Studies of the effect of variation of blend ratio on permselectivity and heterogeneity of ion-exchange membranes

Heterogeneous ion-exchange membranes (both cationic and anionic types) have been synthesized by solution casting techniques using polyvinyl chloride (PVC) as binder and ion-exchange resin (-300+400 mesh). The binder:resin ratio varied from 60:40 to 30:70. The transport behavior of the membranes has been evaluated chronopotentiometrically in sodium chloride (NaCl) solutions of different concentrations. The different parameters E(0) (potential drop across the membrane at the instant of application of current I), E(max) (maximum potential drop across the membrane after the application of current I), DeltaE (magnitude of the potential jump across the membrane at transition time tau), Itau(1/2), tau, etc., have been evaluated. The isoconductance points were determined and based on the microheterogeneous model proposed by Zabolotsky and Nikonenko (J. Membrane Sci. 79 (1993) 181) the distribution factors beta has been evaluated for both types of ion exchange membranes. The electroconductivity of the joint gel (kappa ) and pure gel phases (kappa ' ) has been determined. At any particular solution concentration the transport number as well as the permselectivity of membranes increases with increased resin content of the membrane. The microheterogeneity factor beta exhibits synchronization among the each set of four different membranes for both the cationic and anionic type.     

Transport of NaNO3 solutions across an activated composite membrane: electrochemical and chemical surface characterizations

Electrochemical characterization of two different samples of an activated membrane, which consists of a polymeric support containing different amounts of Di-(2-ethylhexyl) phosphoric acid as a carrier, was made by measuring the electrical resistance, salt diffusion and membrane potential for the activated membranes and the polymeric support in contact with NaNO₃ solutions. Transport parameters such as the ion transport numbers and concentration of fixed charge in the membrane, salt and ionic permeabilities at different NaNO₃ concentrations were obtained. A comparison of the different electrochemical parameters obtained with both activated membranes and the polymeric support shows how the carrier affects the transport of NaNO₃ solutions across the activated membranes. On the other hand, chemical composition of the membrane surfaces as a function of the amount of carrier was determined by X-ray photoelectron spectroscopy technique, which also allows an envisagement of the chemical bonding between the carrier and the membrane top layer (polyamide).     

Membrane characteristics of composite collodion membrane I: Water and salt transports across membranes incorporating perfluorobenzoic acid

Two different composite collodion membranes were prepared to examine the correlation of material transport to membrane charges or membrane structure from comparative study. One is a composite collodion membrane incorporating only perfluorobenzoic acid (PFBA) and the other is the same membrane but with pores formed by adding NaClO₄. The membrane parameters defined by irreversible thermodynamic consideration such as filtration coefficient of water, salt permeability and reflection coefficient were estimated. The differences between two membranes or the dependence of membrane transports on salt species were discussed on the basis of the frictional coefficients between water-membrane skeleton or salt-water interactions.     

Solute molecular transport through polyimide asymmetric organic solvent nanofiltration (OSN) membranes and the effect of membrane-formation parameters on mass transfer

This work studies the effect of two membrane-formation parameters, evaporation time and casting thickness, on the diffusive mass transport of organic solutes through an organic solvent nanofiltration (OSN) membrane. These parameters showed a coupled effect on the final membrane thickness, which was explained in terms of top-layer formation. In a concentration-driven dialysis, both parameters, as well as the resulting membrane thickness, had a significant effect on mass transport. Casting thickness had the greatest effect on membrane mass transport rates. Multivariate regression was used to model the dialysis process with acceptable fit. A representation of the membrane morphology was obtained from SEM pictures and used to formulate an alternative mechanistic mass-transport model. A resistance-circuit analogy was used to describe transport through the top and microporous layers, which also considered diffusion through the pores and the polymer for each layer. From the analyses of the models and considering that no differences in top-layer thickness were observed by SEM, it is concluded that membrane asymmetry, determined by the formation parameters, controls mass transport, rather than top-layer thickness.     

