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

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• 作者：Adel Mesbah ; Stephanie Szenknect ; Nicolas Clavier ; Janeth Lozano-Rodriguez ; Christophe Poinssot ; Christophe Den Auwer ; Rodney C. Ewing ; Nicolas Dacheux
• 刊名：Inorganic Chemistry
• 出版年：2015
• 出版时间：July 20, 2015
• 年：2015
• 卷：54
• 期：14
• 页码：6687-6696
• 全文大小：549K
• ISSN：1520-510X

文摘

Coffinite, USiO₄, is the second most abundant U⁴⁺ 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 掳C, 7 days, and 100% excess silica.