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Structure and Thermochemistry of Perrhenate Sodalite and Mixed Guest Perrhenate/Pertechnetate Sodalite Eric M. Pierce, Kristina I. Lilova, David M. Missimer, Wayne W. Lukens, Lili Wu, Jeffrey P. Fitts, Claudia J. Rawn, Ashfia Huq, Donovan Nicholas Leonard, Jeremy R. Eskelsen, Brian F. Woodfield, Carol M. Jantzen, and Alexandra Navrotsky Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b01879 • Publication Date (Web): 05 Dec 2016 Downloaded from http://pubs.acs.org on December 10, 2016
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Structure and Thermochemistry of Perrhenate Sodalite and Mixed Guest Perrhenate/Pertechnetate Sodalite
4 5 6
Eric M. Piercea‡*, Kristina Lilovab‡, David M. Missimerc, Wayne W. Lukensd, Lili Wub, Jeffrey Fittse, Claudia Rawnf, Ashfia Huqg, Donovan N. Leonardf, Jeremy R. Eskelsena, Brian F. Woodfieldh, Carol M. Jantzeni, and Alexandra Navrotskyb
1 2
7 8 9 10 11 12 13 14 15 16 17 18
a
Environmental Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, MS: 6038, Oak Ridge, TN 37831 b
Peter A. Rock Thermochemistry Laboratory and NEAT ORU, University of California at Davis, Davis, CA 95616 c
Analytical Development Center, Savannah River National Laboratory, Aiken, SC 29808
d
Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720
e
Department of Civil & Environmental Engineering, Princeton University, Princeton, NJ 08544
f
Department of Materials Science & Engineering, University of Tennessee, Knoxville, TN 37996
g
Chemical & Engineering Materials Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831 h i
Chemistry & Biochemistry, Brigham Young University, Provo, UT 84602
Environmental Technology Center, Savannah River National Laboratory, Aiken, SC 29808
19
20 21
KEYWORDS1: sodalite, perrhenate sodalite, pertechnetate sodalite, neutron powder diffraction, x-ray absorption fine structure, high-temperature calorimetry
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1
This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-publicaccess-plan).
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ABSTRACT: Treatment and immobilization of technetium-99 (99Tc) contained in reprocessed
25
nuclear waste and present in contaminated subsurface systems represents a major environmental
26
challenge. One potential approach to managing this highly mobile and long-lived radionuclide is
27
immobilization into micro- and meso-porous crystalline solids, specifically sodalite. We
28
synthesized and characterized the structure of perrhenate sodalite, Na8[AlSiO4]6(ReO4)2, and the
29
structure of a mixed guest perrhenate/pertechnetate sodalite, Na8[AlSiO4]6(ReO4)2-x(TcO4)x.
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Perrhenate was used as a chemical analogue for pertechnetate. Bulk analyses of each solid
31
confirm a cubic sodalite-type structure (ܲ4ത3݊, No. 218 space group) with rhenium and
32
technetium in the 7+ oxidation state. High-resolution nanometer scale characterization
33
measurements provide first-of-a-kind evidence that the ReO4- anions are distributed in a periodic
34
array in the sample, nanoscale clustering is not observed, and the ReO4- anion occupies the
35
center of the sodalite β-cage in Na8[AlSiO4]6(ReO4)2. We also demonstrate, for the first time,
36
that the TcO4- anion can be incorporated into the sodalite structure. Lastly, thermochemistry
37
measurements for the perrhenate sodalite were used to estimate the thermochemistry of
38
pertechnetate sodalite based on a relationship between ionic potential and the enthalpy and Gibbs
39
free energy of formation for previously measured oxyanion-bearing feldspathoid phases. The
40
results collected in this study suggest that micro- and mesoporous crystalline solids maybe viable
41
candidates for the treatment and immobilization of
42
streams and contaminated subsurface environments.
43 44
INTRODUCTION Development of a sustainable nuclear fuel cycle, which must include closing the back-end by
45
recycling and/or disposing of used nuclear fuel, is a key component of the nuclear energy
46
renaissance (12% of the electrical energy worldwide1 and 19% in the United States2). Disposition
99
Tc present in reprocessed nuclear waste
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of radioactive waste generated by the nuclear fuel cycle and nuclear weapons production during
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the Cold War era is one of the most pressing environmental challenges facing the United States
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and the international community.3-4 Furthermore, proposed waste management strategies are
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complicated by the inventory of long-lived radionuclides, such as technetium (99Tc), and the
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time-scales considered for disposal. Since its discovery in 1937 by Perrier and Segre,5-6 the global inventory of 99Tc has increased
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steadily. Technetium-99 (β = 293.7 keV, t1/2 = 2.1 × 105 years), a byproduct of
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fission, comprises a significant component of radioactive waste due to its high fission yield—
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~5%. The world-wide inventory of 99Tc requiring disposition is estimated to have quadrupled to
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~305 MT from 1994 to 2010 because of nuclear energy production.7 Additionally, US weapons
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production sites must dispose of ~5.1 MT of 99Tc (~3.5 MT at the Savannah River Site and ~1.6
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MT at the Hanford Site).8 Treatment and immobilization of
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nuclear waste presents a major challenge because 99Tc volatilizes at the temperatures (~1100 ºC)
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required for vitrification, the preferred international treatment method.8-11 The immobilized
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nuclear waste glass is destined for long-term storage in a geologic repository. The chemistry of
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99
99
235
U and
239
Pu
Tc contained in reprocessed
Tc suggests that under aerobic environmental conditions, the stable
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heptavalent Tc 7+ pertechnetate anion (99TcO4-) is dominant. This oxyanion is soluble and
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readily migrates through the environment because it does not adsorb well onto mineral surfaces,
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soils, or sediments. Because of the long half-life, abundance, and high environmental mobility of
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99
Tc, incorporating it into durable matrices other than glass is an attractive waste management
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strategy.12 For example, recent studies have examined the possibility of incorporating 99Tc in the
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4+ oxidation state into the structure of iron-based minerals.13-19 Although various countries are
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pursuing vitrification as the primary waste management strategy for other radionuclides (e.g.,
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Cs, 90Sr, U-isotopes, etc.), one approach that has been considered previously but not pursued
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for disposition and remediation of TcO4- is encapsulating the radionuclide into micro- and
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mesoporous crystalline solids, such as the feldsphathoid phase sodalite.
