Heterogeneous Uptake of Carbonyl Sulfide on Hematite and Hematite

Aug 17, 2007 - Heterogeneous uptake of carbonyl sulfide (COS) on hematite, NaCl, and a series of hematite−NaCl mixtures was investigated in a static...
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Environ. Sci. Technol. 2007, 41, 6484-6490

Heterogeneous Uptake of Carbonyl Sulfide on Hematite and Hematite-NaCl Mixtures HAIHAN CHEN,† LINGDONG KONG,† J I A N M I N C H E N , * ,† R E N Y I Z H A N G , ‡ A N D LIN WANG‡ Center of Atmospheric Chemistry Studies, Department of Environmental Science & Engineering, Fudan University, 220 Handan Road, Shanghai 200433, People’s Republic of China, and Department of Atmospheric Sciences, Texas A&M University, College Station, Texas 77843

Heterogeneous uptake of carbonyl sulfide (COS) on hematite, NaCl, and a series of hematite-NaCl mixtures was investigated in a static reaction chamber at 298 K using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The adsorbed COS was oxidized on the surface of hematite and hematite-NaCl mixtures, forming surface hydrogen thiocarbonate (HSCO2-), carbonate (CO32-), and sulfate (SO42-) species as well as gaseous CO2. The reactivity of hematite-NaCl mixtures was lower than that of hematite alone. No uptake of COS was observed on the pure NaCl sample. For mixtures, the 40% hematite + 60% NaCl sample exhibited the highest reactivity. Preadsorption experiments using CO2 as a probe molecule indicated that about 70% of adsorbed COS was oxidized by surface oxygen ions on hematite. In contrast, the Fe-ClO species formed during the sample preparation procedure is proposed to be the active site on the hematite-NaCl mixtures. A plausible reaction mechanism of the heterogeneous oxidation of COS is proposed, and atmospheric implications based on these results are discussed.

Introduction Carbonyl sulfide (COS) is one of the most important sulfurcontaining species in the atmosphere, because it has low reactivity and is the only sulfur compound which is transported into the stratosphere, hence the dominant nonvolcanic source of stratospheric sulfate (1-4). An increase in stratospheric sulfate levels has important climatic implications as well as heterogeneous chemical effects that may alter the stratospheric ozone concentration (3-6). Understanding the global sources and sinks of COS is therefore of considerable importance and thus has generated increasing interest. Significant advances have been made in research on the conversion of COS in the atmosphere, mainly focusing on homogeneous processes such as formation of sulfate aerosols through photodissociation and reactions with OH and oxygen radicals (7, 8). However, it is now known that heterogeneous processes are highly significant for atmospheric chemistry (9, 10). In previous work, we found that COS can be catalytically oxidized on the surface of typical * Corresponding author phone: +86-21-65642521; fax: +86-2165642080; e-mail: [email protected]. † Fudan University. ‡ Texas A&M University. 6484

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mineral oxides such as Al2O3, CaO, SiO2, Fe2O3, and MnO2, to produce CO2, S, and SO42- (11). Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) investigations indicated that COS can be oxidized on the surface of preoxidized Al2O3 and atmospheric particles to form hydrogen thiocarbonate (HSCO2-), hydrogen carbonate (HCO3-), and surface sulfate (SO42-) species and suggested that surface hydroxyl groups and oxygen ions on particles are the key species for COS oxidation (12, 13). It is reasonable to assume that a portion of COS otherwise destined for transport into the stratosphere undergoes heterogeneous uptake by atmospheric particles while in the troposphere. However, qualitative and quantitative data regarding heterogeneous uptake of COS by atmospheric particles including sea salt are limited. In coastal regions and the marine boundary layer, abundant COS derived from photodecomposition and photooxidation of dimethyl sulfide (DMS) (14) has the potential to interact with sea-salt particles. In addition, sea salt is believed to mix with mineral dust transported from arid and semiarid regions (15), altering the chemical activity of the particles. A better understanding of the heterogeneous removal of COS by mixtures of sea salt and mineral dust aerosols is therefore highly desirable. In this study, the heterogeneous uptake of COS on hematite, an important component of mineral dust, NaCl, a major component of sea salt, and their mixtures was investigated. Preadsorption experiments using CO2 as a probe molecule were conducted to gain a mechanistic insight into these reactions and specify the nature of surface oxygen ions. Raman spectroscopy was used to characterize surface species formed on the hematite-NaCl mixture. Experiments conducted on samples with predesorption at different temperatures were performed to study the role of surface water and hydroxyl groups in COS oxidation. A plausible reaction mechanism is proposed, and atmospheric implications of this study are discussed.

