Hubbry Logo
UranateUranateMain
Open search
Uranate
Community hub
Uranate
logo
8 pages, 0 posts
0 subscribers
Be the first to start a discussion here.
Be the first to start a discussion here.
Uranate
Uranate
Uranate
View on Wikipedia
from Wikipedia
Uranates of calcium, strontium, barium and lead

A uranate is a ternary oxide involving the element uranium in one of the oxidation states 4, 5 or 6. A typical chemical formula is MxUyOz, where M represents a cation. The uranium atom in uranates(VI) has two short collinear U–O bonds and either four or six more next nearest oxygen atoms.[1] The structures are infinite lattice structures with the uranium atoms linked by bridging oxygen atoms.

Uranium oxides are the foundation of the nuclear fuel cycle ("ammonium diuranate" and "sodium diuranate" are intermediates in the production of uranium oxide nuclear fuels) and their long-term geological disposal requires a thorough understanding of their chemical reactivity, phase transitions, and physical and chemical properties.[2]

Synthesis

[edit]

A method of general applicability involves combining two oxides in a high temperature reaction.[3] For example,

Na2O + UO3 → Na2UO4

Another method is the thermal decomposition of a complex, such as an acetate complex. For example, microcrystalline barium diuranate, BaU2O7, was made by thermal decomposition of barium uranyl acetate at 900 °C.[4]

Ba[UO2(ac)3]2 → BaU2O7 ... (ac=CH3CO2−)

Uranates can be prepared by adding alkali to an aqueous solution of a uranyl salt. However, the composition of the precipitate that forms is variable and depends on the chemical and physical conditions used.[3]

Uranates are insoluble in water and other solvents, so pure samples can only be obtained by careful control of reaction conditions.[citation needed]

Formula U-ox. state Space group Symmetry Formula U-ox. state Space group Symmetry Formula U-ox. state Space group Symmetry
Li2UO4 VI α: Fmmm, Pnma

β:

orthorhombic

hexagonal

BaU2O7 VI I41/amd tetragonal Sr2UO5 VI P21/c monoclinic
Na2UO4 VI α: Cmmm

β: Pnma

orthorhombic

orthorhombic

SrU2O7 VI Li6UO6 VI hexagonal
K2UO4 VI α: I4/mmm

β:

tetragonal

orthorhombic

CaU2O7 VI Ca3UO6 VI P21 monoclinic
Cs2UO4 VI I4/mmm tetragonal MgU3O10 VI hexagonal Sr3UO6 VI P21 monoclinic
MgUO4 VI Imma orthorhombic Li2U3O10 VI α: P21/c

β: P2

monoclinic

monoclinic

Ba3UO6 VI Fm-3m cubic
CaUO4 VI R-3m rhombohedral SrU4O13 VI monoclinic NaUO3 V Pbnm orthorhombic
SrUO4 VI α: R-3m

β: Pbcm

rhombohedral

orthorhombic

Li2U6O19 VI orthorhombic KUO3 V Pm3m cubic
BaUO4 VI Pbcm orthorhombic K2U7O22 VI Pbam orthorhombic RbUO3 V Pm3m cubic
Li2U2O7 VI orthorhombic Rb2U7O22 VI Pbam orthorhombic CaUO3 IV cubic
Na2U2O7 VI C2/m monoclinic Cs2U7O22 VI Pbam orthorhombic SrUO3 IV orthorhombic
K2U2O7 VI R-3m hexagonal Li4UO5 VI I4/m tetragonal BaUO3 IV Pm3m cubic
Rb2U2O7 VI R-3m hexagonal Na4UO5 VI I4/m tetragonal Li3UO4 V tetragonal
Cs2U2O7 VI α: C2/m

β: C2/m

γ: P6/mmc

monoclinic

monoclinic

hexagonal

Ca2UO5 VI P21/c monoclinic Na3UO4 V Fm-3m cubic

Uranium(VI)

[edit]

Structures

[edit]
CaUO4 structure[5]
BaUO4 structure[5]

All uranates(VI) are mixed oxides, that is, compounds made up of metal(s), uranium and oxygen atoms. No uranium oxyanion, such as [UO4]2− or [U2O7]2−, is known. Instead, all uranate structures are based on UOn polyhedra sharing oxygen atoms in an infinite lattice.[1] The structures of uranates(VI) are unlike the structure of any mixed oxide of elements other than actinide elements. A particular feature is the presence of linear O-U-O moieties, which resemble the uranyl ion, UO22+. However, the U-O bond length varies from 167 pm, which is similar to the bond length of the uranyl ion, up to about 208 pm in the related compound α-UO3, so it is debatable as to whether these compounds all contain the uranyl ion. There are two principal types of uranate which are defined by the number of nearest-neighbour oxygen atoms in addition to the "uranyl" oxygens.[1]

In one group, including M2UO4 (M=Li, Na, K) and MUO4 (M=Ca, Sr) there are six additional oxygen atoms. Taking calcium uranate, CaUO4, as an example, the six oxygen atoms are arranged as a flattened octahedron, flattened along the 3-fold symmetry axis of the octahedron which also runs through the O-U-O axis (local point group D3d at the uranium atom). Each of these oxygen atoms is shared between three uranium atoms, which accounts for the stoichiometry, U 2×O 6×1/3 O = UO4. The structure has been described as a hexagonal layer structure. It can also viewed as a distorted fluorite structure in which two U-O distances have decreased and the other six have increased.[1]

In the other group, exemplified by barium uranate, BaUO4, there are four additional oxygen atoms. These four oxygens lie in a plane and each is shared between two uranium atoms, which accounts for the stoichiometry, U 2×O 4×1/2 O = UO4. The structure may called a tetragonal layer structure.[1]

MgUO4 structure.[6]

Magnesium uranate, MgUO4, has a quite different structure. Distorted UO6 octahedra are linked into infinite chains; the "uranyl" U-O bond length is 192 pm, not much shorter than the other U-O bond length of 218 pm.[1]

A number of so-called diuranates are known. They fall into two categories, compounds of exact composition, synthesized by combination of metal oxides or thermal decomposition of salts of uranyl complexes and substances of approximate composition, found in yellowcake. The name refers only to the empirical formula, MxU2O7; the structures are completely different from ions such as the dichromate ion. For example, in barium diuranate, BaU2O7, UO6 octahedral units are joined by sharing edges, forming infinite chains in the directions of the crystallographic a and b directions.[4]

Uranates with more complicated empirical formulas are known. Essentially these arise when the cation:uranium ratio is different from 2:1 (monovalent cations) or 1:1 (divalent cations). Charge-balance constrains the number of oxygen atoms to be equal to half the sum of charges of the cations and uranyl groups. For example, with the cation K+, compounds with K:U ratios of 2, 1 and 0.5 were found, corresponding to empirical formulas K2UO4, K2U2O7 and K2U4O13.[7] The uranate structures in these compounds differ in the way the UOx structural units are linked together.

Properties and uses

[edit]
A drum of yellowcake

Yellowcake is produced in the separation of uranium from other elements, by adding alkali to a solution containing uranyl salts.[8]

When the alkali used is ammonia, so-called ammonium diuranate, known in the industry as ADU, is the main constituent of yellowcake. The exact composition of the precipitate depends to some extent on the conditions and anions that are present and the formula (NH4)2U2O7, is only an approximation. The precipitates obtained on addition of ammonia to uranyl nitrate solution under different conditions of temperature and final pH, when dried, were considered as loosely bound compounds with an ammonia/uranium ratio of 0.37 containing varying amounts of water and ammonium nitrate.[9] In other studies it was found to approximate to the gross formulas 3UO3·NH3·5H2O,[10] The asymmetric stretching frequency of the uranyl ion was found to decrease with increasing NH4+ content. This decrease is continuous and no band splitting was observed, indicating that the ammonium uranate system is homogeneous and continuous.[11]

A collection of uranium glassware

ADU is an intermediate in the production of uranium oxides to be used as nuclear fuel; it is converted directly into an oxide by heating. β-UO3 is produced at about 350 °C and U3O8 is obtained at higher temperatures. When the alkali used is sodium hydroxide, so-called sodium diuranate, SDU, is produced. This can also be converted into an oxide. Another choice of alkali is magnesium oxide, making magnesium diuranate, known as MDU.

