Synthesis and properties of anhydrous rare-earth phosphates, monazite and xenotime: a review.

The synthesis methods, crystal structures, and properties of anhydrous monazite and xenotime (REPO4) crystalline materials are summarized within this review. For both monazite and xenotime, currently available Inorganic Crystal Structure Database data were used to study the effects of incorporating different RE cations on the unit cell parameters, cell volumes, densities, and bond lengths. Domains of monazite-type and xenotime-type structures and other AXO4 compounds (A = RE; X = P, As, V) are discussed with respect to cation sizes. Reported chemical and radiation durabilities are summarized. Different synthesis conditions and chemicals used for single crystals and polycrystalline powders, as well as first-principles calculations of the structures and thermophysical properties of these minerals are also provided.

Effects of NO2 aging on bismuth nanoparticles and bismuth-loaded silica xerogels for iodine capture.

The capture of long-lived radioactive iodine (129I) from oxidizing off-gasses produced from reprocessing used nuclear fuel is paramount to human health and environmental safety. Bismuth has been investigated as a viable iodine getter but the phase stability of bismuth-based sorbents in an oxidizing environment have not yet been researched. In the current work, bismuth nanoparticle-based sorbents, as free particles (Bi-NPs) and embedded within silica xerogel monoliths made with a porogen (TEO-5), were exposed to I2(g) before and after aging in 1 v/v% NO2 at 150 °C. For unaged sorbents, BiI3 was the dominant phase after iodine capture with 8-30 mass% BiOI present due to native Bi2O3 on the surface of the unaged nanoparticles. After 3 h of aging, 82 mass% of the Bi-NPs was converted to Bi2O3 with only a small amount of iodine captured as BiOI (18 mass%). After aging TEO-5 for 3 h, iodine was captured as both BiI3 (26 %) and BiOI (74 %) and no Bi2O3 was detected.". Additionally, bismuth lining the micrometer-scale pores in the TEO-5 led to enhanced iodine capture. In a subsequent exposure of the sorbents to NO2 (secondary aging), all BiI3 converted to BiOI. Thus, direct capture of iodine as BiOI is desired (over BiI3) to minimize loss of iodine after capture.

Dehalogenation reactions between halide salts and phosphate compounds.

Reactions between phosphoric acid [H3PO4] or ammonium hydrogen phosphates [i.e., NH4H2PO4, (NH4)2HPO4] and halide salts can be used to dehalogenate (remove halides from) salt-based waste streams, where the process of removing halides yields products that have more efficient disposal pathways for repository storage. In this context, the term efficiency is defined as higher waste loadings and simplified immobilization processes with potential for recycle of certain salt components (e.g., 37Cl as H37Cl or NH4 37Cl). The main streams identified for these processes are nuclear wastes generated during electrochemical reprocessing of used nuclear fuel as well as used halide salts from molten salt reactor operation. The potential byproducts of these reactions are fairly consistent across the range of halide species (i.e., F, Cl, Br, I) where the most common are hydrogen halides [e.g., HCl(g)] or ammonium halides (e.g., NH4Cl). However, trihalide compounds (e.g., NCl3), nitrogen triiodide ammine adducts [NI3·(NH3) x ], and ammonium triiodide (NH4I3) are also possible. Several of these byproducts (i.e., NCl3, NBr3, NI3, and NH4I3) are shock-sensitive contact explosives so their production in these processes must be tracked and carefully controlled, which includes methods of immediate neutralization upon production such as direct transport to a caustic scrubber for dissolution. Several benefits arise from utilizing H3PO4 as the phosphate additive during dehalogenation reactions for making iron phosphate waste forms including more oxidized iron (higher Fe3+:Fe2+ ratios), higher chemical durabilities, and the avoidance of trihalides, but the byproducts are hydrogen halides, which are corrosive and require special handling.

Role of Zeolite Structural Properties toward Iodine Capture: A Head-to-head Evaluation of Framework Type and Chemical Composition.

