Modelling of pressure-driven membrane separation of electrolytes.: `High temperature' approximation

A model of pressure-driven membrane process of electrolyte separation is presented. The electric field potential assumed as being known, exact solution for permeate composition is readily obtained. All species are assumed to have the same convection velocity. Local electroneutrality condition is not used. The electric potential has been taken into account under high temperature approximation, thus reducing the problem to algebraic equation in exp(Ψ), where Ψ is dimensionless flow potential, and making it possible to calculate concentrations of ions in permeate. Negative retention is shown to be possible for one-component electrolyte solution. For electrolyte mixtures, concentration of ion with high charge is shown to “govern” the membrane selectivity in respect to low-charge ions. Results obtained are in qualitative accordance with the earlier experimental data on membrane separation of reaction mixtures in homogeneous catalysis.     

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.     

The conditions governing the specific adsorption of counter-ions in the interaction of two parallel plate-like colloidal particles

Frens and Overbeek have proposed that during the Brownian collision of two colloidal particles in a hydrophobic sol, the surface charge density due to potential-determining (p.d.) ions remains virtually unchanged. It is argued here that the cause of this behaviour is the low concentration of p.d. ions in the diffuse layer. However, equilibrium can be maintained with respect to counter-ions adsorbed into the Stern region from the supporting electrolyte, because the concentration of such electrolyte in the dispersion medium is considerably greater than that of p.d. ions.A general expression is quoted from earlier work for the electric double layer interaction between two parallel plate-like particles in the case where surface charge due to p.d. ions is fixed, but where counter-ions adsorbed into the Stern region can equilibrate with ions of the same species in the diffuse layer. Incorporating discreteness-of-charge and ion-size effects into the adsorption isotherm of the counter-ions, the double layer interaction energy of the two plates is calculated at contact of the two outer Helmholtz planes (o.h.p.'s). It is shown that although this energy exceeds the classical expression obtained by assuming the potential at the o.h.p. to be independent of plate separation, it remains finite.     

Dielectric analysis of nanofiltration membrane in electrolyte solutions: influences of electrolyte concentration and species on membrane permeation

Dielectric properties of a nanofiltration membrane immersed in dilute aqueous electrolyte solutions were measured, and frequency dependence of capacitance and conductance of the systems was analyzed, based on the interfacial polarization theory, giving values of permittivity and conductivity of the membrane and the solutions. Permittivity, epsilon m, of the membrane slightly decreased whereas conductivity, km, of the membrane increased with increasing electrolyte concentration, as a result of entrance of ions into the membrane. The ratio of membrane/solution conductivity, km/kw, also depended on the electrolyte concentration, showing that distribution of ions in the membrane and in solutions follow Donnan equilibrium, due to the presence of negative fixed charges in the membrane. New expressions were derived from Donnan equilibrium principle to explain this phenomenon, and negative fixed charge concentration ce of the membrane was obtained; thus the Donnan potential, DeltaPhi Don, of the membrane in solutions at various concentrations could be calculated. The new expressions could be expected to be usable to analyze ion permeation property through membrane.     

Coupling of ion and water fluxes in synthetic membranes*

The behaviour of ion-exchange membranes has, in most circumstances, been interpreted in terms of the fixed charge theory of Teorell, Meyer and Sievers. Generally it has been recognised that some allowance should be made in the theory for coupling between ion and water fluxes. In making approximate corrections for this, it has usually been assumed that close coupling between all ion fluxes and the osmotic or electro-osmotic water flux occurs. In this paper the specific nature of ion/water flux coupling is explored and it is shown that close coupling is a good approximation for co-ion and water fluxes but that the coupling of the counter-ion and water fluxes is less strong. p]The interactions between ions, water and membrane matrix in a cation exchange membrane are revealed by determining the values of the pairwise molecular friction coefficients. These findings support long standing qualitative ideas about the interplay of electrostatic, hydration and hydrophobic interactions in membranes. p]The picture that emerges shows that an exact extension of the usual equations of the fixed charge theory to include flux coupling would be too complex to be useful. An approximate procedure is to divide conceptually the counter-ions into two groups, those balancing the fixed charges and those accompanying the sorbed co-ions, i.e. constituting sorbed electrolyte. If coupling of the former group of counter-ions with the water flux is ignored whilst close coupling is assumed between the sorbed electrolyte and the water flux, a very good correction is achieved for the overall effects of coupling.     