Oxides and uranates of uranium(VI) have been used in the past as yellow ceramic glazes as in Fiesta and to make yellow-green uranium glass.[12] Uranates are also important in radioactive waste management.[13]

Uranium(V)

[edit]

Several series of uranates(V) have been characterized. Compounds with the formula MIUO3 have a perovskite structure. Compounds MI3UO4 have a defect rock-salt structure. MI7UO6 structures are based on a hexagonally close-packed array of oxygen atoms. In all cases the uranium is at the centre of an octahedron of oxygen atoms. Also MIIIUO4 have been recently synthesized and characterized (MIII=Bi, Fe, Cr etc.).[14][15] Few other compounds of uranium(V) are stable.[3]

Uranium(IV)

[edit]

Barium uranate, BaUO3, is made from barium oxide and uranium dioxide in an atmosphere that contains absolutely no oxygen. It has a cubic crystal structure (space group Pm3m).[16]

References

[edit]

Further reading

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Uranate

View on Grokipedia
from Grokipedia
Uranates are inorganic salts containing oxyanions of , primarily in the +6 , such as [UO₄]²⁻, derived from hypothetical uranic acids. These compounds typically feature coordinated with oxygen in tetrahedral or (dioxo) motifs, forming ternary oxides with metal cations and exhibiting bright yellow to orange hues due to charge-transfer transitions in the . Notable uranates include (Na₂U₂O₇), a key intermediate known as in refining, which is precipitated from leach solutions and calcined to for further processing in the . Ammonium diuranate, similarly, serves as a precursor to (UO₂) powder used in fuel fabrication, involving to control particle morphology and surface area. Historically, uranates of like lead and calcium have been utilized as pigments for vibrant colors in ceramics and glassware, prized for their stability under high firing temperatures despite the associated . Uranates are chemically stable but highly toxic, with uranium's nephrotoxic effects stemming from its affinity for groups in renal tubules, compounded by alpha that increases long-term health risks upon or of particulates. Their synthesis often occurs via from solutions, and structural variations—such as layered perovskite-like forms in alkaline earth uranates—underpin diverse applications, though proliferation concerns limit non-nuclear uses today.

Overview

Definition and Nomenclature

Uranates are inorganic compounds comprising in the hexavalent (+6) bonded to oxygen and paired with cationic species, typically yielding ternary oxides with stoichiometries such as M2UO4 or M2U2O7 (M = monovalent cation). These represent salts of uranic , a with the formula H2U2O7, formed conceptually from trioxide (UO3) hydration or base treatment of salts. The uranate moiety involves uranium(VI) oxyanions, often polymeric in nature, such as the diuranate [U2O7]2-, rather than discrete monomeric forms like [UO4]2-, which appear primarily in specialized contexts like gas-phase ions or certain lattices. Nomenclature follows conventions for oxyanion salts, designating the compound as " uranate" for simpler uranates (e.g., calcium uranate, CaUO4) or incorporating multipliers for polyuranate structures (e.g., diuranate, (NH4)2U2O7). This parallels naming in other oxide systems, emphasizing the uranium-oxygen framework while specifying the counterion; notation (U(VI)) is implied but may be explicit in ambiguous cases involving lower-valent uranium ternaries occasionally termed uranates. Empirical formulas reflect the layered or chain-like (UO22+) cores extended by bridging oxides, distinguishing uranates from uranyl salts or lower-oxidation oxides.

General Physical and Chemical Properties

Uranates(VI) comprise a class of ternary uranium oxides where uranium adopts the +6 oxidation state, commonly represented by formulas such as M2_2UO4_4 (M = alkali or alkaline earth metal) or polymeric variants like diuranates M2_2U2_2O7_7. These compounds manifest as crystalline solids, typically exhibiting yellow to orange coloration attributable to charge-transfer transitions involving the uranyl-like (UO2_2)2+^{2+} moiety, though some, such as calcium uranate, appear white. The uranium atom is coordinated in a distorted octahedral or tetrahedral geometry, featuring two short axial U=O bonds (approximately 1.70–1.80 Å) and longer equatorial U–O bonds (2.20–2.50 Å), which underpin their structural diversity including scheelite-type (tetragonal) and layered polymorphs. Chemically, uranates(VI) display limited solubility in neutral or acidic aqueous media, with dissolution often requiring alkaline conditions to form uranate complexes or high-temperature hydrothermal setups, as evidenced by solubility measurements of calcium uranate at 300 °C yielding concentrations below 10−4^{-4} mol/dm3^3 in pure water. They react with strong acids to liberate uranyl ions (UO22+_2^{2+}), undergoing hydrolysis, but resist basic environments, reflecting their derivation from hypothetical uranic acid H2_2UO4_4. Thermal stability varies by composition; for instance, sodium uranates persist up to 700–800 °C before decomposing to UO3_3 or U3_3O8_8, while thallium uranates remain intact under nitrogen to 660 °C. Certain uranates, particularly those with scheelite structures like MgUO4_4 or CaUO4_4, exhibit intense green fluorescence under ultraviolet light due to forbidden f-f transitions in the uranium(VI) center.

Historical Context

Discovery and Initial Characterization

Uranate compounds, ternary oxides containing in the +6 , were first prepared in the early through the fusion of uranium trioxide (UO₃) with alkali or alkaline earth carbonates. Around 1820, and contemporaries synthesized yellow-orange alkali uranates by heating UO₃ with or carbonates, marking the initial recognition of these materials as distinct from simple uranium oxides. These early syntheses exploited the reactivity of UO₃ under high temperatures, yielding compounds such as (sodium diuranate), which exhibited bright pigmentation suitable for glazes. Further characterization advanced in the 1840s through the work of Eugène-Melchior Péligot, who investigated uranium sesquioxide (U₄O₉) reactions with bases, confirming the formation of uranates as salts analogous to dichromates in structure and reactivity. Péligot noted that these compounds arose from the acidic behavior of uranium oxides toward bases, producing stable, often insoluble salts with empirical formulas like M₂UO₄ or M₂U₂O₇ (M = alkali metal). Initial analyses highlighted their vivid yellow to orange hues, attributed to charge-transfer transitions in the uranate anion, and their resistance to acids except strong mineral acids, distinguishing them from uranyl salts. Early uranates were empirically characterized by solubility tests, thermal stability, and spectroscopic observations, revealing octahedral coordination around hexavalent uranium without the linear uranyl (UO₂²⁺) ion prominent in aqueous uranium(VI) chemistry. These properties facilitated their application in pigments and glass coloration by the mid-19th century, though precise stoichiometries remained debated until later crystallographic studies. Péligot's contributions clarified the +6 oxidation state dominance in uranates, resolving anomalies in oxide basicity and paving the way for systematic uranium oxyanion chemistry.

Key Developments in Synthesis and Study

The synthesis of uranates initially relied on fusion methods, where trioxide (UO₃) was heated with alkali carbonates or hydroxides to form compounds such as sodium uranate (Na₂U₂O₇). These techniques emerged in the , enabling applications like production, though systematic characterization lagged until the 20th century. Ammonium diuranate ((NH₄)₂U₂O₇), a key intermediate in processing, gained prominence during as part of large-scale purification efforts for nuclear materials, precipitated from solutions using gas. This aqueous precipitation method marked a shift from high-temperature fusions to scalable, solution-based syntheses, improving yield and purity for production. In the mid-20th century, detailed studies of uranates began, focusing on phase compositions, thermal stability, and dehydration products through and . These efforts clarified polymorphism in compounds like Na₂U₂O₇ and revealed mixed-valence phases involving U(V) alongside U(VI). Post-1950s advancements included solid-state reactions under controlled atmospheres to isolate pure phases, such as reacting U₃O₈ with metal oxides, and hydrothermal methods for hydrated variants like Na₂U₂O₇·6H₂O. Structural analyses confirmed (UO₂²⁺) coordination in layered motifs, informing thermodynamic models for cycles.

