Comparison with the oxidation of fluorotelomers. We compared the oxidative transformation patterns of fluorotelomers as the "legacy PFAS precursors". For the extensively studied C6F13-CH2CH2-SO3 -(6:2 FTSA), the degradation and defluorination by SO4 -• were much more difficult than by HO• (Table , zone E versus F). When HO• was used for the heated oxidation, all shorter-chain PFCAs were generated, with the n-2 PFCA as the most abundant product (e.g., C4F9-COO -from C6F13-CH2CH2-SO3 -). The defluorination, PFCA product distribution, and F atom balance (90-98%) were rather consistent at various S2O8 2-doses. In contrast, when SO4 -• was used, short-chain PFCAs were negligible, and the defluorination appeared to have a ~10% limit despite more 6:2 FTSA degraded at higher S2O8 doses. An increasingly lower F atom balance (from 74 to 31%) was also observed, suggesting unknown oxidative pathways and mechanisms.