•A method is proposed to probe intermolecular interaction of two solutes where a solute has no peak.•2D asynchronous spectrum can be used reliably to characterize intermolecular interactions.•This approach is proved to be applicable for real chemical systems.We explore whether it is possible to use 2D correlation spectrum to characterize intermolecular interactions between two solutes dissolved in the same solution when one substance does not possesses any characteristic peak. We demonstrate that the interaction can be manifested by characteristic cross peaks in 2D asynchronous correlated spectrum. The above cross peaks reflect the subtle spectral variations on the characteristic peak of another solute under intermolecular interaction. On the other hand, 2D synchronous spectrum is not suitable to characterize intermolecular interaction since the cross peaks contain irremovable interfering parts. The terbium-chloride/benzamide/methanol system is used to demonstrate that this approach is applicable in the real chemical system.

The present work puts forward a new strategy to achieve group separation of light, middle, and heavy rare-earth ions by liquid–liquid–liquid three-phase systems. Experimental results indicated that heavy rare-earth ion, Yb(III), can be selectively extracted into the organic top phase, while middle rare-earth ion, Eu(III), and light rare-earth ion, La(III), can be enriched, respectively, into PEG-rich middle phase and (NH4)2SO4-rich bottom aqueous phase of Cyanex272/PEG/(NH4)2SO4–H2O three-liquid-phase system. Various influences of aqueous pH, (NH4)2SO4 concentrations, complexing agents, polymers, and their added amount on three-liquid-phase separation of La, Eu, and Yb were investigated. Besides, a possible partitioning mechanism for three rare-earth ions was discussed. The present work highlights the possibility of employing the discrepancy in selective partitioning behaviors of different rare-earth ions (such as La, Ce, Eu, Gd, Yb, and Lu) into different liquid layers of three-liquid-phase system to achieve one-step group separation of light, middle, and heavy rare-earth ions. In comparison with traditional organic-aqueous biphasic extraction for group separation of rare-earth ions, which require multiple steps of pH adjustments and repeated separation, the three-liquid-phase approach exhibits obvious advantages for future industrial application.

The present work reports an interesting phenomena that the addition of hydrophobic ionic liquid, [C4mim]PF6, into the oil–water two phase system of diisopentyl sulfide (S201)–nonane organic solutions and hydrochloric acid aqueous solutions containing Pt (IV), Pd (II) and Rh (III) chloric-complexing anions could induce the formation of a stable liquid–liquid–liquid three layered liquids coexisting medium for extraction and one-step separation of platinum, palladium and rhodium. The influences of H+ and Cl− concentrations in equilibrating aqueous solutions, the initial Pt (IV) concentration, the volume ratio of added [C4mim][PF6] to hydrochloric acid aqueous solution and the different salt ions in equilibrating aqueous solutions on the three-liquid-phase partitioning behaviors of three platinum metal ions were evaluated. The transferring of PtCl62- into the [C4mim]PF6 bottom phase in TLPS was proposed to be according with an anion exchange between PtCl62- and PF6- in ionic liquid. Therefore, the separated ionic liquid phase is not the solvent of organic extractants, but provides an extended separation capacity than the literature reported ionic liquid based two-phase extraction systems. The present work indicated that this ionic liquid based three-liquid system can be used to separate multi-metal coexisting solutions.Highlights► An ionic liquid based three-liquid-phase system for separation of Pt, Pd and Rh. ► Selective partitioning of Pt, Pd and Rh in three liquid phases, respectively. ► Possible mechanism for Pt(IV) extraction by [C4mim][PF6] is ion-exchange.