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Micro- and meso-porous solids represent a family of >150 crystalline phases, which support a
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variety of industrial processes (petrochemical cracking, ion exchange for water softening and
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purification, and gas separation). These porous materials contribute an estimated $350 billion to
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the global economy as part of the world’s chemical industry. The porous structure consists of a
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three-dimensional (3D) framework composed of alternating TO4 (T = Al or Si) tetrahedral units
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that share corner oxygens. The 3D framework structure contains a pore or cavity system that can
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expand (microporous = 2.5 to 20 Å; mesoporous = 20 to 500 Å) to encase various guest anions
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and organic molecules by cooperative changes in the T-O-T bond angle. For example,
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aluminosilicate sodalites, both natural and synthetic, can vary widely in composition but have the
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general formula of M8(Al6Si6O24)X2, where M is a monovalent cation (such as Cs+, K+, Na+, etc.)
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and X can vary between monovalent or divalent anions (such as OH-, Cl-, Br-, I-, MnO42-, ReO4-,
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or theoretically TcO4-).20-31
85
Here we use Re as a nonradioactive analogue for
99
Tc, because under oxidizing conditions
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both elements are oxyanions and they have similar metal oxygen bond lengths (Tc–O = 1.702 Å;
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Re–O = 1.719 Å) and ionic radii (TcO4- = 2.52 Å; ReO4- = 2.60 Å).7,32-34 However, under
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reducing conditions it is easier to reduce Tc in comparison to Re from 7+ to 4+ because of the
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difference in standard reduction potential of ReO4-/ReO2 = 0.510 V versus TcO4-/TcO2 = 0.738
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V.8,35-36 Thus, the use of Re as a nonradioactive analogue for
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oxidizing conditions where both species are expected to remain in the 7+ oxidation state.
99
Tc is only applicable under
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Results collected by Dickson et al.
37-39
suggest that the ReO4- anion—and by analogy the
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TcO4- anion—is incorporated into the sodalite β-cage and that anion selectivity for the sodalite
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β-cage is size-dependent. However, the conclusions are based on bulk characterization results,
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specifically changes in the chemical composition and bulk X-ray powder diffraction spectra. The
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data collect by Dickson et al.
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regarding the location and distribution of the ReO4- anion in the crystalline matrix. Additionally,
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the key question of whether or not the TcO4- anion can be incorporated into the sodalite structure
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also remains elusive.
37-39
provide key insights, but do not provide definitive evidence
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The primary objective of the present study is to definitively determine the location and
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distribution of ReO4- anion in the perrhenate sodalite structure and to demonstrate that the
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pertechnetate anion can be incorporated into the sodalite structure. An additional objective is to
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estimate the thermochemistry of pertechnetate sodalite, which is a key data set for evaluating
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sodalite as a potential host matrix for the highly mobile TcO4- anion. To fulfill the
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aforementioned objectives, we synthesized and characterized the structure of perrhenate sodalite
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using a combination of time-of-flight (TOF) neutron powder diffraction (NPD) and aberration-
107
corrected high annular angular dark field (HAADF) scanning transmission electron microscopy
108
(STEM). Additionally, we synthesized and characterized the oxidation state of Re and 99Tc in a
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mixed guest perrhenate/pertechnetate sodalite using x-ray absorption spectroscopy, extended x-
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ray absorption fine structure, and x-ray powder diffraction. Lastly, calorimetric measurements
111
for the perrhenate sodalite were used to estimate the thermochemistry of pertechnetate sodalite
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based on a relationship between ionic potential and the enthalpy and Gibbs free energy of
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formation for previously measured oxyanion-bearing feldspathoid phases.
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EXPERIMENTAL Synthesis of Perrhenate Sodalite and Mixed Guest Perrhenate / Pertechnetate Sodalite.
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Perrhenate sodalite was synthesized using hydrothermal methods by treating Zeolite 4A with 8M
117
NaOH in the presence of excess sodium perrhenate at 225°C and 400 psi in an autoclave for 7 d
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(168 h). The mixed guest perrhenate/pertechnetate sodalite was also prepared hydrothermally
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using a similar synthesis approach as above with a 11:1 mole ratio of NaReO4 (0.021 moles) to
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NaTcO4 (0.0019 moles). For additional details on the synthesis see the Supporting Information
121
section.
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X-ray Diffraction. The powder XRD spectrum of the homogenized perrhenate sodalite sample
123
was measured with a Panalytical X’Pert PRO diffractometer using CuKα radiation (λ = 1.54060
124
Å). Data were collected in 0.017° steps over the 2θ range 5–110°. The powder XRD spectrum
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for the mixed guest perrhenate/pertechnetate sodalite sample was measured with a Bruker D8
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Advance x-ray diffractometer using CuKα radiation (λ = 1.54060 Å). The samples were ground
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in an agate mortar and pestle, mixed with a 1:10 collodion/amyl acetate mixture, and smeared
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onto a square glass slide. The XRD data were collected in 0.02° step size and a dwell time of 1 s
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over the 2θ range 5–70°.
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X-ray Absorption Spectroscopy. Rhenium x-ray absorption fine structure (XAFS) analysis
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was conducted by placing approximately 200 mg of sample in a Teflon holder sealed with
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Kapton tape. The bulk Re LII-edge (11,959 eV) X-ray Absorption Near Edge Structure (XANES)
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spectrum of the perrhenate sodalite was collected in transmission at Stanford Synchrotron
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Radiation Lightsource (SSRL) on beamline 11-2. The beamline configuration consisted of a
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cryogenically cooled Si(220), φ = 90°, double-crystal monochromator with the second crystal
136
detuned by 70% to reduce the harmonic content of the beam. Incident and transmitted beam
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intensity was determined using nitrogen-filled ion chambers. Data were normalized and
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corrected for self-absorption using Athena.40
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Technetium XAFS analysis was conducted by mixing 100 mg of mixed Re/Tc sodalite with
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100 mg of chloride sodalite and adding the 200 mg mixture to a Teflon holder sealed with
141
Kapton tape. The bulk Tc K-edge (21,047) XANES spectrum of the mixed guest
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perrhenate/pertechnetate sodalite was collected in fluorescence at the National Synchrotron Light
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Source on beamline X27A with a HPGe detector. Data were averaged using Athena and are not
144
corrected for detector dead time or self-absorption because neither effect was significant at the
145
detector count rates and pertechnetate concentrations used in this study (1 mol % pertechnetate in
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the mixed guest perrhenate/pertechnetate sodalite).
147
For additional details on the XAS spectrum fitting see Supporting Information.
