Environ. Sci. Technol. 2010, 44, 432–438
Sonochemical Degradation of Perfluorooctanesulfonate in Aqueous Film-Forming Foams CHAD D. VECITIS,† YAJUAN WANG,‡ JIE CHENG,§ HYUNWOONG PARK,| BRIAN T. MADER,⊥ AND M I C H A E L R . H O F F M A N N * ,§ Department of Chemical Engineering, Environmental Engineering Program, Yale University, P.O. Box 208286, New Haven, Connecticut 06520, Department of Chemical Physiology, Department of Molecular and Experimental Medicine, and The Skaggs Institute for Chemical Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, W. M. Keck Laboratories, California Institute of Technology, Pasadena, California 91125, School of Physics and Energy Science, Kyungpook National University, Daegu, 702-701, Korea, and 3M Environmental Laboratory, 3M Center, Building 260-05-N-17, Maplewood, Minnesota 55144-1000
Received August 17, 2009. Revised manuscript received October 22, 2009. Accepted November 2, 2009.
Aqueous film-forming foams (AFFFs) are fire extinguishing agents developed by the Navy to quickly and effectively combat fires occurring close to explosive materials and are utilized today at car races, airports, oil refineries, and military locations. Fluorochemical (FC) surfactants represent 1-5% of the AFFF composition, which impart properties such as high spreadability, negligible fuel diffusion, and thermal stability to the foam. FC’s are oxidatively recalcitrant, persistent in the environment, and have been detected in groundwater at AFFF training sites. Ultrasonic irradiation of aqueous FCs has been reported to degrade and subsequently mineralize the FC surfactants perfluorooctanoate (PFOA) and perfluorooctanesulfonate (PFOS). Here we present results of the sonochemical degradation of aqueous dilutions of FC-600, a mixture of hydrocarbon (HC) and fluorochemical components including cosolvents, anionic hydrocarbon surfactants, fluorinated amphiphilic surfactants, anionic fluorinated surfactants, and thickeners such as starch. The primary FC surfactant in FC-600, PFOS, was sonolytically degraded over a range of FC-600 aqueous dilutions, 65 ppb < [PFOS]i < 13 100 ppb. Sonochemical PFOS-AFFF decomposition -PFOS -PFOS , are similar to PFOS-Milli-Q rates, RMQ , rates, RAFFF indicating that the AFFF matrix only had a minor effect on the -PFOS -PFOS /RMQ < 2.0, sonochemical degradation rate, 0.5 < RAFFF even though the total organic concentration was 50 times the PFOS concentration, [Org]tot/[PFOS] ∼ 50, consistent with the superior FC surfactant properties. Sonochemical sulfate production is quantitative, ∆[SO42-]/∆[PFOS] g 1, indicating that bubble-water interfacial pyrolytic cleavage of the C-S bond in PFOS is the initial degradation step, in agreement with previous * Corresponding author e-mail:
[email protected]. † Yale University. ‡ The Scripps Research Institute. § California Institute of Technology. | Kyungpook National University. ⊥ 3M Center. 432
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studies done in Milli-Q water. Sonochemical fluoride production is significantly below quantitative expectations, ∆[F-]/ ∆[PFOS] ∼ 4 vs 17, suggesting that in the AFFF matrix, PFOS’ fluorochemical tail is not completely degraded, whereas Milli-Q studies yielded quantitative F- production. Measurements of time-dependent methylene blue active substances and total organic carbon indicate that the other FC-600 components were also sonolytically decomposed.
