Uranyl Incorporation into Calcite and Aragonite: XAFS and

Jan 20, 2000 - (8) began to address these questions using X-ray absorption fine structure (XAFS) spectroscopy, which allows determination of the local...
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Environ. Sci. Technol. 2000, 34, 638-644

Uranyl Incorporation into Calcite and Aragonite: XAFS and Luminescence Studies R I C H A R D J . R E E D E R , * ,† MELISSA NUGENT,† GERALDINE M. LAMBLE,‡ C. DREW TAIT,§ AND DAVID E. MORRIS§ Department of Geosciences, State University of New York, Stony Brook, New York 11794-2100, Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, and Chemical Science and Technology Division, Seaborg Institute for Transactinium Science, Los Alamos National Laboratory, Los Alamos, New Mexico 87545

X-ray absorption, luminescence, and Raman spectroscopic studies of U(VI)-containing calcite and aragonite show that the UO2+ 2 ion, the dominant and mobile form of dissolved uranium in near-surface waters, has a disordered and apparently less stable coordination environment when incorporated into calcite in comparison to aragonite, both common polymorphs of CaCO3. Our findings suggest that calcite, a widely distributed authigenic mineral in soils and near-surface sediments and a principal weathering product of concrete-based containment structures, is not likely to be a suitable host for the long-term sequestration of U(VI). The more stable coordination provided by aragonite suggests that its long-term retention should be favored in this phase, until it inverts to calcite. Consequently, future remobilization of U(VI) coprecipitated with calcium carbonate minerals should not be ruled out in assessments of contaminated sites. Our observation of a similar equatorial coordination of UO2+ 2 in aragonite and the dominant aqueous species [UO2(CO3)43 ] but a different coordination in calcite indicates that a change in UO2+ 2 coordination is required for its incorporation into calcite. This may explain the observed preferential uptake of U(VI) by aragonite relative to calcite.

Introduction Until recently, the primary interest in uranium present in carbonate minerals, typically below 10 ppm in natural samples, has been the determination of absolute ages for geologic processes, including biogenic calcification in the marine environment, calcification in soils and sediments, and even speleothem formation. More recently, concerns about the mobility of radionuclides in contaminated soils and groundwaters have focused attention on their potential uptake by carbonate minerals, which are among the most common secondary phases forming in near-surface environments and are expected to be among the earliest * Corresponding author phone: (631)632-8208; fax: (631)632-8240; e-mail: [email protected]. † State University of New York, Stony Brook. ‡ Lawrence Berkeley National Laboratory. § Los Alamos National Laboratory. 638

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weathering products of concrete-based structures designed for containing radionuclide wastes (1). Differences in uranium uptake behavior and concentration between the two common mineral forms of CaCO3, calcite and aragonite, have long been recognized (2, 3), leading several workers to speculate about possible differences in the substitution mechanism of uranium in calcite and aragonite (4-7) and its influence on the effectiveness of uranium retention in carbonate minerals. Such structural considerations may also be critical for the interpretation of studies showing decreased mobility and bioavailability of metal contaminants in soils treated with calcium carbonate. Recently Sturchio et al. (8) began to address these questions using X-ray absorption fine structure (XAFS) spectroscopy, which allows determination of the local structure around a specific element. Using a natural calcite sample, they showed that the U4+ ion substitutes in the Ca site of calcite with some distortion of the nearest-neighbor oxygen shell but with no significant disruption of the local structure. This suggests that calcite may offer a stable structural environment for U(IV), which is considered to be one important factor for its long-term sequestration. In oxidizing waters more characteristic of earth’s surface, however, the stable oxidation state of uranium is 6+, and the dominant aqueous speciation involves the uranyl moiety, UO2+ 2 . Owing to its configuration and size, this linear ion is not expected to substitute readily into either calcite or aragonite without significant local disruption of the structures, raising concerns about its long-term retention. In contrast to U(IV), which is rather immobile in nearsurface environments, U(VI) aqueous species are known to be highly mobile, in part because of a strong tendency for complexing of UO2+ with various ligands common in 2 natural waters (especially CO23 ). Uptake of such dissolved metal contaminants by many fine-grained mineral phases (e.g., clays, oxides, and hydroxides) is most commonly achieved by adsorption. For carbonates, however, recent evidence suggests that incorporation into the solid (coprecipitation) dominates contaminant uptake (9-11). Consequently, the degree of localized structural disruption around coprecipitated uranyl species bears significantly on the long-term retention of uranium in carbonate minerals, with implications for the potential for uptake and release of uranium in soil and groundwater environments as well as for the interpretation of uranium age-dating systematics. In the present study we use X-ray absorption fine structure (XAFS) spectroscopy, Raman spectroscopy, and luminescence spectroscopy to characterize the speciation and local structure of U(VI) in calcite and aragonite grown separately from uranyl-containing aqueous solutions at room temperature. With these techniques we are able to identify differences in the local coordination of the UO2+ 2 ion between calcite and aragonite, specifically that U(VI) is structurally more compatible with aragonite than calcite. Not only does this finding support observations from nature indicating that the uranyl ion is preferentially taken up by aragonite relative to calcite, but it also suggests that calcite will be less effective for the long-term sequestration of U(VI) relative to U(IV).

