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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]

Extraction and separation of V(V) and Cr(VI) in the alkaline solution obtained from the Sub-molten Salt Method to treat the high chromium vanadium slags has been a thorny problem in the past decades. In this work, a new liquid-liquid solvent extraction strategy was proposed by using acidified primary amine N1923 (denoted as A-N1923) to separate V(V) from alkaline solution containing Cr(VI) and Al(III). It was indicated that co-extraction of Cr(VI) and Al(III) with V(V) can be avoided with the addition of trisodium citrate previously into the feed solutions. The influence from various factors, including the concentration of trisodium citrate, the initial aqueous pH value, concentration of A-N1923 in the organic phase, diluents in the organic phase, and the initial concentration ratio of V(V) to Cr(VI) in alkaline solutions, on the separation of V(V), Cr(VI) and Al(III) were investigated. The extraction loading capacity of V(V) in the organic phase could reach 6.67 g/L at 298 K. Al(III) in the organic phase can be stripped selectively using 0.18 mol/L trisodium citrate solution, and then V(V) can be recovered by 1.0 mol/L NaNO 3 solutions. The regeneration and reusability of the organic phases after stripping were discussed. The present work highlights a novel approach to separate and recover V(V) from alkaline leaching solution containing Cr(VI) and Al(III), especially being applicable for those strongly alkaline solution produced by treating the high chromium vanadium slags using the Sub-molten Salt Method. [ABSTRACT FROM AUTHOR]

Abstract: Three-liquid-phase extraction is a new technique to separate Pt(IV), Pd(II) and Rh(III) from their hydrochloric acid leaching solutions. The selection of phase-forming components is crucial to induce a stable three-liquid-phase system and the controllable three-phase partitioning of Pt(IV), Pd(II) and Rh(III). In present work, various phase-forming salts with different cations and anions, amphiphilic block copolymers with different molecular weight and PPO content, organic extractants with different metal-selective functional groups were investigated to discuss their influence on three-phase mass transfer behaviors of three platinum metals. Experimental results indicated that the formation of three-layered coexisting liquid phases in system of S201/nonane-EOPO2500-Na2SO4-H2O resulted in three-liquid-phase selective partitioning of Pt(IV), Pd(II) and Rh(III). Most of Pt(IV) and Pd(II) were extracted into EOPO-rich phase with the two-phase splitting of EOPO2500-Na2SO4 mixtures, whereas Rh(III) remained in bottom salt aqueous phase. Addition of S201/nonane promoted the separation between Pd(II) and Pt(IV), and most of Pd(II) transferred from EOPO-rich phase into S201 organic top phase of TLPS. Under the optimal conditions, 98.5% of Pd(II), 90.8% of Pt(IV) and 87.1% of Rh(III) were enriched into the top organic, middle polymer and bottom salt aqueous phases, respectively. [Copyright &y& Elsevier]

Abstract: A novel technique, gas-assisted three-liquid-phase extraction (GATE), was proposed to improve three-liquid-phase separation of p-nitrophenol (p-NP) and o-nitrophenol (o-NP) from wastewater. The partition behavior of p-NP and o-NP into a three-liquid-phase system composed of hexane–PEG2000–(NH4)2SO4–H2O at the aid of gas bubbling was discussed. It was demonstrated that the process of gas bubbling was effective for the mass transfer of p-NP and o-NP into PEG phase and hexane phase, respectively. Influences of various parameters including the phase-forming salt concentrations, gas flow rate, gas bubbling time, initial volume of hexane and PEG phase on distribution behavior of p-NP and o-NP were investigated. Under the optimal conditions for GATE, the removal ratio of p-NP and o-NP from wastewater could reach 81% and 90%, respectively. While the mass fraction of p-NP into PEG phase and o-NP into hexane phase arrived at 75% and 74%, respectively. The distribution ratio and concentration factor were all higher than conventional three-liquid-phase extraction system. During GATE processes, the dispersed PEG and hexane in (NH4)2SO4 aqueous solution can be captured by ascending gas streams. GATE is demonstrated to be an effective method for the simultaneously extraction and separation of two or more organic chemicals in one-step''s three-liquid-phase extraction, and also provides a technique to prevent loss of dispersed polymers and organic solvents in aqueous phase in extraction process for treatment of the phenolic wastewater. [Copyright &y& Elsevier]

• 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] • 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]