Environ. Sci. Technol. 2005, 39, 4499-4504
Cr(VI) Reduction and Immobilization by Magnetite under Alkaline pH Conditions: The Role of Passivation Y . T H O M A S H E * ,†,‡ A N D SAMUEL J. TRAINA§ Environmental Science Graduate Program, The Ohio State University, Columbus, Ohio 43210, and Sierra Nevada Research Institute, University of California, Merced, California 95344
This study investigated Cr(VI) reduction and immobilization by magnetite under alkaline pH conditions similar to those present at the Hanford site. Compared to acidic and neutral pH, chromium(VI) reduction by magnetite at high pH conditions is limited (13; and were at temperatures well in excess of 50 °C (2). The high temperatures were caused by the decay of fission products. The estimated Cr concentrations at the time of the discharges ranges from 5.09 × 10-2 to 4.13 × 10-1 mol/L (3). Jones et al. (4) estimated that the total mass of Cr lost to the vadose zone from tanks SX-104, SX-107, SX-108, SX-109, SX113, and SX-115 was about 2685 kg. Recent reports have indicated that chromate has contaminated the groundwater * Corresponding author phone: (510)486-6472; fax: (510)486-5172; e-mail:
[email protected]. † The Ohio State University. ‡ Present address: Earth Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720. § University of California, Merced. 10.1021/es0483692 CCC: $30.25 Published on Web 05/11/2005
2005 American Chemical Society
under the tank farm and is a major concern for the Columbia River (5). The oxidation state of Cr determines its toxicity, solubility, mobility, and fate. Chromate is very mobile in the environment, especially at higher pH conditions where sorption is not favored. On the other hand, Cr(III) is much less mobile. Thus, redox transformations are the most important processes controlling the transport and fate of chromium in the environment. Many studies have demonstrated that ferrous iron [Fe(II)] is an important reductant of Cr(VI) in natural environments (6, 7), because Fe(II)-containing minerals are abundant in many suboxic and anoxic soils and sediments. Magnetite is one of the major Fe(II)-containing minerals in Hanford sediments, and has the potential to reduce and immobilize Cr(VI). Reduction of Cr(VI), Tc(VII), and As(V) by structural Fe(II) in magnetite via a coupled reductionsorption process is an important mechanism that has been investigated under various environmental conditions. For example, Farrell et al. (8) found that pertechnetate was removed through electrostatic adsorption of pertechnetate at an anodically polarized magnetite surface, followed by reduction of adsorbed Tc(VII) by structural Fe(II) in magnetite. White and Peterson (9) demonstrated that structural Fe(II) in magnetite and ilmenite heterogeneously reduced ferric, vanadate, and chromate ions at the oxide surfaces over the pH range 1-7 at 25 °C. Deng et al. (10) and Kendelewicz et al. (11) had similar findings in their respective investigations involving Cr(VI) reduction by magnetite. However, most of these studies were conducted under acidic to near-neutral pH conditions. Information about Cr(VI) reduction at high pH is very sparse, and Cr(VI) reduction by magnetite at alkaline pH has not been reported previously. The leaking of the S and SX tanks caused intense interaction between HLW fluids and the underlying sediment minerals. Zachara et al. (2) examined sediment samples recovered from a HLW plume beneath tank SX 108. These materials exhibited extensive alteration of mineral surfaces. Chromium K-edge, X-ray absorption near edge structure (XANES) spectroscopy indicated that 29-75% of the total Cr was reduced to Cr(III) with the remainder as Cr(VI). Chromium reduction was attributed to OH--induced dissolution of Fe(II)-containing minerals (e.g., biotite, magnetite, chlorite, hornblende) and subsequent reaction of dissolved Fe(II) with Cr(VI). However, direct evidence of Cr(VI) reduction by Fe(II)-containing minerals such as magnetite was lacking at high pH. The objective of this study was to investigate Cr(VI) reduction/immobilization by magnetite under HLW conditions indicative of those thought to have existed in the vadose zone of the DOE’s Hanford site. Chemical reactions were conducted at high base concentration and at low PO2 and PCO2 to limit the effects of these gases on possible side reactions. Additionally, comparisons were made to magnetite-Cr(VI) interactions in neutral and acidic fluids. The formation of passivation layers on magnetite and the subsequent effect on Cr(VI) reduction is also investigated. The result of this study provides additional basis for Cr(VI) transport modeling and remediation efforts at the Hanford site.