Materials and Methods Samples and Preparation. Uranyl-doped calcite and aragonite samples were synthesized separately from roomtemperature aqueous solutions using a modified procedure 10.1021/es990981j CCC: $19.00

 2000 American Chemical Society Published on Web 01/20/2000

of the constant-addition method described by Zhong and Mucci (12). Initially, nuclei were allowed to form in the growth solution; UO2+ 2 was added as a small volume of aqueous uranyl nitrate plasma standard once a steady-state pH was achieved. Ca2+, HCO3 , and U concentrations in the reaction vessel were maintained in the range 10-15 mM, 10-15 mM, and 10-82 µM, respectively, by addition of CaCl2 and NaHCO3 solutions from syringes at precalibrated rates. The CaCl2 syringe contained a small predetermined amount of UO2+ 2 to account for loss from solution during growth. Air was continuously bubbled through solutions, which were stirred. Solution pH remained relatively constant (8.1-8.2) throughout the coprecipitation. By using a high-purity Ca reagent, we avoided Sr contamination, which is common in natural samples and results in an interference for fluorescence detection of XAFS. Saturation states with respect to pure calcite and aragonite were in the range Ω ) 15-28. Growth solutions were undersaturated with respect to other potential solubilitylimiting phases, e.g., rutherfordine (UO2CO3) and schoepite (UO3‚2H2O). Solutions for aragonite coprecipitation differed from those for calcite only by addition of Mg2+. The calcite and aragonite products were finely crystalline. XRD analysis using a low-background sample holder showed no indication of any second phase. The concentration of U in the solids was determined by ICP-MS after dissolution in a high-purity 10% nitric acid solution. The two calcite samples examined were Ucc3 (700 ppm U) and Ucc6 (1890 ppm U); aragonite samples included Uar4 (985 ppm U) and Uar5 (10,810 ppm U). X-ray Absorption Spectroscopy. X-ray absorption spectra were collected at beamline X11A of the National Synchrotron Light Source, Brookhaven National Laboratory. Scans were made of the U L3-edge (17166 eV) using Si(111) monochromator crystals, with one crystal detuned by 30-45% to eliminate harmonic contributions. For most samples, absorption was measured by the fluorescence yield using a 13 element solid-state Ge detector, although a Lytle detector was used for the aragonite sample containing 10 810 ppm U. Transmission mode was employed for model compounds. All solid samples were mounted in a cryostat held at near liquid nitrogen temperature for data collection. Scans were also collected at room temperature for aqueous uranyl nitrate (pH ∼ 1) and aqueous uranyl triscarbonato complex (pH 9.1). Based on calibration of the monochromator against yttrium metal foil (K-edge 17038 eV), the observed U L3-edge position (first peak in the derivative) for all the U(VI) model compounds as well as the U-doped calcite and aragonite samples was ∼17 171 eV. By simultaneous measurement of edge positions, this value is approximately 1-2 eV higher than the U(IV) L3-edge position observed for UF4. Individual scans were calibrated and averaged (typically 12-15 scans for fluorescence detection and 3-5 for transmission). Subsequent reduction of the averaged spectra involved subtraction of preedge background, normalization, and conversion to k space. The χ(k) function was extracted using a cubic spline and Fourier transformed to R space (typical k range 2.5-14.5 Å-1). Fitting was done in R space (typical R range 0.3-5.0 Å) using the program WinXAS 97 (13) and employed theoretical backscattering phases and amplitudes calculated using FEFF7 (14). These were checked against several model compounds, including uranyl nitrate hexahydrate and the aqueous uranyl triscarbonato complex. Additional fitting was done independently using FEFFIT (15), yielding results that were identical within error. A global threshold energy, ∆E0, was allowed to vary during fitting. Fits using separate ∆E0 values for each path were also attempted, but any improvement in the fits was only very slight and statistically insignificant. Errors for the fit parameters coordination number (CN), distance (R), and