Synthesis Methods

Aqueous and Precipitation Techniques

Aqueous precipitation methods for uranates primarily involve the reaction of salts, such as (UO₂(NO₃)₂) or uranyl acetate, with bases or metal hydroxides in aqueous media to form insoluble, often hydrated, uranate precipitates. These techniques exploit the low of uranates under alkaline conditions, where ions (UO₂²⁺) hydrolyze and condense with counterions to yield compounds like M₂U₂O₇·nH₂O (M = or NH₄). The process is influenced by factors including (typically 7–12), , reagent concentration, and mixing rates, which control particle morphology, purity, and yield. is often followed by , , and treatment to obtain crystalline phases, though the aqueous step yields amorphous or poorly crystalline hydrates. Ammonium diuranate ((NH₄)₂U₂O₇, ADU) is synthesized industrially by adding gaseous or aqueous to solutions, a key step in processing for converting to precursors. The reaction proceeds via rapid at 6.5–8.5 and temperatures of 20–60°C, with yields exceeding 99% under optimized conditions; for example, ultrasound-assisted enhances uniformity and reduces agglomeration. Variations in final and temperature affect composition and , with higher favoring orthorhombic ADU phases. This method's simplicity and high efficiency make it preferable for large-scale purification, though impurities like can co-precipitate if not controlled. For uranates, precipitation occurs by mixing acetate or nitrate solutions with hydroxides (e.g., KOH or NaOH) under aqueous or mildly hydrothermal conditions. uranates such as K₂U₆O₁₉ and K₂U₄O₁₃·2.2H₂O form via reaction of acetate with or solutions at 200°C in sealed vessels, yielding precipitates after cooling and ; the U:K ratio and (10–12) dictate the specific . Similarly, sodium uranate Na₂U₂O₇·6H₂O precipitates from solutions with NaOH under hydrothermal conditions at 200°C for extended periods (e.g., 15 days), producing hydrated crystals confirmed by . Room-temperature analogs exist but often result in less defined phases requiring subsequent aging or heating. These methods highlight the role of size in stabilizing complex uranate anions like U₆O₁₉²⁻. Alkaline earth uranates, such as calcium variants, are prepared via reactions where solutions are combined with suspensions of Ca(OH)₂ or Ca²⁺-containing media, promoting of U(VI)-rich hydroxo-uranates under near-neutral to alkaline . Supersaturated conditions yield precursors like Ca₂(UO₂)₃O₃.₇₅(OH)₂.₅·3.5H₂O, with Ca/U ratios near 1 favoring colloidal intermediates that precipitate as hydrous particles; at 700°C then forms crystalline Ca₂U₃O₁₁. This approach underscores hydrolysis-driven condensation, distinct from simple base addition, and is relevant for waste form studies due to phase stability. and analogs follow similar precipitation with their hydroxides, though less documented in aqueous routes.

Solid-State and Thermal Methods

Solid-state synthesis of uranates typically involves intimate mixing of stoichiometric amounts of metal oxides or carbonates with oxides, such as UO₃ or U₃O₈, followed by high-temperature annealing in controlled atmospheres to promote and phase formation. This method is particularly suited for preparing crystalline alkaline monouranates like AUO₄ (A = Ca, Sr, Ba, Pb), where mixtures are heated to facilitate solid-state reactions yielding single-phase products. For instance, CaUO₄ and BaUO₄ are obtained by calcining appropriate oxide mixtures in air at 900 °C in a . Thermal methods, including and , enable the formation of more complex uranates by prolonged heating, often exceeding 1000 °C, to achieve and minimize impurities. MgUO₄, an orthorhombic uranate, was first prepared via solid-state techniques involving reaction of MgO and UO₃, highlighting the method's efficacy for magnesium-based compounds. High-temperature solid-state reactions have also yielded single crystals of layered uranates, such as Rb₄U₅O₁₇, through careful control of and furnace conditions. Variant approaches combine solid-state mixing with chlorination at intermediate temperatures (e.g., 400–425 °C) to enhance purity and phase selectivity in monouranates like SrUO₄. These techniques contrast with aqueous methods by favoring bulk phase transformations over , often resulting in denser, more stable polymorphs, though they require precise to avoid or side phases like U₃O₈. gravimetric analysis complements these syntheses by delineating stability ranges, as seen in studies of alkaline earth uranates where new phases emerge upon extended heating.

Advanced and Specialized Syntheses

Advanced syntheses of uranates often employ sol-gel techniques to produce microspheres or nanostructured precursors, such as ammonium uranate (NH₄)₂U₂O₇, which serve as intermediates for uranium oxides. In the internal gelation variant, solutions are mixed with organic gelling agents like , followed by heating to induce gelation and precipitation at controlled pH, yielding uniform spherical particles with diameters of 100–1000 μm after thermal treatment at 800–1200°C. This method enhances homogeneity and enables doping with actinides or rare earths, as demonstrated in the preparation of Nd- or Ce-doped ammonium uranate microspheres converted to UO₂ via carbothermic reduction. Hydrothermal methods facilitate the formation of complex uranate structures under elevated temperatures (200–300°C) and pressures (autogenous, ~10–20 MPa) in sealed vessels, promoting of polymorphs or hybrid phases not accessible via ambient . For instance, high-temperature, high-pressure hydrothermal treatment of sources with halides or silicates yields uranyl silicates akin to uranate frameworks, such as novel porous U-based germanates with microporous channels. Similarly, hydrothermal reaction of UO₃ hydrates with KCl in water at 150–250°C produces uranates like K₂U₆O₁₉, characterized by layered uranate sheets. These approaches allow precise control over and morphology, with loadings up to 60 mol% in solid solutions like (Zr,U)SiO₄ analogs. Flux-mediated crystal growth represents a specialized technique for obtaining single crystals of uranates, involving dissolution of uranium oxides in molten salt fluxes (e.g., alkali carbonates or chlorides) at 600–1000°C, followed by slow cooling to nucleate crystals. This method has been applied to actinide uranates, revealing polymorphism in compounds like BaUO₄, and is particularly useful for structural elucidation via X-ray diffraction. Electrochemical variants, such as chronopotentiometry on uranyl electrodes in ammoniacal media, enable in situ deposition of ammonium uranate decorated with graphene oxide, offering potential for composite materials with enhanced electrochemical performance. These techniques prioritize yield and purity over scalability, often requiring inert atmospheres to mitigate uranium's sensitivity to hydrolysis.

Structural Chemistry by Oxidation State

Uranium(VI) Uranates

Crystal Structures and Polymorphism

Uranium(VI) uranates of the general MUO4 (M = divalent cation such as Ca, Sr, Ba) consist of isolated UO4 tetrahedra, with uranium coordinated to four oxygen atoms in a nearly regular tetrahedral geometry, typical of the U6+ without . These tetrahedra are linked via M2+ cations in a framework structure, often analogous to (CaWO4), featuring tetragonal symmetry in the I41/a for larger cations. The U–O bond lengths in the tetrahedra average 1.90–1.95 Å, with slight distortions influenced by the M cation size and coordination. Polymorphism arises primarily from lattice strain due to cation mismatch between M2+ and the UO42– anion, leading to reductions from tetragonal to orthorhombic, rhombohedral, or cubic forms, often stabilized by or synthesis conditions. For calcium uranate (CaUO4), the low-temperature vorlanite adopts a cubic (space group Ia-3), but irreversible transformation to a rhombohedral polymorph (space group R-3m) occurs upon heating above 750 °C, driven by and anion packing rearrangement. Strontium uranate (SrUO4) exhibits a rhombohedral-to-orthorhombic transition, with the orthorhombic β-phase (Pbca) forming under oxidizing conditions or high burn-up in nuclear fuels; this shift involves oxygen vacancy ordering and polyhedral tilting in UO8 units transitional to the ideal tetrahedra. Barium uranate (BaUO4) maintains the undistorted tetragonal structure at ambient conditions, with Ba in irregular 8-fold coordination and minimal polymorphism reported, reflecting better lattice matching. Similar scheelite-type structures occur in PbUO4, while smaller MgUO4 shows greater distortion toward orthorhombic symmetry. In alkali uranates like Na2UO4, rock-salt (Fm-3m) arrangements feature octahedral Na coordination around tetrahedral UO4, with limited polymorphism. Phase transitions in these compounds are often , accompanied by volume changes of 1–3%, and influence stability in high-temperature applications such as nuclear ceramics.