The main purpose of present review article is to summarize our previous efforts to develop a new three-liquid-phase extraction strategy for recovery and recycling of platinum, palladium and rhodium from the hydrochloric acid leach solutions of various wasted auto-catalysts.It was found that Pt, Pd and Rh can be separated and selectively enriched into the three different liquid phases of the proposed three-liquid-phase system (TLPS), respectively. The partitioning behaviors of Pt(IV), Pd(II), Rh(III) are controllable and have a close correlation with the phase-forming behaviours of TLPS. A microphase mass transfer mechanism was proposed to explain the differences in partitioning behaviours of three platinum metal ions. The self-assembly of amphiphlic extractant molecules in aqueous solutions result in the controllable enrichment of target metal ions by those separated microphase aggregates. The differences in interfacial mass transfer of three metals are due to a controllable microscopic interaction of platinum metal ions with the microphase aggregates.The TLPS provide a unique separation medium to control the interphase mass transfer of different metal ions. The separation selectivity of the three platinum metals was improved remarkably by using the three-liquid-phase extraction technique in comparison with the traditional liquid–liquid two phase extractions. The present researches highlight a potential possibility for future industrial application of the multiphase extraction for one-step separation of different target metals from complex mixtures.

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.Highlights► S201, EOPO, Na2SO4 are suitable components for three-phase separation of Pt, Pd, Rh. ► ATPS of EOPO-Na2SO4 provides high selectivity for Pt and Pd. ► Addition of S201 promotes the separation between Pt and Pd. ► 98.5% of Pd, 90.8% of Pt and 87.1% of Rh were separated into three respective phases.

This paper proposed a novel sugaring-out assisted three-liquid-phase system (TLPS) of diisopentyl sulfide (S201)–acetonitrile–glucose–water to extract and separate platinum, palladium and rhodium from their hydrochloric acid solution. Experimental results demonstrated that Pd(II), Pt(IV) and Rh(III) could be separated in one-step extraction process, respectively into the S201 organic top phase, sugaring-out acetonitrile-rich middle phase and aqueous bottom phase of the TLPS. The influences of different phase-forming agents, glucose concentration, HCl concentration in equilibrating aqueous phase and the initial volume ratio of added acetonitrile to aqueous glucose solution on partitions of platinum-group metal (PGM) ions were discussed. It was found that the three-liquid-phase partition behaviors of Pd(II), Pt(IV) and Rh(III) had a close relation with the phase-forming behavior of TLPS. The recovery of Pt and Pd were investigated. Sugaring-out assisted TLPS exhibited obvious advantages over the traditional salting-out systems.Highlights► A sugaring-out assisted three-liquid-phase system for separation of Pt, Pd and Rh. ► Three-liquid-phase partitions of Pt, Pd and Rh have a close relation with the phase-forming behavior. ► Sugaring-out induced three-liquid-phase extraction exhibits obvious advantages over the salting-out systems.

Partitioning of two model organic compounds, phenol and o-nitrophenol (o-NP), in aqueous two-phase system (ATPS) of PEG2000–(NH4)2SO4 and three-liquid-phase system (TLPS) of butyl acetate (BA)-PEG2000–(NH4)2SO4 were systematically investigated. Experimental results indicated that the enrichment of phenol and o-NP in the salting-out PEG2000 top phase of ATPS as well as the selectively transfer of the two compounds respectively into BA top phase and PEG2000 middle phase of TLPS were closely related to the phase-forming behavior of ATPS and TLPS. The aggregation behavior of PEG macromolecules is crucial for the phase evolution and distribution of phenol and o-NP. Variation in distribution ratios resulted from the change in microenvironment of salting-out PEG2000 phase induced by increasing concentrations of PEG, (NH4)2SO4 and pH. Addition of BA promoted the unevenly mass transfer of phenol from PEG2000 aqueous phase to organic BA phase, leading to the separation between phenol and o-NP. A suggested mass transfer model describes the entrapment of phenolic compounds from salt aqueous phase to PEG2000 phase under the modulation of salt concentrations and system pH. A possible microscopic mechanism was proposed to describe the selective mass transfer of phenol from PEG2000 phase to BA organic top phase in the resultant TLPS of BA-PEG2000–(NH4)2SO4.Graphical abstractThe mechanism of PEG in aqueous two-phase system for trapping phenolic molecules induced by salt concentration (A, B) and equilibrium of phenol and o-nitrophenol between PEG microphase and salt aqueous phase with the change of system pH (C, D).Highlights► Aggregation of PEG is crucial for the phase behavior of two or three-phase system. ► Phase properties in microphase interfaces are correlated with phase behavior. ► Selective partitioning of phenolic compounds related to interfacial phase property. ► Distribution process can be controlled by modulating phase property. ► Separation between phenol and o-NP is achieved in two or three-phase system.