Debye-Waller factor (σ2) were estimated on the basis of comparisons of fit results for well-characterized model compounds with structure parameters known from X-ray structure determinations. Estimated errors for distance (R) are (0.01 Å for first and second shells and (0.02-0.03 Å for more distant shells. Coordination numbers (CN) generally have errors of (20% (or approximately (1 atom for the equatorial shell) but may be larger for more distant shells. Errors for Debye-Waller (D-W) factors (σ2) are estimated to be (0.001-0.002 Å2. Further explanation of the methods employed are given elsewhere (16). Luminescence and Raman Spectroscopy. Time-resolved luminescence spectra were collected using a SPEX Industries Fluorolog system with a Model 1934D phosphorimeter attachment for the flashlamp excitation source (400-420 nm) as described elsewhere (17). The collection window was gated from 0.04 to 5.0 ms unless otherwise stated. Species with lifetimes 97% of the uranium is present as the uranyl triscarbonato complex, UO2(CO3)43 (aq), with ∼2% as the biscarbonato complex, UO2(CO3)22 (aq). No polynuclear uranyl complexes are predicted to be significant, nor is the recently characterized species Ca2UO2(CO3)3(aq) (18). The structure of the uranyl triscarbonato complex, UO2(CO3)43 , is well-known (19), consisting of three essentially coplanar CO3 groups in bidentate coordination in the equatorial plane of the uranyl moiety (Figure 1a). Six equatorial oxygens (Oeq) are at 2.43 Å, three carbons at 2.88 Å, and three distal oxygens at 4.12 Å. Because of its predominance in our growth solutions, it is likely that the aqueous uranyl triscarbonato species is primarily involved in the incorporation process during coprecipitation. This same molecular unit, with essentially the same configuration, is almost exclusively adopted by uranyl carbonate compounds (20-24). Hence this configuration provides a good starting model for evaluating the local structure of UO2+ 2 incorporated as a minor component in calcite and aragonite. XAFS Spectroscopy. The near-edge regions of the raw U L3-edge absorption spectra for the UO2+ 2 -containing calcite and aragonite samples are shown in Figure 2a, with the corresponding k2-weighted EXAFS shown in Figure 2b along with that for the aqueous uranyl triscarbonato complex. A strong similarity is evident for the aragonite sample and the aqueous triscarbonato species; in contrast the calcite sample exhibits subtle but important differences. The Fourier transform magnitudes (FT, not corrected for phase shifts) for UO2+ 2 -containing calcite and aragonite are shown in Figure 3. The FT for the aragonite and the aqueous triscarbonato species (not shown) both exhibit two distinct major peaks in the shorter distance region of R (real) space, the first corresponding to the two axial oxygens (Oax) and the second to the shell of equatorial oxygens (Oeq), with a small contribution from carbon atoms. In contrast, the FT for the calcite sample shows the same prominent peak for Oax, but the second peak for the equatorial shell is smaller and at a distinctly lower R than for either the aragonite or aqueous triscarbonato species. VOL. 34, NO. 4, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. (a) Schematic model of the uranyl triscarbonato ion [UO2 (CO3)43 ], having essentially the same configuration in aqueous solution and as a minor impurity species in aragonite. View shows three CO3 groups in bidentate coordination in the equatorial plane of the linear OdUdO unit (view slightly displaced from perpendicular to the page). The uranium atom is shown as light blue. (b, c) Possible coordination models of UO2+ in calcite having five equatorial 2 oxygen atoms in different combinations of monodentate and bidentate coordination. Our observations cannot distinguish between these, and evidence suggests that more than one configuration may occur in the calcite. Fits for all the UO2+ 2 -containing calcite and aragonite samples (Table 1) showed the expected two axial oxygen atoms of the UO2+ 2 unit at a distance of 1.80-1.81((0.01) Å, which is consistent with previous XAFS determinations of uranyl species (19, 22, 25, 26). Debye-Waller factors were consistently small (σ2 ) 0.002-0.003 Å2), indicating no significant difference between the distances of the axial oxygens in any of the uranyl-doped carbonates. A multiplescattering feature within the uranyl moiety was always found to be important at ∼3.6 Å and was fitted with the four-legged Oax1-U-Oax2-U path (19, 27). Aragonite. The most significant differences between the uranyl-doped calcite and aragonite samples were evident in the first equatorial shell. The fit results for both aragonite samples are essentially the same: 5.9-6.0 oxygen atoms at 2.42-2.44 ((0.01) Å, with σ2 ) 0.004-0.005 Å2. These U-Oeq distances are the same as in the uranyl triscarbonato complex (Figure 1a) and are very close to the average U-Oeq distance (2.47 Å) determined for coordination by six equatorial oxygens (24). In both aragonite samples ∼3 carbon atoms occur at 640

