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Microscale characterization of uranium (VI) silicate solids and associated neptunium (V) Article  in  Radiochimica Acta · January 2005 DOI: 10.1524/ract.93.5.265.64281

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Radiochim. Acta 93, 265–272 (2005)  by Oldenbourg Wissenschaftsverlag, München

Microscale characterization of uranium(VI) silicate solids and associated neptunium(V) By Matthew Douglas1 , Sue B. Clark1 , ∗, Judah I. Friese2 , Bruce W. Arey2 , Edgar C. Buck2 , Brady D. Hanson2 , Satoshi Utsunomiya3 and Rodney C. Ewing3 1 2 3

Department of Chemistry and Center for Multiphase Environmental Research, Washington State University, Pullman, Washington 99163, USA Radiochemical Science and Engineering Group, Pacific Northwest National Laboratory, Richland, Washington 99352, USA Nuclear Engineering and Radiological Sciences Department, University of Michigan, Ann Arbor, Michigan 48109, USA

(Received June 1, 2004; accepted in revised form October 16, 2004)

Uranyl / Solid solution / Spent nuclear fuel / Uranium minerals / Uranophane

Summary. The uranium(VI) silicate phases uranophane, Ca[(UO2 )(SiO3 OH)]2 ·5H2 O, and sodium boltwoodite, Na[(UO2 )(SiO3 OH)]·1.5H2 O, were synthesized in the presence of small, variable quantities (0.5–2.0 mol % relative to U) of pentavalent neptunium (Np(V), as NpO2 + ), to investigate the nature of its association with these U(VI) solid phases. Solids were characterized by X-ray powder diffraction (XRD), gamma spectrometry (GS), scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM) with electron energy-loss spectroscopy (EELS). Neptunium concentration was determined in the bulk solid phases by GS and was found to range from 780–15 800 µg/g. In some cases, Np distributions between the aqueous and solid phases were monitored, and 78%–97% of the initial Np was associated with the isolated solid. Characterization of individual crystallites by TEM/EELS suggests the Np is associated with the U(VI) phase. No discrete Np phases, such as Np oxides, were observed. Because the U(VI) silicates are believed to be important solubility-controlling solids on a geologic timescale, these results suggest that the partitioning of the minor actinides to these solids must be considered when assessing the performance of a waste repository for spent nuclear fuel.

Introduction Uranium (U)-based spent nuclear fuel (SNF) is approximately 95% UO2 by mass [1]. Depending on the total irradiation time, the remainder consists of approximately 1% plutonium (Pu), 2%–3% fission products, and other transuranic elements [2]. Neptunium-237 is present in SNF at 400–600 ppm and will be one of the major contributors to dose after 50 000 years due to its long half-life of 2.14 × 106 years [3]. At present, the nuclear fuel cycle in the US is operated in a once-through mode with SNF intended for direct disposal in a geologic repository [4]. The Yucca Mountain (YM) facility in the state of Nevada is the *Author for correspondence (E-mail: [email protected]).

