Environ. Sci. Technol. 2005, 39, 7692-7697
Efficient Photochemical Decomposition of Long-Chain Perfluorocarboxylic Acids by Means of an Aqueous/Liquid CO2 Biphasic System HISAO HORI,* ARI YAMAMOTO, AND SHUZO KUTSUNA National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba West, 16-1 Onogawa, Tsukuba 305-8569, Japan
Photochemical decomposition of persistent and bioaccumulative long-chain (C9-C11) perfluorocarboxylic acids (PFCAs) with persulfate ion (S2O82-) in an aqueous/liquid CO2 biphasic system was examined to develop a technique to neutralize stationary sources of the long-chain PFCAs. The long-chain PFCAs, namely, perfluorononanoic acid (PFNA), perfluorodecanoic acid (PFDA), and perfluoroundecanoic acid (PFUA), which are used as emulsifying agents and as surface treatment agents in industry, are relatively insoluble in water but are soluble in liquid CO2; therefore, introduction of liquid CO2 to the aqueous photoreaction system reduces the interference of colloidal PFCA particles. When the biphasic system was used to decompose these PFCAs, the extent of reaction was 6.4-51 times as high as that achieved in the absence of CO2. In the biphasic system, PFNA, PFDA, and PFUA (33.5-33.6 µmol) in 25.0 mL of water were 100%, 100%, and 77.1% decomposed, respectively, after 12 h of irradiation with a 200-W xenonmercury lamp; F- ions were produced as a major product, and short-chain PFCAs, which are less bioaccumulative than the original PFCAs, were minor products. All of the initial S2O82- was transformed to SO42-. The system also efficiently decomposed PFCAs at lower concentrations (e.g., 4.28-16.7 µmol of PFDA in 25.0 mL) and was successfully applied to decompose PFNA in floor wax.
Introduction Perfluorinated acids, especially perfluorocarboxylic acids (PFCAs) and perfluorosulfonic acids and their salts, have been widely used in industry as emulsifying agents in polymer synthesis and as surface treatment agents in photolithography, paper coatings, waxes, fire-fighting foams, and polishes (1-3). As a consequence, perfluorooctanoic acid (C7F15COOH; PFOA) and perfluorooctanesulfonate (PFOS) are widespread in the environment and in wildlife and humans (4-7). The bioaccumulation and toxicities of these compounds have been determined (8-12), and the number of publications reporting environmental concentrations of perfluorinated acids has increased rapidly (13). Recently, long-chain PFCAs, such as perfluorononanoic acid (C8F17COOH; PFNA), perfluorodecanoic acid (C9F19COOH; PFDA), and perfluoroundecanoic acid (C10F21COOH; PFUA), which are more bioac* Corresponding author phone: +81-298-61-8161; fax: +81-29861-8258; e-mail:
[email protected]. 7692
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cumulative than PFOA, were detected in wildlife at higher concentrations than PFOA (14). The long-chain PFCAs have also been detected in human serum samples (15). These anthropogenic pollutants, whose high stabilities are ascribed to their C-F bonds, have no known natural decomposition processes. High temperatures (∼1200 °C) are required to thermally decompose them (3). Therefore, the development of techniques to decompose PFCAs derived from stationary sources is desirable (6, 12). The method should work under mild conditions and should involve cleavage of the C-F bonds to form F- ions, because F- ions readily combine with Ca2+ to form environmentally harmless CaF2. We have previously reported that a heteropolyacid, H3PW12O40, works as an efficient photocatalyst to decompose C2-C8 PFCAs (i.e., from CF3COOH to PFOA) in water to Fions and CO2 at room temperature in the presence of oxygen gas (16-19). Furthermore, persulfate ion (S2O82-) promoted more rapid photochemical decomposition of these PFCAs in water to F- ions and CO2 (20). The reaction using S2O82- is not photocatalytic; the photolysis of S2O82- produces two sulfate radical anions (SO4•-) (eq 1), and the SO4•- formed acts as a strong oxidant, with the decomposition of PFCA proceeding via formation of PFCAs with chain lengths that are shorter than the initial chain lengths.
