Secondary potential in electrodialysis membranes and the effect on permselectivity

The most important factor in the electrodialysis (ED) process is the permselectivity of the ion exchange membranes, which permit not only the separation of cations and anions in a solution, but also the separation of ions with the same sign but different valences. In this work, the mechanism of the permselectivity has been studied through the measurement of the potentials at different planes of the membrane. The experimental results have shown that there was a secondary potential inside ion exchange membranes in an electrodialysis process. At the membrane side touched with dilute solution, this secondary potential enhanced the external electrical field, and thus speeded up the passage of the corresponding ions in the dilute solution through the membranes; at the membrane side touched with concentrated solution, the secondary potential was contrary to the external electrical field and thus counteracted it, which could be very helpful by preventing the ions in the concentrated solution from entering the membranes. Obviously, the existence of the secondary potential might play an important role in the permselectivity of ion exchange membranes in ED processes.     

Studies with inorganic ion-exchange membranes

The pyridinium molybdoarsenate membrane shows a response to pyridinium ions and can be used to determine the concentration of these ions in the range 10(-3)-1M. The potentials generated across the membrane are reproducible and the response time is less than 1 min. There is no interference from certain inorganic and organic ions. The electrode can be used in the pH range 3-6 as well as in non-aqueous medium. Small additions of cetyltrimethylammonium bromide cause large shifts in the membrane potentials. A membrane, after being treated with this surfactant, shows a wider range of response to pyridinium ions. Precipitation titration of pyridinium nitrate has been monitored by using this membrane electrode.     

The effect of aprotic solvent on the selectivity of ion-exchange membranes

The effect of N,N-dimethylacetamide on the selectivity of heterogeneous (MK-40, MA-40, and MA-41) and homogeneous (MF-4SK) ion-exchange membranes is studied for the first time. Concentration dependences of electrical conductivity and diffusion permeability of the membranes were measured experimentally over wide lithium chloride concentrations; on their basis, electrodiffusion coefficients of the co- and counterions were calculated. The interrelation between the electrodiffusion coefficients and the specific moisture capacity of the heterogeneous and homogeneous membranes (which affect their selectivity) is revealed. The calculated electrodiffusion coefficients were used in the calculations of the electromigration transport numbers of counterions in the initial membranes and those processed in mixed solvent. It is shown that the heterogeneous membrane selectivity either increased under the action of the aprotic solvent to polymer material (MA-40, MA-41) or remained practically unchanged (MK-40); the selectivity of homogeneous perfluorinated membranes (e.g., MF-4SK) decreased, thus approaching that of the studied heterogeneous anion-exchange membranes.     

The permselectivity of ion-exchange membranes for non-electrolyte liquid mixtures. II. The effect of counterions (separation of alcohol/water mixtures with nafion membranes)

The effect of counterions on the transport of alcohol/water mixtures through a Nafion ion-exchange membrane is reported. Correlations between the counterions and the membrane's selectivity, permeability and the permeate's state of existence in the membrane were established. The membrane's selectivity toward water, in steady state pervaporation experiments, is much higher than that recorded in sorption experiments when employing an isopropanol/water azeotropic composition as the feed mixture. The values recorded for the apparent energy of activation are conspicuously low and follow the order Cs >K >Na. For all counterions the membrane exhibits a large fraction of free water (freezing water). A substantial freezing-point depression was recorded following the counterion series H > Li > Na > K ≈ Cs. The Cs⁺ version exhibits freezing at 0°C. The swollen membrane equilibrated with the feed mixture shows multiple freezing points, indicative of the heterogeneous nature of the membrane.     

The effects of aspect ratio of inorganic fillers on the structure and property of composite ion-exchange membranes

A new type of nanocomposite ion-exchange membranes containing sulfonated polyethersulfone (sPES) polymer matrix and sulfonated surface-functionalized mesoporous silica (SS) inorganic fillers was prepared. Various characterizations revealed that the addition of inorganic fillers with different shapes had a significant influence on the membrane structure. The mesoporous inorganic fillers not only created extra pore and water channels, assisting the ionic migration and improving conductivity of the composites, but also provided additional fixed charge groups upon surface modification. This allows the Donnan exclusion to work effectively and thus improve the selectivity of membranes. It was proved that the incorporation of appropriate amount of SS additive could significantly improve the conductivity (up to 20 folds) and permselectivity (about 14%) of the sPES membranes. The performance of these newly developed membranes in desalination by electrodialysis was comparable with that of a commercial membrane (FKE).     

Studies with model membranes. X. Evaluation of the thermodynamically effective fixed charge density and permselectivity of mercuric and cupric iodide parchment-supported membranes

Thermodynamically effective fixed charge densities of mercuric and cupric iodide parchment supported membranes were estimated by methods of Teorell, Meyer, and Sievers; Altug and Hair; and the most recent one of Kobatake and co-workers based on the thermodynamics of irreversible processes. The two limiting forms of Kobatake's equation for dilute and concentrated ranges gave identical values of charge densities. It is interesting to note that these two values of limiting cases are closer to the Teorell-Meyer-Sievers and Altug-Hair values. The theoretical predictions for membrane potential by the Kobatake equation were borne out quite satisfactorily by experimental results obtained with both the membranes.     