proposed repository and is undergoing license application in anticipation of receiving waste by the year 2010 [5]. Prior to SNF emplacement in a repository, the physical and chemical processes that control the release and subsequent migration of radionuclides in the environment should be understood. Water intrusion is the most credible scenario for mobilization of activity from SNF. Under oxidizing conditions, tetravalent U will oxidize to U(VI), forming the dioxo uranyl cation, UO2 2+ . This dioxo cation could precipitate as any one of numerous U(VI) crystalline solid phases depending on available anions and cations in the contacting water. The uranyl silicate uranophane, Ca[(UO2 )(SiO3 OH)]2 ·5H2 O, is a secondary U(VI) phase that has been observed in natural U deposits as well as on the surface of UO2 and SNF when contacted with YM groundwater [6–10]. Several U(VI) silicates including uranophane are believed to be among the thermodynamically stable phases that will form following the alteration of SNF in the YM repository on a geologic timescale [11]. Under conditions where SNF is being altered to U(VI) solids, Np is expected to exist as the pentavalent cation. Neptunium(V) should be quite soluble and potentially mobile due to the low effective charge of the neptunyl cation, NpO2 + [12, 13]. Currently the solubility of Np in YM speciation models is assumed to be controlled by discrete Np(V) phases such as Np2 O5 ·xH2 O [14]. The possibility of other Np phases such as NpO2 (cr) in contact with Np(V) containing solutions has also been investigated [15, 16]. It has been suggested that Np partitioning to secondary U phases to form a solid solution may lower the dissolved concentration of Np below that predicted for discrete Np(V)containing solids [17]. Such a prediction is consistent with SNF dissolution experiments, which have shown Np concentrations below that expected if Np2 O5 was the solubility limiting solid. Wilson measured Np concentrations between 10−10 –10−8 M in experiments with SNF and YM groundwater [18]. In contrast, Efurd et al. measured Np concentrations of 10−5 M in solubility tests of Np2 O5 in YM groundwater [19]. This suggests that either Np2 O5 was not the solubility controlling phase or that Np concentrations were limited by slow dissolution of the SNF matrix in Wilson’s studies.

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Retention of activation products by secondary uranium phases has been the subject of much speculation and experimentation [10, 20–26]. Bond-valence calculations [20] indicate that the uranophane structure can accommodate NpO2 + by substitution for the UO2 2+ cation and incorporation of other charge balancing cations. Burns et al. acknowledge that the lower cationic charge of Np(V) may have important structural consequences due to the reduced charge contribution to the axial oxygens of the neptunyl dioxo cation compared to the uranyl cation. Unlike the uranyl axial oxygens, those of Np will require additional coordination to satisfy bond-valence requirements. Such changes in bonding could introduce strain into the structure as neptunyl ions substitute for uranyl ions, perhaps limiting the extent of Np incorporation. While several U(VI) solids have been shown experimentally to incorporate low valence cations such as cesium and strontium into their structures [22, 23], the results of such studies with Np have thus far led to ambiguous results. Buck et al. [21] first reported Np retention by U(VI) alteration products during spent fuel corrosion studies. Analysis of the solids by TEM-EELS suggested that the Np was associated with dehydrated schoepite at approximately 550 ppm concentration. However, another study characterized alteration products from SNF by XAS and set an upper limit for Np in dehydrated schoepite of 200–333 ppm [24]. Finch et al. [10] did not observe retention of Np by uranyl silicates during spent fuel corrosion experiments and attributed this to lack of a charge balancing mechanism. As the formal cationic charges of the neptunyl and uranyl cations differ by one, substitution of the U(VI) moiety by Np(V) will result in a charge deficit. A proposed coupled substitution which achieves local charge balance within the structure is [20]: NpO2 + + OH− ↔ UO2 2+ + O2− .

(1)

However, because O2− ions bridge Si4+ and UO2 2+ ions in the uranyl silicate sheet structure, this substitution may not be possible, thus preventing neptunyl incorporation. A second mechanism for the maintenance of electroneutrality in the structure following neptunyl substitution could involve an additional low-valence cation in the interlayer, such as: NpO2 + + Na+ ↔ UO2 2+ + H2 O .

More recently, Burns et al. [26] have investigated Np incorporation into four synthetically prepared uranyl phases with different interlayer environments. They found that Np was likely incorporated into uranophane and Nacompreignacite, which are solids composed of sheets with a negative structural charge; two other phases without structural charge, metaschoepite and β-(UO2 )(OH)2 , did not contain significant Np. Precipitation of a discrete Np phase was deemed unlikely due to lack of measurable Np in the UOH phases that were synthesized under similar conditions. This study builds on the conclusion of the authors by further characterizing the solid at a higher spatial resolution to obtain information with regard to partitioning of Np to individual crystallites. Here, we report on characterizations of the solid phases formed when U(VI) silicates were precipitated in the presence of small quantities of dissolved NpO2 + . In this work, charge balancing cations, e.g. Na+ , were included in the syntheses. Powder X-ray diffraction (XRD) and gamma spectrometry (GS) have been used to identify the bulk solids and their associated levels of Np, respectively. The high spatial resolution and sensitivity of transmission electron microscopy (TEM) with electron energy loss spectroscopy (EELS) capability makes it an ideal technique to investigate low levels of Np in a U matrix due to the sub-micron and multi-phase nature of these solids. Utilizing EELS, Buck and Fortner [27] were able to detect Np in simulated waste glass at 0.01 wt. % (100 ppm). In this study, resultant solid phases and supernatant liquids were analyzed, and there was no evidence that NpO2 + precipitated as a discrete solid, but rather in association with the U(VI) silicate solids. These results are discussed in the context of predicting the behavior of Np during SNF corrosion and its possible impact on dissolved Np concentrations expected in the YM geologic repository.

