Early steps in the advanced reduction process of the hydrated electron: lessons learned from transient spectroscopy

Numerous water remediation technologies continue to be developed with the aim to mineralize contaminants with particular emphasis on eliminating perfluoroalkyl substances (PFAS). In this review, we describe recent experimental and theoretical studies pertinent to ultraviolet-advanced reduction processes (UV-ARP) in the context of the initial PFAS reduction and defluorination by hydrated electrons, e_aq^-. Specifically, we highlight approaches using transient absorption spectroscopy that measure the kinetic parameters related to the formation of e_aq^- and PFAS reduction via e_aq^- quenching. These studies provide important information, such as the rate constants corresponding to the quenching of e_aq^- by PFAS, that is crucial in developing a mechanistic framework to describe PFAS degradation by UV-ARP. Additionally, we summarize recent theoretical studies that have calculated the energetics associated with relevant reactions and provided insights regarding the mechanism of reductive defluorination in order to identify prevalent processes and chain- length dependences for electron reactions with contaminants.