Spectroscopic Evidence for Ni (II) Surface Speciation at the Iron

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Environ. Sci. Technol. 2008, 42, 1151–1156

Spectroscopic Evidence for Ni(II) Surface Speciation at the Iron Oxyhydroxides-Water Interface YUJI ARAI* Department of Entomology, Soils and Plant Sciences, 270 Poole Agricultural Center, Clemson University, Clemson, South Carolina 29634-0315

Received August 5, 2007. Revised manuscript received December 1, 2007. Accepted December 3, 2007.

Understanding in situ metal surface speciation on mineral surfaces is critical to predicting the natural attenuation of metals in the subsurface environment. In this study, we have demonstrated the novel Ni K-edge X-ray absorption spectroscopy (XAS) measurements needed to understand Ni(II) surface speciation in three synthetic iron oxyhydroxides (ferrihydrite, goethite, and hematite). The adsorption of Ni gradually increases with increasing pH from 5 to 8, and the adsorption edge appears at near the point of zero salt effect (PZSE) of the solids. The results of XAS analysis indicate four different Ni innersphere surface species are present. While total Ni surface species in hematite at pH 6.85 surfaces consist of ∼63% facesharing (interatomic distance of Ni-Fe (RNi-Fe) ∼2.9 Å) and ∼37% corner-sharing (RNi-Fe ∼4.0 Å) surface species on iron octahedra, a combination of two different edge-sharing (between NiO6 and FeO6 octahedra, in chains or in rows) and corner-sharing surface species are observed in goethite and ferrihydrite at pH 5.09–6.89. In ferrihydrite, approximately 70% of surface species areedge-sharingsurfacespecies(inchains)(RNi-Fe ∼3.0Å),followed by ∼30% of edge-sharing species (in rows) (RNi-Fe ∼3.2 Å) and ∼3–5% of corner-sharing surface species (RNi-Fe ∼4.0Å). Goethite contains ∼54% edge-sharing (RNi-Fe ∼3.0 Å), ∼26% edgesharing (RNi-Fe ∼3.2 Å), and 20% corner-sharing surface species. These findings indicate that the reactivity and surface speciation of Ni are sensitive to the crystallinity of iron oxyhydroxides. The spectroscopic evidence for multi-Ni surface speciation should be factored into predictions of the transport of Ni in soil–water environments.

Introduction Heavy metal contamination persists in the terrestrial aquatic environment due to anthropogenic inputs including sewage sludge application, mine waste, and atmospheric deposition. The development of in situ remediation strategies has been difficult, as various abiotic and biotic factors affect the fate and transport of metals in surface and subsurface environments. Of all natural processes, the partitioning process of metals in soils and sediments is one of the most important abiotic factors controlling the mobility and solubility of metals. The reactivity and reaction dynamics of divalent metals such as Ni on various soil adsorbents have been extensively studied to better understand retention and release processes in low temperature geochemical environments (e.g., refs 1–3). * Corresponding author phone: 864-656-2607; fax: 864-656-3443; e-mail: [email protected]. 10.1021/es0719529 CCC: $40.75

Published on Web 01/18/2008

 2008 American Chemical Society

Nickel and Zn sorption on aluminosilicate minerals, aluminum oxides, and soil clay fractions is known to facilitate the formation of Ni/Zn-Al hydroxide phases and/or layered double hydroxide (LDH) phases (e.g., refs 4–8). While divalent metals like Ni might appear to remain in soil clay minerals as secondary precipitate phases, it is not clearly understood how Ni is sequestered in natural materials since most soils and sediments contain other solid components (e.g., iron oxides and organic matter) in addition to aluminosilicate minerals. The effects of humic and soil fulvic acid (SFA) on Ni(II) sequestration mechanisms in kaolinite and boehmite have been recently investigated by several researchers (9, 10). Spectroscopic evidence indicated that the formation of Ni(II) LDH phases can be suppressed by the amount of humic acid coatings on kaolinite surfaces via direct Ni sorption on humic acid functional groups, and/or by the formation of Ni-SFAboehmite ternary species. Several macroscopic studies show strong Ni retention on iron oxyhydroxide surfaces (e.g., refs 11–13). The Ni sorption generally increases from 5 to near neutral pH values (e.g., refs 11, 14), suggesting the importance of Ni reactivity in iron oxyhydroxide-rich environments at near neutral pH. Pedersen and Pind (2000) reported that Ni sorption on montmorillonite surfaces was significantly enhanced at pH 6.5–8 when the clay mineral surfaces are coated or mixed with iron oxyhydroxide (ferrihydrite and lepidocrocite) (15). Unfortunately, the in situ Ni solid-state speciation in Febased geomedia is not well understood due to analytical limitations in molecular-scale techniques. Nickel K-edge XAS measurements on Ni-reacted iron oxyhydroxides has been considered challenging among the synchrotron-based X-ray communities because the energy of the Ni KR signal (∼7478 eV and ∼7460 eV) and the Fe Kβ (∼7058 eV) background noise overlap. Conventional Al foil filtration absorbs not only Fe KR and Kβ X-rays but also Ni KR X-rays, resulting in poor data quality. One way to improve the signal-to-noise ratio is to rely on the high flux X-ray sources at a state of the art, third generation synchrotron facility (beam lines (BL) 11–2 at Stanford Linear Accelerator). In this study, we performed the novel Ni K-edge XAS measurements essential to investigations of Ni surface speciation in three different synthetic iron oxyhydroxides as a function of pH.

