Elsevier

Journal of Hazardous Materials

Volume 340, 15 October 2017, Pages 336-343
Journal of Hazardous Materials

Reductive defluorination of perfluorooctanoic acid by titanium(III) citrate with vitamin B12 and copper nanoparticles

https://doi.org/10.1016/j.jhazmat.2017.06.020Get rights and content

Highlights

  • Reductive defluorination of PFOA using titanium(III) citrate with vitamin B12 and Cu0 was investigated.

  • The combination of B12 and Cu0 generate extraordinary reactive radicals for PFOA removal.

  • The performance of reductants for PFOA removal was titanium(III) citrate  nano zero-valent Iron > NaBH4.

  • Mechanisms of PFOA hydrodefluorination through sequential electron transfer pathways were proposed.

Abstract

Perfluorooctanoic acid (PFOA) is widespread in the environment, which causes serious health and safety concerns. A mechanistic study on reductive defluorination of PFOA by titanium(III) citrate in the presence of catalysts was conducted. Vitamin B12 was used to catalyze reduction reactions by shuttling electrons from a reducing agent (electron donor) to PFOA to produce a Co-carbon bond intermediates. In the presence of copper nanoparticles, a precursor complex, B12-C7F14COOH, adsorbed on the metal surface, followed by a hydrogenolytic reaction to form less-fluorinated products. The synergistic effect between vitamin B12 and copper nanoparticles enhances the reductive activities by electron-transfer reactions and hydrogenolysis. The efficient reduction of PFOA to less-noxious compounds was demonstrated with a copper dose of 2 g L−1, titanium(III) citrate (45 mM), and vitamin B12 (0.2 mM) with an initial pH of 9.0 and 70 °C. In this anoxic aqueous solution, the biomimetic reductive system effectively removed 65% of PFOA. The mass balance on fluoride matched the observed degradation of PFOA, while no short-chain intermediates were detected.

Introduction

Perfluorinated alkyl acids (PFAAs) are considered as emerging persistent organic pollutants [1]. Several long-chain PFAAs, particularly perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS), have been exploited commercially since1950s due to their unique high surface-active effect as well as thermal and chemical stability [2].

As the uses of PFOA increase, it is now present in the environment globally [3], [4], [5]. The perfluorinated chain is responsible for PFOA’s stability, persistence, bioaccumulative characteristics and its adverse impacts on human health and ecosystem [6], [7], [8]. Its presence in the environment has drawn considerable attention from the public and regulatory agencies [9], [10] and the consensus is to suppress its spread and accumulation in the environment.

Current research approaches for PFAAs removal/degradation, such as biological degradation [11] and chemical oxidation and reduction [12], are ineffective to destruct PFAAs under ambient conditions due to their good stability. Several oxidative methods, including direct photolysis [13], photocatalytic oxidation [14], [15], thermal decomposition [16], [17], [18], and electrolysis [19], [20], have been employed to remove PFAAs from water and wastewater, especially for PFOS and PFOA. Most of treatment methods have technical and/or economic constraints, mainly due to high energy-consumption and severe reaction conditions. In addition, although these methods might be able to degrade PFOA, their defluorination efficiencies are relatively lower. One plausible reason for low defluorination is that the carbon chain of PFOA twists into a zigzag shape in the form of a helix due to the full fluorine substitution. Consequently, the electronegative fluorine substituents completely envelop the carbon skeleton and shield the carbon-fluorine bonds from chemical attacks from oxidizing agents. Therefore, energy-intensive and costly treatment methods may be needed for PFOA degradation. The main objective of this study was to develop a cost-effective, eco-friendly and practical process for POA degradation.

Strong reductants are capable of completely decomposing perfluorinated compounds via fluoride elimination [21], in which the strong electronegativity of fluorine atom(s) may act as the reductive reaction center(s) for defluorination. Since the highly-fluorinated alkanes have polarizability and high bond energies, which increase with increasing substitution by fluorine, the cleavage of C–F bond is the critical step for PFOA degradation. A reduction process could be a viable alternative for PFOA degradation.

In the quest for PFOA removal, microbial reductive defluorination has also been considered [22]. Reductive dehalogenation is a known mechanism contributing to biodegradation of certain significant pollutants including highly halogenated compounds [23].

On the other hand, a chemical method has been developed and reported which catalyzed reductive dehalogenation by electron transfer mediators under anoxic conditions [22]. That approach utilized the highly-fluorinated chemicals as electron acceptors, and the catalytic cycle started when a bulk electron donor rapidly reduced the electron transfer mediator and transfered the electron to the target pollutants. In addition, the catalytic degradation of halogenated hydrocarbons by using transition metals has received considerable attention [24]. These findings prompted us to investigate uses of a corrinoid cofactor, vitamin B12 (VB12) [25], [26], [27], [28], and copper nanoparticles [29], [30], [31] as catalysts in vitro with titanium(III) citrate (TC) as a reducing agent in anoxic aqueous solutions for PFOA degradation.

