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[Display omitted] • Interfacial salt effect is closely related to ion diffusion rate in the near-interface boundary layer. • Diffusion rate of target ions is subject to the concentration of coexisting salt ions. • Low concentration of coexisting salt ions can improve the diffusion rate of target ions and its separation. • Oil film thickness ultimately affects the diffusion rate of target ions and separation. • A quantitative correlation of the diffusion rate with the concentration of coexisting salt ions was established. Specific ion effects are known to influence the interfacial behavior of ions in the aqueous phase and thus affect their extraction and separation. There are many studies on enhanced extraction and separation of the target ions based on the specific ion effects. A microscopic level understanding of the competitive mass transfer and selectivity of ions at the interface between heterogeneous phases is crucial importance for the precise control of numerous chemical reaction and separation processes. The conventional salt effect (i.e., background coexisting ions promoting the mass transfer of the target ions) are only observed at higher coexisting ion concentrations. However, the present work reveals an interesting phenomenon: the adsorption enrichment and competitive hydration of coexisting salt cations at the liquid–liquid interface can significantly influence the mass transfer rate of the target rare earth ions in the near-interface boundary layer. Notably, such a distinct interfacial salt effect can promote the separation between coexisting and target ions at lower coexisting ion concentrations, and it cannot be explained by the traditional theory for the salt effect. A thin-layer organic oil film extraction system was employed here as a simplified model experiment to investigate this new interfacial salt effect on the non-equilibrium extraction and separation of target rare earth ions. The results demonstrated that the diffusion rate of the target ions (Er3+) is subject to the local concentration gradient of coexisting salt cations (Mg2+) in the near-interface boundary layer. At the lower bulk concentrations, the Mg2+ ions enriched at the interface would compete for water molecules around the hydration shell of target Er3+ ions, resulting in the dehydration of Er3+ ions. Therefore, an enhanced diffusion rate of Er3+ ions and their adsorption affinity towards the interface were observed. However, at a higher bulk concentration of Mg2+ ions, the diffusion resistance of Er3+ ions increased due to the competitive adsorption and hydration of Mg2+ ions at the interface. The thickness of the organic oil film layer also plays an important role in the diffusion of Er3+ ions. In particular, a thinner oil layer makes it easier to perceive the interfacial salt effect, which enhances the separation between Er3+ and Mg2+ compared to the case of a thicker oil layer. A quantitative correlation between the diffusion rate of Er3+ ions and the concentration of coexisting Mg2+ ions was established to describe the interfacial salt effect. This work highlights the microscopic nature of the salt effect induced by the competitive adsorption kinetics of coexisting ions in the near-interface boundary layer on promoting the extraction and separation of target metal ions. The results will help the development of new approaches to control the separation selectivity and enhance the mass transfer of target metal ions in practical applications. [ABSTRACT FROM AUTHOR]

• BSOLM extraction promotes extraction and separation of rare-earth ions. • An enhanced interfacial salt effect occurs during BSOLM extraction. • Extraction and separation of Er(III) enhanced at low concentrations of coexisting salts. • Specific interfacial salt effects of Mg(II), Fe(III), Al(III) cations are discussed. Understanding the essence of specific salt effect on mass transfer of metal ions acrossing liquid–liquid interface is of crucial importance to intensify extraction and separation of target metal ions. In present work, the effect from various typical coexisting electrolyte salt cations, Mg(II), Al(III), Fe(III) cations, in the in-situ leaching solutions of ion-adsorption type rare earth ores on extraction and separation of heavy rare earths (Er(III) ions as a representative) by our previously suggested new method, bubble-supported organic liquid membrane (BSOLM) extraction, was investigated. It is found that extraction and separation of low concentration Er(III) ions can be significantly enhanced, even when the concentrations of Mg(II), Al(III) and Fe(III) cations are low enough. It cannot be explained by the traditional salt effect theory. Experiments revealed that BSOLM extraction results in an enhanced interfacial salt effect due to its essence of mass transfer only occurring on the surface of thin-layer organic extractant liquid membrane. It is benefit for intensifying the interfacial competitive adsorption of coexisting salt cations. The competitive hydration of those salt cations at interface promotes the extraction of low concentration Er(III) ions, whereas their competitive adsorption is unfavorable of the separation of Er(III). The present work highlights the microscopic nature of specific interfacial effect of various coexisting electrolyte salt cations on extraction of target rare-earth ions. It is benefit for development of new methods to intensify extraction and selective separation of low concentration rare-earth ions from the electrolyte salt in-situ leaching solutions of ion-adsorption type rare earth ores. [ABSTRACT FROM AUTHOR]

[Display omitted] • A new strategy of soft membrane based molecular recognition was proposed. • Recognition of anions on surface of hydrochloric acid is controllable by shear flow. • Hydration shells of easily hydrated anions are easily recognized. • Shear flow has an influence on recognition of the hydration configuration of anions. • Molecular recognition of anions depends on the hydration ability of anions. Understanding the self-assembly behavior of macrocyclic molecules under shear flow and its impact on the mass transfer of target ions near the liquid–liquid interface is of great importance in host-guest chemistry and mordern separation processes. In this work, a new strategy based on soft-film molecular recognition (SF-MRT) was proposed to investigate the effect of aqueous shear flow on specific recognition of octahedral RhCl 6 3-, octahedral PtCl 6 2-, and planar quadrilateral PdCl 4 2- anions with different hydration configurations using 18-crown-6 as an extractant spreaded on the surface of concentrated hydrochloric acid. Remarkably, the SF-MRT process achieved an extraction order of RhCl 6 3- > PdCl 4 2- > PtCl 6 2-, whereas the impregnated resin coated with a layer of oil film containing 18-crown-6 yielded a reverse extraction order of PtCl 6 2- > PdCl 4 2- > RhCl 6 3-. Experimental results demonstrated the functional group (-SO 3 Na+) present on the surface of the resin particles exhibits a preferential adsorption of H 3 O+ ions through the cation exchange mechanism, resulting in crown ether molecules only recognizing H 3 O+ ions, and subsequently facilitating the preferential extraction of PtCl 6 2- anions by 18-crown-6·H 3 O+ through the ionic association mechanism. However, during the SF-MRT process, the hydrophilic and flexible cavity formed by the ether oxygen group (-O-) in crown ether molecules primarily recognizes the water molecules in the hydration shell surrounding platinum metal anions due to the lower concentration of H 3 O+ dissociated on the surface of concentrated hydrochloric acid. Compared to the sparse water molecules around the hydrated PtCl 6 2- anions, the dense water molecules around the hydrated RhCl 6 3- anions are more easily recognized. The directional and flexible hydrogen bond network formed between the crown ether molecules and platinum metal anions through the "water bridges" interaction is more amenable to be controlled by the laminar shear flow beneath the thin-layer extractant oil film. This discovery lays the foundation for the development of new flexible and controllable molecular recognition methods. [ABSTRACT FROM AUTHOR]