Environ. Sci. Technol. 2006, 40, 1049-1054
Efficient Decomposition of Environmentally Persistent Perfluorooctanesulfonate and Related Fluorochemicals Using Zerovalent Iron in Subcritical Water HISAO HORI,* YUMIKO NAGAOKA, ARI YAMAMOTO, TAIZO SANO, NOBUYOSHI YAMASHITA, SACHI TANIYASU, AND SHUZO KUTSUNA National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba West, 16-1 Onogawa, Tsukuba 305-8569, Japan ISSEY OSAKA AND RYUICHI ARAKAWA Department of Applied Chemistry, Faculty of Engineering, Kansai University, 3-3-35 Yamate-cho, Suita 564-8680, Japan
Decomposition of perfluorooctanesulfonate (PFOS) and related chemicals in subcritical water was investigated. Although PFOS demonstrated little reactivity in pure subcritical water, addition of zerovalent metals to the reaction system enhanced the PFOS decomposition to form Fions, with an increasing order of activity of no metal ≈ Al < Cu < Zn , Fe. Use of iron led to the most efficient PFOS decomposition: When iron powder was added to an aqueous solution of PFOS (93-372 µM) and the mixture was heated at 350 °C for 6 h, PFOS concentration in the reaction solution fell below 2.2 µM (detection limit of HPLC with conductometric detection), with formation of Fions with yields [i.e., (moles of F- formed)/(moles of fluorine content in initial PFOS) × 100] of 46.2-51.4% and without any formation of perfluorocarboxylic acids. A small amount of CHF3 was detected in the gas phase with a yield [i.e., (moles of CHF3)/(moles of carbon content in initial PFOS) × 100] of 0.7%, after the reaction of PFOS (372 µM) with iron at 350 °C for 6 h. Spectroscopic measurements indicated that PFOS in water markedly adsorbed on the iron surface even at room temperature, and the adsorbed fluorinated species on the iron surface decomposed with rising temperature, with prominent release of F- ions to the solution phase above 250 °C. This method was also effective in decomposing other perfluoroalkylsulfonates bearing shorter chain (C2-C6) perfluoroalkyl groups and was successfully applied to the decomposition of PFOS contained in an antireflective coating agent used in semiconductor manufacturing.
Introduction Perfluoroalkylsulfonates and perfluorocarboxylic acids, especially perfluorooctanesulfonate (C8F17SO3-, PFOS) and perfluorooctanoic acid (C7F15COOH, PFOA), have recently received a great deal of attention because they are recognized * Corresponding author phone: +81-29-861-8161; fax: +81-29861-8258; e-mail:
[email protected]. 10.1021/es0517419 CCC: $33.50 Published on Web 01/06/2006
2006 American Chemical Society
as ubiquitous environmental contaminants (1-3). These compounds have been widely used in industry as products or raw materials for surface treatment agents in photolithography, emulsifying agents in polymer synthesis, paper coatings, waxes, fire-fighting foams, and polishes because of their specific characteristics such as a high surface-active effect, high thermal and chemical stability, and high light transparency (1, 2). Their high stability consequently results in environmental persistence. The largest fluorochemical manufacturer ceased most PFOS production in the year 2000 (4). However, it is still a necessity in the semiconductor industry as surface treatment agents (photoacid generator, antireflective coating agent, etc.) in photolithography processes (5). Perfluoroalkylsulfonates bearing a shorter perfluoroalkyl chain than PFOS are being introduced as alternatives. Because PFOS and PFOA and their related chemicals show not only environmental persistence but also bioaccumulation (shorter chain compounds are less bioaccumulative) (6-8), the development of techniques to decompose them to environmentally harmless species at stationary sources is desired. Ideally, the method should involve cleavage of the C-F bonds to form F- ions, because F- ions have a well-established waste-treatment process, based on a reaction with Ca2+ to form environmentally harmless CaF2. We have reported previously that perfluorocarboxylic acids such as PFOA in water were slowly decomposed to Fions and CO2 by direct photolysis (9), and the photochemical decomposition was greatly enhanced by use of a heteropolyacid photocatalyst (9), persulfate (10), and persulfate with aqueous/liquid CO2 biphasic media (especially for aqueous less soluble long-chain compounds) (11). However, perfluoroalkylsulfonates such as PFOS were quite different from perfluorocarboxylic acids in their photochemical character: When an aqueous solution of PFOS (0.37 mM) was irradiated by UV-vis light from a 200 W Hg-Xe lamp (main wavelength region 220-460 nm) for 12 h under an oxygen atmosphere (0.5 MPa), we observed no decomposition. Alternatively, when the same photochemical treatment was applied to PFOA, 36.8% of the initial concentration was decomposed. To our knowledge, there is only one example of the decomposition of PFOS in water, by use of high-power sonochemical action, in which PFOS was transformed into PFOA and shorter chain perfluorocarboxylic acids (12). Reactions in sub- or supercritical water have been recognized recently as an innovative and environmentally benign reaction technique (13), where subcritical water is defined as hot water with sufficient pressure to maintain the liquid state and supercritical water is defined as water at temperatures and pressures higher than the critical point (374 °C, 22.1 MPa). The dechlorination of trichloroethylene, trichloroethane, and poly(chlorobiphenyl) (PCB) in sub- or supercritical water, via an oxidation (14) or reduction (15, 16) process, was investigated, and pilot-plant-scale treatment for hazardous compounds on contaminated soil (4-6 kg) in sub- or supercritical water was also achieved (17). We report herein the reductive decomposition of PFOS and other perfluoroalkylsulfonates in subcritical water. When iron powder was added to the reaction system, these stable fluorinated compounds were efficiently decomposed to form F- ions, with no formation of perfluorocarboxylic acids such as bioaccumulative PFOA. We also applied this method to the decomposition of PFOS contained in an antireflective coating agent used in photolithography processes in semiconductor manufacturing. VOL. 40, NO. 3, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Experimental Section Materials. All perfluoroalkylsulfonates used were obtained in the form of potassium salts. Nonafluorobutanesulfonate (C4F9SO3-; Aldrich, Milwaukee, WI) and heptadecafluorooctanesulfonate (perfluorooctanesulfonate, PFOS; Fluka, Buchs, Switzerland) were purchased from commercial sources and were used as received. Pentafluoroethanesulfonate (C2F5SO3-), heptafluoropropanesulfonate (C3F7SO3-), and tridecafluorohexanesulfonate (perfluorohexanesulfonate, C6F13SO3-) were supplied by JEMCO Inc. (Akita, Japan). Fine metal powderssaluminum (>99.99% purity,