Recent developments on ion-exchange membranes and electro-membrane processes

Rapid growth of chemical and biotechnology in diversified areas fuels the demand for the need of reliable green technologies for the down stream processes, which include separation, purification and isolation of the molecules. Ion-exchange membrane technologies are non-hazardous in nature and being widely used not only for separation and purification but their application also extended towards energy conversion devices, storage batteries and sensors etc. Now there is a quite demand for the ion-exchange membrane with better selectivities, less electrical resistance, high chemical, mechanical and thermal stability as well as good durability. A lot of work has been done for the development of these types of ion-exchange membranes during the past twenty-five years. Herein we have reviewed the preparation of various types of ion-exchange membranes, their characterization and applications for different electro-membrane processes. Primary attention has been given to the chemical route used for the membrane preparation. Several general reactions used for the preparation of ion-exchange membranes were described. Methodologies used for the characterization of these membranes and their applications were also reviewed for the benefit of readers, so that they can get all information about the ion-exchange membranes at one platform. Although there are large number of reports available regarding preparations and applications of ion-exchange membranes more emphasis were predicted for the usefulness of these membranes or processes for solving certain type of industrial or social problems. More efforts are needed to bring many products or processes to pilot scale and extent their applications.     

Comparative investigations of ion-exchange membranes

The equilibrium and transport properties (conductivity, transport number, diffusion) of crosslinked ionomer membranes based on sulfinated and sulfonated PSU in aqueous solutions of HCl, NaCl and KCl have been investigated and compared with a Nafion 117 membrane. It has been found that these membranes are more compact and their conducting paths are of smaller dimension than that of the Nafion 117. The influence of length of crosslinking chain, changing from –(CH₂)₄– to –(CH₂)₁₂–, is particularly indicated by the diffusion coefficients; the conductivity and transport numbers of counterions are influenced only slightly. Practically no dependence of this effect on the transport number of H⁺ has been found.     

Studies on cellulose acetate and sulfonated poly(ether ether ketone) blend ultrafiltration membranes

New ultrafiltration membranes based on chemically and thermally stable arylene main-chain polymers have been prepared by blending the sulfonated poly(ether ether ketone) with cellulose acetate in various compositions in N,N^′-dimethylformamide as solvent by phase inversion technique. Prepared membranes have been subjected to ultrafiltration characterizations such as compaction, pure water flux, water content, and membrane hydraulic resistance. The pore statistics and molecular weight cut-off (MWCO) of the membranes have been estimated using proteins such as trypsin, pepsin, egg albumin and bovine serum albumin. The pore size increased with increasing concentrations of sulfonated poly(ether ether ketone) in the casting solution. Similarly, the MWCOs of the membranes ranged from 20 to 69 kDa, depending on the various polymer compositions. Surface and cross-sectional morphologies of membranes were analyzed using scanning electron microscopy. The effects of polymer compositions on the above parameters were analyzed and the results are compared and discussed with those of pure cellulose acetate membranes.     

Homogeneous ion-exchange membranes of improved flexibility

Poly(styrenesulfonic acid) ion-exchange membranes having various degrees of porosity and flexibility have been prepared by using aliphatic and aromatic esters of p-styrenesulfonic acid. The membranes formed from the aliphatic ester monomers were found to exhibit an increase in water uptake, permeability, and flexibility with increase in the size of the alcohol group of the ester monomer. With membranes formed from the phenyl and β-naphthyl ester monomers the reverse trend was indicated. The flexibility of the membranes formed from the aromatic ester monomers was much greater than that obtained with the aliphatic esters.     

Studies with model membranes: Part IX. Evaluation of the thermodynamic effective fixed charge density and permselectivity of parchment supported cobalt tungstate and mercuric chromate membranes

Thermodynamic effective fixed charge densities of cobalt tungstate and mercuric chromate parchment supported membranes were evaluated by a number of methods particularly those of Teorell-Meyer-Sievers [19]. Altug and Hair [23] and the most recent one of Kobatake et al. [24, 27] based on the thermodynamics of irreversible processes. The value of the permselectivity was also obtained for the two membranes based on Kobatake et al.'s procedure. Kobatake et al.' equation was used under two limiting conditions of concentration of electrolyte solution, namely (1) in the concentrated range, and (2) in the dilute range. The charge densities obtained under these conditions namely θ_c and θ_d were found to differ (θ_c=1/2θ_d), but it was interesting to note that θ_c values were more closer to those of the TMS values. The theoretical predictions for membrane potential using Kobatake et al.'s equation are borne out quite satisfactorily by our experimental results for both membranes.     

