TiO2-Induced Heterogeneous Photodegradation of a Fluorotelomer

Sep 22, 2006 - Degradation of C4F9C2H4OH in air over TiO2 particles was examined in this .... 20; BET surface area, 50 m2 g-1) was obtained from Nippo...
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Environ. Sci. Technol. 2006, 40, 6824-6829

TiO2-Induced Heterogeneous Photodegradation of a Fluorotelomer Alcohol in Air SHUZO KUTSUNA, YUMIKO NAGAOKA, KOJI TAKEUCHI, AND HISAO HORI* National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba West, 16-1 Onogawa, Tsukuba 305-8569, Japan

Degradation of C4F9C2H4OH in air over TiO2 particles was examined in this first report of gas-solid heterogeneous photochemical degradation of fluorotelomer alcohols (FTOHs), which may be precursors of perfluorocarboxylic acids (PFCAs) in the environment. Photoirradiation (>290 nm) of C4F9C2H4OH in air flowing over TiO2 produced CO2, via C4F9CH2CHO, C4F9CHO, CnF2n+1COF (n ) 2 and/or 3), and COF2, in that order. X-ray photoelectron spectroscopy of the TiO2 surface showed a decrease in the amount of fluorine bonded to carbon and an increase in the amount of Fas the degradation of C4F9C2H4OH in air proceeded. Of the carbon content in the initial C4F9C2H4OH (78.8 ppmv), 90.7% was transformed to CO2, and the predominant fluorine species produced on the TiO2 surface was F-. Fluorotelomer unsaturated acids, which are considered to be toxic and have been observed in the biodegradation of FTOHs, did not form. Increased relative humidity in the air accelerated the decomposition of the reaction intermediates, which led to increased CO2 and F- formation. This result indicates that humidity is a key factor for counteracting FTOHs in indoor air. Although perfluoroalkyl substances such as PFCAs in water reportedly undergo little photodegradation over TiO2, our data show that mineralization of C4F9C2H4OH in air can be achieved with TiO2.

Introduction Perfluorocarboxylic acids (PFCAs), such as perfluorooctanoic acid (C7F15COOH, PFOA), have recently received much attention because they are recognized as ubiquitous contaminants in water, wildlife, and humans (1-3). These compounds have been widely used as emulsifying agents in polymer synthesis, wax additives, surface-treatment agents in the electronic industries, and so forth, because of such characteristics as their high surface-active effect, high thermal and chemical stability, and high light transparency (1, 2). Manufacturing, processing, and waste-disposal sites have been identified as sources of PFCAs in the environment (47), and a reduction of facility emissions is strongly desired (8). To develop a technique for neutralizing stationary sources of PFCAs, we have previously reported that the heteropolyacid photocatalyst H3PW12O40 efficiently decomposes PFCAs in water to F- ions and CO2 at room temperature (9). We have also reported that persulfate (S2O82-) promotes more rapid photochemical decomposition of PFCAs in water (10, 11). PFCAs have been detected not only at industrialized sites but also at remote sites such as the Arctic (12) and Mid* Corresponding author phone: +81-29-861-8161; fax: +81-29861-8866; e-mail: [email protected]. 6824

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Atlantic Ocean (13). It is unlikely that PFCAs are transported to such remote sites via the atmosphere because PFCAs have low pKa values (for example, the pKa value for PFOA is 2.5) and are thus expected to be removed from the atmosphere via wet and dry deposition on a time scale of a few days (14). Research on the origin of PFCAs at remote sites has increasingly focused on indirect sources, that is, volatile fluorochemicals that degrade into PFCAs. Fluorotelomer alcohols (FTOHs), linear fluorinated alcohols with the formula CnF2n+1C2H4OH (where n ) 2, 4, 6, ...), have recently been proposed to be precursors of PFCAs (15). These volatile compounds (vapor pressures, 102-103 Pa) (16, 17) are used as intermediates in the synthesis of a variety of surfactants and polymeric materials (18, 19), and they remain as impurities in these products (15, 20, 21). Smog chamber studies indicate that FTOHs oxidize under experimental conditions to form PFCAs (15, 22, 23), and an atmospheric model indicates that this could be a significant source of PFCAs worldwide (24). Biodegradation studies also indicate that FTOHs transform to PFCAs (19, 20). FTOHs have been detected in the North American troposphere (18), and emission of these species from fluorinated materials (such as surface treatment agents) in indoor environments has been suggested to be the source (21). We report herein the TiO2-induced photocatalytic degradation of a model FTOH, C4F9C2H4OH, in air, and we discuss the degradation mechanism on the basis of time profiles of the products. This is the first report on the degradation of PFCA-related chemicals in the gas phase by means of a photocatalyst, and the data presented here can be expected to assist in the development of a system for counteracting FTOHs in the indoor air of stationary sources, such as manufacturing and processing sites for FTOHs and FTOHrelated fluorinated surfactants in order to reduce emissions of FTOHs from these sites.

