Structural and Morphological Influences on Neptunium Incorporation

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Structural and Morphological Influence on Neptunium Incorporation in Uranyl Molybdates Nathan A. Meredith, Ginger E. Sigmon, Antonio Simonetti, and Peter C Burns Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 22 Sep 2015 Downloaded from http://pubs.acs.org on October 2, 2015

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Crystal Growth & Design

Structural and Morphological Influences on Neptunium Incorporation in Uranyl Molybdates Nathan A. Meredith,a,*,† Ginger E. Sigmon,b Antonio Simonetti,b and Peter C. Burns a, b a

Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States

b

Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, Indiana 46556, United States

Abstract. The in situ incorporation of pentavalent neptunium has been studied in the structurally related uranyl molybdate frameworks (NH4)4[(UO2)5(MoO4)7](H2O)5 and (NH4)2[(UO2)6(MoO4)7](H2O)2 prepared under similar synthetic conditions. The presence of Np(V) was confirmed by UV-vis-NIR spectroscopy in the first compound whereas Np(VI) was identified in the second based on the observation of a unit cell contraction and the lack of a spectral signature for Np(V). The incorporation of neptunium does not affect the overall structure of the host compound based on the crystallographic unit cell parameters. Neptunium appears to preferentially incorporate in the structure of (NH4)2[(UO2)6(MoO4)7](H2O)2 due to the formation of Np(VI) during synthesis, although higher total uptakes were observed in (NH4)4[(UO2)5(MoO4)7](H2O)5 due to a higher initial concentration of neptunium in solution despite maintaining the same ratio of U:Np.

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Introduction One of the preeminent concerns regarding the use of nuclear energy is the long-term storage and disposal of the waste produced by reactors. The most widely accepted, but unimplemented, solution worldwide is the permanent disposal of irradiated nuclear fuel in a geological repository that utilizes both natural and engineered barriers to prevent the transport of radionuclides from the storage site in the event that the waste container is compromised.1-3 One potential barrier to radionuclide migration is the precipitation of secondary uranium(VI) (uranyl) phases during the oxidative corrosion of the irradiated UO2 fuel matrix. This is expected to occur in a repository where the waste is exposed to a moist oxidizing environment, such as in the Yucca Mountain site, which was previously under consideration in the United States.4-10

Uranyl alteration

compounds are likely to form a corrosion rind on the surface of the waste that impedes further oxidation, and these phases may incorporate various radionuclides into their structures through various substitution mechanisms, thereby reducing the potential transport of those radionuclides away from the near field of the repository.10-12 Neptunium-237 is of considerable concern for repository performance as it is a major contributor to the long-term radioactivity of the waste due to its long half-life (t1/2 = 2.14 × 106 years) and high ingestion radiotoxicity. The initial concentration of neptunium in irradiated nuclear fuel is approximately 0.5 kg per metric ton, and it increases for hundreds of years due to the decay of americium-241 (t1/2 = 430 years).10 Under oxidizing conditions, neptunium will form the NpO2+ (neptunyl) cation, which is soluble in water and is minimally susceptible to either hydrolysis or complexation over a wide range of conditions.13-16 Many studies have, therefore, investigated the incorporation of Np(V) into various uranyl compounds to understand the potential impact on Np(V) release, and have demonstrated that Np(V) may substitute for


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U(VI) based on crystallographic and chemical arguments, experiments, and computational simulations.17-29 There are three major factors that may limit the incorporation of Np(V) into U(VI) compounds. First, the difference in charge between Np(V) and U(VI) will require additional substitutions or changes within the structure of the host compounds.12 Second, the differences in bond lengths – Np(V)-Oeq bonds are approximately 0.1 Å longer than the corresponding U(VI)-Oeq bonds – will potentially introduce strain within the structure. Third, the bonding requirements of the oxygen atoms in NpO2+ are less fully satisfied than those in UO22+, and these atoms frequently bridge to other metal centers in various structures whereas the oxygen atoms in UO22+ are more fully satisfied and rarely bridge to other uranyl ions.12, 30-31 The role of charge-balancing mechanisms for Np(V) substitution for U(VI) has been wellinvestigated experimentally and computationally;17,

20, 23-24, 26-29

however, few studies have

investigated the structural limitations on Np(V) incorporation into U(VI) compounds. Most notably, Klingensmith and Burns showed that the uptake of Np(V) in soddyite, (UO2)2(SiO4)(H2O)2, steadily increased with temperature, because elevated temperatures alleviated the strain associated with the substitution by allowing for more thermal motion of the atoms during crystallization.24

The goal of the present work is to further elucidate the

relationship between the structure of the U(VI) compound and its tendency toward Np(V) incorporation. Uranyl molybdates were selected for study here because molybdenum is one of the major products present in irradiated nuclear fuel (approximately 3,500 ppm following 30 MWd/kg U burnup) and is expected to form metallic precipitates (ε-particles) or oxide precipitates within the fuel matrix.10 Moreover the specific isotopes that form – 95Mo, 96Mo, 97Mo, 98Mo, and

100

Mo –


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are stable end products resulting from the β-decay of the corresponding niobium isotopes; thus, they will persist indefinitely in a repository.32-33 In laboratory experiments focused on the alteration of an approved testing material, a uranyl oxide hydrate containing molybdenum was observed suggesting the potential formation of uranyl molybdates in a repository.33 Molybdenum is sparingly soluble in the borosilicate glasses used for the vitrification of high level waste in the United States and elsewhere.