Materials and Methods Materials. Two-line ferrihydrite and hematite were synthesized according to the methods described by Schwertmann and Cornell (16). Goethite was synthesized using the method described by Atkinson et al. (17). All iron oxyhydroxide suspensions were concentrated by centrifugation, and then washed with deionized (DI) water until the specific conductivity was reduced to less than 0.78 dS m-1. The mineral nature of iron hydroxides were confirmed by XRD. The fivepoint Brunauer–Emmett–Teller (BET) surface areas of ferrihydrite, hematite, and goethite were 160.1(2), 64.2(5), and 42.1(2) m2 g-1, respectively. Points of zero salt effect are ∼7.2 for ferrihydrite, ∼6.5 for goethite, and ∼6.9 for hematite (18). Ni Adsorption Experiments. Nickel adsorption on hematite, goethite, and ferrihydrite (suspension density: 1.5 g/L for goethite and hematite systems, and 0.31 g/L for the ferrihydrite system, I ) 0.01 M NaNO3 solution) was studied (total Ni concentrations of 200 µM, abbreviated as Nitot) for a range of pH conditions (5–8). Iron oxyhydroxide suspensions were pre-equilibrated overnight in 0.01 M NaNO3 solutions at desired pH values. Sufficient amounts of 10 mM NaHCO3 solution were added to achieve the desired bicarVOL. 42, NO. 4, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Least-squares analyses of Ni K-edge bulk XAS analysis. Initial Ni concentration: 180 (µM). Γ: Surface coverage (mg m-2)a samples pH 5.09 ferrihydrite Γ ) 0.007 pH 5.92 ferrihydrite Γ ) 0.013 pH 6.89 ferrihydrite Γ)0.03 pH 6.68 goethite Γ ) 0.10 pH 6.85 hematite Γ ) 0.063 pH 6.85 hematite (air-dry) Γ ) 0.063

CN R(Å) σ2(Å2) CN R(Å) σ2(Å2) CN R(Å) σ2(Å2) CN R(Å) σ2(Å2) CN R(Å) σ2(Å2) CN R(Å) σ2(Å2)

O

Fe1

Fe2

Fe3

MS

5.9(6) 2.050(7) 0.0054(9) 5.7(5) 2.061(7) 0.0060(8) 5.8(5) 2.060(7) 0.006(1) 5.7(5) 2.057(6) 0.0051(7) 6.5(6) 2.045(7) 0.0074(9) 6.1(7) 2.043(8) 0.007(1)

1.0(5) 3.08(4) 0.005* 1.4(3) 3.05(2) 0.005* 1.5(4) 3.06(3) 0.005* 1.2(3) 3.03(2) 0.005* 1.4(4) 2.92(2) 0.01* 1.4(4) 2.92(2) 0.01*

1.8(5) 3.21(3) 0.005* 1.9(4) 3.21(2) 0.005* 1.3(5) 3.19(3) 0.005* 1.1(4) 3.18(3) 0.005*

1.0(7) 4.03(4) 0.008* 1.1(6) 4.05(4) 0.008* 0.9(7) 4.07(4) 0.008* 1.7(6) 4.06(2) 0.008* 1.4(6) 4.07(3) 0.008* 1.9(7) 4.07(3) 0.008*

----0.01* ----0.01* ----0.01* ----0.01* ----0.01* ----0.01*

∆Eo (eV)

ff

fed1

fed2

fc

R-factor

-3(1

68.5 ((2.6)

28.5 ((2.6)

3.4 ((2.6)

0.020

-3(1

68.2 ((2.3)

28.5 ((2.3)

3.4 ((2.3)

0.019

-2(1

70.2 ((2.1)

25.2 ((2.1)

4.5 ((2.1)

0.013

-2(1

54.4 ((5.2)

26.4 ((5.2)

19.4 ((5.2)

0.017

-4(1

62.9 ((4.8)

37.1 ((4.8)

0.024

-4(1

67.5 ((4.9)

