Environ. Sci. Technol. 2004, 38, 6118-6124
Decomposition of Environmentally Persistent Perfluorooctanoic Acid in Water by Photochemical Approaches H I S A O H O R I , * ,† E T S U K O H A Y A K A W A , † HISAHIRO EINAGA,† SHUZO KUTSUNA,† KAZUHIDE KOIKE,† TAKASHI IBUSUKI,† HIROSHI KIATAGAWA,‡ AND RYUICHI ARAKAWA‡ National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba West, 16-1 Onogawa, Tsukuba 305-8569, Japan, and Department of Applied Chemistry, Faculty of Engineering, Kansai University, 3-3-35 Yamate-cho, Suita 564-8680, Japan
The decomposition of persistent and bioaccumulative perfluorooctanoic acid (PFOA) in water by UV-visible light irradiation, by H2O2 with UV-visible light irradiation, and by a tungstic heteropolyacid photocatalyst was examined to develop a technique to counteract stationary sources of PFOA. Direct photolysis proceeded slowly to produce CO2, F-, and short-chain perfluorocarboxylic acids. Compared to the direct photolysis, H2O2 was less effective in PFOA decomposition. On the other hand, the heteropolyacid photocatalyst led to efficient PFOA decomposition and the production of F- ions and CO2. The photocatalyst also suppressed the accumulation of short-chain perfluorocarboxylic acids in the reaction solution. PFOA in the concentrations of 0.34-3.35 mM, typical of those in wastewaters after an emulsifying process in fluoropolymer manufacture, was completely decomposed by the catalyst within 24 h of irradiation from a 200-W xenon-mercury lamp, with no accompanying catalyst degradation, permitting the catalyst to be reused in consecutive runs. Gas chromatography/mass spectrometry (GC/MS) measurements showed no trace of environmentally undesirable species such as CF4, which has a very high global-warming potential. When the (initial PFOA)/(initial catalyst) molar ratio was 10: 1, the turnover number for PFOA decomposition reached 4.33 over 24 h of irradiation.
Introduction Perfluorinated acids (mainly perfluorocarboxylic and perfluorosulfonic acids) and their salts have been widely used in industry as surfactants; they are used as emulsifying agents in polymer synthesis and as surface treatment agents in photolithography. Their other applications include the use as fire retardants, carpet cleaners, and paper coatings (1-3). As the use of perfluorinated acids has increased, some of them, typically, perfluorooctanoic acid (C7F15COOH; PFOA) and perfluorooctanesulfonic acid (C8F17SO3H; PFOS), have recently been detected in environmental waters, animals, and humans (4-8). Analytical studies have revealed that they * Corresponding author phone: +81-298-61-8161; fax: +81-29861-8258; e-mail:
[email protected]. † National Institute of Advanced Industrial Science and Technology (AIST). ‡ Kansai University. 6118
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bioaccumulate, and their toxicological properties are being clarified (9-14). Compounds bearing longer perfluoroalkyl groups have been shown to be more bioaccumulative (10, 13, 14). These compounds are anthropogenic, and, because of their high stability, they have no known natural decomposition processes. For example, when they were boiled in nitric or sulfuric acid, no sign of C-F bond cleavage was found (1, 2), and in attempts to decompose them thermally, a very high temperature (1200 °C) was required (3). Fluorochemical manufacturing and processing sites are significant stationary sources of perfluorinated acids (7, 12). To suppress the accumulation of these compounds in the environment, development of techniques for decomposing them (as waste, especially in wastewaters) to harmless species under mild conditions is desirable. The method should involve cleavage of the C-F bonds to form F- ions, because F- ions readily combine with Ca2+ to form environmentally harmless CaF2. In the present work, we examined the decomposition of PFOA in water by UV-visible light irradiation (direct photolysis), by H2O2 with UV-visible light irradiation, and by a water-soluble tungstic heteropolyacid photocatalyst H3PW12O40‚6H2O (1). To the best of our knowledge, this report is the first example of PFOA decomposition in water by photochemical approaches. We detail an effective photochemical decomposition of PFOA by a system consisting of 1, water, and oxygen under UV-visible light irradiation at room temperature.
