Influence of Dissolved Sodium and Cesium on Uranyl Oxide Hydrate

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Environ. Sci. Technol. 2004, 38, 171-179

Influence of Dissolved Sodium and Cesium on Uranyl Oxide Hydrate Solubility DANIEL E. GIAMMAR* AND JANET G. HERING California Institute of Technology, 1200 E. California Boulevard, Environmental Engineering Science, Pasadena, California 91125

The solubility of uranium-containing minerals can control the mobility of uranium in contaminated soil and groundwater. The identity and solubility of these minerals are strongly influenced by solution composition. The influence of dissolved sodium and cesium on the solubility of uranyl oxide hydrates has been investigated in a series of batch experiments conducted with synthetic metaschoepite ((UO2)8O2(OH)12‚10H2O). During reaction of metaschoepite in NaNO3, CsNO3, and NaF solutions, an initial increase in the dissolved uranium concentration was followed by a decrease as uranium was incorporated into a secondary solid phase. Given sufficient reaction time, metaschoepite was completely transformed to a clarkeite-like sodium uranyl oxide hydrate or a cesium uranyl oxide hydrate that has not previously been described. These secondary solid phases exhibited X-ray diffraction patterns and Raman spectra that were distinct from those of the original metaschoepite. Dissolved uranium concentrations in equilibrium with the sodium and cesium uranyl oxide hydrates can be more than 2 orders of magnitude lower than those in equilibrium with metaschoepite. Initial changes in metaschoepite solubility may also result from particle growth induced by sodium and cesium incorporation into the solid phase.

Introduction Uranium (U) contamination of soil and groundwater has resulted from mining, refining, and waste disposal activities associated with nuclear weapons and energy programs. U is a principal contaminant in soils at Department of Energy weapons processing plants (1), and the proposed high-level radioactive waste repository at Yucca Mountain would receive large amounts of U in the form of spent nuclear fuel and vitrified high-level waste (2). The mobility of U in oxic soil and groundwater can be controlled by U(VI) minerals, and U(VI) oxide hydrates are particularly important because of their occurrence at contaminated sites and crucial role in the corrosion of spent nuclear fuel. Schoepite ((UO2)8O2(OH)12‚12H2O) or schoepite-like phases have been identified in soils at the Fernald Environmental Management Site (3, 4), Oak Ridge National Laboratory (5), and in catch-box soils used in the ballistics testing of depleted uranium metal projectiles (6). Schoepite is the first phase in a sequence of secondary U(VI) phases formed during the corrosion of spent * Corresponding author phone: (314)935-6849; fax: (314)935-5464; e-mail: [email protected]. Present address: Environmental Engineering Science, Washington University in St. Louis, Campus Box 1180, One Brookings Drive, St. Louis, MO 63130. 10.1021/es0345672 CCC: $27.50 Published on Web 11/26/2003

 2004 American Chemical Society

nuclear fuel (7) and the weathering of natural ore deposits (8). The formation of schoepite may control the solubility of U in a corrosive environment, and schoepite is the starting material for the formation of more stable secondary phases. While U sorption on Fe(III) oxide (9-11) and clay minerals (12-14) has been widely observed, surface precipitation of schoepite and schoepite-like phases has also been observed on Fe(III) oxide (11, 15) and phyllosilicate minerals (16). Despite their simple compositions, U(VI) oxide hydrates are a diverse family of minerals, whose structures are composed of sheets of uranyl bipyramidal polyhedra connected through edge and corner sharing of equatorial oxygen atoms (17). Metaschoepite ((UO2)8O2(OH)12‚10H2O) and schoepite are closely related phases with nearly identical interlayer spacings and minor differences in sheet arrangements (18, 19). Water molecules occupy the interlayers of schoepite and metaschoepite (20), but these solids have a strong tendency to incorporate cations into their interlayer spaces (8, 21). New mineral phases are formed by incorporation of Ca2+ (19, 22-25), Na+ (8), K+ (23), Ba2+, and Pb2+ (22). While not observed in nature, synthetic layered cation U(VI) oxide hydrates have also been formed with Sr2+ (26, 27), Cs+ (28), Na+ (29, 30), K+ (31), Mg2+, Mn2+, and Ni2+ (32). Schoepite can also undergo transformation without the incorporation of interlayer cations, including transformations to U(VI) phosphates (24) and carbonates (33). In contaminated environments, cations available for incorporation may be present in the contacting groundwater or may be co-contaminants with U. In unsaturated corrosion tests of UO2 (the primary component of spent nuclear fuel) with groundwater from the Nevada Test Site adjacent to the Yucca Mountain repository, initial schoepite formation was followed by the formation of becquerelite (Ca(UO2)6O4(OH)6‚ 8H2O) and compreignacite (K2(UO2)6O4(OH)6‚7H2O) (7). Formation of U(VI) oxide hydrates with interlayer cations is also likely in high ionic strength aqueous waste solutions containing U. The formation of Na-containing U(VI) oxide hydrates may be expected in systems that store or have received U(VI)-containing aqueous solutions with high NaNO3 and/or NaOH concentrations such as the tank wastes at the Hanford Site and Savannah River Site (34, 35). In Narich geological settings, the mineral clarkeite (Na[(UO2)O(OH)]‚H2O) can form through the alteration of preexisting uranium minerals (8). Cations produced by fission such as Cs+ and Sr2+ that are present in waste solutions and spent nuclear fuel (7) may also be incorporated into U(VI) oxide hydrates that form in these systems. While cation incorporation influences U(VI) oxide hydrate solubility, incorporation may also serve to retard the mobility of cations (36), a process which is particularly important for fission products such as 137Cs and 90Sr. The objectives of this work were to examine the influence of dissolved Na+ and Cs+ on the solubility of U(VI) oxide hydrates and to relate changes in solubility to transformations of the solid phase. The influence of Na+ and Cs+ was probed by performing a series of experiments with synthetic metaschoepite exposed to solutions of varying chemical composition. By combining bulk solution chemistry measurements with solid-phase characterization, observed changes in U solubility were related to metaschoepite transformation and the formation of U(VI) oxide hydrates containing Na+ and Cs+.

