Sonochemical Decomposition of Perfluorooctane Sulfonate and

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Environ. Sci. Technol. 2005, 39, 3388-3392

Sonochemical Decomposition of Perfluorooctane Sulfonate and Perfluorooctanoic Acid H I R O S H I M O R I W A K I , * ,† YOUICHI TAKAGI,‡ MASANOBU TANAKA,† KENSHIRO TSURUHO,† KENJI OKITSU,‡ AND YASUAKI MAEDA‡ Osaka City Institute of Public Health & Environmental Sciences, 8-34, Tojo-cho, Tennoji-ku, Osaka 543-0026, Japan and Graduate School of Engineering, Osaka Prefecture University, 1-1, Gakuen-cho, Sakai, Osaka 599-8531, Japan

Perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) are shown to be globally distributed, environmentally persistent, and bioaccumulative. Although the toxicities of these compounds were reported, the cleanup procedure from the environment is not developed because of their inertness. In this report the sonochemical degradations of PFOS and PFOA to the products through the fission of the perfluorocarbon chains were observed and the half-life times of the PFOS and PFOA degradations under an argon atmosphere determined to be 43 and 22 min, respectively. The shortening of perfluorocarbon chain of PFOS and PFOA leads to the lowering of the toxicity in view of the decrease of the persistence, and the technique would contribute to the remediation of the environmental pollution by these compounds.

Introduction Perfluorooctane sulfonate (PFOS, C8F17SO3-) and perfluorooctanoic acid (PFOA, C7F15COOH) are widely used in a variety of applications, including polymer additives, lubricants, fire retardants and suppressants, pesticides, and surfactants (1). Furthermore, these compounds are metabolically and photochemically inert, resisting both biotic and abiotic degradation (2). The publicly available data on PFOS persistence in humans is the half-life (t1/2) value reported to be approximately 1428 days (3). As a consequence, they have been detected in wildlife, such as fish, birds, and mammals, from urban and remote areas around the world (4, 5). Recent studies have characterized trace levels of the compounds in the serum of nonoccupationally exposed human (6). It has been known that these compounds have several toxicities. It was reported that PFOS and PFOA were capable of inducing peroxisome proliferation in rat and mice (7). Furthermore, Lau et al. reported that in utero exposure to PFOS severely compromised the postnatal survival of neonatal rats and mice (8, 9). Therefore, these compounds are regarded as potential hazards to human health, and it is very important to develop a method for the decomposition of PFOS and PFOA in order to avoid the discharge and polluting. Recent studies on the photochemical degradation of perfluoroalkanoic acids, such as perfluoropropionic acid and * Corresponding author: phone: 81 6 6771 3374; fax: 81 6 6772 0676; e-mail: [email protected]. † Osaka City Institute of Public Health & Environmental Sciences. ‡ Osaka Prefecture University. 3388

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trifluoroacetic acid, have been published (10, 11). However, to our knowledge, there is no report concerned with PFOS and PFOA degradation. Recently, the degradation of environmental contaminants, such as phthalic acid esters, chlorobenzenes, and chlorinated volatile hydrocarbons, by sonochemical action has been studied as one of the most effective oxidation processes (1223). The chemical effects of ultrasonic irradiation are generally due to the cavitation phenomenon, which is comprised of the formation, growth, and collapse of bubbles in a liquid. When the cavitation bubbles are violently collapsing by the ultrasound, the insides of the bubbles reach more than several thousand degrees and several hundred atmospheres (24, 25). In general, sonochemical degradation of organic pollutants proceeds via reaction with OH radicals, which are formed from water pyrolysis in the collapsing hot bubbles. In the case of volatile or hydrophobic pollutants, the degradation proceeds not only via OH radical reaction but also via a direct pyrolysis reaction inside and in the vicinity of the collapsing bubbles. In this study we investigated the decomposition of PFOS and PFOA in aqueous solutions by sonochemical action based on the rates, products, and possible mechanisms of the sonochemical degradations.