Experimental Section Materials. Tungstic heteropolyacid 1 was prepared by the literature method (15) and was purified by ether extraction and recrystallized from water. Spectroscopic characteristics of 1 (31P NMR, IR) were in good agreement with those in the literature (15). Trifluoroacetic acid (CF3COOH, >99.0%), pentafluoropropionic acid (C2F5COOH, >98%), heptafluorobutyric acid (C3F7COOH, >99%), nonafluoropentanoic acid (C4F9COOH, >98%), and undecafluorohexanoic acid (C5F11COOH, >98%) were purchased from Tokyo Kasei Kogyo Co., Ltd. (Tokyo, Japan). Tridecafluoroheptanoic acid (C6F13COOH, >96%) and PFOA (>95%) were obtained from Wako Pure Chemical Industries Ltd. (Osaka, Japan). Oxygen-18enriched water, H2(18O) (95%), was purchased from Nippon Sanso Corporation (Tokyo, Japan). Oxygen (99.9%) and Ar (99.99%) gases for the reaction were purchased from Tomoe Shokai Co., Ltd. (Tokyo, Japan). Standard gas mixtures, CHF3 (512 ppmv)/N2, CF4 (500 ppmv)/N2, C2F6 (1030 ppmv)/Ar, and CO2 (1.010%)/N2 were obtained from Takachiho Trading Co., Ltd. (Tokyo, Japan). Photochemical Procedures. A cylindrical pressureresistant Inconel reactor (176-mL volume, 5.9 cm i.d., manufactured by Nitto Kouatsu Co., Ltd., Tsukuba, Japan) equipped with a sapphire window (4.0 cm i.d.) on the top for introduction of light was used. The inner wall of the reactor was coated with poly(tetrafluoroethylene) (PTFE). A gold vessel (105 mL, 5.5 cm i.d.), which is stable to highly acidic solutions, was introduced into the reactor. The PTFE wall of the reactor was tightly attached to the outer wall of the gold vessel so that no light reached the PTFE wall, and no solution in the gold vessel was in contact with the PTFE wall. In a typical photochemical run, an aqueous solution (22 mL) of PFOA (29.6 µmol; 1.35 mM) was filled into the gold vessel. The PFOA concentration was selected to lie within concentration range present in wastewaters after an emulsifying process in fluoropolymer manufacture (16). The direct photolysis of PFOA was examined by using this solution. In other experimental runs, H2O2 (1.0 M) or 1 (1.47 × 10-4 mol; 10.1021/es049719n CCC: $27.50
2004 American Chemical Society Published on Web 10/09/2004
6.68 mM) was added to the solution. For the reaction using 1, various amounts of PFOA (7.44 µmol to 1.47 mmol) were also used. After the reactor was purged and then pressurized to 0.48 MPa with oxygen gas, the solution was irradiated with UVvisible light from a xenon-mercury lamp (200 W, L200101L, San-Ei Electric Co., Ltd., Osaka, Japan). For the light irradiation, a water filter and an optical-quartz glass fiber were used. After irradiation, the pressure was released, and the reaction gas was collected in a sampling bag and subjected to gas chromatography/mass spectrometry (GC/MS) and GC measurements. The gas volume was measured by an integrating flowmeter. The liquid phase was also subjected to ion chromatography (IC), ion-exclusion chromatography (IEC), high performance liquid chromatography (HPLC), electrospray ionization (ESI) mass spectrometry, and UVvisible spectroscopy. Analytical Procedures. An ion-chromatograph system (Tosoh IC-2001, Tosoh Corporation, Tokyo, Japan) consisting of an automatic sample injector (sample injection volume: 30 µL), a degasser, a pump, a guard column (TSKguard column Super IC-A, 4.6 mm i.d., 1.0 cm length, Tosoh Corporation, Tokyo, Japan), a separation column (TSKgel Super IC-Anion, 4.6 mm i.d., 15 cm length), a column oven (40 °C), and a conductivity detector with a suppressor device was used to measure the F- ion concentrations. The mobile phase was an aqueous solution containing Na2B4O7 (6 mM), H3BO3 (15 mM), and NaHCO3 (0.2 mM), and the flow rate was 0.8 mL min-1. A Tosoh IEC system consisting of a guard column (TSKgel OApak-P, 7.8 mm i.d., 1.0 cm length), a separation column (TSKgel OApak-A, 7.8 mm i.d., 30 cm length), a pump, a column oven (40 °C), and a conductivity detector was also used to measure the concentrations of short-chain perfluorocarboxylic acids produced in the reaction: CF3COOH, C2F5COOH, and