Synthetic Influences on Neptunium Incorporation in Naturally

Dec 5, 2012 - Neptunium incorporation in the torbernite/meta-torbernite [Cu(UO2)2(PO4)2·nH2O] system has been investigated under both hydrothermal an...
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Synthetic Influences on Neptunium Incorporation in Naturally Occurring Copper Uranyl Phosphates Nathan A. Meredith,‡ Matthew J. Polinski,‡ Justin N. Cross,‡ Eric M. Villa,‡ Antonio Simonetti,‡ and Thomas E. Albrecht-Schmitt*,† †

Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, United States Departments of Chemistry and Biochemistry and of Civil & Environmental Engineering & Earth Sciences, University of Notre Dame, Notre Dame, Indiana 46556, United States

‡

S Supporting Information *

ABSTRACT: Neptunium incorporation in the torbernite/meta-torbernite [Cu(UO2)2(PO4)2·nH2O] system has been investigated in this study under both hydrothermal and slow diffusion conditions (at room temperature and 90 °C) to examine the role synthetic conditions play on neptunium uptake by uranyl phases and whether hydrothermal incorporation studies feasibly model environmental incorporation. The hydrothermally prepared crystals contained approximately 19 ± 2 and 73 ± 3 times more neptunium on average than crystals grown during slow diffusion at room temperature and elevated temperature, respectively. UV−vis spectroscopy on the mother solution showed that, on average, 51.53(±0.04)% of the original neptunium(V) remains in solution following hydrothermal synthesis versus 37.24(±0.04)% following slow diffusion at room temperature and 73.62(±0.07)% at 90 °C. Additionally, solid-state UV−vis-NIR indicated that the incorporated neptunium was predominantly present in the +6 oxidation state in the hydrothermal samples despite the fact that the initial oxidation state of neptunium in solution was +5 and no oxidizing species were present in the reactions. The oxidation state of neptunium in the slow diffusion samples was not able to be determined due to the low incorporation levels. These results suggest that neptunium(VI) may play a more significant role than previously expected in geological repositories.

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during the oxidative corrosion of nuclear waste.4 Burns, et al. hypothesized that these uranyl phases may be able to host transuranium elements (neptunium and plutonium) present in nuclear waste via substitutional incorporation for uranium, thereby reducing their environmental mobility.5 To date, numerous studies have been conducted showing the successful uptake of neptunium by uranyl phases, including natural hydroxides and silicates, and various synthetic compounds. Most of these studies claim that the incorporation of neptunium is based on the substitution of NpO2+ for UO22+, although low incorporation levels have generally precluded the identification of the oxidation state of neptunium in the final solid phase. Such a substitution creates a positive charge deficit that must be accommodated by additional alterations elsewhere in the crystal structure to restore charge balance.6 Alternatively, coupling the substitution of NpO2+ with simultaneous substitutions of suitable anions or cations has been shown to enhance the uptake level of neptunium by removing the necessity of the crystalline phase to provide its own alterations.7 In their study on the corrosion of UO2 as a surrogate for spent nuclear fuel, Wronkiewicz, et al. conducted their experiments at 90 °C on the basis that the initial emplacement of waste is expected to significantly increase the temperature in

INTRODUCTION Neptunium-237 is a neutron capture product formed in nuclear reactors that is of particular concern during the long-term storage and disposal of spent nuclear fuel due to its radiotoxicity and long half-life (t1/2 = 2.14 × 106 years). There is as much as 0.5 kg of neptunium-237 present in each metric ton of spent nuclear fuel, and approximately 62 000 t of spent nuclear fuel is awaiting storage in the United States alone as of 2006. Furthermore, the concentration of neptunium-237 in nuclear waste will initially increase during storage as a result of the decay of shorter-lived radionuclides, namely, americium241 (t1/2 = 430 years).1 During the long-term storage of spent nuclear fuel, the waste container will degrade, leading to the oxidative corrosion of the uranium matrix and release of the radionuclides contained therein.1 A summary of the transportation and retardation mechanisms of radionuclides present in spent nuclear fuel has shown that mineralization and precipitation are expected to be important for some radionuclides.2 However, NpO2+, the dominant form of neptunium under oxidizing conditions, is stable in aqueous solution toward both hydrolysis and complexation; therefore, it is expected to remain highly mobile in the environment.3 Several studies involving natural uraninite (UO2+x) deposits and synthetic UO2 samples as analogues for the uranium spent fuel matrix have suggested that secondary uranyl phasessuch as hydroxides, silicates, carbonates, or phosphatesmay form © 2012 American Chemical Society

