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）常作为络合剂应用于从乏核反应堆燃料中回收铀与钚的普雷克斯（Plutonium Uranium Extraction, PUREX）液液萃取工艺中。本研究针对该萃取工艺涉及的有机相分子结构与微观形貌展开探究，借助分子动力学（molecular dynamics）模拟手段证实：当磷酸三丁酯与烷烃稀释剂混合时，其会自组装形成双连续相。磷酸三丁酯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，致使丁基基团向外辐射至有机溶剂体系中。模拟结果预测，磷酸三丁酯的扩散常数相较于其分子量常规应有的扩散常数异常偏低；实验核磁共振（Nuclear Magnetic Resonance, NMR）研究同样显示出极低的扩散常数，与分子聚集模型的结论相符。在油/水界面处的磷酸三丁酯模拟实验表明，低浓度条件下会形成双层膜结构；当浓度升高时，则可观察到与表面双层膜相连的体相双连续结构。我们推测，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异常快速的动力学过程密切相关。