Experimental Section Materials. Two synthetic metaschoepite materials were prepared following an adaptation of published methods (24, VOL. 38, NO. 1, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Summary of Conditions and Observations for Batch Experiments electrolyte expt (ID) M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 E1 E2 E3 E4

starting solid MS-Na MS-Na M2 residual MS-Na MS-Na M5 residual MS-Na MS-Na MS-TBA MS-TBA MS-TBA MS-TBA MS-TBA MS-TBA MS-TBA

(mM) 100 10 10 10 0.04 0 10 100 0 1 5 1 10 1 10

type NaNO3 NaNO3 NaNO3 NaNO3 NaNO3 none CsNO3 CsNO3 none NaF NaF NaNO3 NaNO3 CsNO3 CsNO3

pH MES (mM) 5 5 5 5 0 0 5 5 0 0 0 0 0 0 0

UT (mM) 0.20 0.28 0.25 0.28 0.28 0.42 0.28 0.28 0.44 0.44 0.44 0.49 0.49 0.49 0.48

tfina (d) 42 26 56 45 84 23 114 114 108 108 108 90 90 90 90

[U]diss (µM)

init

fin

init

max

fin

6.00 6.00 6.00 6.00 6.00 6.00 6.00 6.00 5.93 5.95 6.00 5.98 6.15 6.05 6.04

NMb

NMb

NMb NMb 5.84 6.03 5.85 5.89 5.93 6.10 5.96 6.03 5.88 5.52 5.12 5.00

0.02 0.06 0.02 0.02 0.10 0.02 0.02 0.60 0.61 0.62 42.3 26.7 42.5 28.9

14.5 11.3 5.4 13.1 25.9 9.0 4.1 2.9 38.9 107.3 460.9 44.5 44.4 49.3 39.1

3.8 3.5 1.1 6.7 2.1 5.8 1.2 0.9 38.9 100.8 123.7 20.9 44.4 48.8 36.4

a The final times (t ) for experiments M1-10 were determined by stabilization of pH and [U] fin diss over a series of at least 2 measurements over at least 7 d. Measurements did not stabilize after 108 d for experiment M10 or 90 d for experiments E1-4. b NM - not measured.

37), in which 0.01 M UO2(NO3)2‚6H2O solutions (Alfa Aesar) were titrated with freshly prepared 0.1 M strong base to pH 5.5-6.0. One batch was synthesized using NaOH (Mallinckrodt) and another using tetrabutylammonium hydroxide (TBAOH) (Aldrich), hereafter referred to as MS-Na and MSTBA, respectively. The bright yellow precipitate that formed was aged for 7-14 days, then rinsed, centrifuged, and resuspended 6-8 times with ultrapure water. The synthesized solids were identified as metaschoepite by X-ray diffraction (Figure 1), with the best match to reference card 43-0364 (labeled synthetic metaschoepite) of the International Centre for Diffraction Data database (38). Additional identification was provided by Raman and infrared spectra of the MS-Na material, which were in agreement with previously published spectra (39-41). As determined by BET-N2 adsorption on freeze-dried powders, the surface areas of the MS-Na and MS-TBA materials were 12.9 and 8.8 m2 g-1 respectively. Experimental Methods. Metaschoepite dissolution and secondary phase formation were investigated in a series of batch experiments. Experiments were performed in two modes: (1) “metaschoepite addition” experiments were initiated by adding an aliquot of stock metaschoepite suspension to U-free solution and (2) “electrolyte addition” experiments which involved the addition of concentrated NaNO3 or CsNO3 solution to metaschoepite suspensions that had been preequilibrated for 43 days in ultrapure water. This pre-equilibration time was found, in previous experiments conducted in ultrapure water, to be sufficient to achieve constant dissolved U concentrations. The U-free solution in the metaschoepite addition experiments was either ultrapure water or a solution of NaNO3, CsNO3, or NaF. All experiments had an initial pH of 6.00, and some experiments were buffered with 5 mM 2-(n-morpholino)ethanesulfonic acid (MES) (Avocado Research Chemicals) adjusted to pH 6.00 by addition of NaOH (providing an additional 2.3 mM Na). MES was selected as a pH buffer because of its lack of metal complexation (42, 43). Table 1 summarizes the conditions of all experiments performed in this study. All batch experiments were performed at the ambient laboratory temperature (22 ( 2 °C) and under the ambient laboratory atmosphere. Samples for dissolved U were filtered using syringemounted 0.2 µm polycarbonate filter membranes; 5 mL or 10 mL aliquots of well-mixed suspension were collected and filtered, the first 3 mL of filtrate were discarded, and the remainder of the filtrate was collected and acidified to 1% HNO3. Samples for total U were collected by diluting a 1 mL aliquot of the suspension in 10% HNO3, which dissolved all particles. Over the course of an experiment, as many as 20 172

