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a Dep. of Geological and Environmental Sciences, Stanford Univ., Stanford, CA 94305-2115
b Dep. of Civil Engineering, 208 Harbert Engineering Center, Auburn Univ., Auburn, AL 36849-5337
c Environmental Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6038
* Corresponding author (fendorf{at}stanford.edu)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: 
2, Debye-Waller factor • 
(k), EXAFS spectral contribution • CN, coordination number • E0, energy of the U LIII edge • EXAFS, extended x-ray adsorption fine structure spectroscopy • FT, fourier transform • HF, Hanford • OR, Oak Ridge • R, bond length • RSF, radial structure function • SR, Savannah River • XANES, x-ray absorption near-edge structure • XAS, x-ray absorption spectroscopy • Z, type of the atoms coordinating U
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Under typical environmental conditions, U is stable in either the tetra- or the hexavalent state. Uranium(IV) is found in reducing environments and is exceedingly insoluble in natural waters (for a complete review of U solution chemistry, see Grenthe et al., 1992, and Shock et al., 1997). Under oxic conditions, U is usually present as UO2+2, the uranyl cation, which forms stable solution complexes and solids with a range of anions. In acidic environments, uranyl speciation is dominated by the free uranyl cation. At pH > 4, cationic uranyl hydroxide and uranyl carbonate complexes form, of which the latter complexes are anionic above pH 9. The solution speciation of U largely determines how it partitions with the solid phase (Payne and Waite, 1991; Waite et al., 1994; Duff and Amrhein, 1996; Davis et al., 1998).
Detailed investigations of uranyl sorption have been performed for a range of soil solids. Iron (hydr)oxides, including ferrihydrite (Waite et al., 1994; Morrison et al., 1995; Allard et al., 1999; Moyes et al., 2000), goethite (Combes et al., 1992; Kohler et al., 1992), and hematite (Tripathi, 1983; Hsi and Langmuir, 1985; Ticknor, 1994; Bargar et al., 1999) sorb U(VI) strongly under neutral to slightly basic conditions. Dissolved carbonate complexes form under strongly alkaline conditions, leading to desorption of U. A variety of surface complexes have been postulated, although recent spectroscopic data suggests that U carbonate ternary complexes may be the principal surface species (Duff and Amrhein, 1996; Bargar et al., 1999, 2000). Clay minerals also sorb UO2+2 at both permanently and variably charged sites (Tsunashima et al., 1981; Chisholm-Brause et al., 1994; McKinley et al., 1995; Turner et al., 1996; Hudson et al., 1999). Outer-sphere complexes dominate at the permanently charged sites [the (001) plane] and inner-sphere complexes form at variably charged, edge sites. Evidence for U clustering also was found at higher pH and high ionic strength conditions (McKinley et al., 1995). Sulfide minerals such as pyrite, present in anoxic environments, lead to the reduction of uranyl cation and subsequent precipitation of mixed valence oxides (Wersin et al., 1994; Moyes et al., 2000). Carbonates (Carroll et al., 1992; Sturchio et al., 1998; Reeder et al., 2000) and phosphates (Drot and Simoni, 1999) also retain U.
Mechanistic investigations of uranyl sorption on heterogeneous soils and sediment are complicated by the presence of multiple minerals, biological activity, and organic matter. In these systems, chemical and microbial processes can reduce U to form insoluble U(IV) and mixed-valence oxides (Bertsch et al., 1994; Morris et al., 1996; Duff et al., 1997). Uranyl precipitates, including uranyl phosphates (Morris et al., 1996; Arey et al., 1999), hydroxides (Hunter and Bertsch, 1998; Sowder et al., 1999), and carbonates (Abdelouas et al., 1998) can form in oxic, highly contaminated soils and sediments. However, uranyl carbonate complexes dominate and limit sorption under aerated soil conditions and neutral or higher pH (Pratopo et al., 1990; Duff and Amrhein, 1996; Mason et al., 1997). Dissolved organic matter (Li et al., 1980; Owen and Otton, 1995; Arey et al., 1999), sulfate (Geipel et al., 1996), or phosphate (Sandino and Bruno, 1992) may also complex significant fractions of the uranyl cation. Soluble complexes not only limit sorption, but often result in enhanced transport of U (Choppin and Wong, 1998; Gabriel et al., 1998; McCarthy et al., 1998).