Experimental Section Materials. 1H,1H,2H,2H-Nonafluoro-1-hexanol (C4F9C2H4OH, >98%) was purchased from Tokyo Kasei Kogyo Co. (Tokyo, Japan). 3-Perfluoropropyl-3-fluoro-(Z)-propenoic acid (C3F7CFdCHCOOH, >95%) was obtained from Wako Pure Chemical Industries (Osaka, Japan). COF2 and C2F5COF were obtained from SynQuest Laboratories (Alachua, FL). Powdered TiO2 (Degussa P25; anatase/rutile ratio, 80: 20; BET surface area, 50 m2 g-1) was obtained from Nippon Aerosil Co. (Tokyo, Japan). Synthetic air (>99.999%; N2/O2 mixture; O2 content, 21 ( 0.5%) and CO2 (1.010%)/N2 were purchased from Takachiho Trading Co. (Tokyo, Japan). Humidified synthetic air (relative humidity, 50 or 80%) was prepared by passing the synthetic air through water. Photochemical Reactor. All photochemical reactions were carried out in a closed-circulation gas reactor (Figure 1). This reactor, similar to that reported previously (25), consists of a Pyrex glass sample cell with a quartz glass window on the top, a magnetically driven circulation glass pump, a multireflection long-path cell (path length, 3 m) for in situ IR measurements, a gas sampling port for gas chromatography/ mass spectrometry (GCMS), and valves for switching the gas flow routes. The gas mixture circulated through either the main route or the secondary route at a flow rate of 700 mL min- 1. The sample cell contained a small Pyrex glass plate on which TiO2 particles were mounted. The total gas volumes circulating through the main and the secondary routes were 850 and 654 mL, respectively. The sample was irradiated with light (>290 nm, at which C4F9C2H4OH is not degraded by direct photolysis) from a 500 W xenon short-arc lamp 10.1021/es060852k CCC: $33.50