32.5 ((4.9)

0.027

log SI -8.77 -6.67 -5.25 -6.75 -4.65 -3.23 -4.56 -2.46 -3.23 -5.29 -5.25 -1.78 -5.37 -3.28 -1.73 -5.37 -3.28 -1.73

a Saturation index values (log SI) (from top to bottom) for Ni(OH)2(am), β-Ni(OH)2 and NiCO3 are estimated using thermodynamic solubility constants (log k: 12.89, 10.8, and -11.2, respectively). CN: Coordination number. R: interatomic distance (Å). σ2: Debye–Waller factor (Å2). Fit quality confidence limit for parameters: Ni-O/Fe shells, CN: (20%, R (0.02 Å *: Fixed parameter. Fe1 is face- or edge-sharing coordination between NiO6 and FeO6 (R: ∼2.9 and 3.0Å), Fe2 is an edge-sharing coordination (R: ∼3.2Å), and Fe3 is a corner sharing coordination (R: ∼4.05Å) (see Figure 2). CN and R of Ni-O-Fe MS path are correlated with Ni-Fe3 SS path.

bonate concentration in equilibrium with the partial pressure of carbon dioxide gas in air (pCO2 ) 10-3.5 atm) at specified experimental pH values. After 24 h of equilibration with humidified air, 30 mM Ni(NO3)2 stock solution at pH ∼3 was added to the mineral suspensions to ensure Nitot ∼200 µM. Under the initial reaction conditions at t ) 0, systems were undersaturated with respect to Ni(OH)2 (am), β-Ni(OH)2, and NiCO3 based on solubility calculations using the thermodynamic solubility constants (log K: 12.89, 10.8, and -11.2, respectively) (19–22). The samples were reacted for 24 h on an end-over-end shaker operating at 12 rpm. The final pH was measured in the quiescent suspension, and then the suspension was centrifuged at 20 190g for 10 min. The supernatants, which were filtered through 0.45 µm MillexGX filters (Millipore corp., Bedfod, MA), were analyzed for total Ni and Fe using inductively coupled plasma mass spectrometry (ICP-MS). Extended X-ray Absorption Fine Structure Spectroscopic Measurements. Nickel surface speciation on the iron oxyhydroxide surfaces was studied at pH 5–7. The Nitot and ionic strength are 180 µM and 0.01 M NaNO3, respectively. Using aqueous speciation calculations, these systems were calculated to be undersaturated with respect to Ni(OH)2 (am), β-Ni(OH)2 (s), and NiCO3 under all conditions (Table 1). Sufficient amounts of 10 mM NaHCO3 solution were added to achieve the desired bicarbonate concentration in equilibrium with the partial pressure of carbon dioxide gas in air at experimental pH values. Chemical equilibrium calculation indicates that total dissolved carbonate concentration ranges from ∼14–59 µM in the XAS samples at pH 5–6.9. All samples were prepared in 1 L batch systems using the methods described above. The iron oxyhydroxide paste samples were recovered via centrifugation for 10 min at 20,190g. Based on the mass balance calculation, the residual mass of Ni in the paste is less than 3% of total Ni signals in the XAS measurements. Dissolved Ni and Fe concentrations in the supernatant were analyzed using ICP-MS. The samples were loaded in 3 mm polycarbonate sample holders, which were then sealed with 38 µm Mylar windows. The samples were wrapped with moist tissues and kept at ∼3 °C prior to XAS data collection. Room temperature Ni K-edge (8333 eV) 1152

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fluorescence spectra were collected within 36 h of sample preparation at the Stanford Synchrotron Radiation Laboratory (SSRL) beamline 11–2 (Si(220) double crystal monochromator) using a Canberra 30-element Ge detector array equipped with a Co 3 µx filter and 4 Al foils. The storage ring was operated at 3 GeV energy with a current of 80–100 mA. A Ni reference foil was used to calibrate at the Ni K- absorption edge energy positions (8333 eV) every 24 h. The program FEFF 6 (23) was used to estimate backscattering phases and amplitude functions of single scattering (SS) Ni-O/Fe and Ni-O-Fe multiple scattering paths (MS), which were derived from structural refinement data for trevorite, NiFe2O4 (24). XAS data reduction and analyses were performed using the IFEFFIT engine-based interface, SixPACK (25). k3-weighted unsmoothed XAS spectra were averaged and background-subtracted by a linear fit through the pre-edge region and a cubic spline through the spectrum above the absorption edge. Spectra were normalized using the height of the edge step just above the absorption maximum. Normalized EXAFS spectra were filtered over a k-range of 2 to 11.2–11.8 Å-1 depending on data quality (∼330 data points) and Fourier-transformed to produce radial structure–functions. Fourier-transformed XAS spectra were fit in R-space over the range of 1–4.5 Å. In the fit, ∆E o was allowed to float and linked to all shells during the fit. As described in the XAS results and discussion section below, four surface species (a face-sharing, two edge-sharing, and a corner sharing inner-sphere sphere surface species) were considered in the fit. Based on the preliminary fit, we found a Ni-Fe interatomic distance (∼2.9 Å) for a face-sharing coordination (Figure 3a) and two Ni-Fe interatomic distances (∼3.0 and 3.2 Å) corresponding to specific edge-sharing coordination between Ni octahedra and Fe octahedra, in chains or in rows (i.e., inner-sphere bidentate mononuclear coordination) (Figure 3b and 3d) and a longer Ni-Fe interatomic distance at ∼4.05 Å corresponding to cornersharing coordination (i.e., inner-sphere monodentate mononuclear coordination) (Figure 3c). These Ni and Fe octahedra arrangements in chains and in rows are similar to the double chains of edge-sharing Fe octahedra in the goethite structure, with distinct distances between edge-sharing Fe octahedra