Physicochemical Properties

Uranium(VI) uranates are typically colored yellow to orange due to ligand-to-metal charge transfer bands in the associated with the (UO₂²⁺) moiety. These compounds exhibit high thermal stability, with many decomposing rather than melting at elevated temperatures; for instance, calcium uranate (CaUO₄) remains stable up to approximately 1000°C before decomposing into and . Barium uranate (BaUO₄) demonstrates similar refractory behavior, with measurements conducted up to 1570 K without or decomposition under inert conditions, yielding thermodynamic functions such as and increments for modeling high-temperature applications. Chemically, uranium(VI) uranates are sparingly soluble in neutral and alkaline aqueous media, acting as solubility-limiting phases for U(VI) in high-pH environments such as cementitious waste forms, where calcium uranate controls uranium release at levels below those of sodium or free uranate ions. Solubility increases under acidic conditions due to protonation and dissociation of the uranate structure, but remains low in pure even at elevated temperatures and pressures; for example, calcium uranate solubility at 300°C and 0.5 kbar in pure is on the order of 10⁻⁵ to 10⁻⁴ mol/kg, depending on exact phase purity. They are stable in dry air but can undergo oxidation-reduction reactions or in moist environments, reverting to species. Physical properties vary by cation; alkaline earth uranates like BaUO₄ possess anisotropic coefficients (e.g., average linear expansion of ~10 × 10⁻⁶ K⁻¹ from 300 to 1000 K) and elastic moduli in the range of 100-200 GPa, reflecting scheelite-type structures with rigid uranate frameworks. Density typically ranges from 5-7 g/cm³, influenced by packing efficiency in their layered or three-dimensional uranate networks.
CompoundColorWater SolubilityDecomposition Temperature
Na₂U₂O₇YellowInsolubleHigh temperature (>800°C)
K₂UO₄Orange-yellowInsolubleOn heating (~700-900°C)
CaUO₄YellowInsoluble (low µg/L at 25°C)>1000°C
BaUO₄Insoluble

Representative Compounds

Sodium diuranate (Na₂U₂O₇), often encountered as the hexahydrate Na₂U₂O₇·6H₂O, is a prominent uranium(VI) uranate serving as an intermediate in processing and uranium extraction from ores. This manifests as a yellow-orange solid, insoluble in yet soluble in dilute acids, with applications in ceramics for producing colored glazes. Ammonium diuranate ((NH₄)₂U₂O₇) represents another key diuranate precipitate formed during uranium purification, thermally decomposing to uranium trioxide (UO₃) upon heating, which underscores its role in converting uranyl solutions to solid oxides. Alkaline earth uranates exemplify simple M²⁺UO₄ stoichiometries, where M denotes divalent cations. Calcium uranate (CaUO₄) adopts a trigonal crystal structure in space group R-3m (No. 166), characterized by a three-dimensional network wherein Ca²⁺ cations occupy body-centered cubic sites coordinated to eight O²⁻ anions, while uranyl units integrate into the lattice. Barium uranate (BaUO₄) crystallizes in the orthorhombic space group Pbcm (No. 57), with unit cell dimensions a = 5.744(3) Å, b = 8.136(4) Å, and c = 8.237(3) Å at ambient conditions; its structure features uranyl (UO₂)²⁺ ions equatorially coordinated by four oxygen atoms lying in a shared plane between adjacent uranium centers, forming layered (UO₂O₄)²⁻ sheets interconnected by barium cations. These compounds exhibit thermal stability, with BaUO₄ displaying green luminescence under UV excitation at 4.2 K, shifting toward orange at higher temperatures like 77 K. Magnesium uranate (MgUO₄) and strontium uranate (SrUO₄) follow analogous scheelite-related structures, synthesized via solid-state reactions, and contribute to studies on uranate polymorphism and thermodynamic properties across the series. Lead uranate (PbUO₄) similarly forms layered architectures akin to BaUO₄, with applications in assessing uranium mobility in environmental contexts.

Uranium(V) Uranates

Synthesis and Stability Challenges

The synthesis of uranium(V) uranates requires precise control over redox conditions to stabilize the uncommon U(V) oxidation state, which is prone to disproportionation into U(IV) and U(VI) species or oxidation to U(VI) under ambient oxygen levels. Common approaches involve partial reduction of U(VI) precursors, such as uranyl ions (UO₂²⁺), using reducing agents like zinc under hydrothermal conditions at elevated temperatures (typically 150–250°C) and pressures (autogenous, ~10–20 MPa), yielding mixed-valence compounds like Na₅[U₅O₁₆(OH)₂] from UO₃ or uranyl nitrate starting materials. Electrochemical reduction of U(VI) solutions in the presence of alkali cations, such as lithium, can precipitate lithium uranate(V) phases, often detected via electron spin resonance (ESR) spectroscopy for confirmation of the U(V) state. Solid-state syntheses typically employ high-temperature reactions (500–1000°C) between uranium oxides (e.g., UO₃ or U₃O₈) and oxides or carbonates under low oxygen partial pressures to limit over-oxidation, forming oxygen-deficient or mixed-valence . These conditions, often in molten salts or inert atmospheres, allow intermediate U(V) phases during the oxidation of lower oxides, though pure stoichiometric U(V) like Li₅U₅O₁₇ remain elusive and require annealing to prevent . Stability poses significant challenges, as U(V) uranates are metastable and decompose via (2U(V) → U(IV) + U(VI)) upon dissolution or exposure to reductants/oxidants, or oxidize rapidly in air at temperatures above 200°C. In solid matrices like , U(V) incorporates as a uranate-like moiety, remaining stable under reducing conditions but reverting to soluble U(VI) upon matrix breakdown, with isotopic signatures persisting briefly before full . instability limits handling, with many phases converting to U(VI) uranates upon heating in oxygen or decomposing to binary uranium oxides under vacuum, necessitating inert storage and restricting applications. These factors explain the scarcity of well-characterized pure U(V) uranates, with most reported examples being mixed-valence or structurally incorporated rather than discrete compounds.

Structural Features

Uranium(V) uranates lack the linear (UO₂)²⁺ moiety characteristic of uranium(VI) compounds, instead featuring uranium in octahedral UO₆ coordination. This geometry arises from the d¹ electronic configuration of U(V), which favors symmetric octahedral environments without the strong axial bonding seen in higher oxidation states. X-ray absorption spectroscopy (XAS) and (DFT) calculations confirm this local structure, with U-O bond distances averaging approximately 2.1 , longer than the ~1.7 Å U=O bonds in uranyl groups. Compounds such as CrUO₄ and FeUO₄, where the counter-cation is trivalent (Cr³⁺ or Fe³⁺) to balance the U⁵⁺ charge, adopt an orthorhombic in space group Pbcn (No. 60). These isostructural phases consist of a three-dimensional framework of corner-sharing UO₆ and MO₆ octahedra (M = Cr or Fe), analogous to distorted or rutile-derived arrangements. Powder X-ray diffraction (XRD) reveals lattice parameters for FeUO₄ of a ≈ 5.24 , b ≈ 5.42 , c ≈ 7.62 , with slight variations in CrUO₄ due to differences in M-O bonding. The octahedra alternate in a rock-salt-like ordering, with no evidence of Jahn-Teller distortion at U(V) sites, as confirmed by ⁵⁷Fe for FeUO₄ showing Fe³⁺ in octahedral sites. In contrast, MgUO₄ exhibits a monoclinic structure with space group C2/c, featuring similar UO₆ octahedra but with Mg²⁺ requiring potential mixed U(IV)/U(VI) valence to maintain charge neutrality, as pure U(V) is not thermodynamically favored. X-ray photoelectron spectroscopy (XPS) and XAS indicate partial in MgUO₄, underscoring the role of the counter-cation in stabilizing U(V). Overall, these structural motifs highlight the rarity of stable U(V) in solid-state uranates, confined primarily to systems with redox-compatible transition metals.