Barnett et al. (2000) studied uranyl adsorption to three subsurface media under static and dynamic flow conditions. For each medium, U sorption was appreciable between approximately pH 4.5 and 8.5, reversible, and modeled with a Freundlich isotherm. Uranium solution concentration influenced the sorption envelope position. Magnesium, calcium, and dissolved silica competitively inhibited U(VI) retention, suggesting that at least some of the U was bound to exchangeable adsorption sites. Solids adsorbed significantly more U in hydrodynamic experiments than in batch experiments, possibly due to competitive effects of silica (Gabriel et al., 1998). The Hanford (HF) medium retarded U transport nearly twice as much as Oak Ridge (OR) and Savannah River (SR) media. Uranium transport was asymmetric, due to nonlinear adsorption, multiple adsorption sites, and kinetic factors. These data suggest that there were multiple site-types of different reactivity, and complex kinetic factors influenced sorption.
Reversible adsorption and the inhibition by weakly coordinating cations have traditionally been used to suggest that outer-sphere complexation is an important adsorption mechanism. However, inner-sphere complexes can also be labile and thus cannot be ruled out based on these macroscopic measurements. In fact, the extensive partitioning of U(VI) to the solid phase suggests that some U is likely bound as an inner-sphere complex. However, any inner-sphere complexes formed must be relatively labile to describe the reversible sorption observed in both in batch and flow experiments.
Uranyl adsorption in each of the media was remarkably similar despite widely varying mineralogies (Barnett et al., 2000). Clay-sized particle content (largely clay minerals) was not correlated to sorption, and consequently, clay minerals are not likely to be the principal sorbent; silica, organic matter, and manganese oxides can similarly be ruled out. However, sorption was highly correlated to iron content in these media, which is near 25 g kg-1 for each. This study tests the premise that iron (hydr)oxides are responsible for the observed adsorption properties.
X-ray absorption spectroscopy provides an in-situ, element-specific probe of the local structure of the surface complex, and with it the principal sorption mechanisms (Fendorf et al., 1994; Conradson, 1998). Accordingly, we used XAS to further study the sorption mechanism of uranyl on several soils and sediments that are associated with U contamination. The direct application of XAS to the study of uranyl sorption complexes in soils and sediments is complicated by the heterogeneity of natural soil materials; nevertheless, these studies help to determine the structure of adsorbed U in these complex natural media. Accurately determining the surface structure of U is vital to assess the fate of this hazardous element in the environment. Strong sorption complexes and surface precipitates are the least labile and thereby decrease U bioavailability and transport. A detailed understanding of the sorption mechanism will provide insight into its stability, and thus its transport and bioavailability. It will also help to determine the principal soil minerals that control U sorption.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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48 h. The suspensions were filtered and the residual solution U quantified using a kinetic phosphorescence analyzer. Adsorbed U was inferred by difference between initial and final U concentrations. The wet solids were packed into a 5 x 5 x 30 mm polycarbonate sample holder and sealed with Kapton film for analysis. Media reacted with U in transport columns were also studied. A 1-cm diameter by 1.7-cm long column was packed with solids. These columns were then equilibrated with 0.01 M NaNO3. Uranium was added to this solution (5 mg L-1 net concentration) and passed through the column at a flow rate of 4.3 cm h-1, corresponding to a 0.18 h residence time. The U content of the effluent was monitored and sorption was determined by difference between the integrated influent and effluent concentration. Once appreciable sorption was achieved, the solids were removed from the column and placed in a polycarbonate sample holder for x-ray absorption spectroscopic analysis. For both column and batch studies, samples were analyzed by XAS within a week of their inception to minimize changes due to increased reaction time with residual solution.
MINTEQA2 (Allison et al., 1991) was used to determine the speciation of uranyl cation as a function of pH, and to determine the saturation indices of U minerals. The MINTEQA2 database was modified to reflect the uranyl equilibrium constants presented by Grenthe et al. (1992). The partial pressure of CO2, PCO2, was assumed to be in equilibrium with the atmosphere, 3.55 x 10-4 atm, and an oxidized redox potential was established by the complete exchange with atmospheric oxygen (0.2 atm O2). Calculations assumed room temperature and a solution composition representative of the highest ion concentrations found during the course of reaction.