FIGURE 1. Ni(II) adsorption edge on synthetic ferrihydrite, goethite and hematite surfaces in 0.01 M NaNO3 as a function of pH. Suspension density: 1.5 g/L for goethite and hematite systems, and 0.31 g/L for the ferrihydrite system. Initial Ni concentrations: 200 µM/L. when measured either parallel or across to the double chains. The assignment of specific coordination environments for these distances is discussed in the XAS section below. To facilitate the comparison under different reaction conditions, Debye–Waller factors for second and third shells were fixed except for the Ni-O SS path; (i.e., face-sharing Ni-Fe SS path: 0.01 Å2, edge-sharing Ni-Fe SS path 0.005 Å2 and cornersharing Ni-Fe SS path: 0.008 Å2). These Debye–Waller factors for Ni-Fe shells were determined in the preliminary fit, and the same fixed values were used for the same adsorbents. To account for the amplitude at ∼4 Å, we considered the twodimensional structure of Ni octahedral, which has a collinear arrangement with respect to an iron octahedral. This arrangement is consistent with a bond distance of 4.05 Å (Ni-O: ∼2.04 and O-Fe: ∼2 Å) (see the XAS discussion section for more details). In the final fit, MS contributions of a Ni-O-Fe bonding were included with a Ni-Fe SS path at ∼4.06 Å. The σ2 for Ni-O-Fe MS paths are explicitly calculated from σ2 of SS path by summing the disorder parameters of each SS path (26). Based on the assumption that σ2Fe-o < σ2Ni-O-Fe ) σ2 Ni-Fe (0.008 Å2), σ2 for Ni-O-Fe MS paths is fixed at 0.01 Å2, and CN and R for Ni-O-Fe MS and for the Ni-Fe SS path was correlated. To understand the distribution of different surface species, we also conducted CN-correlated XAS analysis. In this model, we set the “total fraction (f ) ) 1” as a summation of all fractions of surface species (i.e., 1 ) ff + fed1 + fed2 + fc), where the fraction of face-sharing Ni-Fe surface species with distance of ∼2.9 Å (ff), the fraction of edge-sharing Ni-Fe distance at ∼3.0 Å (fed1), edge-sharing Ni-Fe distance at ∼3.2 Å (fed2), and corner-sharing Ni-Fe distance at ∼4.06 Å (fc) surface species. While the amplitude reduction factors for ff, fed1, and fed2 are correlated with corresponding Ni-Fe SS paths, fc was correlated with both a corner-sharing Ni-Fe SS path and a Ni-O-Fe MS path.

Results and Discussion pH Dependent Ni Sorption. Figure 1 shows the results of Ni batch adsorption experiments on iron oxyhydroxide surfaces as a function of pH. The adsorption gradually increases with increasing pH from 3 to 8. At near PZSE of the solids, the adsorption edge develops for all three solids. Whereas the total adsorption was nearly negligible at pH 3–5, the Ni uptake gradually increases with increasing pH from 5 to 8. A similar

