Under UV/S treatment of the 100-fold diluted AFFF (total fluorine at 100 mg L -1 ), the concentration of 6:2 FTS increased in the first 8 hours and then slowly decreased (Fig. ). PFSAs such as the C6 PFHxS, although in low concentrations, showed a similar generation-degradation profile (Fig. ). It exhibited higher recalcitrance than that observed in previous studies using pure PFHxS in the deionized water matrix. The slow apparent degradation of these species can be attributed to (1) competing species in the organic matrix of diluted AFFF and (2) the continuous generation of PFHxS from n=6 sulfonamide surfactant precursors. This reasoning is further supported by the rather consistent concentration of PFOS, which has higher reactivity than PFHxS in previous UV/S studies. The sustained PFOS throughout the 24 h is most probably attributed to the conversion of n=8 sulfonamide precursors. PFCAs also showed generation-degradation patterns under UV/S treatment (Fig. ). Because the initial concentrations of all PFCAs were negligible, the generated PFCAs could be attributed to the conversion of fluorotelomeric and sulfonamide precursors. A series of n=4-7 surfactant molecules (detected by Q-ToF-HRMS following literature ) demonstrated high recalcitrance or even net increase (Fig. ). The UV/S module resulted in 40% of overall defluorination after 24 h (Fig. ). Extended reaction beyond 24 h did not further increase defluorination (Fig. ).