(2)

This equation indicates the Na+ ion may occupy the site of one of the water molecules not coordinated to Ca. Due to a lack of charge-balancing cations in the experiments conducted by Finch et al. [10], such a coupled substitution shown in Eq. (2) was not possible, resulting in little observed Np incorporation. In a separate study, U3 O8 doped with Np (Np0.33 U2.67 O8 ) was investigated by Finch et al. [16, 25] who found that the Np was partitioned into two separate phases following moist air corrosion at 150 ◦ C. Dehydrated schoepite was the predominant U(VI) compound in the oxidized solid and contained approximately 2 wt. % Np. The bulk of the Np was present as crystalline NpO2 . This may have been due to the relatively high concentration of Np in the starting material, which was present in a Np : U mole ratio of 1 : 8.

Experimental section Solid phase syntheses were based on the work of Nguyen et al. [28]. Due to availability, most syntheses used tetravalent U as UO2 powder for the source of U. The U(IV) was oxidized with hydrogen peroxide, H2 O2 , and subsequently dissolved in concentrated nitric acid before being diluted to 0.018 M UO2 2+ in ∼ 0.5 M HNO3 . In other syntheses, the source of U was a 0.010 M uranyl acetate solution, UO2 (CH3 COO)2 . In either case, a volume of the uranyl solution containing 3 × 10−4 moles U was adjusted to pH ∼ 8–9 with NaOH, causing hydrolysis and precipitation of the uranyl oxide hydrate metaschoepite, [(UO2 )8 O2 (OH)12 ]·10H2 O. Stoichiometric quantities of 0.050 M solutions of sodium metasilicate, Na2 SiO3 , and calcium acetate, Ca(C2 H3 O2 )2 , were added to the uranyl suspension. A 0.0158 M (2.633 µCi/mL) solution of Np(V) in 5% HNO3 was added in variable amounts, corresponding to Np : U mole ratios of 0.02, 0.01, and 0.005. Control samples without the addition of Np were also prepared. The pH of the suspension was then adjusted to either 8 [28], or 10, as previous experience indicated the use of higher pH yielded more uranophane [29]. This pH range was selected based on previous studies without the addition of Np, in which

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Microscale characterization of U(VI) silicate solids and associated Np(V)

a more pure uranophane phase was formed; however, such conditions are not anticipated at Yucca Mountain. The resulting suspensions were capped, left at room temperature for 16 hours, placed in a 90 ◦ C oven for the desired length of time (from 24 hours to 10 days), and agitated once daily. At the conclusion of the reaction time, samples were removed from the oven and allowed to cool to room temperature. An aliquot of the supernatant was filtered through a 0.45 µm filter, and subsequently analyzed for “dissolved” U and Np concentrations by kinetic phosphorescence analysis (KPA) (data not shown) and GS, respectively. The precipitate was separated from the remaining supernatant by centrifugation and rinsed 3 times with boiling DIW. In some cases, the washes were analyzed for U and Np concentrations such that a mass balance for these elements could be calculated. Following the final rinse/centrifugation cycle, the solids were placed in 23 mL Teflon-lined Parr bombs with 15 mL of DIW and heated to 130 ◦ C for 7 days to improve crystallinity. Finally, the supernatant was decanted and solids were dried at room temperature and then stored in vials for subsequent analysis by XRD, GS, and electron microscopy.