148
Neutron Powder Diffraction. Time-of-flight powder neutron diffraction (PND) data were
149
collected using 1.312 gram samples of sodium perrhenate [Na8(AlSiO4)6(ReO4)2] contained in an
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8 mm diameter vanadium sample can at 298 K. The PND patterns were collected using the
151
POWGEN (BL-11A) neutron powder diffractometer at the Spallation Neutron Source (SNS) at
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Oak Ridge National Laboratory, Oak Ridge, TN. Diffraction profiles were collected using center
153
wavelengths 1.066 Å and 2.665 Å, providing a d-spacing range from 0.57 to 6.18 Å. Rietveld
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refinements of the data were performed using the GSAS software package along with the
155
EXPGUI interface.41-42 The bound coherent scattering lengths 3.62 ± 0.02 fermi (fm) for sodium,
156
3.449 ± 0.005 fm for aluminum, 4.149 ± 0.001 fm for silicon, 9.2 ± 0.2 fm for rhenium, and
157
5.803 ± 0.004 fm for oxygen. The large bound coherent scattering length of oxygen allows the
158
oxygen positions to be determined using neutron diffraction with more accuracy than with XRD.
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Additionally, the difference in the bound scattering length for the aluminum, rhenium, and
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silicon atoms allows these elements to be distinguished better using neutron powder diffraction
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than with XRD. The atomic structure for Na8(AlSiO4)6(ReO4)2 previously reported was used as
162
the starting model for calculating the diffraction patterns.29-30
163
Microscopy imaging. Atomic resolution aberration-corrected STEM images were obtained
164
with a Nion Ultra STEM 60-100 electron microscope. This was equipped with a cold field
165
emission gun and operated at 100kV with a 3rd generation C3/C5 aberration corrector. The
166
aberration-corrected HAADF STEM images were performed with a probe current of 80 pA.
167
Beam damage was readily apparent when imaging in the STEM (see Fig. SI2) and electron dose
168
was minimized by using fast scan speeds at the largest field of view that atomic columns could
169
still be resolved. For additional details on beam damage see Supporting Information. Perrhenate
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sodalite powder was suspended in 50 mL of isopropyl alcohol and a 10 µL droplet was cast onto
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a 3mm copper lacey carbon TEM grid and the solution was allowed to evaporate. The drop cast
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TEM grid was then prebaked in a vacuum oven station at 160°C for 8 hours at ~10-6 torr and
173
then allowed to cool under vacuum for 10 hours. STEM imaging was performed on a single
174
grain of perrhenate sodalite that was positioned on the lacey carbon in a crystallographic
175
orientation close to [111] zone axis. STEM stage tilts were then used to properly orient the
176
sample into a [111] zone axis for atomic column resolution imaging. The [111] zone axis was
177
selected because it was ideal for observing the ReO4- anion present inside the sodalite β-cage.
178
Additional details on aberration-corrected HAADF—also referred to as Z-contrast imaging—and
179
scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) measurements
180
are provided in the Supporting Information.
181
Calorimetry. High temperature oxide melt solution calorimetry was performed using a Tian
182
Calvet twin calorimeter.43-44 Samples in the form of pellets (between 4 and 6 mg) were dropped
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from room temperature (298 K) into the molten 2PbO·B2O3 at the calorimeter temperature in a
184
platinum crucible. Air was flushed over the solvent at 90 mL/min. The calorimeters were
185
calibrated using the heat content of 5 mg α–Al2O3 pellets.
186
A preliminary furnace test was performed before conducting the calorimetry measurements to
187
verify the complete dissolution of the sample. Pellets of approximately 5 mg were prepared and
188
dropped in molten lead borate solvent (2PbO•B2O3), maintained at 973 K in a furnace. The
189
dissolution process started immediately and finished in a minute. After quenching the melt, no
190
undissolved material was found.
191
The drop solution enthalpy, the standard entropies, and the enthalpies of formation from
192
elements for perrhenate sodalite, nepheline, NaReO4, and component oxides are shown in Table
193
SI9. The enthalpies of formation from elements of the component oxides and sodium salt are
194
taken from Robie and Hemingway 45 or calculated from FactSage 46 (NaReO4). The enthalpies of
195
formation of the perrhenate sodalite from components and from elements (reactions 2 and 3) are
196
calculated using the thermodynamic cycles shown in Table SI9:
( ) ( ) +6SiO ( s,298.15K ) → Na AlSiO ( ReO )
(
3Na2O s,298.15K + 2Na ReO4 s,298.15K + 3Al2O3 s,298.15K 197
2
(
8
)
(
4 6
)
) (1)
4 2
(
)
(
8Na s,298.15K + 6Al s,298.15K + 6Si s,298.15K + 2Re s,298.15K 198
(
)
(
+16O2 g,298.15K → Na8 Al6 Si6O24 ReO4
) ( s,298.15K )
) (2)
2
199
201
RESULTS AND DISCUSSION Characterization of Perrhenate Sodalite. In this section we discuss the characterization
202
results obtained from perrhenate sodalite synthesized hydrothermally, to confirm the rhenium
203
oxidation state, to determine the crystal structure, and to definitively determine the location and
200
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distribution of ReO4- anions. For additional details on the hydrothermal synthesis, particle size,
205
and chemical composition see Supporting Information Fig. SI1 and Table SI1.
206
The Re L2-edge X-ray absorption near-edge spectroscopy (XANES) spectra of the four
207
standards are shown in Fig. SI3.35 Two major changes are observed in the rhenium standard
208
reference spectra as one proceeds from Re metal (Re [0] oxidation state) to ReO4- (Re [7+]
209
oxidation state). First, the absorption edge shifts to higher energy because the binding energy of
210
the electron increases as the formal oxidation state increases; there are fewer electrons to screen
211
the charge of the nucleus from the 2p electrons. Second, the area of the large peak at the
212
absorption edge—the “white line,” which is associated with the 2p to 5d transition—increases as
213
the rhenium oxidation state increases because the area is proportional to the number of vacancies
214
in the 5d orbitals (Fig. SI3). Analysis of the perrhenate sodalite XANES spectrum indicates that
215
only ReO4- is present (Fig. SI3) but the fit using the standard reference spectrum suggests that
216
KReO4 is less than optimal (Table SI2). The discrepancy is due to the difference between the
217
local environments of ReO4- in perrhenate sodalite and KReO4, as discussed below.
218
In addition to these major differences, the XANES region just above the edge contains features
219
due to extended x-ray absorption fine structure (EXAFS), especially those caused by multiple
220
scattering. The major changes are clearly seen in Fig. SI3 as the spectrum from standard
221
compounds transition through the range of oxidation states from the 4+ (ReO2) to 7+ (KReO4).