S2O82- + hν f 2SO4•-
(1)
When the persulfate was used, PFCAs were effectively decomposed not only under oxygen but also under an argon atmosphere, and all of the initial S2O82- was transformed to SO42-, for which there is a well-established waste-treatment process. However, application of this homogeneous photochemical system to the decomposition of the long-chain PFCAs is hindered by their lower solubilities in water; for example, the solubilities of PFDA and PFUA are 4 × 10-4 M and 4 × 10-5 M at pH 3, respectively (21). Because colloidal particles scatter light, such a heterogeneous condition is not ideal for homogeneous photochemical reactions. Although these long-chain PFCAs are hardly soluble in water, they are soluble in compressed (liquid or supercritical) CO2, owing to the presence of CO2-philic perfluoroalkyl groups. This property of long-chain PFCAs has led to the use of PFDA as a functional ligand to improve the solubility of inorganic and organometallic catalysts in compressed CO2 (22, 23). In addition, the combination of PFDA and compressed CO2 has been successfully used for coating micromechanical devices: PFDA acts as the coating agent, and compressed CO2 acts as the carrier solvent (24). Compressed CO2 is recognized as an innovative reaction medium because of its nonflammability and nontoxicity (25), and engineering processes using compressed CO2 represent large leaps from laboratory-scale batch operation to plant-scale continuousflow operation (26, 27). Here we describe the effective photochemical decomposition of PFNA, PFDA, and PFUA using S2O82- in an aqueous/ liquid CO2 biphasic system. Compared to the aqueous-phase system (without use of CO2), the biphasic system more efficiently decomposes these long-chain PFCAs. Finally, we applied this method to the decomposition of PFNA contained in floor wax as a representative example of PFCA waste.
Experimental Section Materials. Potassium persulfate (>99.0%) was purchased from Wako Pure Chemical Industries (Osaka, Japan) and used as received. Trifluoroacetic acid (CF3COOH, >99.0%), pentafluoropropionic acid (C2F5COOH, >98%), heptafluorobu10.1021/es050753r CCC: $30.25
2005 American Chemical Society Published on Web 09/02/2005
tyric acid (C3F7COOH, >99%), nonafluoropentanoic acid (C4F9COOH, >98%), undecafluorohexanoic acid (C5F11COOH, >98%), and PFNA (>95%) were purchased from Tokyo Kasei Kogyo Co. (Tokyo, Japan). Tridecafluoroheptanoic acid (C6F13COOH, >96%) and PFOA (>95%) were obtained from Wako Pure Chemical Industries. PFDA (98%) and PFUA (95%) were purchased from Aldrich (Milwaukee, WI). Carbon dioxide (99.99%) and argon (99.99%) gases were purchased from Takachiho Trading Co. (Tokyo, Japan) and Tomoe Shokai Co. (Tokyo, Japan), respectively. Photochemical Procedures. Photochemical reactions were carried out in a high-pressure Inconel reactor (109-mL volume, 4.5-cm i.d.) equipped with a sapphire window and needle valves. The specifications of the reactor were similar to those reported previously (20), except that the window size and volume were smaller to allow high-pressure reactions using liquid CO2. A gold vessel (71 mL, 4.1-cm i.d.), which is stable to highly acidic solutions, was introduced into the reactor. In a typical run, an aqueous solution (25.0 mL) of the long-chain PFCA (PFNA, PFDA, or PFUA; 33.5-33.6 µmol) and K2S2O8 (1.25 mmol) was introduced into the gold vessel, together with a poly(tetrafluoroethylene) (PTFE) stirring bar. Under these conditions, K2S2O8 was soluble, but the hydrophobic long-chain PFCA showed colloidal particles in water. Using a compressor, the reactor was filled through the needle valve with a weighed amount of CO2 (24.7-35.4 g). After 40 min of stirring at 25 °C, the pressure inside the reactor stabilized at 6.4 MPa, and then the mixture was irradiated with UV-visible light from a xenon-mercury lamp (200 W, L2001-01L, San-Ei Electric Co., Osaka, Japan) with vigorous stirring. For light irradiation, a water filter and an opticalquartz glass fiber were used. In