Multistage separation of metal ions with a series of composite supported liquid membranes

The possibility of performing multistage separations of metal cations, present at low concentrations in aqueous solutions, using a series of composite supported liquid membranes (SLM), interposed between compartments containing identical aqueous electrolyte solutions, is discussed. The multistage separation of Eu³⁺. from Am³⁺ and of Sm³⁺ from Nd³⁺ and Ce³⁺ is theoretically analyzed. A single separation stage, using a thin, three-layer asymmetrical composite SLM, which allows differential permeation of Eu³⁺. and Am³⁺, is described and experimentally studied. The composite SLM consists of a 1 mm thick aqueous slab sandwiched between acidic and neutral supported liquid membranes. The results demonstrate that the permeation of each cation through the thin composite SLM can be described by a single permeation parameter which is equal to the permeability coefficient through the first acidic supported liquid membrane of the composite layer.     

A simplified model for the coupled transport of metal ions through hollow-fiber supported liquid membranes

This paper describes a simplified model for the carrier-facilitated transport of metal ions through hollow-fiber supported liquid membranes, HFSLM. The model leads to approximate and simple equations describing the concentration variations expected when an aqueous feed solution is flowing through the lumens of a HFSLM module. The equations incorporate simple and independently measurable parameters and apply to two situations: (a) a once-through mode, i.e., the feed solution passes only once through the module, and (b) a recycling mode, i.e., the feed solution is continuously recirculated through the module. The equations have been tested by measuring the transport of Cu²⁺ ions through microporous polypropylene hollow fibers containing a 0.3 F solution of bis(2-ethylhexyl)phosphoric acid in n-dodecane. HFSLM modules containing a variable number of fibers, fibers of different lengths and operated at different linear flow velocities have been used.     

Permeation of complexed metal ions through a hydrophobic membrane

Selective concentration of a heavy metal complex with acetylacetone (acac) through a hydrophobic polystyrene membrane was carried out under a pressure gradient. A chelate forming heavy metal was selectively concentrated about 2 fold by this method. When using a coating membrane with a high water flux, the permeabilities increased with increasing complex fraction in the aqueous solution, while using a membrane with a low water flux, a bulky complex was not highly concentrated because of steric hindrance. The complex partitioned on the membrane surface was transported and concentrated under a pressure gradient and a linear relationship was found to exist between permeabilities and partition coefficients. It will be possible to concentrate hydrophobic organic solutes by this method, for acac was concentrated when the Cu—acac complex was formed. As the permeabilities increased with decreasing pressure and membrane compaction was strong for a coating membrane, it seems effective to permeate at a low pressure.     

Induction time in metal permeation processes through supported liquid membranes

The origin of the induction time observed in the permeation process of cadmium species through trilaurylammonium chloride in triethylbenzene supported liquid membranes is discussed. A model for the non-steady state transference process, where aqueous film diffusion coupled to an interfacial chemical reaction are the main rate determining processes, was developed. By comparison with experimental transference data, the rate constant of the interfacial reaction between cadmium chloride aqueous complexes and the membrane carrier, trilaurylammonium chloride, was evaluated. The time evolution of the concentration profiles through the aqueous diffusion film is also described.     

金属离子通过大块液膜迁移动力学的研究

本文指出金属离子通过大块液膜的迁移动力学模型。其中A, B, C分别表示在原料相、有机膜相、反萃相中的金属离子, 下标i表示界面; 界面扩散是离子迁移速率的控制步骤。从相应的动力学方程关联实验数据可以得到三个参数(k1, kx, kp),其中kp即为分配比, kp越大, 金属离子在机相中的残留量越小,反之亦然。     

Permeation of hydrocarbons through liquid surfactant membranes and formation of liquid crystalline structures

The separation of benzene from its mixture with heptane can be achieved by emulsifying the feed mixture to form an oil-in-water emulsion and then dispersing this emulsion in kerosene. At high agitation intensity a gel-like structure develops which prevents phase separation and thereby interferes with the overall mass transfer process. These gel-like structures have very high viscosities, and show rheopectic behaviour; polarising microscopic examination of these gel-like structures revealed the existence of liquid crystalline structures on the surface of the emulsion globules dispersed in kerosene. A possible mode of development of such liquid crystalline structures is presented, 1-Pentanol added to kerosene prevents gel formation and assists phase separation. Its action is explained on the basis of Winsor's theory of solubilization. Typical results for selectivity and benzene yield are presented.     

