Crystallization of a Neptunyl Oxalate Hydrate from Solutions Containing Np^V and the Uranyl Peroxide Nanocluster U₆₀Ox₃₀

Graphical Abstract

Interactions between U₆₀Ox₃₀ clusters and Np^V reveal different mechanisms based on Np^V concentrations. Low Np^V concentrations cause the clusters to aggregate into larger macrostructures. Higher concentrations of Np^V outcompete U^VI for coordination with oxalate and lead to cluster dissociation. Neptunyl oxalate crystals and studtite precipitate from cluster remnants.

Abstract

Uranyl peroxide nanoclusters are an evolving family of materials with potential applications throughout the nuclear fuel cycle. While several studies have investigated their interactions with alkali and alkaline earth metals, no studies have probed their interactions with the actinide elements. This work describes a system containing U₆₀Ox₃₀, [((UO₂)(O₂))₆₀(C₂O₄)₃₀]⁶⁰⁻, and neptunium(V) as a function of neptunium concentration. Ultra-small and small angle X-ray scattering were used to observe these interactions in the aqueous phase, and X-ray diffraction was used to observe solid products. The results show that neptunium induces aggregation of U₆₀Ox₃₀ when the neptunium concentration is≤10 mM, whereas (NpO₂)₂C₂O₄ ⋅ 6H₂O(cr) and studtite ultimately form at 15–25 mM neptunium. The latter result suggests that neptunium coordinates with the bridging oxalate ligands in U₆₀Ox₃₀, leaving metastable uranyl peroxide species in solution. This is an important finding given the potential application of uranyl peroxide nanoclusters in the recycling of used nuclear fuel.