XAS Spectroscopy
X-ray absorption spectroscopy was performed at the Stanford Synchrotron Radiation Laboratory on beamlines 4-2 or 4-3. The storage ring operated at 3.0 GeV and at currents between 50 and 100 mA. Spectra were taken with a Si(220) double-crystal monochromator with an unfocused beam. Incident and transmitted intensities were measured with 15-cm N2-filled ionization chambers. Sample fluorescence was measured with a multi-element Ge detector oriented 45° off the sample and orthogonal to the incident radiation. The beam was detuned 50% to reject higher-order harmonic frequencies and to prevent detector saturation.
X-ray absorption spectra were collected from –200 to +1000 eV about the LIII-edge of U (17 166 eV). At least 5 spectra were collected for each sample and averaged for analysis. Internal calibration was achieved with uranyl nitrate between the second and third ionization chambers; its inflection point was set at 17 176 eV.
Data Analysis
X-Ray Absorption Near-Edge Structure Spectroscopy
The x-ray absorption near-edge structure (XANES) spectra were analyzed using Peak Fit 4.0 (Jandel Scientific, San Rafael, CA) and WinXAS (Ressler, 1997). The background was subtracted and the jump height normalized to unity for comparison. No smoothing of the raw spectra was done to preserve spectral line-shapes. These spectra were fit using linear combinations of U(IV) and U(VI), as UO2 and uranyl nitrate standards, respectively (Bertsch et al., 1994). The fractions of each species in the fit were used to determine the oxidation state of the U in the subsurface media. Fits were verified to be accurate within 4% by quantifying the fractions of U(IV) and U(VI) in a series of mixtures of known composition. The fitting is based on the shift in binding energy of the U LIII-shell (2p orbital) as a function of oxidation state. The higher oxidation state, U(VI), has a larger effective nuclear charge, and thus has a slightly higher binding energy. First-derivative fits are more sensitive to the edge position, and consequently were also used in spectral comparisons. Smoothed (3% Savitsky-Golay smoothing) first-derivative XANES spectra were then fit using linear combinations of the first-derivative U(IV) and U(VI) standard spectra, similar to the method used for the raw spectra.
Extended X-Ray Adsorption Fine Structure Spectroscopy
Following spectral averaging, the background was subtracted from the spectra using a polynomial fit, and the resulting spectral jump heights normalized to unity. A six-point cubic spline function that followed the envelope of the decaying spectrum was used to isolate the extended x-ray adsorption fine structure spectroscopy (EXAFS) spectral contribution (the (k) function). The energy (eV) scale was transformed to k-space using 17 166 eV as the energy of the U LIII edge (E0). The (k) spectrum was then weighted by k2 in order to amplify the upper k-range and Fourier-transformed without smoothing to produce a radial structure function (RSF); a k-range of 3 to 15 Å-1 was used. Distinct shells of the RSF function were then back-transformed to isolate the spectral contributions of each atomic shell. Final fits were completed using unfiltered k-weighted (k) spectra.
The WinXAS software package was used for EXAFS data analysis using phase and amplitude functions derived using FEFF 7.02 (Zabinsky et al., 1995; Rehr et al., 1991). Single and multiple scattering paths were considered, although no multiple scattering paths were required for fitting. The accuracy of these phase and amplitude functions were confirmed by comparing fits of uranyl nitrate, U(IV) oxide, and U(VI) oxide with known structures. Phase and amplitude functions calculated using FEFF were also similar to those extracted from the spectra of known standards, in agreement with previous findings (Thompson et al., 1997).