Introduction AFFFs were developed in the 1960s for the Navy as powerful fire-extinguishing agents for use on hydrocarbon fires occurring near explosive materials to quickly and effectively combat fire (1). AFFFs are now utilized at oil refineries, airports, automotive races, and military locations. Fluorochemical (FC) and hydrocarbon surfactants are the primary “active” components of the AFFF mixture. However; FCs are oxidatively stable (2) and have been detected in ground and surface waters near military fire-fighting training sites (3-7) (see Table S1 of the Supporting Information for concentrations). Fluorochemical (FC) surfactants comprise 1-5% w/w of the AFFF. The low FC surface tension [15-20 vs 30 mN/m for hydrocarbons (HC)] gives the foam a high spreadability on a hydrocarbon fuel (8). FC interfacial films reduce hydrocarbon fuel diffusion through the foam to negligible levels (9, 10) due to the immiscibility of FCs and HCs. The chemical and thermal stability of FCs yield a more thermally resistant foam. Studies have suggested replacement of FCs in AFFF formulations to reduce environmental impact (11); however, development of containment and on-site treatment technologies rather than removing FCs from AFFF formulations may allow for continued use of the highly effective FC-based fire-fighting product. Thus, it is desirable to develop technologies to treat waters with significant levels of fluorochemicals. Fluorochemicals are recalcitrant toward conventional organic degradation technologies such as biological remediation (12, 13) and ozone/hydroxyl radical oxidation (14). Persulfate photolysis or thermolysis can degrade perfluoroalkylcarboxylates (PFCs) (15-17) and fluorotelomers (18). PFCs complexed with ferric iron can be photolytically oxidized (19). Both PFCs and perfluoroalkylsufonates (PFSs) can be oxidized at boron-doped diamond (20, 21) and TiO2/Ni-Cu anodes (22). PFSs can be reductively degraded using Fe(0) in subcritical water (23) and by UV-alkaline IPA photolysis (24). Both PFCs and PFSs can be reductively degraded during UV-KI photolysis (25). Ultrasonic irradiation of aqueous PFCs and PFSs yields in situ thermal decomposition of these species (26) and shortly thereafter the transformation into their inorganic constituents; F-, SO42-, CO, and CO2 (27). Ultrasonic irradiation of aqueous solutions produces sonochemistry via cavitation (28). The ultrasonic pressure waves interact with the cavitated bubbles, causing highvelocity radial oscillations, which collapse quasiadiabatically during the compression period, producing bubble vapor temperatures near 4000 K (29, 30) and bubble interface temperatures near 1000 K (31, 32), which will pyrolytically degrade contaminants such as CCl4 (33). The high bubble vapor temperatures will homolytically split water vapor contained within the bubble to produce H-atoms and hydroxyl radicals (34, 35), which can degrade aqueous contaminants such as phenol and dyes (36, 37). Ultrasonic irradiation of aqueous solutions is effective toward fluorochemical degradation (38). Sonochemistry has been observed to be effective over a wide range of FC 10.1021/es902444r
2010 American Chemical Society
Published on Web 12/04/2009
concentrations, 10 ppb to 1000 ppm (39). FC sonochemical kinetics is only impacted by relatively high coconcentrations of semivolatile organics, 0.4 < k′/k0 < 1 when 102 < [SVOC]/ [PFS] < 103 (40). Thus, sonochemical treatment may be appropriate for the degradation of AFFF-based FCs, which have been detected over a range of concentrations in the environment and contain higher concentrations of organic species. Herein, we report on the sonochemical degradation of PFOS in aqueous dilutions of FC-600, an AFFF formulation. Sonochemical PFOS degradation kinetics is determined over a range of aqueous FC-600 dilutions. Time-dependent fluoride and sulfate concentrations are also monitored to determine solution matrix effects on the PFOS sonochemical degradation mechanism. Measurement of time-dependent methylene blue active substances (MBAS) and total organic carbon (TOC) are used to evaluate the sonochemical degradation of organics within the FC-600 matrix.
Experimental Details FC-600 (AFFF) concentrates and HPLC-MS standards containing perfluorooctanesulfonate (PFOS), perfluorooctanoate (PFOA), perfluorohexanesulfonate (PFHS), perfluorohexanoate (PFHA), perfluorobutanesulfonate (PFBS), and perfluorobutanoate (PFBA) were provided by 3M. Ammonium acetate (>99%), sodium fluoride (>99%), sodium sulfate (>99%), and methanol (HR-GC, >99.99%) were obtained from EMD Chemicals Inc. Potassium hydrogen phthalate (>99%) and chloroform were obtained from Sigma-Aldrich. Aqueous dilutions of the AFFF concentrate were prepared with purified water from a Millipore Milli-Q system (18.2 MΩ cm resistivity). Ultrasonic waves were generated with an Undatim ultrasonic transducer of frequency f ) 505 kHz (A ) 25 cm2, Undatim Ultrasonics S.A., Louvain-La-Neuve, Belgium) attached to a 400 mL jacketed glass reactor containing the target aqueous solutions. The applied power and power density, PD, were 75 and 187.5 W L-1, and the calorimetric power density was determined to be 75 ( 5% of the applied power density. The applied power intensity, I, was 3 W cm-2. The temperature was controlled at 10 °C with a Haake A80 refrigerated bath; the actual solution temperature was 2-3 °C greater. All reaction solutions were sparged with argon for at least 30 min prior to and during the ongoing sonochemical reactions unless otherwise noted. Aqueous-phase concentrations of PFOS, PFOA, PFHS, PFHA, PFBS, and PFBA were quantified using HPLC-ES-MS. The MBAS test (41) was used to quantify the total anionic surfactant concentration in solution. A Dionex DX-500 ion chromatograph was used for the analysis of fluoride and sulfate. Total organic carbon was determined (OI Analytical Aurora model 1030) with an autosampler (OI Analytical model 1096). Details of the analytical procedures can be found in the Supporting Information.