all runs, the reaction temperature was held constant at 25 °C. After irradiation, the pressure was slowly released through the needle valve, and the reaction mixture was analyzed by ion chromatography, ion-exclusion chromatography, and high-performance liquid chromatography (HPLC) instruments equipped with appropriate detectors. Control reactions were also carried out under an argon (0.5 MPa) atmosphere, where the conditions (amounts of PFCAs and K2S2O8, volume of water, etc.) were essentially the same as those in the reaction described above using CO2. The procedure for determining whether PFCAs were lost during the pressure-releasing operation was as follows. To an aqueous solution (25.0 mL) of PFDA in the reactor, CO2 (25.6 g) was added, and the mixture was stirred for 1 h. A midget bubbler with a fine-porosity frit containing methanol (20.0 mL) was connected to the needle valve attached to the reactor. The pressure in the reactor was slowly released through the needle valve, and CO2 gas from the reactor was passed through the bubbler. The methanol solution was analyzed by HPLC. No PFDA was detected in the solution, which indicates that the loss of PFDA accompanying the pressure-releasing operation was negligible. Wax Sample. The decomposition of PFNA in floor wax was examined as a model for the decomposition of PFCAs in waste. The wax, containing 75.4 mg L-1 of PFNA, was the same as that used in our previous study (20). An aqueous solution (25.0 mL) containing the wax (1.0 mL) and K2S2O8 (1.25 mmol) was introduced into the reactor, together with a PTFE stirring bar. The reactor was filled with CO2 (28.2 g), and the mixture was irradiated for 12 h. After irradiation, the pressure was slowly released, and the reaction mixture was centrifuged (3000 rpm, 10 min). An InertSep RP-1 cartridge (GL Sciences, Tokyo, Japan) was preconditioned first with methanol (10.0 mL) and then with water (10.0 mL). A portion of the liquid phase (15.0 mL) after the centrifugation was diluted to 50.0 mL with water, and the diluted solution was passed through the cartridge. The PFNA was eluted with methanol (5.0 mL), and the solution was concentrated under
FIGURE 1. Schematic view of the photochemical PFCA decomposition with S2O82- in the aqueous/liquid CO2 biphasic system. Species in parentheses are soluble in that phase. argon to 1.0 mL. The concentrated solution was analyzed by HPLC, and PFNA was quantified. Analytical Procedures. An ion-chromatography system was used to measure the F- and SO42- concentrations. The limits of detection (LODs, injected at 30 µL), which were calculated from a signal-to-noise (S/N) ratio of 3, were 0.74 and 2.62 µg L-1 for F- and SO42-, respectively. An ion-exclusion chromatography system was used to measure the concentrations of short-chain PFCAs (CF3COOH to C3F7COOH). The LODs (S/N ) 3; injected at 5 µL) were 0.27, 0.28, and 0.81 mg L-1 for CF3COOH, C2F5COOH, and C3F7COOH, respectively. Details of the ion-chromatograhy and ion-exclusion chromatography systems have been reported elsewhere (20). To quantify PFCAs ranging from C4F9COOH to PFUA, we used an HPLC system with conductometric detection and a mobile phase consisting of a mixture of methanol and aqueous NaH2PO4 (20 mM, adjusted to pH 3.0 with H3PO4) at several mixing ratios (20, 28). We further varied the mobilephase composition to extend this method to longer chain PFCAs (PFDA and PFUA). The LODs (S/N ) 3, injected at 30 µL) were 0.30 and 0.63 mg L-1 for C4F9COOH and C5F11COOH (mobile phase 55:45 v/v methanol/aqueous NaH2PO4); 0.18, 0.15, 0.63, and 0.57 mg L-1 for C5F11COOH, C6F13COOH, PFOA, and PFNA (mobile phase 65:35 v/v methanol/aqueous NaH2PO4); and 0.14, 0.20, 0.33, 0.24, and 0.49 mg L-1 for C6F13COOH, PFOA, PFNA, PFDA, and PFUA (mobile phase 70:30 v/v methanol/aqueous NaH2PO4), respectively. When the reaction was carried out under an argon atmosphere, the CO2 that formed was quantified by a gas chromatograph using an active carbon column and a thermal conductivity detector (19).