Three-liquid-phase partitioning of Pd(II), Pt(IV) and Rh(III) in systems of S201(diisoamyl sulfide)/nonane–EOPO(polyethylene oxide–polypropylene oxide random block copolymer)–Na2SO4–H2O was investigated. Experimental results indicated that the selective enrichment of Pd(II), Pt(IV) and Rh(III) respectively into the S201 organic top phase, EOPO-based middle phase and Na2SO4 bottom phase was achieved by control over the phase behavior of the three-liquid-phase systems (TLPS). The microphase mass transfer behavior of Pt(IV), Pd(II) and Rh(III) was closely related to the micellization of EOPO molecules. A suggested micro-mechanism model and a mass transfer model describe the micellization of EOPO molecules and the effect on mass transfer of platinum ions across the microphase interfaces. The salting-out induced continuous dehydration and ordered arrangement of the hydrophilic PEO segments in amphiphilic EOPO micelle, and these are the main driving forces for mass transfer of platinum metal ions onto the exposed activity sites of the dehydrated PEO segments. The differences in microphase interfacial structure of EOPO micelles are crucial for the efficient separation between Pt(IV), Pd(II) and Rh(III).Graphical abstractA mass transfer model describes the anionic exchange process between platinum metal ion complexes and the phase-forming salt anions across the microphase interfaces of copolymer micelles.Highlights► The appearance of three liquid phases permits one-step separation of Pd, Pt and Rh. ► The phase behavior has a correlation with three-phase partitioning of Pd, Pt and Rh. ► Microphase interfacial structure is an important factor in mass transfer of metals.

Three-liquid-phase extraction has been considered to be a promising method for isolation and separation of multicomponents. Selective extraction–separation of Ti(IV), Fe(III), and Mg(II) in the three-liquid-phase system containing trialkylphosphine oxide (TRPO), poly(ethylene glycol) with an average molecular mass of 2000 (PEG 2000), and (NH4)2SO4 was achieved by adding a water-soluble complexing agent, ethylenediaminetetraacetic acid (EDTA). The simple and environmentally benign complexing method was proved to be an effective strategy for enhancing the selectivity of the PEG-2000-rich middle phase for Fe(III) without reducing the affinity of the TRPO-rich top phase to Ti(IV). The related chemistry was detailed, and effects of some important parameters such as the aqueous phase pH, EDTA amount, mixing time, and temperature were examined. It revealed that Ti(IV) and Fe(III) could be enriched respectively into the TRPO-rich top phase and the PEG-2000-rich middle phase through optimization of the aqueous pH and the EDTA amount. Ti(IV) was extracted into the top phase with a slow kinetics and a positive enthalpy change (ΔH), whereas Fe(III) was rapidly transferred to the middle phase as Fe(III)–EDTA complexes formed by an exothermic reaction. Mg(II) tended to remain in the bottom phase.

Three-liquid-phase extraction and one-step separation of platinum, palladium, and rhodium in the system composed of diisoamyl sulphide (S201), polyethylene oxide-polypropylene oxide random block copolymer (EOPO), Na2SO4, and H2O were investigated. Experimental results indicated that phase-forming salt type, salt concentrations, coexisting H+ and Cl– concentrations in equilibrious Na2SO4 aqueous solutions have significant influences on the three-liquid-phase partitioning behaviors of Pt(IV), Pd(II), and Rh(III). Under the optimized operation parameters, over 99 wt % of Pd(II), about 90 wt % of Pt(IV), and 85 wt % of Rh(III) in initial feed solutions were respectively concentrated into S201 top phase, EOPO middle phase, and Na2SO4 bottom aqueous phase. The present work explores a possibility to develop a three-liquid-phase extraction approach for one-step separation of platinum metal ions in highly concentrated acidic chloride media obtained by hydrometallurgical processes.

