Micro-structural analysis of aqueous uranyl ions (UO₂²⁺) during the course of forward and back extraction in A...

The extraction and back - extraction are the two important interfacial phenomena which guides the separation of metal ions in a liquid-liquid extraction process having wide ranges of technological applications. The determination of structural aspects at the atomistic level of these two interfacial events is very crucial to develop an understanding from molecular level besides the mechanism of occurrence. The pair correlation function (PCF) between two interacting species has been exploited to carry out the structural analysis of these interfacial event. The 1st peak height of PCFs between two interacting uranium atom (U) of the uranyl ion and oxygen atom (OP) of phosphoryl group of TiAP molecule is found to be increased during extraction of uranyl ion up to 6M of acidity as reached to 0.35 and then decreased. The trend is similar to the profile of the distribution coefficient (K_D) of uranyl ion for wide range of acid concentrations. Further, the 1st peak height of U-OP PCFs are seen to be decreased during the progress of back-extraction as the uranyl ions move into the aqueous phase from the organic phase. The U-OP PCFs for different ligand-solvent systems could explain the efficiency of back-extraction obtained by TBP/dodecane system which is further confirmed by the PCFs between uranium atom (U) and oxygen atom (OW) of water molecule and PCFs between oxygen atom (OP) of phosphoryl group of TiAP molecule and oxygen atom (ON) of nitrate ion. The former gives comparatively higher peak height for TBP (TBP - 31.3 and TiAP - 29.1) and the later offers higher peak height for TiAP (TiAP - 1.2 and TBP <0.3) which indicates that the TiAP molecule retained some uranyl ions and confines them into the aqueous phase due to the molecular aggregation near the interface. Moreover, the 1st peak height of PCFs between oxygen atom (OP) of phosphoryl group of ligand molecule and hydrogen atom (HW) of water molecule disclosed the higher hydration of TiAP (0.8) at the interface which leads to higher surface roughness compared to TBP (0.27). The structural behavior captures the complex interfacial phenomena through the density changes as a function of distance from any reference atom. Further, the spatial distribution function (SDF) is computed to understand the spatial position of uranyl ion with the progress of simulation length during back - extraction. The present atomic level findings might contribute to the fundamental understanding of the complex interfacial phenomena for the metal ion transport and recovery.