516—Synthetic and biological membranes

Biological cell membranes are compared to artificially produced permselective membranes. In spite of the difference in their thickness and electric resistance it is shown that they are similar in many important aspects, and the behaviour of synthetic membranes can explain very many points in the behaviour of cell membranes, especially axons. It is stressed that it is seldom necessary to assume unproven channels in biological membranes which are specific in the transport of a single ion species. Most of the phenomena of selectivity, opening and closing of such membranes can be explained by the influence of pH on membrane sites, by their poisoning with higly charged and/or large counter-ions. The borderline between two oppositely charged regions of a mosaic membrane opens up because it can be traversed by both positive and negative ions and permselectively is lost. Bipolar membranes rectify the current. Amphoteric membranes undergo very complex changes in conducting properties and permselectively with pH, especially when allowance is made for the weakly dissociated sites contained within these membranes. It is suggested that black lipid membranes made in the laboratory could be used to elucidate new points of similarity if they were used under similar conditions to our technical membranes.     

Ion separations with membranes

Ion separations are important for resource recovery, water treatment, and energy production and storage. Techniques such as chemical precipitation, selective adsorption, and solvent extraction are effective, but membranes may separate ions continuously with less waste and lower energy costs. Separation of monovalent and multivalent ions with nanofiltration or electrodialysis membranes already enables water softening and edible salt purification. Similar membranes are attractive as separators in vanadium redox flow batteries. Selective partitioning of divalent counter-ions into ion-exchange membranes even allows transport of these ions against their concentration gradients in salt mixtures. However, separations of ions with the same charge is more challenging. Recent research demonstrated highly selective ion “sieving” at small scales. Separations using electrical potentials and differences in ion electrophoretic mobilities are promising, but relatively unexplored. Carrier-mediated transport affords high selectivity in liquid membranes, but these systems are not very stable, and selective transport via hopping between anchored carriers has proven elusive. Finally, this paper discusses how concentration polarization decreases selectivities in many membrane processes. Although development of selective, inexpensive ion-separation membranes is a work in progress, successes in water softening and edible salt purification suggests that future selective membranes will serve as complementary methods to traditional purification techniques.     

Use of ionic membranes in the fractionation of water-alcohol mixtures by the pervaporation process

Five membranes, constituted of ionic chains grafted onto inert matrix, were studied in pervaporation of water - ethanol mixtures of different compositions. The permeability and selectivity of all five membranes showed a marked dependence on the nature of the counter-ions. Water permeates preferentially through all the types of membrane and counter-ions, except the protonated carboxylate membrane. For cation exchange membranes with alcaline counter-ions, the selectivity decreases in the following sequence: K+<Na+<Li+<H+. The permeability of the sulfonate membrane follows the reversed sequence, while that of the carboxylate membrane follows the same sequence, i.e. there is no trade-off between permeability and selectivity for the latter membrane. The anion-exchange (quaternized amine) membrane behaved similarly to the sulfonate membrane, i.e. the permeability and the selectivity vary in opposite directions. An attempt to interpret the influence of the nature of counter-ions on the basis of physico-chemical properties of the ion pairs was made.     

Controlling the transport of cations through permselective mesoporous alumina layers by manipulation of electric field and ionic strength

The electric field-driven transport of ions through supported mesoporous gamma-alumina membranes was investigated. The influence of ion concentration, ion valency, pH, ionic strength, and electrolyte composition on transport behavior was determined. The permselectivity of the membrane was found to be highly dependent on the ionic strength. When the ionic strength was sufficiently low for electrical double-layer overlap to occur inside the pores, the membrane was found to be cation-permselective and the transport rate of cations could be tuned by variation of the potential difference over the membrane. The cation permselectivity is thought to be due to the adsorption of anions onto the pore walls, causing a net negative immobile surface charge density, and consequently, a positively charged mobile double layer. The transport mechanism of cations was interpreted in terms of a combination of Fick diffusion and ion migration. By variation of the potential difference over the membrane the transport of double-charged cations, Cu2+, could be controlled accurately, effectively resulting in on/off tunable transport. In the absence of double-layer overlap at high ionic strength, the membrane was found to be nonselective.     