Properties and Reactivity

Uranium(V) uranates, such as compounds MUO₃ (M = Na, K, Rb) and divalent metal uranates like MgUO₄, CrUO₄, and FeUO₄, exhibit U⁵⁺ confirmed by , , and , with uranium coordinated in distorted octahedral or perovskite-like structures depending on the cation. These solids display antiferromagnetic ordering at low temperatures, as observed in neutron diffraction studies of NaUO₃, KUO₃, and RbUO₃, with magnetic moments lower than the spin-only value of 2.54 μ_B due to covalency and crystal field effects. Thermodynamic stability increases with the ionic radius of the counter-cation; for divalent metal uranates, enthalpies of formation become more exothermic (e.g., more negative ΔH_f) as the M²⁺ radius grows, rendering CrUO₄ and FeUO₄ energetically favorable relative to component oxides, as determined by high-temperature oxide melt solution calorimetry and density functional theory calculations. Trisodium uranate Na₃UO₄ shows polymorphic behavior, transitioning from a low-temperature disordered cubic phase (metastable) to a semi-ordered high-temperature β-phase in Fd̅3m space group, with heat capacity measurements revealing anomalies near 30 K akin to those in UO₂. Reactivity of U(V) uranates stems from the inherent instability of the U⁵⁺ state, which favors to U(IV) and U(VI) species (2U⁵⁺ → U⁴⁺ + U⁶⁺), a process kinetically accelerated in aqueous or solution environments but suppressed in solid lattices by and cation interactions. Solid-state examples like FeUO₄ demonstrate enhanced U⁵⁺ persistence against changes, relevant to incorporation in iron-bearing minerals for nuclear waste repositories, though exposure to oxidants or reductants can induce decomposition. Unlike stable U(VI) uranates, U(V) variants show limited hydrolytic stability and sensitivity to atmospheric oxygen, often requiring inert handling, with no widespread reports of catalytic or synthetic utility due to lability.

Uranium(IV) Uranates

Formation and Structures

Uranium(IV) uranates are synthesized primarily through high-temperature solid-state reactions involving uranium(IV) oxide (UO₂) and the oxide of an alkaline earth or other metal, conducted under strictly inert or reducing atmospheres to prevent oxidation to higher uranium valence states such as U(V) or U(VI). For instance, barium uranate (BaUO₃) is prepared by reacting UO₂ with barium peroxide (BaO₂) under an argon atmosphere at approximately 900°C, ensuring the maintenance of the U(IV) oxidation state. These conditions are critical, as exposure to even trace oxygen can lead to over-oxidation and formation of mixed-valence or higher-oxidation-state phases. The crystal structures of uranium(IV) uranates typically feature close-packed arrangements of metal-oxygen polyhedra. BaUO₃, a prototypical example, adopts the cubic perovskite structure (space group Pm-3m), consisting of a three-dimensional framework of corner-sharing UO₆ octahedra with barium cations occupying the 12-coordinate sites in the lattice. This structure is analogous to other ABO₃ perovskites, where U(IV) occupies the B-site with a coordination number of six, exhibiting U-O bond lengths around 2.20-2.25 Å as determined by X-ray and neutron diffraction. Slight non-stoichiometry, such as BaUO_{3.023}, may occur due to oxygen incorporation, but the core perovskite motif persists. Other uranium(IV) uranates, though less commonly reported, may exhibit fluorite-related or layered structures depending on the counter-cation, but pure U(IV) phases remain rare compared to mixed-valence lower-valent uranates like Ba₂U₃O₉, which incorporate U(IV) alongside U(VI). In these systems, the U(IV) centers often form distorted octahedral coordinations, influenced by the overall lattice charge balance and synthesis conditions.

Stability and Decomposition

Uranium(IV) uranates, typically realized in mixed-valence frameworks with alkaline earth cations, possess limited thermal stability and are prone to transformations under varying atmospheric conditions. Reduction of triuranates such as A₂U₃O₁₁ (where A = Ca, Sr, or Ba) in an atmosphere containing 7% initiates at 650–950 K, resulting in black residues characteristic of lower-valency uranium phases incorporating U(IV), accompanied by weight losses of approximately 3.5% for the calcium analogue due to oxygen removal. These reduced phases, including novel hexagonal A₂U₃O₉ structures (average U oxidation state ≈ +5.33), revert upon oxidation in dry air, transforming to intermediate A₂U₃O₁₀ (for Ca and Sr) or stable A₂U₃O₁₁ (for Ba), underscoring their instability in oxidizing media and tendency toward phase segregation into uranium dioxide-like U(IV) components and higher-valent . Decomposition pathways often involve or oxygen loss under inert or reducing environments, yielding uranium(IV) oxide (UO₂) and metal oxides as ultimate products at elevated temperatures exceeding 900 K, while exposure to air promotes reoxidation to thermodynamically favored U(VI)-dominant species. Thermal gravimetric and diffraction analyses confirm that synthesis of these compounds requires strictly controlled reducing conditions from higher-valent precursors, as ambient stability is compromised by facile .

Specific Examples

Barium uranate (BaUO₃) is a key example of a uranium(IV) uranate, synthesized by the reaction of and under oxygen-free conditions to maintain the U(IV) . It possesses a perovskite-type structure, often with slight orthorhombic distortion akin to the GdFeO₃ prototype, where uranium(IV) occupies octahedral sites coordinated by six oxygen atoms. This compound demonstrates thermodynamic stability relative to its binary oxide components, with lattice parameters reflecting the accommodation of the large U⁴⁺ cation ( approximately 0.89 Å in octahedral coordination). Strontium uranate (SrUO₃) serves as another representative uranium(IV) uranate, sharing the distorted perovskite structure of BaUO₃ and exhibiting similar stability against decomposition into SrO and UO₂. The smaller Sr²⁺ cation (ionic radius 1.18 Å) compared to Ba²⁺ induces a greater degree of octahedral tilting in the structure, influencing its electronic properties, including potential metallic conductivity due to the d¹ configuration of U⁴⁺. Both BaUO₃ and SrUO₃ highlight the role of alkaline earth cations in stabilizing ternary uranium(IV) oxides, though CaUO₃ is less common owing to lattice strain from the even smaller Ca²⁺. These compounds generally display paramagnetic behavior attributable to unpaired 5f electrons in U⁴⁺, with measurements for BaUO₃ revealing Curie-Weiss-like dependence over a wide range (4.2–300 ), indicative of localized moments without long-range magnetic order at . Limited and reactivity under ambient conditions underscore their utility in studies of actinide , though they decompose upon exposure to oxygen, reverting to higher uranium oxidation states.

Applications and Uses

Role in Nuclear Fuel Processing

In the front end of the , uranates serve as key intermediates during the milling of , where is extracted and concentrated into a transportable form known as . Following , ore is crushed, ground, and leached—typically with for low-carbonate ores or alkaline solutions like for ores—to solubilize as uranyl complexes. The is then recovered via or solvent extraction, after which yields diuranates such as ammonium diuranate ((NH₄)₂U₂O₇) from acid circuits or (Na₂U₂O₇) from alkaline ones. Precipitation of ammonium diuranate, the most common form, occurs by neutralizing uranyl sulfate solutions with gas or ammonium hydroxide, raising the to 6.5–8.0 at 50–85°C over 2–6 hours, consuming approximately 0.22 kg NH₃ per kg U₃O₈ equivalent. For , adjusts the in eluates at 20–80°C. The resulting precipitates are filtered, washed to remove impurities like sulfates and silica, and dried, often followed by at 400–850°C to yield concentrate (U₃O₈) with 70–90% content by weight. This process produces tens of thousands of tonnes annually from over 50 active mills worldwide, enabling efficient shipment to conversion facilities. At conversion plants, uranates are dissolved in , purified through solvent extraction to remove residual impurities (e.g., <0.2%, sodium <0.5%), and transformed into (UF₆) for gaseous enrichment. This step ensures nuclear-grade material, as unrefined uranates contain variable impurities that could affect downstream fuel fabrication into (UO₂) pellets. While primarily an early-stage product, uranates like ammonium diuranate can also form incidentally during storage or reprocessing of spent , influencing protocols.