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FIGURE 1. X-ray diffraction patterns of (a) synthetic metaschoepite (MS-Na), (b) synthetic metaschoepite (MS-TBA), (c) a reference pattern (JCPDS card 43-0364) for synthetic metaschoepite, (d) a calculated pattern for clarkeite, (e) a reference pattern (JCPDS card 43-0347) for Na2U2O7, (f) solids after metaschoepite reaction for 484 h in 10 mM NaNO3 buffered at pH 6 with 5 mM MES (expt. M2), (g) solids after reaction of residual materials from experiment M2 for 1348 h in 10 mM NaNO3 buffered at pH 6 with 5 mM MES (expt. M3), (h) solids after metaschoepite reaction for 2599 h in 5 mM NaF solution (expt. M11), and (i) solids after metaschoepite reaction for 2740 h in 100 mM CsNO3 buffered at pH 6 with 5 mM MES (expt. M8). The broad peak from 16 to 18° in patterns f-i is from the polycarbonate filter membrane upon which the solids were collected. Note: Diffractograms displaced along y-axis for legibility. samples were collected, which always constituted less than half of the initial suspension volume. The solids collected on the filter membranes used for dissolved U sampling were air-dried at room temperature and kept for subsequent analysis by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and Raman spectroscopy. This sampling

TABLE 2. Aqueous Phase U(VI) Reactions and Thermodynamic Stability Constants Considered in This Worka,b Log K

reaction UO2OH+

H+

UO2 + H2O ) + UO22+ + 2H2O ) UO2(OH)2(aq) + 2H+ 2+ UO2 + 3H2O ) UO2(OH)3- + 3H+ UO22+ + 4H2O ) UO2(OH)42- + 4H+ 2UO22+ + H2O ) (UO2)2OH3+ + H+ 2UO22+ + 2H2O ) (UO2)2(OH)22+ + 2H+ 3UO22+ + 4H2O ) (UO2)3(OH)42+ + 4H+ 3UO22+ + 5H2O ) (UO2)3(OH)5+ + 5H+ 3UO22+ + 7H2O ) (UO2)3(OH)7- + 7H+ 4UO22+ + 7H2O ) (UO2)4(OH)7+ + 7H+ UO22+ + CO32- ) UO2CO3(aq) UO22+ + 2CO32- ) UO2(CO3)22UO22+ + 3CO32- ) UO2(CO3)343UO22+ + 6CO32- ) (UO2)3(CO3)66UO22+ + F- ) UO2F+ UO22+ + 2F- ) UO2F2(aq) UO22+ + 3F- ) UO2F3UO22+ + 4F- ) UO2F422+

-5.20 -11.5 -19.2 -33.0 -2.7 -5.62 -11.9 -15.55 -31.0 -21.9 9.68 16.94 21.60 54.0 5.09 8.62 10.9 11.7

a Species that account for >2% of the total dissolved U in any of the experiments are listed in bold. b Constants (for zero ionic strength) are from the critical review by Grenthe et al. (45) with the exception of the UO2(OH)2(aq) formation constant, which is from Silva et al. (46).

procedure was used throughout batch experiments and to sample the stock metaschoepite suspensions and U-free solutions prior to metaschoepite addition experiments. Dissolved concentrations of U, Na, and Cs were determined by inductively coupled plasma mass spectrometry (ICP-MS) with a Hewlett-Packard HP4500 instrument. Samples for ICP-MS analysis were prepared in 1% HNO3, and 10 ppb Tl was used as an internal standard. Solution pH was measured with a Ross glass electrode and Orion 720A pH meter. XRD analyses were performed on a Scintag Pad V X-ray powder diffractometer with a Cu KR X-ray source and Ge detector. Diffuse reflectance infrared spectra were collected with a Bio-Rad FTS-45 instrument. SEM images were collected on Au- and C-coated samples with a Camscan Series II scanning electron microscope. Raman spectra were measured on a Renishaw MicroRaman Spectrometer with a 514.5 nm argon ion laser. To collect bulk Raman spectra and avoid dehydrating the sample under a focused laser, the Raman spectrometer was operated with a defocused beam. Surface area measurements by N2-BET adsorption were performed with a Micromeritics Gemini instrument. Equilibrium calculations were performed using the software program MINEQL+ (44). The MINEQL+ database was modified to include the reactions and equilibrium constants for dissolved U(VI) species from the critical review of Grenthe et al. (45) and from Silva (46) (Table 2). Ionic strength corrections used in MINEQL+ and in other calculations in this work were made using the Davies equation (47). Input parameters for calculations were total concentrations of components, ionic strength, pH, and 10-3.5 atm PCO2.