C3F7COOH. The sample injection volume was 5.0 µL. The mobile phase was phthalic acid (2 mM) with a flow rate of 0.6 mL min-1. The concentrations of longer-chain perfluorocarboxylic acids (C4F9COOH, C5F11COOH, C6F13COOH, and PFOA) were measured by a HPLC system with conductometric detection (Tosoh IC-2001 chromatograph) consisting 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 or 65:35, v/v) at a flow rate of 0.4 mL min-1. The retention times, concentration ranges of calibration curves, coefficients of determination of the calibration curves, and limits of detection and quantification for the IC, IEC, and HPLC measurements are shown in the Supporting Information (Tables S1-3). ESI mass spectrometry was also used to identify the products in the liquid phase. The full scan (m/z 50-650) mass spectra were obtained with a triple-stage quadrupole mass spectrometer TSQ700 (Finnigan MAT, San-Jose, CA). Analyses were carried out in negative ion mode. The reaction samples (initial PFOA concentration: 1.35 mM) were diluted with acetonitrile (1000-fold by volume) and were electrosprayed at a flow rate of 10 µL min-1 for the mass spectrometric analysis. The sheath gas (N2) was used, and the pressure was 0.34 MPa. The electrospray potential was -4.5 kV, and the cone voltage was 20 V. The heated capillary temperature used was set to 150 °C, because the formation of fragment species [C7F15]-, the CO2-removed anion species from PFOA, was markedly observed at higher temperature such as 250 °C. A GC/MS system consisting of a gas chromatograph (HP5890, Hewlett-Packard, Wilmington, DE), a mass spec-
FIGURE 1. Wavelength-distribution for the absorptions of (A) PFOA (1.35 mM in water), (B) H2O2 (1.0 M in water), (C) heteropolyacid 1 (6.68 mM in water), and (D) the emission from the xenon-mercury lamp. The concentrations of PFOA, H2O2, and 1 were the same as those in the subsequent photochemical reactions. The path length for the measurement of the absorption spectra was 1.0 cm. The spectrum D was taken from specifications attached with the lamp. trometer (HP 5972A), and a workstation (HP G1034CJ) was used to qualitatively identify the gas products. The GC separation was performed on a fused silica capillary column (Poraplot Q, 0.32 mm i.d., 25 m length, Chrompack, Bergen op Zoom, The Netherlands) using He as a carrier gas. The oven temperature was held constant at 30 °C. The sample gas (30 µL) was introduced into the GC/MS system with splitless mode. The injector temperature was held constant at 120 °C, and the electron impact (EI) source was operated at 70 eV. The analyses were conducted in full scan mode (m/z 1.2-200). Standard gas mixtures, CHF3 (512 ppmv)/N2, CF4 (500 ppmv)/N2, C2F6 (1030 ppmv)/Ar, and CO2 (1.010%)/ N2 were also subjected to measurements for comparison. Selected ion monitoring mode was also used to confirm the no formation of fluorinated species such as CF4, CHF3, and C2F6 in the reaction gas phase, by monitoring the characteristic m/z values for each compound. The quantification of CO2 was carried out by a gas chromatograph (GC 323, GL Sciences, Tokyo, Japan) consisting of an injector (150 °C), a column oven (110 °C), and a thermal conductivity detector (TCD) (130 °C). The column was an active carbon column (GL Sciences, 60/80 mesh, 2.17 mm i.d., 2 m length), and the carrier gas was Ar. The sample injection mode was splitless, and the injection volume was 0.4 mL. Quantification was performed by comparison to an external calibration. The retention time, concentration range, coefficient of determination of the calibration curve, and limits of detection and quantification for the CO2 measurement are shown in Supporting Information (Table S-4). UV-visible spectra were measured using a Shimadzu UV2550 spectrophotometer (Shimadzu Corporation, Kyoto, Japan). The samples were transferred into quartz cells (path length: 1.0 cm) without dilution and then subjected to measurements. When the reaction was carried out under Ar instead of oxygen, the transfer of the reaction mixture into a quartz cell was also carried out under Ar.
