Deep Insight of the Mechanism for Nitrate-Promoted PFASs Defluorination in UV/Sulfite ARP: Activation of the Decarboxylation–Hydroxylation–Elimination–Hydrolysis Degradation Pathway

The UV/sulfite advanced reduction process (ARP) holds promise for the removal of per- and polyfluoroalkyl substances (PFASs) by a hydrated electron (e_aq^-)-induced H/F exchange process under anoxic conditions. Traditionally, the presence of coexisting nitrate in water has always been regarded as a major inhibitory factor for PFASs defluorination. However, this study observed an unexpected promotive effect of nitrate on defluorination, challenging the previous phenomenon. Notably, the addition of 100 μM nitrate resulted in a remarkable 54% enhancement in PFOA defluorination. A novel mechanism was discovered that nitrate-derived reactive nitrogen species (RNS) activated the decarboxylation-hydroxylation-elimination-hydrolysis (DHEH) process, an important degradation pathway for PFASs in UV/sulfite ARP. Induced by e_aq^-, the PFAS molecule first became a perfluorinated radical and then was transformed into unstable perfluorinated alcohol by reacting with water. Due to the high reactivity driven by unpaired electrons of RNS, water molecules were destabilized with the H-O bond stretched from 0.98 to 1.04 Å. This effectively enhanced the spontaneity of the reaction between perfluorinated radical and water molecules and consequently made the whole DHEH process more thermodynamic favorable (ΔG, -23.53 → -376.28 kJ/mol). Such a process breaks through the view that the nitrate directly reacts with e_aq^- to affect PFASs defluorination in ARP systems. This finding offers an innovative perspective for optimizing PFAS defluorination by strategically regulating nitrate levels in water bodies.