XRD analysis Solid samples were ground in a mortar and pestle with corundum, Al2 O3 , added as an internal standard. A collodian/amyl acetate mixture was used to adhere the powder to a glass slide. The diffractometer used was a Scintag PAD V and monochromatic Cu K α X-radiation allowed diffraction data between two-theta angles of 5–65◦ to be collected at 0.02◦ steps with a 20 second count time per step. JADE6 software from Materials Data, Inc. was used to plot the diffraction patterns, identify d-values, and fit reference patterns to identify the phase(s) present. The International Centre for Diffraction Data – Powder Diffraction File (ICDDPDF) database of reference patterns was used to identify phases present in samples. Detection of minor phases (less than 2–3 wt. %) and amorphous phases by this method is not possible, an inherent limitation of powder XRD.

GS Solid samples were weighed into glass scintillation vials and counted on a Ge(Li) detector for variable lengths of time to obtain acceptable counting statistics. The software program GENIE 2000 was used to identify 237 Np based on gammaray peak energies (86.48, 92.29, 95.87, and 108.00 keV) and calculate concentrations present in samples based on sample mass, energy calibrations and measured detector efficiencies. Based on counting times of 16 hours, detection limits were approximately 2.88 × 10−6 µCi/mL (1.3 × 10−7 M) and 2.54 × 10−3 µCi/g (10 µg/g) for 237 Np in the supernatants and solids, respectively.

SEM/EDS analysis Samples to be analyzed by SEM/EDS were mounted on stainless steel stubs and carbon-coated. Using a JEOL 840 scanning electron microscope with Robinson backscatter detector and an Oxford ISIS 300 energy-dispersive spectrometer (EDS) at 20 kV accelerating voltage, images were obtained showing the morphology of crystallites within each

sample. Qualitative EDS was performed on numerous crystallites in each sample (at 29 kV accelerating voltage) to determine if variable elemental compositions existed among the crystallites. Bremsstrahlung radiation limits the sensitivity of the technique, and EDS has an approximate detection limit of 0.1 wt. % for each element analyzed [27].

TEM/EELS analysis Samples to be analyzed by TEM/EELS were crushed and placed on carbon-coated copper grids. The presence of U and Np was confirmed by determining the energy gap (∆E-M4,5 ) between the M4 and M5 absorption edges (U ∆E-M4,5 = 176 eV; Np ∆E-M4,5 = 184 eV) recorded with a Gatan Image Filter (GIF2000) on a JEOL2010 TEM at the Environmental Molecular Sciences Laboratory at Pacific Northwest National Laboratory. The Np-M5 (3665 eV) edge can be subject to interference from the plural scattering from U-M5 (3552 eV) and the U-O4,5 edge (95–111 eV). However, the Np-M4 edge (3850 eV) is not subject to similar interference and is separated from any plural events on the U-M4 edge by 7–8 eV. We adopted an energy dispersion of 0.5 eV (∼ 100 nm diameter analysis area, integration time of 5–10 s, sum of 50 spectra) to increase the channel separation between features, including the Np-M4 peak and the peak near 3665 eV. All spectra were calibrated internally from the assumed 176 eV energy separation of the uranium M5 and M4 edges.

Results and discussion Characterization of synthesis products indicated that different solids precipitated depending on the uranyl starting materials used. The only crystalline phase observed by XRD for the samples prepared from uranyl acetate stock solutions was uranophane, whereas the 8 samples prepared from a solution of oxidized UO2 powder yielded a mixture of uranophane with a phase similar in structure to sodium boltwoodite, (Nax K1−x )(UO2 )(SiO3 OH)·1.5H2 O. While no K+ was included in the synthesis, it is likely that an isostructural Na+ end-member formed due to the large amount of sodium cations introduced with the silicate source. X-ray diffraction patterns for the solids are shown in Fig. 1, with the pattern of a synthetic potassium-free sodium boltwoodite phase [28] included for reference. This phase has the same uranyl silicate sheet structure as uranophane [30] and has been observed previously by others [31, 32]. Additionally, Li and Burns [33] have synthesized single crystals of the solid Na4 (UO2 )2 (Si4 O10 )2 (H2 O)4 , which was a potential phase in the work described here. Elemental analysis of the solids by EDS indicates they are uranyl silicates containing U, Si, Ca, and Na. No signal from Np was observed by this technique (data not shown); however, analysis of the solids by GS showed that Np was associated with the bulk U(VI) solid phase (Table 1.) The presence of sodium boltwoodite was more pronounced in the pH 8 syntheses than the pH 10 syntheses (Fig. 1.) In fact, the syntheses at pH 8 contained no detectable uranophane by XRD. Sodium boltwoodite may be another important solid phase in the YM repository given the high sodium content of the groundwater in that region [11].