The type (Z), coordination number (CN), distance (R), and the Debye-Waller factor (2) of the atoms coordinating U were determined by fitting the experimental spectrum. The 2 is effectively the variance in the bond length and is a measure of the disorder of the coordination environment. Each variable was independently varied except E0 and 2. E0 was constrained to the same value for each shell. Fitting did not appreciably change the values of 2; thus, 2 was constrained to minimize the number of variables required for fitting. The 2 was set to 0.002 for the axial oxygens and 0.005 for other shells while the CN was varied; these values were selected based on (UO2)(CO4-3)3 solutions (Bargar et al., 1999). For OR samples, the equatorial oxygens had greater disorder, thus 2 was varied in fitting of this shell. Once the filtered spectra were fit, the resulting parameters were combined and refit to the unfiltered (k) spectrum. The accuracy of the fits was estimated using the 2 statistical parameter, for which smaller values correspond to the best fits. Each fit had a reduced 2 of 6000 for unsmoothed k2(k) spectra, and 420 using RSFs. By comparison with model compounds, the interatomic distances are accurate within 0.02 Å, and the coordination number is within 30% for the first shell, but is less accurate for more distant shells. Elements of similar atomic number (Z ± 1) cannot be distinguished due to similarities in the phase and amplitude functions, although differences in local structure (i.e., interatomic distances) may help to determine more information about which element is present.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Local Structure
Extended x-ray absorption fine structure spectroscopy is useful for determining the average local structure of U sorbed to subsurface media. The EXAFS spectra of hydrated uranyl cations contain three principal shells. The first shell is characteristic of the axial oxygens of the uranyl cation, and the second shell corresponds to the oxygens from water and other ligands in the equatorial plane. The second shell may also contain contributions from second-nearest neighbor ligands such nitrate, carbonate, or carboxylate. More distant neighboring atoms also form a third coordination shell. For comparison, the structural parameters for selected uranyl species are included in Table 2.
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2 oxygens at a R = 1.78 Å, d(U-Oax), similar to that reported for other UO2+2 species (Table 2). Furthermore, the axial oxygen coordination environment of uranyl is not altered appreciably when sorbed (Waite et al., 1994; Chisholm-Brause et al., 1994; Hudson et al., 1999) or incorporated into solids (Mercier et al., 1995; Allen et al., 1996; Reeder et al., 2000); thus, changes in the axial coordination environment are not expected.
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2.30 Å and the second at a somewhat longer distance, R = 2.42 Å. For HF1 and SR1, both equatorial O distances were fit with similar coordination numbers. However, OR soils exhibit larger coordination numbers and disorder for the U-Oeq shell at 2.41 Å. These coordination environments imply that two distinct species coordinate the uranyl cation in the equatorial plane, and that the quantity of these species is comparable for HF1 and SR1 media, while one species is more significant for the OR soil.
Uranium sorption on the subsurface media studied here is reversible under hydrodynamic conditions (Barnett et al., 2000), similar to labile outer-sphere surface complexes that often form on clay minerals (Chisholm-Brause et al., 1994; Hudson et al., 1999). However, outer-sphere sorption of hydrated uranyl cations is inconsistent with the spectroscopic data. Such outer-sphere complexes contain a single U-O distance for equatorial oxygen at R 2.43 Å. The presence of two distinct d(U-Oeq) signifies that formation of fully-hydrated outersphere complexes is not the only mechanism of sorption.
The split in the equatorial oxygen shells further implies that ligands other than water are present in the equatorial plane. Chemically bound ligands disrupt the equatorial waters, and result in the formation of an elongated U-Oligand distance and shortened equatorial U-OH2O distance. For example, uranyl nitrate hexahydrate contains two bidentate nitrate groups with a d(U-ONO3) of 2.52 Å, while equatorial waters have contracted Rs of 2.39 Å (Table 2, Fig. 3). Similar variation in the coordination environments of equatorial oxygen is found for uranyl carbonate (Bargar et al., 1999, 2000) and uranyl phosphate (Mercier et al., 1985) complexes.
Several structural models explain the presence of a split equatorial oxygen shell. Surface hydroxyls of many soil minerals could form inner-sphere complexes with the uranyl cation, with longer bonds to the surface bridging oxygens and shorter U-O bonds to the residual waters. Such inner-sphere complexes form on ferrihydrite (Waite et al., 1994) and could similarly form on other surface hydroxyl sites such as aluminol or silanol groups (Hudson et al., 1999). However, recent evidence also suggests that U sorption may form inner-sphere ternary surface complexes (Bargar et al., 1999, 2000) and even polymeric species (Waite et al., 1994; Hudson et al., 1999) coordinated to both a surface and a ligand.
