Grazing-Incidence XAFS Study of Aqueous Zn(II) Sorption on α-al 2 O 3 Single Crystals

Transcription

1 Journal of Colloid and Interface Science 244, (2001) doi: /jcis , available online at on Grazing-Incidence XAFS Study of Aqueous Zn(II) Sorption on α-al 2 O 3 Single Crystals Thomas P. Trainor,,1,2 Jeffrey P. Fitts, Alexis S. Templeton, Daniel Grolimund, and Gordon E. Brown, Jr., Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305; Environmental Molecular Sciences Institute, Columbia University, New York, New York 10027; Swiss Light Source (SLS), Paul Scherrer Institute, CH-5232 Villigen, Switzerland; and Stanford Synchrotron Radiation Laboratory, Stanford, California Received June 21, 2001; accepted September 3, 2001; published online November 9, 2001 Grazing-incidence XAFS spectroscopy was applied to study the sorption of Zn(II) on two crystallographically distinct surfaces of highly polished sapphire single crystals as a simplified analog for metal ion sorption on natural aluminum-(hydr)oxides. Experiments were performed both in situ (in contact with bulk solution) and ex situ in a humidified N 2 atmosphere. The identification of an Al shell at roughly 3 Å in all samples indicates that Zn(II) binds as an inner sphere complex on both the (0001) and (1 102) surfaces. The first shell Zn O distances of Å suggest that Zn is in fourfold coordination with oxygen in the in situ samples. However, sample drying appears to have induced the formation of polynuclear surface complexes with first shell Zn O distances closer to values expected for sixfold coordination ( Å). The results presented here show that in situ characterization of sorption products on single crystal surfaces using Grazing-incidence XAFS is feasible if solution conditions are chosen carefully. C 2001 Elsevier Science 1. INTRODUCTION The distribution and mobility of dissolved metals in aquatic systems is controlled to a large extent by reactions with mineral surfaces (1 3). Reactions of dissolved metals with oxide surfaces are also important in several technological applications, such as preparation of metal-oxide on metal-oxide catalysts (4). Therefore, understanding the key variables involved in the sorption of dissolved metals on oxide surfaces is needed in a variety of environmental and technological applications. The relative stability of sorbed species is related to the mode of sorption (e.g., inner-sphere vs outer-sphere adsorption complexes or the formation of multinuclear clusters or surface precipitates), which is in turn influenced by the structure of the reactive substrate surface and solution conditions. The ability to distinguish among the various sorption modes provides useful information for constraining the overall sorption reaction stoichiometry and understanding the ultimate stability of surface reaction products. 1 To whom correspondence should be addressed. 2 Current address: Consortium for Advanced Radiation Sources, The University of Chicago, Building 434-A, 9700 South Cass Avenue, Argonne, IL Fax: (630) trainor@cars.uchicago.edu. In a number of studies, x-ray absorption fine structure (XAFS) spectroscopy has been successfully applied to the examination of the local structure of sorption products associated with high surface-area powdered substrates (5 7). However, one difficulty with this approach is the lack of surface-specific information, since the powdered materials often have multiple surface terminations and the composition and structure of the reactive surfaces are poorly constrained. Several studies have utilized Grazing- Incidence XAFS (GI-XAFS) spectroscopy to examine the local structures of sorption products on single crystal surfaces (cf. (8 13)), since this technique allows for using well characterized substrate surfaces and provides enhanced surface sensitivity compared to conventional XAFS on powder substrates. This approach can potentially provide information on the local structure of adsorption complexes at specific adsorption sites on single crystal surfaces. However, due to the low total surface area, it can be difficult to make measurements on samples in the presence of bulk solution (in situ), especially at low sorption densities, whereas ex situ studies are prone to artifacts introduced through sample drying. Furthermore, the results of GI-XAFS measurements are an average over a large area of the substrate surface, and therefore, will include contributions from sorbed species on both terrace and defect sites, which can potentially complicate the analysis. In this study we have used GI-XAFS spectroscopy to characterize the local structure of Zn(II) sorption products on oriented single crystals of sapphire (α-al 2 O 3 ). This work is a continuation of a previous study in which we