Received: October 29, 2012 Revised: December 4, 2012 Published: December 5, 2012 386

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temperature reactions and after 2−3 days for the room-temperature reactions. For the hydrothermal reactions, 1 mL of 0.2 M UO2(NO3)2·6H2O, 50 μL of 0.2 M NpO2OH, 1 mL of 0.1 M Cu(NO3)2·2.5H2O, and 2 mL of 0.1 M H3PO4 were placed in 23 mL PTFE autoclave liners. The liners were then sealed in Parr 4749 autoclaves and heated in a box furnace at 180 °C for 3 days, followed by controlled cooling to room temperature at 7.7 °C/h (∼20 h). Three identical incorporation reactions were run under each set of experimental conditions along with control reactions omitting neptunium. The experimental details for both the hydrothermal and the slow diffusion reactions are summarized in Table 1 with more specific details provided in the Supporting Information.

a repository. Consequently, liquid water will not contact the spent fuel material until the temperature decreases to the boiling point of water. Their study demonstrated that gradually dripping simulated groundwater over a timespan of 10 years led to the formation of uranyl compounds with anions that were present in the water.4 Their findings suggest that, under repository conditions, secondary uranyl phase formation and potential neptunium incorporation may be driven by slow diffusion. To the best of our knowledge, however, all previous studies involving neptunium incorporation have been conducted under hydrothermal conditions, raising the question of whether or not incorporation under hydrothermal conditions reasonably models the processes that may occur during waste storage. To attempt to address this question, we have turned to the torbernite/meta-torbernite system for our preliminary investigation. The compound Cu[(UO2)(PO4)]2·nH2O (CuUPO) is easily formed under both hydrothermal and slow diffusion conditions. Furthermore, studies on contaminated sediments from the U.S. Department of Energy Hanford site in Washington State have identified the presence of uranyl minerals, including meta-torbernite, indicating that it may be an important secondary uranyl phase when phosphate is present in high concentrations in groundwater.8

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Table 1. Experimental Details for Np Incorporation in CuUPO (mol/L, mmol) synthesis

slow diffusiona

hydrothermal

UO2(NO3)2·6H2O Cu(NO3)2·2.5H2O H3PO4 NpO2OH initial pHb

0.010, 0.200 0.005, 0.100 0.010, 0.200 5.0 × 10−4, 0.01 1.82

0.050, 0.200 0.025, 0.100 0.050, 0.200 2.5 × 10−3, 0.01 1.27

a

For the slow diffusion reactions, the concentration is calculated to be that at equilibrium in 20 mL of water, the total volume in the vial. b The pH was measured in a mock solution prepared by combining all reagents except neptunium in the appropriate quantities in a 20 mL vial. Note that the slow diffusion reactions also included 16 mL of distilled water.