Materials and Methods Experimental Details. All solutions were prepared using reagent-grade NaOH (5 M volumetric solution, J. T. Baker), Na2CrO4 (Aldrich, 98%), NaNO3 (Fisher, crystal, 99.2%), and Al(NO3)3 (Fisher, crystal, 98.2%). NaOH used for magnetite synthesis was from Aldrich with a purity of 99.998%. VOL. 39, NO. 12, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Experimental Setup and pH Conditionsa treatment 1 2 3 4 5 6 7
NaOH (mol L-1) 0 0.1 0.5 1 2 2 HCl 0.1 mol L-1
NaNO3 (mol L-1)
Na2CrO4 (mol L-1)
0 0 0 0 0 1
10-3
10-3 10-3 10-3 10-3 10-3 10-3
pH 7.64 13.01 13.45 13.59 13.65 13.58 0.93
a The experiments were conducted at 50 °C. Magnetite to solution ratio equals 1:100 in all treatments.
Deionized water was boiled and purged with Ar(g) to eliminate dissolved O2 and CO2 and then stored in a plastic container in an Ar(g)-filled glovebox. Reagents were mixed in an Ar(g)-filled glovebox, and the stock solutions for each reagent were prepared under Ar(g). For each treatment, an appropriate amount of stock solution was added to a highdensity polypropylene reaction vessel, sealed under Ar(g), and then moved to a 50 °C incubator (under atmospheric conditions) to monitor reactions. A magnetite to solution ratio of 1:100 (by mass) was used in all treatments in this study. Table 1 shows the experimental details for each treatment. The reactions were monitored at the interval of 1, 6, and 24 h and 4, 16, and 32 d, at which time the reacted suspensions were centrifuged to initiate phase separation prior to analysis. The solution phase was collected to analyze total Cr and Cr(VI) concentration. The concentration of total dissolved Cr(VI) was measured using a colorimetric method developed by the USGS (12). Diphenylcarbazide reagent was prepared by dissolving 0.2 g of diphenylcarbizide and 1.0 g of phthalic anhydride in 200 mL of ethanol. Cr(VI) was measured at 540 nm with a Cary 3 UV-visible spectrophotometer. Total Cr was analyzed by ICP-OES (Perkin-Elmer Optima 3000). Samples for X-ray diffraction (XRD) were prepared by drying the centrigfuged solid pastes under an Ar(g) atmosphere in order to avoid exposure to air. X-ray diffraction patterns of the synthesized magnetite and other reaction products were collected using a Philips XRG X-ray generator, from 2 to 70° 2θ with 4-s steps. The mineral phase were identified by searching and matching to a standard Powder Diffraction File 1993 database. Transmission electron micrographs were collected with a CM12 Philips microscope at 60 keV. The samples were prepared using a diluted aqueous suspension of the reacted magnetite sample. One to two drops of the suspension was added to coated transmission electron microscopy (TEM) grids and air-dried. Particle surface area was measured by Micromeritics FlowsorbII 2300 using a N2(g) BET single-point method. X-ray Absorption Spectroscopy Experiments. To collect X-ray absorption spectra (XAS), wet pastes from centrifugation separations were mounted to Teflon sample holders inside an Ar-filled glovebox. The samples were stored in sealed plastic bags under highly moisture conditions to prevent desiccation. Synchrotron-based XAS measurements were performed at the Stanford Synchrotron Radiation Laboratory (SSRL) on beam lines IV-3 and XI-2 equipped with a Si(220) double-crystal monochromator. Fluorescence-yield Cr Kedge XAS data were collected using a Canberra Ge 13-element energy-dispersive detector. Higher-order harmonics in the incoming beam were removed with a Pt-coated mirror and by detuning the main beam by 30-50%. Energy was calibrated by collecting the transmission spectrum of a Cr metal foil. The first inflection point of the K-edge of the Cr metal foil was assigned as 5989 eV. X-ray absorption fine structure 4500