Variable Charge-Densities in Hydrophilic Weak-Base Ion-Exchange Membranes. V. Equilibrium Properties with Pure 1-1 Electrolytes. Swelling and Ion-Exclusion

Abstract

The equilibrium distribution of chloride or nitrate counter-ions of K or NH₄ co-ions and of water is determined experimentally at different degrees of ionization X[xbar] of hydrophylic weak-base aminated polyvinyl-alcohol membranes. These data are analyzed and explained in terms of thermodynamics interrelating the molality of fixed-charge densities and of counter-ions to that of the amount of water and of diffused salt in the membrane.

Three domains have to be considered: the polyelectrolyte domain with small external concentration compared to the internal net fixed charge concentration (notion of “net-charge” densities), the domain of homogeneous interstitial liquid of the concentrated electrolyte type with sufficiently concentrated salt solution, and the intermediate domain.     

Nitrate and chloride transport through a smart membrane

To develop membranes having ionic selective properties under control of external stimuli is a challenge of the membrane and material scientific community. Conducting polymers swell and shrink under electrochemical control, so they are good candidates to prepare such smart membranes. The ionic transport through a new free-standing polypyrrole film working as a membrane in a diffusion cell was studied. The driving forces were transversal electric fields or concentration gradients across the film. The obtained ionic conductivity was dependent on both the electrolyte nature and concentration, as well as on the oxidation degree of the film, which was controlled by the applied external electric potential. Reverse and continuous changes of up to one order of magnitude on the transversal ionic conductivity are obtained when the membrane is in stationary oxidation states attained by polarisations at a constant potential in the range between −0.6 V and +0.4 V, respectively. A prevalent conductivity of anions (t₋ = 0.94) was obtained from Donnan potential measurements. The experimental results indicate that the oxidised film behaves as a nanoporous membrane highly permeable to nitrate ions, while the rejection of these ions is very high in the reduced film. The free-standing polypyrrole film works then as a smart membrane selective to nitrate ions under concentration gradient.     

Molecular dynamics simulation of an archaeal lipid bilayer with sodium chloride

We have performed molecular dynamics simulations of a bilayer formed by the synthetic archaeal lipid, diphytanyl phosphatidylcholine, in NaCl electrolyte solution at four different concentrations (0-4 M) to investigate how structural and dynamic properties of the model archaeal membrane are changed due to the ionic strength of the solution. The archaeal lipid bilayer shows minor changes in their physical properties, indicating the unusual high stability of the membrane against salt, though small reductions of molecular area and lateral diffusion of the lipid are detected at the highest electrolyte concentration of 4 M. Sodium ions penetrate to the ether-rich region, where the ions are likely bound to the ether oxygen in the sn-1 chain rather than to that in the sn-2 chain. The observed salt bridges among two or three neighboring lipids account for the small reduction in the molecular area. The bound ions together with the counter (chloride) ions give rise to a diffusive electric double layer; as a result, the membrane dipole potential is slightly increased with increasing NaCl concentration.     