222
The smaller EXAFS contributions may be seen in the size and spacing of the features at energies
223
above that of the white line. As noted above, the EXAFS contributions result in slightly different
224
spectra for KReO4 versus perrhenate sodalite. Although both materials contain the tetrahedral
225
coordinated ReO4- anion, in the KReO4, the potassium ion interacts strongly with the ReO4-
226
anion (the only anion present in ReO4-); in perrhenate sodalite, the sodium ions interact with both
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the negatively charged sodalite framework and the ReO4- anion. The resulting weakening of the
228
interaction between ReO4- and the sodium ions is reflected in the decrease of the Re–O bond
229
distance in perrhenate sodalite, 1.729(7) Å, versus NaReO4, 1.728(2) Å,47 and in KReO4,
230
1.723(4) Å48-49 (Fig. 1 and Table SI3). The observed shortening of the Re–O bond length in the
231
sodalite crystal structure is consistent with the shortening of the Mn–O bond length in
232
permanganate sodalite.31,50
233
234
235
Fig. 1. Re L2-edge EXAFS spectrum of perrhenate sodalite, Na8[AlSiO4]6(ReO4)2. The EXAFS
236
data and fit are depicted as a solid gray line and black circles, respectively. Fit range: 2 < k < 11
237
(a); 0.8 < R < 2.0 (b); the number of independent points was 8.6, and the number of parameters
238
was 4.
239
A combination of powder x-ray diffraction (pXRD) (not shown) and neutron powder
240
diffraction (NPD) (Fig. 2) measurements were performed on the perrhenate sodalite sample. The
241
crystallography data are provided in Table 1. The refined atomic positions, site occupancies, and
242
atomic displacement parameters are given in Table SI4 for the NPD results. It was possible to
243
refine the anisotropic displacement parameters for all of the atoms with the exception of 11 ACS Paragon Plus Environment
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aluminum with the NPD results (see Table SI5). For the aluminum atom, the atomic
245
displacement parameter was ~5 times smaller than the silicon atom; one explanation for this
246
difference is that portion of the aluminum sites have been replaced by silicon atoms, which has a
247
smaller neutron cross-section. The data were subsequently refined with the aluminum site
248
occupied by both aluminum and silicon atoms, which resulted in site occupancies of 83(7)%
249
aluminum and 17(7)% silicon. The refined value of Uiso only slightly increased to 0.47(9) × 100,
250
and the χ2 decreased slightly to 2.109. The formula obtained by refining the site occupancy of
251
the oxygen atom, labeled as O2 in Table SI4 – SI6, is Na8(AlSiO4)6(ReO3.75)2, compared with
252
Na8(Al0.83Si1.17O4)6(ReO3.75)2 when the aluminum site is shared by both aluminum and silicon.
253
Similar to the NPD, the pXRD results indicate a sodalite-type structure with a P 43n (No. 218)
254
space group and lattice parameter a = 9.15283(8) Å (Table SI6). These bulk measurements are
255
consistent with the results obtained previously by Mattigod and colleagues.29-30
256
257 258
Fig. 2. Rietveld refinement profiles of powder neutron diffraction data for perrhenate sodalite,
259
Na8[AlSiO4]6(ReO4)2. The neutron diffraction data were collected using two different center
260
wavelengths to access different d-spacing ranges (d-spacing from 0.5 to 3.0Å on [a] and d-
261
spacing from 1.1 to 6.2Å [b]). 12 ACS Paragon Plus Environment
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262 263 264 265 266
Table 1. Refinement details and crystal data for perrhenate sodalite, Na8[AlSiO4]6(ReO4)2, determined by neutron and x-ray powder diffraction. Refinement details and crystal data determined from x-ray powder diffraction data collected on mixed perrhenate/pertechnetate sodalite, Na8[AlSiO4]6(ReO4)2-x(TcO4)x.
Refinement wRp Rexp χ2 Variables Crystal Data Crystal system Space Group a, Å Volume, Å3 Z
Na8[AlSiO4]6(ReO4)2 Neutron X-ray 2.40% 7.50% 1.40% histogram 1 1.75% 2.13% histogram 2 2.111 18.4 64 28
Na8[AlSiO4]6(ReO4)2-x(TcO4)x X-ray 11.91% 4.33
Cubic
Cubic
Cubic
P43n , number 218
P43n , number 218
a
a
9.1553(2) 767.40(3)a 1
9.1544(2) 767.18(3)a 1
7.618 23
P43n , number 218 9.155(6)a 767(2)a 1 Na8[AlSiO4]6(ReO4)1.48(TcO4)0.5
Formula Na8[AlSiO4]6(ReO3.75)2 Na8[AlSiO4]6(ReO3.71)2 2 Formula weight 1390.5 1389.4 1339.4 Calculated density, 2.899 g/cm3 3.009 3.007 a Estimated standard deviation for the lattice parameters and volume reported as 3σ. Other results from the NPD and XRD refinements are reported as 1σ. 267
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The structure drawn using the refined atomic positions is shown in Fig. SI4. The silicon and
269
aluminum atoms are both tetrahedrally coordinated, and the Al–O bond length is 1.613(2) Å,
270
compared with the Si–O bond length of 1.719(2) Å. The tetrahedra link together to form a
271
framework with the sodium atoms located in channels along the z direction. The rhenium atom is
272
located in the center and at the corners of the unit cell and is surrounded by the partially occupied
273
oxygen (labeled as O2 in Table SI4 – SI6) at a bond distance of 1.655(6) Å. The Re–O bond
274
distance may be compared to previously published values (1.695(7) Å) obtained using a bench-
275
top XRD system.29-30 The Ueq for O2 is large; however, this is most likely due to the dynamic
276
disorder of the site as previously observed in NH4TcO4.51 This disorder, which is caused by
277
circular oscillating movements (libration) of oxygen atoms around Re, artificially shortens the
278
observed Re–O distance. When corrected for the large thermal parameters of O2 using the
279
“riding model,”52 the Re–O bond distance is 1.741 Å, which is within error of the distance
280
determined by EXAFS and 0.01 to 0.02 Å longer than the distances in KReO4 and NaReO4,
281
which are not corrected for libration.
282
To better understand the location and distribution of the ReO4- anion in perrhenate sodalite, a
283
combination of high-resolution nanometer scale characterization techniques were employed (i.e.,
284
SAED, atomic force microscopy [AFM], and aberration-corrected HAADF STEM). A grain of
285
perrhenate sodalite oriented along the [111] crystallographic plane was used to image atomic
286
columns. Fig. 3 shows a wider view of the grain with beam damage, the raw, and processed
287
images. In the acquired raw image, the ReO4- atomic columns are clear with the fainter signal
288
emanating from the Na, Al, Si, O atoms contained in the sodalite framework
289
this aberration-corrected HAADF-STEM image also illustrates that the ReO4- anions are evenly
290
distributed in the sample rather than clustered in certain locations. To further corroborate the 14
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aforementioned conclusion, nanometer scale SAED and AFM measurements were also
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performed to confirm the crystal structure (Fig. SI5 and SI6). Both SAED and AFM
293
measurements showed that the perrhenate sodalite crystals are cubic and have lattice spacings
294
that are consistent with the values measured in the bulk sample (see Supporting Information).