Instead, the molybdenum, which forms

hexavalent MoO42- upon vitrification, reacts with alkali metals forming soluble compounds of the form A2MoO4. These in turn destabilize the glass and increase the rate of corrosion and leaching of other radionuclides. Ceramic-glasses are an alternative waste form in which a source of calcium is added to the borosilicate vitrification process to precipitate powellite – CaMoO4 – and thereby increase the molybdenum fixing capacity of the waste form.34-39 Uranyl molybdate phases have a rich structural diversity that includes structural units consisting of finite clusters, infinite chains, infinite sheets, and frameworks.30 The relative sizes of uranyl polyhedra and molybdate tetrahedra result in very flexible U-O-Mo linkages that range from 120-180°.40 This flexibility may allow for higher Np(V) uptake during incorporation than in another, more rigid system. The two uranyl molybdate frameworks selected for investigation in this work are (NH4)4[(UO2)5(MoO4)7](H2O)5 (UMO1)40 and (NH4)2[(UO2)6(MoO4)7](H2O)2 (UMO2)41 and their structures are shown in Figure 1. Depending on the synthetic conditions, UMO1 may crystallize in either of two different morphologies – truncated bipyramidal (UMO1A) or bipyramidal (UMO1B) as seen in Figures 2a and 2b, respectively. UMO2 typically forms bladed crystals (Figure 2c).


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Figure 1. The structures of (a) UMO1 viewed down [100], (b) UMO1 viewed down [001], (c) UMO2 viewed down [100], and (d) UMO2 viewed down [001]. NH4+ cations and water molecules are omitted for clarity (yellow pentagonal bipyramids = uranium, purple tetrahedra = molybdenum, red spheres = oxygen).


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Figure 2. Optical images of (a) truncated bipyramidal crystal of UMO1A, (b) bipyramidal crystals of UMO1B, and (c) bladed crystals of UMO2. Acquired using the imaging software on a Craic Technologies microspectrophotometer. Detailed structural descriptions of UMO1 and UMO2 are available elsewhere.40-41 In brief, both structures contain UO7 pentagonal bipyramids and MoO4 tetrahedra, and the extended structures are formed exclusively through vertex-sharing between UO7 and MoO4 polyhedra. In both compounds the charge-balance needed for the substitution of Np(V) for U(VI) may be accomplished by the inclusion of additional NH4+ cations within the structural channels. The primary difference between these two compounds is the stoichiometry of the fundamental chains that make up the two frameworks – [(UO2)5(MoO4)7]4- versus [(UO2)6(MoO4)7]2-.

This

difference accounts for the topological variation that gives rise to the two structures.40-41 Higher Np(V) uptake may occur in UMO1 because the less-dense structure may be more flexible and better able to accommodate the substitution, and the large channels may permit interstitial neptunium incorporation as well. We herein report our observations regarding the impact of the crystal structure and morphology on neptunium incorporation in these two compounds. Experimental Procedures Caution!

While the UO2(CH3COO)2·2H2O used for these reactions contains depleted

uranium, precautions for handling radioactive materials were followed.

237

Np represents a

serious health risk owing to its α and γ emission, and especially because of its decay to the shortlived isotope

233

Pa (t1/2 = 27.0 days), which is a potent β and γ emitter. This work utilized a

previously prepared stock solution that was stored in a glovebox until needed at which point transfers to the reaction vessel were carried out in an isolated fume hood. All studies were


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conducted at the University of Notre Dame, which has appropriate facilities and personnel for handling radioactive materials. In addition to standard personal protective equipment (lab coats, gloves, and eye protection), whole body and extremity radiation monitoring were employed while manipulating all material used in this work. Syntheses. (NH4)6Mo7O244H2O (Fisher Scientific Company) was used as received without further purification.