FIGURE 2. (a) Normalized, background-subtracted k3-weighted Ni K-edge EXAFS spectra of Ni(II)-reacted hematite, goethite, and ferrihydrite, (b) Fourier-transformed k3-weighted Ni K-edge EXAFS spectra of Ni(II)-reacted hematite, goethite and ferrihydrite (solid lines) and nonlinear least-squares fits (dotted lines). pH-dependent adsorption behavior has been reported in other studies on iron oxyhydroxides (e.g., ref 11). Although ferrihydrite has the highest surface area of the three solids, the total Ni adsorption is much less than that of the other two solids. This is due to differences in suspension density used in the experiments. While the suspension density of ferrihydrite is 0.31 g/L, that of goethite and hematite are 1.5 g/L. The differences yield in the total surface area of ferrihydrite, (49.6 m2 L-1), goethite (63.1 m2 L-1) and hematite (96.3 m2 L-1). The order of total surface area follows the Ni adsorption trend in three different adsorbents. It is also important to mention that dissolved [Fe]total was less than 4 µg/L in all samples, suggesting that the systems were undersaturated with respect to Fe/Ni-bearing secondary precipitates (e.g., ferrihydrite). XAS Analysis. Figure 2a and 2b show the k3-weighted EXAFS spectra of Ni(II)-reacted iron oxyhydroxide samples. The results of fit parameters are shown in Table 1 (in units of Å), and excellent fit (R-factor: 0.013–0.027) was obtained in all samples. Interatomic distances mentioned in this section are corrected for phase shift (in units of Å). In all samples, we observed the Ni-O distance at ∼2.05 Å and CN ∼6. This indicates the 6-fold oxygen coordination of a Ni atom. Hematite. Fourier Transform (FT) spectra of Ni-reacted hematite samples resemble a spectrum of 6 mol % Nisubstituted hematite (27). As reported in the literature, similar face-sharing Ni-Fe distances (∼2.92 Å) are found in our Nisorbed hematite samples (indicated by arrows in Figure 2b). The CN of the Ni-Fe shell (∼1.4 ( 0.4) indicates that a Ni atom coordinated to an Fe atom. The face-sharing metal octahedral coordination environment is illustrated in Figure 2a. Assuming the Fe-O distance of ∼1.95 Å (28) and the Ni-O distance of ∼1.98 Å (27), the estimated Ni-Fe distance VOL. 42, NO. 4, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Schematic polyhedron representation of Ni(II) surface species on the iron octahedra structure of iron oxyhydroxide surfaces. Four surface species found in XAS analysis are shown. (a) Face-sharing Ni(II) inner-sphere surface complex via bidentate mononuclear Ni-O3-Fe linkage. (b) Edge-sharing (between two NiO6 and FeO6 octahedral, in the chains) Ni(II) inner-sphere surface complex via bidentate mononuclear Ni-O2-Fe linkage. (c) Corner sharing Ni(II) inner-sphere surface complex via monodentate mononuclear Ni-O-Fe linkage (collinear arrangement of NiO6 and FeO6 octahedral structures are assumed to yield in the interatomic distance of Ni-Fe ∼4.05 Å). (d) Edge-sharing (between NiO6 and FeO6 octahedral, in the rows) Ni(II) inner-sphere surface complex via bidentate mononuclear Ni-O2-Fe linkage. of ∼2.9 Å is in good agreement with the results of our XAS analysis (2.92 Å). In addition to the Ni-Fe short distance, there are strong FT features at ∼4.0 Å in both paste and air-dry samples (indicated by a vertical dotted line in Figure 2b). Since the presence of this FT feature is independent of moisture content, it is unlikely that outer-sphere surface species are contributing to a Ni-O SS at this distance. When the size of two intermediate oxygen atoms (∼2.8 Å) and the radii of Ni and Fe atoms are considered, the Ni-Fe interatomic distances of outer-sphere complexes yield in >6 Å which is much larger than the distance discussed. Therefore, the presence of outer-sphere complexes was ruled out. We have also considered a Ni-Ni SS path at ∼4.0 Å. However, it could not be incorporated into the fit. Based on previous XAS studies of Ni-substituted goethite and hematite (e.g., refs 27–29,), the feature at ∼4.0 Å cannot be explained by any of the reported Ni-Fe distances (∼3–3.7 Å). Interestingly, Cormier et al. (1999) reported focusing effects arising from a nearly collinear Ni-O-Ni arrangement in nickel oxide-containing alkaline borate glasses (30). They were able to account for the FT feature at 4.08 Å with the Ni-O-Ni MS path. Using a similar assumption, we posited a flexible corner-sharing Ni octahedral arrangement on an Fe octahedra, forming two-dimensional adsorption complexes in our Ni sorbed hematite samples. Assuming the collinear arrangement between a Ni-O-Fe linkage (Figure 2c: corner-sharing), we were able to account for the FT feature at ∼4.07 Å with average CN of ∼1.6 ( 0.7. Overall, hematite spectra fit well with two Ni-Fe distances of ∼2.9 Å and 4.07 Å (Table 1 and Figures 2a and 2b), suggesting two different Ni surface species coexist on hematite surfaces. In addition to shell-by-shell fit analysis, CN-correlated analysis was conducted to estimate the fraction of facesharing and corner-sharing Ni surface species. In the paste sample, the fractions of face-sharing and corner-sharing surface species are ∼62.9 and ∼37.1%, respectively. The drying process has resulted in an increase in the fraction of face-sharing surface species from 62.9 to 67.5% (i.e., a 1154