Coupled transport of Ag(I) ions through triethanolamine–cyclohexanone-based supported liquid membranes

Facilitated transport of silver(I) ions in acidic medium, across a supported liquid membrane (SLM) by using triethanolamine (TEA) as carrier, dissolved in cyclohexanone, has been investigated. The parameters studied are HNO₃ concentration variation in the feed, pH of the feed solution, carrier concentration in the membrane phase, silver(I) ions concentration in the feed phase and KCN concentration in the stripping phase. Increase in H⁺ concentration by increasing HNO₃ concentration from 0.5 to 1 M results into an increase in silver ions flux but a decrease in flux has been found beyond 1 M HNO₃ concentration in the feed, providing a maximum flux of 3.21 × 10⁻⁷ mol/m² s at 1 M HNO₃. Increase in TEA concentration inside the membrane enhances flux with its maximum value at 2.25 M TEA. Further increase in the concentration of TEA leads to a decreased rate of transport due to the increase in viscosity of membrane liquid. The optimum conditions for Ag(I) ions transport are 1 M HNO₃ (feed), 2.25 M TEA (membrane) and 1.5 M KCN in the stripping phase. It has been observed that Ag(I) flux across the membrane tends to increase with increase in Ag(I) ions concentration in the feed phase. Applying the studied conditions to silver plating waste solutions, Ag ions have been removed up to 99% in a time interval of 5 h.     

Uranyi ion transport through tri-n-octylamine-xylene based supported liquid membranes

Tri-n-octylamine (TOA) dissolved in xylene has been used as carrier, constituting liquid membrane supported in Celgard 2400 polypropylene microporous film for the transport of uranyl ions against their concentration gradient from aqueous acid solutions to an alkaline aqueous phase. Effect of sttrring rate, nitric acid concentration and TOA concentration in the organic membrane phase, on the flux of uranyl ions through the membrane has been studied. Viscosity and density data have been obtained to estimate diffusion coefficients and hence the permeability coefficients to compare the same with experimental values, using distribution coefficient data, measured from solvent extraction experiments and available in the literature. Analysis of the flux data has been performed to study the stoichiometry of the chemical reaction involved in complex formation reaction. The results have been compared with simple liquid-liquid extraction data.     

Transport of strontium cation through hollow fiber supported liquid membranes

Transport of strontium through supported hollow fiber dichlorobenzene liquid membranes has been studied. The possible mechanism of strontium transport with 18-crown-6 ether as a carrier and picrate as co-counter-ion as well as the construction of a pertraction device with on-line radiometric detection of strontium using⁸⁵Sr tracer is described. Preliminary results of strontium pertraction in a recycling and one-pass mode with different concentrations of crown are shown.     

Permeation of gases through metallized polymer membranes

The permeation of He, H₂, CO₂, Ar, N₂ and Kr at 50°C through polyethyleneterephthalate, PET, membranes metallized with Pd, Ni and Cu was studied. It was found that metallizing a PET membrane changed its permeability for the gases studied, and that the permeability for H₂ varied slightly with differing H₂ pressure. In the range of 0-50°C the temperature dependence of the permeability for He and H₂ was determined. The results obtained were interpreted by assuming that the permeation of all gases, including H₂, through the metal layers of the membranes takes place by diffusion through fine defects which exist in their structure and, moreover, that H₂ also permeates through the Pd and Ni layers themselves. An important point is that by this method an increase of up to an order of magnitude of the membrane selectivity for H₂ was obtained     

Studies on phenol permeation through supported liquid membranes containing functionalized polyorganosiloxanes

In this study, the functionalized, linear, hydrophobic fluid organosiloxane polymers, namely, methylhydrosiloxane–dimethylsiloxane copolymers supported on a polypropylene microporous flat sheet membrane (Celgard 2502 and 2402) have been tested as supported liquid membranes (SLMs) for phenol recovery from aqueous phases into a 0.1 M NaOH phase. The functionalized polymers include, Me₃SiO[MeSi(OR)O]_x[Me₂SiO]_ySiMe₃ (containing x = 15–18, 25–35 and 50–55 mol% of R, where R is –(CH₂)_nNMe₂ (n = 3 or 4 or 6) or –(CH₂)₂OEt pendent organofunctional groups. The functionalities, R, tested were derived from the commercially available 3-dimethylamino-1-propanol and 2-ethoxyethanol as well as newly synthesized 4-dimethylamino-1-butanol and 6-dimethylamino-1-hexanol which have been made for the purpose of this study.