UO2(CH3COO)22H2O (MV Laboratories, Inc) was recrystallized with

glacial acetic acid before use in the reactions. The source of neptunium was a 0.177 M stock solution of Np(V) prepared in 1 M HCl from reclaimed reaction products by the method of Sullivan and Skanthakumar.42 The presence of Np(V) was confirmed from the UV-vis spectrum of the stock solution. All reactions were conducted under hydrothermal conditions in Teflonlined autoclaves. For each neptunium incorporation reaction, the quantity of Np(V) was 10% of the amount of U(VI) in solution. Control reactions excluding neptunium were also conducted for all three samples. UMO1A and UMO1B were both prepared from an approximately 2:1 molar ratio of UO2(CH3COO)22H2O to (NH4)6Mo7O244H2O and each neptunium bearing reaction was conducted in duplicate. The amount of UO2(CH3COO)22H2O and (NH4)6Mo7O244H2O used in the synthesis of UMO1B was 2/5 of the quantity used for UMO1A. After measuring the solids for each reaction into individual 7 mL Teflon liners with screw-threaded lids, 2 mL/2.03 mL of ultrapure water and 50 µL/20 µL of the Np(V) solution were added to the reactions for UMO1A and UMO1B, respectively. The liners were then sealed and distributed into two 125 mL Teflon liners for secondary containment. The outer liners were filled with 30 mL of water to provide counter-pressure during heating and were subsequently sealed inside Parr 4748 steel autoclaves that were placed in a convection oven at 180°C for four days before being cooled to room


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temperature at the oven’s natural cooling rate (~2 hours). The control reactions were conducted separately according to the same procedure. UMO2 was synthesized from an approximately 3:1 molar ratio of UO2(CH3COO)22H2O to (NH4)6Mo7O244H2O. Only one neptunium incorporation reaction was prepared for UMO2 due to the limited amount of Np-237 available. After measuring the solids into a 7 mL Teflon liner with a screw-threaded lid, 2.03 mL of ultrapure water and 20 uL of the Np(V) solution were added to the reaction and the liner was sealed. The liner and the corresponding control reaction were placed in a 125 mL Teflon liner filled with 30 mL of water and sealed inside a Parr 4748 steel autoclave. The vessel was placed in a convection oven at 180°C for five days before being cooled to room temperature at the oven’s natural cooling rate (~2 hours). The amounts of the reagents measured in each of the five neptunium incorporation reactions as well as the control experiments are summarized in Table 1. Table 1. Experimental Details for the Np(V)-Spiked Syntheses of UMO1 and UMO2 UO2(CH3COO)22H2O

(NH4)6Mo7O244H2O

H2O

(g, mmol)

(g, mmol)

(mL)

0.177 Np(V)

M pH*

(µL, mmol) UMO1A

0.0392, 0.0925

0.0581, 0.0470

2.05

-

4.28

UMO1A+Np1

0.0396, 0.0934

0.0587, 0.0475

2

50, 0.00885

-

UMO1A+Np2

0.0391, 0.0922

0.0582, 0.0471

2

50, 0.00885

-

UMO1B

0.0152, 0.0358

0.0228, 0.0184

2.03

-

4.23

UMO1B+Np1

0.0152, 0.0358

0.0231, 0.0187

2.03

20, 0.00354

-

UMO1B+Np2

0.0158, 0.0373

0.0233, 0.0189

2.03

20, 0.00354

-

UMO2

0.0151, 0.0356

0.0153, 0.0124

2.05

-

3.95

UMO2+Np

0.0158, 0.0373

0.0159, 0.0129

2.03

20, 0.00354

-


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*pH values were measured using a separate solution prepared in 4 mL of water so that the probe could be fully immersed. At the conclusion of the reactions, the supernatant was drawn off using a pipette and discarded. The solid was then rinsed twice with ultrapure water and then transferred by pipette to Petri dishes. The water was again drawn off and the products were rinsed with ethanol to disperse them throughout the dish. The ethanol was then allowed to evaporate prior to manipulating the crystals for further analysis. Crystallographic Studies. One crystal from each of the neptunium containing compounds was fixed to the end of a glass fiber using epoxy. The fiber was then mounted and optically aligned on a Bruker APEXII Quazar X-ray diffractometer. The data collection was performed using an IµS X-ray source containing a 30 W microfocused sealed tube (Mo Kα, λ = 0.7103 Å) with high-brilliance and high-performance focusing Quazar multilayer optics. The Bruker APEX II software was used for the initial unit cell determination and control of the data collection. The intensities of the reflections of a hemisphere of data were collected by a combination of four sets of exposures, with each set having a different φ angle for the crystal. Each exposure covered a range of 0.5° in ω, resulting in the collection of 1464 frames per sample. The Bruker SAINT software was used for data integration, including Lorentz and polarization corrections. Semi-empirical absorption corrections were applied using the program SCALE (SADABS).43 The unit cells for each sample were then refined based on the full data sets. The initial structure models atomic coordinates from previously reported structures and were subsequently refined by least squares with SHELXL.44


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Optical Spectroscopy. Solid-state ultraviolet-visible-near infrared (UV-vis-NIR) spectra were obtained for single crystals harvested from each reaction, including the control (Np free) reactions. The crystals were placed on quartz slides under Infineum oil. Spectra were collected using a Craic Technologies microspectrophotometer from 200 to 1400 nm with the exposure time automatically optimized by the Craic software. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). Several crystals of varying sizes from each sample including the control reactions were placed on a 1inch round glass slide with double-sided tape for LA-ICP-MS analysis. The measurements were conducted using a ThermoFinnigan high-resolution magnetic sector Element2 ICP-MS coupled to a New Wave Research UP213 Nd:YAG laser ablation system.