Quantum model for transport through membranes

In the present paper a new model for the transport through membranes, introduced previously by Hosur, is considered. In this model, the membrane is assimilated to a potential barrier, and the material flow through the membrane is determined from the transmission coefficient of the barrier, assuming the existence of an energy difference among the molecules placed at both sides of the membrane. An equation for the transport, in the case of small energy differences, is obtained, which is particularized to the cases that the energy difference is caused by a temperature gradient, a concentration gradient, and both gradients acting together. In all cases, under certain limitations, formally identical equations to those of the thermodynamics of irreversible processes are obtained.     

Anisotropic gas transport in a bilayer membrane in the free molecular flow regime

Asymmetric phenomena associated with gas transport in the free molecular flow in multilayer membranes have been investigated. Bilayer track membranes have been examined. A model describing anisotropic gas transport across a multilayer membrane has been constructed and analyzed. The interaction parameters characterizing the effect of the geometry of the inner surface of the pores on the gas flow through the membrane have been determined.     

Synthesis,characterization and electrochemical properties of cation selective ion exchange composite membranes

In this work polystyrene based strontium phosphate membranes (SPMs) were prepared by applying different pressures. The membrane potential is measured with uni-univalent electrolytes (KCl, NaCl, and LiCl) solutions using saturated calomel electrodes (SCEs). The effective fixed charge density of these membranes is determined by the Torell, Meyer and Sievers method and it showed the dependence of membrane potential on the porosity, the charge on the membrane matrix, charge and size of permeating ions. The membranes are characterized by X-ray diffraction, scanning electron microscopy and IR spectroscopy. The order of surface charge density for electrolytes is KCl > NaCl > LiCl. Other parameters such as transport number, distribution coefficient, charge effectiveness and related parameters are calculated. The membrane was found to be mechanically stable, and can be operated over a wide pH range.     

Liquid membrane transport: a survey

The literature pertaining to facilitated transport and liquid membrane separations is reviewed and summarized, especially work reported since 1977. Liquid membranes of all geometries are discussed, including immobilized liquid membranes and liquid surfactant or emulsion liquid membranes. Emphasis is placed on facilitated, or carrier-mediated transport in both configurations although other mechanisms such as coupled-transport and transport due to solubility differences are discussed. Mathematical modeling and analytical solutions for facilitated transport models are summarized. The possibility of industrial application of liquid membrane technology is mentioned and the most important experimental techniques for liquid membrane research are discussed. Also, directions for future research are recommended.     

Electrokinetic parameters of ion transport across isolated pepper cuticular membranes

The transport of NaCl and CaCl₂ solutions across isolated pepper cuticular membranes was studied by means of conductivity, membrane potential and diffusion experiments. Some characteristic membrane parameters such as the electrical resistance, ionic and salt permeabilities were obtained as a function of the electrolyte concentrations. Cuticle morphological asymmetry accounts for differences in membrane potential values under external reverse gradients. The influence of temperature on the membrane structure was also considered, but only small changes in the electrokinetic parameters were obtained. From the NaCl diffusion experiments two activation energies were determined (54.8 kJ/mol for temperature ranging between 15 and 35°C, and 20.6 kJ/mol for the interval of temperature between 40 and 60°C), which could be associated to thermal transitions in the molecular structure of the cuticle for the interval 30–40°C.     

On the electrochemical characterization of ion-selective cellulose acetate membranes with and without stigmasterol liquid membranes

Stigmasterol, a plant product, has been used as a surfactant to generate liquid membranes supported on a cellulose acetate matrix. Electrochemical characterization of the membrane has been attempted with a view to simulating its behavior with natural membranes by measuring membrane potentials and membrane conductance. The selectivity of cellulose acetate membrane kept in contact with magnesium chloride solutions of different mean concentrations has been found to change from anion to cation. Transport numbers have been estimated from membrane potential data. Permselectivity and fixed charge density values for the cellulose acetate membrane with and without stigmasterol have been determined from transport numbers. The variation of these parameters with concentration and pH has also been examined.     