Results and Discussion Direct Photolysis. In the present reaction condition, the aqueous solution of PFOA (1.35 mM) was irradiated with UV-visible light from a xenon-mercury lamp through a water filter. Under these conditions, the lamp emitted mainly 220-460 nm light (Figure 1D). PFOA has strong absorption VOL. 38, NO. 22, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Irradiation-time dependence of PFOA decomposition (direct photolysis): detected molar amounts of PFOA, F-, CO2, C6F13COOH, and C5F11COOH. An aqueous solution (22 mL) containing PFOA (29.6 µmol; 1.35 mM) was irradiated with a xenon-mercury lamp under oxygen (0.48 MPa). In addition to these compounds, small amounts of perfluorocarboxylic acids bearing C1-C4 perfluoroalkyl groups were detected (see Table 1).
FIGURE 2. HPLC chromatograms of the sample solutions without irradiation and after irradiation: (A) an aqueous solution containing PFOA (1.35 × 10-4 M) was subjected to the HPLC measurement, (B) an aqueous solution (22 mL) containing PFOA (29.6 µmol; 1.35 mM) was irradiated for 48 h with a xenon-mercury lamp under oxygen (0.48 MPa), and the reaction solution was subjected to the measurement without dilution. Mobile phase was a mixture of methanol and aqueous NaH2PO4 (20 mM, adjusted to pH 3.0 with H3PO4) (55:45, v/v). A small peak between the solvent and C4F9COOH peaks in B was attributed to C3F7COOH; however, the quantitative analysis of C3F7COOH was carried out by ion-exclusion chromatography. from the deep UV-region to 220 nm and a weak, broad absorption from 220 to 270 nm (Figure 1A). As mentioned above, PFOA is chemically and thermally stable. However, irradiation of an aqueous solution of PFOA under 0.48 MPa of oxygen at room temperature caused decomposition of PFOA and the formation of F-, CO2, and short-chain perfluorocarboxylic acids. The decomposition of PFOA and formation of the short-chain perfluorocarboxylic acids bearing C4-C6 perfluoroalkyl groups were successfully quantified by the HPLC with conductometric detection (Figure 2); perfluorocarboxylic acids bearing C1-C3 perfluoroalkyl groups were quantified by ion-exclusion chromatography. The formation of short-chain perfluorocarboxylic acids was also confirmed by ESI mass spectral measurements of the reaction solution, which showed peaks for the corresponding anions (Figure S-1 in Supporting Information). Figure 3 shows the irradiation-time dependence of the photoreaction under oxygen. The amounts of PFOA de6120
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creased, and the amounts of CO2 and F- increased with increasing irradiation. After 72 h of irradiation, 89.5% of the initial PFOA was decomposed. The amount of C6F13COOH increased up to 48 h of irradiation and then decreased, whereas those of perfluorocarboxylic acids bearing shorter perfluoroalkyl groups, for example, C5F11COOH, continued to increase after 48 h. This observation suggests that the formation of short-chain pefluorocarboxylic acids occurs by stepwise removal of CF2. Table 1 lists the product distribution under several reaction conditions. From the results shown in entry 2, the fluorine recovery after 72 h of irradiation (the molar ratio of total fluorine content after irradiation in the F- and short-chain perfluorocarboxylic acids formed and in unchanged PFOA to that in the PFOA before irradiation) was 95.9%. On the other hand, the carbon recovery (the molar ratio of total carbon content in the CO2 and short-chain perfluorocarboxylic acids formed and in unchanged PFOA to that in the PFOA before irradiation) was 104%. Hence, the initial fluorine and carbon in PFOA can be well accounted for by unchanged PFOA, F-, and short-chain perfluorocarboxylic acids in the liquid phase and CO2 in the gas phase. Consistently, GC/MS analysis of the gas phase revealed that the present system produced no environmentally undesirable gaseous species such as CF4, a stable species that has a global-warming potential at least 3900 times higher than that of CO2 (17). CF4 is often observed in the decomposition of perfluorocompounds by extremely high-energy techniques such as electron beam irradiation (18). When the oxygen pressure was raised to 0.95 MPa (entry 3, Table 1), no significant changes occurred compared with 0.48 MPa of oxygen (entry 1, Table 1). On the other hand, when 0.48 MPa of argon was used (entry 4, Table 1), the amount of PFOA decomposed was slightly decreased, whereas the amounts of F- and, especially, CO2 relevantly decreased. This suggests that oxygen plays an important role, probably not in the first PFOA decomposition step (C-C bond cleavage between C7F15 and COOH) but in the subsequent steps that produce F- and CO2. In the reaction under argon (entry 4, Table 1), the oxygen recovery (the molar ratio of total oxygen content in the CO2 and short-chain perfluorocarboxylic acids produced and in unchanged PFOA to that in the PFOA before irradiation) was
TABLE 1. Product Distribution after PFOA Decomposition by UV-Visible Light Irradiation (Direct Photolysis) and H2O2 + UV-Visible Light Irradiationa
no.