EXPERIMENTAL SECTION

Syntheses. UO2(NO3)2·6H2O (98%, International Bio-Analytical Industries), Cu(NO3)2·2.5H2O (98.2%, Fischer Chemical), and H3PO4 (85%, Fischer Chemical) were used as received without further purification. Stock solutions were prepared for each species at concentrations of 0.2, 0.1, and 0.1 M, respectively. A stock solution of NpO2+ was prepared by dissolving previously recycled NpO2OH in distilled water with a minimal amount of concentrated nitric acid. The final concentration of Np(V) in this solution was 0.2 M. Caution! While the UO2(NO3)2·6H2O used for these reactions contains depleted U, standard precautions for handling radioactive materials should be followed. Old sources of depleted U should not be used as the daughter elements of natural decay are highly radioactive and present serious health risks. In our laboratory, uranium starting materials are stored in a ventilated cabinet until needed. All manipulations are carried out wearing lab coats, gloves, and eye protection along with whole body and f inger radiation monitoring. All uranium products are stored in sealed boxes in ventilated cabinets. Caution! 237Np represents a serious health risk owing to its α and γ emission, and especially because of its decay to the short-lived isotope 233Pa (t1/2 = 27.0 days), which is a potent β and γ emitter. Neptunium starting materials are stored in a glovebox until needed. All manipulations with f ree-flowing solids are carried out in the glovebox. When the material is taken out of the glovebox, solids are either sealed in glass vials or covered in water. All neptunium products are stored under water or oil in a f ume hood. All studies were conducted at the University of Notre Dame, which has appropriate material and personnel for handling radioactive materials. Slow diffusion samples were prepared by segregating solutions containing the cations (UO22+, 1 mL; Cu2+, 1 mL; and NpO2+, 50 μL) and anions (PO43−, 2 mL) in two separate 3 mL borosilicate glass vials and allowing them to diffuse into a larger 20 mL borosilicate glass vial initially containing only distilled water (16 mL). See Figure S1 in the Supporting Information for a schematic depicting the reaction vessel. Reactions were conducted at room temperature and 90 °C. Roomtemperature reactions were sealed with Parafilm and placed in a fume hood for 1 week. High-temperature reactions were covered with matching screw-on polyethylene lids and placed in a 250 mL Pyrex glass beaker for additional containment. The beaker was covered with a watch glass, placed in a box furnace, and heated at 90 °C for 1 week before being cooled to room temperature at the furnace’s natural cooling rate. Green crystals were observed to form exclusively on the rim of the uranium/copper/neptunium vial after 1 day for the elevated

The crystals formed in all reactions were isolated in Petri dishes and layered with a sufficient amount of mother solution to completely cover all of the crystals. When being manipulated for further analysis, the solution was drawn off and the crystals were dispersed with ethanol. The crystals were then reimmersed in the mother solution for storage. Crystallographic Studies. Crystals from the hydrothermal, room temperature, and elevated temperature slow diffusion reactions were mounted on CryoLoops with Krytox oil and optically aligned on a Bruker APEXII Quazar X-ray diffractometer using a digital camera. Initial intensity measurements were performed using an IμS X-ray source, a 30 W microfocused sealed tube (Mo Kα, λ = 0.71073 Å) with high-brilliance and high-performance focusing Quazar multilayer optics. Standard APEX II software was used for determination of the unit cell and data collection control. The intensities of reflections were collected by a combination of three (room-temperature and hydrothermal) or four (elevated temperature) sets of exposures. Each set had a different φ angle for the crystal, and each exposure covered a range of 0.5° in ω. Totals of 536, 535, and 536 frames were collected for the room-temperature, high-temperature, and hydrothermal reactions, respectively. Each frame had an exposure time of 10 s. SAINT software was used for data integration, including Lorentz and polarization corrections. Semiempirical absorption corrections were applied using the program SCALE (SADABS). The crystal structures of all three samples were solved by direct methods, and all non-hydrogen atoms were refined anisotropically.9 Optical Spectroscopy. Solid-state UV−vis-NIR data were acquired for individual, freshly harvested crystals from each reaction using a Craic Technologies microspectrophotometer. Crystals were placed on quartz slides under Krytox oil, and spectra were collected from 200 to 1400 nm. The exposure time was optimized automatically by the Craic software. Solution UV−vis spectra were acquired for the mother solutions of each reaction using a Cary 6000i spectrometer. Spectra were collected from 400 to 1400 nm. A baseline correction using a water-filled cuvette was automatically applied to the spectra using the Cary software. The Np(V) peak at approximately 980 nm was used to calculate the concentration of neptunium in solution (ε = 395 M−1 cm−1).10 An 387

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Table 2. Crystallographic Information for CuUPO and Neptunium-Incorporated CuUPO space group a (Å) c (Å) V (Å3) a

torbernitea

CuUPO SDRT

meta-torbernitea

CuUPO SDHT

CuUPO HYDRO

P4/ncc 7.0267(4) 20.807(2) 1027.3(1)

P4/ncc 7.0214(4) 20.6882(12) 1019.93(10)