Experimental Section Materials. Perfluorooctanoic sulfonate sodium salt (PFOS), PFOA, tridecafluoroheptanoic acid, undecafluorohexanoic acid, nonafluorovaleric acid, pentafluoropropionic acid, trifluoroacetic acid sodium salt, and 1-octanesulfonic acid sodium salt were purchased from Tokyo Kasei Kogyo (Tokyo, Japan). Heptafluorobutyric acid was obtained from Nakalai tesque (Kyoto, Japan). Ammonium acetic acid, n-octanoic acid, and acetonitrile (HPLC grade) were obtained from Wako Pure Chemical Industries (Osaka, Japan). Standard stock solutions (1000 mg/mL) were prepared by dissolving solid standards in methanol. Eight working standard solutions were prepared by dilutions in water to a concentration range from 1 ng/mL to 10 mg/mL. Sonochemical Procedures. The ultrasonic apparatus consisted of an ultrasonic generator and an oscillator operating at 200 W (Kaijo model 4021, 200 kHz). The average power introduced to the reactor was 3 W/cm2. The temperature of the solution was kept at 20 °C by a cooling water bath during sonication. The PFOS or PFOA aqueous solutions (60 mL, 10 or 100 mg/L) were sonicated in a cylindrical glass vessel. The bottom of the vessel was made as thin as possible (1 mm) for effective transmission of the ultrasonic waves. The vessel was fixed at 4.0 mm from the oscillator and closed during sonication. The sidearm of the vessel was used for argon bubbling to irradiate under an argon atmosphere and for withdrawing liquid samples. Two milliliters of the test solution was extracted from the vessel at irradiation times of 0, 5, 10, 20, 30, 45, and 60 min. ESI/MS Analysis. The scan range of the mass spectra ranged from 100 to 480 (for PFOS), from 100 to 410 (PFOA), from 50 to 190 (1-octanesulfonic acid), and from 50 to 140 (n-octanoic acid) for the easy detection of the product peaks without any strong peaks of the substances. The solutions were injected by a pump attached to the LC/MS system, and the flow rate was 10 µL/min. The scan time was 1.0 s, and the mass spectra were obtained from the average of 30 scans. In the ESI/MS measurement for the PFOS and PFOA solution before ultrasonic irradiation the same ESI conditions were used. 10.1021/es040342v CCC: $30.25

 2005 American Chemical Society Published on Web 03/30/2005

LC/MS/MS Analysis. The PFOS and PFOA in the solution were analyzed using a LC/MS/MS technique. The analytical procedure was the same as that previously reported (26). Liquid chromatography was carried out on a HPLC apparatus equipped with an Agilent model 1100 series (Agilent, Yokogawa Analytical Systems, Tokyo, Japan). A TOSOH TSKGEL ODS-80TsQA (Tosoh, Tokyo, Japan: 5 µm particle size, 2.0 × 150 mm i.d.) was used for the LC separation of PFOS and PFOA. The HPLC separation was carried out at 40 °C using a gradient composed of solution A (1 mM ammonium acetate solution adjusted to pH 4 by the addition of acetic acid) and solvent B (methanol). The gradient expressed as changes in solvent B was as follows: 0-10 min, a linear increase from 50% to 90% B; 10-20 min, hold at 90% B. The flow rate was 0.2 mL/min. The ESI-MS/MS analyses were performed using a MDS-Sciex API2000 (Sciex, Applied Biosystems Japan, Tokyo, Japan). Ionization of the analytes was achieved by electrospray in the negative-ion mode. The electrospray conditions were as follows: nitrogen curtain gas, 35 L/min; ion-spray voltage, -3800 V; declustering voltage, -26 and -21 V; the collision energy,-90 and -30 eV for PFOS and PFOA, respectively. The LC/MS/MS acquisition was performed in the multiple reaction monitor (MRM) mode by following the reactions m/z 499-80 characteristic of PFOS and m/z 413-169 characteristic of PFOA. The sonochemical products of PFOS and PFOA were also measured by LC/MS/MS. The LC and electrospray conditions were the same as those used in the PFOS and PFOA analysis. The LC/MS/MS acquisition was performed in MRM mode by following the reaction m/z 363-319 (tridecafluoroheptanoic acid), m/z 313-269 (undecafluorohexanoic acid), m/z 263-219 (nonafluorovaleric acid), m/z 213-169 (heptafluorobutyric acid), m/z 163-119 (pentafluoropropionic acid), and m/z 113-69 (trifluoroacetic acid). A stock solution at a concentration of 100 mg/L was prepared by dissolution of these compounds in water. Working standards were prepared on a daily basis by dilution of the stock solution with water. Calibration curves for perfluorocompounds at concentrations from 0.5 to 10 mg/L were constructed, and a good linearity was observed. The detection limits of these compounds determined for a signal-to-noise ratio of 3:1 in the MRM ranged from 0.01 to 0.5 mg/L. 1-Octanesulfonic acid and n-octanoic acid were analyzed by LC/MS. When working in the selected-ion monitoring mode (SIM), m/z 193 for 1-octanesulfonic acid and m/z 143 for n-octanoic acid were monitored. The calibration curves of the analytes were obtained before and after analysis of samples, and we checked the linear correlation (>0.99) and confirmed that there were no significant differences of the inclination of calibration curves. Ion Chromatography. Ion chromatography was performed using a LC-10A system and CDD-10Avp conductivity detector (Shimadzu, Kyoto, Japan). Shim-pack IC-AC (Shimadzu, Kyoto, Japan: 5 µm particle size, 4.6 × 150 mm i.d.) was used for the separation of ion products. The IC separation was carried out at 40 °C. The eluents were 8.0 mM phydroxybenzoic acid and 3.2 mM bis(2-hydroxyethyl)iminotris(hydroxymethyl)methane, and the flow rate was 1.5 mL/ min.