conditions
1b
O2 (0.48 MPa) O2 (0.48 MPa) O2 (0.95 MPa) Ar (0.48 MPa) H2O2 (1 M) + O2 (0.48 MPa)
2c 3b 4b 5b
a
h.
d
short-chain CnF2n+1COOH (µmol) n) 5 4 3 2
PFOA decomposed (µmol)
F(µmol)
CO2 (µmol)
6
13.3
66.1
69.4
6.30
2.91
1.15
0.45
0.20
0.25
94.8
7.06
6.58
4.23
2.42
1.47
1.24
26.5
149
1
13.0
59.5
50.5
5.39
3.01
1.30
0.49
0.27
0.31
12.8
52.6
24.5
4.43
2.21
0.88
0.33
0.13
ndd
10.5
50.1
35.9
5.51
2.16
0.87
0.30
0.13
ndd
The initial amount of PFOA was 29.6 µmol, and the volume of the reaction solution was 22 mL. b Reaction time ) 24 h. c Reaction time ) 72 nd ) not detected.
spectrum of the reaction mixture was almost identical with that of the reaction under argon, and the CO2 produced consisted of C(16O)2, C(16O)(18O), and C(18O)2 with relative abundance of 11.0, 8.4, and 80.6%, respectively. The higher selectivity of CO2 toward oxygen-18-containing CO2 in the reaction under oxygen than under argon supports the view that oxygen enhances not the first PFOA decomposition step (which produces C(16O)2 only) but the subsequent processes to form CO2 and F-. Thus, both water and oxygen play important roles in the PFOA decomposition. Based on the reported experimental results and on data from the literature (19, 20), it is possible to propose a formation mechanism for the short-chain perfluorocarboxylic acids. Irradiation of PFOA in water cleaves the C-C bond between the C7F15 and COOH. The C7F15 radical in water forms the thermally unstable alcohol C7F15OH, which undergoes HF elimination to form C6F13COF (eq 1) (19). This acid fluoride undergoes hydrolysis to give the perfluorocarboxylic acid with one less CF2 unit, C6F13COOH (eq 2) (20).
FIGURE 4. ESI mass spectra of the reaction solution of PFOA after direct photolysis for 24 h under Ar in H2(18O): peaks for (A) [C6F13COO]and (B) [C5F11COO]-. 167%. This indicated that the oxygen in the products mainly came from water. To elucidate the origin of oxygen in the products, we carried out the photoreaction of PFOA in H2(18O) under both argon and oxygen atmospheres. Figure 4 shows the ESI mass spectra of the reaction mixture using H2(18O) after 24 h of irradiation under argon. As expected, the short-chain product [C6F13COO]- showed large peaks at m/z ) 365 and 367, which correspond to [C6F13C(16O)(18O)]and [C6F13C(18O)(18O)]-, respectively, and a very small peak at m/z ) 363, which corresponds to [C6F13C(16O)(16O)](Figure 4A). Another product, [C5F11COO]-, also showed large peaks at m/z ) 315 for [C5F11C(16O)(18O)]- and 317 for [C5F11C(18O)(18O)]- and a very small peak at m/z ) 313 for [C5F11C(16O)(16O)]- (Figure 4B). These observations clearly indicate that water acts as an oxygen source for the shortchain perfluorocarboxylic acids, that is, the products were formed through hydrolysis. In addition, GC/MS analysis of the gas phase after the reaction in H2(18O) showed CO2 peaks corresponding to C(16O)2, C(16O)(18O), and C(18O)2 at m/z ) 44, 46, and 48, with relative abundance of 21.0, 6.5, and 72.5%, respectively. Therefore, water also acts as the oxygen source for the CO2 produced. On the other hand, when the reaction in H2(18O) was carried out under oxygen gas (16O2), allowing an increase in the formation of F- and CO2, the ESI mass
C7F15OH f C6F13COF + H+ + F-
(1)
C6F13COF + H2O f C6F13COOH + H+ + F-
(2)
In the same manner, perfluorocarboxylic acids bearing shorter perfluoroalkyl groups are formed in a stepwise manner from perfluorocarboxylic acids that bear longer perfluoroalkyl groups. Hydrogen Peroxide + UV-Visible Light Irradiation. The combination of H2O2 and UV irradiation has been applied to the decomposition of many organic pollutants in water (21), where the photolysis of H2O2 causes cleavage of the molecule into chemically active OH radicals. This method was effective for the decomposition of chlorinated organic compounds such as chlorophenols (22) under acidic conditions similar to those of our PFOA solutions. In our experimental condition, the solution containing H2O2 (1.0 M) had a strong absorption below 300 nm (Figure 1B), which could fully absorb the light from the lamp. By using this system, PFOA was also decomposed to form F-, CO2, and short-chain perfluorocarboxylic acids (entry 5 in Table 1). However, the amounts of PFOA decomposed and of F- and CO2 formed all were lower than those in the direct photolysis (see entries 1 and 5 in Table 1). Hydroxyl radicals in aqueous solution have a poor reactivity with CF3COOH (23), so that TiO2 photocatalyst has a very poor ability to decompose CF3COOH (24, 25). Heteropolyacid Photocatalyst. Water-soluble heteropolyacids are attractive candidate photocatalysts for the decomposition of perfluorocarboxylic acids because of their mulVOL. 