The per- and polyfluoroalkyl substances (PFASs) attractive much attention in view of the persistency, the toxicological profile of certain PFASs, and their potential to accumulate in food chains. Defluorination and mineralization is of great importance for the degradation of PFASs and reduction of (eco-) toxicity. In this research, hypophosphite radical (H2PO2●-), sulfate radical anion (SO4●-) and hydroxyl radical (●OH) were produced simultaneously by in-situ electrochemical method with atomically dispersed silver modified titanium-based manganese oxides electrode (SAg-MnTiOx) as working electrode and titanium plate as counter electrode. Taking perfluorooctanoic acid (PFOA) as model pollutant, the defluorination of PFOA reached 90% within 60 min, accompanied by 84.5% removal of total organic carbon at current density of 40 mA cm-2, electrode spacing of 0.5 cm and geometric area of 15 cm2 L-1. Furthermore, the energy cost at SAg-MnTiOx anode was only 31.2% compared with traditional electrode such as RuOx/IrOx/Ti. The synergistic oxidation and reduction processes were confirmed to be responsible for the efficient defluorination and mineralization by in-situ electron-nuclear double resonance (ENDOR) with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as trapper. And the effluent was linked with time of flight mass spectrometry. Typically, the DMPO●-OH was fitted by g=2.0065, AN=AHβ=1.49 mT. The DMPO●-OSO3H was fitted with g=2.0056, AN=1.38 mT, AH1=1.01 mT, AH2=0.14 mT and AH3=0.073 mT. The adduct of H2PO2●- and DMPO was fitted by AN=1.55 mT, AP=3.15 mT, AH1=2.77 mT, AH2=1.61 mT, AH3=0.23 mT. Combined with the data of mass spectrometry, we speculate that sulfate radical mainly oxidizes PFOA by electron transfer, followed by Kolbe-like decarboxylation. The hypophosphite radical stimulated by hydroxyl radical promoted the defluorination process, especially when the concentration of hypophosphite radical reached 1016 spins L-1. This synergistic oxidation-reduction method provides a strategy for effectively reducing or eliminating the environmental risk of PFASs. The simultaneous utilization of anode and cathode current reduces the waste of electric energy in side reactions such as oxygen evolution and hydrogen evolution.

Synergistic oxidation-reduction of PFAS: Radical initiation mechanism, efficient defluorination and mineralization

ACS Spring Meeting 2021

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