Materials and Methods Materials. Hematite was synthesized as described elsewhere (16). X-ray diffraction (Rigaku D/MAX-IIX-ray diffractometer with Cu KR) was used to verify the purity of the synthesized hematite, which has an average particle size of 20-30 nm as determined by transmission electron microscopy (TEM, JEOL, JEM-2010). NaCl was purchased from Alfa Aesar with a purity of 99.999%. A series of hematite-NaCl mixtures with 1090% mass fractions of NaCl was prepared by wet impregnation of hematite with a saturated aqueous solution of NaCl. The mixtures were dried at 373 K for 6 h and kept in a desiccator at 68% RH for 48 h before further use. A solution of NaClO and its mixture with hematite (NaClO 5%), dried at 298 K for 24 h, were subsequently characterized by Raman spectroscopy. Gaseous oxygen (99.999% purity), argon (99.999% purity), carbonyl sulfide (99.99% purity), and carbon dioxide (99.999% purity) were introduced through an air dryer before use. Other chemicals were of analytical grade and used without further purification. Analytical Methods. In situ DRIFT spectra were recorded in the spectral range from 4000 to 650 cm-1 using a Nicolet FTIR spectrometer equipped with a MCT detector and a Spectra-Tech Diffuse Reflectance Accessory. IR spectra were recorded at a resolution of 4 cm-1, and 100 scans were averaged for each spectrum resulting in a time resolution of 1 min. A 30 mg sample was placed in a sample holder of the in situ chamber. The chamber was then flushed with argon (50 mL min-1) for 60 min to blow off water and other 10.1021/es070717n CCC: $37.00

 2007 American Chemical Society Published on Web 08/17/2007

FIGURE 1. In situ DRIFT spectra of hematite as a function of time after exposure to 1000 ppm COS + 21% O2 in a reaction chamber at 298 K. physisorbed impurities, and a background spectrum of the sample was recorded. Subsequently COS in Ar at a known concentration and O2 were introduced into the chamber for 3 min, after which the inlet and outlet of the chamber were closed. IR spectra were collected as a function of time. A sample temperature controller was used to control the reaction temperature. Preadsorption experiments using CO2 as a probe molecule were carried out at 298 K. After the blow-off process with argon, CO2 was flushed into the chamber using Ar as a carrier gas, after which the inlet and the outlet of the chamber were closed. Samples were exposed to CO2 for 180 min before the chamber was evacuated under 1.3 × 10-2 mbar for 30 min. The samples were then exposed to gaseous reactants. Experiments were also performed to investigate the role of surface water adsorbed on hematite and the hematiteNaCl mixtures. Following the blow-off process, the chamber was evacuated under 1.3 × 10-2 mbar for 60 min. During the evacuation, the sample was kept at 298, 373, 473, 573 and

673 K, respectively, and then cooled to 298 K before being exposed to gaseous reactants. Nitrogen adsorption-desorption isotherms of hematite, NaCl, and the hematite-NaCl mixtures were obtained at 77 K over the whole range of relative pressures, using an ASAP 2010 automatic equipment. Specific areas were computed from these isotherms by applying the Brunauer-EmmettTeller (BET) method. The BET results are shown in Table 1. Raman spectra of hematite, NaClO, the hematite-NaClO mixture, and the hematite-NaCl mixture were recorded with a Dilor LabRam-1B Raman microscope. A 632.81 nm laser was used as the excitation source at a standard power setting of 6 mW. Calibration Curve of Gaseous COS Concentration. A series of in situ DRIFT spectra with varied concentrations of COS (40-2000 ppm) was recorded at 298 K. The absorption areas of gaseous COS in the range of 2092-2015 cm-1 are shown to have a linear correlation with the concentration of COS (R2 > 0.999). Thus, the concentration of gaseous COS VOL. 41, NO. 18, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Uptake Coefficients for Heterogeneous Reaction of COS on Hematite and a Series of Hematite-NaCl Mixtures mass fraction of NaCl (%)