examined the reaction products associated with Zn(II)/alumina powders (14) where it is proposed that Zn(II) binds as an inner-sphere mononuclear complex to AlO 6 polyhedral edge sites at adsorption densities <1.1 µmol/m 2 (ph 7 8) based on the observed Zn Al distances of Å. Further, we observed a distinct change in the first-shell Zn O bond lengths and coordination numbers upon adsorption, suggesting that the adsorbed species is fourfold coordinated by first-shell oxygen (R Zn O = 1.97 Å) at these low sorption densities, whereas aqueous Zn(II) is sixfold coordinated with first-shell oxygen (R Zn O = 2.06 Å). At higher sorption densities on alumina powders we observe a local structure about Zn(II) which is consistent with the formation of a /01 $35.00 C 2001 Elsevier Science All rights reserved

2 240 TRAINOR ET AL. mixed metal Zn/Al-hydroxide having a hydrotalcite type structure (brucite-type double layer hydroxide structure). This phase was identified based on the presence of Zn in sixfold coordination with first-shell oxygen neighbors, and a mixed second shell of Zn and Al at Å, as well as more distant shells characteristic of the structure type. This phase formed as a result of the dissolution of the alumina substrate and reprecipitation of the mixed-metal phase (also cf. (15 19)). Based on the results of the previous study we could constrain the overall reaction of Zn(II) with alumina under the conditions of the study; however, the types of reactive sites involved in the adsorption reaction are poorly constrained. The objective of this work is to examine the sorption products on single crystal surfaces of alumina to further constrain the predominant sorption reactions in the Zn(II)/aluminum-(hydr)oxide system. We have chosen to use the α-al 2 O 3 (0001) and (1 102) surfaces as model substrates. These surfaces are well characterized and have been used in previous studies of Pb(II), Co(II), and Cu(II) adsorption (8, 9, 13, 20, 21). The structures of the hydrated surfaces have been examined using x-ray scattering (22, 23), which shows that the two surfaces differ in the predominant coordination environments of the surface oxygen groups and therefore serve as good model surfaces for the investigation of how metal binding may be influenced by surface structure. 2. MATERIALS AND METHODS Highly polished single-crystal α-al 2 O 3 (Union Carbide Crystal Products) substrates of the specified orientation (2-inch diameter) were washed in 0.01 M nitric acid, with multiple water rinses prior to equilibration with metal solutions. Samples were characterized by XPS (Surface Science S-Probe, monochromatic AlK α radiation) prior to reaction to ensure that the surfaces were free of contamination (estimated detection limit of zinc is 0.05 µmol/m 2 ) and after reaction to estimate metal ion uptake (Table 1). The clean substrates typically had rms roughness values of <10 Å based on modeling the grazing-angle reflectivity. Details of the characterization of these surfaces are given by Eng et al. (22) and Trainor et al. (23). The Zn(II) stock solutions were prepared from Zn(NO 3 ) 2 salts using 18 M water from a Barnstead water system. All solutions were prepared under a N 2 atmosphere to prevent CO 2 contamination, and the ph adjusted to 7 using 0.01 M NaOH. Solutions were prepared with 0.01 M NaNO 3 as the background electrolyte unless otherwise noted. All chemicals used to prepare stock solutions were of reagent grade quality. In the concentration range of interest in this work the predominant species of dissolved Zn(II) is the hexacoordinated cation Zn 2+ (H 2 O) 6 (24, 25). Experiments were performed at the Stanford Synchrotron Radiation Laboratory (SSRL) on beamlines 6-2 and 4-2 using Si(111) or Si(220) double-crystal monochromators, the SSRL grazing incidence apparatus, and a 13-element Ge array detector (Canberra). A Pt-coated focusing mirror located upstream of TABLE 1 Curve Fitting Results for Zn(II)/α-Al 2 O 3 Samples Sample Orient Shell N eff R (A ) σ 2 (A 2 ) A. Zn(II)/α-Al 2 O 3 Zn O 5.7(4) 2.06(1) (1 102), ex-situ A1 Zn Al 3.2(7) 3.09(2) 0.01 Ɣ = 0.2 horz Zn Zn 2.0(6) 3.83(2) 0.01 Zn O 6.8(4) 2.06(1) A2 Zn Al 3.6(6) 3.05(2) 0.01 horz Zn Zn 2.4(6) 3.82(2) 0.01 Zn O 6.2(4) 2.07(1) A3 Zn Al 2.1(7) 3.07(2) 0.01 vert Zn Zn 0.6(6) 3.80(6) 0.01 B. Zn(II)/α-Al 2 O 3 B1 Zn O 5.2(2) 1.97(1) (1 102), in-situ horz Zn Al 1.4(8) 3.05(4) 0.01 Ɣ = 1.9 a B2 Zn O 4.4(3) 1.97(2) vert Zn Al 1.9(6) 2.97(3) 0.01 C. Zn(II)/α-Al 2 O 3 C1 Zn O 5.8(5) 2.06(1) vert Zn Al 1.0(8) 2.98(6) 0.01 (0001), ex-situ Zn O 6.3(6) 2.08(1) Ɣ = 0.3 C2 Zn Al 5(1) 3.07(2) 0.01 horz Zn Zn 4(1) 3.84(2) 0.01 D. Zn(II)/α-Al 2 O 3 D1 Zn O 5.3(4) 2.07(1) (0001), ex-situ horz Zn Al 4.6(8) 3.10(2) 0.01 Ɣ = 0.2 Zn Zn 3.3(6) 3.86(2) 0.01 E. Zn(II)/α-Al 