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Fig. 1. Powder XRD patterns for solids synthesized in the presence of neptunium. (a) Sodium boltwoodite was the predominant phase in the solids precipitated at pH 8, as the reference pattern at the bottom from Nguyen et al. [28] shows. (b) A mixture of sodium boltwoodite and uranophane precipitated at pH 10. Initial Np concentrations where 2.0, 1.0, 0.5, and 0 mol % Np (compared to U) and are shown in order of decreasing mol % Np from top to bottom. Oxidized UO2 powder was the source of UO2 2+ in these experiments.

Table 1. Np supernatant and solid concentrations for the U(VI) silicat syntheses. Sample description

Uranyl source

Total reaction Final Initial supernatant Final supernatant % Np remaining Bulk solid phase Np time (days) pH Np concentration (M) Np concentration (M) in solution concentration (µg/g)

2.0 mol % Np/pH 8 1.0 mol % Np/pH 8 0.5 mol % Np/pH 8 0.0 mol % Np/pH 8

U(IV)/H2 O2 U(IV)/H2 O2 U(IV)/H2 O2 U(IV)/H2 O2

11 11 11 11

5.6 5.1 5.6 5.0

(2.0 ± 0.1) × 10−4 (1.0 ± 0.1) × 10−4 (5.0 ± 0.3) × 10−5 N/A

(7.8 ± 0.2) × 10−5 (5.3 ± 0.2) × 10−5 (1.7 ± 0.1) × 10−5 N/A

39 ± 2 53 ± 3 34 ± 3 N/A

2700 ± 100 840 ± 50 880 ± 50 N/A

2.0 mol % Np/pH 10 1.0 mol % Np/pH 10 0.5 mol % Np/pH 10 0.0 mol % Np/pH 10

U(IV)/H2 O2 U(IV)/H2 O2 U(IV)/H2 O2 U(IV)/H2 O2

11 11 11 11

9.7 10.0 9.9 9.7

(2.0 ± 0.1) × 10−4 (1.0 ± 0.1) × 10−4 (5.0 ± 0.3) × 10−3 N/A

(1.4 ± 0.2) × 10−7 < 1.3 × 10−7 < 1.3 × 10−7 N/A

0.07 ± 0.01 < 0.13 < 0.26 N/A

6300 ± 200 3000 ± 100 1700 ± 100 N/A

NM a (1.4 ± 0.1) × 10−4 NM N/A

(1.7 ± 0.2) × 10−7 N/A

0.12 ± 0.01 N/A

780 ± 50 N/A

(2.0 ± 0.1) × 10−4 (1.0 ± 0.1) × 10−4 (5.0 ± 0.3) × 10−5 N/A

(2.6 ± 0.5) × 10−7 (2.6 ± 0.5) × 10−7 (1.3 ± 0.2) × 10−7 N/A

0.13 ± 0.02 0.26 ± 0.05 0.26 ± 0.04 N/A

15 800 ± 700 10 200 ± 400 4700 ± 200 N/A

2.0 mol % Np/pH 10 U(VI)-acetate 0.0 mol % Np/pH 10 U(VI)-acetate

2 2

2.0 mol % Np/pH 10 1.0 mol % Np/pH 10 0.5 mol % Np/pH 10 0.0 mol % Np/pH 10

7 7 7 7

U(VI)-acetate U(VI)-acetate U(VI)-acetate U(VI)-acetate

9.8 9.3 9.2 9.4

a: Not measured

Significant substitution of the uranyl entity by the neptunyl dioxo cation could result in a shift of the structural lattice parameters due to local structural modifications result-

ing from charge differences. For example, Burns and Li [34] observed a change in the structural space group and lattice parameters when Sr2+ ions were substituted for Ca2+ ions by