BET area (m2 g-1)

kfa (S-1)

Ageometric (m2)

γgeometricb

ABET (m2)

γBETb

0 10 20 30 40 50 60 70 80 90 100

37.6 33.3 31.6 24.3 22.2 19.5 14.3 9.01 4.85 2.84 2.17

(6.86 ( 0.76) × 10-5 (6.85 ( 0.21) × 10-6 (4.90 ( 0.80) × 10-6 (3.41 ( 0.51) × 10-6 (6.94 ( 1.27) × 10-6 (1.03 ( 0.11) × 10-5 (1.10 ( 0.08) × 10-5 (4.50 ( 0.38) × 10-6 (1.13 ( 0.71) × 10-6 (3.53 ( 0.56) × 10-7 0

0.785 × 10-4 0.785 × 10-4 0.785 × 10-4 0.785 × 10-4 0.785 × 10-4 0.785 × 10-4 0.785 × 10-4 0.785 × 10-4 0.785 × 10-4 0.785 × 10-4 0.785 × 10-4

(2.69 ( 0.30) × 10-7 (2.69 ( 0.08) × 10-8 (1.92 ( 0.32) × 10-8 (1.34 ( 0.20) × 10-8 (2.73 ( 0.50) × 10-8 (4.04 ( 0.46) × 10-8 (4.34 ( 0.36) × 10-8 (1.77 ( 0.15) × 10-8 (4.44 ( 0.75) × 10-9 (1.39 ( 0.22) × 10-9 0

1.128 0.999 0.948 0.729 0.666 0.585 0.429 0.2703 0.1455 0.0852 0.0651

(1.87 ( 0.21) × 10-11 (2.11 ( 0.06) × 10-12 (1.59 ( 0.26) × 10-12 (1.44 ( 0.22) × 10-12 (3.22 ( 0.58) × 10-12 (5.42 ( 0.62) × 10-12 (7.94 ( 0.65) × 10-12 (5.14 ( 0.43) × 10-12 (2.40 ( 0.41) × 10-12 (1.28 ( 0.21) × 10-12 0

a k is the first-order rate coefficient for the OCS oxidation. b γ f geometric and γBET are the uptake coefficients calculated by geometric and BET surface areas, respectively.

can be determined by measuring the in situ DRIFT spectra peak areas of gaseous COS.

Results and Discussion DRIFTS Study of the Heterogeneous Reaction of COS on Hematite. In situ DRIFT spectra following exposure of hematite to 1000 ppm COS at 298 K are shown in Figure 1. The adsorption band between 2100 and 2000 cm-1 with features at 2071 and 2051 cm-1, corresponding to gaseous COS (17), decreased drastically in intensity as the reaction proceeded. The bands at 2360 and 2341 cm-1 attributed to gaseous CO2 (18) continuously grew in intensity as the gaseous COS decreased. Negative bands at 3677 and 1638 cm-1 are characteristic of the loss of surface hydroxyl species (OH) (19, 20), implying that the hydroxyl groups are involved in this reaction. In addition, several weak adsorption bands with peaks near 3532, 1572, 1438, 1375, 1314, and 1134 cm-1 appeared in the spectra and increased in intensity with time. These bands remained in the spectra even upon evacuation of the gas phase, indicating that surface species were formed and adsorbed on hematite. The broad band centered around 3532 cm-1 is attributed to hydrogen binding of COS or sulfate (21, 22). The hydrogen thiocarbonate (HSCO2-) with a band at 1572 cm-1 is proposed to be the intermediate species of COS oxidation according to previous studies (12, 23). The appearance of the bands at 1438, 1375, and 1314 cm-1 is characteristic of the formation of carbonate species, CO32(20, 24). The band at 1134 cm-1 is attributed to sulfate (SO42-) (25). After 20 h, the amounts of CO2 and SO42- increased significantly, while the bands assigned to COS, HSCO2-, and CO32- disappeared. The above results indicate that COS can interact with the surface sites of hematite to form HSCO2-. In the presence of O2, the resulting HSCO2- is converted into CO2, surface CO32-, and SO42- species. Given sufficient exposure time, a limited amount of COS is completely oxidized. As SO42- accumulates on the hematite, the surface pH decreases (26), resulting in the conversion of surface CO32- to gaseous CO2, which merges into the gaseous CO2 directly produced from the COS oxidation. Reactivity of Hematite-NaCl Mixtures at Different Mass Fractions. In order to investigate the mixing effect, DRIFTS experiments were carried out on a series of hematite-NaCl mixtures with 10-90% mass fractions of NaCl. A blank experiment performed in the absence of samples indicated that the closed chamber consumes approximately 2.8% of the COS in the system in 150 min through adsorption by the inner surface of the chamber. Compared to the blank experiment, there is no evidence of the consumption of COS and the formation of new species on NaCl, suggesting that NaCl has no ability to interact with COS at 298 K. For the 6486