Studies on the ion-exchange behaviour of chromium ferrocyanide

The sorption of univalent, bivalent and trivalent ions has been studied on chromium ferrocyanide gel. The studies reveal a high sorption capacity for Cs⁺, Tl⁺, Ag⁺, Cu²⁺, Zn²⁺, Cd²⁺, Fe³⁺ and Th⁴⁺. The sorption of monovalent cations show purely ion-exchange mechanism while the uptake of bivalent and trivalent cations is non-equivalent in nature. Single elution of Rb⁺, Cs⁺ and Tl⁺ has been performed from the columns of this exchanger and the recovery is almost complete in all the cases. Cu²⁺ and Ag⁺ get completely adsorbed on the gel column and their elution is not possible probably due to the formation of some new solid phases. Depending on the K_d values of the metal ions, a large number of separations of radiochemical as well as analytical importance can be performed on the columns of this exchanger material.     

Studies on the fixed charge density and permselectivity of parchment supported manganese ferrocyanide membrane

Summary Membrane potentials across parchment supported manganese ferrocyanide membrane for various electrolytes over a wide concentration range have been studied. It was found that the membrane potential increases with dilution of the external electrolyte solutions. The fixed charge density of the membrane has been evaluated byAltug andHair's method and comes out to be −0.06 N. The variation of the transport number of the counterions in the membrane phase and perm-selectivity of the membrane with dilution have also been studied.

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Zusammenfassung Es wurden Membranpotentiale von Manganferrocyanid-Membranen auf Pergamenttr?gern in L?sungen verschiedener Elektrolyte über weite Konzentrationsbereiche untersucht. Das Membranpotential nahm mit der zunehmenden Verdünnung zu. Die fixierte Ladungsdichte wurde nachAltug undHair ermittelt und betrug −0.06 N. Die ?nderung der überführungszahl der Gegenionen in die Membranphase und die Permselektivit?t der Membran wurden ebenfalls bestimmt.

     

Application of ion-exchange reactions between membranes and resins

The separation of a solution of an electrolyte from an ion-exchange resin by an ion-exchange membrane, where the charge sign of the fixed sites is the same, results in a process that is comparable to Donnan dialysis in its overall effect. That is, the counter-ions from the resin can be exchanged for ions of the same charge sign in the electrolyte. The reaction is demonstrated and the efficiency of the process evaluated by monitoring the metathesis of carbonic acid from sodium carbonate. An application of the metathesis of a non-electrolyte to a preconcentration method for ion chromatography is demonstrated.     

Reformulating the permselectivity-conductivity tradeoff relation in ion-exchange membranes

Polymer membranes used in separation applications exhibit a tradeoff between permeability and selectivity. That is, membranes that are highly permeable tend to have low selectivity and vice versa. For ion-exchange membranes used in applications such as electrodialysis and reverse electrodialysis, this tradeoff is expressed in terms of membrane permselectivity (i.e., ability to selectively permeate counter-ions over co-ions) and ionic conductivity (i.e., ability to transport ions in the presence of an electric field). The use of membrane permselectivity and ionic conductivity to illustrate a tradeoff between counter-ion throughput and counter-ion/co-ion selectivity in ion-exchange membranes complicates the analysis since permselectivity depends on the properties of the external solution and ionic conductivity depends on the transport of all mobile ions within a membrane. Furthermore, the use of these parameters restricts the analysis to ion-exchange membranes used in applications in which counter-ion/co-ion selectivity is required. In this study, the permselectivity-conductivity tradeoff relation for ion-exchange membranes is reformulated in terms of ion concentrations and diffusion coefficients in the membrane. The reformulated framework enables a direct comparison between counter-ion throughput and counter-ion/co-ion selectivity and is general. The generalizability of the reformulated tradeoff relation is demonstrated for cation-exchange membranes used in vanadium redox flow batteries.     

Electroluminescence of ion-exchange polymeric membranes in the swollen state

The first results on the electroluminescence of the MK-40L, MK-40K, MF-4SK, MA-40L, MA-41L, MA-40K, and MA-41K ion-exchange polymeric membranes in the swollen state are reported. It was found that electroluminescence had the character of flashes. Intensity and time characteristics of electroluminescence were determined over the range of NaCl solution concentrations from 0 to 0.1 M. The special features of electroluminescence from the ion-exchange membranes were determined from their photographs. It was found that the membranes in the air-dry state did not exhibit electroluminescence.     

Effect of an aprotic solvent on the properties and structure of ion-exchange membranes

The effect of the aprotic solvent dimethylacetamide on the equilibrium and transport properties of heterogeneous (MK-40, MA-40, and MA-41) and homogeneous (MF-4SK) ion-exchange membranes is investigated. On the basis of concentration dependences of the conductivity and diffusion permeability of membranes, model calculations of transport-structural parameters that reflect the structural and kinetic characteristics of conducting phases of the swollen polymer are performed. The effect of the aprotic solvent on the flow of current through the structural fragments of the ion-exchange material is estimated. The causes of changes that are induced in the properties of the membranes by the aprotic solvent are ascertained.     

Effect of diamond blend on the gas-separation properties of composite membranes

The morphological structure and gas transport properties of polyimide- and polyamide-imidebased rigid-chain polymers containing a fine carbon filler (a diamond blend) are studied. Gas transport properties are measured, and the effect exerted on these properties by intermolecular interaction between the functional groups of polymer chains and the fine filler is analyzed.