 2006 American Chemical Society Published on Web 09/22/2006

FIGURE 1. Schematic view of the closed-circulation gas photochemical reactor: (a) sample cell, (b) cutoff filter, (c) xenon lamp, (d) magnetically driven circulation glass pump, (e) multireflection long-path cell for IR measurements, (f) pressure gauge, (g) gas sampling port for GCMS measurements, (h1, h2, h3) valves for switching the gas flow route, (i) air flow ports for the pretreatment of TiO2, (j) water bath for the control of reaction temperature or heater for the pretreatment of TiO2, and (k) gas mixture inlet. The gas mixture circulated through the main route (h1 valve closed, h2 and h3 valves open) or secondary route (h1 open, h2 and h3 closed). When the gas mixture circulated through the main route, the total gas volume was 850 mL. (UXL500D, Ushio, Tokyo, Japan) through a cutoff filter (UV30, Kenko Co., Tokyo, Japan). The irradiance at the sample position was 3.4 mW cm-2 at 355-375 nm, as measured by a UV sensor. During each reaction, the temperature of the sample cell was kept constant at 298 K by means of a water bath. Photochemical Reactions. The gas mixture of C4F9C2H4OH and synthetic air was prepared by evaporation of C4F9C2H4OH and subsequent dilution with synthetic air. The TiO2 particles (3 mg) were mounted on a glass plate, which was inserted into the sample cell in the reactor, and then pretreated by the following procedure. Synthetic air was flowed over the TiO2 particles at a rate of 186 mL min-1 at atmospheric pressure via a route separate from the gas circulation routes. While exposed to the air flow, the TiO2 particles were heated at 623 K for 60 min and then cooled to 298 K. When the reactions were carried out under humidified air, the initial pretreatment was followed by further pretreatment with flowing humidified synthetic air at a flow rate of 186 mL min-1 for 60 min. After the pretreatment, the time courses of the partial pressures of C4F9C2H4OH and its degradation products were measured in the following three steps. In the first step, the C4F9C2H4OH-air mixture at atmospheric pressure was circulated through the secondary route for 30 min at an initial C4F9C2H4OH partial pressure of about 80 mTorr. In the second step, the gas flow route was switched to the main route so that the gas mixture flowed over the pretreated TiO2 for 60 min. In the third step, the sample cell was irradiated for 145-1305 min while the gas mixture circulated through the main route. We also carried out control experiments without irradiation and/or without TiO2. All reactions were carried out under atmospheric pressure. Analytical Procedures. In situ IR spectra of the gas mixture were measured with an FTIR spectrometer (Winspec50, JEOL, Tokyo, Japan) at 0.5 cm-1 resolution with 50 scans. The partial pressure of C4F9C2H4OH was determined from the maximum absorbance of the gas mixture and the absorption coefficient [ (base 10) ) 5.14 × 10-3 Torr-1 cm-1] at about 1045 cm-1, where there is no interference from reaction products. The partial pressures of COF2 and CnF2n+1COF (n ) 2 and/or 3) were determined using  ) 1.49 × 10-2 Torr-1 cm-1 at about

FIGURE 2. Time course of the partial pressure of C4F9C2H4OH. (A) The C4F9C2H4OH-air mixture circulated in the secondary route for 30 min [step (i)]. The flow route was then switched to the main route, which included TiO2 (3 mg) in the sample cell, and the gas mixture was circulated until 90 min [step (ii)]. The sample was irradiated (>290 nm) from 90 to 235 min [step (iii)]. Runs B and C were control experiments with no irradiation: (B) TiO2 present; (C) TiO2 absent. Dry synthetic air was used for each run. 1929 cm-1 and  ) 7.44 × 10-3 Torr-1 cm-1 at about 1889 cm-1, respectively. The partial pressure of CO2 was determined using two calibration curves for different partial pressure ranges (Table S-1 in the Supporting Information). GCMS measurements were carried out using an electronimpact ionization GCMS (5973 Inert, Agilent Technologies, Palo Alto, CA) with a capillary column (Rx-1, 0.32 mm i.d., 60 m length); the column temperature was kept at 253 K for 3 min and then raised by 10 K min-1 to 473 K. The carrier gas, He, was flowed at 1 mL min-1. X-ray photoelectron spectroscopy (XPS) was carried out at Nissan Arc (Yokosuka, Japan) to investigate the changes in the TiO2 surface due to reactions. The instrument was a Quantum-2000 model (Physical Electronics, Chanhassen, MN) with Al KR radiation. The calibration of binding energy in the measurement was performed using the TiO2 peak (2p) as 459.2 eV.

Results and Discussion Time Course of FTOH Partial Pressure. The time course of the partial pressure of C4F9C2H4OH in the presence of irradiation over TiO2 particles is shown in Figure 2 (run A), together with the time courses for the control experiments (Figure 2, runs B and C). In all runs, the partial pressure dropped when the gas circulation was switched from the secondary to the main route at 30 min. This drop was due to an increase in the gas volume from 654 to 850 mL, which resulted in a 23% decrease in the partial pressure. Although no TiO2 was present and no irradiation was performed in run C, the partial pressure of C4F9C2H4OH decreased slightly over time; we ascribe this gradual decrease to the slow adsorption of C4F9C2H4OH onto the inner wall of the reactor, a supposition that is supported by the fact that no degradation product was detected. When TiO2 was present (runs A and B), the partial pressure of C4F9C2H4OH in step (ii), during which the gas mixture flowed over TiO2 in the dark, was lower than the pressure observed in run C. This phenomenon indicates that C4F9C2H4OH was adsorbed onto the TiO2 surface. Because no reaction product was observed in this step, we conclude that no reaction occurred between C4F9C2H4OH and TiO2 in the dark. VOL. 40, NO. 21, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. IR spectrum obtained by subtracting the features of C4F9C2H4OH, CO2, and COF2 from Figure 3B.