Experimental Results and Discussion AFFF (FC-600) Compositional Analysis. The AFFF provided by 3M was a standard commercial FC-600 formulation. The FC-600 concentrate received was a foamy, orange gel, which is diluted 20 times with fresh water or seawater to ∼6% of the initial concentrate for fire extinguishing use. Listed in Table 1 are the FC-600 MSDS reported concentration ranges for most of the components. A quantitative analysis of the FC concentrations and a semiquantitative analysis of other FC-600 components were also conducted. HPLC-MS was used to identify anionic surfactants present in the FC-600 and to quantify the FC surfactants. Three alkyl sulfate esters, octyl sulfate [OS, CH3(CH2)7OSO3-], decyl sulfate [DS, CH3(CH2)9OSO3-], and dodecyl sulfate [DoDS, CH3(CH2)11OSO3-]; three perfluorinated sulfonates,
TABLE 1. Analytically Determined FC-600 Composition with Respect to [PFYX] concentration fluorochemical
ppm
mM
PFOS PFHS PFBS PFOA PFHA total PFYX
3650 ( 710 820 ( 140 254 ( 13 161 ( 33 53 ( 13 4940 ( 910
7.3 ( 1.4 2.1 ( 0.4 0.85 ( 0.04 0.39 ( 0.08 0.17 ( 0.04 10.8 ( 2.0
perfluorobutanesulfonate [PFBS, CF3(CF2)3SO3-], perfluorohexanesulfonate [PFHS, CF3(CF2)5SO3-], and perfluorooctanesulfonate [PFOS, CF3(CF2)7SO3-]; and two perfluorinated carboxylates, perfluorohexanoate [PFHA, CF3(CF2)4CO2-] and perfluorooctanoate [PFOA, CF3(CF2)6CO2-], were identified by HPLC-MS. An LC chromatogram of the MS peaks is shown in Figure 1. An eight-point calibration curve from 1 to 250 ppb was used to quantitate the FC surfactants present after serial dilutions of the AFFF concentrate. PFOS was determined to be the dominant FC surfactant in the FC-600 concentrate; [PFOS] ) 3650 ( 710 ppm (7.3 ( 1.4 mM) and ∑[PFYX] ) 4940 ( 910 ppm (10.8 ( 2.0 mM). For the FC-600 perfluorinated anionic surfactant concentrations listed in Table 1, PFYX will be used as a general term to denote these surfactants with Y ) octane (O), hexane (H), and butane (B) and X ) sulfonate (S) and acid (A) or in this case the deprotonated acid. The MBAS test was used to analyze the total FC-600 anionic surfactant concentration, [MBAS-]. The MBAS test is only active toward species that are strongly ionized in solution. At circumneutral pH (7-8), the alkyl sulfate esters (pKa ) -3 to -4), perfluorinated carboxylates (pKa < 0) (41), and perfluorinated sulfonates (pKa ) -3 to -4) are all active toward the MBAS test. Total anionic surfactant concentrations in the FC-600 concentrate were determined to be 89.1 ( 6.5 mM. The total alkyl sulfate ester concentration, [AS-], is the difference between the MBAS anionic surfactant concentration, [MBAS-], and the total LC-MS FC surfactant concentration, ∑[PFYX], eq 1. [AS-] ) [MBAS-] -
∑ [PFYX]
(1)
The total alkyl sulfate ester concentration was determined to be 78.3 ( 8.5 mM, which is 7.3 times greater than the measured total FC surfactant concentration and 10.7 times greater than the measured PFOS concentration. If the alkyl sulfate esters are assumed to be an equimolar mixture of the C8, C10, and C12 species observed by LC-MS (i.e., 〈MW〉 ) 237 g/mol), then the measured concentration would be 23,000 ppm AS-. This value falls within the MS-DS reported range for alkyl sulfate salts of 1-5%. The total nonfluorinated organic concentration was determined by TOC analysis. Thermally activated persulfate can oxidize perfluorinated organics (16); however, the