Examination of more distant neighbors provides a means to determine which surface complexes are most significant under the conditions of this study. For HF1, an additional spectral feature is present as a high-distant shoulder to the equatorial oxygen in the Fourier transform (FT) spectrum (Fig. 3). We fit this spectral feature with 1 C atom at 2.89 Å, similar to carbonate ternary surface complexes proposed for uranyl sorption on iron oxides (Barger et al., 1999, 2000). The structural environments for other bidentate carbonate complexes also have a similar d(U-C) and split in the equatorial oxygen shell. A U-Fe shell with R 3.42 Å is also present, similar to that of uranyl carbonate ternary complexes on ferrihydrite (Bargar et al., 1999, 2000). Thus, EXAFS data suggest that uranyl carbonate ternary complexes form on iron (hydr)oxides for HF1.
SR and OR solids exhibit different second-neighbor coordination; each containing a spectral feature in the FT spectra is noted at a R (uncorrected for phase shift) of 3.1 Å of SR and OR solids (Fig. 3 and 4). This feature is best described with a U-P shell at a R of 3.61 Å (Fig. 5 , Table 4). The U-P distance correlates with other bidentate mononuclear phosphate complexes (Table 2, Mercier et al., 1985) and for uranyl adsorbed on phosphate minerals (Drot et al., 1998). The spectral feature attributed of OR and SR media at 3.1 Å could also arise from multiple scattering (MS) phenomena. Such MS features are significant in highly symmetric systems (Thompson et al., 1997), although they are less significant in more disordered systems. Bargar et al. (2000) have observed a MS peak arising from double-focusing from the axial oxygens at a slightly shorter uncorrected distance; however, they found this peak was a relatively insignificant portion of the intensity of the spectral feature. Similarly, multiple scattering peaks do not adequately describe our data, particularly at k < 6 (Fig. 5, Table 4). Although the feature alone is not statistically significant, it does significantly improve the fit of the shell when used in addition to P and Fe, suggesting MS probably does contribute to the spectral feature observed in the RSF at 3.1 Å. However, the shells can effectively be ignored because they have very low intensity and their inclusion does not appreciably influence the fit distance for either P or Fe.
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1 µM phosphate would be required to precipitate autunite, a less stable uranyl phosphate that is often formed for kinetic reasons. Consequently, phosphate precipitates are not likely to explain the phosphate coordination in these samples. In contrast, soluble uranyl phosphate complexes are reasonable explanations of P spectral features observed in the EXAFS data. Aqueous uranyl phosphate complexes commonly formed in solutions having 
ratios greater than 10 (Sandino and Bruno, 1992; Brendler et al., 1996) and may be similar to the surface complexes in these studies. The equilibrium ratios predicted by MINTEQA2 are somewhat lower, 0.2 to 1 for these media; thus, formation of uranyl phosphate complexes would be confined to the surfaces where higher concentrations of adsorbed phosphate drive the formation of ternary complexes in these media.
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0.5, indicates that only a fraction of adsorbed U is complexed to phosphate. Hence, other uranyl surface complexes, including those directly on iron (hydr)oxides or clay minerals, are also likely to be present. Extended x-ray absorption fine structure spectroscopy cannot distinguish between P, Si, or Al; it is thus possible that U-Al or U-Si contributions explain a portion of the observed U-P shell in OR and SR media. A bidentate, mononuclear surface complex formed on aluminosilicate clay minerals would result in a d(U-Si/Al) between 3.4 and 3.7 Å, similar to the measured d(U-P). Exented x-ray absorption fine structure spectroscopy spectral features attributed to Si or Al have rarely been observed for inner-sphere uranyl complexes on clay minerals (Chisholm-Brause et al., 1994; Hudson et al., 1999; Sylwester et al., 2000); however, Moyes et al. (2000) detected d(U-Si) at both 2.75 Å (to Si) and 3.66 Å (to Al). While the 3.66 Å peak may be observed in our data, the U-Si peak at 2.75 Å is absent. Additionally, inner-sphere uranyl complexes on silica contain shorter U-Si distances, R = 3.2 Å (Sylwester et al., 2000), compared with the R = 3.6 Å distance observed in these experiments. Therefore, it is unlikely that Al and Si backscattering features alone describe the U-P shell. The lack of Al and Si spectral features does not preclude the presence of inner-sphere sorption to clay minerals, but the extent of sorption is less than our detection limits. Similarities in the sorption behavior of uranyl within each medium suggest a common phase may be important in retention; iron (hydr)oxides fit this criterion, having similar concentrations within each medium and also having an affinity for uranyl (Barnett et al., 2000). Extended x-ray absorption fine structure spectroscopy data partially supports this hypothesis in that the addition of an iron contribution to the phosphate shell improved the fits for the filtered spectra of all subsurface media examined (Table 4; Fig. 5). However, the U-Fe shell is insignificant when fitting was performed on the raw, unfiltered spectra for all samples except HF1. The complexity of uranyl coordination chemistry contributes to this statistical insignificance; only a given number of variables can be fit for a given k-range. Therefore, an iron shell is suggested but not conclusively present and is not included in the final fits of the raw spectrum; it is included in Table 3 only for reference.