FIGURE 2. Evolution of [U]diss during (a) experiment M6 ([U]T ) 0.28 mM), addition of metaschoepite to unbuffered dilute solution (pH varied from 5.85 to 6.10), (b) experiments M4 and M7 ([U]T ) 0.28 mM), addition of metaschoepite (MS-Na) to 10 mM NaNO3 (9) and 10 mM CsNO3 (b) solutions buffered at pH 6 with 5 mM MES, and (c) experiments M9, M10, and M11 ([U]T ) 0.44 mM), addition of metaschoepite to unbuffered solutions of 0 (b), 1 mM (O), and 5 mM (9) NaF solution. The dashed lines in panel c represent the calculated [U]diss in equilibrium with metaschoepite. and M11. The NaF solution was initially selected in an effort to prevent U loss from solution by formation of stable dissolved U(VI)-fluoride complexes. In the presence of fluoride, metaschoepite dissolution was substantial (indeed, complete at the highest fluoride concentration), but U was still subsequently lost from solution. Although these batch experiments do not allow quantification of dissolution rates, the evolution of [U]diss prior to reprecipitation suggests that dissolution of synthetic schoepite and metaschoepite is rapid; a minimum bound of 1.6 µmol m-2 h-1 at pH 6 (48) has been estimated from flow through experiments (data not shown). The pattern in [U]diss with time (i.e., increasing followed by decreasing concentrations) may be interpreted as the dissolution of metaschoepite followed by the nucleation and growth of a secondary phase (eq 1):

Results and Discussion Dissolution and Secondary Phase Formation. Although designed to investigate metaschoepite dissolution, almost none of the experiments exhibited simple dissolution behavior. The evolution of [U]diss with time occurred in three phases: (1) a rapid initial increase, (2) a continuing, slower increase, and (3) a decrease to a constant concentration. This pattern results in an apparent overshoot of the final equilibrium concentration and was observed following addition of metaschoepite to ultrapure water (Figure 2a), NaNO3 and CsNO3 solution (Figure 2b), and even NaF solution (Figure 2c). The overshoot phenomenon was reproducible and was most pronounced in experiments M4

d[U]diss ) Rdiss,ms - Rpptn,ms + Rdiss,secondary dt Rpptn,secondary (1) where the rates of dissolution and precipitation (Rdiss and Rpptn) for metaschoepite and a precipitated secondary phase are denoted by the subscripts “ms” and “secondary”. Initially, the increase in [U]diss with time reflects only Rdiss,ms. If no secondary phase were formed, then Rdiss,ms would eventually be balanced by Rpptn,ms and equilibrium with metaschoepite would be attained. However, with formation of a secondary solid phase, Rpptn,secondary can substantially retard the net VOL. 38, NO. 1, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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increase of [U]diss in the system; when Rpptn,secondary exceeds Rdiss,ms, [U]diss starts to decrease. Ultimately, the system will reach equilibrium where Rpptn,secondary ) Rdiss,secondary if only the secondary phase is present and Rpptn,secondary + Rpptn,ms ) Rdiss,secondary + Rdiss,ms if metaschoepite coexists with the secondary phase. Similar dissolution-reprecipitation patterns have been observed during the reaction of soddyite ((UO2)2SiO4‚2H2O) in NaNO3 solution with soddyite dissolution succeeded by precipitation of a clarkeite-like solid (49). The hypothesis of secondary phase precipitation was confirmed by XRD analysis (Figure 1). The first indication of a new phase during reaction in NaNO3 solution was the appearance of a peak at 15° 2θ after reaction for 484 h (expt. M2, Figure 1f). The intensity of the 15° peak increased with time, while the intensities of the original metaschoepite peaks at 12.0° and 24.4° decreased. Resuspension of the residual solids from experiment M2 in 10 mM NaNO3 solution (expt. M3) and the reaction of metaschoepite with 5 mM NaF (expt. M11) resulted in complete transformation to the same secondary phase (Figure 1g, h). Because F- was not present in experiments M2 and M3 and MES was not present in experiment M11, the transformation can be attributed to the effect of Na+ and not F- or the MES buffer. The observation of a secondary phase during metaschoepite reaction in CsNO3 solution was not only more rapid but also more subtle, with the dominant metaschoepite peak shifting from 12.0° to 11.9° and new peaks appearing at 23.9° and 28.3° (Figure 1i). An examination of a review of XRD patterns of Ucontaining minerals (50) indicated that clarkeite (Na[(UO2)O(OH)]‚H2O) was the only phase that was both consistent with the diffraction pattern and could have formed given the compositions of the solutions. The two dominant peaks (15.0°, d ) 5.903 Å, (003); 26.43°, d ) 3.37 Å, (101)) are in agreement with a calculated pattern for clarkeite (51), but the dominant peak in the calculated pattern (27.85°, d ) 3.20 Å, (012)) is absent in the observed patterns. The calculated pattern for clarkeite is similar, but not identical, to the reference pattern (pdf card 43-0347) for anhydrous sodium diuranate (Na2U2O7) (38). Clarkeite, like metaschoepite, has a structure consisting of sheets of edge- and corner-sharing uranyl bipyramidal polyhedra, but in clarkeite the interlayer space is occupied by Na+ and water molecules (51). The XRD pattern does not definitively identify the new solid as clarkeite, and so we will refer to the clarkeite-like phase as NaUOH. The transformation of metaschoepite to a clarkeite-like solid has also been observed in aqueous systems with high pH and Na+ concentration (52) even in the presence of high carbonate concentrations (35, 53). No reference pattern for the phase that formed from metaschoepite reaction in CsNO3 could be found, but it seems plausible that this phase incorporates Cs+ in the interlayer; we will refer to this phase as CsUOH. Na- and Cs-containing U(VI) oxide hydrates have recently been synthesized at hydrothermal conditions with compositions of Na2[(UO2)3O3(OH)2] (29) and Cs3[(UO2)12O7(OH)13](H2O)3 (28); however, the X-ray diffraction patterns of these hydrothermal phases are very different from those of the secondary phases formed in this study. The XRD patterns of the phases formed in this study are also different from those of anhydrous Na- and Cs-containing uranates (R-Na2UO4, Na2U2O7, Cs2UO4, and Cs2U2O7) recently studied by Volkovich and co-workers (54, 55). A series of phases may exist from the highly hydrated phases formed in this work at ambient temperature, to the less hydrated hydrothermally synthesized phases, and ultimately to the anhydrous uranates (e.g., Na2U2O7) synthesized at very high temperatures. Raman spectra provide additional information regarding transformations suggested by changes in solution chemistry and XRD (Figure 3). The Raman spectra of unaltered metaschoepite and NaUOH are clearly different, although both have two peaks corresponding to the symmetric stretch 174