This study evaluated zeolite-based sorbents for iodine gas [I2(g)] capture. Based on the framework structures and porosities, five zeolites, including two faujasite (FAU), one ZSM-5 (MFI), one mesoMFI, one ZSM-22 (TON), as well as two mesoporous materials, were evaluated for I2(g) capture at room temperature and 150 °C in an iodine-saturated environment. From these preliminary studies, the three best-performing zeolites were ion-exchanged with Ag+ and evaluated for I2(g) capture under similar conditions. Energy-dispersive X-ray spectroscopy data suggest that Ag-FAU frameworks were the materials with the highest capacity for I2(g) in this study, showing ∼3× higher adsorption compared to Ag-mordenite (Ag-MOR) at room temperature, but X-ray diffraction measurements show that the faujasite structure collapsed during the adsorption studies because of dealumination. The Ag-MFI zeolites are decent sorbents in real-life applications, showing both good sorption capacities and higher stability. In-depth analyses and characterizations, including synchrotron X-ray absorption spectroscopy, revealed the influence of structural and chemical properties of zeolites on the performance for iodine adsorption from the gas phase.

Review of recent developments in iodine wasteform production.

Radioiodine capture and immobilization is not only important to consider during the operation of reactors (i.e., I-131), during nuclear accidents (i.e., I-131 and I-129) or nuclear fuel reprocessing (i.e., I-131 and I-129), but also during disposal of nuclear wastes (i.e., I-129). Most disposal plans for I-129-containing waste forms (including spent nuclear fuel) propose to store them in underground repositories. Here, iodine can be highly mobile and, given its radiotoxicity, needs to be carefully managed to minimize long-term environmental impacts arising from disposal. Typically, any process that has been used to capture iodine from reprocessing or in a reactor is not suitable for direct disposal, rather conversion into a wasteform for disposal is required. The objectives of these materials are to use either chemical immobilization or physical encapsulation to reduce the leaching of iodine by groundwaters. Some of the more recent ideas have been to design capture materials that better align with disposal concepts, making the industrial processing requirements easier. Research on iodine capture materials and wasteforms has been extensive. This review will act as both an update on the state of the research since the last time it was comprehensively summarized, and an evaluation of the industrial techniques required to create the proposed iodine wasteforms in terms of resulting material chemistry and applicability.

Dechlorination Apparatus for Treating Chloride Salt Wastes: System Evaluation and Scale-Up.

This paper describes an apparatus used to remove chlorine from chloride salt-based nuclear wastes from electrochemical reprocessing and/or chloride-based molten salt reactors (MSRs) through dechlorination by reacting the salts with ammonium dihydrogen phosphate (NH4H2PO4 or ADP) at temperatures up to 600 °C to produce NH4Cl as a byproduct. The benefits of removing the Cl from these salts include 37Cl recovery from Cl-based MSR salts, formation of UCl3 from the NH4Cl, as well as removal of Cl from the salts and conversion of the salt cations to oxides to allow for immobilization in a chemically durable iron phosphate waste form. This generation-2 system is an improvement over the generation-1 system and provides a means for scaling up salt throughput as well as NH4Cl recovery. The generation-2 system includes a five-zone furnace so the temperature of the four-zone gradient furnace can be tailored to control the location of NH4Cl condensation on a four-piece fused quartz off-gas system. Both ADP and NH4Cl decomposition reactions include the production of NH3 and acids (i.e., H3PO4 and HCl, respectively), so careful temperature control is needed during the ADP-salt reactions to maximize the NH4Cl production and minimize NH4Cl decomposition. In two sets of experiments run in the generation-1 and generation-2 apparatuses, NH4Cl yields were ≥5.5-fold higher for the new system compared to the original prototype system and the batch sizes can be ≥2.5-fold higher. In addition, some thermodynamic experiments evaluating the reactions of ADP + KCl as well as decomposition of pure NH4Cl were performed to assess the temperatures of the reactions and identify off-gas products.

Iodine Capture with Mechanically Robust Heat-Treated Ag-Al-Si-O Xerogel Sorbents.

