Specific Salt Effect on the Interaction between Rare Earth Ions and Trioctylphosphine Oxide Molecules at the Organic–Aqueous Two-Phase Interface: Experiments and Molecular Dynamics Simulations

Abstract

Understanding the essence of the specific salt effect on the enhancement of the transport of metal ions across the liquid/liquid interface during the process of solvent extraction is of crucial importance for the development of a new approach to extract and selectively separate various valuable metals from complex aqueous solutions. However, some abnormal experimental phenomena involved in the salt effect on the liquid/liquid solvent extraction could not be understood only from the conventional interpretation based on the salting-out ability of salt ions. The knowledge into the microscopic mechanism behind the specific salt effect was urgent. Herein, as an example, the effect of adding various salts on the extraction performance of rare earth ion Pr using trioctylphosphine oxide (TOPO) as the organic extractant was investigated. It was revealed that the difference in the interface propensity of different salt anions enriched at the organic-aqueous two-phase interface played a crucial role in promoting the interaction of TOPO molecules with Pr ions, despite the occurrence of the salting-out effect of those salt anions with the increase of their concentrations in aqueous solutions. The interfacial interaction mechanism obtained by molecular dynamics simulations revealed that both the interface propensity and the salting-out ability of the coexisting salt anions contributed to the enhancement in the extraction of Pr into the TOPO organic phase. However, when the concentrations of coexisting salts in aqueous solutions were low enough, the extraction of Pr was mainly dominated by the interface propensity of those added salt anions but not their salting-out ability. With the increase in the concentration of salts, the salting-out effect gradually became significant and, therefore, began to join with the interface propensity of salt anions to co-dominate the transport of Pr ions across the liquid/liquid interface. The present study highlights the microscopic nature of the salt effect on promoting the extraction of rare earth ions and suggests that the interaction of organic extractant molecules with rare earth ions at liquid/liquid interface was dependent not only upon the salting-out ability of the coexisting salt ions but also their interface propensity.

Ion-specific clustering of metal–amphiphile complexes in rare earth separations

“…[51][52][53] Ion-specific effects in bulk phase 54 and at the interfaces 50,55 can play a significant role in SX. 14,56,57 TOMA-NO3 system stands out in all known lanthanide SX systems, as it preferentially extracts light lanthanides over heavy lanthanides (DLa/DLu ~ 500) (Figure 1c, dataset D). Some plausible sources of this preference are differences in aqueous phase speciation, interfacial phenomena, and organic phase speciation and aggregation.…”

Anions Enhance Rare Earth Adsorption at Negatively Charged Surfaces

“…Although ion-specific effects have been known since the famous experiments of Hofmeister, 3,16 it is difficult to describe them with a unified, molecular-scale model, especially when the surface functionalization and ion-ion correlations originating from direct electrostatic interactions creates a complex network of interactions. [15][16] The recent advancements in surface specific spectroscopy 15,[17][18][19][20][21] and computational 22 methods, provide new opportunities in understanding these complex interfacial interactions.…”

Origins of Clustering of Metalate-Extractant Complexes in Liquid-Liquid Extraction

“…12,14 This suggests that the trends observed at the nanoscale interfaces in the organic phase could be related to ion-extractant interactions at the macroscopic interfaces. 18,[64][65][66] , respectively. The variation of (a) core radius of the aggregate, (b) electron density of the core, (c) the apparent hard-sphere volume fraction, and (d) aggregates per cluster are shown.…”