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FIGURE 3. Raman spectra of synthetic metaschoepite (MS-Na), synthetic NaUOH from reaction of residual materials from experiment M2 for 1348 h in 10 mM NaNO3 buffered at pH 6 with 5 mM MES (expt. M3), and solids from metaschoepite reaction for 1082 h in 10 mM NaNO3 (expt. M4) and for 2740 h in 100 mM CsNO3 (expt. M8) both buffered at pH 6 with 5 mM MES. The dominant peaks in the 800-900 cm-1 region are attributed to the symmetric uranyl stretch for uranyl groups in different coordination environments. of the linear UO22+, which can have energies in the range 900-750 cm-1 (41). The splitting of this Raman band indicates that while U remains present as UO22+ in bipyramidal coordination, the UO22+ moiety is present in two sites with different equatorial coordination environments (39). The difference in the two coordination environments is greater in NaUOH than in metaschoepite. The material reacted in CsNO3 solution has splitting of the symmetric UO22+ stretch into as many as four separate peaks. Lowering of the UO22+ symmetry as a result of secondary phase formation may also make the antisymmetric stretch Raman active (41, 54). Recent Raman spectroscopy studies of anhydrous Na- and Cscontaining uranates observed both symmetric and antisymmetric UO22+ stretches (54, 55); however, these solids were synthesized at high temperatures and have compositions and structures unlike those of the low-temperature NaUOH and CsUOH phases formed in this work. Transformations of schoepite and metaschoepite following the incorporation of cations into interlayers between sheets of uranyl polyhedra have been observed previously. When contacted with Ca(NO3)2 or CaCl2 solutions of 10 mM or greater, schoepite was transformed to becquerelite within 1-3 months (19, 22-24), a time-scale comparable to that of the present work. Exposure of schoepite to 1 M KCl completely transformed the solid to compreignacite within 3 months (23), and 1 week of exposure to BaCl2 and Pb(NO3)2 at 60° C yielded billietite (Ba(UO2)6O4(OH)6‚8H2O) and wo¨lsendorfite ((Ca,Ba)xPb7-x[(UO2)14O19(OH)4]‚12H2O) (22). Synthetic Mg, Mn, and Ni uranyl oxide hydrates have also been formed from synthetic schoepite through contact with 0.5 M metal salt solutions at 60° C for 2 weeks (32). Scanning electron micrographs of metaschoepite (MSNa) before and after reaction are shown for the MS-Na material used in the current work as well as material from an earlier synthesis following the same procedure (Figure 4). These electron micrographs suggest an increase in particle size with reaction in NaNO3 solution. Elemental analysis of the solids using energy-dispersive X-ray analysis was performed, but it was impossible to distinguish the Na or Cs signal associated with the solid phase from the signal contributed by entrained water from the batch reactor solution. The effects of particle size and surface area on U solubility will be discussed in a following section. Uptake of Sodium and Cesium Ions. To confirm that the secondary phases observed by XRD and Raman spectroscopy