295
Multi-slice frozen phonon model simulations were conducted with the Quantitative STEM
296
(QSTEM) program to provide additional insight into the aforementioned STEM results.55
297
Simulations were performed on an area that was x = 73.235 Å, y = 73.235 Å, and z = 1475.2 Å.
298
Output from the simulations consist of images at varying thicknesses and included 20 nm, 60
299
nm, and 147 nm. The simulation parameters include a box size of 38.98 Å with a scan window
300
105 pixels × 105 pixels, window size of 25.6 Å × 25.6 Å, 8 slabs and 140 slices per slab
301
resulting in a slice thickness of 1.3078 Å. Results illustrate that the background increases with
302
increasing thickness (Fig. 4). However, the ReO4- atomic columns are still clearly visible in a
303
QSTEM simulation image at 147 nm thick with Poison noise added. The results from these
304
simulations compare well with the experimental images collected and provide further indication
305
that the atomic columns observed in Fig. 3 arise from the ReO4- anion.
306
Collectively these results provide clear evidence that the ReO4- anions are uniformly
307
distributed throughout the sample, nanoscale clustering is not observed, and ReO4- occupies the
308
center of the perrhenate sodalite framework. This is consistent with the XAFS and diffraction
309
results.
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Fig. 3. Aberration-corrected HAADF STEM images showing atomic columns in the perrhenate
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sodalite along the [111] crystal face. Image (a) wider field of view, (b) is the raw image of scan
314
area, and (c) is the raw image after being cropped and processed using a Gaussian blur and
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histogram/gamma adjustment.
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Fig. 4. Output images from Quantitative STEM simulations of perrhenate sodalite along the
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[111] crystal face for three different sample thicknesss; (a) = 20 nm, (b) = 60 nm, and (d) = 147
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nm, with Poisson noise included in each image. For comparison, Poisson noise was both
320
excluded and included in the 147 nm thick images, (c) and (d) respectively.
321 322
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Characterization of Mixed Guest Perrhenate/Pertechnetate Sodalite. A mixed guest
324
perrhenate/pertechnetate sodalite was synthesized (see Supplemental Information) and then was
325
characterized by XAFS, which shows that Tc and Re are both present in the 7+ oxidation state,
326
scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), and pXRD.
327
Here we focus the XAFS discussion on technetium; the Re L2-edge XAFS spectra and results
328
are given in Fig. SI1 and Tables SI2 and SI3. Analysis of the XANES spectrum of TcO4- in the
329
mixed guest perrhenate/pertechnetate sodalite is almost identical to that of TcO4- adsorbed on ion
330
exchange resin
331
first peak above the edge is slightly narrower for the mixed guest perrhenate/pertechnetate
332
sodalite. Additionally, the EXAFS fitting showed that the number of neighbors and bond
333
distance are in very good agreement with the TcO4- anion. Furthermore, inclusion of the
334
neighboring sodium atoms did not improve the EXAFS fit, evident by the lack of change in the
335
reduced chi-squared value. Collectively, the XANES and EXAFS measurements confirmed that
336
99
56
(Tc 7+ oxidation state) (Fig. 5 and Table SI7). The main difference is that the
Tc and Re were in the 7+ oxidation state.
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339
340 341
Fig. 5. Tc K-edge XANES (a) and EXAFS (b, c) spectra of the Na8[AlSiO4]6(ReO4)1.48(TcO4)0.52
342
sample. The EXAFS data and fit are depicted as a solid grey line and black circles, respectively.
343
Fit range: 2 < k < 14; 1.0 < R < 3.0; the number of independent points was 16.9, the number of
344
parameters was 4, and R factor = 0.003.
345 346
The SEM-EDS measurements confirmed that the structure and size of the crystals were
347
consistent with the perrhenate sodalite sample discussed in the previous section (Fig. SI7). The
348
pXRD data shown in Fig. 6 illustrates a broadening of the diffraction peaks with the addition of
349
TcO4-. The observed peak broadening is most evident at 23.78 °2θ when compared with the 19
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pXRD spectrum for Na8[AlSiO4]6(ReO4)2 and Na8[AlSiO4]6(ReO4)2-x(TcO4)x. Structural
351
refinement of the mixed guest perrhenate/pertechnetate sodalite pXRD spectrum is consistent
352
with a mixed sodalite sample that contains ~26% TcO4- and 74% ReO4- in the sodalite cage
353
structure, suggesting a stoichiometry of Na8[AlSiO4]6(ReO4)1.48(TcO4)0.52. The refinement and
354
crystal data results are provided in Table 1 and the structural parameters are provided in Table
355
SI8.
356
357 358
Fig. 6. Structural refinements for powder x-ray powder diffraction of mixed guest
359
perrhenate/pertechnetate sodalite, Na8[AlSiO4]6(ReO4)1.48(TcO4)0.52 (a). A comparison of the
360
pXRD
361
Na8[AlSiO4]6(ReO4)1.48(TcO4)0.52 is also provided (b).
spectra
for
perrhenate
sodalite,
Na8[AlSiO4]6(ReO4)2,
and
362 363
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Chemical Stability of Perrhenate and Mixed Guest Perrhenate/Pertechnetate Sodalites.
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Estimating the thermodynamic stability of pertechnetate sodalite is a key component in
366
evaluating the long-term stability; therefore, the heat capacity and enthalpy of formation were
367
measured for perrhenate sodalite and these results are discussed next. The heat capacity data
368
between 2 and 300 K were fit and used to calculate the thermodynamic functions. The standard
369
molar entropy of both perrhenate sodalite and NaReO4 was calculated from the heat capacity
370
measurements using the following equation: T
Sm =
371
∫ 0
C p,m T
dT
(3)
372
as 1,190.77 and 152.43 J/mol•K (Table 2). Recalculating the enthalpy of formation for
373
perrhenate sodalite from nepheline rather than the components results in a ∆Hneph,comp of–3.76 ±
374
1.55 kJ/mol.