The study showed that phenol permeation expressed as permeate flux through the membranes increases with the larger number of carbon spacers in the alkyl chain of the aminoalcohol pendent, larger porosity of the polypropylene support films, higher mol% of the methylhydrosiloxane portion functionalized and faster flow rates of both the feed and the receiving phases. Phenol permeation was enhanced significantly when the mol% of the methylhydrosiloxane portion was 50–55 or 25–35 with 6-dimethylamino-1-hexanol functionality supported on Celgard 2502.     

支撑液膜研究及应用进展

支撑液膜技术是高选择性膜分离技术,本文对支撑液膜分离技术的研究进展进行了回顾,详述了用于金属离子分离的支撑液膜所采用的载体、影响支撑液膜稳定性的原因以及改善途径,并对支撑液膜的发展前景进行了展望。     

Nitrate ion transport across TBP-kerosene oil supported liquid membranes coupled with uranyl ions

The role of nitrate ions in uranyl ions transport across TBP-kerosene oil supported liquid membranes (SLM) at varied concentrations of HNO₃ and NaNO₃ has been studied. It has been found that nitrate ions move faster compared to uranyl ions at the uranium feed solution concentrations studied. The nitrate to uranyl ions flux ratio vary from 355 to 2636 under different chemical conditions. At low uranium concentration the nitrate ions transport as HNO₃ · TBP, in addition to as UO₂(NO₃)₂ · 2TBP type complex species. The flux of nitrate ions is of the order of 12.10 · 10^–3 mol · m^–2 · s^–1 compared to that of uranium ions (4.56 · 10^–6 mol · m^–2 · s^–1). The permeability coefficient of the membrane for nitrate ions varies with chemical composition of the feed solution and is in the order of 2.5 · 10^–10 m^–2 · s^–1. The data is useful to estimate the nitrate ions required to move a given amount of uranyl ions across such an SLM and in simple solvent extraction.     

Extraction of Pd(II) ions using tri-n-octylamine xylene base supported liquid membranes

Membranes, based on tri-n-octylamine (TOA) xylene liquid, supported in hydrophobic microporous films have been used to study the transport of Pd(II) ions, after extraction into the membrane. Various parameters, such as the effect of hydrochloric acid concentration in the feed solution, TOA concentration in the membrane phase, effect of stripping agent like nitric acid concentration, and temperature on the flux of Pd(II) ions across the liquid membranes have been investigated. The optimum conditions of transport for these metal ions determined are, TOA concentration, 1.25 mol·dm^–3, HCl concentration in the feed solution, 5 mol·dm^–3, and concentration of nitric acid used as a stripping, agent 5 mol·dm^–3. The maximum values of the flux and permeability determined under the optimum condition are 23·10^–6 mol·m^–2·s^–1 and 2.40·10³ m²·s^–1 at 25°C. The results obtained have been used to elucidate the mechanism of palladium transport.     

Selective separation of aromatic hydrocarbons through supported liquid membranes based on ionic liquids

Ionic liquids are emerging as alternative solvents for volatile organic compounds traditionally used in liquid–liquid extraction and liquid membrane separation. In this paper, we examine whether room-temperature ionic liquids as a membrane solution can be utilized for hydrocarbon separation by using a supported liquid membrane. Aromatic hydrocarbons, benzene, toluene and p-xylene were successfully transported through the membrane based on the ionic liquids. Although the permeation rates through the membrane based on the ionic liquids were less than those of water, the selectivity of aromatic hydrocarbons was greatly improved. The maximum selectivity to heptane was obtained using benzene in the aromatic permeation and 1-n-butyl-3-methylimidazolium hexafluorophosphate in the liquid membrane phase.     