Various radionuclides are released as gases during reprocessing of used nuclear fuel or during nuclear accidents including iodine-129 (129I) and iodine-131 (131I). These isotopes are of particular concern to the environment and human health as they are environmentally mobile and can cause thyroid cancer. In this work, silver-loaded heat-treated aluminosilicate xerogels (Ag-HTX) were evaluated as sorbents for iodine [I2(g)] capture. After synthesis of the base NaAlSiO4 xerogel, a heat-treatment step was performed to help increase the mechanical integrity of the NaAlSiO4 gels (Na-HTX) prior to Ag-exchanging to create Ag-HTX xerogels. Samples were characterized by powder X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, Brunauer-Emmett-Teller analysis, gravimetric iodine loading, nanoindentation, and dynamic mechanical analysis. The structural and chemical analyses of Ag-HTX showed uniform distribution of Ag throughout the gel network after Ag-exchange. After I2(g) capture, the AgI crystallites were observed in the sorbent, verifying chemisorption as the primary iodine capture mechanism. Iodine loading of this xerogel was 0.43 g g-1 at 150 °C over 1 day and 0.52 g g-1 at 22 °C over 33 days. The specific surface area of Ag-HTX was 202 m2 g-1 and decreased to 87 m2 g-1 after iodine loading. The hardness of the Na-HTX was >145 times higher than that of the heat-treated aerogel of the same starting composition. The heat-treatment process increased Young's modulus (compressive) value to 40.8 MPa from 7.0 MPa of as-made xerogel, demonstrating the need for this added step in the sample preparation process. These results show that Ag-HTX is a promising sorbent for I2(g) capture with good iodine loading capacity and mechanical stability.

Environmental Remediation with Functional Aerogels and Xerogels.

Several different types of aerogels and xerogels are demonstrated as effective sorbents for the capture and/or immobilization of radionuclides and other contaminants in gaseous form [e.g., Hg(g), I2(g), Xe, Kr] as well as ionic form (e.g., Cd2+, Ce4+, Cs+, Cu2+, Fe2+, Hg2+, I-, IO3 -, Kr, Pb2+, Rb+, Sr2+, 99Tc7+, U6+, Zn2+). These sorbents have unique properties, which include high specific surface areas, high pore volumes, a range of pore sizes, and functionalities that provide methods for binding radionuclides and other contaminants, generally through physisorption, chemisorption, or a combination thereof. This combination of properties and functionalities makes these types of materials ideal for use as sorbents for capturing radionuclides. The primary base materials that will be discussed in this paper include Ag0-functionalized silica aerogels, Ag+-impregnated aluminosilicate aerogels, Ag0-functionalized aluminosilicate aerogels, metal-impregnated (non-Ag) aluminosilicate aerogels and xerogels, sulfide-based aerogels, and carbon-based aerogel composites. For the capture of I2(g), the materials reported herein show some of the highest iodine loadings ever reported for inorganic sorbents. For the capture of ionic species, these materials also show promise as next-generation materials for active radionuclide remediation. This progress report describes materials fabrication, general properties, and environmental remediation applications.

Metal-Organic Framework-Polyacrylonitrile Composite Beads for Xenon Capture.

Mechanically robust forms of HKUST-1 metal-organic frameworks (MOFs) were fabricated by embedding the MOF crystals in a passive polyacrylonitrile (PAN) matrix at different MOF loadings of 10-90 mass %. PAN is highly porous and acts as a scaffold that holds the active MOF adsorbent in place. These MOF-PAN composites were then evaluated for capturing Xe. Data presented herein show that the PAN matrix does not notably interfere with the Xe capture process, where the Xe capacities scale somewhat linearly with the increase in MOF loadings within the composites. Also, γ radiation exposures to the composites revealed that they are highly tolerant to these types of radiation fields.

Gaseous Iodine Sorbents: A Comparison between Ag-Loaded Aerogel and Xerogel Scaffolds.