Material Science and Catalysis

Uranates of , such as lead and iron uranates, form the basis for yellow glazes achieved through oxidation firing of mixtures. At elevated temperatures in processing, converts to uranium uranate phases, enabling vibrant coloration in enamels, , and formulations. In catalysis, bismuth uranate (Bi₂UO₆) functions as a selective heterogeneous catalyst for the oxidative demethylation of to , operating effectively at 400–500 °C in streams. The reaction proceeds via reduction of the uranate by , yielding , CO₂, and H₂O, followed by catalyst reoxidation with oxygen; optimal selectivity occurs with Bi/U ratios near 2. Kinetic studies indicate the rate-determining step involves interaction with lattice oxygen in the uranate structure. While broader applications of uranium oxides in VOC combustion have been explored, specific uranate variants like bismuth uranate highlight their potential in catalysis despite radiological constraints limiting industrial adoption.

Historical and Niche Applications

Uranates, particularly (Na₂U₂O₇), found historical application in glassmaking as colorants from the 1830s to the 1940s, where they were incorporated into molten glass mixtures to produce characteristic yellow-green under light. This use persisted in products like vaseline glass, which typically contained about 2% by weight, enabling vibrant coloration without significant radiological concerns at the time due to limited understanding of long-term hazards. In ceramics, uranates of heavy metals such as lead, barium, and calcium formed the basis for durable pigments and glazes, especially uranium oxide yellows applied under glazes and fired in oxidizing conditions to yield stable, bright hues. These compounds were valued for their vitrifiable properties in enamels, lusters, and decorative tiles, with uranium oxide enabling red-orange tones in items like Fiesta dinnerware produced until the 1970s. Production declined sharply after World War II as awareness of uranium's radioactivity grew, leading to regulatory restrictions. Niche applications extended to early 20th-century and studies, where uranates like calcium uranate (CaUO₄) were used for their luminescent properties in phosphors, though these were supplanted by safer alternatives post-1950. Today, surviving artifacts serve primarily as collectibles, with their low-level emissions posing minimal risk under normal handling, as confirmed by radiation surveys showing doses below regulatory limits.

Safety, Toxicity, and Environmental Considerations

Chemical and Radiological Hazards

Uranate compounds, such as (Na₂U₂O₇), exhibit chemical toxicity primarily as heavy metal poisons, with targeting the proximal tubules in the kidneys, leading to glomerular and tubular upon sufficient exposure. Acute ingestion of soluble uranium species can cause renal failure, with animal studies showing LD50 values around 100-500 mg/kg for , though insoluble uranates like demonstrate lower oral due to poor gastrointestinal absorption (typically <1%). of uranate dusts poses a greater , as particulates can deposit in the lungs, causing localized inflammation and potential systemic uptake via dissolution in lung fluids, with repeated exposure linked to organ damage including and . Safety data for sodium uranate classify it as fatal if swallowed or inhaled, harmful via skin contact, and capable of causing eye irritation, with precautions emphasizing avoidance of dust generation. Radiological hazards from uranates stem from the alpha decay of uranium isotopes (primarily ²³⁸U and its daughters ²³⁴U and ²³⁴Th), which emit low-energy alpha particles with minimal external penetration but significant internal damage potential if internalized. Natural uranium in uranates has a low specific activity (approximately 0.00015 Ci/g), resulting in committed effective doses from inhalation of insoluble particles on the order of 0.001-0.01 Sv per gram inhaled, far lower than acute chemical thresholds for most scenarios. For insoluble uranates, lung retention times can exceed years, prolonging alpha irradiation to bronchial epithelium and increasing stochastic risks like lung cancer, though epidemiological data from uranium workers indicate chemical nephrotoxicity often dominates over radiological effects at typical exposure levels. Combined exposures may synergize, with chemical damage impairing cellular repair against radiation-induced DNA lesions, but quantitative assessments prioritize chemical limits (e.g., OSHA PEL of 0.05 mg/m³ for soluble uranium, 0.25 mg/m³ for insoluble). Overall, for uranates used in nuclear processing, handling protocols treat radiological risks as secondary to chemical and particulate inhalation hazards.

Handling and Mitigation Strategies

Handling uranium(IV) uranates, such as alkaline earth compounds like CaUO4 and BaUO4, requires adherence to protocols minimizing both chemical and radiological exposure, as these insoluble powders pose risks of , , and dermal absorption leading to kidney damage and alpha radiation effects. Personnel must employ (PPE) including anti- clothing, gloves, eye protection, and respirators equipped with high-efficiency particulate air () filters when handling powders or in areas with potential airborne . Operations should occur in controlled environments like glove boxes, fume hoods, or biological safety cabinets (Class I, Type B) to contain particulates and prevent dispersal. The ALARA (As Low As Reasonably Achievable) principle guides all activities, involving routine monitoring of air, surfaces, and personnel with Geiger-Mueller counters or alpha probes, followed by immediate of gloves and workspaces using wet wiping or vacuum systems with filters to reduce contamination levels. Hands must be washed thoroughly after handling, and eating, drinking, or smoking is prohibited in work areas to avoid inadvertent . For spills, evacuate non-equipped personnel, ventilate the area, and absorb material with inert agents like before disposal as , avoiding organic solvents that may enhance solubility and uptake. Storage entails sealed, labeled containers in secure, locked areas with secondary , segregated from oxidizers, acids, or flammables to prevent , and under conditions limiting moisture to avoid or dust generation. Waste mitigation involves as mixed radioactive and chemical , with treatment via immobilization in or matrices for long-term disposal, complying with regulatory limits such as OSHA's permissible exposure of 0.25 mg/m³ for insoluble compounds. In cases of suspected exposure, medical surveillance includes urine bioassays for levels and with agents like diethylenetriaminepentaacetic acid (DTPA) if internal contamination exceeds thresholds, though efficacy diminishes beyond 24 hours post-exposure.

Empirical Risk Assessments

Empirical assessments of risks associated with uranate compounds, such as (Na₂U₂O₇), indicate low to moderate primarily through occupational exposure in processing facilities, where these insoluble uranium(VI) salts serve as intermediates. Animal studies classify poorly water-soluble uranates like and ammonium diuranate as having moderate-to-low systemic , with oral LD₅₀ values exceeding those of highly soluble forms like , reflecting limited gastrointestinal absorption and reduced nephrotoxic potential. Inhalation exposures in rabbits to dusts analogous to uranate-derived particulates at concentrations of 2.0 mg U/m³ produced , hemorrhage, and , though milder effects like slight occurred in dogs at similar levels. Epidemiological data from processing workers, involving handling of uranates during and steps, reveal some evidence of elevated risk, potentially attributable to chronic inhalation of uranium aerosols and associated decay products rather than uranate-specific chemical effects. Reviews of 27 studies on such cohorts found inconsistent links to other malignancies or non-malignant respiratory diseases, with no conclusive evidence of damage or systemic effects at exposure levels below occupational limits (e.g., 0.2 mg U/m³ ). Chemical , the primary concern for soluble uranium species, appears minimal for insoluble uranates due to poor , as supported by absence of renal impairment in worker studies. Radiological risks from uranates remain low in empirical contexts, as uranium's alpha emissions pose negligible external and internal doses from uranate are mitigated by low and rapid clearance, with monitored worker exposures typically below 1 mSv/year in modern facilities handling sodium dissolution. No large-scale empirical data document acute environmental releases of uranates leading to population-level health impacts, though localized contamination from processing effluents underscores potential for in aquatic systems, prompting risk mitigation via . Overall, controlled handling aligns with guidelines showing adverse effects primarily at high acute exposures exceeding industrial norms.