2 O 3 E1 Zn O 4.8(3) 1.98(1) (0001), in-situ horz Zn Al 2.7(7) 3.13(2) 0.01 Ɣ = 0.3 E2 Zn O 5.5(4) 1.99(1) vert Zn Al 1.4(6) 3.07(4) 0.01 Note. Estimated errors at the 95% confidence interval for the last significant figure are given in parenthesis. Parameters without reported errors were held fixed in final fit. Solution conditions for the samples are 30 µm Zn(NO 3 ) 2 at ph 7 in 0.01 M NaNO 3 background electrolyte (sample D was prepared without the addition of the NaNO 3 ). Surface coverage estimates (Ɣ) are given in µmol/m 2 and were determined by XPS measurements on dried samples (see (9) for a discussion of the procedure). Equilibration times were 16 h for samples A and C and 1 h for samples B, D, and E. a This high coverage number is likely a result of precipitation of Zn-nitrate salt upon removal of the sample from the in situ cell. the monochromator was used on both beam lines. The beam was collimated to roughly 150 µm vertically before the Io chamber (N 2 filled ionization chamber). In situ samples were mounted in a Teflon solution cell and sealed with a thin (1.5 µm) polypropylene film. The cell was purged with N 2 prior to being filled with the metal-bearing solution. After equilibration a negative pressure was applied to the cell to generate a thin water film over the crystal surface. The thickness of the water film (typically about 1 2 µm) was characterized by x-ray reflectivity prior to collection of XAFS data. During data collection the air space above the cell was purged with humidified N 2 to prevent evaporation of the water film. Ex situ samples were prepared by equilibration in the metal-bearing solution in a N 2 -purged glove box, and withdrawn under a N 2 jet to remove excess solution. During data collection the samples were maintained in a humidified (>80% r.h.) N 2 environment.

3 AQUEOUS Zn(II) SORPTION ON α-al 2 O 3 SINGLE CRYSTALS 241 XAFS data were collected with the incident angle set slightly below the critical angle of the oxide substrate ( 0.2 ). Data collection was performed in both horizontal (E-vector parallel to substrate surface) and vertical (E-vector perpendicular to substrate) sample orientations. Additionally, horizontal data on the ex situ α-al 2 O 3 (1 102) sample were collected with the E-vector perpendicular ( ) and parallel ( ) to the [ 111] direction. Data analysis was performed using EXAFSPAK (26) with phase and amplitude functions calculated using Feff 7.0 (27, 28) for Zn O, Zn Al, and Zn Zn pair correlations (see Trainor, et al. (14) for additional details). Individual shells were identified by fits to Fourier-filtered shells from the raw EXAFS. Final fits were performed on the raw EXAFS data over the k-range of approximately 3 10 Å 1 with seed values from fits to the filtered shells. Estimated accuracy of results based on fits to model compounds for the first, second, and third shells are ±0.02, ±0.05, and ±0.1 Å for R and ±10, ±30, and ±50% for the coordination number, respectively. It is noted that coordination numbers obtained from fits to polarized EXAFS data are effective coordination numbers (N eff ), dependent on the orientation of the polarization vector relative to the absorber backscatterer vector (29). For K-edge XAFS the effective coordination number is given by N eff = N i=1 3 e r 2, where the sum is over all identical neighbors in a given shell, e is the unit vector along the direction of the electric field and r is a unit vector between the absorber and the backscatterer. Therefore, the largest N eff results when the polarization and absorber backscatterer vectors are parallel; however, for a symmetric environment (such as an octahedral or tetrahedral coordination sphere) N eff will give the same result as the true coordination number (cf. (10)). 3. RESULTS AND DISCUSSION Solution conditions were chosen to avoid saturation with respect to Zn(II)-hydroxides, carbonates, and basic salt precipitates (24) while obtaining sufficient metal uptake for XAFS data collection. For in situ measurements it is necessary that the signal from the surface-bound species dominate over the signal from solution species. The ratio of surface-bound to dissolved Zn(II) is a function of the thickness of the overlying solution layer, solution concentration, and sorption density (Ɣ). We found that we could typically maintain a solution layer thickness of roughly 1 µm, while thinner layers were difficult to generate and maintain. Figure 1 shows the XAFS spectra and corresponding Fourier transforms (FTs) for aqueous Zn(H 2 O) 2+ 6 and Zn(II) adsorbed on alumina powder versus in situ GI-XAFS spectra for 30 and 300 µm Zn(II) on an α-al 2 O 3 (1 102) substrate. While the signal quality is significantly poorer for the 30 µm GI-XAFS spectrum, the EXAFS frequency