Photochemical Oxidation Or Reduction Degradation Of Perfluorooctanoic Acid Using UV Irradiation

Perfluorooctanoic acid (PFOA) is widely used as fire retardants, surfactants, emulsion, carpet cleaners, fluoroelastomers, and packaging materials due to its unique high stability, high surface-active effect, hydrophobic and oleophobic properties. As the use of PFOA has increased, it has been detected in the environment across the globe. In recent years, the stability, persistence and bioaccumulative properties of PFOA make it have an adverse effect on human health and ecosystem, and hence received much attention. It is thus crucial to evolve effective methods for the degradation of PFO A. This compound shows high chemical stabilization and has no known natural decomposition processes. It is not degradable by conventional treatment methods such as O3, O3/UV, O3/H2O2, and H2O2/Fe2+. Moreover, even partial degradation can be obtained when PFOA processed under high temperature and high pressure conditions. Compared with obove mentioned methods, some good progress has been made on photochemical degradation of PFOA using UV irradiation. This method degrades PFOA effectively, mainly due to the generation of strong oxidizing or reducing active species through reaction medium activated by UV irradiation. Besides, UV itself also can activate PFOA and then strengthen the degradation effect of PFOA. Hence, the key point of using UV photochemical degradation of PFOA is to exploit the reaction medium. It should produce enough active species which show the effectiveness for PFOA degradation. However, disadvantages of harsh reaction conditions needing when using persulfate (S2O82-) as reaction medium hinder its widely use and the serious lack of effective reaction medium for photocatalytic oxidation and photochemical reduction degradation of PFOA. Therefore, the goals of the present work are to develop the UV-assisted Fe2+ions for the activation of S2Og2-and use bismuth oxychliride (BiOC1) and sulfite (SO32-) as photocatalyst and reductant, respectively. After that, highly efficient oxidation and reduction systems were constructed for remediation of PFOA contaminated water. Related degradation mechanism and process were also investigated. The major contents and results are described as follows:(1) The S2O82-activation results in generation of strong oxidizing sulfate radical anion (SO4·-) which can degrade PFOA effectively. Compared with the single way, the UV-assisted Fe2+ions as activation way can enhance the formation of SO4·-and then improve the PFOA degradation under mild conditions. Under the conditions of pH5.0,1.0mmol L-1Fe2+and30.0mmol L-1S2O82-, it was found that the Fe2+/S2O82-,and UV/S2O82-processes cause defluorination ratios of1.6%and23.2%for20.0μmol L-1PFOA within5h, respectively, but a combined system of UV/Fe2+/S2Og2-dramatically promoted the defluorination ratio up to63.3%. The beneficial synergistic behavior between Fe2+/S2O82-and UV/S2O82-was demonstrated to be dependent on Fe2+dosage, S2O82-initial concentration, and solution pH. The decomposition of PFOA resulted in generation of shorter-chain perfluorinated carboxylic acids (PFCAs), formaic acid and fluoride ions; the generated PFCAs intermediates could be further defluorinated by adding supplementary Fe2+, S2O82-and re-adjusting solution pH in later reaction period. The much enhanced PFOA defluorination in the UV/Fe2+/S2O82-system was attributed to the fact that the simultaneous employment of UV light and Fe2+not only greatly enhanced the activation of S2O82-to form SO4·-, but also provided an additional decarboxylation pathway caused by electron transfer from PFOA to in situ generated Fe3+.(2) TiO2photocatalysis as an effective method to eliminate the adverse effect of various organic contaminates was ineffective for PFOA degradation. This is mainly due to that PFOA contains no hydrogen atoms available for abstraction by hydroxyl radicals (·OH) generated in the TiO2photocatalysis process. When PFOA degradation conducted in a highly acidic medium, it can be degraded effectively through the photogenerated holes (hvB+direct oxidation. However, such extreme conditions limited its practical application. The tight coordination between the electron donor and the catalyst is an essential requisite for direct hole oxidation during the photocatalytic process. Because the low bond energy and long bond length of the Bi-O bond, the exposed oxygen atoms on the surface of BiOC1can escape from lattice to form vacancies which binds PFOA tightly by inserting the O atom of-COOH into the position of vacancy and thus facilitate the hole direct oxidization degradation of PFOA. Besides, the layer structure of BiOC1can separate the hole-electron pair efficiently. Therefore, an photocatalytic oxidation method using BiOC1as photocatalyst for PFOA degradation was established. The BiOC1nanosheets was prepared by a simple hydrolysis method and then characterized by X-Ray diffraction (XRD), scanning electron microscopy (SEM), UV-vis diffuse reflectance spectra (UV-vis DRS) and nitrogen adsorption-desorption isotherms. Under the conditions of pH4.8and catalyst load0.5g L-1, the defluorination efficiency of PFOA on P25and In2O3after24h irradiation was8.9%and31.7%, respectively. However, when using BiOC1as a catalyst,59.3%defluorination of PFOA was obtained within24h. By using HPLC/MS, six shorter-chain PFCAs were detected after irradiation. These compounds can further be degraded by increasing the concentration of oxygen vacancy of BiOC1. Catalytic mechanism of BiOC1through oxygen vacancy-mediated hvB+direct oxidation was confirmed based on the diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and X-Ray photoelectron spectra (XPS) analysis results.(3) Due to its strong electronegativity, the fluorine atom has high electron withdrawing capability. As a consequence, reductive defluorination of PFOA is more feasible than oxidative defluorination. Because of its high reduction potential, some reductive remediation methods such as using zero-valent iron (Fe0) and photogenerated electrons (eCB-) show low effectiveness for PFOA defluorination. Alternatively, the reductive reaction of PFOA with hydrated electrons (eaq-) as a powerful reducing agent (-2.9V) could be easily carried out. However, the existing methods suffer from some problems such as the limited applications of laser flush photolysis in the practical treatment of massive PFO A and the potential insalubrious effect of iodides on humans. Thus, a more economical and eco-friendly reductive processes for the PFOA defluorination by using UV/SO32-was established. Under the conditions of pH10.3and10.0mmol L-1SO32-,this process led to a PFOA removal of98.9%at about1.5h and a defluorination ratio of88.5%at reaction time of24h under N2atmosphere. It was confirmed that the reductive defluorination of PFOA was achieved by eaq-being generated from the photo-conversion of SO32-as a mediator using NO3-and NO2-as radical caption agents. Accompanying the reduction of PFOA, a small amount of short-chain perfluorocarboxylic acids, and less fluorinated carboxylic acids were generated, all of which were able to be further degraded with further releasing of fluoride ions. Based on the generation, accumulation and distribution of intermediates, eaq-induced defluorination pathway of PFOA was proposed in a SO32--mediated UV photochemical system.