The overall objective of this proof-of-concept project is to confirm the effectiveness and investigate the underlying mechanisms of a novel ultraviolet (UV) light source (222 nm-emitting KrCl* excimer lamps) for hydrated electron-mediated reductive defluorination of per- and polyfluoroalkyl substances (PFAS) in different water matrices. This objective is guided by the hypothesis that, compared to conventional 254 nm lamps, 222 nm irradiation will generate greater concentrations of aqueous electrons due to enhanced light absorption by common electron sources such as sulfite and iodide at 222 nm versus 254 nm. In addition, it is hypothesized that direct photolysis of selected PFAS structures (perfluorocarboxylic acids) by 222 nm will aid in PFAS destruction. Furthermore, it is hypothesized that 222 nm irradiation can be employed to treat multiple PFAS-impacted media, ranging from aqueous film-forming foams (AFFF) to ion exchange still bottoms.

In-depth characterization of the fundamental photochemical kinetic parameters of the system, such as aqueous electron formation rate, concentration, and scavenging capacity, will further be used to guide the development of a combined treatment train approach for PFAS destruction in ion exchange still bottoms. Overall, it is anticipated that the energy demand for PFAS defluorination using UV-reductive systems will be significantly reduced by using 222 nm irradiation compared to 254 nm irradiation. The preliminary data show that the defluorination of C4 and C8 perfluoroalkyl acids is enhanced by three-fold under 222 nm irradiation compared to 254 nm irradiation at the same fluence, which describes the amount of inputted photons (i.e., energy). These preliminary results suggest that the approach promises to be a more cost-effective remediation alternative for matrices impacted by PFAS.

The overall project objective of evaluating KrCl* 222 nm irradiation for PFAS destruction will be achieved by focusing on four technical, experimentally driven tasks and one task focused on energy use analysis. In Task 1, the project team will optimize PFAS defluorination rates under 222 nm irradiation for a diverse array of PFAS structures. In Task 2, the project team will apply these optimized conditions to AFFF to facilitate a comparison of defluorination and energy cost compared to 254 nm irradiation. In Task 3, the team will optimize pre-treatment modules designed to mitigate the impact of water quality constituents that lower PFAS defluorination, such as dissolved organic carbon and nitrate. Task 4 will focus on evaluating this combined treatment train for PFAS defluorination in an ancillary waste stream, most likely ion exchange still bottoms. Finally, Task 5 will focus on energy cost analysis and reporting. Overall, these tasks aimed at (i) verifying the effectiveness and underlying mechanisms of 222 nm irradiation for PFAS defluorination in UV-reductive systems, (ii) investigating the synergistic effect of direct photolysis of diverse PFAS structures by 222 nm light on defluorination efficiencies, and (iii) using fundamental photochemical kinetic models that quantify aqueous electron reaction pathways and concentrations to guide process development.

Successful completion of this project will advance the fundamental science of UV-reductive process for PFAS treatment, providing the basis for incorporating 222 nm light sources into the remediation of PFAS-impacted matrices. Although UV-reductive processes have thus far focused mainly on water with minimal water quality constituents, this project will evaluate a combined treatment train to mitigate these water quality constituents prior to the UV-reductive system. Finally, the scientific results from this project will be grounded in fundamental photochemical models that quantify fluencenormalized aqueous electron exposure. Incorporation of these models in this project and in future work may help explain discrepancies observed between laboratory- and pilot-scale UV-reductive systems and pave the way for future innovations. Successful completion of this proof-of-concept effort will ultimately lead to more cost effective PFAS treatment technologies, directly benefiting the warfighter and installation communities. (Anticipated Project Completion - 2026)