and Manganese Oxide Minerals Manganite and Hausmannite

Mar 17, 2005 - Neptunium-237 is present in nuclear waste and consti- tutes a potential ..... synthetic brines of interest to the Waste Isolation Pilot...
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Environ. Sci. Technol. 2005, 39, 2608-2615

Interfacial Interactions between Np(V) and Manganese Oxide Minerals Manganite and Hausmannite P. A. WILK,§ D. A. SHAUGHNESSY,§ R . E . W I L S O N , †,‡ A N D H . N I T S C H E * ,†,‡ Chemical Biology and Nuclear Science Division, Lawrence Livermore National Laboratory, Livermore, California 94551, Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, and Department of Chemistry, University of California, Berkeley, CA 94720-1460

Manganese oxides, present as minor phases in the vadose zone, have been previously shown to sequester large quantities of plutonium under environmental conditions. We are now continuing these studies with Np(V). Sorption onto manganite (MnOOH) and hausmannite (Mn3O4) at solidto-solution ratios of 2.5-3.3 mg/mL has been studied as a function of neptunium concentration and pH. The sorption of Np increased as a function of pH for both minerals, attaining a maximum at neutral pH, and then decreased with increasing alkalinity. X-ray absorption fine structure spectroscopy (XAFS), taken at the Np LIII- edge, has been used to determine the oxidation state of the sorbed Np. Our experimental results indicate reduction of the Np(V) because of interaction with the X-ray beam. These findings significantly impact the interpretation of results reported elsewhere on Np(V) investigated though the use of highintensity X-ray beams.

Introduction To understand important fundamental chemical processes governing the potential migration of transuranic contaminants in the environment, we are investigating the interactions between actinides and manganese oxide/hydroxide minerals. It has been previously shown that plutonium will sorb to iron oxyhydroxides, manganese oxide soil components, and other naturally occurring materials, which could possibly retard its migration through the environment (16). Recent synchrotron-based X-ray microprobe studies have shown that in a mixed mineral system, manganese oxides preferentially sequester plutonium over iron oxides even if the iron is present in larger quantities (7). There is a wide range of manganese oxyhydroxide minerals found in the environment (8, 9) that can also be present as coatings on soil and sediment grains. These manganese oxides have high negative surface charges over a wide pH range (pH g 2), resulting in a large cation sorption capacity (10). Because of this, an effective transport model and remediation system for neptunium contaminants specifically requires an understanding of the interactions of neptunium * Corresponding author phone: (510)486-5615; fax: (510)486-7444; e-mail: [email protected]. § Lawrence Livermore National Laboratory. † Lawrence Berkeley National Laboratory. ‡ University of California. 2608

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with manganese oxyhydroxides. Neptunium (11) is expected to sorb to these mineral surfaces in an inner-sphere coordination similar to what has been observed with other actinides (12-16). Neptunium-237 is present in nuclear waste and constitutes a potential hazard to the environment because of its long half-life (2 144 000 ( 7000 years (17)) and relatively high solubility in near-neutral aqueous solution (18-22). Predicting the potential hazard of neptunium, as well as other transuranic elements in the environment, requires reliable transport models. Fundamental knowledge of the interfacial reactions between neptunium and the surrounding geologic material is essential for modeling the transport of this element through the vadose zone as well as understanding the potential risk that this radionuclide poses. We have begun systematic studies of the interfacial reactions between aqueous neptunyl(V) species and the manganese oxyhydroxide minerals manganite (MnOOH) and hausmannite (Mn3O4). These minerals are present at many nuclear waste sites such as Savannah River and Hanford, where they are found in vadose zone conditions beneath the tank farms (23, 24). We investigated the sorption of Np(V) onto manganite and hausmannite as a function of neptunium concentration and pH. X-ray absorption fine structure spectroscopy (XAFS), taken at the Np LIII- edge, was used to probe the oxidation state and structures of the sorbed neptunium complexes.

