Synthesis of Coffinite, USiO4, and Structural Investigations of UxTh(1–x)SiO4 Solid Solutions

Labs, Sabrina; Hennig, Christoph; Weiß, Stephan; Curtius, H.; Zänker, Harald; Bosbach, Dirk

doi:10.1021/es403995b

Cited by 36 publications

(53 citation statements)

References 31 publications

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“…Given these results, the quantity of coffinite that forms increases with the amount of excess Na 2 SiO 3 . On the basis of these results and those recently reported by Labs et al, 37 we can infer from the data presented in Figure 9 that coffinite free of coexisting UO 2 with a smaller quantity of amorphous silica can be obtained, using a U/Si molar ratio of 1/2. To understand this result, tetravalent uranium and silicon speciation were examined for all the synthesis conditions using the Phreeqc 2 software 56 and the thermodynamic data reported in Table S2 of the Supporting Information.…”

Section: Resultssupporting

confidence: 87%

Coffinite, USiO₄, Is Abundant in Nature: So Why Is It So Difficult To Synthesize?

et al. 2015

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Coffinite, USiO4, is the second most abundant U(4+) mineral on Earth, and its formation by the alteration of the UO2 in spent nuclear fuel in a geologic repository may control the release of radionuclides to the environment. Despite its abundance in nature, the synthesis and characterization of coffinite have eluded researchers for decades. On the basis of the recent synthesis of USiO4, we can now define the experimental conditions under which coffinite is most efficiently formed. Optimal formation conditions are defined for four parameters: pH, T, heating time, and U/Si molar ratio. The adjustment of pH between 10 and 12 leads probably to the formation of a uranium(IV) hydroxo-silicate complex that acts as a precursor of uranium(IV) silicate colloids and then of coffinite. Moreover, in this pH range, the largest yield of coffinite formation (as compared with those of the two competing byproduct phases, nanometer-scale UO2 and amorphous SiO2) is obtained for 250 °C, 7 days, and 100% excess silica.

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Section: Resultssupporting

confidence: 87%

Coffinite, USiO₄, Is Abundant in Nature: So Why Is It So Difficult To Synthesize?

et al. 2015

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“…Coffinite-F and -G samples were prepared by hydrothermal synthesis routes, described elsewhere (33,35). Because these syntheses routes inevitably have UO 2 and amorphous silica as byproducts, further purification is needed.…”

Section: Experimental Methodsmentioning

confidence: 99%

“…It appears not to form by reacting the oxides under dry high-temperature conditions (24, 29). Synthesis from aqueous solutions usually produces UO 2 and amorphous SiO 2 impurities, with coffinite sometimes being only a minor phase (24,(30)(31)(32)(33)(34)(35). It is not clear whether these difficulties arise from kinetic factors (slow reaction rates) or reflect intrinsic thermodynamic instability (33).…”

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confidence: 99%

Thermodynamics of formation of coffinite, USiO ₄

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et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