Effect of hydration of polyamide membranes on the surface electrokinetic parameters: surface characterization by x-ray photoelectronic spectroscopy and atomic force microscopy

The surface and the solid/liquid interface of two polyamide membranes, one experimental (B0) and one commercial (NF45), have been characterized by X-ray photoelectronic spectroscopy (XPS), atomic force microscopy (AFM), and zeta potential, respectively. The surface roughness, determined by AFM data analysis, is different for the two membranes, and results show that the commercial NF45 membrane presents a much lower roughness than the experimental B0 membrane. XPS data indicate that the surface of membrane NF45 is similar to that of pure polyamide, while membrane B0 contains a considerable amount of impurities. The homogeneity in depth of both membranes was also studied by determining the composition profile at different analysis angles. Streaming potential along the membrane surface or tangential streaming potential (TSP) measurements with NaCl solutions at different concentrations were carried out with both membranes to determine the zeta potential and the electrokinetic surface charge density, and a correlation between membrane surface and interface parameters is made. Some differences in atomic concentrations of membrane surface elements and X-ray photoelectronic spectra of the samples used in TSP measurements and after a drying process at 90 degrees C for 24 h can be observed when they are compared with those for fresh membranes. Electrokinetic parameters for membrane NF45 (TSP, zeta potential, and surface electrokinetic charge density) obtained from three different series of measurements strongly decrease as a result of membrane use, but for membrane B0 they are practically independent of the number of measurements. This difference in the electrokinetic behavior of the two membranes has been related to the hydration process of the surface for each sample studied by XPS and AFM.     

Experimental analysis, modeling, and theoretical design of McMaster pore-filled nanofiltration membranes

A side-by-side comparison of the performance of McMaster pore-filled (MacPF) and commercial nanofiltration (NF) membranes is presented here. The single-salt and multi-component performance of these membranes is studied using experimental data and using a mathematical model. The pseudo two-dimensional model is based on the extended Nernst–Planck equation, a modified Poisson–Boltzmann equation, and hydrodynamic calculations. The model includes four structural properties of the membrane: pore radius, pure water permeability, surface charge density and the ratio of effective membrane thickness to water content. The analysis demonstrates that the rejection and transport mechanisms are the same in the commercial and MacPF membranes with different contributions from each type of mechanism (convection, diffusion and electromigration). Solute rejection in NF membranes is determined primarily by a combination of steric and electrostatic effects. The selectivity of MacPF membranes is primarily determined by electrostatic effects with a significantly smaller contribution of steric effects compared to commercial membranes. Hence, these membranes have the ability to reject ions while remaining highly permeable to low molecular weight organics. Additionally, a new theoretical membrane design approach is presented. This design procedure potentially offers the optimization of NF membrane performance by tailoring the membrane structure and operating variables to the specific process, simultaneously. The procedure is validated at the laboratory scale.     

Effect of age and chemical treatments on characteristic parameters for active and porous sublayers of polymeric composite membranes

Changes in the transport parameters and the chemical nature of the surface of composite polyamide/polysulfone membranes due to both aging and treatment with chemical products (HCl, H(3)NO, and NaOH) have been considered. Hydraulic and salt permeability were obtained from water flow and salt diffusion measurements, respectively, and their values seem to indicate a modification in the structural parameters (porosity/thickness) of aging samples, while HCl and HNO(3) treatments will act in the opposite way. Chemical modifications in the membrane surfaces were studied by X-ray photoelectron spectroscopy (XPS), which mainly show the effect of H(3)NO and HCl on the polyamide active layer of the membranes (polyamide oxidation), but no chemical damage for that sublayer. Electrical characterization of both sublayers of the composite membranes were determined from impedance spectroscopy (IS) measurements using equivalent circuits as models, and these results indicate: (i) a strong increase of the membrane electrical resistance as a consequence of aging, mainly that associated with the active sublayer (30 times higher for an old sample than for a fresh one) and treatment with NaOH; (ii) the reduction of this effect when the samples were treated with HCl and HNO(3) solutions. Changes in the values of the electrical resistance of the composite membranes are in agreement with those obtained for permeabilities, but the electrical parameter also allows the determination of the contribution of each sublayer.     