Ablated particles were

transported from the ablation cell to the ICP-MS using a He carrier gas with a flow rate of 1 L/min. Ion signals were measured for the following isotopes: spectral overlap between

238

U and

237

95

Mo,

235

U,

238

U, and

237

Np. To avoid

Np the analyses were acquired in medium resolution mode

(resolution = mass/peak width ~ 4000). A typical analysis consisted of a 60 second measurement of background ion signals followed by a 60 second measurement during laser ablation. Between analyses, a minimum 10-20 second wait time was allotted to minimize memory affects from previous ablations. Each analysis represents 90 scans (90 runs × 1 pass) with a sampling (dwell) time of 0.01 seconds and 20 samples per mass peak. The measured intensities for each mass were determined by a peak top integration. Laser ablation analyses were conducted using spot sizes between 12 and 15 µm, a laser firing rate of 1-2Hz, and 100% power output. These parameters correspond to an energy density of 8.5-9 J/cm2. Four crystals were analyzed from each sample with four ablation measurements


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conducted per crystal necessitating the use of relatively large crystals (0.2-0.6 mm in size). Measurements were conducted by stationary spot mode analysis, which resulted in the production of a single hole into the sample (~40 to 50 microns in depth). For each individual laser ablation analysis, the corresponding time-resolved spectra (time vs. ion signal intensities in counts per second – cps) were closely monitored for quality control. In the optimal case, timeresolved spectra show a flat plateau region (e.g. Figure 4b) or slightly negative slope (Figures 4a and 4c); the latter is most probably related to a decrease in particle extraction from a deepening hole with progressive ablation (time). Crystal fracturing during the laser ablation interval was sometimes an issue, in particular for crystals of UMO2+Np as these were thin plates and, therefore, more susceptible to fracturing. The average signal for these samples was determined up to the point of fracture. Conversely, for some crystals, notably those from the syntheses of UMO1B+Np, some Np-rich powder was present on the surface yielding initially higher signals for

237

Np. In these cases, the calculated

average ion signal was determined from the data collected during the subsequent period of the laser ablation interval. Finally, if less than 12 consecutive data points (~15.5 seconds) were available in a given ablation, the results of that measurement were discarded. The lack of a suitable, matrix-matched external standard prohibits the quantification of the 237Np concentration in the samples. Therefore, a semi-quantitative determination of the percent substitution was derived from the ratio of the

237

Np signal over the total actinide signal (235U +

238

U+

237

Np).

Further discussion and examples of the data manipulation employed in this analysis are provided in the Supporting Information. Results


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Syntheses. All of the synthesis products consisted of clear solutions over solid particles. For the incorporation reactions and the control reaction for UMO1B, the crystals were also covered with a pale orange crust that was easily removed. The crystals obtained from the control reactions for UMO1A and UMO2, on the other hand, were clean and no other solid material was present in the reactions. This difference likely arises from the cooling rate of the ovens. The control reactions for UMO1A and UMO2 were conducted in an oven that was adjacent to several other ovens. The heat from the other ovens likely contributed to a slightly slower cooling rate for these reactions; in fact, upon cooling the oven only reached 33°C due to the adjacent ovens. By comparison, the incorporation reactions and the control reaction for UMO1B were conducted in an adjacent lab in which the oven was isolated from other heat sources. As such, it cooled more quickly and was able to reach a temperature of 25°C. Moreover, these reactions stood for several days before they were opened, which may have contributed to precipitation of excess reagents from solution. In any case, the target crystals were readily recovered from each batch of products. The control reaction for UMO1A yielded dark yellow truncated bipyramidal crystals that further darkened to orange after the ethanol used to wash them evaporated. The crystals from the corresponding Np-spiked reaction were initially orange and did not change color over time. Both the control and incorporation reactions for UMO1B yielded orange bipyramidal crystals. One of the Np-spiked reactions also contained translucent yellow blades of UMO2. In the following presentation of results and subsequent discussion, these crystals will be denoted UMO2+Np*. Finally, the control and incorporation reactions for UMO2 yielded translucent yellow blades that frequently formed clusters.


