Exploring the reductive transformation pathways of perfluoroalkyl Sulfonylimides to legacy perfluoroalkyl sulfonates and perfluoroalkyl sulfonamide compounds.

Bis(perfluoroalkylsulfonyl)imides (NH(CnF2n+1O2S)2) have been increasingly adopted as alternatives to legacy PFAS in industrial applications; however, their fate during water treatment remains poorly understood. Herein, we systematically investigated the reductive transformation of three sulfonyl imides (i.e., NH(C4F9O2S)2, NH(C2F5O2S)2, and NH(CF3O2S)2) under a UV/sulfite-based advanced reduction process (ARP). Bis(perfluoroalkylsulfonyl)imides exhibited high resistance to both direct photolysis and oxidative degradation, as evidenced by negligible degradation (<7%) and nearly no defluorination. In contrast, the UV/sulfite system achieved complete removal of bis(nonafluorobutanesulfonyl)imide within 24 h, with defluorination efficiencies of up to 46.4%. Notably, LC-MS and LC-Q-Orbitrap HRMS analyses revealed substantial formation of legacy PFAS, including perfluoroalkyl sulfonamides (CnHF2n+1NO2S-), perfluoroalkyl sulfonates (CnF2n+1SO3-), which accounted for 5.0-10.3% and 12.9-48.5% of total fluorine, respectively. Density functional theory (DFT) calculations corroborated these findings, demonstrating that their secondary formation through reductive cleavage of the sulfonyl imide linkage is both thermodynamically favorable and kinetically accessible. However, short-chain perfluoroalkyl sulfonamides and perfluoroalkyl sulfonates exhibited substantially lower reactivity toward eaq⁻ than their parent compound, thereby limiting overall defluorination. Complete degradation pathways for bis(perfluoroalkylsulfonyl)imides with n = 1, 2, and 4 were proposed based on identified TPs and DFT calculations. This study provides the first evidence that bis(perfluoroalkylsulfonyl)imides serve as precursors to short- and ultrashort-chain sulfonamides and sulfonates during reductive treatment, highlighting the potential for fluorine mass redistribution into persistent legacy PFAS during ARP-based remediation of emerging novel PFASs.