Enhanced degradation of perfluorooctanoic acid (PFOA) and its emerging substitutes using VUV/UV/micro-nano bubble process.

The degradation of per- and polyfluoroalkyl substances (PFAS) with high efficiency and low cost is very challenging due to their chemical stability. Their tendency to enrich at the gas-liquid interface renders the vacuum-ultraviolet/ultraviolet/air micro-nano bubbles (i.e., VUV/UV/MNBs) process promising for accelerating their degradation. Compared with VUV/UV alone, VUV/UV/MNBs markedly enhanced both degradation kinetics and defluorination efficiencies of the studied PFAS, with enhancement factors for rate constant (ηₖ) and defluorination (ηdF%) of 7.2-7.7 and 1.9-2.5, respectively. This enhancement was attributed to three factors: (1) the substantially lower VUV absorption coefficient of air MNBs than that of water improved VUV photons penetration, enlarging the irradiated volume and increasing the fraction of VUV photons absorbed by PFAS molecules; (2) the extensive interfacial area of air MNBs exploited the inherent surface activity of PFAS, promoting their spatial redistribution and accumulation at the bubble-liquid interface, where direct VUV photolysis is more effective; and (3) the hydrodynamic behavior of air MNBs enhanced mass transfer and mixing, increasing PFAS transport to the irradiated volume. VUV photons penetration contributed ∼20% of the enhancement for all three PFAS, while spatial redistribution was the dominant pathway, accounting for ≥ 46%. The process remained effective for degradation and defluorination under moderate humic acid levels (20 mg L-1) and high Cl- and SO42- concentrations (100 mM), but was inhibited by strongly light-absorbing species such as NO3-. Overall, the VUV/UV/MNBs process offers a sustainable, chemical-free, and efficient approach for treating PFAS-laden high salinity wastewater.