P4/n 6.9756(5) 17.349(2) 844.2(1)

P4/n 6.9486(2) 17.2383(7) 832.32(5)

P4/n 6.9396(11) 17.220(3) 829.3(2)

Reference 12.

additional, manual baseline correction was applied prior to the calculation by averaging 10 baseline points (five from each side) around the peak and subtracting the average from the peak absorbance. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). Laser ablation inductively coupled plasma mass spectrometric analyses of crystals from all samples were conducted using a ThermoFinnigan high-resolution magnetic sector Element2 ICP-MS instrument coupled to a UP213 Nd:YAG laser ablation system (New Wave Research). Selected crystals were affixed on 1 in. round glass slides with double-sided tape. Individual analyses consisted of a 60 s measurement of background ion signals, followed by a 60 s measurement of ion signals (31P, 63Cu, 65 Cu, 235U, 238U, and 237Np) during ablation. Each analysis represented a total of 55 scans (55 runs × 1 pass) with a sample (dwell) time of 0.01 s with 20 samples per ion signal peak. Analyses were conducted in medium mass resolution mode (resolution = mass/peak width ∼ 4000) to eliminate possible spectral overlap, in particular between Np and U ion signals. Ablated particles were transported from the ablation cell to the ICP-MS instrument using He carrier gas at a flow rate of 1.0 L/min. Crystals were ablated using a range of spot sizes between 8 and 15 μm, a repetition rate of 1−2 Hz, and 100% power output corresponding to an energy density of approximately 10−15 J/cm2. To avoid memory effects, a minimum 20 s wait time was allotted between measurements. In general, three ablations were conducted per crystal for three crystals of each sample. However, the small size of some crystals, notably those grown hydrothermally, precluded this, and one ablation was conducted per crystal for nine crystals of the sample. The average ion signal was determined over the interval from 60 to 120 s. In some cases, the crystal fractured under ablation. In these instances, the average ion signal was determined up to the point of fracture. Occasionally, the crystal fractured immediately upon ablation, and the data from such measurements was omitted. The percent substitution of neptunium was calculated from the ratio of the 237Np ion signal over the total actinide signal (235U + 238U + 237Np). The data were then averaged for all ablations in a given sample to provide an average incorporation level for the entire reaction.

As described by Locock and Burns,12 torbernite and metatorbernite adopt a layered topology composed of vertex sharing uranyl square bipyramids and phosphate tetrahedra. Copper cations in the interlayer form Jahn−Teller distorted octahedra coordinated by four water molecules in the equatorial plane. The axial positions are capped by Oyl atoms from uranyl moieties on different layers, thereby linking the uranyl phosphate sheets together. Additional water molecules occupy the interlayer to achieve the appropriate hydration for each compound ([Cu(UO2)2(PO4)2·nH2O]; n = 8 for metatorbernite and n = 12 for torbernite).12 Optical Spectroscopy. An example of a solid-state UV− vis-NIR spectrum for the hydrothermal reaction (control and neptunium-doped sample) is given in Figure 1. This spectrum

RESULTS Syntheses. Meta-torbernite crystallizes in the hydrothermal reactions and high-temperature slow diffusion reactions as square green plates, whereas torbernite crystallizes in the roomtemperature slow diffusion reactions as arrowhead-shaped green tablets. The meta-torbernite crystals formed in the high-temperature slow diffusion reactions tended to be significantly larger than those that formed under hydrothermal conditions. In all cases, neptunium incorporation does not affect the coloration or habit of the crystals formed when compared to those formed in the control reactions. Crystallographic Studies. As with most previous studies, the exception being Cs[(UO2)(HSeO3)(SeO3)],11 neptunium incorporation does not introduce a significant change in the unit cell, as may be seen in Table 2, because of its compatibility within the structure. The minor variations that are observed are more likely the result of differences in measurement conditions and collection parameters or dehydration of the samples during the collection rather than neptunium incorporation.