Silver-exchanged aluminosilicate aerogels and xerogels were investigated as gaseous iodine [I2(g)] sorbents. The structures, morphologies, compositions, and porosities of aerogels (as-made and heat-treated at 350 °C) and xerogels are compared by using powder X-ray diffraction (PXRD), scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, and specific surface area (SSA) as well as pore size analyses. The as-made aerogels, xerogels, and heat-treated aerogels were ion exchanged with Ag in AgNO3 solutions of deionized water and methanol (5:1 by volume), and PXRD patterns showed the presence of nanocrystalline Ag0 after the Ag exchange. Gravimetric iodine loadings of Ag-aerogels and Ag-xerogels were 0.33-0.41 g g-1. The Ag-aerogels without heat treatment showed an ∼8 mass % higher iodine loading than Ag-impregnated xerogels and ∼3 mass % higher than heat-treated Ag-impregnated aerogels. All gels after iodine uptake showed the presence of AgI, indicating chemisorption of iodine to silver. The SSA values of the as-made gels were 420-600 m2 g-1 but decreased significantly to 34-120 m2 g-1 after Ag impregnation and iodine uptake. Overall, changes in physical and chemical properties of aerogels and xerogels after iodine uptake were similar and the differences in iodine loading capacities of the aerogels and xerogels were minimal, providing a driver for using xerogels due to their less complex synthesis process as compared to aerogels.

Dehydration synthesis and crystal structure of terbium oxychloride, TbOCl.

Terbium oxychloride, TbOCl, was synthesized via the simple heat-treatment of TbCl3·6H2O and its structure was determined by refinement against X-ray powder diffraction data. TbOCl crystallizes with the matlockite (PbFCl) structure in the tetra-gonal space group P4/nmm and is composed of alternating (001) layers of (TbO) n and n Cl-. The unit-cell parameters, unit-cell volume, and density were compared to the literature data of other isostructural rare-earth oxychlorides in the same space group and showed good agreement when compared to the calculated trendlines.

Evaluation of Getter Metals in Na-Al-Si-O Aerogels and Xerogels for the Capture of Iodine Gas.

In this paper, sodium aluminosilicate aerogels and xerogels were evaluated as scaffolds for a variety of different getters including Ag+, Cs+, Cu2+, Fe3+, K+, Li+, Rb+, Sb3+, Sn2+, and Sn4+ for the capture of gaseous iodine coming from nuclear facilities. The exchange capacities varied widely from a near complete exchange in the case of Ag+ to much lower exchange levels for some of the Sn compounds [i.e., colloidal SnO2, Sn(II) acetate, and Sn(IV) acetate]. Several of the additives showed great promise at allowing for high iodine loadings in the base materials including the following: AgNO3, colloidal SnO2, Sn(II) acetate, Sn(IV) acetate, Cu(NO3)2, and CuSO4. From the standpoint of iodine uptake as a function of getter loading, Sn4+ was the most promising with a getter utilization (mass of iodine divided by mass of Sn, in atomic %) of 8.4, a chemical uptake of 60.7 mass % (oxygen excluded), and an mI ms-1 (mass of iodine per mass of sorbent) value of 0.881; these are some of the highest values reported to date for inorganic iodine sorbents.

Crystal structures and comparisons of huntite aluminum borates REAl3(BO3)4 (RE = Tb, Dy and Ho).

Three huntite-type aluminoborates of stoichiometry REAl3(BO3)4 (RE = Tb, Dy and Ho), namely, terbium/dysprosium/holmium trialuminium tetrakis(borate), were synthesized by slow cooling within a K2Mo3O10 flux with spontaneous crystallization. The crystal structures were determined using single-crystal X-ray diffraction (SC-XRD) data. The synthesized borates are isostructural to the huntite [CaMg3(CO3)4] structure and crystallized within the trigonal R32 space group. The structural parameters were compared to literature data of other huntite REAl3(BO3)4 crystals within the R32 space group. All three borates fit well into the trends calculated from the literature data. The unit-cell parameters and volumes increase linearly with larger RE cations whereas the densities decrease. All of the crystals studied were refined as inversion twins.

Synthesis and crystal structure of a mixed alkaline-earth powellite, Ca0.84Sr0.16MoO4.