Removal of aromatic compounds in the aqueous solution via micellar enhanced ultrafiltration: Part 1. Behavior of nonionic surfactants

The effects of nonionic surfactants having different hydrophilicity and membranes having different hydrophobicity and molecular weight cut-off on the performance of micellar-enhanced ultrafiltration (MEUF) process were examined. A homologous series of polyethyleneglycol (PEG) alkylether having different numbers of methylene groups and ethylene oxide groups was used for nonionic surfactants. Polysulfone membranes and cellulose acetate membranes having different molecular cut-off were used for hydrophobic membranes and hydrophilic membranes, respectively. The concentration of surfactant added to pure water was fixed at the value of 100 times of critical micelle concentration (CMC). The flux through polysulfone membranes decreased remarkably due to adsorption mainly caused by hydrophobic interactions between surfactant and membrane material. The decline of solution flux for cellulose acetate membranes was not as serious as that for polysulfone membranes because of hydrophilic properties of cellulose acetate membranes. The surfactant rejections for the cellulose acetate membranes increased with decreasing membrane pore size and with increasing the hydrophobicity of surfactant. On the other hand the surfactant rejections for polysulfone membranes showed totally different rejection trends with those for cellulose acetate membranes. The surfactant rejections for the polysulfone membranes depend on the strength of hydrophobic interactions between surfactant and membrane material and molecular weight of surfactants.     

Preparation and characterization of monovalent cation selective sulfonated poly(ether ether ketone) and poly(ether sulfone) composite membranes

Highly charged cation permeable composite membranes were prepared by blending of sulfonated poly(ether sulfone) (SPES) with sulfonated poly(ether ether ketone) (SPEEK) in 0 to 90% weight ratio, to adjust the hydrophobic properties and ion selective nature. Extent of sulfonation was confirmed by 1H NMR and ion exchange capacity and degree of sulfonation depending on blending composition. These membranes were characterized as a function of weight fraction of SPEEK by recording ion-exchange capacity, water uptake, thermogravimetric analysis, membrane conductivity and membrane potential in equilibration with different electrolytic solutions. Membrane permselectivity and solute flux were estimated using these data on the basis of non-equilibrium thermodynamic principles and for observing the selectivity of different membranes for mono- or bivalent counter-ions. It was observed that relative selectivity for monovalent in comparison to bivalent counter-ions were increased with the decrease in SPEEK content in the composite membrane matrix. The range of SPEEK content in the blend from 60 to 80% appears the most suitable for the selective separation of monovalent ions from bivalent ions. Furthermore, highly charged nature and stabilities of these membranes extend their applications for the electro-assisted separations of similarly charged ions as well as other electro-membrane processes.     

Characterization of Zinc Ion Transport Via Dialysis Process

Treatment of metal ions' wastes is getting more interest due to the tight regulations for environmental protection. Dialysis, a membrane based process with the concentration difference as the driving force, may be used for separation of metal ions from wastewater. In this study membranes with different pore sizes including Accurel, Celgard, GVHP, PM30 and PTHK membranes were employed to characterise the transport of zinc ion in various (0.01, 0.1, 0.5, 1, 5 and 10 w/v percent) initial feed concentrations. The results show that low initial feed concentration causes less passage of ions through the membrane due to low driving force, i.e. concentration gradient across the membrane. This result is expected. However the effect of membrane pore size is somehow unexpected. It was found that the large pore size membranes provide less penetration of the metal ions through the membrane. This reproducible result has been explained based on the transport mechanism. Two types of mechanisms, i.e. extensive and intensive mechanisms, have been suggested for metal ion transport through different pore size membranes.     

Studies on Pervaporation Separation of Ethanol/Water Mixtures with Membranes of Polypropylene Sulfonate Salts

Sulfonated polypropylene membranes loaded with different kinds of counter-ions were prepared by heterogeneous chiorosulfonation reaction of polypropylene membrane followed by hydrolysis and ion-exchange reaction. The membranes obtained were used for selective separation of ethanol/water mixtures by pervaporation. The effects of counter-ion species, ion-exchange capacity, solution composition, thickness of membrane and temperature on the separation behavior of the membranes were investigated. Microstructure and morphology of the membranes were examined by X-ray and SEM as well.     