38, NO. 22, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. Irradiation-time dependence of PFOA decomposition in the presence of heteropolyacid 1: detected molar amounts of PFOA, F-, CO2, and C6F13COOH. An aqueous solution (22 mL) containing PFOA (29.6 µmol; 1.35 mM) and 1 (1.47 × 10-4 mol; 6.68 mM) was irradiated with a xenon-mercury lamp under oxygen (0.48 MPa). tielectron redox capabilities and their high stability under highly acidic conditions (26, 27). We have previously demonstrated that the tungstic heteropolyacid 1 can effectively decompose highly stable CF3COOH and C2F5COOH in water to produce F- and CO2 by UV-visible irradiation under oxygen (28, 29). The heteropolyacid 1 had a strong absorption in the deep UV region to 380 nm (Figure 1C). In the reaction condition involving 1, the absorbance of 1 at 254 nm (a maximum emission line of the lamp) was 1 × 104 times higher than that of PFOA. Hence, 1 was virtually the only species that absorbed the light from the lamp during the photochemical reactions. Figure 5 shows the irradiation-time dependence of the photoreaction using 1, in which we used the same PFOA concentration and light intensity as those in the direct photolysis (Figure 3). After irradiation started, the amount of PFOA detected decreased markedly, and F- and CO2 were found as products; the reaction was much faster than that induced by direct photolysis. After 24 h of irradiation, PFOA disappeared completely, and the amount of F- formed was 4.8 times that produced by the direct photolysis. The amount of PFOA decreased linearly with respect to time at the short period of irradiation, where the initial PFOA decomposition rate using 1 was 1.63 µmol h-1, which was 2.9 times that of the direct photolysis. As described above, direct photolysis of PFOA produced short-chain perfluorocarboxylic acids. When 1 was used, such species were also detected. A short period of irradiation, such as 4 h, produced the one-CF2unit-shortened species C6F13COOH (entry 1,Table 2), and
further irradiation produced other perfluorocarboxylic acids bearing shorter perfluoroalkyl groups (entry 2, Table 2), and a prolonged irradiation period, such as 48 h, caused the disappearance of longer chain species, C6F13COOH and C5F11COOH (entry 3, Table 2). The amounts of short-chain perfluorocarboxylic acids were much lower, compared with the amount of F- formed: the molar ratio of F- to the total fluorine content of the products after 24 h of irradiation was 88% (as calculated from entry 2 in Table 2). In contrast, when the direct photolysis was carried out, the corresponding value was 34% (from entry 1 in Table 1). Finally, when using 1, the molar ratio of F- to the total fluorine content of the products reached 97% after 48 h of irradiation (from entry 3 in Table 2). Hence, the use of 1 not only caused an increase in the PFOA decomposition and the formation of F- and CO2 but also suppressed the accumulation of short-chain perfluorocarboxylic acids. No environmentally harmful species (CF4 etc.) were detected. In the absence of light irradiation, no reaction occurred. When the reaction in the presence of 1 was carried out under 0.48 MPa of argon instead of 0.48 MPa of oxygen, only 7.7% of the initial PFOA was decomposed after 24 h of irradiation (calculated from entry 4 in Table 2), whereas the corresponding yield for the reaction under an oxygen atmosphere was 100%. Thus, a combination of 1, oxygen, and light irradiation is required to achieve efficient PFOA decomposition. The low activity using 1 under argon can be explained in terms of the reaction mechanism as follows. The anion part of 1, [PW12O40]3-, is stable below pH 2 (30). In the present system, the concentration of 1 was sufficient to give highly acidic conditions (pH ∼0.8); therefore [PW12O40]3- remained as the stable species. Photoexcitation from the ground-state species [PW12O40]3- to the ligand-tometal charge-transfer excited-state species [PW12O40]3-* is generally accepted as the initiation process of photocatalysis by 1 (eq 3) (26, 27).