FIGURE 4. Scanning electron micrographs of filtered and dried materials before and after reactions of metaschoepite in aqueous solution: (a) stock suspension of synthetic metaschoepite (MSNa), (b) stock suspension of earlier synthesis of metaschoepite that also used NaOH, (c) solids after MS-Na reaction for 2015 h in unbuffered 35 µM NaNO3 solution, and (d) solids after reaction of material from earlier metaschoepite synthesis for 944 h in 100 mM NaNO3 solution buffered with 5 mM MES. had incorporated Na+ and Cs+ it is desirable to have a measurement of changes in the dissolved concentrations of Na+ and Cs+. Since the direct measurement of small (µMscale) changes in a large (mM-scale) background was not feasible, an indirect measurement approach based on electroneutrality was employed. To maintain charge neutrality of the solid phase, the incorporation of Na+ or Cs+ into metaschoepite must result in the loss of a positively charged species such as a proton (eq 2a) or uranyl ion (eq 2b).

(UO2)8O2(OH)12‚10H2O(s) + 8Na+ )

8Na[(UO2)O(OH)]H2O(s) + 8H+ (2a)

3(UO2)8O2(OH)12‚10H2O(s) + 16Na+ ) 16Na[(UO2)O(OH)]H2O(s) + 8UO22+ + 24H2O (2b) At all conditions, electroneutrality of the dissolved phase must hold (eq 3, only dominant species for pH ) 6, PCO2 ) 10-3.5 atm are shown).

[Na+] + [Cs+] + [H+] + 2[UO22+] + [UO2OH+] + 2[(UO2)2(OH)22+] + 2[(UO2)3(OH)42+] +

[(UO2)3(OH)5+] + [(UO2)4(OH)7+] ) [OH-] + [HCO3-] +

[NO3-] (3)

It is assumed that nitrate is unreactive in the system and that its concentration is constant. For each sample, the concentrations of all dissolved species except Na+ or Cs+ are calculated based on the measured pH and [U]diss. The concentration of Na+ or Cs+ is then calculated by difference. This approach was used to track the incorporation of Na+ and Cs+ into the solid phase during reaction in the electrolyte addition experiments. Figure 5 shows the changes in pH and [U]diss together with the calculated uptake of Na+ or Cs+ during two batch experiments. In the first hour, rapid uptake of Na+ and Cs+ was accompanied by decreasing pH and decreasing

FIGURE 5. Evolution of [U]diss (b), Na+ or Cs+ (0) lost from solution by uptake into the solid, and pH (-) following the addition of (a) 10 mM NaNO3 (expt. E2) and (b) 10 mM CsNO3 (expt. E4) to metaschoepite suspensions that had been preequilibrated in ultrapure water for 43 days. [U]diss. Following this initial phase, the pH decreased slightly while continuing Na+ or Cs+ incorporation was balanced by release of U to solution. While the electrolyte-addition experiments were useful in tracking the incorporation of Na+ and Cs+ into the solid phase, it is interesting that these experiments did not result in the transformation of metaschoepite to NaUOH or CsUOH phases observed in the metaschoepite-addition experiments. The amount of Na+ or Cs+ taken up by the solid corresponds to Na (or Cs)-to-U molar ratios in the solid of only 0.07 to 0.1, far less than the 1:1 ratio in clarkeite, and XRD analysis of the solids showed no evidence for formation of secondary solid phases. The principal differences between the electrolyte-addition and metaschoepite-addition experiments were the absence of a pH buffer, lower Na+ or Cs+ to U ratios, and the synthesis method of the metaschoepite used (Table 1). The pH decrease that occurred in the absence of a pH buffer appears to have prevented the solution from reaching supersaturation with respect to NaUOH or CsUOH. Calculated ion activity products for NaUOH and CsUOH for the solution compositions in the electrolyte addition experiments were always lower than solubility products of NaUOH and CsUOH estimated in the next section. Lower Na+ or Cs+ to U ratios also make the formation of NaUOH and CsUOH phases less energetically favorable. In addition, the starting material for the electrolyte addition experiments was prepared using TBAOH as the base to avoid the presence of Na+. Thus the Na+ and Cs+ uptake by the MS-TBA may be analogous to cation exchange with clay minerals; Cs+ has a particularly strong affinity for binding at interlayer sites of phyllosilicates (56, 57). Equilibrium Solubility of Uranyl Oxide Hydrates. The influence of Na+ and Cs+ on U(VI) oxide hydrate solubility must be considered in relation to the equilibrium solubility of metaschoepite and schoepite phases. Solubility (i.e., the equilibrium value of [U]diss) corresponds to the dissolution reaction (eq 4), which is often abbreviated (eq 5) with the VOL. 38, NO. 1, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 3. Schoepite Equilibrium Constants experimental conditions material

pH

electrolyte

MS-Na MS-TBA synthetic synthetic synthetic synthetic synthetic synthetic formed on UO2(s) crystalline amorphous

