Carbonate radicals as the "Knocker" driving rapid PFOA removal in the VUV/SPC process: Performance and mechanism

Perfluorooctanoic acid (PFOA) is a persistent emerging pollutant with strong ecotoxicity, difficult to eliminate due to the extremely strong C-F bonds in its molecule. Here, a vacuum ultraviolet (VUV) activation of sodium percarbonate (SPC) process was employed to achieve efficient degradation and detoxification of PFOA. 91.9% of PFOA was rapidly removed within 5 min at 1 mM SPC and pH 8.8. The pseudo-first-order rate constant (k_obs) increased with SPC concentration, reaching 0.475 min^-1. Based on electron spin resonance (EPR) and probe method analysis, combined with free radical quenching experiments, the involvement of carbonate radical (·CO₃⁻), superoxide radical (·O₂⁻), and hydroxyl radical (·OH) in PFOA degradation was confirmed. Density Functional Theory (DFT) calculations and liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis indicated that the degradation of PFOA followed a stepwise chain shortening pathway. ·CO₃^- acted as a "knocker" by triggering decarboxylation, thereby enabling subsequent C-F bond cleavage and defluorination, while chain transformation was driven by ·OH, yielding short-chain perfluorocarboxylic acids as the main intermediates. Importantly, the acute bioluminescence inhibition rate of the treated solution decreased from 89.8% to 26.9%. ECOSAR predictions further confirmed that the degradation products pose substantially lower environmental hazard. The presence of humic acid (HA), chloride (Cl^-), and nitrate (NO₃⁻) inhibited PFOA elimination to varying extents. HA exhibited the strongest suppression (k_obs decreased to 0.01527 min^-1), followed by Cl⁻ (k_obs = 0.02578 min^-1) and NO₃⁻ (k_obs = 0.01682 min^-1). Dissolved oxygen was crucial for sustaining radical chain reactions. These findings provide an innovative treatment process for the rapid removal of PFOA and defluorination from water.