紫外辐照接枝制备亲水性荷正电纳滤膜

通过在酚酞基聚芳醚酮超滤膜表面紫外辐照接枝亲水性单体二烯丙基二甲基氯化铵(DADMAC)制备了一种表面荷正电的纳滤膜. ATR-FTIR和表面水接触角的研究结果表明膜表面的接枝率和亲水性随着辐照时间和单体在接枝溶液中的浓度的增加而增加. 荷正电纳滤膜对盐溶液有很好的截留, 对盐溶液中的高价阳离子和低价阳离子的截留率分别为95%和65%. 但当溶液中存在高价负离子时, 膜的截留性能会明显下降. 表明静电效应在荷电纳滤膜的分离过程中起了重要的作用.     

Influence of steric, electric, and dielectric effects on membrane potential

The membrane potential arising through nanofiltration membranes separating two aqueous solutions of the same electrolyte at identical hydrostatic pressures but different concentrations is investigated within the scope of the steric, electric, and dielectric exclusion model. The influence of the ion size and the so-called dielectric exclusion on the membrane potential arising through both neutral and electrically charged membranes is investigated. Dielectric phenomena have no influence on the membrane potential through neutral membranes, unlike ion size effects which increase the membrane potential value. For charged membranes, both steric and dielectric effects increase the membrane potential at a given concentration but the diffusion potential (that is the high-concentration limit of the membrane potential) is affected only by steric effects. It is therefore proposed that membrane potential measurements carried out at high salt concentrations could be used to determine the mean pore size of nanofiltration membranes. In practical cases, the membrane volume charge density and the dielectric constant inside pores depend on the physicochemical properties of both the membrane and the surrounding solutions (pH, concentration, and chemical nature of ions). It is shown that the Donnan and dielectric exclusions affect the membrane potential of charged membranes similarly; namely, a higher salt concentration is needed to screen the membrane fixed charge. The membrane volume charge density and the pore dielectric constant cannot then be determined unambiguously by means of membrane potential experiments, and additional independent measurements are in need. It is suggested to carry out rejection rate measurements (together with membrane potential measurements).     

Colloidochemical properties of ultra-and microfiltration membranes in electrolyte solutions

Dependences of the structural, electrokinetic, and adsorption characteristics on solution pH and background electrolyte (NaCl) concentration are extensively studied for Sartorius and Vladisart cellulose acetate microfiltration membranes with pore sizes of 0.45 and 0.2 μm and a Vladisart ultrafiltration membrane with the rejection of 20 kD. It is revealed that effective hydrodynamic pore radii and maximum pore radii of the microfiltration membranes are 1.5-to 2-and 2.5-to 4-fold, respectively, larger than those presented in the catalog, which is related to the membrane calibration relative to the sizes of rejected particles. For the ultrafiltration membrane, it is shown that, when the pressure increased from 0.5 to 8.0 atm, filtration factor of a liquid and streaming potential substantially decrease owing to the contraction of the polymer network. Measurements of membrane conductivity by the difference and contact methods suggest that a structural anisotropy is virtually absent in the microfiltration membranes and that the ultrafiltration membrane has a nonuniform structure. Negative electrokinetic potentials, whose absolute values increase with the pH and dilution of a background electrolyte solution, are observed for all studied membranes. Isoelectric points of the ultrafiltration and microfiltration membranes are observed at pH ≤ 3 and 2.1 ± 0.2, respectively.     

Mass diffusion-based separation of sugars in a microfluidic contactor with nanofiltration membranes

Processes such as chromatographic separation and nanofiltration can remove low molecular weight sugars from liquid mixtures of oligosaccharides. As an alternative for the separation of such liquid mixtures, we studied mass diffusion separation of such sugars in a microfluidic device with incorporated nanofiltration membranes. This separation method is based on differences between diffusivities of components and does not require high transmembrane pressures. The effects of channel depth and flow rate were studied in experiments. The key parameters selectivity and rejection increased with increasing channel depth due to increased external mass transfer limitations. Among the studied membranes, the obtained selectivities and rejections correlated to the specified retention values by the manufacturers. Compared to more conventional nanofiltration where high pressure forces solutes through membranes, we obtained corresponding selectivities and fluxes of only an order of magnitude smaller. Simulated results indicated that with optimized microchannel and membrane dimensions, the presented separation process can compete with currently available separation technologies.