375 376 377
Table 2. Drop solution enthalpies and enthalpies of formation of perrhenate sodalite, recalculated for four oxygen basis. Compound ∆Hds, kJ/mol dehydrated ∆Hf,comp, kJ/mol ∆Hf,el, kJ/mol Na8[AlSiO4]6(ReO4)2 215.35 ± 2.02 –136.17 ± 0.70 –2437.45 ± 2.82 Na2O –112.86 ± 0.97b –414.80 ± 0.30 Al2O3 107.45 ± 0.76c –1675.70 ± 1.30 SiO2 39.70 ± 1.00d –910.70 ± 1.00 NaReO4 126.55 ± 0.55 –1036.00 ± 1.00 a All errors reported are 2σ (i.e., 2 standard deviations from the mean). b Average drop solution enthalpy from Fialips et al. 2001, Kiseleva et al. 2001, and Kiseleva et al. 1996. c Drop solution enthalpy of Al2O3, which is an average of the values measured over 5 years in The Peter A. Rock Thermochemistry Laboratory. d Average drop solution enthalpy from Kiseleva et al. 1996, Chai and Navrotsky 1993, and Trofymluk et al. 2005. e The results are calculated for the stoichiometric compound and for the composition, obtained from the microprobe or elemental analysis. 21
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The enthalpy of formation from components of perrhenate sodalite is slightly more negative
379
than that of the nepheline: -136.17 ± 0.70 and -132.41 ± 1.34 kJ/mol, respectively. Additionally,
380
the enthalpy of formation of the perrhenate sodalite from the NaReO4 and nepheline can be
381
calculated using the drop solution enthalpies and the following reactions:
382
(
6NaAlSiO4 + 2Na ReO4 ↔ Na8 AlSiO4 ReO4 6
)
2
(4)
383
This results in an enthalpy of reaction of -22.57 ± 9.03 kJ/mol. Recalculated on a 4-oxygen basis
384
(as used for nepheline), the reaction is
385
1 1 NaAlSiO4 + Na ReO4 ↔ Na8 AlSiO4 ReO4 6 3 6
(
)
2
(5)
386
and the drop solution enthalpy is -3.76 ± 1.51 kJ/mol. Thus, the incorporation of NaReO4 into the
387
nepheline structure is slightly exothermic (e.g., stabilizing).
388
The standard entropies of the three compounds were used to calculate the entropy of
389
formation, according to reaction 5 as 23.30 J/mol•K (recalculated to a 4-oxygen basis). The
390
Gibbs Free Energy of formation at 298.15 K was determined to be -10.71 kJ/mol.
391
The data discussed in the previous sections illustrate that TcO4- can be incorporated into the
392
sodalite structure; however, to evaluate the environmental stability of pertechnetate sodalite
393
requires an estimate of the ∆Hf and ∆Gf. To determine the ∆Hf and ∆Gf for pertechnetate
394
sodalite, we use an approach that relates the ionic potential (i.e., ratio of charge to anion radius)
395
with the ∆Hf and ∆Gf for oxyanion-bearing aluminosilicate sodalites. The ∆Hf, ∆Gf, ∆Sf,
396
thermodynamic cycles, ionic radius, and ionic potentials used to develop the estimates are
397
provided in Tables SI10 – SI12.
398
Fig. 7 shows a plot of ∆Hf and ∆Gf as a function of ionic potential for feldspathoids phases.
399
The ionic potential is 0.397 Å-1 for Na8[AlSiO4]6(TcO4)2.00 and resulted in ∆Hf = -815.92 ± 14.20 22
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kJ/mol at 298 K. This value is similar to ∆Hf = -814.02 ± 10.68 kJ/mol at 298 K measured for
401
Na8[AlSiO4]6(ReO4)2.00. The error in the ∆Hf values for Na8[AlSiO4]6(TcO4)2.00 value is from the
402
errors of the slope and the intercept of the linear fit, which depends on the two standard
403
deviations of the experimental ∆Hf for the anion containing feldspathoids.
404
405
406
Fig. 7. Enthalpy (a) and Gibbs free energy (b) of formation as a function of ionic potential. The
407
line represents a linear regression to the data. The equations of the lines for ∆Hf,comp and ∆Gf,comp
408
are provided in each plot.
409
All Gibbs energies of formation from components are calculated using the standard equation
410
∆Gf,comp = ∆Hf,comp – 298•∆Sf,comp. Based on the ionic potential x = 0.397 Å-1, the Gibbs energy
411
of formation from components at 298 K of the pertechnetate sodalite will be -879.93 ± 10.80
412
kJ/mol (Fig. 7). This value is also similar to that of Na8[AlSiO4]6(ReO4)2.00 (-878.82 ± 10.68
413
kJ/mol). Although the entropy of the Na8[AlSiO4]6(TcO4)2.00 is expected to be similar to that of
414
the Na8[AlSiO4]6(ReO4)2.00 given the similarity in structure, it was not used to calculate the
415
Gibbs energy of formation. The Gibbs free energy of formation value for Na8[AlSiO4]6(TcO4)2.00 23
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416
was estimated from the fit of the four Gibbs energies of formation of the anion-based sodium
417
aluminosilicates. It is also important to note that the entropy of formation for nitrate cancrinite
418
was estimated to be ~1,000 J/mol•K, which is similar to the low temperature heat capacity values
419
measured for nosean, Na8[AlSiO4]6SO4, carbonate cancrinite, Na8[AlSiO4]6CO3, and
420
Na8[AlSiO4]6(ReO4)2.00. Lastly, the difference in both enthalpies and entropies of formation
421
originates from the sodium salt values. Because the ReO4- anion has a smaller ionic potential,
422
whereas the other compounds used in the analysis are higher than TcO4- (see Table SI12), the
423
existing data bracket the TcO4- anion, thus this ∆Hf and ∆Gf estimation approach represents an
424
interpolation rather than an extrapolation.
425
In closing, these results conclusively show that both the ReO4- and TcO4- anion are
426
incorporated into the sodalite β-cage. Furthermore, the thermochemical stability is similar for the
427
two solids. On the basis of the present work, we can say that sodalite minerals and more
428
generally micro- and mesoporous materials offer a potentially viable immobilization option for
429
the treatment of complex reprocessed nuclear waste streams that contain 99Tc.
430
Environmental Implications. At former nuclear weapons production sites, such as the
431
Hanford site, technetium represents one of the most problematic contaminants to treat because of
432
its complex chemistry in tank waste, its volatility at the high temperatures required for
433
vitrification, and its high mobility in subsurface systems.57 Currently, a large inventory of
434
technetium is being stored in high-level tank waste or present in subsurface systems as a result of
435
intentional and unintentional discharges. Because high-level tank waste streams contain anions,
436
such as NO3- and NO2-
437
pretreatment process that destroys NO3- or NO2- and concentrates TcO4- may be required prior to
37-38
, which can out compete with TcO4- for the sodalite β-cage, a
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immobilizing TcO4- in a sodalite waste form. Pretreatment options include the application of the
439
Fluidized Bed Steam Reformer Technology12 which directly destroys NO3- and NO2-, or the
440
existing vitrification facilities. For example, the off-gas liquid waste stream downstream of the
441
vitrification melter in the low-activity waste portion of the Hanford Waste Treatment and
442
Immobilization Plant will be enriched in TcO4- and other volatile anions (e.g., SO42-)57, and offer
443
an opportunity to immobilize TcO4- into a sodalite waste form. Additionally, at the Hanford site
444
intentional and unintentional discharges of alkaline tank solutions to the subsurface may have
445
resulted in the formation of pertechnetate-bearing feldspathoid phases. Previous investigations
446
demonstrated that when simulated tank waste reacts with Hanford sediments, a range of
447
feldpathoid phases form including sodalite.28,58 This study provides the data needed to identify
448
the chemical conditions required to maximize TcO4- incorporation into sodalite as well as the
449
ability to predict the long-term stability of this phase in environmental systems.
