Photochemical Defluorination Of Aqueous Perfluorooctanoic Acid（PFOA） By VUV/Fe³⁺ System

Perfluorinated compounds have been widely used in industry such as surfactant foam and fire extinguishing agent because of their unique physical and chemical properties. perfluorooctanoic acid(PFOA) and perfluorooctane sulfonate(PFOS), have continually been found in the aquatic environment, animals and humans in the past few years. PFCs have stable attribute and are very difficult to degrade in nature. PFCs are environmental persistent and bioaccumulated, as well as toxic to biosome. Hence, they have become the focus of environmental research in recent years.In this study, a combined process of vacuum ultraviolet and trivalent iron ions was firstly used for defluorination of aqueous PFOA. The effects of initial PFOA concentration, Fe3+ concentration, and p H on the defluorination were explored. The optimizing strategy of influencing factors was determined using response surface method(RSM). The intermediate products were identified and quantified by LC-MS/MS. The mechanism of PFOA defluorination was demonstrate base on the analysis of experimental result and quantum chemistry. In the end, the impacts of inorganic anion and organic matter in water environment on PFOA photodecomposition were investigated.The results showed that PFOA could be efficiently defluorinated by VUV/Fe3+, with the defluoroization rate of 51.21% in 4 hours. Compared with VUV system, the defluoroization efficiency and its kinetics constant of PFOA increased by 1 and 1.6 times respectively. The defluorination of PFOA was significantly enhanced under the PFOA/Fe3+ molar ratio of 1.8:1~1.2:1 and p H 3~4.Response surface analysis showed that the fitting degree of regression model was available. p H was the most important factor for the PFOA defluorination followed by Fe3+ concentration and initial PFOA concentration.The synergetic effect of p H and Fe3+ dosage on defluorination was significant. Up to 54.13% PFOA were defluorinated after 4h radiation under the predicted optimum action condition composites. The predicted values agreed well with the experimental values with only 0.36% deviation.Six shorter-chain perfluorinated carboxylic acids(PFCAs) with C7, C6, C5, C4, C3 and C2 were detected as the main intermediates in VUV/Fe3+. ·OH played an important role in the transformation of Fe2+ and Fe3+. The quantum chemical calculation showed that the decarboxylation taken place first in the defluorination of PFOA and PFCAs. Shorter chain PFCAs asked more energy for their degradation, which postponed the whole mineralization of PFOA. Our results indicated that direct light degradation played a major role in PFOA defluorination under VUV radiation(λ=185 nm), while the defluorination of PFOA was simultaneously photocatalytically decomposed and defluorinated in the presence of ferric ion(λ=254 nm).Inorganic anion hindered the photo mineralization of PFOA following the order of HCO3- > SO42- > NO3- ~ Cl- > Cl O4-. Micro-molecular volatile organic compounds had considerable inhibiting effect on the defluorination of PFOA, while the negative impact of macromolecular volatile organic compounds was negligible. DOM was observed to have insignificant effect on PFOA decomposition as long as the irradiation time was long enough.