In heterogeneous soil matrices, it is likely that a mixture of sorption complexes is present. In any case, the spectral data indicate that phosphate or carbonate ternary complexes form an appreciable fraction of the adsorbed uranyl species. These ternary complexes could be inner- or outer-sphere complexes on soil minerals. However, the relatively high ionic strength used in these experiments (0.01 M) would impede the formation of outer-sphere complexes on clay minerals (Turner et al., 1996). Therefore, much of uranyl adsorption is probably inner-sphere to the iron (hydr)oxides as is suggested by the presence of Fe in the local coordination sphere. Examples of possible inner- and outer-sphere complexes on a typical iron (hydr)oxide are depicted in Figure 6
. Ternary uranyl carbonates on iron (hydr)oxides have also been suggested previously (Waite et al., 1994; Duff and Amrhein, 1996; Bargar et al., 1999, 2000). Outer-sphere adsorption of uranyl carbonate or phosphate complexes on aluminosilicate clays, suggested by Sylwester et al. (2000) for uranyl ions, is not likely to form in these experiments because the clays are negatively charged under these experimental conditions and are unlikely to adsorb significant quantities of anionic uranyl phosphate or carbonate complexes. However, inner-sphere ternary complexes could be present at the edges of clay minerals. Additionally, outer-sphere surface complexes on iron hydroxides could also be present due to the favorable interaction between the positively-charged iron (hydr)oxide surface and negatively-charged adsorbed ternary complexes. Unfortunately, spectroscopic verification of outer-sphere complexes is not possible using XAS. The low coordination number of U to Fe in fitting provides indirect evidence supporting the existence of outer-sphere complexes; it suggests that inner-sphere complexes represent only a portion of the sorbed U. It should be noted, however, that constrained 2s could further reduce the calculated coordination numbers relative to the actual value (i.e., the reported values are very conservative estimates and are likely much lower than the actual values). Additional complexes are likely present but not detected due to disorder or destructive interference.
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+5, UO2+ and UO2CO03 (Grenthe et al., 1992; Shock et al., 1997). Despite the stability of multinuclear solution complexes in OR3 and OR4 (pH 6.55 and 5.72, respectively), they did not comprise a significant fraction of sorbed U; this is consistent with the surface complexation modeling of Waite et al. (1994).
Loading Effects
The effects of loading method were studied by examining the conditions under which UO2+2 is sorbed on OR soils. Samples OR1 and OR3 were loaded with U(VI) in batch experiments equilibrated for 48 h to allow for near steady-state sorption. Samples HF1, SR1, OR2 and OR4, were reacted under hydrodynamic conditions. The loading method influenced the partition coefficient in the transport models of Barnett et al. (2000), and the models that were used to describe transport phenomena indicate kinetic limitations in uranyl sorption. While these factors suggest some differences in retention mechanism may arise from batch and column loading, the spectra exhibit no significant differences that can be attributed to the method of loading. In fact, the spectral similarities between uranyl adsorbed to OR media suggest that similar surface complexes are found regardless of loading method. This does not preclude the presence of different surface complexes; differences probably exist that are too subtle to be detected using XAS. However, structural similarities may be a result of reversible control on sorption in both column and batch loaded samples. Alternatively, differences in spectra may have been moderated by the time between reaction and analysis.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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Received for publication April 16, 2001.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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17188 eV.