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FIGURE 1. Chromium(VI) reduction by synthetic magnetite at three pH conditions (pH 1, pH 7, and pH 13), magnetite-to-solution ratio equal to 1:100, and initial Cr(VI) concentration added as Na2CrO4 was 1 mmol L-1 in all treatments. (XAFS) spectra were collected over the energy range of 5.76.5 keV. Averaging, normalization, and background subtraction of the raw XAFS spectra were performed with the computer program EXAFSPAK (13). Each averaged XAFS spectrum was separated into two regions, the X-ray absorption near-edge structure (XANES) spectral region and the extended XAFS (EXAFS) spectral region. The XANES spectra (5980-6100 eV) were normalized and compared for qualitative information. The EXAFS oscillations were isolated with a spline function and converted from energy to k-space. A spline was fit in the computer program IFEFFIT (14). The converted EXAFS spectra were then exported back to the EXAFSPAK program for Fourier transform and curve-fitting procedures. The k3weighted EXAFS spectra of the model compounds sodium chromate and chromium(III) oxide were fit (k-range ) 3-12 Å-1) with phase shift and amplitude functions generated by using the ab initio computer code FEFF8 (15). These fitting results were used to test the theoretical phase shift and amplitude functions that were later used to fit unknown samples. Values for coordination number (CN) and distance to scattering atoms (R) were determined from a least-squares fit of the EXAFS and the Fourier-filtered EXAFS of each shell. Magnetite Synthesis and Characterization. Magnetite was synthesized after the method by Schwertmann and Cornell (16). The synthesized magnetite was characterized by XRD and TEM (details provided in the Supporting information). Potentiometric titrations of the synthetic magnetite were made at three different ionic strengths with a Mettler automatic titrator (0.6, 5, and 20 µmol L-1 NaClO4) to determine the point of zero salt effect (PZSE). Inasmuch as NaClO4 is an indifferent salt on magnetite, the PZSE corresponds to the point of zero charge (PZC) (17). From the potentiometric titration curves, we determined that the PZSE was 6.5, which is within the range (6.3-6.8) reported by Regazzoni et al. (18) for the PZC. X-ray diffraction patterns of the synthesized magnetite agree well with the reference magnetite pattern from the International Center for Diffraction Database (ICDD, 1993). Transmission electron micrographs of the synthesized precipitate show cubic and spherical morphology. These cubic and spherically shaped particles are in the tens of nanometer size ranges. Surface area (BET) measurements indicated a specific surface area of 26.1 m2 g-1.
Results and Discussion Chromium (VI) Reduction and Immoblization. Figure 1 shows Cr(VI) reduction by magnetite under acidic, neutral, and alkaline pH conditions. Substantial amounts of chromium(VI) were removed from the aqueous phase under both the acidic and neutral pH conditions. At high pH conditions,
FIGURE 2. Chromium K-edge XANES spectra of Cr(VI) reacted magnetite at 50 °C at different pH conditions: (a) chromate model compound, (b) pH 1, (c) pH 13, and (d) pH 7. 13), the magnetite surface should be negatively charged, leading to significant electrostatic repulsion of the chromate oxyanions, causing even further decreases in the rate of Cr(VI) reduction. X-ray Absorption Spectroscopy. Chromium K-edge XANES and EXAFS spectra were collected from magnetite that had been reacted with aqueous Cr(VI) in acidic, neutral, and highly alkaline solutions. XANES spectra can provide information about in situ Cr oxidation states due to the difference in Cr(III) and Cr(VI) spectra. As we can see from Figure 2, chromium XANES spectra showed that Cr(VI) was completely reduced to Cr(III) under neutral and alkaline pH conditions. Under acidic conditions, however, a small preedge feature suggests that Cr(VI) is not completely reduced even though all Cr(VI) is removed from the solution phase; therefore, some Cr may be removed by a sorption mechanism. The EXAFS measurements indicated that after the reduction of Cr(VI), Cr(III) became associated with the solid phase. The fitting results (Figure 3a,b and Table 2) show that Cr reduced by magnetite has similar structural environments at all three different pH conditions. The first shell corresponds
FIGURE 3. Chromium EXAFS spectra of Cr(VI) reacted magnetite at 50 °C at different pH conditions. Solid line is the data, and dotted line shows the least-squares fit: (a) Cr EXAFS spectra and (b) Fourier transform.