Results and discussion Biphasic Photoreaction System. The reaction system is shown in Figure 1. The initial aqueous phase contains one of the PFCAs under study and persulfate, and the white colloidal particles of the PFCA are visible. After introduction of CO2, the system consisted of two liquid phases (water and CO2) and one gas phase (CO2). In a typical run, the amount of CO2 introduced was 24.7 g. The solubility of CO2 in water under 6.4 MPa at 25 °C is 5.9 g/100 g of H2O (29). Therefore, the amount of CO2 in the aqueous phase (25.0 mL) was approximately 1.5 g, and the total amount of liquid and gaseous CO2 was 23.2 g. Because liquid and gaseous CO2 densities under 6.4 MPa at 25 °C are 0.71 and 0.24 g mL-1, VOL. 39, NO. 19, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Wavelength distribution for (A) the absorption of S2O82(1.25 mmol in 25.0 mL water), the emission from the xenon-mercury lamp, and (B) the absorptions of long-chain PFCAs (33.5 µmol in 25.0 mL of water). The concentrations of PFCAs and S2O82- were similar to those in the initial concentrations in photochemical reactions before introduction of CO2. Measurements for B were carried out after removal of colloidal particles by centrifugation. The path length for the measurement of the absorption spectra was 1.0 cm. respectively (30), and the internal reactor volume except for the aqueous phase was 84.0 mL, the liquid CO2 volume in the reactor was estimated to be 6.5 mL. The oxidant, S2O82-, is soluble in water but is insoluble in liquid CO2. Therefore, decomposition of PFCA occurs predominantly in the aqueous phase, and PFCA is supplied from the liquid CO2 phase. The wavelength distribution for absorption of S2O82- in water, emission from the lamp, and absorptions of PFCAs in water are shown in Figure 2. The concentrations of PFCAs and S2O82- were similar to those in the initial aqueous phase, where PFCAs were at saturation levels. The lamp emits mainly 220-460-nm light (Figure 2A). On the other hand, S2O82strongly absorbs from the deep-UV region to 320 nm. The long-chain PFCAs have weak absorptions in this wavelength region (Figure 2B). Therefore, S2O82- is the dominant absorbing species in the aqueous phase, which is the reaction phase in the biphasic system. Among the UV-visible spectra of PFCAs (Figure 2B), only the spectrum of PFUA, the longestchain PFCA, shows an absorption maximum at about 255 nm. Although there have been no reports describing the origin of this specific absorption maximum, fluorine NMR spectrometry of these compounds has shown that although C6-C10 PFCAs (from C5F11COOH to PFDA) show only small changes in the chemical shift of the terminal -CF3, a noticeable change occurs between C10 and C11 PFCAs (PFDA and PFUA), which suggests a distinct change in geometry and chain rigidity (31). Decomposition of PFCAs. The irradiation-time dependence of the photoreaction of PFNA, where a 37-fold molar excess of S2O82- was used, is shown in Figure 3. When the aqueous/liquid CO2 biphasic system was used, the amount of PFNA decreased with irradiation, and F- was found as a major product. After 12 h of irradiation, the PFNA concentration fell below the LOD of the HPLC; the amount of PFNA in the aqueous phase (25.0 mL) decreased dramatically from the initial 33.6 µmol to