A novel and simple three-liquid-phase extraction (TLPE) approach was developed for the simultaneous removal and separation of Cr(III) and Cr(VI) from aqueous solutions. The proposed three-liquid-phase system (TLPS) consists of di(2-ethylhexyl)phosphoric acid (D2EHPA), poly(ethylene glycol) (PEG) with a molecular weight of 2000, and (NH4)2SO4. The effects of various factors including the aqueous solution pH, amounts of (NH4)2SO4 and PEG, initial chromium amount, D2EHPA concentration, phase-mixing time, and diluent on the three-phase partitioning behavior of Cr(III) and Cr(VI) were evaluated. The distribution behavior of the two oxidation states of chromium was found to be highly pH-dependent. Cr(III) preferred the D2EHPA-rich top phase through a cation-exchange reaction, whereas Cr(VI) was enriched in the PEG-rich middle phase through ion-pair formation. By appropriate selection of the extraction conditions, nearly all of the Cr(III) was extracted into the D2EHPA top phase, and more than 90% of the Cr(VI) was transferred into the PEG-rich middle phase within 5 min. The present work highlights the possibility of using the TLPE approach for the extraction and separation of two different oxidation states of metal ions, such as Cr(III) and Cr(VI), in a single extraction step.

A new mixer-settler-mixer three chamber integrated extractor is proposed in this work for liquid-liquid-liquid three phase countercurrent and continuous extraction. Experiments revealed the influences of the structural design of the three-liquid-phase extractor and some key operational parameters on three-phase partition of two phenolic isomers, p-nitrophenol (p-NP) and o-nitrophenol (o-NP). The model three-liquid-phase extraction system used here is nonane (organic top-phase)-polyethylene glycol (PEG 2000) (polymer middle-phase)-(NH4)2SO4 aqueous solution (aqueous bottom-phase). It is indicated that agitating speed and retention time in three-phase mixer are key parameters to extraction fraction of nitrophenol. Dispersion band behavior is related to agitating intensity, and its occurrence does not affect the extraction fraction of target compounds. The present work highlights the possibility of a feasible approach of scaling up of the proposed three-phase extraction apparatus for future industrial-aimed applications.

The present work reports an interesting phenomena that the addition of hydrophobic ionic liquid, [C4mim]PF6, into the oil–water two phase system of diisopentyl sulfide (S201)–nonane organic solutions and hydrochloric acid aqueous solutions containing Pt (IV), Pd (II) and Rh (III) chloric-complexing anions could induce the formation of a stable liquid–liquid–liquid three layered liquids coexisting medium for extraction and one-step separation of platinum, palladium and rhodium. The influences of H+ and Cl− concentrations in equilibrating aqueous solutions, the initial Pt (IV) concentration, the volume ratio of added [C4mim][PF6] to hydrochloric acid aqueous solution and the different salt ions in equilibrating aqueous solutions on the three-liquid-phase partitioning behaviors of three platinum metal ions were evaluated. The transferring of PtCl62- into the [C4mim]PF6 bottom phase in TLPS was proposed to be according with an anion exchange between PtCl62- and PF6- in ionic liquid. Therefore, the separated ionic liquid phase is not the solvent of organic extractants, but provides an extended separation capacity than the literature reported ionic liquid based two-phase extraction systems. The present work indicated that this ionic liquid based three-liquid system can be used to separate multi-metal coexisting solutions.Highlights► An ionic liquid based three-liquid-phase system for separation of Pt, Pd and Rh. ► Selective partitioning of Pt, Pd and Rh in three liquid phases, respectively. ► Possible mechanism for Pt(IV) extraction by [C4mim][PF6] is ion-exchange.