TABLE 2. Chromium EXAFS Fitting Parametersa sample
bond
N
R
σ2
Na2CrO4 pH 1
Cr-O Cr-O Cr-Cr/Fe Cr-O Cr-Cr/Fe Cr-O Cr-Cr/Fe
4b 3.28 1.44 4.09 1.96 3.64 1.00
1.66 2.02 3.06 1.99 3.05 1.99 3.05
0.0031 0.0010 0.0070 0.0027 0.0069 0.0028 0.0029
pH 7 pH 13
a N is the number of backscattering atoms around the absorbing Cr atom, R is the absorber-backscatterer distance, σ2is the Debye-Waller value. b N values were fixed at known values for model compound Na2CrO4. Estimated errors for N ∼ 20%, R ∼ 0.01 based on least-squares fits of EXAFS spectra; error represents 95% confidence interval.
to a Cr-O bond at 1.99 Å, and the second shell corresponds to a Cr-Cr/Fe bond at 3.05 Å. Since the photoelectron backscattering function of Fe and Cr are similar, it is not possible to use EXAFS spectra to distinguish between these two possible second-shell atoms. In the sodium chromate model compound, the Cr-O bond distance is 1.66 Å. These results are in general consistent with Cr(VI)-O, Cr(III)-O, and Cr(III)-Cr(III) bonds reported in other studies (23). For the Cr(VI)- and Cr(III)-containing model compounds, Peterson et al. (23) determined the Cr(VI)-O distance to be 1.63 ( 0.03 Å and the Cr(III)-O distance to be 1.99 Å. In investigating the reaction of Cr(VI) with magnetite, Peterson et al. (24) showed the distance between Cr and its nearest shell of oxygen atoms to be 1.98 ( 0.01 Å. Second-shell distances between Cr and Cr and/or Fe atoms were 3.01 ( VOL. 39, NO. 12, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. X-ray diffraction patterns of synthetic magnetite at different NaOH concentrations in the presence of 1 mmol L-1 Na2CrO4. Goethite formed in all NaOH treatments and relative intensity increased with increased NaOH concentration: (a) Reference pattern of goethite, plotted from a standard Powder Diffraction File 1993 database; (b-e) 0, 0.1, 0.5, 1, and 2 mol L-1 NaOH treatment, respectively. Dotted lines in the plot indicate major maghemite peak position.
FIGURE 4. Transmission electron micrograph showing needleshaped particles (goethite) formed by reacting synthetic magnetite with 2 mol L-1 NaOH in the presence of 1 mmol L-1 Na2CrO4 at 50 °C for 16 d. 0.01 Å. The Cr-O distance measured in the current system is similar to that of the coprecipitates formed in the homogeneous system, where Cr(VI) is reduced by aqueous Fe(II) under alkaline pH (25); however, the Cr-Cr/Fe distance is slightly larger than that was measured for the precipitates in the homogeneous system. This may because that in the heterogeneous system, reduction of Cr(VI) causes precipitation of Fe1-xCrxOOH or Cr(OH)3 deposited on the magnetite surface. But in the homogeneous system, where Cr(VI) is reduced by Fe2+ or ferrous hydrolysis species, reduced Cr and oxidized Fe forms coprecipitates. Patterson et al. (6) found that on amorphous iron sulfide, chromate reduction occurred dominantly at the FeS surface resulting in a solid phase with the composition of [Cr0.75,Fe0.25](OH)3. While less extensive, reduction of Cr(VI) by Fe(II)(aq) was noted and produced a solid with the opposite Cr:Fe ratio, [Cr0.25,Fe0.75](OH)3. It is possible that differences in the Cr:Fe ratio may lead to different Cr-Cr/Fe distances. Hansel et al. (26) suggested that an increase in the mole fraction of Cr would cause the Cr-Cr/Fe bond to expand. Role of Passivation and Cr(VI) Reduction. Passivation is an important process in heterogeneous redox reactions and is observed for redox reactions involving magnetite, green rust, and zero-valent iron (27, 28). In studies of Fe(0)-induced reduction of Cr(VI), Chambers (27) found Fe0 was coated with a thin oxide skin consisting mostly of magnetite. The redox process converted magnetite and zero-valent iron, which are conductive and facilitate electron-transfer processes, to an insulating iron(III)-chromium(III) oxyhydroxide. The insulating nature of these layers prevented electron transfer from deeper layers, thus bringing the redox process to a halt. Williams (28) suggested that rate limitations observed at high Cr(VI) concentrations may have been due to surface passivation caused by precipitation of a sparingly soluble solid (such as a mixed Cr(III)-Fe(III) precipitate) or by a depletion of available Fe(II) within the green rust structure. In the current investigation, we think that some of the limited Cr(VI) reduction under alkaline pH conditions may 4502