FIGURE 3. IR spectra of the C4F9C2H4OH-air mixture at (A) 75 min (45 min after the C4F9C2H4OH-air mixture started to flow on the TiO2 particles) and (B) 125 min (35 min after the onset of irradiation). The time course of the treatment was the same as that of Figure 2A. In run A, after irradiation started at 90 min [step (iii)], the decrease in the partial pressure was more prominent than the decrease observed in run B (without irradiation), and the pressure decrease followed pseudo-first-order kinetics with a rate constant of 1.02 h-1. These results indicate that C4F9C2H4OH was photocatalytically degraded on the TiO2 surface. Products in the Gas Phase. The IR spectra for the gas mixture before and after irradiation of C4F9C2H4OH in the presence of TiO2 are shown in Figure 3. Before irradiation, the spectrum showed an intense absorption around 10001400 cm-1, owing to the CF stretching in C4F9C2H4OH (Figure 3A). The spectrum after irradiation showed several new peaks (Figure 3B), and the peaks at 2300-2380 and 1925-1935 cm-1 were identified as CO2 and COF2, respectively, by comparison to the spectra of standard gas mixtures. To identify other products, we subtracted the spectral features attributable to C4F9C2H4OH, CO2, and COF2 from Figure 3B, and in the 16002000 cm-1 region of the difference spectrum, residual peaks were observed at 1889, 1853, 1801, 1778, 1752, and 1713 cm-1 (Figure 4). The peak at 1889 cm-1 corresponds to CnF2n+1COF (n ) 2 and/or 3; the CdO stretching absorption of C2F5COF is the same as that of C3F7COF so they cannot be distinguished from each other). The small peak at 1778 cm-1 and the large peak at 1752 cm-1 were assigned as C4F9CHO and C4F9CH2CHO, respectively (23). The formation of C4F9CH2CHO was also supported by GCMS measurements: characteristic peaks were observed at m/z ) 262 (M+, 7.1%), 261 ([M - H]+, 3.1%), 242 (M+ - HF, 4.4%), 241([M - H]+ - HF, 7.7%), 214 (C5H2F8+, 20.7%), 95 (C3H2F3+, 100%), and 69 (CF3+, 53.6%), where M is the mass number of C4F9CH2CHO. These aldehyde species have been found not only in the present heterogeneous reaction induced by TiO2 but also in the homogeneous (gas-phase) reactions of C4F9C2H4OH induced by the Cl atom in a smog chamber (15, 23). We measured the IR spectrum of C3F7CFdCHCOOH, a fluorotelomer unsaturated acid. Fluorotelomer unsaturated acids are considered to be more toxic than their parent compounds (26) and have been detected as biodegradation products of FTOHs (19). An authentic sample of C3F7CFd 6826

