Benarib Sofian, Munoz Maëva, Kieffer Isabelle, Hazemann Jean-Louis, Dacheux Nicolas, Clavier Nicolas
ICSM, Univ Montpellier, CEA, CNRS, ENSCM, Bagnols/Cèze, France.
Université Grenoble Alpes, UAR CNRS 832, OSUG, 38041 Grenoble, France.
Dalton Trans. 2024 Aug 20;53(33):13982-13995. doi: 10.1039/d4dt01451k.
Hydrothermal conversion of actinide oxalates has recently gained attention as an innovative fabrication route for nuclear fuels but has remained mainly limited to tetra- or tri-valent cations. We report herein the reductive conversion of mixtures of uranyl and oxalate ions into UO oxides under mild hydrothermal conditions ( = 250 °C). A multi-parametric study first led to specifying the optimal conditions in terms of pH, oxalate/U ratio and duration to provide a quantitative precipitation of uranium in the hyper-stoichiometric dioxide form with pH = 0.8; = / = 3, and = 72 hours. Particularly, pH was evidenced as a key parameter, with 3 different compounds obtained over a range of only 0.4 units. The mechanism leading to the formation of UO was then investigated thanks to an XANES study. Analysis of the supernatant showed that U(VI) was quickly reduced into U(IV) thanks to the presence of oxalates and/or their decomposition products in solution, following first-order kinetics. Tetravalent uranium was then hydrolysed, leading to the precipitation of UO as the only crystalline phase. This study thus demonstrates that the hydrothermal conversion of actinide oxalates into oxides is an extremely versatile tool that can be implemented in a large variety of chemical systems, particularly in terms of the oxidation state of the cations initially present in solution.
锕系元素草酸盐的水热转化作为一种用于核燃料的创新制备途径，近来受到了关注，但主要仍局限于四价或三价阳离子。我们在此报告了在温和的水热条件下（250°C），将铀酰离子和草酸根离子的混合物还原转化为UO氧化物。一项多参数研究首先确定了在pH值、草酸根/铀比和反应持续时间方面的最佳条件，以提供超化学计量二氧化物形式的铀的定量沉淀，此时pH = 0.8；草酸根/铀 = 3，反应持续时间 = 72小时。特别地，pH值被证明是一个关键参数，在仅0.4个单位的范围内得到了3种不同的化合物。借助XANES研究，随后对UO形成的机理进行了研究。对上清液的分析表明，由于溶液中存在草酸盐和/或其分解产物，U(VI)按照一级动力学迅速还原为U(IV)。然后四价铀发生水解，导致UO作为唯一的晶相沉淀出来。因此，这项研究表明，锕系元素草酸盐向氧化物的水热转化是一种极其通用的工具，可以在多种化学体系中实现，特别是在溶液中最初存在的阳离子的氧化态方面。