identifies neptunium in the +6 oxidation state from the series of peaks between 500 and 650 nm, which are a characteristic vibronic coupling pattern for Np(VI). The primary identifying peak for Np(VI) that occurs at 1200 nm is absent because the square-bipyramidal symmetry of the uranyl/neptunyl sites quenches f−f transitions.10 Solid-state UV−vis-NIR spectra for the slow diffusion reactions did not provide any information about the neptunium oxidation state due to lower incorporation levels. Sample spectra are available in the Supporting Information. Data from solution UV−vis studies are provided in Table 3. In general, the room-temperature slow diffusion reactions had the lowest percentage of neptunium(V) remaining in solution, whereas the high-temperature slow diffusion reactions had the greatest. No peaks for oxidation states other than +5 were observed in the spectra. Examples of spectra from each reaction are shown in Figure 2. LA-ICP-MS. Time-resolved LA-ICP-MS spectra for each reaction condition are provided in Figure 3. In each case, it may

Figure 1. Sample solid-state UV−vis-NIR spectra for CuUPO and neptunium-doped CuUPO from hydrothermal reactions showing peaks characteristic for Np(VI).

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Table 3. Neptunium Incorporation in CuUPO sample CuUPO CuUPO CuUPO CuUPO CuUPO CuUPO CuUPO CuUPO CuUPO

+ + + + + + + + +

Np Np Np Np Np Np Np Np Np

SDRT-1 SDRT-2 SDRT-3 SDHT-1 SDHT-2 SDHT-3 HYDRO-1 HYDRO-2 HYDRO-3

% Np in solution

% Np substitution

37.21(±0.02)% 37.10(±0.07)% 37.41(±0.08)% 80.92(±0.04)% 75.85(±0.20)% 64.08(±0.03)% 46.80(±0.10)% 51.99(±0.04)% 55.79(±0.04)%

0.15(±0.04)% 0.12(±0.01)% 0.054(±0.002)% 0.028(±0.002)% 0.029(±0.002)% 0.028(±0.001)% 1.50(±0.06)% 2.43(±0.10)% 2.19(±0.07)%

Figure 2. Solution UV−vis spectra for mother solutions from hydrothermal, room-temperature slow diffusion, and high-temperature slow diffusion reactions showing only peaks for Np(V) in solution. Note that, although the hydrothermal reaction has the greatest absorption, it contained a higher initial concentration of neptunium due to the lower reaction volume, and the percentage remaining in solution is intermediate between the two slow diffusion reactions.

be seen that the 237Np signal is consistent with the other analyzed ions. The flat profile of the various ion signals (cps: counts per second) during the laser ablation interval, including that for Np, indicates that the latter is uniformly incorporated throughout the crystal and not present only on the surface. The percent substitution of neptunium for uranium based on the relative ion signals is given in Table 3. The highest uptake of neptunium occurs in hydrothermal reactions and is approximately 19 ± 2 and 73 ± 3 times greater, on average, than the incorporation level in the room-temperature and hightemperature slow diffusion reactions, respectively.

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Figure 3. LA-ICP-MS spectra for Np incorporation in CuUPO showing a uniform distribution of neptunium throughout the crystals of each sample: (a) hydrothermal (crystal from reaction 1), (b) roomtemperature slow diffusion (crystal from reaction 3), (c) hightemperature slow diffusion (crystal from reaction 1).

DISCUSSION This study has demonstrated the successful incorporation of neptunium into torbernite/meta-torbernite crystals grown under hydrothermal and slow diffusion conditions. Most notably, we have observed that it is Np(VI), not Np(V), that incorporates and that hydrothermal conditions favor higher uptake levels, although some discrepancies have been observed when determining the amount of neptunium remaining in solution to evaluate the environmental significance of this work. In uranyl compounds, the incorporation of neptunium is presumed to occur by the substitution of neptunium onto uranium crystallographic sites, as depicted in Figure 4 for torbernite/meta-torbernite. This mechanism is based on the similarity of coordination environments adopted by actinide elements.5 For actinides in the +5 or +6 oxidation state, this

coordination environment is centered on the linear actinyl moiety, AnO2+/2+, giving rise to bipyramidal environments.13 In most previous studies, the oxidation state of neptunium was assumed to be +5 based on the starting materials used and the absence of oxidizing species in the reaction; however, few of these studies, if any, were able to identify the oxidation state due to low incorporation levels.6,7 In this study, we have been able to conclusively identify the presence of Np(VI) in the hydrothermal samples from solid-state UV−vis-NIR data. When NpO2+ substitutes for UO22+, there is a resultant charge imbalance that must be accommodated by other alterations or 389