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2.89-2.90 Å (σ2 ) 0.002-0.003 Å2). U-C distances in this range are characteristic of bidentate coordination by three CO3 groups (19). Oxygens were fitted at 4.1-4.2 Å, the range expected for the distal oxygens of CO3 groups parallel to the equatorial plane. In addition the linear, three-legged multiple scattering path (Odist-C-U) at essentially the same distance was found to be an important contribution. Consequently, the same basic structural unit as in the growth solution occurs for the U(VI) minor impurities in the aragonite. The three peaks from 2.8 to 4.3 Å in the FT magnitudes for both aragonite samples (Figure 3) reflect backscattering from Ca shells in addition to the distal oxygens. Best fits were obtained with split Ca shells. In the lower concentration aragonite sample, these included ∼3 Ca atoms each at 3.82 and 4.03 Å, with a weaker contribution of ∼1 Ca at 4.75 Å. In the higher concentration sample, the same basic arrangement gave the best fit but with slight differences in fit parameters: ∼2 and ∼3 Ca atoms at 3.78 and 3.96 Å, respectively, with 1 Ca at 4.75 Å. An effort was made to fit some of the high-R features in the FT with U shells. Although it was possible to fit the weak feature at ∼4.3 Å in the FT (not phase-shift corrected) with a U-U backscattering path at 4.7 Å, the resultant fit was no better than that using the U-Ca path. Moreover, the absence of polynuclear uranyl complexes in the growth solution and the low total uranium concentration (985 ppm) in this sample make it unlikely that this is a U-U backscattering feature. Calcite. The second peak in the FT for the calcite samples is smaller and at a closer distance from the uranium than for the aragonite (Figure 3). The best-fit distance for the equatorial oxygen shell was decreased significantly, yielding ∼5 oxygen atoms at 2.33 (( 0.01) Å but with notably large D-W factors (σ2 ) 0.009-0.010 Å2), the latter suggesting disorder among the distances. This fitted U-Oeq distance is shorter than those observed for the uranyl-doped aragonites (2.42-2.44 Å) and the uranyl triscarbonato complex (2.43 Å) and suggests equatorial coordination by fewer than six oxygens. A survey of U(VI) coordination in well-refined crystal structures (24) reported average U-Oeq distances of 2.28, 2.37, and 2.47 Å, respectively, for coordination by four, five, and six equatorial oxygens. Hence, a change in equatorial coordination number (CN) by one oxygen can be expected to result in a change in distance of approximately 0.1 Å. Our observed distance of 2.33 Å is 0.1 Å shorter than the U-Oeq distance in the uranyl triscarbonato complex and therefore is consistent with a coordination by five oxygens. This agrees well with our fitted CN value of ∼5.2 (( 1); however, because of the estimated error associated with fitting the number of neighbors, the CN value by itself does not uniquely identify five oxygens. Assuming coordination exclusively by CO3 groups, a CN value of five requires at least one CO3 to be in monodentate coordination (Figure 1b,c). It is possible that the U-Oeq distance may also be influenced by the character of the CO3 group coordination (monodentate vs bidentate). Geipel et al. (28) observed a similar U-Oeq distance (2.34 Å) using XAFS when they examined calcite exposed to uranyl perchlorate solutions, and we speculate that the uranyl may actually have been coprecipitated with the calcite. Weak components in the calcite spectra were best fitted with carbon atoms at ∼2.9 and ∼3.2 Å. These also suggest multiple types of coordination by CO3, but because carbon is a weak backscatterer some caution is needed to avoid overinterpretation. Nevertheless, we note that the shorter distance is typical for bidentate CO3 groups, and the longer distance would be consistent with monodentate coordination (although not uniquely so). There is little structure in the Fourier transforms beyond 3.2 Å; the position and amplitudes of the small peaks that do appear are very sensitive to the selected k range for the FT and hence are probably artifacts. Therefore, we see no evidence of significant backscattering