Microscale characterization of U(VI) silicate solids and associated Np(V)

269

ion-exchange in becquerelite, Ca[(UO2 )6 O4 (OH)6 ](H2 O)8 , due to a different coordination environment of the Sr2+ in the interlayer. For this study, uranophane lattice parameters were calculated for the phases in Fig. 1b using the MDI JADE6 software package. Although small differences were seen from sample to sample, no systematic trend was observed from lowest to highest Np loading and the differences are not statistically significant (data not shown). This is not surprising given the small quantities of NpO2 + used in this study. The syntheses at the two initial pHs differed significantly in the magnitude of pH shift during the reaction. Formation of uranophane leads to the release of protons as shown in the following balanced reaction: Ca2+ + 2UO2 2+ + 2H4 SiO4 + 11H2 O ↔ Ca[(UO2 )(SiO3 OH)]2 ·5H2 O + 6H3 O+

(3)

As a result, syntheses with an initial pH of 8 ranged from 5.0–5.6 at the conclusion of the reaction period. Under these conditions, significant quantities of Np remained in solution, although GS indicated some Np was associated with the solid, as shown in Table 1. For the system with an initial pH of 10, the pH did not deviate significantly during the course of the reaction. At this higher pH, approximately 70% of the silicic acid is in the form H3 SiO4 − . This may account for the release of fewer protons into solution. Virtually none of the initial Np remained in solution for the solids synthesized under these conditions. The Np content of the bulk solids was higher than that for the solids precipitated at the lower pH and decreased systematically in proportion to the initial Np concentration. As indicated in Table 1, initial Np(V) concentrations used in the syntheses ranged from 5 × 10−5 to 2 × 10−4 M. This concentration of Np is expected to hydrolyze at pH 10 and precipitate (at least initially) as NpO2 OH·xH2 O (am) [35]. As the log K sp◦ of fresh and aged precipitates of this phase are −8.7 ± 0.2 and −9.3 ± 0.5, respectively [36], the dissolved Np concentration in equilibrium with the oxyhydroxide solid can be estimated. For the higher pH syntheses with a pH between 9.7–10.0 at the conclusion of the reaction, a Np concentration between 5 × 10−6 –4 × 10−5 M is expected at equilibrium using the K sp values for the crystalline and amorphous Np phases, respectively. Table 1 shows that the equilibrium dissolved Np concentration after 10 days is lower than that predicted if NpO2 OH·xH2 O (am) is the Np-controlling solid (< 1.3 × 10−7 M). At a final pH ranging from 5.0–5.6, as was the case for experiments in which the initial pH was 8, the Np concentration is insufficient to precipitate NpO2 OH·xH2 O(am) as a discrete phase. Measured concentrations of Np after 10 days, as shown in Table 1, indicate dissolved Np concentrations ranged from 1.7–7.8 × 10−5 M, below that of the initial concentration of Np. Solids were observed by SEM and representative morphologies are shown in Fig. 2. Crystallites grown in the presence of Np (at left) are smaller than those grown in its absence (at right). If the NpO2 + was incorporated into the U(VI) silicate solids, its concentration in those phases would be too low to detect by EDS. On the other hand, if the Np were to precipitate as a discrete solid,

Fig. 2. Scanning electron micrographs showing the morphology of the mixture of uranophane and sodium boltwoodite synthesized at pH 10. The two images at left (a and b) are two locations of the solid synthesized with 2.0 mol % Np while those at right (c and d) are two locations of the sample prepared in the absence of Np. All images are shown at the same magnification.

