Unveiling a Novel Phosphorus-Mediated Strategy for Perfluorocarboxylate (PFCA) Defluorination via a VUV/Hypophosphite System: Phosphorus Evolution, Pathway, and Mechanisms

The emergence of advanced reduction processes (ARPs) offers a promising strategy for mitigating the environmental risks of per- and polyfluoroalkyl substances (PFAS). However, conventional UV-based ARPs (e.g., UV/sulfite) are often constrained by their narrow pH applicability (typically alkaline), N 2 atmosphere, and high reagent dosages. Here, PFAS defluorination and nonorthophosphate conversion could be achieved via synergistic generation of hydrated electrons (e aq – ) and reactive phosphorus species (RPS) using a vacuum-ultraviolet (VUV)/hypophosphite system. Under the conditions of pH 7.3 with only 0.1 mM reagent and for 24 h without aeration, the defluorination of perfluorooctanoic acid (PFOA) reached 72.3%, accompanied by a 78.78% conversion of hypophosphite (P(I)) to phosphate (P(V)). Quenching and probe experiments confirmed that e aq – generated from the VUV activation of P(I) played a critical role in PFOA defluorination, while the abundant •OH generated reacted with P(I), promoting the complete oxidation of P(I) to P(V). Combined experimental and density functional theory (DFT) studies revealed that P(I) not only served as a source of e aq – but also generated RPS, such as HPO 3 • – and HPO 4 • –, which readily participated in PFAS degradation via direct phosphorylation and H/F exchange-initiated pathways. The lowest unoccupied molecular orbital (LUMO) energy of perfluoroalkyl radicals suggested that they were more prone to undergo subsequent phosphorylation compared to hydrogenated intermediates. This work proposes a novel strategy with broad pH applicability, low reagent dosages, and no aeration for PFAS remediation, offering dual environmental benefits through simultaneous PFAS degradation and non-orthophosphate pollution control.