[PW12O40]3- + hν f [PW12O40]3-*
(3)
After an electron transfer from PFOA to the excited-state species (eq 4), the resulting reduced complex, [PW12O40]4-, is reoxidized to [PW12O40]3- in the presence of oxygen (eq 5).
[PW12O40]3-* + PFOA f [PW12O40]4- + PFOA+
(4)
[PW12O40]4- + O2 f [PW12O40]3- + O2-
(5)
In the absence of oxygen, the reoxidation process is very slow. In fact, when the photoreaction was carried out under argon (conditions under which low PFOA decomposition occurred), the UV-visible spectrum after the irradiation showed a broad absorption band in the region from 400 to 1000 nm, with absorption maxima at 493 and 752 nm (Figure 6A). The new absorption reflects the appearance of the oneelectron-reduced complex, [PW12O40]4-, which was identified
TABLE 2. Product Distribution after PFOA Decomposition by Heteropolyacid Photocatalyst 1a
no. 1 2 3 4
short-chain CnF2n+1COOH (µmol) n) 5 4 3 2
reaction time (h)/ atmosphere (pressure, MPa)
PFOA decomposed (µmol)
F(µmol)
CO2 (µmol)
6
4/ O2 (0.48) 24/ O2 (0.48) 48/ O2 (0.48) 24/ Ar (0.48)
4.82
48.3
28.4
0.73
ndb
ndb
ndb
ndb
ndb
153
0.53
1.13
0.83
1.54
0.36
1.07
184
ndb
ndb
0.62
0.41
0.43
0.75
0.68
ndb
ndb
ndb
ndb
ndb
29.6 29.6 2.28
315 366 11.7
10.0
1
a The initial amount of PFOA was 29.6 µmol, the initial amount of 1 was 1.47 × 10-4 mol, and the reaction solution volume was 22 mL. ) not detected.