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be due to passivation of the magnetite surface by formation of goethite, maghemite, and/or Fe1-xCrxOOH in the system. Transmission electron micrographs of magnetite reacted with different amounts of NaOH and 1 mmol L-1 Na2CrO4 (Figure 4) show acicular shaped crystals coexisting with apparently residual cubic and spherical particles of magnetite/ maghemite. These acicular shaped crystals are consistent with the typical morphology of goethite. The formation of goethite in the system was confirmed by XRD (Figure 5). Powder XRD patterns of magnetite reacted with Cr(VI) in different NaOH concentrations show that goethite formed in all of the NaOH treatments with the relative intensity of the goethite reflections increasing with NaOH concentration. Due to the significant structural difference between goethite and magnetite, the formation of goethite from magnetite is not likely through topotactic reaction but likely through a reconstructive transformation mechanism, which often involves dissolution and reprecipitation; the initial phase dissolves completely, and the new phase nucleates to form precipitates from solution. In the current system, goethite is most likely formed through the reconstructive mechanism with the dissolution of magnetite and the release of Fe to the solution phase as prerequisite. The release of Fe(II) to solution would also lead to the homogeneous reduction of Cr(VI) as observed by He et al. (25). Unfortunately, dissolved Fe(II) was not detected in the present study, and its presence can only be surmised by the formation of goethite. The overall reactions for magnetite dissolution and maghemite, goethite, and CrxFe1-xOOH formation under alkaline conditions can be written as:
Fe3O4 + OH- + H2O ) γ-Fe2O3 + Fe(OH)3-
(1)
2Fe(OH)3- + 1/2O2 + 2H+ ) 2R-FeOOH + 3H2O (2) 3Fe(OH)3- + CrO42- + 5H+ ) 2R-FeOOH + 2CrxFe1-xOOH + 5H2O (3) As we can see from reaction 1, maghemite should also be formed in the system. This is confirmed by XRD (Figure 5) at lower NaOH concentrations. But at higher NaOH concentrations, the maghemite peaks disappear. The disappearance of the maghemite peaks in 1 and 2 mol L-1 NaOH treatments (see Figure 5. XRD) and the lack of any detectable hematite suggest that maghemite might dissolve to form
goethite. The reactions can be written as:
γ-Fe2O3 + 2OH- + 3H2O ) 2Fe(OH)4-
(4)
Fe(OH)4- + H+ ) R-FeOOH + 2H2O
(5)
Thus, at least some conversion of magnetite to goethite may have passed through a maghemite intermediate. Similar mechanisms have been proposed in some earlier studies. For example, Kiyama (29) suggested that R-FeOOH and R-Fe2O3 are formed upon the further oxidation of γ-Fe2O3, possibly through the hydrolysis of ferric hydroxo complexes, formed by the dissolution of γ-Fe2O3. Blesa and Matijevic(30) found that transformation of maghemite to goethite and hematite occurs through dissolution of maghemite in basic solutions to yield Fe(OH)4-, which then forms goethite or hematite. Reaction 2 shows the formation of R-FeOOH. In reaction 3, Cr(VI) is reduced, and both R-FeOOH and Fe1-xCrxOOH are produced. The overall behavior of the system is likely determined by the competition of these two reactions (i.e., the competition for Fe2+ between O2 and Cr(VI)). Under high pH conditions, the kinetics of Fe(II) oxidation are very rapid, and O2 is much more efficient in oxidizing Fe(II) than Cr(VI) (31). The reaction of Fe(II) with O2 at 25 °C and pH 8 is faster by factors of 10-20 than those with Cr(VI) at 0.01 and 0.7 M ionic strengths, respectively, while the rates are similar at pH 7, and 7-14 times slower at pH 6 (32). Lin (33) showed that at pH greater than 8.0 and in the presence of dissolved oxygen, Cr(VI) reduction by Fe(II) was greatly suppressed due to the rapid oxidation of Fe(II). Even though the experiment was conducted in a closed system and caution was taken to avoid exposure to O2, diffusion of O2 through the walls of the reaction vessels over the lifetime of the experiments could have significant impacts on the system through reaction 2. The competition to form goethite and Fe1-xCrxOOH is decided by the competition of O2 and Cr(VI) for Fe2+, in which O2 is more effective under high pH conditions. The formation of maghemite and goethite may have consumed Fe2+ and/or formed a passivation layer on the magnetite surface, inhibiting further Cr(VI) reduction by magnetite. Many studies of Cr(VI)-induced passivation of Fe(II) minerals have reported the formation of X-ray amorphous surface precipitates. For example, Kendelewicz et