Experimental Procedures Minerals. The manganite and hausmannite minerals used in this study have been previously characterized by our group and are described elsewhere (16, 25). Powder X-ray diffraction (XRD) patterns confirmed the respective mineral phases, and the surface areas were determined by nitrogen adsorption using the standard BET method. The surface area of manganite was 9.5 ( 1.4 m2/g and hausmannite was 20.4 ( 0.8 m2/g. The point of zero charge (pHPZC) of manganite occurs at a pH of 7.4 ( 0.3 and the pHPZC of hausmannite was greater than 10 (16, 25). While the solubility of these minerals has not been quantified in the literature, they are considered to be largely insoluble in water (26). Sorption Experiments. All sorption experiments were performed with minerals that were crushed and sieved to 63-212 µm in size. These experiments were performed in polycarbonate centrifuge cones (Nalgene Oak Ridge polycarbonate tubes with round bottoms) that were preconditioned with 0.1 M HClO4. There is significant adsorption of neptunium onto the walls of the cones that had to be taken into account, especially at neutral pH. This result differs from those reported by others using the same tubes (13, 27, 28). Adsorption of neptunium to the vessel walls was a factor of 2 greater when polypropylene tubes were used. Both the polycarbonate and polypropylene tubes had polypropylene caps with Teflon sealing rings, but in neither case was it found that the neptunium sorbed onto the caps or the rings. The 237Np(V) was prepared from a stock solution that also contained some decay products. The neptunium was purified using the technique previously described by Shaughnessy (29) and Hall (30) and is described briefly here. The neptunium was loaded on an anion exchange column (DOWEX AG-1-X8 chloride form, free column volume approximately 2 mL) in concentrated HCl. The protactinium was removed first by eluting with one free column volume of 0.2 M HF in concentrated HCl. The plutonium was then removed by eluting with one free column volume of a 7:1 solution of concentrated HCl and HI. The clean neptunium 10.1021/es040080x CCC: $30.25

 2005 American Chemical Society Published on Web 03/17/2005

TABLE 1. Sorption as a Function of BET Measured Surface Area for 100% Sorption of Neptunium from Solution mineral

surface area (m2)

MnOOH Mn3O4 Np concentration solid/solution ratio

9.5 ( 1.4 20.4 ( 0.8

amount sorbed at 100% sorption (mg/m2) 4.737 2.206 1.5 × 10-4 M 10 mg/3 mL

FIGURE 1. Sorption of neptunium on manganite and hausmannite as a function of pH. The error bars are contained within the points and are approximately 2%. was then stripped off the column with a dilute (2 M) solution of HCl and stored in a glass vial. The oxidation state of the stock solution was verified using optical absorption spectroscopy (Ocean Optics S2000 Fiber Optic Spectrometer). Only Np(V), with its characteristic peak at 980 nm (31), was present in solution. An aliquot of the clean neptunium solution was added to an appropriate amount of 0.1 M NaClO4 to make three stock solutions at neptunium concentrations of 1.5 × 10-4 M, 1.5 × 10-5 M, and 1.5 × 10-6 M. Three milliliters of stock solution (4 mL of the 1.5 × 10-6 M solution) were pipetted into the polycarbonate cones and the pH was adjusted with small amounts of NaOH and HClO4 of varying concentrations. The pH was determined with a Ross style electrode (NaCl electrolyte bridge). At pH values greater than 7, 0.5 M NaHCO3 or 3 M Na2CO3 was added to achieve equilibrium with carbonate in air (calculated ahead of time using carbonate equilibrium with log KH ) -1.47, log K1 ) -6.16, and log K2 ) -9.94). The tubes were then rocked end over end for at least 2 days to equilibrate the neptunium with the tubes. Two 50-µL aliquots of each solution were added to 4 mL of Ecolume scintillation cocktail and were counted using a digital Wallac 1414 liquid scintillation counter with alpha-beta and energy discrimination to verify the concentration of neptunium in solution after any initial sorption of neptunium onto the tube walls. Alpha-beta discrimination is essential to remove interference from the beta-decaying daughter 233Pa. To each neptunium solution, 10 mg of manganite or hausmannite was then added for solid-to-solution ratios of 2.5 or 3.3 mg/mL respectively, and

