Hydrogen-polarized vacuum ultraviolet photolysis system for enhanced destruction of perfluoroalkyl substances

Abstract

Reductive water treatment using hydrated electrons ($e_{\mathrm{aq}}^{-}$) is a promising technology to destruct perfluoroalkyl substances; however, it faces challenges of slow reaction kinetics, undesirable chemical addition, and high energy consumption. Herein, we developed a hydrogen (H₂)-polarized water photolysis system using vacuum UV (VUV) light at 185 nm for reductive destruction of perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS). The 185-nm photons directly photolyzed H₂O and OH^- into HO·, H·, and $e_{\mathrm{aq}}^{-}$. H₂ elevated the quasi steady-state concentration of $e_{\mathrm{aq}}^{-}$ 18 times in untuned VUV systems through eliminating the scavenging effect of dissolved oxygen and converting hydroxyl radicals (HO·/O·^-) into $e_{\mathrm{aq}}^{-}$. The polarization effect of H₂ increased the degradation of PFOA from 10 % to 95 % and the defluorination from 17 % to 94 % and led to 87 % of defluorination for PFOS. The pH impacted VUV photon adsorption between H₂O and OH^- and shifted the equilibrium between H· and $e_{\mathrm{aq}}^{-}$, which led to an optimal pH of 10.3 for PFOA destruction. The presence of chloride and sulfate enhanced the production of $e_{\mathrm{aq}}^{-}$ and promoted PFOA destruction. H₂-polarized VUV water photolysis systems produced high levels of $e_{\mathrm{aq}}^{-}$ from clean water constituents and significantly reduced energy consumption for PFAS treatment under mild alkaline conditions.