al. (11) investigated Cr(VI) reduction by magnetite and found a mixed, amorphous, insulating iron(III)/chromium(III) oxyhydroxide layer on the surface that eventually blocked electron transfer across the interface. Their spectroscopic data indicated a lack of Fe in the topmost chromium (oxy)hydroxide layer on magnetite. But there are also indications for the formation of crystalline, Cr(III)-substituted goethite phases as well (34-36). Yamashita et al. (37) found that Cr3+ promotes the formation of fine R-FeOOH and R-(Fe1-xCrx)OOH particles. Since goethite formation is favored at high pH (38-40), the high pH conditions in this study facilitated formation of goethite and reduced the potential for Cr(VI) reduction by magnetite. Even though we do not detect an Fe1-xCrxOOH phase with X-ray diffraction as in homogeneous systems, it is likely that an Fe1-xCrxOOH phase formed in the current system according to reaction 3. Implications. Transformation of magnetite to goethite and maghemite will limit the role of magnetite in redox reactions. These base-induced transformations decrease the role of magnetite in the reduction of soluble oxidants such as Cr(VI), because the formation of a goethite or maghemite phase may create a passivation layer on the magnetite surface and stop further reduction of Cr(VI). Similar problems with surface oxidation and passivation are likely to occur in the application of zero-valent metal for in situ remediation
strategies. There are numerous studies investigating Fe(0) for remediation of groundwaters contaminated by chromate, arsenic and organic contaminants (20, 27). Many of these studies have observed that the reductive capacity of zerovalent Fe degrades over time, and some authors have postulated that ferrous or ferric reaction products precipitate on the metal, slowing the contaminant reduction rates (20). On the other hand, formation of goethite can affect transport of many contaminant ions through sorption reactions, because goethite structure has channels (grooves) that can serve as excellent attachment sites for adsorbed species (17). In the downstream of tank leakage with less direct HLW impact, this mechanism may be more important. Thus, transformation of magnetite to goethite in the Hanford sediments may have altered contaminant migration in the subsurface environment impacted by HLW plumes. The results of this study further the understanding of Cr behavior at high-level tank waste conditions and supplement efforts to model the transport of Cr at the Hanford site. This study also gives insight into the behavior of other contaminants in the Hanford tank waste, especially redox active contaminants such as Tc and U. Chemical reactions between minerals and alkaline solutions occur in a variety of geological and engineered environments. For example, Lake Magadi and alkaline flooding of sandstone reservoirs causes geologically high pH environments (41), and emplaced concrete stabilization of soil with lime produces local high pH conditions (42, 43). Therefore, this study will be applicable to the behavior of chromium and other contaminants in these high pH environments as well.
Acknowledgments The authors thank Dr. J. Bigham of The Ohio State University for his help with the XRD analysis, Dr. C. Chen at University of Texas for assistance with XAS experiments, and Mr. D. Beak of The Ohio State University for help with magnetite synthesis. The study was funded by EMSP, Department of Energy (Grant DE-FG07-99ER15010). Portions of this research were carried out at the Stanford Synchrotron Radiation Laboratory, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. The SSRL Structural Molecular Biology Program is supported by the Department of Energy, Office of Biological and Environmental Research, and by the National Institutes of Health, National Center for Research Resources, Biomedical Technology Program.
Supporting Information Available The characterization of synthesized magnetite by XRD and TEM. This material is available free of charge via the Internet at http://pubs.acs.org.
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(27)
(28)
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(30)
(31) (32)
(33)
(34)
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(38)
(39)
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(41) (42)
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Received for review October 19, 2004. Revised manuscript received March 16, 2005. Accepted March 17, 2005. ES0483692