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entirely ruled out as the oxidation state of neptunium was not conclusively identified in those studies. The absence of Np(IV) from both the mother solution and the solid phases is somewhat puzzling as it should form concomitantly with Np(VI) during the disproportionation of Np(V). Np(IV) is much less soluble than Np(V) and is expected to rapidly undergo hydrolysis to form Np(OH)4 even in moderately acidic solutions. However, in these reactions, the high acidity should stabilize the Np4+ ion in aqueous solution.3 The PTFE liners and covers for the borosilicate glass vials used in this study are permeable to air; thus, we believe that the Np(IV) in solution is likely oxidized by oxygen back to Np(V): NpO2+ + e− → Np4+, E° = +0.604 V; O2 + 4H+ + 4e− → 2H2O, E° = +1.229 V.10,16 That said, the possibility that Np(V) is oxidized in air to Np(VI) rather than undergoing disproportionation should also be considered as this process may be solubility driven by the incorporation of Np(VI) into the solid phase. However, the formation of Np(VI) by oxidation does not account for the observed differences in neptunium uptake levels, which are better explained in the context of disproportionation. Many studies have investigated the factors that influence Np(V) disproportionation and have found that, in addition to high acidity, disproportionation increases with both increasing temperature and concentration of Np(V) and also in the presence of complexing species.14 Although the oxidation state of neptunium in the slow diffusion reactions was not accessible by our available techniques due to the low uptake levels, these considerations may explain why so much more neptunium incorporated in the hydrothermal reactions compared to the slow diffusion reactions. The significantly higher temperature and 5 times greater concentration of neptunium in the hydrothermal reactions favor disproportionation, and incorporation, in turn, favors the direct substitution of the Np(VI) that forms. The discrepancies observed between incorporation under hydrothermal and slow diffusion conditions suggest that hydrothermal incorporation may not reliably model incorporation in a geological repository where slow diffusion conditions are expected to exist. While high levels of incorporation are desirable, the percentage of neptunium remaining in solution is the more environmentally relevant parameter. One should expect that the hydrothermal samples that give the highest incorporation would correspondingly have the least neptunium remaining in solution; however, this is not the case as the lowest percentage of neptunium in solution, on average, 37.24(±0.04)%, was observed for the room-temperature slow diffusion reactions. It appeared that a second, powdery phase formed during the room-temperature reactions; however, we have not been able to isolate or analyze it as yet. The presence of neptunium in this secondary phase would account for the additional decrease in the amount of neptunium in solution. In a preliminary study in which the room-temperature slow diffusion reaction was allowed to proceed for 1 month (as opposed to 1 week in this work), the percentage of neptunium in solution was observed to be 65.18(±0.16)%, corresponding more closely with that observed for the high-temperature slow diffusion reaction, 73.62(±0.07)%. Furthermore, the powdery phase was absent in both the preliminary study and the high-temperature reactions. It is possible that this secondary phase is itinerant and that time and temperature favor its dissolution, making it inconsequential in the context of long-term geological disposal of nuclear waste. We are currently investigating the role played

Figure 4. Presumed mechanism of neptunium incorporation in the torbernite/meta-torbernite layer showing substitution of the neptunyl species onto uranyl sites.

