Enhanced PFHxS degradation by DBD combined with microbubbles and sulfite: Synergistic effects and mechanisms

Perfluorohexane sulfonate (PFHxS), a persistent member of the PFAS family, remains challenging to degrade, and its transformation mechanisms are not yet fully understood. Here, a dielectric barrier discharge coupled with microbubbles and sulfite (DBD/MBs/sulfite) system was developed for PFHxS degradation. Among all tested systems, the ternary process achieved the best performance, reaching 96.7% degradation and 26.3% defluorination within 60 min, with rate constant of 0.049 min^-1. The enhancement mainly arose from the coupling of MB-enhanced gas-liquid interfacial transport and sulfite-mediated reactive-species conversion under DBD conditions. Higher discharge power and sulfite concentration enhanced PFHxS degradation, whereas higher initial PFHxS concentration reduced removal efficiency but increased energy yield. Reactive-species identification by optical emission spectroscopy (OES), electron spin resonance (ESR), and scavenging experiments showed that PFHxS degradation proceeded through a complex oxidative-reductive network involving SO₄^-•, •OH, e_aq^-, ¹O₂, •O₂^-, and nitrogen-related oxidizing species, among which SO₄^-• and •OH were the dominant radicals. Electronic-structure analysis indicated that PFHxS reactivity was highly localized at the sulfonate end, especially around O14, O16, and S15. Combined with LC-MS identification of intermediates, the pathways involved desulfonation, H/F exchange, hydroxylation, sequential -CF₂ elimination, and C-C bond cleavage, leading to shorter-chain and structurally simplified products. Acidic conditions favored PFHxS degradation, whereas coexisting anions exerted inhibition. Nevertheless, the system still achieved 77.8% degradation in the most inhibitory realistic water matrix, demonstrating good matrix tolerance. Overall, the DBD/MBs/sulfite system provided an efficient and adaptable strategy for PFHxS removal and offered mechanistic insight into plasma-assisted degradation of perfluorinated sulfonates.