Results and Discussion Sonochemical Degradation of PFOS and PFOA. The degradation of PFOS was carried out at an initial concentration of 10 mg/L under an air and argon atmosphere. Figure 1a shows the time dependence of the sonochemical degradation of PFOS. The decomposition of PFOS showed the linearity in a pseudo-first-order plot as shown in Figure 1b, and this indicates that the reaction follows pseudo-first-order kinetics. The decomposition of the substrate (%), rate constants, and

FIGURE 1. (a) Changes in the concentration of PFOS during sonolysis. (b) Pseudo-first-order plots of degradation of PFOS with irradiation time: (b) PFOS sonication, under air atmosphere; (O) PFOS, sonication under argon atmosphere; (2) PFOA, formation under air atmosphere; (4) PFOA, formation under argon atmosphere. half-life times by sonication under various conditions are summarized in Table 1. Twenty-eight percent of the PFOS was decomposed by ultrasonic irradiation (60 min) under an air atmosphere (Table 1, entry 1), and PFOA was formed by the reaction. The formation of PFOA was confirmed by observation of the LC/MS/MS peak at the same retention time as a PFOA standard solution. The sonochemical degradation of PFOS was accelerated by argon saturation (Table 1, entry 2). It is well known that the presence of argon gas produces a higher temperature and gives a high reaction yield during the sonochemical reaction because of the high polytropic index of argon gas. For the PFOS degradation the degradation rate under argon was observed to be approximately 2 times as fast as that under air. The reacted solutions were reduced from 60 to 46 mL by sampling for observation of the change of substrates. In the case of the sonochemical decomposition of PFOS without sampling, the yield of decomposition (57%) was nearly the same as that with sampling (60%). The volume of the solution would affect the reaction rates, but the significant influence was not observed under the conditions used in this study. The sonolysis of PFOS in the presence of tert-butyl alcohol (10 mM), which is known as a radical scavenger (27, 28), was carried out in order to evaluate the radical reaction (Table 1, entry 3). The degradation of PFOS in the presence of tertbutyl alcohol was suppressed about 12% after a 60 min sonolysis. Formation of PFOA from PFOS was observed in the presence of tert-butyl alcohol. The yield of PFOA (6%) was similar to that in the absence of tert-butyl alcohol (7%). These facts suggest that the sonochemical degradation of PFOS is mainly promoted by thermal decomposition. Similar to PFOS, PFOA was also decomposed by ultrasonic irradiation. The results are shown in Figure 2 and entries 4 and 5 in Table 1. The rate constants of the PFOA degradation were 2 times higher than those of PFOS under corresponding conditions. The saturated argon gas accelerated the sonochemical degradation of PFOA similar to the PFOS degradation. As shown in Figure 1, the formation of PFOA by the PFOS degradation was decreased by the argon saturation. This could be explained by the fact that the formed PFOA undergoes further degradation more efficiently under an argon atmosphere. For the tert-butyl alcohol (10 mM) addition the pseudo-first-order plots of the PFOA degradation did not show any linearity by which the degradation seems to be accelerated. Ninety-eight percent of the PFOA was decomposed by ultrasonic irradiation (60 min) in the presence of tert-butyl alcohol. This might indicate that PFOA reacted with tert-butyl alcohol. Therefore, it was impossible to evaluate the radical reaction during the PFOA degradation with the addition of tert-butyl alcohol. Sonochemical Products of PFOS and PFOA. To observe the products of the PFOS and PFOA degradations, the mass spectra of the sonicated solutions were obtained using the electrospray ionization/mass spectrometry (ESI/MS) techVOL. 39, NO. 9, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Rate Constants and Half-Life Times of PFOS and PFOA Degradation by Ultrasonic Irradiation (irradiation time: 60 min) entry

compounds

atmosphere

decomposition of substratea (%)

rate constantb (min-1)

half-life time (min)

1 2 3 4 5

PFOS PFOS PFOS PFOA PFOA

air argonc argonc,d air argonc

28 60 48 63 85

0.0068 0.016 0.012 0.0155 0.032

102 43 58 45 22

a The initial concentration was 10 mg/L. b Pseudo-first-order constant of degradation. c Argon gas was saturated in the solution. d In the presence of tert-butyl alcohol (10 mM).