FIGURE 1. HPLC-MS chromatogram of prominent peaks in negative ion mode. VOL. 44, NO. 1, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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characteristic oxidation half-life, 30-60 min, is much longer than that utilized in a TOC analyzer, 1-2 min. Therefore, the TOC measurement will be a sum of the nonfluorinated carbon in the AFFF. The measured TOC in the FC-600 concentrate that was diluted 500 times with water was 272 ( 20 ppm, which corresponds to 13.6 ( 1.0% w/w in the concentrate. Total organic carbon estimates using MSDS values yield a TOC range of 10 to 15% w/w, which is consistent with the measured value. The FC-600 fluoride, F-, and sulfate, SO42-, were determined by ion chromatography to be 0.83 ( 0.05 and 3.4 ( 0.2 ppm, respectively, in a 500 times dilution of FC600 concentrate with water. This corresponds to [F-] ) 420 ( 20 ppm and [SO42-] ) 1,700 ( 100 ppm in the FC600 concentrate. The FC-600 MSDS chemical composition, formula, structure, function, and measured composition can be found in Table 2. Sonochemical Degradation of PFOS in Aqueous Dilutions of FC-600. PFOS, PFHS, PFBS, and PFOA are observed to be sonochemically degraded in a 1/5000 dilution of the FC600 concentrate with water (f ) 505 kHz, PD ) 188 W L-1, I ) 3 W cm-2, 10 °C, argon, [PFOS]i ) 1110 ( 460 ppb, [PFHS]i ) 210 ( 16 ppb, [PFBS]i ) 80 ( 4 ppb, and [PFOA]i ) 29.5 ppb). The normalized time-dependent sonolytic decomposition of PFOS, PFHS, and PFBS are depicted in Figure 2a. PFBS sonolytically decomposes following eq 2, where kPFBS ) 0.43 ppb min-1 (R2 ) 0.993). d[PFYX] ) -kPFYX dt
(2)
PFHS sonolytically decomposes following eq 3, where kPFHS′ ) 0.0089 min-1 (R2 ) 0.992).
d[PFYX] ) -kPFYX′[PFYX] dt
(3)
PFOS sonochemical decomposition kinetics could not be accurately fit by eq 2 or 3, but a linear combination of the two, eq 4. [PFYX]t /[PFYX]i ) (1 - a) + a exp(-kPFYX′t) + kPFYXt (4) The time-dependent PFOS concentration is fit well with eq 4 where a ) 0.76, kPFOS′ ) 0.075 min-1, kPFOS′ ) 0.0015 min-1, and R2 ) 0.991. In Figure 2b, the normalized sonochemical degradation kinetics of PFOS in a range of aqueous FC-600 dilutions, from 250× to 50,000× ([PFOX]i ) 65 ( 17, 845 ( 45, 5,500, and 13,100 ( 750 ppb is presented; ultrasound conditions are the same as Figure 2a). Sonochemical degradation of the 1 /50 000 FC-600 dilution follows eq 2, with kPFOS ) 0.34 ppb min-1 (R2 ) 0.97). The sonochemical decomposition of the 1 /5000 and 1/500 dilutions follows eq 4, where a ) 0.65/0.61, kPFOS′ ) 0.11/0.08 min-1, and kPFOS′ ) -0.0024/0.0024 min-1, respectively (R2 ) 0.996/0.997). Sonolysis of the 1/250 aqueous FC-600 dilution follows eq 3, with kPFOS′ ) 0.010 min-1 (R2 ) 0.997). In Figure 2c, -d[PFOS]/dt (nM min-1) is plotted as a function of [PFOS]i (nM). d[PFOS]/dt is taken as kPFOS or kPFOS′[PFOS]i. The PFOS concentration-dependent sonochemical kinetic saturation in the AFFF matrix (O, 515 kHz, 180 W L-1, 10 °C, argon) qualitatively agrees with observations in “pure” water (0, 358 kHz, 250 W L-1, 10 °C, argon) (39). The absolute sonochemical degradation rate initially increases with increasing [PFOS]i until kinetic saturation at -PFOS -PFOS < RMQ , and [PFOS]i ∼ 10 µM. When [PFOS]i < 10 µM, RAFFF
TABLE 2. FC-600 Components: Structures, Properties, and Concentrationsd
a from MS-DS. b from HPLC-MS, PFOS is the dominant FC anionic surfactant. c from HPLC-MS and MBAS; assumes HC surfactants are of equal concentration and they have an average MW (decyl sulfate) ) 237. d Concentrate was diluted 20 times (or to 6%) prior to fire-extinguishing use.