0.474 0.221 1.5 × 10-5 M 10 mg/3 mL

0.063 0.029 1.5 × 10-6 M 10 mg/4 mL

the slurries were slowly rocked end over end for 24 h. The mineral absorption was very rapid and complete within 24 h. The amount of neptunium sorbed onto the minerals was determined by calculating the difference between the original concentration of neptunium in solution and the amount of neptunium remaining in solution after contact with the minerals. After sorption was complete, a 120-µL aliquot was removed from the sample and the solid and supernatant were separated using a Microcon YM-30 cellulose centrifugal filter at 10 000 rpm for 12 min. The filters were used without any prior treatment and neptunium adsorption on the filters was negligible in the pH ranges used in these experiments. Two 50-µL aliquots of the resulting solution were each added to 4 mL of Ecolume scintillation cocktail and were counted using the Wallac liquid scintillation counter to determine the concentration of neptunium remaining in solution. X-ray Absorption Fine Structure (XAFS). Samples for XAFS measurements were prepared as described above, except in larger volumes. For these experiments, 35-mL polycarbonate cones were used (Nalgene 35-mL Oak Ridge graduated tubes with conical bottoms) and 250 mg manganite or 100 mg hausmannite was added to 20 mL of solution for solid-to-solution ratios of 12.5 and 5 mg/mL, respectively. After 24 h of contact time with the mineral, the solids were filtered and packed into XAFS sample tubes as wet pastes. The samples were then sealed inside multiple polyethylene bags for containment. Neptunium LIII-edge XAFS spectra were collected at the Stanford Synchrotron Radiation Laboratory (SSRL) on wiggler beam line 4-1 using a half-tuned Si (220) double-crystal monochromator and a 0.7-mm vertical slit opening. Fluorescence yield spectra were collected with a four-pixel Ge fluorescence detector (32). Energy calibration was performed simultaneously by measuring transmission data from a NpO2 reference. All data reduction and fits to the data utilize standard procedures (33, 34), and data from the Ge detector was corrected for dead time prior to data reduction. Backscattering amplitudes and phases for fitting the raw data were calculated with FEFF8 (35).

Results and Discussion Neptunium sorption on manganite and hausmannite as a function of pH and neptunium concentration is shown in Figure 1. Because of long counting times used, the error in the experiment is almost entirely due to the volume error associated with the aliquot sampling. This error is less than 1% per aliquot. The amount sorbed was calculated by comparing the activity in the supernatant to the activity in the solution before contact with the minerals resulting in a sampling error associated with this measurement of less than 2%. It was observed that the amount of neptunium sorbed onto manganite and hausmannite steadily increased from pH 3 to a maximum value of 100% at pH 7 and then decreased in the pH region between 9 and 10. Table 1 summarizes this sorption as a function of surface area. The kinetic behavior of neptunium sorbed onto manganite and hausmannite was studied at several pH values over the course of a week. It was observed that the majority of the sorption occurs within the first 24 h of contact. On the basis of this, subsequent sorption experiments were measured on a 24-h period to avoid any secondary reactions. The rapid VOL. 39, NO. 8, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Solution speciation of Np(V) at an ionic strength of 0.1 M and [Np] ) 10-4 M, equilibrated with atmospheric carbonate.

TABLE 2. Distribution of Oxidation States of Neptunium on the Mineral Surfaces after Sorption, Determined by Fitting XANES Data with a Linear Combination of Neptunium Standardsa Mineral

pH

Np(IV)

Np(V)

NpO2 solution in HNO3

+

pH 3

78 ( 12%

22 ( 12%

MnOOH

pH 7 pH 8 pH 10

78 ( 7% 53 ( 3% 0%

22 ( 7% 47 ( 3% 100%

Mn3O4

pH 5 pH 7 pH 8 pH 10

11 ( 3% 0% 43 ( 5% 0%

89 ( 3% 100% 57 ( 5% 100%

a Errors in the XANES fits were calculated from the diagonal terms of the covariance matrix assuming that χ2 divided by the number of degrees of freedom was equal to unity.

FIGURE 3. An example of the XANES data, collected from a manganite mineral sample, at pH 8. The fit to the data is a linear combination of the neptunium(IV) and (V) oxidation states, which are shown in their relative amounts. Redox behavior is observed in the XANES spectra in the majority of the samples. sorption can most likely be attributed to a diffusion-controlled surface reaction (36). It has been previously shown with plutonium that the amount of sorption and sorption kinetics did not depend on the mineral particle size (16). This rapid sorption behavior has been observed previously with uranium (12), neptunium (37, 38), and plutonium on goethite (39). The sorption behavior of neptunium on manganite and hausmannite is due to the interaction of the neptunium species present in solution and the chemistry of the mineral surface itself. Figure 2 shows the results of a calculation of 2610