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mixtures, spectra trends similar to those shown in Figure 1 were observed as exposure time increased, indicating that COS can also be oxidized on hematite-NaCl mixtures. The observed reactions are first order with respect to COS in 150 min, and the correlation coefficients (R) are greater than 0.990. The kinetic equation therefore can be described as below

-

d[COS] ) kf[COS] dt ln

[COS]0 [COS]

) kf t

(1)

(2)

where kf is the first-order rate coefficient, [COS]0 is the initial concentration of COS, and [COS] is the concentration of COS at corresponding exposure time. The concentrations of COS can be obtained by comparing the absorbance peak area of COS between 2092 and 2015 cm-1 to the calibration curve of gaseous COS. The obtained first-order rate coefficients vary with the mass fractions of NaCl as shown in Table 1. In order to further compare the reactivity of the mixtures and provide data for existing atmospheric chemistry models, the DRIFTS experimental data were analyzed to determine the reactive uptake coefficients (γ). The reactive uptake coefficient (γ) can be calculated from the first-order rate coefficient of the uptake (kf), the collision frequency (Z), and the reactive surface area (As)

kf Z × As

(3)

1 ν 4V COS

(4)

γ)Z)

νCOS ) x8RT/πMCOS

(5)

where νCOS is the mean molecular velocity of COS, V is the volume of the reactor (0.250 × 10-5 m3), R is the gas constant (J mol-1 K-1), and T is the temperature (K) (27). Two extreme cases of effective sample surface were used. If the reaction rate was high, the reaction is diffusion-controlled, and the effective surface would be the geometric surface area of the sample holder (Ageometric ) 0.785 × 10-4 m2). If the reaction rate was low, the reaction is reaction rate-controlled, and the effective surface would be calculated by the BET surface areas (ABET) listed in Table 1 (28). Three or more replication experiments were conducted for each sample, and the average uptake coefficients γgeometric and γBET for each sample are tabulated (Table 1). As shown in Table 1, the addition of NaCl lowers the sample reactivity.

FIGURE 2. (A) DRIFT spectra of samples recorded after exposure to 400 mbar of CO2 and evacuation of the chamber to 1.3 × 10-2 mbar for 30 min at 298 K: (a) hematite, (b) 80% hematite + 20% NaCl, (c) 40% hematite + 60% NaCl, and (d) 20% hematite + 80% NaCl. (B) Gaussian curve fitting for spectrum of A (a).