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occurs predominantly at the bubble-water interface. The curves in Figure 2c are fits to the Langmuir-Hinshelwood model, eq 5, which does not take into account competition for interfacial sites (see eq 10).
R-PFOS )
FIGURE 2. Sonochemical degradation of fluorochemicals in aqueous dilutions of FC-600. Ultrasonic conditions are f ) 505 kHz, PD ) 188 W L-1, I ) 3 W cm-2, 10 °C, argon. (A) Time-dependent concentration of PFOS, PFHS, and PFBS in a 1/5000 aqueous dilution of FC-600. [PFOS]i ) 1110 ( 460 ppb (O), [PFHS]i ) 210 ( 16 ppb (0), and [PFBS]i ) 80 ( 4 ppb (3). (B) Time-dependent concentration of PFOS in a range of FC-600 aqueous dilutions. [PFOS]i ) 65 ( 17 (O), 845 ( 45 (3), 5500 (]), and 13100 ( 750 ppb (4). (C) d[PFOS]/dt vs [PFOS]i in a pure Milli-Q matrix and a diluted FC-600 matrix. PFOS in Milli-Q (0) and corresponding fit (-) to eq 5, where Vmax ) 230 nM min-1 and Ksono ) 120 000 M-1; data from ref 39. PFOS in FC-600 (b) and corresponding fit (-) to eq 5 where Vmax ) 350 nM min-1 and Ksono ) 100 000 M-1. -PFOS -PFOS when [PFOS]i > 10 µM, RAFFF > RMQ , and in all cases the -PFOS / difference in rates is less than a factor of 2; 0.5 < RAFFF -PFOS < 2.0. Previous reports on the sonochemical degradaRMQ tion of chlorophenols (42), azo dyes (43), PFOS/PFOA (39), and alkylbenzylsulfonates (39) have also reported saturation kinetics with increasing substrate concentration and attributed it to a Langmuir-type mechanism, where degradation
d[PFOS] -PFOS PFOS Θ ) ) Vmax dt PFOS Ksono [PFOS] -PFOS (5) Vmax PFOS 1 + Ksono [PFOS]
Vmax is the maximum degradation rate when all the air-water interface sites are filled, ΘPFOS is the fraction of PFOS-occupied interface sites, and Ksono is the sonochemical bubble-water interface partitioning coefficient. The solid -PFOS line corresponds to Vmax (MQ) ) 230 nM min-1 and PFOS Ksono (MQ) ) 120 000 M-1, and the dashed line corresponds -PFOS PFOS to Vmax (AFFF) ) 350 nM min-1 and Ksono (AFFF) ) 100 000 M-1. Thus, the AFFF matrix increases the maximum PFOS sonochemical degradation and leads to only a slight depression in the sonochemical bubble-water interface partitioning, which is likely due to competition with other surfactants for interfacial sites. More importantly, even though the total alkyl sulfate ester, [AS-]/[PFOS] ∼ 10, and the total organic carbon concentration, [TOC]/[PFOS] ∼ 50, are much greater than the PFOS concentration, the PFOS sonochemical degradation rates remain relatively unaffected due to the superior FC surfactant properties (44). Sonochemical Production of F- and SO42- from FC-600 and Its Mechanistic Implications. The time-dependent aqueous PFOS [CF3(CF2)7SO3-], fluoride (F-), and sulfate (SO42-) concentrations (Figure 3a,b) during ultrasonic irradiation of an approximately 1/250 dilution of FC-600 with Milli-Q water are plotted in units of micromolar (3a, [PFOS]i ) 19 µM) and parts per million (3b, [PFOS]i ) 13 ppm), respectively. The ultrasonic conditions are the same as those in Figure 2. Both fluoride and sulfate have nonzero initial concentrations and are observed to increase over the entire reaction. The sonochemical PFOS decomposition is best fit to an exponential decay (eq 3), with kPFOS′ ) 0.010 min-1 (R2 ) 0.997). At an initial PFOS concentration of 19 µM, this yields an absolute degradation rate, d[PFOS]/dt ) -0.19 µM min-1. The F- and SO42- time-dependent production were best fit linearly (eq 2), with production rates of d[F-]/dt ) 0.80 µM min-1 (R2 ) 0.97) and d[SO42-]/dt ) 0.19 µM min-1 (R2 ) 0.992). The time-dependent sulfate and fluoride mole balances yield insight into the PFOS sonochemical decomposition mechanism; ∆[SO42-]/∆[PFOS] ) 1.0 and ∆[F-]/∆[PFOS] ) 17 are the stoichiometric values. PFOS sonochemistry in ultrapure deionized water completed under similar conditions ([PFOS]i ) 