Factors influencing the transport of tryptophan hydrochloride through supported liquid membranes containing macrocyclic carriers

Commercially available PTFE membranes were used as a support for liquid membranes in amino acid transport. Using tryptophan as a model amino acid, the influence of the type of organic liquid, kind of macrocyclic carrier and counter-ion on transport efficiency was examined. These studies show the strong influence of the kind of the counter-ion co-transported with amino acid cation, and the type of macrocyclic carrier used on the transport efficiency. The transport efficiency depends also on the pH of the source phase and on the nature of the organic liquid used as a membrane solvent. Liquid membranes supported on commercial porous-PTFE-membranes with hydrophobic solvents are stable for more than two months, while those with more hydrophilic solvents, for more than 30 days. The use of dodecylbenzenesulfonic acid as a counter-ion results in the highest flux of tryptophan, but in this case, the stability of membranes appeared to be five times lower.     

Transport of titanium(IV) ions across di-2-ethylhexylphosphoric acid-CCl4 supported liquid membranes

Transport study for Ti(IV) ions using di-2-ethylhexylphosphoric acid (D2EHPA) (carrier)-CCl₄ (diluent) liquid supported membrane in microporous polypropylene hydrophobic film has been performed. The parameters studied are effects of carrier, H₂SO₄, stripping agent (NH₄F) concentrations and temperature variation on flux and permeability coefficients of the metal ion. The optimum concentrations of transport found are 2.04 mol·dm^–3 D2EHPA, 1.0 mol·dm^–3 H₂SO₄ in the feed and 1 mol·dm^–3 NH₄F as stripping agent. The maximum flux and permeability coefficient determined are 1.32·10^–5 mol·m^–2·s^–1 and 8.02·10^–12 mol·m^–2·s^–1, respectively. The transport of this metal ion is increased with increase in temperature. The mechanism of transport appears to be based on coupled counter ion transport phenomenon.     

Extraction and stripping study of strontium ions across D2EHPA-TBP-kerosene oil based supported liquid membranes

A Sr ion transport study across D2EHPA-TBP kerosene oil based liquid membranes supported on microporous polypropylene film has been performed. The parameters studied were the effect of di(2-ethylhexyl)phosphoric acid (D2EHPA) and TBP concentration variation in the membrane liquid, HNO₃ concentration variation in the stripping phase and citric acid concentration variation in the feed solution. The optimum conditions of transport are 0.3 mol/dm³ D2EHPA, 0.1 mol/dm³ TBP, 0.01 mol/dm³ citric acid in feed and 2 mol/dm³ HNO₃ in the stripping phase. The mechanism of transport observed is counter-ion coupled transport. The coupling ions are protons. The maximum flux for Sr ion transport observed is 5.33·10^–5 mol·m^–2·s^–1 and maximum permeability under optimum conditions observed is 8.08·10^–11 m^–2·s^–1.     

Pertraction through liquid membranes

A survey of liquid membranes and related transport mechanisms is presented. Possibilities and advantages of application of pertraction as a separation and concentration operation are discussed. Pertraction into emulsions is analyzed from the process point of view in more detail. A further development of this operation will be connected with a better understanding of interphase phenomena.     

Cyclic voltammetry of highly hydrophilic ions at a supported liquid membrane

Cyclic voltammetry has been used to study the coupling of ion transfer reactions at a liquid membrane. The liquids are either supported by a porous hydrophobic membrane (polyvinylidene difluoride, PVDF) when the organic solvent is non-volatile (o-nitrophenyloctylether) or are merely a free standing organic solvent layer such as 1,2-dichloroethane comprised between two hydrophilic dialysis membranes supporting the adjacent aqueous phases. The passage of current across the liquid membrane is associated with two ion transfer reactions across the two polarised liquid liquid interfaces in series. It is shown that it is possible to study the transfer of highly hydrophilic ions at one interface by limiting the mass transfer of the other ion transfer reaction at the other interface. Indeed, for systems comprising an ion M in one aqueous phase and a reference ion R partitioned between the membrane and the other aqueous phase, the observed and simulated cyclic voltammograms have a half-wave potential determined by the Gibbs energy of transfer of M transferring at one interface and by the limiting mass transfer of R at the other interface. This new methodology opens a way to measure the Gibbs energy of transfer of highly hydrophilic or hydrophobic ions, which usually limits the potential window at single liquid liquid interfaces (ITIES).