The behaviour of tributyl phosphate in an organic diluent

Tributyl phosphate (TBP) is used as a complexing agent in the Plutonium Uranium Extraction (PUREX) liquid–liquid phase extraction process for recovering uranium and plutonium from spent nuclear reactor fuel. Here, we address the molecular and microstructure of the organic phases involved in the extraction process, using molecular dynamics to show that when TBP is mixed with a paraffinic diluent, the TBP self-assembles into a bi-continuous phase. The underlying self-association of TBP is driven by intermolecular interaction between its polar groups, resulting in butyl moieties radiating out into the organic solvent. Simulation predicts a TBP diffusion constant that is anomalously low compared to what might normally be expected for its size; experimental nuclear magnetic resonance (NMR) studies also indicate an extremely low diffusion constant, consistent with a molecular aggregation model. Simulation of TBP at an oil/water interface shows the formation of a bilayer system at low TBP concentrations. At higher concentrations, a bulk bi-continuous structure is observed linking to this surface bilayer. We suggest that this structure may be intimately connected with the surprisingly rapid kinetics of the interfacial mass transport of uranium and plutonium from the aqueous to the organic phase in the PUREX process.

磷酸三丁酯（Tributyl phosphate, TBP）作为络合剂，应用于普雷克斯（Plutonium Uranium Extraction, PUREX）液液萃取工艺中，用于从乏核反应堆燃料中回收铀与钚。本研究针对该萃取工艺涉及的有机相分子与微观结构展开探究，通过分子动力学模拟发现：当TBP与石蜡类稀释剂混合时，TBP会自组装形成双连续相。TBP的内在自缔合行为由其极性基团间的分子间相互作用驱动，使得丁基基团向外辐射至有机溶剂中。模拟预测的TBP扩散系数，相较于其分子尺寸本应对应的正常扩散系数异常偏低；实验核磁共振（NMR）研究同样观测到极低的扩散系数，与分子聚集模型的结论一致。对油/水界面处TBP的模拟显示，在低TBP浓度下会形成双层膜结构；当浓度升高时，则会形成与该表面双层结构相连的体相双连续结构。本研究提出，该结构或与普雷克斯工艺中铀、钚从水相向有机相的界面传质动力学异常快速存在密切关联。