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CHCOOH showed peaks at 1771, 1356, 1247, and 1125 cm-1 in air. The fact that no corresponding peaks were observed in the IR spectra of our reaction mixtures indicates that the present system did not produce such toxic species. The time courses of the partial pressures of C4F9C2H4OH, COF2, and CO2 are shown in Figure 5A. We determined the initial partial pressure of C4F9C2H4OH when the gas mixture was introduced into the main route (at 30 min) to be 59.9 mTorr, by multiplying the average partial pressure of C4F9C2H4OH in the secondary route (0-30 min) by 0.77, owing to the gas volume increase accompanying the switching of the gas circulation route. The partial pressure of 59.9 mTorr corresponds to a concentration of 78.8 ppmv. After irradiation started at 90 min, the partial pressure of C4F9C2H4OH decreased markedly, and COF2 and CO2 were the predominant products. At 270 min, that is, after 180 min of irradiation, C4F9C2H4OH had completely disappeared from the air. The partial pressure of CO2 increased after irradiation started and eventually reached 326 mTorr (429 ppmv) at 1395 min (1305 min after irradiation started), indicating that 90.7% of the carbon content in the initial C4F9C2H4OH was successfully mineralized to CO2. The partial pressure of COF2 reached a maximum (111.0 mTorr) at 265 min (175 min after irradiation started), at which point 30.6% of the carbon content in the initial C4F9C2H4OH had been transformed to COF2 and then began to decrease, disappearing completely at about 900 min (810 min after irradiation started). The partial pressure of CnF2n+1COF (n ) 2 and/or 3) (Figure 5B) was much lower than the partial pressures of CO2 and COF2 (Figure 5A). It reached a maximum (18.32 mTorr) at 245 min (155 min after irradiation started), and then began to decrease, disappearing entirely at 690 min (600 min after irradiation started). In addition, the absorbance of the aldehyde species C4F9CH2CHO and C4F9CHO reached maxima at 145 and 185 min (55 and 95 min after irradiation started), respectively, and then decreased, disappearing at about 400 and 600 min (310 and 510 min after irradiation started) (Figure 5C). Furthermore, the absorbance of tiny peaks of unknown species at 1713, 1801, and 1853 cm-1 reached maxima at 105-185 min (15-95 min after irradiation started) and disappeared at about 300-700 min (Figure S-1 in the Supporting Information). These results indicate that C4F9C2H4OH decomposed to form first C4F9CH2CHO and then C4F9CHO, followed by CnF2n+1COF (n ) 2 and/or 3) and then COF2, which finally decomposed to CO2. Perfluoroalkyl substances such as PFCAs (e.g., trifluoroacetic acid) in water are reported to undergo little degradation by TiO2 photo-

FIGURE 6. XPS spectra [(A) F(1s) and (B) C(1s) regions] of the TiO2 particles before treatment and after photochemical reaction with C4F9C2H4OH in air with irradiation for 145 and 1305 min. The time course of the treatment was the same as that of Figure 5; the irradiation times of 145 and 1305 min correspond to 235 and 1395 min on the time scale of Figure 5, respectively. The intensity of each spectrum was normalized by the use of the Ti intensity of each sample.

FIGURE 5. Time courses of partial pressures of (A) C4F9C2H4OH, COF2, and CO2 and (B) CnF2n+1COF (n ) 2 and/or 3), and (C) time courses of IR absorbance of C4F9CH2CHO and C4F9CHO. The time course for the treatment of the sample was the same as that of Figure 2A, except for the prolonged irradiation time. catalyst (27) because OH radicals react only very slowly with PFCAs (28, 29). However, the present data show that the mineralization of C4F9C2H4OH in air to CO2 is possible by means of TiO2. Products on the TiO2 Surface. To investigate the changes in the TiO2 surface caused by the reaction, we measured the XPS spectra of the TiO2 particles before and after the reaction corresponding to the regions of the F(1s) and C(1s) core electrons (Figure 6). The TiO2 surface before the C4F9C2H4OH-air mixture flowed over it showed no fluorine-containing species in either spectrum. After the C4F9C2H4OH-air mixture flowed over the TiO2 and was irradiated for 145 min, the recovered TiO2 particles showed a peak at about 689 eV, which corresponds to fluorine bonded to carbon (F-C); a peak at about 685 eV, which corresponds to negatively charged monovalent fluorine (F-) in the F(1s) region (Figure 6A); and peaks at about 294, 291, and 289 eV, which correspond to CF3, CF2, and OdCO groups, respectively, in the C(1s) region (Figure 6B). After prolonged irradiation (1305 min), the F-C peak decreased dramatically, and the F- peak increased in the F(1s) region (Figure 6A), where the atomic ratio of F- to total fluorine on the surface was calculated to be 67% on the basis of the peak intensity; at the same time, the peaks of CF3, CF2, and OdCO in the C(1s) region almost completely disappeared (Figure 6B). These findings clearly