With increasing mass fractions of NaCl, the uptake coefficient first decreases, then increases, and then decreases again. For mixtures, the 40% hematite + 60% NaCl sample exhibits the highest reactivity. Its corresponding uptake coefficient calculated by the BET surface area decreases approximate 57% compared with that of hematite. The reactivity of all samples compared by γBET is in the following order: hematite > 40% hematite + 60% NaCl > 50% hematite + 50% NaCl > 30% hematite + 70% NaCl > 60% hematite + 40% NaCl > 20% hematite + 80% NaCl > 90% hematite + 10% NaCl > 80% hematite + 20% NaCl > 70% hematite + 30% NaCl > 10% hematite + 90% NaCl . NaCl. The results demonstrate that the mixing of hematite with NaCl has a suppressing effect on the oxidation of COS, which is distinct from the oxidation of SO2, since previous studies suggested that the uptake coefficient of SO2 on mineral dust can be calculated from the reactivity of the single components along with each component weighed by its abundance in the mixture (29). Preadsorption Experiment and Raman Study To Investigate Reaction Active Sites. To determine what kinds of surface active sites play a role in the COS oxidation and what causes the reactivity variation shown in Table 1, CO2 preadsorption experiments were carried out according to previous work (30). After the blow-off procedure, carbon dioxide was introduced into the chamber in the presence of hematite at 298 K for 180 min, and subsequently the evacuation was conducted. It was observed that upon evacuation gaseous CO2 bands at 2360 and 2341 cm-1 decreased significantly in intensity and almost completely disappeared after 3 min. However, two adsorption bands with features at 1550 and 1327 cm-1 remained (Figure 2A). To further analyze the region from 1750 to 1250 cm-1, a Gaussian curve fitting program (Galactic software) was employed to deconvolute the overlapped bands, and results are shown in Figure 2B. The obtained bands are assigned to monodentate carbonate (1383 and 1446 cm-1) and bidentate carbonate (1316 and 1553 cm-1), respectively, indicating that CO2 can be coordinated with surface oxygen ions to form carbonate species (20, 24).

FIGURE 3. First-order rate coefficients for COS oxidation on hematite pretreated with CO2 at varied partial pressure.

To study the nature of surface oxygen ions on COS oxidation, hematite preadsorbed by CO2 was then exposed to reactant gases. The effect of the amount of CO2 adsorbed on hematite on the oxidation rate was investigated by varying partial pressures of CO2 from 40 to 750 mbar in the preadsorption procedure. Results shown in Figure 3 reveal that the COS oxidation rates decrease gradually with increasing the partial pressures of CO2 in the range of 40-400 mbar. The reactivity of pretreated hematite at both 400 and 750 mbar CO2 decreases about 70% compared with that of hematite without CO2 preadsorption, indicating that saturation occurs when 400 mbar CO2 is present in the chamber. Surface oxygen ions are shown to be the key active sites for heterogeneous oxidation of COS on hematite. When CO2 is introduced into the system, it is coordinated with surface oxygen ions, which are involved in the COS oxidation, leading to the poisoning of surface oxygen ions and subsequently the inhibition of reactivity. However, preadsorbed CO2 does not inhibit COS oxidation completely, suggesting that other active sites such as surface hydroxyl groups exist (11, 12). VOL. 41, NO. 18, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. First-order rate coefficients for COS oxidation on the hematite-NaCl mixtures with and without CO2 preadsorption.

FIGURE 5. Laser Raman spectra: (a) hematite, (b) NaClO, (c) 95% hematite + 5% NaClO, and (d) 40% hematite + 60% NaCl. Preadsorption experiments were also performed on hematite-NaCl mixtures. DRIFT spectra of CO2 adsorption on the mixtures are shown in Figure 2A(b-d). The bands for carbonate species decrease drastically in intensity as the mass fraction of NaCl in the mixture increases. Comparisons of the reactivity of mixtures with and without preadsorbed CO2 indicate that unlike hematite, preadsorbed CO2 has no obvious effect on the reactivity of the mixtures (Figure 4). Evidently, the addition of NaCl blocks surface oxygen ions, the dominant active sites for COS oxidation on hematite, resulting in the observed reactivity differences. In contrast to hematite, surface oxygen ions are not the key active sites for heterogeneous oxidation of COS on hematite-NaCl mixtures. Therefore, the Raman study was conducted to identify their active sites. As shown in Figure 5, hematite presents Raman bands at 217, 278, 387, 481, and 597 cm-1 (Figure 5a) (31). Sodium hypochlorite (NaClO) presents Raman bands at 481, 606, 710, 934, 990, and 1065 cm-1 (Figure 5b). A mixture of NaClO and hematite was also characterized by Raman spectroscopy (Figure 5c). In addition to bands for hematite and NaClO, new bands at 666 and 823 cm-1 were observed, which are due to the formation of FeClO species (32). The mixture of 40% hematite + 60% NaCl was chosen for the Raman study as it showed the highest activity among all the mixtures (Figure 5d), and the Fe-ClO species characterized by bands at 660 and 818 cm-1 was also observed as a new species, resulting from the wet impregnation of hematite with NaCl at ambient temperature. Knipping et al. (33, 34) have proposed an interface reaction between the hydroxyl radical and NaCl to produce Cl2, leading to the formation of NaClO. In this study, we failed to observe Raman 6488