5.5-6.1 6.0-6.2 4.7, 6.3

none none 0.5 M NaClO4

ambient ambient argon

2.8-4.6 4.5-5.5 5.0-10.0 3.3, 8.4 3.3, 8.4 6.0-9.0 6.0-9.0

0.1 M NaClO4 0.1 M NaClO4 1 mM NaCl 1 M NaCl 1 M NaCl 0.5 M NaClO4 0.5 M NaClO4

ambient nitrogen argon oxygen/nitrogen oxygen/nitrogen nitrogen nitrogen

a

Values extrapolated to zero ionic strength by authors.

b

atmosphere

(UO2)8O2(OH)12‚10H2O(s) + 16H+ ) 8UO22+ + 24H2O (4)

Ksp,ms )

{UO22+} {H+}2

9

5.38-5.46 5.48-5.54 4.70 4.81b 4.73-5.14 5.13 5.20 5.38 5.73 5.97 6.33

present work present work (30) (45) (33, 58) (61) (46) (37) (37) (59) (59)

be estimated. The solids obtained at the conclusion of experiment M3 have an XRD pattern that exhibits only peaks of a clarkeite-like phase. The solution composition observed in this experiment can be used to obtain a solubility product (eq 7)

(5) Ksp,NaUOH ) (6)

Solubility products can only be calculated from data obtained at equilibrium; unfortunately, very few of the experiments can be unambiguously interpreted as having reached equilibrium with a single solid phase. Identification of equilibrium conditions is based on stable [U]diss and pH together with XRD patterns and Raman spectra that suggest the presence of only one solid phase. The assumption of equilibrium is most reliable for the experiments using the MS-TBA metaschoepite. Stable concentrations are observed at the conclusion of experiment M9 (conducted without any electrolyte) and after the 43 days of pre-equilibration performed in preparation for the electrolyte addition experiments (experiments E1-4). From the final conditions of these five experiments (Table 1), the average value of log Ksp,ms is 5.52 with a range of values from 5.48 to 5.54. Reported values for the solubility product of schoepite range from 104.70 to 106.33 (Table 3). The wide range of published constants arises from variations in the sets of dissolved phase U(VI) reactions and equilibrium constants considered, the method used to correct values to zero ionic strength, particle size effects, and possibly the influence of Na+ (as observed in this study). The final dissolved U concentrations are lower in metaschoepite addition experiments conducted in the presence of Na+ and Cs+ than in experiments conducted in ultrapure water. This observation has two possible explanations. In the first, the incorporation of monovalent cations into the interlayer space leads to a new crystal structure that is more stable than the original metaschoepite. In the second, the incorporation of Na+ and Cs+ accelerates crystal growth and leads to the formation of larger crystals, which are more stable but retain the original metaschoepite structure. An explanation based on the formation of new phases with Na+ and Cs+ incorporation is consistent with the XRD and Raman evidence for new (non-schoepite) phases. This explanation is also consistent with the observed formation of the minerals becquerelite and compreignacite after initial schoepite formation during UO2(s) weathering and corrosion (7, 8). By examining the solution conditions for which solids show complete alteration to a sodium or cesium uranyl oxide hydrate, preliminary solubility products for these phases can 176

ref

Calculated from ∆Gf° values determined by calorimetry.

associated solubility product (eq 6).

UO3‚2H2O(s) + 2H+ ) UO22+ + 3H2O

Log Kspa

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 1, 2004

{UO22+}{Na+} {H+}3

(7)

of 108.81 corresponding to the reaction for clarkeite dissolution (eq 8).

3H+ + Na(UO2)O(OH)(s) ) Na+ + UO22+ + 2H2O

(8)

The composition and solubility product expression for clarkeite are equivalent to those of the sodium diuranate trihydrate on a half mole basis (0.5Na2U2O7‚3H2O). In a previous study of the dissolution of the U(VI) silicate mineral soddyite conducted in our laboratory (49), clarkeite formation was modeled using a solubility product of 109.02. The reaction of metaschoepite in 5 mM NaF solution also produces a solid that has only clarkeite peaks in its XRD pattern. A solubility product of 107.65 is calculated based on the final solution composition from that experiment, although this lower value would be highly sensitive to any errors in the formation constants for uranyl-fluoride complexes. A solubility product for a clarkeite-like sodium diuranate trihydrate phase (expressed as 0.5Na2U2O7) of 1012.55 has been previously determined for high pH and high Na+ conditions (52, 53). Using critically reviewed free energies of formation for the anhydrous sodium uranate Na2U2O7, a solubility product of 1011.3 is calculated for 0.5Na2U2O7 (45). These previously reported solubility constants are much higher than those determined in the present work and would significantly overpredict [U]diss at the conditions of the current experiments. The differences among reported solubility products most likely results from the large differences in experimental conditions, especially pH. The dominant dissolved species at the conditions of previous studies will be different than those at the conditions of the current study, and the equilibrium constants used for dissolved U(VI) hydrolysis and carbonate species may not be applicable across a wide pH range and the same set of constants may not be used in each study. Dı´az Arocas and Grambow (30) determined a solubility product of 107.13 for the phase NaO.33UO3.16‚2H2O, but the XRD pattern of this solid is unlike that determined in the current work. In the case of the cesium uranyl oxide hydrate no reference XRD pattern could be found. Since the structure and stoichiometry of this solid are not known, we base our calculations for this solid on an analogy to clarkeite, with a