is more similar to the powder sorption sample than the 300 µm sample. The 300 µm sample is dominated by the aqueous Zn(H 2 O) 2+ 6 component, based on a fitted first shell Zn O distance of 2.07 Å and lack of a strong second shell in the FT. Therefore, it appears that with an order of magnitude decrease in the solution FIG. 1. Comparison of EXAFS spectra for Zn(II) on the α-al 2 O 3 (1 102) surface at 30 and 300 µm solution concentration versus aqueous Zn(II) and Zn(II) sorbed on alumina powders. concentration the ratio of surface-bound to solution species has increased significantly. This may be indicative of near saturation of reactive sites at higher concentration or multiple reactive sites, since the ratio of surface-bound to solution species would be expected to remain constant in the linear regime of a pure Langmuir-type adsorption isotherm (2). Based on the results of these measurements, the final solution conditions used for sample preparation were 30 µm Zn(NO 3 ) 2 at ph 7 in a 0.01 M NaNO 3 background electrolyte. Zn K-edge GI-EXAFS spectra and FTs (uncorrected for phase shift) for in situ and ex situ samples on the (0001) and (1 102) surfaces of α-al 2 O 3 are shown in Fig. 2, and curve fitting results are presented in Table 1. The first-shell Zn O distances presented in Table 1 fall into two distinct ranges: Å for the in situ samples and Å for the ex situ samples. The first-shell Zn O distance is well correlated with the first-shell coordination number, with R Zn O 1.96 Å typical of fourfold coordination and R Zn O 2.06 Å typical of sixfold coordination (30). Therefore, based on the first-shell Zn O distances (Table 1), it appears that the dominant Zn coordination is fourfold for the in situ samples, while in the absence of the bulk solution Zn is predominantly in sixfold coordination with nearest-neighbor oxygen. This is also reflected in the fitted values for the coordination numbers, where the average of the first-shell coordination numbers of the in situ samples is about 20% lower than the average ex situ value. In all samples we chose to fix the Debye-Waller factors so that comparisons of the coordination numbers could be more easily made across the series of samples. We note that the coordination numbers and Debye-Waller factors are highly correlated, and the chosen value of Å 2 for the Zn O first shell is based on our previous studies of Zn-adsorption complexes and Zn in model structures (14). Further, it appears that the first-shell environment in all the samples is not significantly distorted due to the lack of strong polarization dependence of the first-shell distance or effective coordination number.

4 242 TRAINOR ET AL. The observation of a difference in the first-shell coordination of Zn(II) upon adsorption for the in situ samples is consistent with our previous findings on alumina powders. We note that there should be some component of Zn(II) in sixfold coordination convolved with the in situ spectra due to the presence of the bulk metal solution. In most cases (for both in situ and ex situ samples) the spectra could be fit with two oxygen shells at roughly 1.97 and Å (this was particularly evident in sample C1). However, due to the limited range of data, a single shell was used, and the Zn O distances and coordination numbers reported in Table 1 are considered average values. A second-shell feature is apparent in the FTs of all samples in Fig. 2 at R Å. This feature corresponds to an Al neighbor with best-fit R Zn Al distances in the range of Å (Table 1). An examination of the fits to the Fourier filtered second-shell feature in Fig. 3 shows that an Al shell at approximately 3 Å reproduces both the phase and the amplitude envelope well. Fits with only a Zn shell were unsatisfactory; however, in the ex situ horizontal samples (A1, A2, D1, and C2) a good fit could be achieved with mixed Zn/Al shell, with the Zn Zn correlation at roughly 3.2 Å. The observation of a Zn Al shell indicates that Zn(II) is binding in an innersphere manner on both surfaces. Furthermore, the Zn Al distances are in good agreement with that found from our powder XAFS study for edge-sharing bidentate linkage association of Zn with AlO 6 polyhedra. The similarity of the second-shell Zn Al distances for both the (0001) and (1 102) surfaces suggest that FIG. 2. Comparison of EXAFS spectra and Fourier transforms (uncorrected for phase shift) for Zn(II)/α-Al 2 O 3 samples. FIG. 3. Comparison of the second- and third-shell fits for two representative spectra. The fits (dashed) are of the filtered shells (solid) as indicated. (a and c) Al gives the best fit for the second shell. (b and d) Both Al and Zn (and combinations of Zn/Al) give reasonable fits. *σ 2 was held fixed in all fits at 0.01 Å 2.