one would expect to detect Np by EDS in those solid aggregates. For this reason, many particles and morphologically distinct areas were sampled by EDS during SEM analysis. For the highest level Np-containing solid, 42 individual EDS spectra were obtained for different areas of the sample. None of them resulted in an observable signal from Np, while Ca, U, and Si were seen for each particle sampled. X-ray diffraction patterns of a Np-containing uranophane at pH 10 using uranyl acetate as the initial source of UO2 2+ are shown in Fig. 3. This method of synthesis yielded a crystalline solid of primarily uranophane with little or no evidence for sodium boltwoodite. A second phase was observed in the solid formed in the absence of Np, and was matched to ICDD-PDF #50-0039, calcium uranyl oxide hydrate. At the conclusion of this experiment, less than 1% of the initial Np remained in solution (Table 1); however, GS of the bulk solid indicated that it contained only 780 ppm Np. It is likely that some Np was lost during the washing stages of the procedure, as the wash solutions were yellow in color indicating dissolution of the U(VI) solid (and most likely, any associated Np). This may have been the result of reduced crystallinity due to the shorter reaction time used to minimize the formation of sodium boltwoodite. The morphologies of these phases were observed by SEM, as shown in Fig. 3b–c. Neptunium could not be detected by EDS analyses at any locations in the solid synthesized in its presence. The solid formed in the absence of Np shows the Ca-UOH phase, which can be observed as large pseudo-hexagonal platelets (Fig. 3c). Reasons for its presence in this solid and not that synthesized with Np are uncertain; however, mixtures of products are not uncommon in uranyl phase syntheses.

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Fig. 3. (a) XRD patterns for uranophane synthesized in the presence of 2.0 mol % Np (top) and absence of Np (bottom) at pH 10. The reference pattern for uranophane is shown underneath the series. Uranyl acetate was the source of UO2 2+ in these samples. (b) Scanning electron micrograph of the morphology of the uranophane phase prepared with 2.0 mol % Np initially present in solution. (c) Scanning electron micrograph showing the morphology of the uranophane phase prepared in the absence of Np (the large platelets were identifie by XRD as a Ca-U(VI)-oxide hydrate which precipitated with uranophane in this sample). Images are shown at the same magnification.

Synthesis of solids using uranyl acetate was repeated using Np : U mole ratios of 0.02, 0.01, and 0.005 and with an initial pH of 10. To increase the efficiency of the process, the entire synthesis was done in Parr bombs and wash solutions were analyzed for Np concentrations so that a mass balance for the element could be obtained. Analysis of these solids indicated that between 78%–97% of the initial Np was associated with the bulk solid (Table 1) and only a small fraction of Np was removed through washing. On a mass basis, the solids contained between 4700–15 800 µg/g Np. This far higher concentration range of Np may be an effect of longer reaction time, the higher temperature and pressure environment of the Parr bomb, or a combination of these factors. The XRD pattern demonstrated the only crystalline phase present was uranophane (data not shown), although variable peak intensities and widths suggested a wide range of crystallinity existed among the solids. A mass balance calculation for total Np demonstrated that nearly all Np was associated with the solids, after determining Np in the supernatants, washes, and isolated solids (Fig. 4). In an effort to investigate the microscale distribution of Np in the solids, several samples were analyzed by TEMEELS. The EELS spectra recorded for two samples with differing bulk Np concentrations are shown in Fig. 5. One of the samples had a bulk Np concentration measured at 6300 µg/g Np while the second sample had 3000 µg/g Np. While the U peaks have similar intensities for the two samples, the Np signal was lower for the sample with smaller bulk Np concentration. For all the crystallites analyzed, a Np signal was never observed without a U signal – that is, no

Fig. 4. Percentages of total Np measured at the conclusion of the reaction in the supernatant, the three RT rinses of the precipitate, and the isolated solid, for the uranophane solids synthesized from uranyl acetate in Parr bombs.