Transport Properties of Selective Membranes Reversible to Nitrogen-Containing Organic Base Cations: Permeability and Ion Flow

Main transport properties were studied for selective membranes with low dielectric constants based on liquid ion exchangers involving nitrogen-containing organic base cations. Permeabilities and ion flows through a membrane were calculated for major and interfering ions. Dependences of the transport properties of membranes on the concentrations of the ion exchanger and near-membrane solution and their potentiometric characteristics are presented. It was demonstrated that the transport properties of liquid membranes are determined by two main factors: the transfer of counterions through the phase boundary by the extraction–exchange mechanism and the leaching of the ion exchanger from the membrane.     

A study of the electrical behaviour of isolated tomato cuticular membranes and cutin by impedance spectroscopy measurements

Different isolated tomato fruit cuticular membranes (ripe and green tomato cuticles and the cutin of these membranes) were studied by impedance spectroscopy measurements when the membranes were in contact with NaCl solutions at different concentrations. Remarkable differences in the impedance plots and the equivalent circuits associated to each membrane sample were obtained: the ripe tomato cuticle and the cutin, only present a relaxation process, but for the green tomato cuticle two relaxation processes were obtained. Using the equivalent circuits as models, electrical and electrochemical parameters for each membrane were determined. These results permit us to assign the relaxation processes to the different components of the tomato membrane (polyester matrix, carbohydrates and pigments), obtaining in this way a detailed picture of the different environments of the plant interface. Variation with NaCl concentration for the different electrical parameters was also studied, and the electrical resistance of the biopolymer matrix was obtained.     

Kinetics of bonded invertase: asymmetric polysulfone membranes

Asymmetric flat ultrafiltration membranes made from bromomethylated polysulfone were used to fix invertase chemically. The invertase reactivity of these membranes was compared with those where enzyme bonding was achieved by reacting bromomethylated polysulfone with ethylene diamine and glutaric dialdehyde which act as spacers. In both cases the invertase fixation was carried out such that its concentration at the surface facing the saccharose feed solution could be neglected The kinetic behavior of the immobilized invertase was modelled by investigating the influence of diffusive and of convective transport across the membranes. Considering axial back-mixing of the convective flow within the membrane, the reaction can be simulated at low substrate concentrations. The heterogeneous distribution of the enzyme within the membrane matrix prevents us from calculating the kinetic data of the enzymatic reaction over the entire range of parameters.     

Synthesis and transport abilities of new membrane materials incorporating mono- and bi-pyrazolic compounds

The membrane materials were obtained by photopolymerisation of formulation that contain the active monomer spread on a polyacrylonitrile support. The facilitated transport and the extraction power of Cd(II), Pb(II) and Hg(II) through the synthesised membranes were reported. We have determined both the diffusion flux of different cations and the selectivity of the prepared membranes towards each cations.     

纳滤膜对电解质溶液分离特性的理论研究(II): 混合电解质溶液

假定纳滤膜具有狭缝状孔, 使用纯水透过系数、膜孔径及膜表面电势来表征纳滤膜的分离特征, 用流体力学半径和无限稀释扩散系数表征了离子特性. 采用扩展Nernst-Planck方程、Donnan平衡模型和Poisson-Boltzmann理论描述了混合电解质溶液中离子在膜孔内的传递现象, 计算了三种商用纳滤膜(ESNA1-LF, ESNA1和LES90)对同阴离子、同阳离子和含四种离子的混合电解质体系中离子的截留率, 并与实验数据进行了比较. 计算结果表明, 电解质溶液中离子在纳滤膜孔内传递的主要机理是离子的扩散和电迁移, 纳滤膜对混合电解质溶液中离子的分离效果主要由空间位阻和静电效应决定. 该模型在低浓度时对含一价离子的混合电解质溶液通过纳滤膜的截留率计算结果比较准确, 但对高浓度或含高价离子的混合电解质溶液则偏差较大.