FIGURE 2. (a) Changes in the concentration of PFOA during sonolysis. (b) Pseudo-first-order plots of degradation of PFOA with irradiation time: (2) PFOA, sonication under air atmosphere; (4) PFOA, sonication under argon atmosphere. nique. Figure 3 shows the mass spectra and assigned ionic compounds of the PFOS and PFOA solution after ultrasonic irradiation for 60 min. In the ESI/MS measurement for the PFOS and PFOA solution before ultrasonic irradiation under the same ESI conditions only peaks of PFOS and PFOA were observed, and the ions shown in Figure 3 were not obtained at all. In addition, in the other perfluorocompounds, such as C8F17COOH, the fragmentation could not be observed under the same ESI/MS condition. On the basis of these results it was considered that the peaks shown in Figure 3 were not formed by fragmentation of the substrates and the sonochemical products at the interface of the ESI/MS. The PFOS degradation solution gave the ion peak of PFOA (m/z 413) and peaks at m/z 363, 313, 263, 213, 163, and 113. The m/z differences between the neighbor peaks are 50. These peaks can be assigned to CF3(CF2)nCOO- (n ) 0-6) ions because the formula weight of CF2 is 50. In addition, the m/z 449 and 399 ions, which were assigned as the C7F15SO3- and C6F13SO3- ions, were observed. It could be envisioned that the ions were formed by dissociation of CF2 from PFOS. As for PFOA, the peaks, which can be assigned to CF3(CF2)nCOO(n ) 0-5) ions, were observed similar to PFOS. ESI/MS can detect only polar compounds having a molecular weight in the applied mass range. Therefore, it should be noted that there is the possibility that other products exist but are not shown in Figure 3. Next, the sonicated solution (initial concentration; 100 mg/L) was analyzed by LC/MS/MS to ascertain the formation of CF3(CF2)nCOO- (n ) 0-5) ions through PFOS and PFOA sonication. The ion peaks of perfluorocarbon acids were observed at the same retention time as the standard solution, and it was confirmed that those compounds were produced through sonochemical degradation of PFOS and PFOA. The time dependences of the concentrations of the sonochemical products are shown in Figure 4a and b for PFOS and PFOA, respectively. The concentrations of the product ions, CF3(CF2)nCOO- (n ) 4-6), increased during 0-20 min and decreased after 30 min. On the other hand, trifluoroacetic acid and pentafluoropropionic acid increased over time. This could be explained by the fact that the formed perfluorocarbonic acids undergo a further degradation and shortening of the perfluorocarbon chain. 3390

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Ion Products Formed from PFOS and PFOA Decomposition. The sonicated solutions (60 min irradiation, argon atmosphere) were analyzed using ion chromatography. A fluoride ion was detected from both sonicated PFOS and PFOA solutions, and the concentrations were 3.3 mg/L for PFOS and 5.0 mg/L for PFOA. Furthermore, the concentration of SO42- ion was 0.95 mg/L in the irradiation of PFOS solution. The concentration of each ion product increased linearly with the irradiation time. The pH of solutions of PFOS and PFOA was changed from 4.8 to 3.5 and from 4.7 to 3.6 by sonication, respectively. Though the values of pKa of PFOS and PFOA have been not clear, PFOS and PFOA are considered to be very strong acids and will be present in the solution at a pH range from 3 to 5 completely in the ionized form. Therefore, it was thought that decomposition by sonication would not be influenced by pH (29, 30). Fenton Reaction. The Fenton process was attempted to destroy PFOS and PFOA. The Fenton reagents (0.2 mM, Fe2+/ H2O2 molar ratio ) 1:1) were added into the PFOS and PFOA solution (0.02 mM). However, degradation of PFOS and PFOA was not observed. These facts indicate that the sonochemical decomposition of PFOS and PFOA needed not only the attack of the hydroxy radical but also high temperatures and high pressures produced by sonication. Sonochemical Degradation of 1-Octanesulfonic Acid (OS) and n-Octanoic Acid (OA). The sonochemical degradation of OS and OA was observed in order to explore the difference of the sonochemical reactivity between perfluoroalkanoic acids and alkanoic acids. The degradation of OS and OA was carried out at an initial concentration of 20 µM, which is the same as the degradation of 10 mg/L PFOS. Figure 5 shows the time dependence of the sonochemical degradation of OS and OA under an argon atmosphere. The rate constants and the half-life times by sonochemical degradation of OS and OA are summarized in Table 2. Ninety nine percent of OS and OA were decomposed by ultrasonic irradiation (60 min). The rate constant observed for the degradation of OS is approximately 32 times faster than that of PFOS, and the rate constant of the OA degradation was 15 times as high as that of PFOA under corresponding conditions. This fact indicates that the inertness of the fluorocarbon chain has much effect on the rate of sonochemical degradation of PFOS and PFOA. In the ESI/MS measurement for the OS and OA solution the ion peaks at m/z 143, 129, 115, 101, and 87 corresponding to CH3(CH2)nCOO- (n ) 3-7) ions were observed. It could be envisioned that the ions were formed by dissociation of CH2 from OA, and the process forthe sonochemical reaction of OS and OA would be similar to those of PFOS and PFOA. Mechanisms of the Sonochemical Degradation of PFOS and PFOA. On the basis of the obtained facts we outlined the possible mechanisms of the ions detected by ESI/MS and ion chromatography. The formation of PFOA during PFOS degradation may be due to oxidation after dissociation of the SO3- group, and the dissociated SO3- group will be oxidized to SO42- by sonication. The generated PFOA would undergo shortening of the perfluorocarbon chain caused by