Recent Research and Future Prospects

Advances in Structural Analysis

Advances in of uranate compounds have leveraged combined and neutron diffraction techniques to overcome challenges posed by uranium's high and sensitivity, enabling precise determination of atomic positions, including oxygen coordination around . In 2015, the of trisodium uranate (Na₃UO₄) was refined using powder neutron diffraction alongside data, revealing a hexagonal high-temperature phase transitioning to a lower-symmetry form at , contrary to earlier cubic models; this polymorphism arises from uranium's distorted tetrahedral UO₄ units linked by sodium cations. Neutron diffraction proved essential for locating light oxygen atoms accurately, as methods alone suffer from scattering dominance by . Synchrotron-based single-crystal has further enhanced resolution for complex uranate frameworks, particularly in variants, confirming two-dimensional layered and three-dimensional motifs in hydrates akin to uranate precursors. A 2024 study introduced refinements treating anomalous dispersion as a free parameter in heavy-element compounds like uranates, improving accuracy under Mo Kα and reducing artifacts from uranium's f-electrons. These methods have been applied to calcium uranate (CaUO₄), whose scheelite-type features isolated UO₄ tetrahedra, with lattice parameters refined via recent to support stability assessments in nuclear waste forms. Electron diffraction complements these techniques for phase identification in polycrystalline uranates, as demonstrated in 2025 analyses of electroneutral phases, validating triclinic symmetries not resolvable by conventional powder methods. Overall, these developments prioritize empirical validation over theoretical assumptions, with peer-reviewed refinements emphasizing causal links between synthesis conditions and structural motifs, such as yielding discrete uranate anions.

Novel Syntheses and Compounds

In 2017, researchers developed an aqueous route for synthesizing calcium monouranate (CaUO₄) nanoparticles, involving in solution followed by thermal curing to induce phase transformations, enabling control over and morphology for potential applications in nuclear materials. This method contrasts with traditional high-temperature solid-state reactions by allowing lower-energy processing and finer particle control, as confirmed by and analyses of the resulting scheelite-structured CaUO₄. A solid-state chlorination approach for alkaline-earth monouranates (e.g., CaUO₄, SrUO₄, BaUO₄) was reported in , optimizing reaction temperatures to selectively form pure phases from precursors, with subsequent to remove byproducts like chlorides. This technique yields high-purity compounds suitable for or uses, with chlorination at 600–800°C preventing over-reduction to lower oxidation states. Hydrothermal synthesis has enabled novel nanocomposites, such as (g-C₃N₄) integrated with UO₃·NH₃·H₂O (a form of ammonium uranate), prepared in ratios of 10:1 and 3:1 via short-duration (e.g., 2–4 hours) reactions at elevated temperatures and pressures. These hybrids exhibit enhanced photocatalytic properties for uranium-related processes, attributed to the interfacial charge transfer between the uranate phase and g-C₃N₄, as characterized by SEM, XRD, and UV-Vis . Ammonium uranate decorated with reduced graphene oxide (rGO) represents another advancement, synthesized via electrodeposition or methods followed by reduction, demonstrating hydrogen storage capacities up to 1.5 wt% at ambient conditions due to the uranate's catalytic sites and rGO's conductivity. Structural confirmation via FTIR and TEM reveals uniform dispersion of uranate particles (10–50 nm) on rGO sheets, improving stability over pure ammonium uranate ((NH₄)₂U₂O₇). Laser desorption/ionization techniques have produced gas-phase molecular uranate anions (e.g., UO₃⁻ to U₅O₁₆⁻) from solid UO₃ or diuranate precursors, providing insights into cluster structures via and offering a pathway for non-aqueous, vapor-phase compound exploration. These clusters feature in +6 oxidation states with layered motifs, differing from solution-based polymeric uranates.

Emerging Applications in Waste Forms and Beyond

Hexavalent uranium waste forms, including uranate phases, are under investigation for immobilizing -rich radioactive wastes generated across the , such as from mining, enrichment, and reprocessing. These forms leverage the high chemical durability of U(VI) compounds without requiring reducing conditions, potentially lowering processing costs compared to tetravalent or mixed-valence ceramics. Recent assessments highlight silicates, phosphates, and related ternary oxides as promising candidates, with natural analogues demonstrating long-term stability over geological timescales. In legacy nuclear sites like Hanford, sodium uranates such as Na2U2O7 have been identified as prevalent uranium-bearing phases in alkaline tank wastes, influencing and mobility under disposal conditions. Emerging formulations incorporating or alkaline uranates aim to enhance retention by forming stable, low- matrices resistant to leaching in repository environments. Studies on waste forms indicate that incorporation into calcium-based phases, potentially evolving into uranate-like structures, controls long-term release, with experiments revealing phases that minimize environmental migration. Beyond waste immobilization, uranates show potential in strategies, where their formation aids in sequestering from contaminated via precipitation or onto mineral surfaces. For instance, microbial processes under alkaline conditions can promote U(VI) reduction and subsequent uranate precipitation, offering pathways for . However, applications remain exploratory, with challenges in and stability requiring further empirical validation.

