and indirect radical reactions. UV provides a promising approach for perfluorooctanoic acid (PFOA) degradation and defluorination and is capable of breaking strong C-F bonds at a higher rate. Efficiencies of advanced oxidation (AO) and advanced reduction (AR) indicate that both processes are capable of PFOA degradation and defluorination. However, efficiencies vary based on the activation method used and their operational conditions, as well as other experimental parameters including the type and concentration of oxidative or reducing agents used, pH of the solution and the competition between co-contaminants or scavenging species, and the target contaminants. 

Degradation of PFAS with a sulfonic head group, such as PFOS and PFHxS, did not show similar efficacy as PFOA, which has a carboxylic head group. PFAS physical and chemical properties including head group of the PFAS appear to have a significant influence on the degradation and defluorination mechanism and kinetics. 

Both UV and US could oxidatively degrade PFAS through OH. radical reactions with or without additives. Photon energy from direct UV irradiation activates reducing agents which act as electron donors, generating highly reactive hydrated electrons (e_aq^-) and reducing radicals to initiate reductive defluorination leading to PFAS degradation. However, competition between PFAS and scavenging species reduces defluorination rates. Both oxidative and reductive degradation involves C-C bond scission and head group detachment leading to chain shortening as well as breaking C-F bonds, resulting in defluorination. 

This study revealed that UV mediated AO and AR are viable PFAS destruction techniques, considering their simplicity, ease of use and good efficacy, while US involves high energy use, high cost, and regular maintenance and replacement of ultrasonic transducers due to rapid deterioration. However, combination of ultrasonication and ultraviolet activation together with a suitable sensitiser that is derived from a natural mineral using sustainable approach, is the novelty in this research, and is the next step forward.

Publications

Author
Wanninayake, D. M. (2021, Jan 28). Comparison of currently available PFAS remediation technologies in water: A review. J Environ Manage, 283, 111977. https://doi.org/http://dx.doi.org/10.1016/j.jenvman.2021.111977 

Co-author
Leung, S. C. E., Wanninayake, D., Chen, D., Nguyen, N.-T., & Li., Q. (2023). Physicochemical properties and interactions of  perfluoroalkyl substances (PFAS) - Challenges and opportunities in sensing and remediation. Science of the Total Environment. 
https://doi.org/10.1016/j.scitotenv.2023.166764

Authors

Dushanthi Wanninayake^{1, 2*}, David Hamilton³, Jimmy Yu¹, Andrew Busch¹

1 School of Engineering and Built Environment, Griffith University, Brisbane, QLD 4111, Australia
2 Queensland Micro and Nanotechnology Centre, Griffith University, Brisbane, QLD 4111, Australia
3 Australian Rivers Institute, Griffith University, Brisbane, QLD 4111, Australia4.    FUTURE by Nature, Melbourne, Victoria
*  Corresponding author: dushanthi.wanninayake@griffithuni.edu.au