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Efficient Decomposition of Perfluorocarboxylic Acids in Aqueous Suspensions of a TiO2 Photocatalyst with Medium-Pressure Ultraviolet Lamp Irradiation under Atmospheric Pressure Tsuyoshi Ochiai,*,†,‡ Yuichi Iizuka,†,§ Kazuya Nakata,†,‡ Taketoshi Murakami,† Donald A. Tryk,^ Yoshihiro Koide,§ Yuko Morito,z and Akira Fujishima†,‡ †
Kanagawa Academy of Science and Technology, KSP East 421, 3-2-1 Sakado, Takatsu-ku, Kawasaki, Kanagawa 213-0012, Japan Division of Photocatalyst for Energy and Environment, Research Institute for Science and Technology, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan § Department of Material and Life Chemistry, Faculty of Engineering, Kanagawa University, 3-27-1, Rokkakubashi, Yokohama, Kanagawa 221-8686, Japan ^ Fuel Cell Nanomaterials Center, University of Yamanashi, 6-43 Miyamae-cho, Kofu, Yamanashi 400-0021, Japan z U-VIX Corporation, 2-14-8 Midorigaoka, Meguro-ku, Tokyo 152-0034, Japan ‡
ABSTRACT: Decomposition of environmentally persistent perfluorooctanoic acid (PFOA) in aqueous suspensions of a TiO2 photocatalyst, with use of medium-pressure ultraviolet (MPUV) lamp irradiation alone, was examined under atmospheric pressure. Compared to direct photolysis, TiO2 photocatalysis led to efficient PFOA decomposition and production of CO2 and F . PFOA decomposition followed pseudo-first-order kinetics, with observed rate constants of 1.3 10 2 and 8.6 10 2 dm3 h 1 in direct photolysis and TiO2 photocatalysis, respectively. The latter (photocatalytic) rate constant is 5 100 times greater than those obtained in other published research on the photocatalytic decomposition of PFOA. In the proposed decomposition pathway, PFOA molecules adsorb onto the TiO2 surface according to adsorption equilibrium in an aqueous suspension and could be easily decomposed by holes and radicals generated by MPUV lamp irradiation. Under the present reaction conditions, a narrow region of TiO2 concentrations around 1.5 wt % showed the maximum extent of PFOA decomposition, CO2 formation, and F formation. The optimum rate can be attributed to a trade-off between an increase of photon absorption by TiO2 and a decrease in UV penetration below the surface of the TiO2 suspension. In the 1.5 wt % TiO2 photocatalyst concentration, 5 mM PFOA was almost totally decomposed during 4 h of MPUV lamp irradiation under atmospheric pressure.
’ INTRODUCTION Perfluorinated acids have been widely used in industry as surfactants, surface treatment agents, and flame retardants. As the use of perfluorinated acids has increased, some of them, typically perfluorooctanoic acid (C7F15COOH; PFOA), have been detected in the environment.1 Analytical studies have revealed their toxicological properties and high stability.2 Thus, techniques for decomposing them to harmless products under mild conditions are desirable. Recently, the strong oxidation ability of TiO2 photocatalysts has received growing attention.3 6 TiO2 photocatalysts can decompose organic contaminants into less harmful products by means of active oxygen species produced by ultraviolet (UV) light irradiation. However, the reaction energies estimated for decomposition of the C F bond in PFOA are in the 3.6 5.5 eV range.7 These values are higher than the energies of the photocatalytically generated active oxygen species and the band gap of TiO2. Therefore, it has been thought that TiO2 photocatalysts are not suitable for decomposition of PFOA. The first instance of efficient photocatalytic decomposition of PFOA was achieved by Hori et al. without using TiO2. They reported that the tungstic heteropolyacid photocatalyst H3PW12O40 3 6H2O led to efficient PFOA decomposition and the production of F and CO2.8 However, the reaction requires a pure oxygen atmosphere at a r 2011 American Chemical Society
pressure of 0.48 MPa. On the other hand, Zhao and Zhang showed that a β-Ga2O3 photocatalyst could decompose PFOA under ambient conditions.9 However, the amount of F generation was much smaller than the amount of decomposed PFOA, and there was no evidence of CO2 generation. Therefore, environmentally undesirable gaseous species such as CF4, a stable species that has a global-warming potential higher than that of CO2, may be produced. Moreover, β-Ga2O3 is too expensive to be used industrially. Panchangama et al. have reported efficient photocatalysis of PFOA using TiO2 and its reaction mechanisms.10 However, they used an extremely low concentration of PFOA (0.12 mM). A typical PFOA concentration found in wastewater from fluoropolymer manufacturing is 1.35 mM.11 Moreover, the addition of 0.15 M perchloric acid and continuous oxygen bubbling were required for facilitation of the ionization of perfluorocarboxylic acids and serving as both an oxidant and electron acceptor, respectively. These experimental conditions are not suitable for industrial application. Received: August 19, 2010 Accepted: August 12, 2011 Revised: August 4, 2011 Published: August 12, 2011 10943
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Figure 1. Schematic illustration of the photocatalytic system.