References

  1. https://www.yourdictionary.com/uranate
  2. https://digitalfire.com/hazard/uranium%2Band%2Bceramics
  3. https://pubchem.ncbi.nlm.nih.gov/compound/160982
  4. https://www.sciencedirect.com/science/article/abs/pii/S0022311521003019
  5. https://www.orau.org/health-physics-museum/collection/consumer/ceramics/index.html
  6. https://www.ncbi.nlm.nih.gov/books/NBK158805/
  7. https://inis.iaea.org/records/pxj5j-47921
  8. https://www.chemeurope.com/en/encyclopedia/Uranate.html
  9. https://www.chemicalbook.com/ChemicalProductProperty_EN_CB4414676.htm
  10. https://pubs.acs.org/doi/abs/10.1021/ic902550g
  11. https://www.sciencedirect.com/science/article/abs/pii/002219027880308X
  12. https://www.merriam-webster.com/dictionary/uranate
  13. https://www.sciencedirect.com/science/article/pii/0022190269803787
  14. https://www.atsdr.cdc.gov/toxprofiles/tp150-c4.pdf
  15. https://www.sciencedirect.com/science/article/abs/pii/0022190271802178
  16. https://pubs.acs.org/doi/abs/10.1021/cm020258j
  17. https://www.sciencedirect.com/science/article/abs/pii/S1293255810003195
  18. https://inis.iaea.org/search/search.aspx?orig_q=RN:29022465
  19. https://www.academia.edu/29305921/Four_thallium_I_uranates_VI_their_preparation_structure_and_properties
  20. https://www.nature.com/articles/srep32898
  21. https://www.sciencedirect.com/science/article/abs/pii/S0022311599000914
  22. https://www.sciencedirect.com/science/article/pii/S0187893X18300089
  23. https://pubs.acs.org/doi/10.1021/ja01124a043
  24. https://www.osti.gov/servlets/purl/1001078
  25. https://www-pub.iaea.org/MTCD/Publications/PDF/Pub553_web.pdf
  26. https://pubs.rsc.org/en/content/getauthorversionpdf/c8dt04675a
  27. https://www.researchgate.net/publication/303090799_Synthesis_and_study_of_sodium_uranate_Na2U2O76H2O_and_of_products_of_its_dehydration_and_thermal_decomposition
  28. https://www.oecd-nea.org/upload/docs/application/pdf/2019-12/uranium.pdf
  29. https://www.sciencedirect.com/science/article/abs/pii/S0149197021003905
  30. http://fulir.irb.hr/2128/1/toma%25C5%25BEi%25C4%258D%252C%2520branica%252C%2520te%25C5%25BEak.%2520precipitation%2520and%2520hydrolysis%2520of%2520uranium%2520vi%2520in%2520aqueous%2520solutions.pdf
  31. https://onlinelibrary.wiley.com/doi/abs/10.1002/jctb.5010120907
  32. https://pmc.ncbi.nlm.nih.gov/articles/PMC11872519/
  33. https://link.springer.com/article/10.1134/S106636221506003X
  34. https://www.sciencedirect.com/science/article/pii/S0925838816321740
  35. https://www.sciencedirect.com/science/article/abs/pii/S0022311518308894
  36. https://ri.conicet.gov.ar/bitstream/handle/11336/68716/CONICET_Digital_Nro.e7d853d7-edca-4b0f-9693-a821a49b6557_A.pdf?sequence=2&isAllowed=y
  37. https://laro.lanl.gov/view/pdfCoverPage?instCode=01LANL_INST&filePid=13158041280003761&download=true
  38. https://www.sciencedirect.com/science/article/abs/pii/S0925838808010840
  39. https://www.researchgate.net/publication/325236434_A_new_method_for_alkaline-earth_monouranates_preparation_by_solid_state_and_chlorination_reactions
  40. https://www.sciencedirect.com/science/article/pii/002219026780143X
  41. https://www.sciencedirect.com/science/article/abs/pii/S0022311599001452
  42. https://www.sciencedirect.com/science/article/pii/S1738573319307168
  43. https://www.researchgate.net/publication/337141010_The_conversion_of_ammonium_uranate_prepared_via_sol-gel_synthesis_into_uranium_oxides
  44. https://pubs.acs.org/doi/10.1021/ic201857e
  45. https://inis.iaea.org/records/cb9ss-xw636
  46. https://pubs.rsc.org/en/content/articlelanding/2024/dt/d4dt01604a
  47. https://pubmed.ncbi.nlm.nih.gov/30702735/
  48. https://www.sciencedirect.com/science/article/abs/pii/S0016236123003174
  49. https://pubs.rsc.org/en/content/articlelanding/2021/nj/d1nj00855b
  50. https://apps.dtic.mil/sti/tr/pdf/ADA413026.pdf
  51. https://www.sciencedirect.com/science/article/abs/pii/0022190276900130
  52. https://pubs.geoscienceworld.org/canmin/pdf-lookup/54/1/177
  53. https://pubs.geoscienceworld.org/msa/ammin/article/97/5-6/1002/45637/Thermally-induced-transformation-of-vorlanite-to
  54. https://pubs.acs.org/doi/abs/10.1021/acs.inorgchem.6b01391
  55. https://academic.oup.com/bcsj/article-abstract/50/11/2940/7356730
  56. https://www.ismar.org/wp-content/uploads/2021/09/BMR_06_162-224_1984.pdf
  57. https://www.jstage.jst.go.jp/article/jaesj1959/20/4/20_4_241/_article/-char/en
  58. https://www.researchgate.net/publication/232366893_The_heat_capacity_of_BaUO4
  59. https://www.pnnl.gov/main/publications/external/technical_reports/PNNL-20726.pdf
  60. https://www.sciencedirect.com/science/article/pii/S0022311513009215
  61. https://legacy.materialsproject.org/materials/mp-3960/
  62. https://dspace.library.uu.nl/bitstream/handle/1874/24851/lam_80_luminescence%2520of%2520barium.pdf?sequence=1
  63. https://www.chemicalbook.com/ChemicalProductProperty_EN_CB2751579.htm
  64. https://www.sciencedirect.com/science/article/abs/pii/0022311572900943
  65. https://www.researchgate.net/publication/304242081_ChemInform_Abstract_THE_LUMINESCENCE_OF_BARIUM_URANATE_BAUO4
  66. https://www.researchgate.net/figure/Plot-of-the-heat-capacity-of-BaUO-4-between-18-K-and-350-K-s-present-results-h_fig1_232366893
  67. https://www.sciencedirect.com/science/article/pii/S0022311516303853
  68. https://pubs.acs.org/doi/10.1021/acs.estlett.7b00348
  69. https://pubmed.ncbi.nlm.nih.gov/23906373/
  70. https://www.researchgate.net/publication/263353731_The_role_of_uraniumV_ion_in_the_chemical_composition_of_meta-autunite_from_pegmatites_of_Quintos_de_Baixo_Brazil
  71. https://pubs.acs.org/doi/10.1021/acs.est.1c06865
  72. https://pubs.rsc.org/en/content/articlelanding/2016/dt/c6dt00066e
  73. https://www.sciencedirect.com/science/article/am/pii/S0022311522003336
  74. https://www.osti.gov/biblio/1243177
  75. https://www.researchgate.net/publication/229236917_Antiferromagnetism_in_MUO3_MNaKRb_studied_by_neutron_diffraction
  76. https://pubs.acs.org/doi/10.1021/acs.inorgchem.5b00136
  77. https://www.sciencedirect.com/science/article/abs/pii/0021961472900249
  78. https://pubs.acs.org/doi/10.1021/ic50139a035
  79. https://www.sciencedirect.com/science/article/abs/pii/S0168127324000229
  80. https://www.sciencedirect.com/science/article/abs/pii/S0022311598000129
  81. https://www.sciencedirect.com/science/article/pii/S0022459683710698
  82. https://www.sciencedirect.com/science/article/pii/S0022311599001452
  83. https://pubs.acs.org/doi/10.1021/acs.langmuir.1c02927
  84. https://www.sciencedirect.com/science/article/abs/pii/S0022459683710698
  85. https://www.iaea.org/sites/default/files/publications/magazines/bulletin/bull23-2/23204882428.pdf
  86. https://world-nuclear.org/information-library/nuclear-fuel-cycle/mining-of-uranium/uranium-mining-overview
  87. https://www.sciencedirect.com/science/article/pii/0021951772901017
  88. https://research.tue.nl/en/publications/the-oxidative-demethylation-of-toluene-ii-kinetics-of-the-reducti
  89. https://pubs.rsc.org/en/content/articlelanding/1987/f2/f29878301263
  90. https://www.czechcenter.org/online-exhibit-sodium-diuranate-uranium-in-czech-glass-glazes
  91. https://www.orau.org/health-physics-museum/collection/consumer/glass/vaseline-uranium-glass.html
  92. https://www.epa.gov/radtown/radioactivity-antiques
  93. https://opg.optica.org/abstract.cfm?uri=josa-35-11-731
  94. https://www.sciencedirect.com/science/article/abs/pii/S0969806X22003425
  95. https://www.atsdr.cdc.gov/toxprofiles/tp150-c2.pdf
  96. https://www.ncbi.nlm.nih.gov/books/NBK158798/
  97. https://ibilabs.com/msds/sodium-uranate-msds/
  98. https://www.wnti.co.uk/wp-content/uploads/2021/01/WNTI-Good-Practice-Guide-Generic-UOC-Safety-Data-Sheet.pdf
  99. https://www.atsdr.cdc.gov/toxprofiles/tp150.pdf
  100. https://wwwn.cdc.gov/TSP/PHS/PHS.aspx?phsid=438&toxid=77
  101. https://www.researchgate.net/publication/366457707_Radiological_safety_during_sodium_diuranate_dissolution_process_A_radiological_data_study
  102. https://sti.srs.gov/fulltext/tr2001331/tr2001331.html
  103. https://archive.cdc.gov/www_atsdr_cdc_gov/csem/uranium/standards.html
  104. https://nj.gov/health/eoh/rtkweb/documents/fs/1969.pdf
  105. https://ehs.unl.edu/sites/unl.edu.business-and-finance.university-operations.ehs/files/media/file/SP_SOP_Uranium.pdf
  106. https://ehs.yale.edu/sites/default/files/files/U-238.pdf
  107. https://science.osti.gov/-/media/nbl/pdf/price-lists/SDS/SDS-Uranium_Oxide_UO2.pdf?la=en&hash=1DB7555C9D9C27D597CC342E960A8DF67E0DEBFD
  108. https://www.energy.gov/nnsa/articles/sds-uranium-metal
  109. https://www.ncbi.nlm.nih.gov/books/NBK158804/
  110. https://www.sciencedirect.com/science/article/pii/S0147651325003793
  111. https://www.tandfonline.com/doi/full/10.3109/09553002.2014.943849
  112. https://journals.lww.com/rpae/fulltext/2022/45020/radiological_safety_during_sodium_diuranate.8.aspx
  113. https://www.sciencedirect.com/science/article/pii/S0020169325003548
  114. https://pubs.acs.org/doi/10.1021/acs.inorgchem.4c01772
  115. https://www.sciencedirect.com/science/article/pii/S0020169325000775
  116. https://www.researchgate.net/publication/313885506_Synthesis_of_Calcium_Monouranate_Particles_via_an_Aqueous_Route
  117. https://scispace.com/papers/synthesis-of-calcium-monouranate-particles-via-an-aqueous-3mipzvssb6
  118. https://www.sciencedirect.com/science/article/abs/pii/S1387700318303393
  119. https://www.sciencedirect.com/science/article/abs/pii/S036031992504460X
  120. https://doi.org/10.1016/j.fuel.2023.127704
  121. https://www.sciencedirect.com/science/article/pii/S0254058424008393
  122. https://www.researchgate.net/publication/323300862_Uranium-bearing_phases_in_Hanford_nuclear_waste
  123. https://pubs.acs.org/doi/10.1021/es5017125
Add your contribution
Related Hubs
User Avatar
No comments yet.