Ti(III) ion is commercially available and a mild reducing agent at low pHs. The reducing power of a Ti(III)/Ti(IV) system increases significantly in strongly basic media [32], [33]. It could reductively remove halogen atoms from both sp2 and sp3 hybridized organic halids [34]. In this research, a feasibility study on efficient PFOA degradation using an enhanced Ti(III) ion reductive system was conducted. Its effectiveness and the associated effects of the catalyst were evaluated. Additionally, PFOA decomposition by the TC/VB12/Cu system was compared to those by the NaBH4/VB12/Cu, the Fe/VB12/Cu, and the persulfate treatment systems under similar conditions. The mechanisms responsible for PFOA degradation were proposed and discussed using the results of vitamin B12 color variations and a mass balance analysis.

Section snippets

Chemical reagents

All experimental chemicals used in this study were of reagent grade. Water used was purified water from Milli-Q systems. Titanium(III) chloride (15% solution in HCl), vitaminB12 (99.0% purity), and PFOA(C7F15COOH (C8), 96% purity) were obtained from Sigma-Aldrich,USA.

In all the high-performance liquid chromatography(HPLC) and ion chromatography (IC) analysis, authentic standard compounds were used for product identification and quantification. For the HPLC analysis, HPLC-grade acetonitrile (CH3

Selection of reductant for PFOA degradation

Because of the high C-F bonding energy (127 kcal/mol) and the low reduction potentials (E  1.1 V) [37], PFOA is extremely difficult to be defluorinated. One of the tasks of this study was to compare three commonly-used reductants for defluorination; they are titanium(III) citrate, sodium borohydride (NaBH4) and nano zero-valent Iron (nZVI). Another agenda was to investigate the roles of catalysts in reductive defluorination of PFOA.

NaBH4 has been used as a reducing agent due to its high reducing

Conclusions

The effective reduction of PFOA to less-noxious compounds was demonstrated with a copper dose of 2 g L−1, titanium(III) citrate (45 mM), and vitamin B12 (0.2 mM) at an initial pH of 9.0 and 70 °C. In this anoxic aqueous solution, the biomimetic reduction system effectively removed 65% of PFOA. The mass balance on fluoride ions matched the observed degradation of PFOA, while no short-chain intermediates were detected. This study developed a viable alternative for reductive defluorination of PFOA by

Acknowledgment

The authors acknowledge the financial support from the Ministry of Science and Technology (MOST) of Taiwan (MOST 100-2221-E-002-043-MY3 and MOST104-2621-M-002-027).

References (51)

  • A. Clerici et al.

    Reductive reactions of substituted pyridines by aqueous titanium trichloride

    Tetrahedron

    (1982)
  • C.C. Huang et al.

    Synergistic effect of zero–valent copper nanoparticles on dichloromethane degradation by vitamin B12 under reducing condition

    Chem. Eng. J.

    (2013)
  • L.C. Seefeldt et al.

    A continuous, spectrophotometric activity assay for nitrogenase using the reductant titanium(III) citrate

    Anal. Biochem.

    (1994)
  • J. Paradies

    Photogeneration of titanium(III) from titanium(IV) citrate in aqueous solution

    J. Inorg. Biochem.

    (2006)
  • Z. Song

    Reductive defluorination of perfluorooctanoic acid by hydrated electrons in a sulfite–mediated UV photochemical system

    J. Hazard. Mater.

    (2013)
  • Y. Qu

    Photo–reductive defluorination of perfluorooctanoic acid in water

    Water Res.

    (2010)
  • F.P. Van der Zee et al.

    Impact and application of electron shuttles on the redox (bio)transformation of contaminants: a review

    Biotechnol. Adv.

    (2009)
  • D.A. Harrington et al.

    ac Impedance of faradaic reactions involving electrosorbed intermediates—I. Kinetic theory

    Electrochim. Acta

    (1987)
  • UNEP, Report of the persistent organic pollutants review committee on the work of its third meeting, POPRC.3, Add.5,...
  • X.Y. Li

    Efficient photocatalytic decomposition of perfluorooctanoic acid by indium oxide and its mechanism

    Environ. Sci. Technol.

    (2012)
  • J.W. Martin

    Identification of long–chain perfluorinated acids in biota from the canadian arctic

    Environ. Sci. Technol.

    (2004)
  • C.J. Young

    Perfluorinated acids in arctic snow: new evidence for atmospheric formation

    Environ. Sci. Technol.

    (2007)
  • K. Kannan

    Perfluorooctanesulfonate and related fluorinated hydrocarbons in mink and river otters from the united states

    Environ. Sci. Technol.

    (2002)
  • K. Steenland et al.

    Epidemiologic evidence on the health effects of perfluorooctanoic acid (PFOA)

    Environ. Health Perspect.

    (2010)
  • F. Oliaei

    PFOS and PFC releases and associated pollution from a PFC production plant in Minnesota (USA)

    Environ. Sci. Pollut. Res. Int.

    (2013)
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