FIGURE 3. (a) ESI-MS spectrum of the PFOS solution after ultrasonic irradiation (60 min). A negative-ion mode was used. Scan rage: 100-480. (b) ESI/MS spectrum of the ion peaks assigned as products of the PFOA degradation by ultrasonic irradiation (60 min). A negative-ion mode was used. Scan range: 100-410.

FIGURE 4. Changes in the concentration of CF3(CF2)nCOO- (n ) 0-5) ions through the PFOS (a) and PFOA (b) sonication.

FIGURE 5. Changes in the concentration of OS and OA during sonolysis under argon: (×) OS, (O) OA. Initial concentration: 20 µM.

TABLE 2. Rate Constants and Half-Life Times of OS and OA Degradation by Ultrasonic Irradiation entry

compounds

decomposition of substratea (%)

rate constantb (min-1)

half-life time (min)

1 2

OS OA

99 99

0.22 0.22

3.2 3.2

a The initial concentration was 20 M. b Pseudo-first-order constant of degradation. Calculated using the decomposition data during the irradiation time: 0-20 min.

repetition of the COO- dissociation and the oxidation of the generated ions or radicals of the perfluorocarbon. The formation of CF3(CF2)6SO3- and CF3(CF2)5SO3- from PFOS would be due to recombination of the fissional fluorocarbon group with SO3- group. The sites of the sonochemical decomposition of PFOS and PFOA would be considered as follows according to the cavitation phenomena. There are three different regions in an aqueous solution system. (1) The inside of the collapsing

cavitation bubbles where several thousands of degrees and more than hundreds of atmospheres are produced. Here, water vapor is pyrolyzed into H atoms and OH radicals. (2) The interfacial region between the cavitation bubbles and the bulk solution where the temperature is lower than the inside of the cavitation bubbles but still high enough for thermal decomposition of the solutes to occur. (3) The bulk solution at ambient temperature where the reactions of the solute molecules with various radicals and unstable species, which escaped from the interfacial region, take place. Because both of the PFOS and PFOA molecules have a hydrophobic group (perfluoroalkyl group) and a hydrophilic group (acid group), they behave like an anionic surfactant in an ultrasonic field. Recently, it has been reported that the surfactants in water are decomposed by reaction with OH radicals and the pyrolysis reaction (31, 32). Since PFOS and PFOA are nonvolatile molecules, the reaction in the cavitation bubbles should be excluded. Furthermore, on the basis of the preliminary experiments of Fenton reactions, it is suggested that PFOS and PFOA are not decomposed by reaction with OH radicals. Therefore, it could be concluded that most of the PFOS and PFOA molecules are pyrolyzed at the interfacial region between the cavitation bubbles and the bulk solution. This research showed the degradation of PFOS and PFOA by ultrasonic irradiation. The sonochemical reactions of PFOS and PFOA would contribute to the cleanup of these compounds from the environment and the lowering of the toxicity of these compounds. Ohmori et al. reported that perfluorocarboxylic acids having shorter carbon chain length showed higher total clearance in rats (33).

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Received for review January 29, 2004. Accepted January 3, 200520052005. ES040342V