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Crystallographic Studies. The unit cells for UMO1A+Np, UMO1B+Np, and UMO2+Np are provided in Table 2, along with the published unit cells for UMO1 and UMO2. As with most previous studies, the incorporation of neptunium does not result in any significant changes to the unit cell parameters of the host compound. Table 2. Unit Cell Parameters for Np-Doped UMO1 and UMO2 UMO140

UMO1A+Np

UMO1B+Np

UMO241

UMO2+Np

Space Group

P61

P61

P61

Pbcm

Pbcm

a (Å)

11.4067(5)

11.591(9)

11.550(8)

13.970(1)

13.9204(18)

b (Å)

11.4067(5)

11.591(9)

11.550(8)

10.747(1)

10.7136(14)

c (Å)

70.659(5)

69.88(5)

71.52(5)

25.607(2)

25.503(3)

V (Å3)

7961.9(7)

8131(11)

8263(10)

3844.4(6)

3803.4(9)

T (K)

293

293

293

Not reported

293

2.123(8)

3.781(8)

-

-1.066(5)

Percent Difference in V (%)

Optical Spectroscopy. Selected UV-vis-NIR spectra for crystals from each reaction are provided in Figure 3. The spectra of the control and Np-doped samples for UMO1B and UMO2 are indistinguishable likely owing to the low uptake of neptunium.

The spectrum for

UMO1A+Np contains an additional peak centered at approximately 1008 nm that is absent in the spectrum of the control sample, which corresponds to the characteristic f-f transition of Np(V).45 Typically, this peak is observed at ∼980 nm in perchlorate or nitric acid solution, but peak shifts are known. One cause is the presence of cation-cation interactions (CCIs) that form when the axial –yl oxygen atom of one NpO2+ moiety forms an equatorial bond to a second NpO2+ center.31 However, CCIs are rarely observed in U(VI) compounds and do not exist in the


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structures of the uranyl molybdates investigated in this work.40, 41 As such, it seems unlikely that CCIs are present in the current case, as this would require major structural rearrangements associated with Np(V) incorporation. A more likely explanation for the 28 nm peak shift is strong ligand complexation between Np(V) and Mo(VI) through a shared oxygen atom. Similarly large shifts have been reported for Np(V) compounds of pertechnetate, chromate, and vanadate as well as molybdate all in the absence of CCIs.46-48


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Figure 3.

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Solid state UV-vis-NIR spectra of (a) UMO1A+Np, (b) UMO1B+Np, and (c)

UMO2+Np compared to the control samples. The insets in each spectrum magnify the region where neptunium f-f transitions are observed.


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LA-ICP-MS Results. Representative time-resolved spectra (time vs. ion signal intensities in counts per second – cps) for LA-ICP-MS analyses are shown in Figure 4, and the results for neptunium substitution are summarized in Table 3. In each example, the 237Np signal presents a flat profile as a function of time and is consistent relative to the other analytes. As time corresponds to depth of penetration in stationary spot mode, these features indicate that neptunium is uniformly incorporated throughout the crystal and not merely adsorbed to the surface.


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Figure 4. Typical time-resolved plots obtained during LA-ICP-MS analysis showing the relative ion signal intensities for the elements investigated in this work:

(a) UMO1A+Np1, (b)

UMO1B+Np1, and (c) UMO2+Np. (Blue ♦ = 95Mo; Red ■ = 235U; Green ▲ = 238U; Purple × = 237

Np)

Table 3. Percent Substitution of Neptunium for Uranium in UMO1A, UMO1B, and UMO2


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%Np (median) UMO1A+Np1

0.149±0.018

UMO1A+Np2

0.216±0.019

UMO1B+Np1

0.051±0.004

UMO1B+Np2

0.079±0.003

UMO2+Np

0.109±0.012

UMO2+Np*

0.147±0.011

The extent of neptunium incorporation in the compounds studied here varied significantly in crystals taken from the same reaction, particularly for UMO1A. This presumably arises because the crystals nucleated and grew at different times during the synthesis, potentially at different temperatures and as the neptunium concentration in solution varied. The results for each individual ablation measurement are provided in the Supporting Information. Discussion The initial solution in each synthesis contained a Np(V) to U(VI) ratio of 1:10. The uranyl molybdates that formed were able to incorporate Np(V) (Table 3), but at dramatically lower levels than were present in solution. This indicates that the majority of the Np(V) remained in solution or was precipitated into phases other than the uranyl molybdates. One possibility is Np2O5, which is known to precipitate under hydrothermal conditions49 and has been previously identified in other incorporation studies.25 The concentration of Np(V) is 0.00432 mol/L in the synthesis of UMO1A and 0.00173 mol/L in the syntheses for UMO1B and UMO2. Based on the solubility curves prepared by Kaszuba and Runde,15 Np2O5 is not expected to precipitate from solutions at the pH range (3.95-4.30) and Np(V) concentrations observed for these reactions at room temperature.