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the solution speciation of 1 × 10-4 M Np(V) in a NaNO3 solution at an ionic strength of 0.1 M in equilibrium with atmospheric carbonate (obtained with the MINEQL+ program version 3.01b using the most recent (40) thermodynamic data for neptunium complexes). According to this calculation, at pH values lower than 7, the neptunium is present in solution as the neptunyl NpO2+ cation. From pH 8 to approximately pH 10, the majority of neptunium in solution is present as a monocarbonato anion. At higher pH values, the dominant species is a triscarbonato anion. At around pH 10, there is a small but significant contribution by the biscarbonato anionic complex. In all cases, the carbonate ligands coordinate in a bidentate manner (41). It is not always the dominant species in solution that interacts with the mineral surface. The charged neptunium solution complexes interact with the surface functional groups of the mineral, which are hydroxide groups for manganese dioxide and a variety of other oxide minerals (42, 43). As previously mentioned, the pHPZC of manganite is 7.4 ( 0.3 (16, 25). At pH values lower than the pHPZC, the surface hydroxyl groups are protonated, resulting in a net positive surface charge; similarly, at pH values larger than the pHPZC, the surface has a net negative charge because of deprotonated surface hydroxyl groups. For hausmannite, the pHPZC (pH > 10) lies outside of our reliable experimental range. The positively charged neptunyl cation at acidic pH levels tends

FIGURE 4. NIR absorption spectra of sample supernatant before EXAFS, showing clearly the single 980-nm peak corresponding to the Np(V) oxidation state. not to sorb because of electrostatic repulsion, but sorption increases after pH 7 with the onset of the neptunium monocarbonate anionic species. Likewise, the decreased sorption in the weakly alkaline region is due to the repulsion between the negatively charged surface and the tris-carbonato anion. It has been observed elsewhere that metal cations can strongly sorb to mineral surfaces even against electrostatic repulsion, contrary to the pHPZC (44, 45). This seems to indicate that the complexation chemistry of metal cations with surface hydroxyl groups can be even more dominant than electrostatic forces in driving metal cation

sorption and could help explain the amount of sorption observed at lower pH values. The results from the sorption batch experiment indicate that manganese-based minerals in the environment could significantly retard transport of neptunium through contaminated environments. Results such as these need to be incorporated into surface complexation models to better predict the migration of neptunium contaminants through the vadose zone to groundwater supplies. In addition, it might be possible to use the sequestering properties of these minerals in a manner to mitigate existing environmental contamination. Neptunium Valence Determination. The oxidation states of both the neptunium sorbed to the mineral surfaces and the neptunium remaining in solution after sorption was complete were determined using XAFS and optical absorption methods. To determine the oxidation state of the neptunium bound to the mineral surfaces, X-ray absorption near-edge structure (XANES) measurements were made. Neptunium oxidation states can be determined by carefully comparing the main LIII absorption edge energy to those of known standards. The incident X-ray energies for the Np LIII-edges of the standard solutions were also calibrated to the NpO2 reference. There is a feature on the high-energy side of the white line in the Np(V) spectrum because of multiple scattering between the neptunium central atom and the axial oxygens of the neptunyl structure that is not observed in Np(IV) spectra. The differences in energy and shape between Np(IV) and Np(V) allow for the determination of neptunium oxidation states on the mineral surfaces by fitting a linear combination of the standard spectra to the experimental data (16, 25). In addition, optical absorption spectroscopy was used to determine the valence of the neptunium remaining in solution after the sorption process was com-

FIGURE 5. NEA selected standard potentials (in volts) (40, 62).

FIGURE 6. Potentials (in volts) from Katz and Seaborg (47) (as well as IUPAC (51)). VOL. 39, NO. 8, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 3. Standard Reduction Potentials of Neptunium in Acidic Solution (in Volts) NpO22 + / NpO2+

NpO2 + / Np4+

NpO22 + / Np4+

Np4+ / Np3+

Np3+ / Np

Np4+ / Np

Np3+ / Np2+

NpO3 + / NpO22 +

1.15 1.24 1.159

0.75 0.64 0.604

0.94 0.882

0.147 0.15 0.219

-1.86 -1.79 -1.772

-1.30 -1.274

-4.7

2.04

NpO2 + / Np3+

NpO22 + / Np3+

Np2+ / Np -0.3

0.411

0.661

ref (49, 53) (47, 49, 51) (40)

FIGURE 7. Formal potentials (in volts) in basic solution (1 M NaOH) (40, 52). pleted. The minerals were filtered out and the resulting supernatant was measured from 400 to 1300 nm. Optical absorption measurements were made before and after the samples were introduced into the X-ray beam. Thus, the combination of XANES and optical absorption measurements allows us to probe the valence of both the sorbed neptunium and the neptunium remaining in solution.