450
451 452
ASSOCIATED CONTENT Supporting Information
453
The Supporting Information is available free of charge on the ACS Publications website.
454
Details of materials synthesis and characterization (e.g., chemical composition, particle size,
455
and surface area), specifics on the XAFS and EXAFS spectra analysis, supplemental STEM and
456
TEM information, AFM topographic images and height profiles, and thermochemistry details,
457
and list of ionic radii and ionic potentials used for oxyanions.
458
AUTHOR INFORMATION
459
Corresponding Author 25
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460
*Corresponding Author:
[email protected], Phone: (865) 574-9968, Fax: (865) 576-8646
461
Author Contributions
462
EMP and KL are co-first authors and contributed equally to this study, EMP conceived and
463
organized the research study; DM, EMP, and CMJ synthesized the samples tested; KL, LW,
464
BW, and AN collected and analyzed the thermochemistry data; WWL, JF, and EMP collected,
465
analyzed, and interpreted the x-ray absorption data; EMP, AH, and CR collected, analyzed, and
466
interpreted the neutron powder diffraction data; EMP, DL, and JE collected, analyzed, and
467
interpreted the STEM and QSTEM results; and JE and EMP collected and analyzed the AFM
468
results. The manuscript was written through contributions of all authors. All authors have given
469
approval to the final version of the manuscript.
470
Funding Sources
471
Support was provided by the Subsurface Biogeochemical Research Program under the US
472
Department of Energy (DOE) Office of Biological and Environmental Research, Climate and
473
Environmental Sciences Division. Portions of this research were supported by Heavy Element
474
Chemistry Program under the Office of Basic Energy Sciences (BES) Chemical Sciences,
475
Biosciences and Geosciences Divisions and the Tank Waste Management Technology
476
Development Program under the Office of Environmental Management.
477
ACKNOWLEDGMENT
478
The powder neutron diffraction data was collected on POWGEN (BL-11A) neutron powder
479
diffractometer at Oak Ridge National Laboratory (ORNL) Spallation Neutron Source (SNS)
480
under proposal numbers IPTS 5857 and 7810. The XAFS data was collected on beam line 20-ID-
481
B at the Advanced Photon Source (APS) at Argonne National Laboratory (ANL) under proposal 26
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number GUP-24070, National Synchrotron Light Source (NSLS) at Brookhaven National
483
Laboratory (BNL) beam line X27A, and at the the Stanford Synchrotron Radiation Lightsource
484
(SSRL). Use of the NSLS, BNL, was supported by the US Department of Energy (DOE), Office
485
of Science, Office of Basic Energy Sciences (BES) under Contract No. DE-AC02-98CH10886.
486
Use of the SSRL, SLAC National Accelerator Laboratory, is supported by the DOE, Office of
487
Science, BES under Contract No. DE-AC02-76SF00515. A portion of this research used
488
resources of the APS, a DOE Office of Science User Facility operated for the DOE Office of
489
Science by ANL under Contract No. DE-AC02-06CH11357. Portions of this work were
490
performed at Lawrence Berkeley National Laboratory under Contract No. DE-AC02-
491
05CH11231. TA portion of this research was performed at ORNL’s SNS was sponsored by the
492
Scientific User Facilities Division, BES, DOE. The ultra STEM imaging was conducted at the
493
Center for Nanophase Material Sciences, which is a DOE Office of Science User Facility. ORNL
494
is managed by UT-Battelle, LLC, for DOE under contract DE-AC05-00OR22725.
495
27
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23. Weller, M.; Wong, G. Intracage reactions in sodalites. Journal of Chemical Society, Dalton Transactions 1990, 593–597. 24. Weller, M. Where zeolites and oxides merge: Semi-condensed tetrahedral frameworks. Journal of Chemical Society, Dalton Transactions 2000, (23), 4227–4240. 25. Trill, H. Sodalite Solid Solution System. Synthesis, Topotactic Transformations, and Investigation of Framewok-Guest and Guest-Guest Interactions. Westfalische Wilhelms Universitat, Munster, 2002. 26. Trill, H.; Eckert, H.; Srdanov, V. Topotactic transformations of sodalite cages: Synthesis and NMR study of mixed salt-free and salt-bearing sodalites. Journal of the American Chemical Society 2002, 124, 8361–8370. 27. Trill, H.; Eckert, H.; Srdanov, V. Mixed halide sodalite solid solution system. Hydrothermal synthesis and structural characterization by solid state NMR. J. Phys. Chem. B 2003, 107, 8779–8788. 28. Rivera, N.; Choi, S.; Strepka, C.; Mueller, K.; Perdrial, N.; Chorover, J.; O'Day, P. A. Cesium and strontium incorporation into zeolite-type phases during homogeneous nucleation from caustic solutions. American Mineralogist 2011, 96, 1809-1820. 29. Mattigod, S. V.; McGrail, B. P.; McCready, D. E.; Wang, L.; Parker, K. E.; Young, J. S. Synthesis and structure of perrhenate sodalite. Microporous and Mesoporous Materials 2006, 91 (1-3), 139-144. 30. McCready, D. E.; Mattigod, S. V.; Young, J. S.; McGrail, B. P. X-ray powder diffraction data for Na8(AlSiO4)6(ReO4)2. International Center for Diffraction Data: Advances in X-ray Analysis 2004, 47, 297-302. 31. Brenchley, M. E.; Weller, M. T. Synthesis and Structruer of M8[AlSiO4]6 (XO4)2, M = Na, Li, K; X = Cl, Mn Sodalites. Zeolites 1994, 14, 1994. 32. Moyer, B. A.; Bonnesen, P. V., Physical Factors in Anion Separations. In Supramolecular Chemistry of Anions; Bianchi, A.; Bowman-James, K.; Garcia-Espana, E., Eds.; Wiley-VCH: New York, 1997; pp 1-44. 33. Marcus, Y. Ionic-radii in aqueous-solutions. Chemical Reviews 1988, 88, 1475-1498. 34. Marcus, Y. Thermodynamics of solvation of ions. Part 5 - Gibbs free energy of hydration t 298. Journal of the Chemical Society Faraday Transactions 1991, 87, 2995-2999. 35. Lukens, W. Dissimilar behavior of technetium and rhenium in borosilicate waste glass as determined by xray absorption spectroscopy. Chemical Material 2007, 19, 559. 36. Wakoff, B.; Nagy, K. L. Perrhenate uptake by iron and aluminum oxyhydroxides: an analogue for pertechnetate incorporation in Hanford waste tank sludges. Environmental Science and Technology 2004, 38 (6), 1765-1771. 37. Dickson, J. O.; Harsh, J. B.; Flury, M.; Lukens, W. W.; Pierce, E. M. Competitive Incorporation of Perrhenate and Nitrate in Sodalite. Environmental Science and Technology 2014, 48, 12851-12857. 