Promotive Effects of Chloride and Sulfate on the Near-Complete Destruction of Perfluorocarboxylates (PFCAs) in Brine via Hydrogen-tuned 185-nm UV Photolysis: Mechanisms and Kinetics

Hydrogen-tuned 185 nm vacuum ultraviolet (VUV/H₂) photolysis is an emerging technology to destroy per- and polyfluoroalkyl substance (PFAS) in brine. This study discovered the promotive effects of two major brine anions, i.e., chloride and sulfate in VUV/H₂ photolysis on the hydrated electron (e_aq^-) generation and perfluorocarboxylates (PFCAs) destruction and established a kinetics model to elucidate the promotive effects on the steady-state concentration of e_aq^- ([e_aq^-]_ss). Results showed that VUV/H₂ achieved near-complete defluorination of perfluorooctanoic acid (PFOA) in the presence of up to 1000 mM chloride or sulfate at pH 12. The defluorination rate constant (k_deF) of PFOA peaked with a chloride concentration at 100 mM and with a sulfate concentration at 500 mM. The promotive effects of chloride and sulfate were attributed to an enhanced generation of e_aq^- via their direct VUV photolysis and conversion of additionally generated hydroxyl radical to e_aq^- by H₂, which was supported by a linear correlation between the predicted [e_aq^-]_ss and experimentally observed k_deF. The k_deF value increased from pH 9 to 12, which was attributed to the speciation of the H^·/e_aq^- pair. Furthermore, the VUV system achieved >95% defluorination and ≥99% parent compound degradation of a concentrated PFCAs mixture in a synthetic brine, without generating any toxic perchlorate or chlorate.