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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]

[Display omitted] • HCl only dissociates to the solvated H 3 O+...Cl− on the surface of hydrochloric acid. • Protonation of A336 promotes the migration of H 3 O+ towards the interface. • Extraction of easily hydrated PdCl 4 2− ions enhanced due to the migration of H 3 O+. • Enrichment of Cl− ions at the interface inhibits the extraction of PtCl 6 2−. • Reverse migration of H 3 O+ and Cl− promotes the separation of Pt (IV) and Pd (II). The separation process in hydrochloric acid environments is determined by the interfacial chemistry of the acid. However, the surface species of concentrated acids are complex. The effect of dominant species on the surface of concentrated acids and their dissociation behavior on interfacial separation has been the focus of academic debate. The extraction and separation of two typical anions, PdCl 4 2− and PtCl 6 2−, with different hydration characters on the surface of concentrated hydrochloric acid were investigated by thin layer extraction (TLX). A new phenomenon was discovered that the effect of hydrochloric acid concentration, extractant concentration, organic film thickness, and shear flow rate on the extraction of easily hydrated PdCl 4 2− anions is greater than that of hardly hydrated PtCl 6 2−. However, during the conventional SX process, the phenomenon cannot be observed due to the continuous dispersion and aggregation of oil droplets. It found that HCl molecules only dissociate into solvated H 3 O+...Cl− ion pairs on the surface of concentrated hydrochloric acid, but not H 3 O+ and Cl− ions respectively. The concentration of dissociated H 3 O+ ions at the interface is lower than that in the bulk phase. The protonation of the A336 molecule promotes the migration of dissociated H 3 O+ ions in the bulk phase towards the interface. Because the PdCl 4 2− anions are more likely to be hydrated than PtCl 6 2− anions and combine with H 3 O+ ions, the orderly migration of H 3 O+ ions in the laminar boundary layer towards the interface drives the mass transfer of easily hydrated PdCl 4 2− anions. The anion exchange between PdCl 4 2− and Cl− ions associated with the A336 molecules results in a higher concentration of Cl− ions enriched at the interface than that in the bulk phase. The migration of easily hydrated Cl− ions towards the bulk phase would drag the water molecules around PtCl 6 2− anions and therefore inhibit the mass transfer of PtCl 6 2− anions. The orderly reverse migration of H 3 O+ and Cl− ions enhances the separation between easily hydrated PtCl 6 2− and hardly hydrated PdCl 4 2− anions. This study offers a new perspective on the dissociation of species on the surface of concentrated hydrochloric acid and its impact on separation processes. [ABSTRACT FROM AUTHOR]