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107

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Coffinite, USiO 4 , is an important U(IV) mineral, but its thermodynamic properties are not well-constrained. In this work, two different coffinite samples were synthesized under hydrothermal conditions and purified from a mixture of products. The enthalpy of formation was obtained by high-temperature oxide melt solution calorimetry. Coffinite is energetically metastable with respect to a mixture of UO 2 (uraninite) and SiO 2 (quartz) by 25.6 ± 3.9 kJ/mol. Its standard enthalpy of formation from the elements at 25°C is −1,970.0 ± 4.2 kJ/mol. Decomposition of the two samples was characterized by X-ray diffraction and by thermogravimetry and differential scanning calorimetry coupled with mass spectrometric analysis of evolved gases. Coffinite slowly decomposes to U 3 O 8 and SiO 2 starting around 450°C in air and thus has poor thermal stability in the ambient environment. The energetic metastability explains why coffinite cannot be synthesized directly from uraninite and quartz but can be made by low-temperature precipitation in aqueous and hydrothermal environments. These thermochemical constraints are in accord with observations of the occurrence of coffinite in nature and are relevant to spent nuclear fuel corrosion.uranium | coffinite | USiO 4 | U(IV) minerals | calorimetry I n many countries with nuclear energy programs, spent nuclear fuel (SNF) and/or vitrified high-level radioactive waste will be disposed in an underground geological repository. Demonstrating the long-term (10 6 -10 9 y) safety of such a repository system is a major challenge. The potential release of radionuclides into the environment strongly depends on the availability of water and the subsequent corrosion of the waste form as well as the formation of secondary phases, which control the radionuclide solubility. Coffinite (1), USiO 4 , is expected to be an important alteration product of SNF in contact with silica-enriched groundwater under reducing conditions (2-8). It is also found, accompanied by thorium orthosilicate and uranothorite, in igneous and metamorphic rocks and ore minerals from uranium and thorium sedimentary deposits (2,4,5,(8)(9)(10)(11)(12)(13)(14)(15)(16). Under reducing conditions in the repository system, the uranium solubility (very low) in aqueous solutions is typically derived from the solubility product of UO 2 . Stable U(IV) minerals, which could form as secondary phases, would impart lower uranium solubility to such systems. Thus, knowledge of coffinite thermodynamics is needed to constrain the solubility of U(IV) in natural environments and would be useful in repository assessment.In natural uranium deposits such as Oklo (Gabon) (4,7,11,12,14,17,18) and Cigar Lake (Canada) (5, 13, 15), coffinite has been suggested to coexist with uraninite, based on electron probe microanalysis (EPMA) (4,5,7,11,13,17,19,20) and transmission electron microscopy (TEM) (8, 15). However, it is not clear whether such apparent replacement of uraninite by a coffinite-like phase is a direct solid-state process or occurs mediated by dis...

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“…U IV oxyhydroxy silicate colloids form a candidate class of colloids accomplishing the colloid‐chemical processes necessary for the formation of coffinitic uranium ore. Furthermore, An IV oxyhydroxide colloids may also be involved in the “coffinitization” of UO 2 ‐based spent fuel, a phenomenon expected in nuclear waste repositories after the access of water which occurs via the dissolution and the reprecipitation of An IV[110, 111] and which is of significance for the behavior of spent fuel.…”

Section: Concluding Remarks and Future Outlookmentioning

confidence: 99%

Oxyhydroxy Silicate Colloids: A New Type of Waterborne Actinide(IV) Colloids

et al. 2016

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At the near‐neutral and reducing aquatic conditions expected in undisturbed ore deposits or in closed nuclear waste repositories, the actinides Th, U, Np, and Pu are primarily tetravalent. These tetravalent actinides (AnIV) are sparingly soluble in aquatic systems and, hence, are often assumed to be immobile. However, AnIV could become mobile if they occur as colloids. This review focuses on a new type of AnIV colloids, oxyhydroxy silicate colloids. We herein discuss the chemical characteristics of these colloids and the potential implication for their environmental behavior. The binary oxyhydroxy silicate colloids of AnIV could be potentially more mobile as a waterborne species than the well‐known mono‐component oxyhydroxide colloids.

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Synthesis of Coffinite, USiO₄, and Structural Investigations of U_xTh_(1–x)SiO₄ Solid Solutions

Cited by 36 publications

References 31 publications

Coffinite, USiO₄, Is Abundant in Nature: So Why Is It So Difficult To Synthesize?

Coffinite, USiO₄, Is Abundant in Nature: So Why Is It So Difficult To Synthesize?

Thermodynamics of formation of coffinite, USiO ₄

Oxyhydroxy Silicate Colloids: A New Type of Waterborne Actinide(IV) Colloids

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Synthesis of Coffinite, USiO4, and Structural Investigations of UxTh(1–x)SiO4 Solid Solutions

Cited by 36 publications

References 31 publications

Coffinite, USiO4, Is Abundant in Nature: So Why Is It So Difficult To Synthesize?

Coffinite, USiO4, Is Abundant in Nature: So Why Is It So Difficult To Synthesize?

Thermodynamics of formation of coffinite, USiO 4

Oxyhydroxy Silicate Colloids: A New Type of Waterborne Actinide(IV) Colloids

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Product

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Synthesis of Coffinite, USiO₄, and Structural Investigations of U_xTh_(1–x)SiO₄ Solid Solutions

Coffinite, USiO₄, Is Abundant in Nature: So Why Is It So Difficult To Synthesize?

Coffinite, USiO₄, Is Abundant in Nature: So Why Is It So Difficult To Synthesize?

Thermodynamics of formation of coffinite, USiO ₄