A mixed alkaline-earth powellite, Ca0.84Sr0.16MoO4 (calcium strontium molybdate), was synthesized by a flux method and its crystal structure was solved using single-crystal X-ray diffraction (SC-XRD) data. The compound crystallized in the I41/a space group as with a typical CaMoO4 powellite, but with larger unit-cell parameters and unit-cell volume as a result of the partial incorporation of larger Sr cations into the Ca sites within the crystal. The unit cell and volume were well fitted with the trendline calculated from literature values, and the powder X-ray diffraction (P-XRD) pattern of the ground crystal is in good agreement with the calculated pattern from the solved structure.

Crystal structures and comparisons of potassium rare-earth molybdates KRE(MoO4)2 (RE = Tb, Dy, Ho, Er, Yb, and Lu).

Six potassium rare-earth molybdates KRE(MoO4)2 (RE = Tb, Dy, Ho, Er, Yb, and Lu) were synthesized by flux-assisted growth in K2Mo3O10. The crystal structures were determined using single-crystal X-ray diffraction data. The synthesized molybdates crystallize with the ortho-rhom-bic Pbcn space group (No. 60). Trendlines for unit-cell parameters were calculated using data from the current study. The unit-cell parameters a and c increase linearly whereas b decreases with larger RE cations, based on crystal radii. The unit-cell volumes increase linearly and the densities decrease linearly with larger RE cations. The average distances between the RE cations and the nearest O atoms increase with larger cations whereas the average distances of Mo-O and K-O do not show specific trends.

Crystal structure and chemistry of tricadmium digermanium tetra-arsenide, Cd3Ge2As4.

A cadmium germanium arsenide compound, Cd3Ge2As4, was synthesized using a double-containment fused quartz ampoule method within a rocking furnace and a melt-quench technique. The crystal structure was determined from single-crystal X-ray diffraction (SC-XRD), scanning and transmission electron microscopies (i.e. SEM, STEM, and TEM), and selected area diffraction (SAD) and confirmed with electron backscatter diffraction (EBSD). The chemistry was verified with electron energy loss spectroscopy (EELS).

Syntheses, crystal structures, and comparisons of rare-earth oxyapatites Ca2 RE 8(SiO4)6O2 (RE = La, Nd, Sm, Eu, or Yb) and NaLa9(SiO4)6O2.

Six different rare-earth oxyapatites, including Ca2 RE 8(SiO4)6O2 (RE = La, Nd, Sm, Eu, or Yb) and NaLa9(SiO4)6O2, were synthesized using solution-based processes followed by cold pressing and sinter-ing. The crystal structures of the synthesized oxyapatites were determined from powder X-ray diffraction (P-XRD) and their chemistries verified with electron probe microanalysis (EPMA). All the oxyapatites were isostructural within the hexa-gonal space group P63/m and showed similar unit-cell parameters. The isolated [SiO4]4- tetra-hedra in each crystal are linked by the cations at the 4f and 6h sites occupied by RE 3+ and Ca2+ in Ca2 RE 8(SiO4)6O2 or La3+ and Na+ in NaLa9(SiO4)6O2. The lattice parameters, cell volumes, and densities of the synthesized oxyapatites fit well to the trendlines calculated from literature values.

Supramolecular Assembly of Geometrically Unstable Hybrid Organic-Inorganic Uranyl Peroxide Cage Clusters and Their Transformations.

An aromatic ligand was introduced into the synthesis of a uranyl peroxide polyoxometalate formulated as K32(UO2)19(O2)26(OH)2(C6H4P2O6)4·65H2O that consists of a unique "open oyster" shaped structure (U19) with intramolecular H-bonds. In the solid state, K-π and π-π interactions as well as K-O bonds enable the formation of a supramolecular network between U19 clusters. U19 adopts an incomplete fullerene topology and was utilized as a precursor from which the geometrically favored U24 structure was produced. A potassium-encapsulated U24 structure was obtained upon heating the solution containing U19.

Synthesis and crystal structure of a neodymium borosilicate, Nd3BSi2O10.

A lanthanide borosilicate, trineodymium borosilicate or Nd3BSi2O10, was synthesized using a flux method with LiCl, and its structure was determined from X-ray powder diffraction (XRD) and electron probe microanalysis (EPMA). The structure is composed of layers with [SiO4]4- and [BSiO6]5- anions alternating along the c axis linked by Nd3+ cations between them.