5 AQUEOUS Zn(II) SORPTION ON α-al 2 O 3 SINGLE CRYSTALS 243 edge-sharing sites on both surfaces are the preferred binding sites for Zn(II). A third shell is evident by the strong feature in the FTs of the ex situ horizontal spectra of both the (0001) and (1 102) samples at R Å, which is also apparent by the strong shoulder in the EXAFS at k 6 Å 1. This feature is best fit by Zn Zn correlation at roughly 3.8 Å (Fig. 3). While a combination of more distant Al shells or mixed Zn/Al shells could also fit the spectra, the relatively large phase shift of Al and large coordination numbers suggests that Zn Zn correlation is the best match (Fig. 3). Further, there appears to be little reason why a Zn Al correlation would appear after removal of bulk solution, since this strong shoulder would likely be evident in the in situ EXAFS as well. Therefore, it appears that an additional species is present in the ex situ samples and suggests that the increase in the first shell Zn O distance between in situ and ex situ samples is likely associated with the observed third-shell feature. The effect of polarization dependence is most evident in the third-shell feature of the ex situ (0001) and (1 102) samples. The third-shell feature in the FTs has a large amplitude in the horizontal orientation which is essentially absent in the vertical orientation. This is also evident in the shoulder at 6 Å 1 in the EXAFS spectra, where there is an apparent decrease in intensity of the shoulder for the ex situ (1 102) samples and a disappearance altogether on the (0001) as the polarization is changed from horizontal to vertical. However, we see little azimuthal polarization dependence of the second- or third-shell features on the (1 102) horizontal spectra (A1 vs A2). The strong vertical polarization dependence in the ex situ samples indicates that the third-shell correlation is highly oriented parallel to the surface plane. The polarization dependence of the in situ samples is much less evident, though there is some variation in the best fit values for the second-shell Zn Al distances (Table 1). This could be associated with Zn binding to multiple sites with various Zn Al distances and different polarization sensitivities; however, given the quality of the data, it is difficult to distinguish multiple Al shells within a small range of distances. The observation of Zn Zn correlations suggests that oriented clusters or precipitates have formed in the ex situ samples where the more distant Zn neighbors tend to lie in the plane of the surface. The ex situ samples were withdrawn from the bulk solution under a N 2 jet, and it is possible that evaporation of the solution led to an increase in the dissolved concentration of ionic species, allowing the solutions to reach saturation. However, this feature is present in the sample prepared without addition of NaNO 3 electrolyte (D) and in a duplicate sample of (A) which was washed with water prior to examination (not shown). The 3.8 Å Zn Zn distance is longer than would be expected for a next nearest neighbor zinc based on comparison to model compounds (Fig. 4), and other Zn-hydroxide or hydroxy-nitrate structures reported in the literature (cf. Table 2 in (19) and Table 1 in (31)), where a strong contribution from a zinc backscatterer in the range of Å would be expected. The observed Zn Zn FIG. 4. Model compound EXAFS and Fourier transforms (scaled for visualization). (i) 10 mm, ph 3.6, Zn(NO 3 ) 2 solution; (ii) Zn/Al coprecipitate; (iii) Zn 5 (NO 3 ) 2 (OH) 8 2H 2 O; (iv) ZnO; (v) ZnAl 2 O 4. distance is in the range of what might be expected for cornersharing linkage of hydroxo-bridged polyhedra. Therefore, while we cannot rule out the formation of oriented precipitates, it appears more likely that these features are due to the formation of multinuclear surface species. As mentioned above, the second shell of the ex situ samples could be fit with a mixed Zn/Al shell with the Zn Zn correlation at roughly 3.2 Å. This Zn Zn distance is most consistent with edge-sharing ZnO 6 polyhedra and we suggest that the presence of the Zn Zn correlation at 3.2 Å likely accounts for the large increase in the fitted values of the second shell Al coordination numbers between the in situ and ex situ samples (Table 1). In addition, the larger coordination numbers for the second- and third-shell features from the ex situ (0001) horizontal spectra as compared with the (1 102) spectra, suggest that there is a greater proportion of the multinuclear species present. 