FIGURE 6. Calculated uranium solubility as controlled by metaschoepite, clarkeite, and CsUOH using the constants determined in this work. Calculations are for conditions of 25 °C, 10-3.5 atm PCO2, 1 mM total uranium, 0.01 M ionic strength, and 0.01 M total Na and Cs for clarkeite and CsUOH calculations, respectively. corresponding dissolution reaction (eq 9) and solubility product (eq 10).

3H+ + Cs(UO2)O(OH)(s) ) Cs+ + UO22+ + 2H2O Ksp,CsUOH )

{UO22+}{Cs+} {H+}3

(9) (10)

The final solution conditions of experiments M7 and M8 then correspond to values of Ksp,CsUOH of 108.61 and 109.27. Verification of a solubility product for this phase requires both characterization of the solid and determination of [U]diss under varying solution conditions (especially pH). Nonetheless, the potential effects of Na+ and Cs+ on U(VI) oxide hydrate solubility can be examined for the constants obtained in this study (Figure 6). Another possible factor affecting solubility is particle size, which can be significant for particles smaller than 1 µm or with specific surface area greater than a few m2 g-1. This effect has been qualitatively discussed for schoepite in previous studies (37, 52, 58, 59). The particle size effect can be quantitatively related to the molar surface area s (m2 mol-1) and interfacial free energy γ j (J m-2) as shown in eq 11.

2 γ j 3 log Ksp(s) ) log Ksp(s)∞) + s RT

(11)

Interfacial energies range from 26 mJ m-2 for gypsum to 1600 mJ m-2 for goethite (60). An interfacial energy of 940 mJ m-2 can be calculated for MS-TBA metaschoepite using the critically reviewed value (which is also one of the lowest available) of 104.81 for Ksp(s)∞) and the values of 105.52 for Ksp(s) with s ) 2800 m2 mol-1. With the admittedly simple assumption of spherical particle morphology and an interfacial energy of 940 mJ m-2, a doubling of the particle diameter would lower the schoepite solubility product from 105.52 to 105.16. At pH 6 and 0.01 M ionic strength, this decrease corresponds to a decrease in equilibrium [U]diss from 46.1 to 5.1 µM. While the incorporation of Na+ and Cs+ into the metaschoepite structure can ultimately lead to a new phase, the initial decrease in solubility may be due to a ripening of metaschoepite to larger particles by Na- or Cs-induced mobilization of U or to a lowering of the interfacial free energy by Na+ and Cs+ interaction with the metaschoepite surface. Environmental Implications. The weathering and corrosion of uraninite and spent nuclear fuel can result in a complex series of secondary phases, with the initial precipitation of schoepite and metaschoepite followed by the

formation of more stable phases. This sequence of precipitation, with the precipitation of more soluble phases preceding that of less soluble phases, is termed the Ostwald Step Rule and is driven by the inverse relationship between solubility and the solid-water interfacial energy (60). While schoepite and metaschoepite may be the first phases to form, the composition of the dissolved phase determines the next phases to form in the weathering progression. Clearly Na+ and Cs+ are not inert, background ions in such systems but rather play an integral role in the formation of more stable phases. The formation of Na uranyl oxide hydrates can be expected in systems that store or have received U-containing aqueous solutions with high sodium concentrations. Cs+, including the fission product 137Cs, may also be encountered as a co-contaminant with U, and the incorporation of Cs+ in uranyl oxide hydrates may be a desirable mechanism for retarding Cs+ migration. The dissolution-reprecipitation pattern observed for uranium in batch experiments has implications for the predictions of U mobility in contaminated environments. Estimates of U mobility must consider the full set of reactions and products, including those discussed in this study. While estimates based only on equilibrium may underestimate U solubility because the formation of the most stable phases may be kinetically hindered, estimates that consider metaschoepite but not Na and Cs uranyl oxide hydrates may overestimate U solubility. The progression of U(VI) solid phases from schoepite and metaschoepite to more stable phases is clearly not only an equilibrium process, and the rates of the dissolution and precipitation reactions must be considered.

Acknowledgments Yi-Ping Liu contributed to this work as part of a Summer Undergraduate Research Fellowship. Raman spectra were collected with the assistance of George Rossman and Liz Arredondo. During the completion of this work, D.E.G. was partially supported by a National Science Foundation Graduate Fellowship. The authors thank the four anonymous reviewers whose comments helped improve this manuscript.

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Received for review June 6, 2003. Revised manuscript received October 1, 2003. Accepted October 13, 2003. ES0345672

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