For the degradation of individual PFAS structures, EO has an overwhelming advantage over UV/S. The strongly oxidative environment rapidly destroyed n=4 FTS and achieved >95% defluorination. But this compound is highly recalcitrant under UV/S because the short C4F9- moiety segregated by the -CH2CH2-linker does not have a weak C-F bond for easy defluorination by eaq -. This experimental finding corroborates the insights from the DFT calculation discussed in the earlier section. Moreover, in comparison to UV/S, EO achieved much faster (10 h versus >24 h) and deeper (~100% versus 78%) defluorination of PFBS. Hence, EO has higher tolerance with short-chain PFAS than two other heterogeneous technologies-plasma and sonication-both encountered challenges from PFBS (C4F9-SO3 -) and PFBA (C3F7-COO -). Based on the above experimental findings, we hypothesized that EO could be placed after UV/S to obtain the best treatment result for multiple reasons. To validate the hypotheses above, we developed a primitive UV/S-EO layout to treat PFOS, PFOA, and PFBA, all of which are representative PFAS and could not be 100% defluorinated by EO (Fig. ). In the first stage of UV/S treatment, PFOA and PFBA were completely removed within 30 min (Fig. ). After the parent PFCAs quickly disappeared, the defluorination from the transformation products continued. In previous studies, the maximum defluorination from PFCAs under the same UV/S condition took 4-8 h. However, for the UV/S-EO layout, we arbitrarily stopped the UV/S treatment after 2-3 h when the increase of defluorination became sluggish. For the more recalcitrant PFOS under UV/S treatment, the defluorination accompanied the parent compound removal. We stopped UV/S when most parent PFOS disappeared at 5 h (Fig. ). The following EO treatment increased defluorination to 100% for all three PFAS. Transfomration product (TP) analyses verified our mechanistic hypotheses. The UV/S treatment of n=7 PFOA generated a series of shorter-chain n=1-6 PFCAs (Fig. ) as quantified by a triple-quadrupole mass spectrometer (QQQ MS/MS). These PFCAs are attributed to the wellknown decarboxylation and a recently identified C-C bond cleavage mechanism. The UV/S treatment removed most of the PFCA TPs within 3 h. Quadruple time-of-flight high-resolution mass spectrometer (Q-ToF-HRMS) found a series of hydrodelfuorination products (Fig. ) from the parent PFOA (C8F15O2 -) and the chain-shortened PFHpA (C7F13O2 -). The MS peaks for C8HF14O2 -and PFHpA showed similar abundance, indicating that the two transformation pathways proceeded in parallel and were equally significant (Fig. ). The UV/S degradation of hydrofluorinated TPs, such as C8HF14O2 -and C8H2F13O2 -, were much slower than the perfluorinated PFOA (Fig. versus Fig. ). The detection of deeply hydrodefluorinated TPs (e.g., C8H12F3O2 -and C7H10F3O2 -) is consistent with the previous study using a different photoreactor setting and a quadrupole Orbitrap HRMS instrument. The three residual C-F bonds with high recalcitrance against UV/S were most probably on the terminal CF3-. The switch to EO mode generated short-chain PFCAs again (Fig. ) from various hydrodefluorinated TPs. The sharp increase of TFA suggested that hydrodefluorination by UV/S occurred on carbon atoms near the terminal CF3-. With the extension of EO treatment, all PFCA TPs (Fig. ) and hydrodefluorinated TPs (Fig. ) were destroyed to negligible concentrations, as evidenced by the defluorination to ~100% (Fig. ). The near-quantitative defluorination of individual PFAS structures motivated us to apply UV/S-EO for AFFF treatment at ambient conditions. For fire suppression, the original AFFF liquid was typically diluted about 100-fold. It was further diluted after entering the water environment. To date, only a few studies have reported treating diluted AFFF (total fluorine 0.16-27 mg L -1 ) by individual EO, UV/S, and plasma technologies. None of these nonthermal methods realized ~100% defluorination (Table ).