Also mentioned in the literature are the potentials from Keller (31) and Latimer’s oxidation potentials (53). Summaries of these data are given in Table 3. In addition, several published formal potentials in acidic solutions have been measured, which agree with these data to a large extent (31, 40, 46, 52-54). Unfortunately, there are very few references to formal potentials in basic solution. Since the speciation of neptunium changes in basic solution, and there is some disagreement on the expected speciation in basic solution, it is difficult to extrapolate to basic solutions from acidic solutions. Of the references previously mentioned, only Katz and Seaborg (47), Keller (31), IUPAC (51), and the NEA (40) list standard redox potentials of neptunium in basic solution. Katz and Seaborg list the same formal potential as IUPAC in 1 M NaOH (see Figure 6). The NEA lists the formal potential in 1 M NaOH that is recommended by Peretrukhin et al. (40, 52) (see Figure 7).

An example of XANES measurements collected from Np(V) sorbed onto the minerals is shown in Figure 3. A comparison of neptunium standards (Np(V)O2+ solution and Np(IV)O2 solid) to the data from the samples containing mineral show that in some of the samples the neptunium has been reduced from the initial Np(V) to the Np(IV) oxidation state (see Table 2). This was verified after the experiment via optical absorption spectroscopy. The concentrated Np(V)O2+ solution was not reduced during the short period in which the reference spectrum was taken. At pH 8, approximately half the neptunium in both the hausmannite and manganite solutions was reduced. This reduction was not observed at pH 10 or pH 5. Strangely, at pH 7, the manganite sample saw ∼80% reduction while the hausmannite sample did not see any reduction at all. After contact with manganite, no reduction of the neptunium was observed in laboratory experiments unless the samples were introduced into the X-ray beam. However, some X-ray reduction was observed in a neptunyl control solution even when no minerals were present. This same control solution was stable against reduction in the laboratory during the same experimental period as verified by optical spectroscopy. The amount of each neptunium oxidation state as determined from XANES spectroscopy are given in Table 2. An optical absorption spectrum of pure Np(V) solution before being bombarded at the synchrotron is shown in Figure 4. These results imply that the X-ray irradiation was responsible for the reduction or at least encouraged the reduction catalytically.

The speciation of neptunium in alkaline solutions in the absence of carbonate is unclear and if it were better known, it would be possible to calculate accurately the potentials from those measured in acidic solution. It appears now that the species postulated in earlier reference works, such as those by Katz and Seaborg, are oversimplified. The data currently available seem to support a much more complex picture. Bolvin et al. (55) and Williams et al. (56) have deduced a formula of NpO4(OH)23- and NpO2(OH)42- for the (VII) and (VI) oxidation states, respectively, in strongly alkaline solutions, which is also supported by the work of Peretrukhin et al. (52) and Shilov (57). This formula is reflected in the Latimer diagram depicted in Figure 7. Also reflected in this figure is the more complex species for Np(V): NpO2(OH)3(H2O)2-, which is expected in strongly alkaline solutions (52). In more dilute solutions, aNaOH < 0.8, and the Np(V) species expected by Peretrukhin et al. are NpO2OH‚xH2O and NpO2(OH)2(H2O)2- (52).

Neptunium Redox Behavior. The early actinides have a more complex chemistry than the lanthanides because of the poor shielding of the 5f electrons. Although neptunium can exist in several oxidation states, the +4 and +5 oxidation states are most stable (31, 46, 47). It is redox active, with the +4 and +5 oxidation states favored, depending on the chemical environment. Extensive reviews of neptunium electrochemical and thermodynamic data have been published, several of them quite recently (48-52). Of these, the reviews published by the Nuclear Energy Agency (NEA) (Figure 5) (40) and International Union of Pure and Applied Chemistry (IUPAC) (Figure 6) (51) make a qualified recommendation for a standard reduction potential for the Np couples on the basis of a careful analysis of the existing data. Also widely referenced in the actinide community are the neptunium standard potentials from Katz and Seaborg (47), which are nearly identical to the IUPAC recommendation.

While it is evident that the neptunium can be reduced solely by the presence of the X-ray beam (we saw reduction of our standard solution in the beam with no Mn present), it is possible that it is catalyzed by a Mn-mediated reaction that is otherwise typically too slow or with an energy of activation that is too high to observe in the absence of the mineral. The disproportionation of Mn(III) to Mn(IV) and Mn(II) could then contribute to the reduction (16, 25) observed in those samples with manganese minerals present compared to the inconsistent, perhaps slower, reduction of the Np(V) solution alone. Mn(II) is highly soluble and its presence in solution could account for the rapid reduction of neptunium to the +4 state. Carbonate species might also contribute to the reduction of neptunium. Mn(II) would not be expected to reduce Np(V) in acidic solutions, but on the basis of the known standard potentials, it is possible for a Mn(II/III) couple to reduce Np(V) in basic solution with a

