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Banayan Esfahani, Ehsan

University of British Columbia

Manufacturing, applications, and disposal of fluorochemicals since the late 1940s have led to global pollution by a class of >9000 chemicals namely per- and poly-fluoroalkyl substances (PFAS), known as forever chemicals. This work focuses on the degradability of different well-known PFAS, role of active species, and transformation mechanisms in photochemical processes, in particular those involving vacuum-UV (VUV) irradiation. Photochemical experiments are conducted with UV and UV/VUV bench-scale collimated beam reactors, facilitating fluence measurement. This work is the first to determine fluence-based decomposition of PFAS and provides insights on the true impact of introducing 185 nm which accelerates evolution of intermediates and boost fluorine recovery. PFOS and its common alternatives (PFHxS, PFBS, and 6:2 FTSA) along with PFOA and its substituents (PFBA and GenX) are selected to represent the vast range of characteristics exhibited by the most common and environmentally relevant PFAS. Substantial differences are observed in degradability and transformation mechanisms of difference PFAS based on their chain length, head-group, and presence of chemical bonds like C-H and C-O bonds. The efficacy and mechanism of different reductive (eaqˉ -based) and oxidative (˙OH and SO₄˙ˉ -based) processes are compared. Cleavage of C-F bond, functional group detachment followed by chain shortening, and center C-C or C-O bond scissions are proposed reductive pathways of PFAS decomposition upon reaction with eaqˉ. On the other hand, hydrogen abstraction and one electron transfer are involved in the attack of ˙OH and SO₄˙ˉ , respectively. Faster transformation of intermediates and in turn a greater extent of defluorination are observed in photo-reductive environments. In addition, impacts of key operational parameters (e.g., mediator type and concentration, solution pH, temperature, and dissolved oxygen), photon wavelength and intensity, PFAS characteristics (e.g., structure and initial concentration), and co-presence of common water solutes (e.g., organic and inorganic matters) are investigated. Further, insights on the application of VUV processes in treatment of real water samples and its integration with physical separation processes are provided. Lastly, but not least, a kinetic modeling is proposed to determine valuable optical parameters (e.g., quantum yields, intrinsic rate constants, and radicals concentrations) during PFAS photodegradation.

Text

2024-07-31

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/83544

Chemical and Biological Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/