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FIGURE 6. First-order rate coefficients for COS oxidation on hematite as a function of chamber evacuation temperature. bands near 710, 934, 990, and 1065 cm-1 corresponding to NaClO (Figure 5d), possibly due to low abundance of NaClO formation. The newly formed Fe-ClO species may play a role in the heterogeneous uptake of COS. More research is needed to elucidate the relationship between the reactivity of the mixtures and the abundance of Fe-ClO in the mixtures. Role of Surface Adsorbed Water. Hydrolysis of COS on oxides, an effective way to remove COS in industrial tail gas, is strongly affected by the amount of adsorbed water (35). In order to study the role that surface water plays in oxidation of COS, experiments were carried out at 298 K over a series of hematite samples that had been pre-evacuated under 1.3 × 10-2 mbar for 60 min at 298, 373, 473, 573, and 673 K, respectively. Results shown in Figure 6 suggest that the preevacuated sample at 373 K greatly enhanced the reactivity of hematite, and the first-order rate coefficient increased by a factor of 2.3 compared with that at 298 K. However, with a further increase in pre-evacuated temperature the reactivity decreased, and the sample pre-evacuated at 673 K showed even lower reactivity than that at 298 K. Pre-evacuation of hematite is known to mainly remove surface adsorbed water, and, at higher temperature, surface dehydration occurs. The observed increase in the reactivity of the sample preevacuated at 373 K is due to the removal of physisorbed water, which may serve as an inhibitor for the COS oxidation by blocking access to the active sites. Samples pre-evacuated at higher temperatures (473-673 K) display removal of surface hydroxyl groups along with the physisorbed water, resulting in a decrease in the sample reactivity. In agreement with other studies, surface hydroxyl groups could also be the active sites (11, 12). On the basis of the above results, a mechanism explaining heterogeneous reaction of COS on hematite is proposed, shown in Figure 7. Uptake of COS occurs on hematite to form HSCO2-. The resulting HSCO2- is an intermediate that can be oxidized to form CO2, CO32-, and SO42- species in the presence of O2. About 70% loss of COS can be attributed to surface oxygen ions, with other active sites such as surface hydroxyl groups accounting for additional residual loss.

Atmospheric Implications We propose that heterogeneous reactions on the mixture of mineral dust and sea salt represent another significant sink for COS. When dimethyl sulfide is continuously emitted from the open ocean into the atmosphere, it undergoes a series of complex oxidation reactions. Carbonyl sulfide, one of the gaseous products, is stable in the ocean air, in which sea salt is the primary aerosol. However, in the light of the formation of the mineral-NaCl mixture by long-range transport of mineral dust from inland, the heterogeneous uptake of COS must be taken into account. Accordingly, sulfate aerosols in the lower troposphere increase because of the conversion of

FIGURE 7. Mechanism of heterogeneous oxidation of COS on the surface of hematite.

COS to sulfate, and the mass of COS transported into the stratosphere decrease. The heterogeneous reaction of COS on atmospheric particles and the mixing effect of different composition aerosols should be included in atmospheric chemistry models. This work is also significant for contributing to the understanding of the cycle of sulfur compounds, the impact of mineral dust mixed with sea salt on aerosol radiative forcing, and the sulfate cooling effect. Further studies performed under more atmospherically relevant conditions to explore the heterogeneous reaction of COS are required.

(12) (13)

(14) (15)

Acknowledgments This work was supported by the National Natural Science Foundation of China (Key Project Grant No. 40533017, Grant No. 40605001).

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Received for review March 23, 2007. Revised manuscript received June 28, 2007. Accepted July 11, 2007. ES070717N