Here we report efficient photocatalytic decomposition of a high concentration of PFOA (5 mM) by use of a TiO2 photocatalyst with only medium-pressure UV (MPUV) lamp irradiation under atmospheric pressure. MPUV lamps emit over a broad spectral range in the UV (ca. 250 375 nm) with a number of intense peaks. This type of spectral emission is suitable for photochemical and photocatalytic processes.
Figure 2. HPLC chromatograms of the sample solutions: (A) an aqueous solution containing PFOA (5 mM); (B) an aqueous solution (0.1 L) containing PFOA (5 mM) irradiated for 4 h with a MPUV lamp; (C) an aqueous suspension (0.1 L) containing PFOA (5 mM) and P25 (1.5 wt %) irradiated for 4 h with a MPUV lamp.
’ EXPERIMENTAL SECTION Decomposition of PFOA. PFOA was obtained by Tokyo Kasei Kogyo Co., Ltd. The TiO2 nanoparticles (P25) were obtained from Evonik Industries. Figure 1 shows a schematic illustration of the system. A glass vessel (inner diameter 133 mm height 40 mm; nominal volume, 500 mL), equipped with a quartz window and sampling ports, was used as a simple batch reactor. In a typical run, an aqueous suspension (0.1 L) containing PFOA (5 mM) and P25 TiO2 nanoparticles (1.5 6.0 wt %) was filled into the reactor. A MPUV lamp (U46C18, 600 mW cm 2 at 254 nm, Heraeus) was used as a light source for photocatalysis. During the reaction, the gas phase was analyzed by a photoacoustic field gas monitor and the aqueous phase was analyzed by ion chromatography and high-performance liquid chromatography (HPLC). Analytical Procedures. To confirm total decomposition of PFOA, the concentration of CO2 in the reactor was monitored by an Innova photoacoustic field gas monitor (model 1412, Innova AirTech Instruments), and the concentration of F was measured by an ion chromatography system (Dionex DX-120) consisting of a pump, a guard column (Dionex IonPak AG12A; 4.0 mm i.d., 50 mm length), a separation column (Dionex IonPak AS12A; 4.0 mm i.d., 200 mm length), and a conductivity detector with a suppressor device (Dionex ASRS300). The mobile phase of the ion chromatography system was an aqueous solution containing Na2CO3 (2.7 mM) and NaHCO3 (0.3 mM), and the flow rate was 1.5 mL min 1. A HPLC system was used to quantify the concentration of PFOA and to identify the decomposition products in the aqueous phase. The HPLC system (Shimadzu LC-2010C) consisted of an automatic sample injector (injection volume: 30 μL), a degasser, a pump, and a column oven (40 C). The separation column was a Tosoh TSKgel Super-ODS (4.6 mm i.d., 10 cm length 2). The samples were isocratically eluted with a mobile phase of a mixture of methanol and aqueous NaH2PO4 (20 mM, adjusted to pH 3.0 with H3PO4) (55:45, v/v) at a flow rate of 0.4 mL min 1 and detected by a variable-wavelength detector set at 204 nm.
Figure 3. Irradiation time dependence of PFOA decomposition in the presence (solid lines) and absence (broken lines) of TiO2: detected molar amounts of (A) PFOA, (B) CO2, and (C) F . An aqueous suspension (0.1 L) containing PFOA (5 mM) and P25 (1.5 wt %) was irradiated with a MPUV lamp.