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b
nd
FIGURE 6. UV-visible spectra of the sample solutions after 24 h of irradiation under (A) argon and (B) oxygen. The initial amounts of 1 and PFOA were the same as those in Figure 5. The spectra of the sample solutions before light irradiation were identical to spectrum B under both argon and oxygen atmospheres. In A, the amount of [PW12O40]4- formed was calculated to be 1.30 µmol from the absorbance at 752 nm; therefore, 0.88% of 1 was found as the reduced complex. by comparison with its reported spectrum (31). When the reaction was carried out under argon, the reoxidation of [PW12O40]4- was suppressed. On the other hand, when the reaction was carried out under oxygen, which allowed efficient decomposition of PFOA, the spectrum after irradiation (Figure 6B) was the same as that before irradiation and no near-IR absorption was observed, showing that oxygen is effectively used in the reoxidation process of [PW12O40]4-. After electron transfer between the excited complex [PW12O40]3-* and PFOA (eq 4), the first bond to be cleaved in the one-electron-oxidized PFOA should be the C-C bond between C7F15 and COOH, because C6F13COOH is the first intermediate to form and be detected. This bond cleavage proceeds probably by the photo-Kolbe mechanism, which has been proposed in the decomposition of acetic acid by polyoxomolybdate (32) and in the decomposition of CF3COOH by 1 (28). The subsequent processes after the formation of the C7F15 radical are not clear; however, they include hydrolysis, because when the photochemical reaction using 1 was carried out in H2(18O) under oxygen for 24 h of irradiation, the CO2 produced showed relative abundances of C(16O)2, C(16O18O), and C(18O)2 of 4.1, 7.6, and 88.3%, respectively: most of the oxygen in the CO2 produced came from water. To check the degradation of 1 after the irradiation, the reaction solution after 24 h of irradiation was diluted with acetonitrile, and its UV-visible spectrum was measured. The heteropolyacid 1 has an absorption maximum at 266 nm owing to ligand-to-metal charge transfer (27). The UV-visible absorption pattern of 1 after 24 h of irradiation was the same as that before irradiation, and the 266 nm absorbance after irradiation was 103% of that before irradiation. In addition, the IR (KBr) spectrum of 1 after the reaction showed no change. Hence, no degradation of 1 was found after 24 h of irradiation. The photochemical decomposition of PFOA using 1 was also effective at various initial PFOA concentrations. As shown in Figure 7, for a constant amount of 1 and a reaction time of 24 h, the amounts of F-, CO2 formed, and PFOA decomposed all increased as the initial amount of PFOA increased. The initial PFOA was completely decomposed in the concentration range 7.44-73.8 µmol (0.34-3.35 mM). After the PFOA had been completely decomposed, the catalyst
FIGURE 7. Dependence of (A) F- formation, (B) CO2 formation, (C) PFOA decomposition, and (D) decomposition yield on the initial amount of PFOA. An aqueous solution (22 mL) containing PFOA [7.44 µmol (0.34 mM) to 2.95 × 10-4 mol (13.4 mM)] and 1 (1.47 × 10-4 mol; 6.68 mM) was irradiated for 24 h with a xenon-mercury lamp under oxygen (0.48 MPa). remained active, and consecutive runs were therefore possible. When an additional amount (73.8 µmol) of PFOA was introduced to the reaction solution after an initial charge of PFOA (73.8 µmol) had been decomposed by 24 h of irradiation, the additional PFOA was also completely decomposed after a second 24 h of irradiation. When the initial amount of PFOA exceeded 2.95 × 10-4 mol (13.4 mM), some PFOA precipitated from the solution. However, despite such a heterogeneous reaction condition, the amount of PFOA decomposed still increased. When the initial amount of PFOA was 1.47 mmol, corresponding to an (initial PFOA)/(initial 1) molar ratio of 10:1, the amount of PFOA decomposed after 24 h of irradiation reached 6.36 × 10-4 mol with a decomposition yield of 43.3%, corresponding to a turnover number [(mole of decomposed PFOA)/(mole of initial 1)] of 4.33. The system using heteropolyacid photocatalyst 1 can effectively decompose PFOA in water. To treat wastewaters, recovery of catalyst from reaction mixtures should be taken into account. Purification of 1 by ether extraction has been proposed in the literature (15), so we adopted this method for the recovery of the catalyst from a reaction mixture. To accomplish this, the reaction solution (20 mL), which contained 0.40 g of 1, was transferred to a separatory funnel, diethyl ether (40 mL) was added, and the mixture was shaken. After the mixture was allowed to stand, a new phase appeared at the bottom of the separatory funnel. We collected the phase and evaporated it to dryness. Spectroscopically (UVvisible, IR, 31P NMR) pure 1 was obtained with a recovery of 78%. The further application of the photochemical decomposition method to other environmentally persistent perfluorinated acids is being investigated in our laboratory.
Acknowledgments This work was supported in part by a Grant-in-Aid for Scientific Research (no. 15310066) from the Japan Society for the Promotion of Science (JSPS).
Supporting Information Available The retention times, concentration ranges of calibration curves, coefficients of determination of the calibration curves, limits of detection and quantification for the IC, IEC, HPLC, and GC measurements, and typical ESI mass spectra of the reaction mixtures. This material is available free of charge via the Internet at http://pubs.acs.org. VOL. 38, NO. 22, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Received for review February 22, 2004. Revised manuscript received July 5, 2004. Accepted August 27, 2004. ES049719N
