Photochemical Decomposition Of Perfluorooctane Sulfonate And Its Substitute In Water

Perfluorinated compounds (PFCs) are one kind of emerging persistent organicpollutants and have received increasing attention due to their global occurrence,accumulation and toxicity. PFCs are resistant to conventional biological treatmentmethods and advanced oxidation processes due to the strong electronegativity offluorine and the strength of the C-F bond. Some researchers reported that PFCs can bedecomposed under extreme conditions. This study found that the representativePFCs-perfluorooctane sulfonate (PFOS) and its substitute (6:2fluorotelomer sulfonate，6:2FTS) can be effectively photodecomposed by the UV/Fe3+and VUV/alkalineprocesses. Besides, the effects of reaction parameters were investigated and thedecomposition mechanisms were proposed.Under the irradiation of254nm UV light, it was found that thephotodecomposition of PFOS was greatly accelerated by addition of trace ferric ions. Inthe presence of ferric ions (100μM), PFOS (20μM) decreased to below the detectionlimit within48h, with the first-order reaction rate constant of1.67d1, which is about50times higher than that by the direct photolysis. The effects of the ferric ionconcentration, the solution pH and the UV wavelength on the PFOSphotodecomposition were investigated. In addition, the photochemical decomposition ofPFOS in the secondary effluent taken from a municipal wastewater plant wasinvestigated. The coexisting organic matters in the secondary effluent reduced the PFOSdecomposition rate to less than1/3of that in pure water under254nm UV irradiation.However,185nm VUV light could quickly degrade the coexisting organic matters, andaccordingly the adverse impacts of coexisting organic matters could be largelyeliminated.During the ferric ion mediated photodecomposition of PFOS under UV irradiation,C2C8perfluorocarboxylic acids (PFCAs) were identified as the main intermediates.The results of UV-vis absorption and UPLC-MS/MS indicated that PFOS formed acomplex with ferric ions. The ESR detection indicated that the electronic state ofFe3+-PFOS complex changed during reaction. Furthermore, the role of oxygen andhydroxyl radical on the defluorination of PFOS was investigated. Accordingly the mechanism for PFOS photodecomposition in the presence of ferric ion was proposed.6:2FTS, a kind of substitute for PFOS, was decomposed under UV light in thepresence of ferric ion. The optimal ferric ion concentration was100μM and pH valuewas3.0. The addition of oxalic acid and hydrogen peroxide both enhanced thedecomposition of6:2FTS. However, both of them have no significant impacts on thedefluorination of PFOS. The C2-C7perfluorocarboxylic acids were identified as theintermediates of6:2FTS by UPLC-MS/MS. Accordingly, the mechanism for ferric ionmediated photodecomposition of6:2FTS under UV irradiation was proposed.Under the irradiation of185nm VUV light, PFOS in an anoxic alkaline solutioncould be efficiently decomposed and defluorinated. However, PFOS isomers showedmuch different reactivity. According to the19F NMR spectroscopy, the technical PFOSsamples used in this study contained34.3%branched isomers and65.7%linear PFOS.The branched isomers in the technical PFOS samples decomposed much faster with thefirst-order reaction rate constant of0.0806min-1at pH12.5, while the linear PFOSdecomposed more slowly with the rate constant of0.0175h-1when the initial technicalPFOS concentration was20μM. The PFOS decomposition is believed to be initiated bythe aquated electrons (e aq) formed during the photolysis of water under185nm VUVlight.