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Page 20 of 31

Overall, the crystals of UMO1A+Np showed the highest uptake; however, this may simply be a factor of the increased concentration of neptunium in those reactions (~2.5 times). UV-visNIR spectra collected for UMO1A+Np confirmed the presence of Np(V) in the crystalline product, which is supported by the observed increase in unit cell volume for UMO1A+Np relative to the control sample as the ionic radius of Np(V) is larger than that of U(VI).30, 31 The low-level of incorporation in UMO1B+Np and relative increase in unit cell volume are also consistent with the incorporation of Np(V) in that sample, although the UV-vis-NIR spectrum was inconclusive owing to the lower concentration of neptunium in the crystals. Substitution of Np(V) for U(VI) requires both a charge-balancing mechanism and structural distortions to accommodate the larger neptunyl ion.

Interstitial addition of ammonium cations into the

interlayer regions of the structures is the most likely charge-balancing mechanism for this system. Crystals obtained from the synthesis of UMO2+Np present spectra and unit cells that are not consistent with the incorporation of Np(V). First, despite the greater incorporation of neptunium in crystals of UMO2+Np than in crystals of UMO1B+Np, no peak indicative of Np(V) is observed in the UV-vis-NIR spectrum. Second, there is a marked decrease in the unit cell volume of UMO2+Np versus the Np-free control. Incorporation of Np(VI) is consistent with these observations, as it has a smaller ionic radius that U(VI)30, 31 and the molar absorptivity of Np(VI) is 45 mol-1 dm3 cm-1 at 1223 nm versus 395 mol-1 dm3 cm-1 for Np(V);45 thus, the peak is not likely to be observed at the low neptunium concentrations seen in this study. Finally, the substitution of Np(VI) for U(VI) is isovalent and does not require associated charge-balance, which would allow for higher uptake.


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Np(VI) may form via either oxidation or disproportionation of Np(V). The standard (25°C, 1 atm) reduction potentials for Mo(VI) to Mo(V) and Np(VI) to Np(V) are +0.4350 and +1.159 V,45 respectively. The oxidation of Np(V) by Mo(VI), therefore, is unfavorable under standard conditions, and is unlikely under the elevated temperatures and pressures used in our syntheses. The extent of disproportionation of Np(V) to Np(IV) and Np(VI) increases with acidity, neptunium concentration, temperature, and ionic strength.49-56

In our previous work, we

observed disproportionation under strongly acidic conditions (pH 1-2) in the synthesis of pure Np(VI) compounds57 as well as in incorporation experiements.28 Under conditions more similar to those of the current study (pH 3.95-4.3), disproportionation did not prevent the recovery of pure Np(V) compounds.49,

58-60

Additionally, for the syntheses of Np(VI) compounds, the

concentration of neptunium was much higher than in the current or previous incorporation studies.57 Although the extent of Np(V) disproportionation should be minimal under the synthetic conditions used in this work, a modest amount of Np(IV) and Np(VI) will have formed, allowing Np(VI) to be incorporated in crystals of UMO2+Np. Moreover, the synthesis of UMO2+Np was conducted at a slightly lower pH than either UMO1A+Np or UMO1B+Np, suggesting that disproportionation may be more likely in the former. Np(VI) should incorporate at levels similar to the initial aqueous solution; however, the concentration in crystals of UMO2+Np was still much less, which likely indicates that disproportionation was still limited in this system. Our initial hypothesis in this work was that the more open framework of UMO1 (ρcalc = 3.294 g/cm3)40 would be able to accommodate more Np(V) than UMO2 (ρcalc = 4.858 g/cm3)41 as it should be more structurally flexible, and there is also the possibility of incorporating Np(V) into the large channels of the framework. However, due to the likely formation of Np(VI) in the


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Page 22 of 31

synthesis of UMO2, and the low levels of incorporation in both compounds, it is impossible to confirm the hypothesis.

It is also tempting to compare the crystals of UMO1B+Np1 and

UMO2+Np* which formed in the same reaction as the uptake of neptunium was ~2.9 times greater in UMO2+Np* than in UMO1B+Np1. Although this appears to show a significant preference for incorporation in the structure of UMO2, it is also possible that the two compounds crystallized at different times during the reaction, potentially at different temperatures or neptunium concentrations. Furthermore, the UV-vis-NIR data obtained for crystals from that reaction provide no oxidation state information due to the low levels of incorporation. In the introduction, we also proposed that, due to the structural flexibility of uranyl pentagonal bipyramids and molybdate tetrahedra, uranyl molybdates may be able to incorporate more neptunium than other, more rigid structures.