FIGURE 2. (a) Near-edge region of the raw U L3 absorption edge for the U(VI)-containing calcite and aragonite samples. (b) The k2-weighted background-subtracted L3-edge EXAFS for the U(VI)-containing calcite and aragonite samples along with the spectrum for the aqueous uranyl triscarbonato species. The spectrum for the aragonite is similar to that for the aqueous species but different than for calcite. from Ca atoms, unlike the result for aragonite. Nor can we identify distal oxygens as would be expected for uniform bidentate coordination of CO3 groups as found in the aragonite. This may result from twisting or tilting of one or more of the CO3 groups out of the equatorial plane, with an attendant loss in the multiple-scattering contribution along the linear Odist-C-U path found to be important in aragonite. In view of the large D-W factors for the equatorial oxygen shell and the multiple although weak contributions from carbon in the uranyl-doped calcite, we attempted to fit the second peak in the FT as a split shell. Although it could be fitted with two slightly separated oxygen shells, no statistically significant improvement in the overall fit resulted. Therefore it seems likely that some variation among the individual U-Oeq distances exists but not sufficiently separated or uniform to distinguish satisfactorily. A further possibility is that more than one local structural environment exists for the U(VI) in these calcite samples.

Raman and Luminescence Spectroscopy. The results from Raman and luminescence spectroscopies not only support the XAFS finding of a difference in the local environment of uranyl in calcite and aragonite but also signal additional complexity. The Raman spectrum from the uranylcontaining aragonite shows a relatively broad peak at 809 to 813 cm-1, indicating strong equatorial bonding to the uranium (29). The luminescence spectrum from the uranyl in aragonite (Figure 4a) is very similar to that found for the mineral liebigite, Ca2UO2(CO3)3‚11H2O, (Figure 4b) and for the recently characterized Ca2UO2(CO3)3 aqueous species (18). Therefore, both the Raman and luminescence data are also consistent with the UO2+ 2 being coordinated by three CO3 groups in a bidentate arrangement in the aragonite. At room temperature, a single-exponential decay of the excited state was noted (τ ) 117 ( 20 µs), which is more similar to that for triscarbonato minerals (liebigite: τ ) 270 µs) than that of the aqueous complex (τ ) 64 ns), indicating that the VOL. 34, NO. 4, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. (a) The Fourier transform magnitude (not corrected for phase shifts) of the k2-weighted XAFS spectrum showing the fit for the aragonite sample. The two large peaks at low R correspond to the axial and equatorial oxygen shells, with a weak contribution from carbon. Peaks between about 3 and 4.5 Å correspond to shells for calcium and oxygen atoms, with contributions from two multiplescattering paths. (b) The FT magnitude (k2-weighted) for the calcite sample shows the same low-R peak for the two axial oxygens, but the peak for the equatorial oxygen atoms is smaller and shifted to lower R, suggesting fewer oxygens positioned closer to the U than in aragonite. The lack of significant features above about 3.2 Å indicates either the absence of well-defined structure (i.e., disorder) or more than one environment. uranyl is strongly bound within the aragonite structure rather than loosely bound as a surface species (18). Cooling to liquid nitrogen temperature revealed a second exponential decay component (τ ) 125 µs and τ ) 590 µs). Spectral sharpening was also apparent when the gate for luminescence detection 642

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was delayed to include light only from 2.5 to 10.0 ms after the excitation pulse. Therefore, there also must be a second uranyl component in the aragonite. Given the XAFS results, this second, short-lived component is minor, and the major species in aragonite is the UO2(CO3)3 unit.

