P. R. Danesi

Separation of Metal Species by Supported Liquid Membranes

Abstract The works performed in the Separation Chemistry Group of the Chemistry Division of Argonne National Laboratory on the transport and separation properties of supported liquid membranes (SLM) are reviewed. The models and equations which describe the permeation through SLMs of metal species are described. These models have been tested with various carriers absorbed on flat-sheet and hollow-fiber SLMs by measuring the permeation of several metal species of hydrometallurgical and nuclear interest. An equation for the separation factor of metal species in SLM processes and examples of separations of metal ions are reported. The possibility of bypassing the single stage character of SLM separations by using multilayer composite SLMs, arranged in series, is also analyzed. Finally, the factors which control the stability of SLMs are briefly discussed.

Mass Transfer Rate through Liquid Membranes: Interfacial Chemical Reactions and Diffusion as Simultaneous Permeability Controlling Factors

Abstract Equations describing the permeability of a liquid membrane to metal cations have been derived taking into account aqueous diffusion, membrane diffusion, and interfacial chemical reactions as simultaneous permeability controlling factors. Diffusion and chemical reactions have been coupled by a simple model analogous to the one previously described by us to represent liquid-liquid extraction kinetics. The derived equations, which make use of experimentally determined interfacial reaction mechanisms, qualitatively fit unexplained literature data regarding Cu2+ transfer through liquid membranes. Their use to predict and optimize membrane permeability in practical separation processes by setting the appropriate concentration of the membrane carrier [LIX 64 (General Mills), a commercial β-hydroxy-oxime] and the pH of the aqueous copper feed solution is briefly discussed.

Lifetime of supported liquid membranes: the influence of interfacial properties, chemical composition and water transport on the long-term stability of the membranes

Abstract The permeability coefficients of some metal species through selected supported liquid membranes (SLM) have been measured over extended periods of time, in the presence and absence of osmotic pressure gradients. The composition of the SLMs was varied in such a way to obtain membranes with variable interfacial properties. The interfacial tensions, viscosities, contact angles and water solubilities in the organic phases of the various liquid membranes were measured. The ability of these SLMs to transport water under the influence of osmotic pressure gradients of different magnitude was also measured. In this way correlations were established among the following factors: the lifetime of the SLMs, their interfacial and water extraction properties, the concentrations of the feed and strip solutions adjacent to the SLM and the ability of the SLM to transport water. The results of long-term operation of several types of SLMs indicate that they are relatively stable when the osmotic pressures of the feed and strip solutions are approximately equal, but become increasingly unstable as the difference between the osmotic pressures of the feed and strip solutions increases. In addition, the rate at which water is transported through the SLMs under the influence of a given osmotic pressure difference is inversely related to membrane lifetime. The following mechanism of water transport through SLMs was proposed: (i) in presence of an osmotic pressure gradient water tends to flow through the organic-filled pores of the SLM, providing the organic phase can appreciably solubilize water; this initial water flow can be imagined as the diffusion of water molecules through a stationary organic diluent; (ii) when the amount of flowing water becomes massive and the SLM is easily wetted by the aqueous phases, the organic phase is eventually displaced from the support pores and replaced by water; at this point the membrane behaves as a semipermeable diaphragm containing water-filled micropores and its ability to perform carrier-mediated coupled transport of metal species is completely lost.

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.

SELECTIVITY-STRUCTURE TRENDS IN THE EXTRACTION OF Co(II) AND Ni(II) BY DIALKYL PHOSPHORIC, ALKYL ALKYLPHOSPHONIC,AND DIALKYLPHOSPHINIC ACIDS∗

Abstract The conditional equilibrium constants for the extraction reactions of Co(II) and Ni(ll) by toluene solutions of homologous series dialkyl phosphoric, alkyl alkylphosphonlc, and dialkyl-phosphlnic acids have been evaluated. From the equilibrium constants the Co/Ni selectivity factors were calculated. Absorption spectra and water extraction data for organic solutions of selected reagents which had extracted the metals were also obtained. The data have allowed us to establish correlations between the structure of the extractants and their selectivity for Co(II) over Ni(II). The two main features controlling the different extractive behavior of Co(II) and Nl(II) with the organo-phosphorus reagents appear to be the different degree of hydration of the extracted complexes and their different coordination geometry. Sterlc hindrance effects, introduced by branchings on the alkyl chains greatly reduce the extractability of Ni(II) which forms octahedral complexes in the organic phase, while the extraction ...

