Passage of TBP–uranyl complexes from aqueous–organic interface to the organic phase: insights from molecular dynamics simulation

Abstract

The present study reports molecular dynamics simulations for biphasic systems comprising tributyl phosphate (TBP) in dodecane and uranyl nitrate in the aqueous phase, which are key chemical species in the well-known Pu-U extraction (PUREX) process. An attempt has been made to understand the nature of interface and mechanism of 'TBP associated uranyl' crossing under neutral and acidic conditions. Results show that the solvent density undergoes large fluctuation near the interface depending on the nature of the aqueous-organic phase. The study provides compelling evidence of experimentally observed reorganization of interfacial complexes at the interface and their structural reformation during extraction. It has been observed that the surface active nature of TBP and their interfacial coverage is modulated by the nature of incorporated solute species and their location with respect to the interface. Also, the TBP structuring near the interface is destroyed when an acidic interface is considered rather than a neutral one which favors the uranyl extraction. With an acidic interface, the water humidity of organic phase was observed to be increased in the experiments. Furthermore, the acid/water solubility in the organic phase was observed to be influenced by selection of acid models and their concentration. Simulations with high acid concentration show water pocket formation in the organic phase. However, in the case of dissociated ions or a mixture of both, no such water pool is observed and the extracted water remains dispersed in the organic phase, having the tendency to be replaced by HNO3 because of preferred TBP·HNO3 complexation over TBP·H2O. Most remarkably, the present study makes evident the TBP-induced charge redistribution of uranyl complexes during migration from the interface to the bulk organic phase, which contributes to drive uranyl complexes such as UO2·NO3·4TBP, UO2·5TBP and UO2·NO3·3TBP·HNO3 in the organic phase, and this was reestablished by DFT calculations. Thus, the present study inspires future simulation and experimental investigations on the extraction of species at the liquid-liquid interface.

Liquid interfaces related to lanthanide and actinide chemistry studied using vibrational sum frequency generation spectroscopy

“…Some molecular dynamics simulation studies h ave suggested that many t y pes of u ranyl-TBP complexes, such as UO 2 (NO 3 ) 2 (TBP) 2 , UO 2 (NO 3 ) 2 (H 2 O)(TBP) 2 , and UO 2 (NO 3 ) (TBP) 4 , are formed at the interface before extraction into the organic phase. [15][16][17][18] However, there have not been any experimental evidence of the complex formation at the interface. Figure 4 shows the vibrational spectra of the air/aqueous-TBP interface, and the bands observed are due to the P=O stretch of TBP, 9 indicating the existence of TBP at the interface.…”

Uranyl Speciation in the Presence of Specific Ion Gradients at the Electrolyte/Organic Interface

“…S3). Although this was noted within the simulation literature, 37,41,62 there lacks strong experimental evidence for such long-range correlations. In combination, these data preclude simple calculation of the equilibrium constants K 1 and K 2 because of the complex equilibria with higher-nitrate containing species and water or nitrate bridged multinuclear U-containing configurations.…”

“…The force field parameters describing uranyl nitrate interactions have a significant impact on the associated association constants and long-range interactions between intact uranyl nitrate complexes. The literature has relied heavily upon force fields developed by Wipff and coworkers, [37][38][39]62 can alter both structural and dynamic properties, has not been studied in detail.…”

Interfacial Heterogeneity is Essential to Water Extraction into Organic Solvents

“…The MD study of TBP is extensive, including the development of several different force fields with parameters empirically fit to various physicochemical properties and under a range of conditions, including pure TBP solutions, 1-5 binary mixtures with organic solvents 2-8 and interfacial TBP/alkane/aqueous systems. [9][10][11][12] TBP is known to adsorb to at the interface with its dipole, aligned with the P=O bond of the phosphate head group, oriented towards the aqueous phase. 13 It has further been predicted that TBP undergoes a significant conformational change in the interfacial region, 14 and prior studies have described the dynamic process by which TBP extracts water molecules from the aqueous/organic interface into the bulk organic.…”

“…9 Relative solubilities and structures of species between two phases can be obtained from simulation techniques including Gibbs ensemble Monte Carlo simulation. [10][11][12] However, to understand the species and structures which form at the interface and inform the kinetics of extraction, explicit simulation of the interface is required. In principle, interfacial organization and spatial fluctuations can be utilized as design features to control interfacial speciation and transport, creating more robust extraction gateways that increase separations efficiency.…”