TABLE 1. Best-Fit Parameters from U L3-Edge XAFS Data for Uranyl-Containing Aragonite and Calcite Samples aragonite (Uar4) shell

CN

Oax Oeq C Ca Ca Ca Odist

2a 5.9 3.2 2.8 2.9 0.9 3.9

R (Å) 1.81 2.44 2.90 3.82 4.03 4.75 4.22

aragonite (Uar5) σ2 (Å2) 0.003 0.004 0.002 0.004 0.007 0.003 0.004

shell

CN

R (Å)

σ2 (Å2)

Oax Oeq C Ca Ca Ca Odist

2a

1.80 2.42 2.89 3.78 3.96 4.75 4.10

0.002 0.005 0.003 0.004 0.006 0.004 0.004

calcite (Ucc6)

6.0 3.2 1.7 3.2 1.0 5.3

calcite (Ucc3)

shell

CN

R (Å)

σ2

(Å2)

shell

CN

R (Å)

σ2 (Å2)

Oax Oeq C C

2a 5.2 2.1 1.6

1.80 2.33 2.91 3.22

0.002 0.009 0.003 0.005

Oax Oeq C C

2a 5.3 1.9 1.3

1.80 2.33 2.89 3.22

0.003 0.010 0.005 0.005

a Fixed during fitting. Estimated errors are CN ((20%), R ((0.01, first two shells; (0.02-0.03, more distant shells), σ2 ((0.001-0.002). Samples: Uar4 (985 ppm U); Uar5 (10,810 ppm U); Ucc6 (1890 ppm U); Ucc3 (700 ppm U). Multiple-scattering paths were found to be important at ∼3.6 Å in both samples and at ∼4.1-4.2 Å in aragonite.

FIGURE 4. (a) Luminescence spectra from uranyl incorporated into aragonite (dashed line) and calcite (solid line) compared with (b) spectra from well characterized mineral samples liebigite (dashed line) and rutherfordine (solid line). Spectra were collected at room temperature, except for rutherfordine, which was collected at liquid nitrogen temperature. The Raman spectrum from the UO2+ 2 -containing calcite shows a relatively broad peak at 818 ((2) cm-1, again indicating strong equatorial bonding to the uranium (29). Furthermore, the luminescence spectrum (Figure 4a) is shifted considerably to the red of that for the aragonite, clearly indicating a different configuration in the calcite. In a previous paper on uranyl sorption on calcite, it was postulated that uranyl hydroxides are formed at the mineral surface (28). Indeed, the luminescence spectrum of the calcite in Figure