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decrease in corner-sharing species from 37 to 32.5%), suggesting the effects of moisture content on the Ni(II) surface species. Goethite. The FT spectrum of goethite exhibits a much less intense Ni-Fe SS feature at ∼3.0 Å compared to the FT spectra features of hematite (Figure 2b). In the k3-weighted EXAFS spectra (Figure 2a), the difference between hematite and goethite can be also observed at k ) 7–9 Å-1. Unlike in the hematite spectra, the face-sharing Ni-Fe distance of 2.9 Å could not be incorporated into the fit for the goethite sample. Instead, the preliminary fit indicates that ∼3.0–3.2 Å Ni-Fe interatomic distances appear to be better candidates. To evaluate the presence of other surface species, the fit was initially performed with a single Ni-Fe shell at ∼3.0 Å. A residual spectrum was prepared by subtracting the Ni-O SS (∼2.06 Å) and Ni-Fe SS (∼3.03 Å) contributions (shown in the Supporting Information. Unlike in the hematite samples, we observed additional FT features at ∼3.2 Å and ∼4.06 Å (Supporting Information). The Ni-Fe distance at ∼3.2 Å has been previously reported in Ni-substituted goethite (natural and synthetic) 28, 29). We found that these Ni-Fe distances correspond to the edge-sharing (two adjacent octahedral, in rows) (Figure 3d) and the corner-sharing Ni(II) inner-sphere surface complexes (Figure 3c), respectively. The residual spectrum was successfully fit using parameters shown in Table 1. With these additional two shells, the fit quality (Rfactor) was improved approximately 30%. Furthermore, CNcorrelated analysis indicated that approximately 54.2% of the Ni consisted of edge-sharing surface species (in chains; Figure 3b), 26.2% consisted of edge-sharing surface species (in rows; Figure 2d), and 19.4% consisted of corner-sharing surface species (Figure 3c). It is important to mention that a peak at ∼2.7 Å in RSF (indicated by a vertical dotted line in Figure 2b) was unable to describe when only a single Ni-Fe shell at 3–3.2 Å was considered, suggesting that the reasonable XAS model prediction with two Ni-Fe distances at 3.05 Å and 3.2 Å. Ferrihydrite. Ni(II)-reacted ferrihydrite spectra show much larger FT features at ∼3.0 Å compared to spectra of other two iron oxides. Furthermore, distinctive features at k of 7 Å-1 (indicated by arrows in Figure 2a) are present in all ferrihydrite samples. Based on the fit result of hematite and goethite, all four surface species (face-, corner- and two edge-sharing arrangements) were considered. As in the goethite sample, we did not observe the Ni-Fe SS at ∼2.9 Å corresponding to the face-sharing linkages between NiO6 and FeO6 structures. During the initial fit, it was clear that ferrihydrite largely contain the Ni-Fe distance at ∼3.0 Å. However, using a single Ni-Fe shell resulted in a poor fit of the data (R-factor: ∼0.3). To reveal the presence of various Ni octahedral coordination environments, residual spectra were prepared by subtracting the contributions of the Ni-O SS at ∼2.06 Å and the Ni--Fe SS at ∼3.07 Å. The Ni-Fe distance at ∼3.07 Å is also reported by Xu and co-workers in Ni(II) reacted ferrihydrite at pH 6–7 (31). Residual spectra of samples at pH 5.09 and 6.89 are shown in the Supporting Information (a residual spectrum at pH 5.92 is not shown). As with the previous goethite sample, two Ni-Fe distances (∼3.2 and 4.05 Å) were observed in FT residual spectra (Supporting Information), suggesting that the ferrihydrite surfaces have more than one Ni(II) surface species. Using two additional Ni-Fe SS paths and a Ni-O-Fe MS path, the residual spectra were successfully fit. The fit parameters for all three ferrihydrite samples (Table 1) indicate the presence of three different Ni octahedral arrangements on iron octahedral structures (i.e., two edgesharing and a corner-sharing coordination environments). Coordination number correlated analysis showed that the distribution of these surface species are not significantly different in samples at pH 5.09 and 5.92 (approximately 68.2%

for edge-sharing surface species at Ni-Fe distance at ∼3.07 Å, 28.5% for edge-sharing surface species at Ni-Fe distance at 3.21 Å, and 3.4% for corner-sharing surface species). However, in the sample with the highest Ni loading level (0.03 mg/m2) at pH 6.89 (Table 1), the distribution of the three different surface species changes to 70.2% for edgesharing surface species (in chains), 25.2% for edge-sharing surface species (in rows) and 4.5% for corner-sharing surface species.