Table 4 summarizes the results from several

previous neptunium incorporation studies in which the compounds were prepared at a similar initial Np(V) concentration and pH to UMO1B+Np and UMO2+Np. Table 4. Selected Results from Previous Incorporation Studies Compound

Structure

Initial [Np(V)]

Temp

pH

Uptake

Ref.

(% of total actinides)

(°C)

(mol/L) Na2[(UO2)3O2(OH)3]2(H2O)7

Edge- and 0.0018 vertexsharing sheets

100

5.25.7

0.0498

17

Ca(UO2)2(SiO3OH)2(H2O)5

Edge- and 0.0015 vertexsharing sheets

100

5.4

0.0406

17

Na[(UO2)4O2(OH)5](H2O)5

Edgesharing sheets

120

4

0.0500

20

0.0021


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(UO2)2(SiO4)(H2O)2

Edge- and 0.00075 vertexsharing framework

80

4

140

0.0210

24

0.0646

(NH4)4[(UO2)5(MoO4)7](H2O)5 Vertex0.0173 sharing framework

180

4.23

0.079±0.003 This work

(NH4)2[(UO2)6(MoO4)7](H2O)2 Vertex0.0173 sharing framework

180

3.95

0.109±0.012 This work

Overall, the uranyl molybdates show modestly higher uptake of Np(V) than the uranyl compounds of previous studies; however, this may be a result of the higher temperature rather than the flexibility of the linkages between uranyl and molybdate polyhedra. By applying a linear extrapolation to the data for (UO2)2(SiO4)(H2O)2, 0.0937 % neptunium is calculated for 180°C which is comparable to the observed uptake levels of UMO1B and UMO2. In the compounds studied, it appears that the anion has little effect on Np(V) incorporation although the sample size is small since most incorporation studies have used highly variable synthetic conditions that make it difficult to draw any meaningful comparisons. Conclusion The data collected in this work shows that neptunium can be incorporated into the structures of two uranyl molybdate frameworks at low levels, and the uptake levels observed do not affect the overall structure of the host compound. The objective of this work was to determine if the structure of the host compound as well as the crystal morphology had any effect on the level of incorporation that could be achieved.

Due to the likely formation of Np(VI) via

disproportionation in the synthesis of UMO2, a direct comparison cannot be made. Moreover, although uranyl molybdates are one of the most structurally flexible systems, the incorporation


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of neptunium was comparable to other crystal systems suggesting that crystal structure plays a much less important role than charge balance with regards to the extent of neptunium uptake. ASSOCIATED CONTENT

Supporting Information. Further explanation and examples of LA-ICP-MS data manipulation and results of individual ablation measurement. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author [email protected] Present Addresses †

Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, United

States Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources U.S. Department of Energy, Subsurface Biogeochemical Research Program under grant DESC0004245 Notes The authors declare no competing financial interests.


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ACKNOWLEDGMENTS We are grateful for support provided by the U.S. Department of Energy, Subsurface Biogeochemical Research Program under grant DE-SC0004245.

We also thank three

anonymous reviewers for their generous comments and recommendations that have significantly improved this article. REFERENCES (1) Report to the Secretary of Energy; Blue Ribbon Commission on America’s Nuclear Future. 2012. (2) Disposal Subcommittee Report to the Full Commission; Blue Ribbon Commission on America’s Nuclear Future. 2012. (3) Feiveson, H.; Mian, Z.; Ramana, M. V.; von Hippel, F. B. Atom. Sci. [Online] 2011. Available at: www.thebulletin.org/webedition/features/managing-nuclear-spent-fuelpolicylessons-10-country-study (accessed Jun 18, 2014). (4) Sidorenko, G. A.; Dubinchuk, V. T.; Kopchenova, E. V. Atomic Energy 1975, 38, 137139. (5) Forsyth, R. S.; Werme, L. O. J. Nucl. Mater. 1992, 190, 3-19. (6) Wronkiewicz, D. J.; Bates, J. K.; Gerding, T. J.; Veleckis, E.; Tani, B. S. J. Nucl. Mater. 1992, 190, 107-127. (7) Finch, R. J.; Ewing, R. C. J. Nucl. Mater. 1992, 190, 133-156.