10 µM, Papp ) 150 W, V ) 0.6 L, 10 °C, argon) yielded ∆[SO42-]/∆[PFOS] > 0.95, and ∆[F-]/∆[PFOS] > 15.5 at all time points indicating that shortly after the initial PFOS molecule decomposed, the fluorinated tail was transformed to its inorganic constituents (27). After 120 min of ultrasonic irradiation, the 1/250 aqueous FC-600 dilution had a sulfur mole balance of ∆[SO42-]/∆[PFOS] ) 1.15 and a fluorine mole balance of ∆[F-]/∆[PFOS] ) 4.68. The diluted FC-600 sulfate mole balance is in agreement with results completed in Milli-Q. In contrast, the FC-600 fluoride mole balance is much lower than that observed in Milli-Q water and 3.63× less than the theoretical value. The consistency of the sulfate results and their equivalence to the stoichiometric value indicate that the -C-S- bond is the first PFOS bond to be sonochemically cleaved. Two possible aqueous mechanisms for -C-S- bond cleavage resulting in the loss of the ionic headgroup are represented in eqs 6 and 7. VOL. 44, NO. 1, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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)))
CF3(CF2)7-SO3- 98 CF3(CF2)5CFdCF2 + SO3F )))
CF3(CF2)7-SO3- + H+ 98 CF3(CF2)6CF2H + SO3
(6)
(7)
eq 8); the electrophilic fluororadicals could then react with a hydrocarbon species (R-H, eq 9) (45, 46) to produce a smaller, nonradical fluorochemical product (RF-H) to be degassed from solution (47). )))
The SO3 or SO3F would hydrolyze immediately to yield SO42-, and the mechanism is thus consistent with observations that ∆[SO42-]/∆[PFOS] g 1. The ratio ∆[F-]/∆[PFOS] was found to be >15.5 during ultrasonic irradiation of PFOS in “pure” water. The primary fluorochemical products (eqs 6 and 7) were quickly degraded through a subsequent series of pyrolytic and combustion reactions to yield CO, CO2, and HF, which was hydrolyzed rapidly to H+ and F-. However, during sonochemical degradation of FC-600, ∆[F-]/∆[PFOS] < 5, even after 120 minutes of sonolysis, indicating that the fluorocarbon tail remains partially intact during the present experiments. The Milli-Q experiments were completed using a closed system, whereas the AFFF experiments were completed on an open system. Thus, if high Henry’s constant fluorochemical intermediates were produced, they could be degassed from solution. They would need to degas quickly, since the sonochemical half-life for the fluorochemical tail was determined to be on the order of 1-2 min (27). For example, the initial fluorochemical intermediate will degrade via a -C-C- bond cleavage to yield two fluoroalkyl radicals (RFC,
CF3(CF2)6CF2H 98 RF1 • + RF2 •
(8)
RF• + R-H f RF-H + R•
(9)
The chemical reaction in eq 9 was not possible during PFOS sonolysis in pure water, since there were no hydrocarbons present. The RF• species produced in eq 8 could also react with unsaturated hydrocarbons (>d 109 M-1 s-1. Thus, in the diluted FC-600 solutions, short chain carboxylate intermediates will have even longer lifetimes as they will preferentially accumulate in the bulk aqueous phase where hydroxyl radical concentrations will be reduced not only due to recombination pathways but also due to competitive scavenging by the higher concentrations of other surface active species in solution. Sonochemical TOC mineralization rates can be increased by simultaneous ozonation (54) and photocatalysis (58).
Acknowledgments This work was supported by the 3M Co. The author’s thank 3M for the donation of the Agilent HPLC-MS ion trap used for all the analyses and for the donation of PFC calibration standards. The authors wish to thank Dr. Nathan Dalleska (Caltech Environmental Analytical Center) for comments and analytical support.
Supporting Information Available Information as outlined in the text. This material is available free of charge via the Internet at http://pubs.acs.org.
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