’ RESULTS AND DISCUSSION Decomposition of PFOA in the Absence and Presence of TiO2. Under the present reaction conditions, the aqueous solu-
tion of PFOA (5.0 mM) was irradiated with UV visible light from a MPUV lamp through a quartz filter at ambient temperature and pressure. After 4 h of irradiation, an HPLC chromatogram of the reaction solution showed a decrease of the PFOA peak and several additional peaks eluting before the PFOA peak (Figure 2A,B). According to liquid chromatography mass spectrometry (LC-MS) measurements (data not shown), based on the work of Hori et al.,8 the peaks eluting prior to the PFOA peak could be assigned as the short-chain perfluorocarboxylic acids bearing C4 C6 perfluoroalkyl groups, indicating that electrochemical decomposition of PFOA occurred. PFOA absorbs from the deep-UV region to 270 nm.12 Thus, the high intensity of UV visible light from the MPUV lamp (600 mW cm 2 at 254 nm) used in this study caused decomposition of PFOA by direct photolysis. Interestingly, 4 h of irradiation of PFOA in an aqueous suspension of TiO2 (1.5 wt %) resulted in significant decomposition relative to direct photolysis (Figure 2C), indicating that photocatalytic decomposition of PFOA occurred. Figure 3 shows the irradiation time dependence of the concentrations of the species in the reaction mixture. In both the absence and presence of TiO2, the amounts of PFOA decreased 10944
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Scheme 1. Representative Scheme of the Photochemical and Photocatalytic Degradation of PFOA and the Formation of Carbon Dioxide and Fluoride
Figure 4. Dependence of PFOA decomposition (solid circles), CO2 formation (open triangles), and F formation (open circles) on the initial amount of TiO2. An aqueous suspension (0.1 L) containing PFOA (5 mM) and P25 (0 6.0 wt %) was irradiated for 4 h with a MPUV lamp.
and the amounts of CO2 and F increased with increasing irradiation. This observation suggests that PFOA was successfully decomposed to CO2 and F in both cases. PFOA decomposition follows pseudo-first-order kinetics, with observed rate constants of 1.3 10 2 and 8.6 10 2 dm3 h 1 (normalized by the sample solution volume) in the absence and presence of TiO2, respectively. Thus, the TiO2 photocatalyst was able to accelerate PFOA decomposition (6 times greater than that for direct photolysis). This rate constant, 8.6 10 2 dm3 h 1, is 5 100 times greater than that obtained in other research reports of photocatalytic decomposition of PFOA using the other photocatalysts.8,9,13 16 For examples, the rate constants for photocatalytic decomposition of PFOA using tungstic heteropolyacid irradiated with a xenon mercury lamp under oxygen (0.48 MPa) and β-Ga2O3 irradiated with a mercury lamp under a nitrogen atmosphere are 2.1 10 3 and 1.7 10 2 dm3 h 1, respectively (calculated by reported data).8,9 TiO2 photocatalysts absorb UV light efficiently. Moreover, the absorption coefficient of P25 TiO2 (Evonik Industries) is almost 2.5 times larger than the other TiO2 at the deep-UV region to 280 nm.17 Thus, the suitability of the large absorption coefficient at the UV region of P25 and the high intensity of UV light from the MPUV lamp (600 mW cm 2 at 254 nm) caused efficient decomposition of PFOA. In addition, the reproducibility in this research is good. Although each set of experiments was repeated two or three times, almost the same decomposition rate constants were obtained. Proposed Decomposition Pathway. On the basis of the experimental results of PFOA decomposition in a H2O2 solution with UV irradiation, Hori et al. concluded that OH radicals generated by TiO2 are not suitable to decompose PFOA.8 However, the present results clearly show PFOA decomposition by TiO2 photocatalysis. According to the experimental results and based on data from the literature,4,8,18,19 it is possible to propose a decomposition mechanism for PFOA (Scheme 1). In the PFOA solution, MPUV light irradiation cleaves the C C bond between C7F15 and COOH and generates CO2. The C7F15 radical in water forms the thermally unstable alcohol C7F15OH, which undergoes F elimination to form C6F13COF. This acid fluoride
Figure 5. Pseudo-first-order photocatalytic degradation rate constant as a function of the initial amount of TiO2.