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(21) Buck, E. C.; Douglas, M.; McNamara, B. K.; Hanson, B. D. Possible Incorporation of Neptunium in Uranyl (VI) Alteration Phases, Pacific Northwest National Laboratory: Richland, WA. 2003. (22) Douglas, M.; Clark, S. B.; Friese, J. I.; Arey, B. W.; Buck, E. C.; Hanson, B. D.; Utsunomiya, S.; Ewing, R. C. Radiochim. Acta 2005, 93, 265-272. (23) Burns, P. C.; Klingensmith, A. L. Elements 2006, 2, 351-356. (24) Klingensmith, A. L.; Burns., P. C. Am. Mineral. 2007, 92, 1946−1951. (25) Alessi, D. S.; Szymanowski, J. E. S.; Forbes, T. Z.; Quicksall, A. N.; Sigmon, G. E.; Burns, P. C.; Fein, J. B. J. Nucl. Mater. 2013, 433, 233−239. (26) Wu, S.; Chen, F.; Simonetti, A.; Albrecht-Schmitt, T. E. Environ. Sci. Technol. 2010, 44, 3192-3196. (27) Meredith, N. A.; Polinski, M. J.; Lin, J.; Simonetti, A.; Albrecht-Schmitt, T. E. Inorg. Chem. 2012, 51, 10480-10482. (28) Meredith, N. A.; Polinski, M. J.; Cross, J. N.; Villa, E. M.; Simonetti, A.; AlbrechtSchmitt, T. E. Crystl. Growth Des. 2013, 13, 386-392. (29) Schuller, L. C.; Ewing, R. C.; Becker, U. J. Nucl. Mater. 2013, 434, 440-450. (30) Burns, P. C. Can. Mineral. 2005, 43, 1839-1894. (31) Forbes, T. Z.; Wallace, C.; Burns, P. C. Can. Mineral. 2008, 46, 1623-1645.


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(32) Guenther, R. J.; Blahnik, D. E.; Campbell, T. K.; Jenquin, U. P.; Mendel, J. E.; Thornhill, C. K. Characterization of Spent Fuel Approved Testing Material ATM-106, Pacific Northwest National Laboratory: Richland, WA. 1988. (33) Buck, E. C. Wronkiewicz, D. J.; Finn, P. A.; Bates, J. K. J. Nucl. Mater. 1997, 249, 7076. (34) Short, R. J.; Hand, R. J.; Hyatt, N. C.; Möbus, G. J. Nucl. Mater. 2005, 340, 179-186. (35) Pinet, O.; Dussossoy, J. L.; David, C.; Fillet, C. J. Nucl. Mater. 2008, 377, 307-312. (36) Taurines, R.; Boizot, B. J. Non-Cryst. Solids 2011, 357, 2723-2725. (37) Ha, Y.-K.; Kim, J.-G.; Park, Y. S.; Park, S. D.; Song, K. Nucl. Eng. Technol. 2011, 43, 309-316. (38) Taurines, T.; Boizot, B. J. Am. Ceram. Soc. 2012, 95, 1105-1111. (39) Tautines, T.; Neff, D.; Boizot, B. J. Am. Ceram. Soc. 2013, 96, 3001-3007. (40) Krivovichev, S. V.; Cahill, C. L.; Burns, P. C. Inorg. Chem. 2003, 42, 2459-2464. (41) Krivovichev, S. V.; Burns, P. C. Can. Mineral. 2001, 39, 207-214. (42) Forbes, T. Z. The Crystal Chemistry of Neptunium Compounds: Structural Relationships to U6+ Mineralogy. Ph. D. Dissertation, University of Notre Dame, Notre Dame, IN, 2007. (43) Sheldrick, G. M.: SADABS 2001, A Program for Absorption Correction Using SMART CCD Based on the Method of Blessing (Blessing, R. H. Acta Cryst. 1995, A51, 33).


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(55) Kihara, S.; Yoshida, Z.; Aoyagi, H.; Maeda, K.; Shirai, O.; Kitatsuji, Y.; Yoshida, Y. Pure Appl. Chem. 1999, 71, 1771-1807. (56) Steele, H.; Taylor, R. J. Inorg. Chem. 2007, 46, 6311-6318. (57) Forbes, T. Z.; Burns, P. C. Can. Mineral. 2007, 45, 471-477. (58) Forbes, T. Z.; Burns, P. C. Am. Mineral. 2006, 91, 1089-1093. (59) Forbes, T. Z.; Burns, P. C.; Soderholm, L.; Skanthakumar, S. Chem. Mater. 2006, 18, 1643-1649. (60) Forbes, T. Z.; Burns, P. C. Inorg. Chem. 2008, 47, 705-712.


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FOR TABLE OF CONTENTS USE ONLY Structural and Morphological Influences on Neptunium Incorporation in Uranyl Molybdates Nathan A. Meredith, Ginger E. Sigmon, Antonio Simonetti, and Peter C. Burns

TOC GRAPHIC

SYNOPSIS The doping of neptunium(V) into crystals of the structurally related uranyl molybdate frameworks (NH4)4[(UO2)5(MoO4)7](H2O)5 and (NH4)2[(UO2)6(MoO4)7](H2O)2 reveals that neptunium is preferentially incorporated in (NH4)2[(UO2)6(MoO4)7](H2O)2 due to the formation of Np(VI) by disproportionation.


31

Structural and Morphological Influences on Neptunium Incorporation

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