Transport of Eu3+ through a Bis(2-ethylhexyl)-phosphoric Acid, n-Dodecane Solid Supported Liquid Membrane

Abstract The coupled transport of Eu3+ and H+ ions through a solid supported liquid membrane consisting of a porous polypropylene film immobilizing an HDEHP solution in n-dodecane has been studied as a function of the membrane area, stirring speed of the aqueous solutions, membrane composition, and acidity of the feed solution. The experimental results are in agreement with predictions derived from a theoretical permeability coefficient equation which assumes that membrane diffusion and aqueous film diffusion are the only rate-controlling factors.

SEPARATION OF ACTINIDES AND LANTHANIDES FROM ACIDIC NUCLEAR WASTES BY SUPPORTED LIQUID MEMBRANES

ABSTRACT Supported liquid membranes, SLM, consisting of a solution of 0.25 M octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO) and 0.75 μ tributylphosphate (TBP) in decalin absorbed on thin micropocous polypropylene supports, have been studied for their ability to perform selective separations and concentrations of actinide and lanthanide ions from synthetic acidic nuclear wastes. The permeability coefficients of selected actinides (Am, Pu, U, Np) and of some of the other major components of the wastes have been measured using SLMs in flat-sheet and hollow- fiber configurations. The results have shown that with the thin (25 μm) flat-sheet SLMs, using Celgard® 2500 as support, the membrane permeation process is mainly controlled by the rate of diffusion through the aqueous boundary layers. With the thicker (430 μmpar; hollow-fiber SLMs, using Accurel® hollow-fibers as support, the membrane permeation process Is controlled by the rate of diffusion through both the SLM and the aqueous boundar...

Transfer rate and separation of Cd(II) and Zn(II) chloride species by a trilaurylammonium chloride—triethylbenzene supported liquid membrane

Abstract The transfer rate of Cd(II) and Zn(II) from aqueous feed chloride solutions to ammonium acetate strip solutions through a supported liquid membrane consisting of a trilaurylammonium chloride (TLAHCl) solution in triethylbenzene (t-e-b), adsorbed on a polypropylene microporous film, was studied. The influence of the stirring speed, feed chloride concentration, and TLAHCI carrier concentration was investigated. The data were explained by a previously derived equation in terms of membrane diffusion, aqueous film diffusion, and aqueous complex formation chemistry. The best experimental conditions to perform a Cd(II)Zn(II) separation were identified. The separation of Zn(II) and Cd(II) was experimentally studied.

Mass transfer rate through solid supported liquid membranes: Influence of carrier dimerization and feed metal concentration on membrane permeability

Abstract The coupled transport of Cu 2+ and H + ions through solid supported liquid membranes consisting of a porous polypropylene film immobilizing a β-hydroxy-oxime (carrier) in toluene has been studied as function of both the hydrodynamic conditions and the chemical composition of the system. Data have been obtained using a wide range of metal concentrations, up to complete loading of the membrane carrier with Cu 2+ ions. The experimental results have been quantitatively explained by a previously derived permeability coefficient equation taking into account both the self-association and the progressive saturation by copper ions of the membrane carrier. The thickness of the aqueous diffusion films and the membrane diffusion coefficient of the Cu 2+ —carrier complex have been evaluated.

Permeation of metal ions through a series of two complementary supported liquid membranes

Abstract The permeation of Am3+ and Eu3+. through two composite supported liquid membranes, SLM, consisting of a series of two complementary SLMs, separated by an aqueous solution, has been studied. The first liquid membrane was a neutral membrane, i.e., a solution of a bifunctional neutral organophosphorous extractant in decalin. The second liquid membrane was an acidic membrane, i.e., a solution of bis(2-ethylhexyl)phosphoric acid in n-dodecane or of dinonylnaphthalene sulphonie acid in decalin. The solid support was a microporous polypropylene film. The composite SLM system had the sequence “Solution A — SLM(A) -Solution B — SLM(B) — Solution A”, where aqueous solution A promotes extraction of th the metal cations into SLM(A) and their stripping from SLM(B), and aqueous solution B promotes stripping of metal cations from SLM(A) and their extraction into SLM(B). SLM(A) and SLM(B) are a neutral or an acidic S/aVis or vice versa. The study has demonstrated that the single-stage character of SLM separations of metal ions in solution can be in principle overcome by repeating the composite SLM arrangement a number of times. The equations describing the concentration variations in the aqueous solutions which are adjacent to the acidic and neutral SLMs are also reported. They allow one to predict quantitatively the degree of enrichment of each aqueous solution as function of time and the degree of separation among different cations achievable with the composite SLM system. The overall permeability of the composite SLM system to a given cation is shown to be a function of the single-membrane permeability coefficients as well as of the volumes of the aqueous solutions and the SLM area.