Photo-reductive decomposition of perfluorooctane sulfonate (PFOS) and its common alternatives by UV/VUV/sulfite process: Mechanism, kinetic modeling, and water matrix effects

The present study investigated the photo-reduction of perfluorooctane sulfonate (PFOS) and its alternatives, focusing on decomposition mechanisms, active species involvement, the influence of background water constituents, and kinetic model development. The decomposition and defluorination rates followed the order of PFOS > PFHxS > 6:2 FTSA > PFBS, with shorter chains and CH₂ linkers enhancing the resistance of PFOS alternatives against the attack of hydrated electrons (e_aq^-). Two primary pathways were identified during the photodegradation of PFAS: (i) H/F exchange at CF bonds with the lowest bond dissociation energies (BDEs) and (ii) functional group cleavage followed by short-chain PFCAs formation, with OH playing a crucial role in transforming intermediates. Adding iodide and elevated temperatures demonstrated a synergistic effect on PFBS decomposition and defluorination, with high temperatures promoting functional group cleavage as the preferred defluorination pathway. The study examined the impact of background water constituents in different aqueous environments, from surface waters to wastewater streams and ion-exchange brine concentrates. Chloride exhibited a concentration-based dual impact on the UV/VUV/sulfite process: promotive effects at low dosages (1-10 mM) by acting as a secondary e_aq^- mediator, and adverse effects at high dosages (20-500 mM) due to the scavenging effect of generated chlorine radicals (Cl). High ionic strength adversely affected e_aq^- quantum efficiency. Additionally, bicarbonate and natural organic matter (NOM) had opposing effects on PFOS photo-reduction, primarily through e_aq^- scavenging and pH alteration. Kinetic modeling revealed reaction rate constants of the studied PFAS with e_aq^- ranging from 1.8 × 10⁶ to 1.3 × 10⁹ M^-1 s^-1, corroborating the concentration profiles of active species and highlighting the reductive nature of sulfite-mediated processes.