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FIGURE 8. Fourier-filtered EXAFS (A, B) and resulting Fourier transforms (C, D) of neptunium sorbed onto manganite and hausmannite at pH 8 after 24 h of contact. Experimental data is represented by the small triangles and the solid lines depict the corresponding fits. The transform range for each sample was from 2.5 to 8.0 Å-1 and was Gaussian narrowed by 0.3 Å-1 during the transform. Solubility was exceeded, and only precipitation is apparent in the EXAFS Fourier transform. potential difference of about 0.6 V. Since both the Mn and Np couple would be expected to share an identical dependence on [OH]-, pH should have no effect on the difference between the two potentials as long as the pH > 7. The corresponding couples in acidic solution have a difference of about negative one volt. Recently, Np(V) reduction has been observed on the surface of iron oxides (37) and iron sulfide (58); although it was attributed to reduction by the mineral, it seems that it was in fact reduced by the X-ray beam. Neptunium(V) reduction in bacterium has also been observed elsewhere in nonmetal reducing bacterium exposed to a synchrotron X-ray beam (59). Neptunium-Mineral Sorption Complexes. To determine the structures of the sorbed neptunium-mineral complexes, extended XAFS (EXAFS) measurements were also made. Figure 8 shows the neptunium LIII-edge k3-weighted EXAFS spectra for neptunium sorbed onto manganite and hausmannite at pH 8. Table 4 summarizes the EXAFS parameters

TABLE 4. EXAFS Structural Parameters for Neptunium Sorbed onto Manganite and Hausmannite at pH ) 8 mineral

pH

shell

R (Å)

Na

σ (Å)

MnOOH pH 8

Np-O 2.33 ( 0.01 7.7 ( 1.3 0.10 ( 0.01 Np-Np 3.83 ( 0.02 3.6 ( 2.2 0.06 ( 0.03

Mn3O4

Np-O 2.33 ( 0.01 4.5 ( 0.8 0.04 ( 0.02 Np-Np 3.83 ( 0.04 3.5 ( 2.8 0.09 ( 0.05

pH 8

a The number of neighbors was calculated using S 2 ) 0.9 ( 0.1 o determined from NpO2.

that were fit to the data. The absence of anything but NpNp interactions implies that the neptunium has simply precipitated on the surface. An attempt was made to include a second Np-O scattering path into the fit to account for any neptunyl cation, but we were unable to fit the data with this type of interaction. This provides additional evidence VOL. 39, NO. 8, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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for the reduction of Np(V) to the Np(IV) oxidation state. According to the Mineql calculations, Np(V) was not expected to precipitate at these concentrations and at these pH values. Accurate transactinide solubility calculations require detailed knowledge of concentration, electrical, and chemical characteristics of all species in the solution. If the calculation does not take all these parameters into account, it is possible to predict a solubility that is off by an order of magnitude or more. Meaningful thermodynamic solubility data can only be collected empirically by carefully constructed experiments (22). For example, uranium at neutral pH will precipitate at a concentration greater by more than an order of magnitude (60) than predicted purely from thermodynamic data (40, 61). The observed reduction of the neptunyl is important. If the minerals were to contribute to the reduction, the migration of neptunium contaminant species in the environment would be far less likely if exposure to reducing materials causes the more soluble neptunyl cations to form insoluble solids. However, the results presented here, especially the reduction of the control solution, show that the synchrotron X-ray beam causes reduction of neptunyl. Further work needs to be done to fully understand this phenomenon. Obtaining structural information on these types of neptunium samples using XAFS may be impossible because of the redox interaction. To study neptunyl(V) interactions with the mineral surfaces, additional experimental methods other than XAFS will have to be investigated. As previously mentioned, the oxidation state of the sorbed neptunium is very important as it may determine whether the contaminates are likely to rapidly migrate through the vadose zone.

Acknowledgments This work is supported by the Office of Science and Technology, within the U.S. Department of Energy (DOE), Environmental Management Science Program. Lawrence Berkeley National Laboratory (LBNL) is operated by the DOE under contract number DE-AC03-76SF00098. This work was performed in part at the Stanford Synchrotron Radiation Laboratory, which is operated by the DOE, Office of Basic Energy Sciences. The authors would like to thank Corwin Booth of Lawrence Berkeley National Laboratory for his valuable assistance with the XAFS data.

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Received for review July 14, 2004. Revised manuscript received January 3, 2005. Accepted January 19, 2005. ES040080X

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