indicate that mineralization of C4F9C2H4OH to F- proceeded on the TiO2 surface, while CO2 evolved into the gas phase. Effect of Relative Humidity in Air. We investigated the photocatalytic degradation of C4F9C2H4OH on TiO2 particles in humidified air with a relative humidity of 50% or 80% and compared the results with those for the reaction in dry air because increased humidity may accelerate the reaction steps, especially those that proceeded by hydrolysis. The changes in the time profiles of the partial pressures of C4F9C2H4OH and degradation products due to the change in relative humidity are shown in Figure 7. No significant differences were observed in the time profiles of the partial pressure of C4F9C2H4OH when the relative humidity was increased from 0% to 80% (Figure 7A). In contrast, the time profiles of the degradation products changed markedly. When the humidity was increased, CnF2n+1COF (n ) 2 and/or 3) was produced after a short period of irradiation, and the pressure was much lower than that observed in the reaction under dry air, which suggests that CnF2n+1COF (n ) 2 and/or 3) decomposed more rapidly (Figure 7B). The pressure of COF2 (Figure 7C) also increased and then decreased more rapidly than in dry air, leading to increased formation of CO2 (Figure 7D). The increase in the decomposition of COF2 with increasing the humidity (Figure 7C) is attributable to the increase in the hydrolysis of COF2 (eq 1) (30):

COF2 + H2O f CO2 + 2HF

(1)

CnF2n+1COF (n ) 2 and/or 3) is known to react with water (30), and this reaction can contribute to the mineralization of the fluorine component (eq 2):

CnF2n+1COF + H2O f CnF2n+1COOH + HF VOL. 40, NO. 21, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 8. XPS spectra [(A) F(1s) and (B) C(1s) regions] for the TiO2 particles after photochemical reaction with C4F9C2H4OH in air with a relative humidity (RH) of 0 or 80%. The time course of the treatment was the same as that of Figure 2A, where the TiO2 particles were irradiated for 145 min and then subjected to measurement. The intensity of each spectrum was normalized by the use of the Ti intensity of each sample.

FIGURE 7. Effect of the relative humidity (RH) in air on the time profiles of the partial pressures of (A) C4F9C2H4OH, (B) CnF2n+1COF (n ) 2 and/or 3), (C) COF2, and (D) CO2. The time course for the treatment of the sample was the same as that of Figure 2A. The hydrolysis of perfluorinated acid fluorides is the final step in the formation of PFCAs from FTOHs in smog chamber experiments (15, 23). The acceleration of the decomposition of the degradation products with increasing humidity was also reflected in the XPS spectra of the TiO2 particles. When the reaction was carried out in dry air, after 145 min of irradiation the TiO2 particles showed a large F-C peak and a small F- peak in the F(1s) region, where the atomic ratio of F- to total fluorine on the surface was 12% (Figure 8A). At the same time, CF3, CF2, and OdCO groups were observed in the C(1s) region (Figure 8B). In contrast, when the reaction was carried out at a relative humidity of 80%, the F-C peak became much smaller and the F- peak became larger than the corresponding peaks for dry air (Figure 8A), and the atomic ratio of Fto total fluorine increased to 39%. In addition, the presence of CF3, CF2, and OdCO groups in the C(1s) region for the sample obtained at a relative humidity of 80% became indistinct in spite of the short period of irradiation (Figure 6828

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8B). These results clearly indicate that the increase in the humidity increased the adsorption of water molecules on the hydrophilic TiO2 surface (31) and accelerated the mineralization of C4F9C2H4OH to F- on the TiO2 surface. This finding is consistent with the behavior of the degradation products in the gas phase. Although the corresponding PFCAs should be formed from the hydrolysis of CnF2n+1COF (n ) 2 and/or 3) (eq 2), these species may further decompose on the TiO2 surface because no CF3, CF2, or OdCO groups remained in the C(1s) region after the reaction at a relative humidity of 80% (Figure 8B). These results indicate that humidity in air is a key factor for counteracting FTOHs in indoor air. Further applications of this method to other FTOHs and modifications of the photocatalyst are being investigated in our laboratory.

Acknowledgments This work was supported in part by a Grant-in-Aid for Scientific Research (No. 17310055) from the Japan Society for the Promotion of Science (JSPS).

Supporting Information Available Calibration curves for the partial pressure of CO2 and time courses of IR absorbance of the species at 1713, 1801, and 1853 cm-1. This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review April 10, 2006. Revised manuscript received August 8, 2006. Accepted August 15, 2006. ES060852K

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