discrete Np-containing particles were observed. In addition, we compared the intensity ratios of the two ‘white-line’ features in the spectrum to confirm the presence of Np. If all features in the spectrum are entirely due to plural scattering from U, then the intensity ratio of the plural peaks should be the same as the intensity ratio of the uranium M4 /M5 peaks (0.40–0.45). The average intensity ratio of the Np-M4,5 obtained on the U(VI) silicate solids was 0.19 (n = 7) with σ = 0.04, indicating that they cannot be plural peaks of U. This is consistent with Np being associated with the U(VI) silicate solids, although it does not necessarily demonstrate that incorporation (as opposed to sorption) is the mechanism of association. Due to the electron beam intensity required

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Microscale characterization of U(VI) silicate solids and associated Np(V)

was shown to be associated with uranyl silicate solids by monitoring its disappearance from solution and by direct measurements of the bulk solids. Precipitation of discrete Np solids such as NpO2 has been suggested as a possible fate for Np in SNF. However, no evidence for discrete Np solids, either by SEM/EDS or TEM/EELS, was observed. In the case of SNF corrosion, Np association with the secondary U solids that form will likely limit dissolved Np concentrations such that precipitation of pure Np solids is unlikely. Therefore, the secondary uranyl silicate phases uranophane and sodium boltwoodite may be important determinants of dissolved concentrations of Np in a geologic repository. Further studies are underway to assess the solubility of the solid phases and the mechanism(s) of association of Np with the solids. Fig. 5. Electron energy-loss spectra (shown in second-difference mode) for single crystallites of two samples with different bulk concentrations of Np. Data shown for the solids synthesized with 2.0 mol % Np (6300 µg/g Np measured in the bulk) and 1 mol % Np (3000 µg/g Np measured in the bulk) synthesized at pH 10. The two spectra have been overlaid and the peaks due to U superimpose identically. The energy difference between the U-M4 and U-M5 peaks is 176 eV, whereas the energy difference between the peaks labeled as Np-M4 and Np-M5 is 184 eV. Further evidence for the assignment of the Np-M4 and Np-M5 peaks is discussed in the text.

to obtain the EELS spectra, the irradiation amorphized the U(VI) silicate rapidly and may have altered the oxidation state of Np during analysis. Based on detection of Np only in association with U-containing aggregates by EELS and lack of a Np signal by SEM/EDS of numerous solid aggregates, it does not appear likely that Np precipitated as a discrete solid. In the work of Burns et al. [26], uranophane and Nacompreignacite were found to contain levels of Np in proportion to the concentration level of the solutions from which they were formed. Metaschoepite and β-(UO2 )(OH)2 contained no significant levels of Np. Solid phases synthesized were characterized by XRD as well as ICP-AES and ICP-MS, which demonstrated Np was associated with the bulk phases. However, it is also possible that a minimal amount of a crystalline Np phase or amorphous Npcontaining phase precipitated that could not be observed by XRD. The present study shows evidence for the association of Np with uranophane crystallites present in the bulk solid with no indication of a discrete Np solid, and thus is consistent with the work of Burns et al. for the possible incorporation of Np into uranophane. In addition, this work demonstrates the importance of microscale characterization for reflecting the heterogeneity of the synthesized solids, discounting the possibility of discrete Np solids, and indicating an association between U and Np in these phases. In summary, it has been shown that U(VI) silicates synthesized in the presence of low concentrations of Np still retain the structural characteristics of the U(VI) solids. The X-ray diffractograms did not reveal a significant structural modification in the Np-associated solids. Morphologies of the solids resulting from preparation with Np were altered, however; crystallites appear somewhat smaller in size than their counterparts synthesized with Np absent. Np

Acknowledgment. Primary funding for this work was provided by the U.S. Department of Energy’s Office of Science Basic Energy Sciences program under contract DE-FG02-01ER15138. Support from the NSF Integrative Graduate Education and Research Training (IGERT) program in the Center for Multiphase Environmental Research at Washington State University under grant DGE-9972817 and from the Yucca Mountain Project supported by DOE, Office of Civilian Radioactive Waste Management is acknowledged. The TEM work was performed at the EMSL, a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory, operated for DOE by Battelle.

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