38. Dickson, J. O.; Harsh, J. B.; Lukens, W. W.; Pierce, E. M. Perrhenate incorporation into binary mixed sodalites: The role of anion size and implications for technetium-99 sequestration. Chemical Geology 2015, 395, 138-143. 39. Dickson, J. O.; Harsh, J. B.; Flury, M.; Pierce, E. M. Immobilization and exchange of perrhenate in sodalite and cancrinite. Microporous and Mesoporous Materials 2015, 214, 115-120. 40. Ravel, B. ATHENA and ARTEMIS interactie graphical data analysis using IFEFFIT. Physica Scripta 2005, T115, 1007-1010. 41. Larson, A.; Von Dreele, R. General Structure Analysis System (GSAS). LAUR 86-748; Los Alamos National Laboratory: Los Alamos, New Mexico, 1994. 42. Toby, B. EXPGUI, a graphical user interface for GSAS. Journal of Applied Crystallography 2001, 34, 210–213. 43. Navrotsky, A. Progress and New Directions in High-Temperature Calorimetry. Phys. Chem. Miner. 1977, 2 (1-2), 89-104. 44. Navrotsky, A. Progress and new directions in high temperature calorimetry revisited. Phys. Chem. Miner. 1997, 24 (3), 222-241. 45. Robie, R.; Hemingway, B. Thermodynamic properties of minerals and related substances at 298.15 K and 1 bar (105 pascals) pressure and at higher temperatures. US Geological Survey Bulletin: Denver, Colorado, 1995; p 461. 46. Bale, C. W.; Belisle, E.; Chartrand, P.; Decterov, S. A.; Eriksson, G.; Hack, K.; Jung, I.-H.; Kang, Y.-B.; Melancon, J.; Pelton, A. D.; Robelin, C.; Petersen, S. FactSage thermochemical software and databases - recent developments. Calphad 2009, 33 (2), 295-311. 47. Atzesdorfer, A.; Range, K. J. Sodium Metaperrhenate, NaReO4 - High Pressure Synthesis of SingleCrystals and Structure Refinement. Z.Naturforsch.(B) 1995, 50 (9), 1417-1418.
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Structure and Thermochemistry of Perrhenate Sodalite and Mixed Guest Perrhenate/Pertechnetate Sodalite................................................................................................... 1
629
INTRODUCTION ...................................................................................................................... 2
630
EXPERIMENTAL ...................................................................................................................... 6
631
RESULTS AND DISCUSSION ................................................................................................. 9
632
ASSOCIATED CONTENT ...................................................................................................... 25
633
AUTHOR INFORMATION ..................................................................................................... 25
634
ACKNOWLEDGMENT........................................................................................................... 26
635
REFERENCES ......................................................................................................................... 28
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List of Figures:
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Fig. 1. Extended x-ray absorption fine structure of perrhenate sodalite, Na8[AlSiO4]6(ReO4)2. The EXAFS data and fit are depicted as a solid gray line and black circles, respectively. Fit range: 2 < k < 11 (a); 0.8 < R < 2.0 (b); the number of independent points was 8.6, and the number of parameters was 4.Error! Bookmark not defined.
643 644 645 646
Fig. 2. Rietveld refinement profiles of powder neutron diffraction data for perrhenate sodalite, Na8[AlSiO4]6(ReO4)2. The neutron diffraction data were collected using two different center wavelengths to access different d-spacing ranges (d-spacing from 0.5 to 3.0Å on [a] and d-spacing from 1.1 to 6.2Å [b]). ......................................................................... 12
647 648 649 650
Fig. 3. Aberration-corrected HAADF STEM images showing atomic columns in the perrhenate sodalite along the [111] crystal face. Image (a) wider field of view, (b) is the raw image of scan area, and (c) is the raw image after being cropped and processed using a Gaussian blur and histogram/gamma adjustment. ..................................................................... 16
651 652 653 654
Fig. 4. Output images from Quantitative STEM simulations of perrhenate sodalite along the [111] crystal face for three different sample thicknesss; (a) = 20 nm, (b) = 60 nm, and (d) = 147 nm, with Poisson noise included in each image. For comparison, Poisson noise was both excluded and included in the 147 nm thick images, (c) and (d) respectively................ 17
655 656 657 658
Fig. 5. Tc K-edge XANES (a) and EXAFS (b, c) spectra of the Na8[AlSiO4]6(ReO4)1.48(TcO4)0.52 sample. The EXAFS data and fit are depicted as a solid grey line and black circles, respectively. Fit range: 2 < k < 14; 1.0 < R < 3.0; the number of independent points was 16.9, the number of parameters was 4, and R factor = 0.003. ........... 19
659 660 661 662
Fig. 6. Structural refinements for powder x-ray powder diffraction of mixed guest perrhenate/pertechnetate sodalite, Na8[AlSiO4]6(ReO4)1.48(TcO4)0.52 (a). A comparison of the pXRD spectra for perrhenate sodalite, Na8[AlSiO4]6(ReO4)2, and Na8[AlSiO4]6(ReO4)1.48(TcO4)0.52 is also provided (b).................................................................. 20
663 664 665
Fig. 7. Enthalpy (a) and Gibbs free energy (b) of formation as a function of ionic potential. The line represents a linear regression to the data. The equations of the lines for ∆Hf,comp and ∆Gf,comp are provided in each plot. ........................................................................... 23
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List of Tables:
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Table 1. Refinement details and crystal data for perrhenate sodalite, Na8[AlSiO4]6(ReO4)2, determined by neutron and x-ray powder diffraction. Refinement details and crystal data determined from x-ray powder diffraction data collected on mixed perrhenate/pertechnetate sodalite, Na8[AlSiO4]6(ReO4)2-x(TcO4)x. ..................... 13
672 673
Table 2. Drop solution enthalpies and enthalpies of formation of perrhenate sodalite, recalculated for four oxygen basis. ............................................................................................................................................................... 21
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