Maximizing Hydrated Electron Concentration in the Advanced Reduction Process Treatment of Per- and Polyfluoroalkyl Substances Under Varying Source Water Matrices

Abstract

Ultraviolet advanced reduction processes (UV-ARP) have received significant attention in recent years for the treatment of recalcitrant contaminants, most notably per- and polyfluoroalkyl substances (PFAS). The effectiveness of UV-ARP contaminant destruction resides in the generation of a powerful reducing species – the hydrated electron (eaq-). This research investigates the factors that influence the available eaq- concentration ([eaq-]) in solution and subsequent contaminant degradation by first presenting results from an extensive literature review on UV-ARP. One major finding of our literature review was the lack of reported photochemical kinetic parameters, namely [eaq-] and eaq- scavenging capacity of a source water. Using a probe compound, we developed the Re,UV method to quantify these key photochemical kinetic parameters for any source water. We discovered that [eaq-] varied significantly throughout a 24 h treatment of perfluorooctane sulfonate, a PFAS known to be recalcitrant in UV-ARP treatment, due to several key eaq- scavengers present in the background source water matrix. One of those eaq- scavengers, dissolved organic matter, was found to both scavenge eaq- well into treatment (≥4 h) and screen UV photons from sensitizer illumination and subsequent eaq- formation. The Re,UV method proved to be useful tool in evaluating contaminant degradation in UV-ARP.

Furthermore, several recent studies have indicated that the most feasible application of UV-ARP for PFAS treatment would be after concentration with membrane technology or ion exchange. While these technologies do concentrate PFAS, the resulting background water matrix will have concentrated eaq- scavengers. We thus decided to deploy our Re,UV method to optimize [eaq-] and subsequent PFAS destruction in a surface water derived reverse osmosis concentrate. We discovered that ultraviolet advanced oxidation processes followed by UV-ARP led to the destruction of all detected PFAS but perfluorobutanesulfonic acid (PFBS) within 24 h in addition to the destruction of ammonium, nitrate, nitrite, and bromate. Furthermore, we observed the conversion of hard-to-treat fluorotelomers to reducible PFAS that were subsequently defluorinated, resulting in a total 90% defluorination at 24 h. Our results indicate that while PFAS can be destroyed in reverse osmosis concentrate by UV-ARP, additional research is needed prior to full scale application of this technology.