undergoes hydrolysis to produce another F and the perfluorocarboxylic acid with one less CF2 unit, C6F13COOH. Through repitition of these processes, finally, PFOA can be totally mineralized into CO2 and F . In the presence of TiO2, PFOA molecules adsorb onto the TiO2 surface according to adsorption equilibrium. Adsorbed PFOA molecules can be easily decomposed by holes and radicals generated by TiO2. Moreover, C7F15 radicals and C6F13COF react with OH radicals on the TiO2 surface efficiently. The lifetime of the OH radical is estimated to be a few microseconds.20 However, this is sufficient for reaction with the adsorbed chemical species on the TiO2 surface. This scheme is clearly with the proposals (1) that the peaks eluting prior to the PFOA peak in a HPLC chromatogram (Figure 2B,C) could be assigned to the short-chain perfluorocarboxylic acids bearing C4 C6 perfluoroalkyl groups by LC MS measurement and (2) the observed molar ratio of generated CO2 to F of 1:2 in Figure 2. Effect of the TiO2 Amount. Figure 4 shows the dependence of PFOA decomposition, CO2 formation, and F formation on the initial amount of TiO2 for a constant amount of PFOA (5 mM) and a reaction time of 4 h. When 1.5 wt % of TiO2 was used, the maximum extent of PFOA decomposition, CO2 formation, and F formation was observed. The maximum decomposition rate was also obtained at 1.5 wt % of TiO2 (Figure 5). These results suggest that the effective photocatalytic decomposition of PFOA using TiO2 occurs in a narrow region of the TiO2 amount. Similar curves for the rate constant versus TiO2 amount have been reported and discussed by several research groups.21 24 The optimum rate, 1.5 wt % of P25 TiO2, can be attributed to an increase in photon absorption by P25 TiO2 with the large absorption coefficient 10945
dx.doi.org/10.1021/ie1017496 |Ind. Eng. Chem. Res. 2011, 50, 10943–10947
Industrial & Engineering Chemistry Research and greater reactive oxygen radical species production at the surface of the TiO2 particles. The decrease in the rate for higher TiO2 amounts can be attributed to a decrease in UV penetration below the surface of the TiO2 suspension. As described in the Introduction, the techniques for decomposing PFOA to harmless species under mild conditions (ambient temperature and pressure) are desirable. To obtain a large reaction rate constant under mild conditions, decomposition of PFOA should be carried out in aqueous suspensions containing a narrowly controlled amount of TiO2, with MPUV lamp irradiation. However, the optimum amount of P25 TiO2 is strongly affected by the reactor design. In this reaction condition, the irradiation area of the MPUV lamp (45 mm 45 mm) is relatively smaller than the reactor diameter (133 mm). Therefore, only a portion of suspended P25 may be excited. Now we are trying to design another chamber for more effective decomposition of PFOA.
’ CONCLUSIONS Decomposition of PFOA in aqueous suspensions of TiO2 photocatalysts with only MPUV lamp irradiation was achieved. Compared to direct photolysis, TiO2 photocatalysis led to efficient PFOA decomposition and the production of CO2 and F . PFOA decomposition followed pseudo-first-order kinetics, with an observed rate constant of 8.6 10 2 dm3 h 1 for TiO2 photocatalysis. This rate constant is 5 100 times greater than those obtained in other published research on the photocatalytic decomposition of PFOA. In the proposed decomposition pathway, PFOA molecules adsorb onto the TiO2 surface according to adsorption equilibrium in an aqueous suspension and could be easily decomposed by holes and radicals generated by MPUV lamp irradiation. The optimum rate can be attributed to a tradeoff between an increase of photon absorption by TiO2 and a decrease in UV penetration below the surface of the TiO2 suspension. Moreover, the decomposition efficiency of PFOA was not decreased by repeating the decomposition test twice or three times by the addition of PFOA into the suspension after total decomposition of the initial PFOA. Although we used a simple batch reactor under atmospheric pressure, without any addition of reagents or gases, it would be attractive to develop a similar new continuous-type photocatalytic reactor system for the practical treatment of wastewater-containing perfluorinated acids. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The research was supported, in part, by the Nippon Sheet Glass Foundation for Materials Science and Engineering (2010). The authors are grateful to Dr. Yasuo Shida (Tokyo College of Pharmacy) for LC MS analysis. ’ REFERENCES (1) Bj€orklund, J. A.; Thuresson, K.; de Wit, C. A. Perfluoroalkyl Compounds (PFCs) in Indoor Dust: Concentrations, Human Exposure Estimates, and Sources. Environ. Sci. Technol. 2009, 43, 2276. (2) Renner, R. Growing Concern over Perfluorinated Chemicals. Environ. Sci. Technol. 2001, 35, 154A.
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