Environmental Implications Our study shows that three iron oxyhydroxides (goethite, hematite and ferrihydrite) can readily sequester dissolved Ni(II) on the surfaces at pH ∼7. The detailed XAS analysis indicates the formation of four different NiO6 coordination environments on Fe octahedra (face-sharing, two edgesharing, and corner-sharing surface species) in these solids. Crystallinity of iron oxyhydroxides seems to play an important role in the formation and distribution of different surface species. As recently demonstrated by several researchers (32, 33), in situ spectroscopic results can be usefully incorporated into surface complexation models to constrain the fitting parameters, resulting in better predictions of metal/ radionuclide sorption at the mineral-water interfaces. Thus far, our knowledge of Ni sequestration on geomedia has been largely limited to the formation of secondary precipitates (e.g., LDH) in aluminosilicate minerals. However, our findings indicate that Ni retention in soils, which generally contain iron oxyhydroxides, might be more complex. The formation of LDH might be suppressed by the presence of iron oxyhydroxides, which may compete for the sorption of dissolved Ni along with other clay minerals and soil organic matter. The findings should improve the understanding of Ni partitioning processes in natural materials.

Acknowledgments I thank Dr. J. Rogers from Stanford Synchrotron Radiation Laboratory (SSRL) for assistance with the XAS measurements. Portions of this research were carried out at the SSRL, 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 residual k3-weighted Ni K-edge EXAFS spectra, and the theoretical fit. Thise material is available free of charge via the Internet at http://pubs.acs.org.

Literature Cited (1) Hayes, K. F.; Leckie, J. O. Modeling ionic strength effects on cation adsorption at hydrous oxide/solution interfaces. J. Colloid Interface Sci. 1987, 115, 564–572. (2) Puls, R. W.; Powell, R. M.; Clark, D.; Eldred, C. J. Effects of pH, solid/solution ratio, ionic strength, and organic acids on Pb and Cd sorption on kaolinite. Water, Air, Soil Pollut. 1991, 57—58 (1), 423–430. (3) Mattigod, S. V.; Gibali, A. S.; Page, A. L. Effect of ionic strength and ion pair formation on the adsorption of nickel by kaolinite. Clays Clay Miner. 1979, 27 (6), 411–416. (4) Ford, R. G.; Sparks, D. L. The nature of Zn precipitates formed in the presence of pyrophyllite. Environ. Sci. Technol. 2000, 34, 2479–2483. (5) Scheinost, A. C.; Sparks, D. L. Formation of layered single- and double-metal hydroxide precipitates at the mineral/water interface: A multiple-scattering XAFS analysis. J. Colloid Interface Sci. 2000, 223, 167–178.