4. SUMMARY Our results show that in situ characterization of metal-ion adsorbates on single crystal surfaces is feasible; however, it requires careful control of the solution conditions, while examination of ex situ samples is prone to alteration of the reaction products induced by sample drying. For Zn(II) adsorption on α-al 2 O 3 (0001) and (1 102) single crystal surfaces we conclude that Zn(II) is bound in an inner-sphere manner, and the observation of an Al shell at roughly 3 Å is consistent with a bidentate linkage of Zn(II) to AlO 6 octahedra. The first-shell Zn O distances and coordination numbers suggest that Zn(II) is fourfold coordinated by oxygen in the in situ samples, whereas

6 244 TRAINOR ET AL. the average coordination appears to be sixfold in the ex situ samples. The results for the in situ samples are consistent with our previous study of Zn(II) sorption on α-al 2 O 3 powders (14) where Zn(II) was found to bind as an inner-sphere complex in fourfold coordination with nearest neighbor on the edge sites of AlO 6 polyhedra. Results for the (0001) and (1 102) samples are similar even though the structure of the two surfaces is significantly different. The hydrated α-al 2 O 3 (0001) surface is terminated by doubly coordinated oxygen (22), whereas the (1 102) surface has singly, doubly and triply coordinated oxygen exposed at the surface (23). While there are potential edge-sharing sites available on both surfaces, the low quality of the data hindered attempts to rationalize the adsorbate local structures in terms of available surface sites. However, at the low adsorption densities under the conditions of our study, defect sites may potentially be the most significant reactive sites. For example, in the recent work by Templeton et al. (21), it was found that Pb(II) binding on these surfaces could be modeled using two surface sites: a low density of high affinity ( defect sites ) and a second terrace site. The best fit values for the density of high affinity sites was on the order of 0.5 µmol/m 2 for both surfaces (21), which is in reasonable agreement with that estimated from photoemission studies of water sorption on clean alumina surfaces (32). In comparison with our estimated adsorption densities from XPS (Table 1), we see that these high-affinity sites are potentially the dominant reactive sites at these low coverages. Our results also show that the predominant speciation of sorbed Zn(II) differs between the in situ and ex situ samples. The increase in first shell Zn O distances suggests that the Zn coordination is predominantly sixfold in the ex situ samples, and the presence of strong third-neighbor contributions indicates the formation of oriented multinuclear complexes or oriented precipitates. We note that the solution conditions used for these experiments were at subsaturation with respect to the formation of homogeneous precipitates, and our results are inconsistent with the formation of mixed metal Zn/Al-hydroxide phases as previously observed in experiments on powder substrates (14). Therefore, as discussed above, we suggest that it is more likely that we have induced the formation of two-dimensional multinuclear complexes at the surface. A similar result was observed by Waychunas et al., who found that Fe(III) formed multinuclear complexes on quartz surfaces, with a local structure similar to that found in hematite, after sample drying (12). Further, given that sorption at low metal ion loadings (<10% of a monolayer) may be dominated by defect sites on these surfaces such as steps, we suggest that these adsorbed species may serve as nucleation sites for the formation of the multinuclear species. ACKNOWLEDGMENTS We thank George Parks for helpful discussion and comments. This work was funded by DOE Grant DE-FG03-93ER14347-A006. XAFS data were collected at the Stanford Synchrotron Radiation Laboratory (SSRL) and we gratefully acknowledge the support of the SSRL staff. SSRL is supported by the Department of Energy (BES and BER) and the National Institutes of Health. REFERENCES 1. Brown, G. E., Jr., et al., Chem. Rev. 99, (1999). 2. Stumm, W., and Morgan, J. J., Aquatic Chemistry, Wiley, New York, Sigg, L., Goss, K. U., Haderlein, S., Harms, H., Hug, S. J., and Ludwig, C., Chimia 51, (1997). 4. Lambert, J. F., and Che, M., in Dynamics of Surfaces and Reaction Kinetics in Heterogeneous Catalysis (G. F. Froment and K. C. 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