Degradation of PFOS in concentrated saline waste streams using UV/ Sulfite process: Critical impact of aggregation

Ultraviolet/sulfite (UV/S)-based advanced reduction has been considered a promising approach for the degradation of per- and polyfluoroalkyl substances (PFAS). Focusing on the UV/S treatment of perfluorooctane sulfonate (PFOS) in solutions representative of ion exchange (IEX) regeneration waste, this study highlights the critical need to account for and overcome PFOS aggregation when assessing degradation performance. Aggregation, rather than true degradation, can result in an apparent near-complete decrease in bulk PFOS concentration, leading to misleading interpretations. Despite adjustments in operational parameters, PFOS aggregation and subsequent surface adsorption persisted under high-ionic-strength conditions, introducing artifacts that confound accurate evaluation of degradation efficiency. The addition of cetyltrimethylammonium bromide (CTAB) as a secondary surfactant effectively mitigated aggregate formation, preventing misleading concentration artifacts. Molecular simulations revealed that CTAB promotes micellar reorganization of PFOS via two synergistic mechanisms: complex salt-bridging interactions and frontier orbital segment pairing. This restructuring also enhanced hydrated electron delivery to PFOS, enabling nearly complete degradation and up to 87 % defluorination. CTAB addition thus emerged as an effective strategy, promoting accelerated PFOS degradation under relatively mild conditions in saline solutions. The mechanistic insights drawn in this study, validated through complementary experiments and simulations, offer novel perspectives on addressing challenges in PFOS degradation.