4 is similar to spectra from several uranyl hydroxides, especially UO2(OH)2 and [(UO2)3(OH)7]- (30). However, our aqueous speciation calculations show that the most abundant uranyl hydroxide is 4 orders of magnitude lower in concentration than the aqueous UO2(CO3)43 species, which accounts for >97% of the uranium in solution. Moreover, it is unlikely that the UO2(CO3)43 would eject CO3 to scavenge OH-, particularly when the former is the more abundant of the two aqueous species by a factor of >100. The luminescence spectrum for calcite provides some support for the mixed monodentate/bidentate coordination suggested from the XAFS results. Local geometry with monodentate CO3 coordination around a uranyl moiety is known from a few solids such as rutherfordine, in which two CO3 groups are coordinated in a twisted bidentate fashion and two additional CO3 groups in a monodentate fashion (31). We note that the luminescence spectrum of rutherfordine is red-shifted from the liebigite spectrum, as is the calcite spectrum from the aragonite spectrum (Figure 4). However, the rutherfordine structure is an extended layer structure with edge-sharing of the uranyl polyhedra, so the carbonates do not bond to the individual uranyls as strongly as they would for isolated polyhedra. The room-temperature lifetime of the uranyl in calcite is 114 µs, very similar to that of the uranyl in aragonite. At liquid nitrogen temperatures, the lifetime is 220 µs, with possibly a second component of 90 µs. Gating the luminescence spectrum at longer delay times resulted in increased resolution of the peaks. Together with the low-temperature lifetime measurements, we can conclude that more than one local configuration probably exists in the calcite. Incorporation Mechanism. We have shown that the configuration of the UO2(CO3)43 unit, the dominant aqueous species, is retained by the uranyl in aragonite, suggesting that the entire unit is incorporated into the structure essentially intact. In contrast, a different equatorial coordination occurs in calcite, characterized by fewer nearest oxygens at a closer distance, at least some of which reflect monodentate CO3 groups, and probably more than one local configuration (i.e., disorder). Hence, the coordination of the UO2+ 2 unit clearly must change during incorporation into calcite. We cannot say whether this coordination change took place at the growing calcite surface or if forced to conform during “burial” into the structure. It is interesting to note that workers examining coprecipitation of the aqueous B(OH)4 species into calcium carbonate (32, 33) also concluded that a change in its primary coordination occurs during incorporation into calcite (resulting in trigonally coordinated boron) but not into aragonite (where the boron remains tetrahedrally coordinated). Identification of a substitution site for the U(VI) in these carbonates is more problematic than for U(IV) substitution (8), owing to the size and geometry of the uranyl carbonate complex. In the case of the uranyl-containing aragonite, the XAFS results showed 4-5 Ca atoms at a distance 3.8-4.0 Å, with weaker Ca backscattering at 4.75 Å. The aragonite structure has six Ca-Ca distances over the range 3.89-4.10 Å, with four more at 4.7 Å. Hence, our U-Ca distances allow an interpretation in which the uranyl triscarbonate complex occupies a portion of the structure corresponding to a Ca polyhedron and some of the coordinated CO3 groups. But we emphasize that this should not necessarily be interpreted as a substitution of U for Ca, since the aqueous uranyl carbonate species incorporated is an anion and bears no geometrical similarity to Ca2+. For the uranyl-containing calcite, identifying a substitution site is even more difficult. The inability to identify any backscattering from Ca atoms or from O atoms beyond the first equatorial shell from the XAFS of the calcite indicates either a disordered or multiple structural environments for VOL. 34, NO. 4, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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U(VI), which is confirmed by the luminescence data. The presence of monodentate CO3 groups would contribute to this disorder because of their greater freedom for tilting and rotation in adapting to the host structure. This disorder in calcite, and its apparent absence in aragonite strongly suggests a less stable structural environment for uranyl ion in calcite than in aragonite. Our conclusion that uranyl ion has a more stable and well-defined local environment when coprecipitated with aragonite than with calcite is unfortunate for the long-term retention of U(VI) by carbonates. At the conditions prevailing near earth’s surface, aragonite is metastable relative to calcite, and in the presence of waters containing little Mg2+, aragonite readily recrystallizes to calcite. Although it is not yet possible to assess the stability of the dilute solid solutions formed by coprecipitation of uranyl with calcite, it seems likely that significant disruption of the local structure occurs, further decreasing its stability. Consequently, calcite may not be effective for sequestering U(VI) species over time scales important for immobilization of radionuclides. Our results also offer the first direct structural insight about the relative differences in uptake of uranyl by aragonite and calcite phases. Several studies have shown that uranyl is preferentially taken up by aragonite relative to calcite (34, 35). Although the factors that control the partitioning behavior of impurities into carbonates reflect much more than simply the local structural environment in the host solid, our finding that a change in uranyl coordination is apparently required for its incorporation into calcite but not in aragonite is entirely consistent with its preferential uptake by the latter. And the disordered or multiple environments for uranyl in calcite are presumably a reflection of the stronger discrimination against its incorporation.

Acknowledgments Support for R.J.R. and M.N. were provided by NSF Grant EAR9706012. C.D.T. and D.E.M. were supported by the LANL Laboratory Directed Research and Development Program.

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Received for review August 23, 1999. Revised manuscript received November 23, 1999. Accepted December 8, 1999. ES990981J