substitutions in the structure; however, when NpO 2 2+ substitutes, as appears to occur in this reaction, no charge imbalance is created, potentially allowing for higher uptake levels when Np(VI) is present rather than Np(V). Further evidence supporting the presence of Np(VI) was observed in several initial studies we conducted in designing this experiment. In these studies, several cocontaminants based on cationic or anionic cosubstitutions (SeO42−, SO42−, Nd3+, Sc3+, Cs+) were included in the reactions to investigate different mechanisms of balancing charge after the incorporation of NpO 2 + . However, very little variation in the percent substitution of neptunium was observed, supporting the presence of Np(VI) for which charge balance is irrelevant. Solid-state UV−vis-NIR spectra for these reactions also indicated the presence of Np(VI). Additional details and spectra from this preliminary work are provided in the Supporting Information. In our reactions, as in previous studies, no species were present that are capable of oxidizing neptunium from +5 to +6 (NpO22+ + e− → NpO2+, E° = +1.159 V).10 The given reduction potential (and those that follow) are for reactions at 25 °C and 1 atm and should only be considered as a guide, especially for the hydrothermal and high-temperature slow diffusion reactions. We, therefore, attribute the presence of Np(VI) in the crystals to the disproportionation of Np(V), as shown in Scheme 1. The syntheses were conducted at highly Scheme 1. Np(V) Disproportionation to Np(IV) and Np(VI)

acidic conditions (pH 1−2) that have been shown to favor disproportionation.14 A similar observation was made by Forbes and Burns when synthesizing a series of neptunium phosphates that are isotypic with torbernite/meta-torbernite.15 Attempts to conduct the reactions at neutral or alkaline conditions using combinations of phosphoric acid and trisodium phosphate have been unsuccessful. The identification of Np(VI) in this study may indicate that previous incorporation studies, including our own, may be incorrect in assuming the incorporation of Np(V). Studies in silicate and hydroxide systems were typically conducted at higher pH in the range of 4−8 and slightly lower temperatures between 80 and 160 °C depending on the compound of interest.6 These conditions reduce the likelihood of disproportionation; however, the incorporation of Np(VI) cannot be 390

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Crystal Growth & Design by the length of synthesis in the slow diffusion reactions in the hope that the results of that work may be able to better address the observed discrepancy between the highest incorporation levels (hydrothermal) and lowest amount of neptunium in solution (room-temperature slow diffusion).

CONCLUSIONS The determination of Np(VI) in the doped samples discussed in this study has implications for neptunium incorporation studies and also for our understanding of the environmental behavior of neptunium, particularly in a repository. In general, Np(V) is believed to be the most relevant oxidation state under oxidizing environmental conditions. In fact, in their review on the aqueous geochemistry of neptunium, Kaszuba and Runde claim, “Np(VI) is not important under most environmental conditions and is stable only in highly oxidizing solutions,” and do not include any data on Np(VI) in their article.3 Our study suggests that the environmental relevance of Np(VI) should be reevaluated. In a repository, the radiolysis of water generates oxidizing molecules and radicals along with radiolytic acids. The presence of these species increases the oxidizing potential and acidity of the solution contacting the waste.17 These factors, coupled with higher temperatures from the energy released during radioactive decay, may, in fact, create an environment that favors the formation of Np(VI) by disproportionation, if not by oxidation. Thus, Np(VI) may play a more significant role in the long-term storage of nuclear waste than was previously thought. Although our results indicate that hydrothermal conditions may not reliably model neptunium incorporation in a repository environment, it is important to note that these results may be specific to the phosphate system we have analyzed. Furthermore, during oxidative corrosion of spent nuclear fuel in a repository, groundwater anions and cations are expected to be at much higher concentrations than uranium and neptunium, whereas our samples were prepared based on stoichiometric ratios. Additionally, our reactions were conducted under highly acidic conditions that may or may not model environmental conditions in a repository. Lastly, the 1 week synthetic period of our study is insignificant when compared to the thousands of years that will elapse during waste storage. Bearing these matters in mind, it is clear that further work is needed to investigate the factors that impact the formation of uranyl alteration products, neptunium incorporation in these phases, and ultimately the stability of the doped compounds in terms of solubility and radiation damage resistance. Understanding these various influences is critical in developing an effective and responsible plan for long-term nuclear waste storage and disposal.

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ASSOCIATED CONTENT

S Supporting Information *

Additional experimental data and schematics, solid-state UV− vis-NIR spectra for slow diffusion reactions, and experimental details and results from preliminary studies. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

We are grateful for support provided by the U.S. Department of Energy, Subsurface Biogeochemical Research Program, under grant ER64804. We also thank three anonymous reviewers for their generous comments and recommendations that have significantly improved this article. N.A.M. thanks Alice L. Meredith for her help in proofreading the final draft of this article.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 391

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