(6) Yamaguchi, N. U.; Scheinost, A. C.; Sparks, D. L. Influence of gibbsite surface area and citrate on Ni sorption mechanisms at pH 7.5. Clays Clay Miner. 2002, 50 (6), 784–790. (7) Elzinga, E. J.; Sparks, D. L. Nickel sorption mechanisms in a pyrophyllite-montmorillonite mixture. J. Colloid Interface Sci. 1999, 213, 506–512. (8) Scheckel, K. G.; Sparks, D. L. Kinetics of the formation and dissolution of Ni precipitates in a gibbsite/amorphous silica mixture. J. Colloid Interface Sci. 2000, 229, 222–229. (9) Nachtegaal, M.; Sparks, D. L. Nickel sequestration in a kaolinitehumic acid complex. Environ. Sci. Technol. 2003, 37, 529–534. (10) Strathmann, T. J.; Myneni, S. C. B. Effect of soil fluvic acid on Nickel(II) sorption and Bonding at the aqueous-boehmite (γAlOOH) interface. Environ. Sci. Technol. 2005, 39, 4027–4034. (11) Bryce, A. L.; Kornlcker, W. A.; Elzerman, A. W. Nickel adsorption to hydrous ferric oxide in the presence of EDTA: Effects of component addition sequence. Environ. Sci. Technol. 1994, 28, 2353–2359. (12) Buerge-Weirich, D.; Hari, R.; Xue, H.; Behra, P.; Sigg, L. Adsorption of Cu, Cd, and Ni on goethite in the presence of natural groundwater ligands. Environ. Sci. Technol. 2002, 36 (3), 328–336. (13) Coughlin, B. R.; Stone, A. T. Nonreversible adsorption of divalent metal ions (MnII, CoII, NiII, CuII, and PbII) onto goethite: Effects of acidification, FeII addition, and picolinic acid addition. Environ. Sci. Technol. 1995, 29 (9), 2445–2455. (14) Trivedi, P.; Axe, L. Ni and Zn sorption to amorphous versu crystalline iron oxides: Macroscopic studies. J. Colloid Interface Sci. 2001, 244, 221–229. (15) Green-Pedersen, H.; Pind, N. Preparation, characterization, and sorption properties for Ni(II) of iron oxyhydroxide-montmorillonite. Colloids Surf., A 2000, 168 (2), 133–145. (16) Schwertmann, U.; Cornell, R. M. Iron Oxides in the Laboratory: Preparation and Characterization; VCH Publisher: Weinheim/ New York, 1991; p 137. (17) Atkinson, R. J.; Parfitt, R. L.; Smart, R. S. C. Infrared study of phosphate adsorption on goethite. J. Chem. Soc. Faraday Trans. 1 1974, 70, 1472–1479. (18) Zelazny, L. W.; He, L.; Vanwormhoudt, A. Charge analysis of soils and anion exchange. In Methods of Soil Analysis Part 3-Chemical methods; Sparks, D. L., Ed.; Soil Science Society of America, Inc.: Madison, WI, 1996; pp 1231–1254. (19) Baes, C. F., Jr.; Mesmer, R. E. Hydrolysis of Cations; Krieger Publishing Company: Melbourne, FL, 1986. (20) Hummel, W.; Curti, E. Nickel Aqueous speciation at ambient conditions: A thermodynamic elegy. Monatsh. Chem. 2003, 134, 941–973. (21) Gamsjäger, H.; Wallner, H.; Preis, W. Solid-solute phase equilibria in aqueous solutions XVII [I]. Solubility and thermodynamic data of nickel(II) hydroxide. Monatsh. Chem. 2002, 133 (2), 225– 229. (22) Smith, R. M.; Martell, A. E., Volume 4: Inorganic Complexes, Critical Stability Constant; Plenum press: New York, 1981. (23) Zabinsky, S. I.; Rehr, J. J.; Ankudinov, A.; Albers, R. C.; Eller, M. J. Multiple-scattering calculations of X-ray-absorption spectra. Phys. Rev. B 1995, 52 (4), 2995–3009. (24) Hill, R. J.; Craig, J. R.; Gibbs, G. V. Systematics of the spinel structure type. Phys. Chem. Miner. 1979, 4 (4), 317–339. (25) Webb, S. M. A graphical user interface for XAS analysis using IFEFFIT. Phys. Scr. 2005, (T115), 1011–1014. (26) Hudson, E. A.; Allen, P. G.; Terminello, L. J.; Denecke, M. A.; Reich, T. Polarized x-ray absorption spectroscopy of the uranyl ion: Comparison of experiment and theory. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 156–165. (27) Singh, B.; Sherman, D. M.; Gilkes, R. J.; Wells, M.; Mosselmans, J. F. W. Structural chemistry of Fe, Mn, and Ni in synthetic hematite as determined by extended X-ray absorption fine structure spectroscopy. Clays Clay Miner. 2000, 48 (5), 521–527. (28) Manceau, A.; Schlegel, M. L.; Musso, M.; Sole, V. A.; Gauthier, C.; Petit, P. E.; Trolard, F. Crystal chemistry of trace elements in natural and synthetic goethite. Geochim. Cosmochim. Acta 2000, 64 (21), 3643–3661. (29) Carvalho-E-Silva, M. L.; Ramos, A. Y.; Tolentino, H. C. N.; Enzweiler, J.; Netto, S. M.; Alves, M. D. C. M. Incorporation of Ni into natural goethite: An investigation by X-ray absorption spectroscopy. Am. Mineral. 2003, 88, 876–882. (30) Cormier, L.; Galoisy, L.; Calas, G. Evidence of Ni-containing ordered domains in low-alkali borate glasses. Europhys. Lett. 1999, 45 (5), 572–578. (31) Xu, Y; Axe, L; Boonfueng, T; Boonfueng, T.; Tyson, T. A.; Trivedi, T.; Pandya, K. Ni(II) complexation to amorphous VOL. 42, NO. 4, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

9

1155

hydrous ferric oxide: An X-ray absorption spectroscopy study. J. Colloid Interface Sci. 2007, 314, 10–17. (32) Fukushi, K.; Sverjensky, D. A. A predictive model (ETLM) for arsenate adsorption and surface speciation on oxides consistent with spectroscopic and theoretical molecular evidence. Geochim. Cosmochim. Acta 2006, 70 (15), 3778– 3802.

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9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 4, 2008

(33) Arai, Y.; McBeath, M.; Bargar, J. R.; Joye, J.; Davis, J. A. Uranyl adsorption and surface speciation at the imogolite-water interface: Self-consistent spectroscopic and surface complexation models. Geochim. Cosmochim. Acta 2006, 70 (10), 2492–2509.

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