Flux reduction of ultrafiltration membranes with different cut-off due to adsorption of a low-molecular-weight hydrophobic solute-correlation between flux decline and pore size

The flux of ultrafiltration membranes may be severely reduced when treating low-molecular-weight hydrophobic solutes even though the cut-off of the membrane is orders of magnitudes greater than the size of the solute molecules. In this investigation, the flux reduction was correlated to the membrane pore size using octanoic acid as a model substance. As a comparison, the pore size was also determined by measuring the retention of a dextran solution and by using the liquid–liquid displacement porometry method (LLDP). The membranes used were four asymmetric polysulphone and polyethersulphone membranes with nominal molecular weight cut-off (NMWCO) between 6 and 50 kDa. It is shown that the use of a low-molecular-weight hydrophobic solute may provide a rapid and simple method of characterising hydrophobic ultrafiltration membranes, both regarding their sensitivity to flux reduction due to adsorption, and their pore-size distribution.     

Calibration of ultrafiltration membranes against size exclusion chromatography columns

Using the extension of the concept of universal calibration parameter, yielding a relation between the hydrodynamic volume of molecules and the elution volume in size exclusion chromatography (SEC), to retention coefficients in ultrafiltration (UF), we propose a direct calibration of UF membranes against chromatography columns. Plotting the retention coefficient by one given UF membrane of a series of probe molecules versus their elution volume in SEC chromatography provides a calibration curve for this membrane. For a wide range of retentions, such calibration can be directly used to predict the retention of any molecule: one only needs to measure its exclusion volume by the SEC column, and read the retention by the calibrated membrane on the calibration curves.     

Change in molecular weight retention curves of ultrafiltration membranes with respect to lignosulfonates under gel formation conditions

The paper considers ultrafiltration of lignosulfonates (LS) under predominantly the gel formation conditions. An effort is to determine the molecular weight retention (MWR) curves of a series of ultrafiltration membranes differing in their pore size under in turn different operating pressures (1–32 bar). The initial separative properties (both retentivity and volume flux) of all membranes are shown to change because of gel formation occurring actually instantly as a cake layer placed mostly onto the membrane surface. The transmembrane pressure-drop sets up primarily these properties but the initial hydrodynamic permeability coefficient of a membrane (i.e. its mean pore size) is also of concern. As a result, an increase in that pressure results in a shift of the molecular weight retention curves of all membranes under study towards lower molecular weights: the more, the higher their mean pore size. Further, these curves become more abrupt in their form, and such a change depends on the mean pore size of a membrane as well.     

Effect of Sodium Chloride on Ultrafiltration of Aqueous Sulfonol Solutions

Effect of NaCl on ultrafiltration of micellar aqueous Sulfonol solutions through a UFM-50membrane is studied, including its effect on the permeate flux, retention factor, hydrodynamic resistanceof the membrane. Dependences of the hydrodynamic resistanse of dynamic membranes on the time of filtration are found.     

Molecular theory of barycentric velocity: monatomic fluids

The notion of barycentric velocity appears in irreversible thermodynamics and fluid mechanics, in which it is a field variable obeying the hydrodynamic equations or, more specifically, the momentum balance equation, which is coupled to the rest of hydrodynamic equations. Therefore, its behavior is not known until the hydrodynamic equations are solved for the flow problem of interest. Unlike diffusion fluxes, heat fluxes, or stresses, it does not have its own constitutive relation similar to Fick's law, Fourier's law, and Newton's law of viscosity. In this work, the constitutive equation is derived for it. In parallel to the phenomenological notion of barycentric velocity, the notion of mean fluid velocity appears in statistical mechanics of irreversible dynamic processes according to the theory of Irving and Kirkwood [J. Chem. Phys. 18, 817 (1950)], and plays the same role of the phenomenological counterpart. In this work, we investigate the statistical mechanical meanings of the mean fluid velocity of a fluid in flow beyond its formal connection with the barycentric velocity. We show that it consists of two components; the center-of-gravity velocity of the packet of fluid molecules, which may be identified with the barycentric velocity in the phenomenological theory, and the diffusive contribution of its collective modes relative to the center of gravity. If the fluid is uniform in space or if the packet of fluid mass is rigid, the diffusive component vanishes. The statistical mechanical (molecular theory) formula for the mean fluid velocity provides the constitutive relation for it in terms of density and temperature gradients present in the fluid in flow. The constitutive relation obtained for the mean fluid velocity can be an important component in the theory of transport processes in liquids. Its significance to fluid mechanics is briefly discussed.     

The role played by viscosity in the redistribution of diffusion fluxes during vaporization

The problem is discussed of taking into account the viscous properties of a binary vapor-gas system in an analysis of diffusion fluxes during vaporization on the condition that the “accepting” gas is not absorbed by a vaporizing liquid. To answer the question of the correctness of using the Navier-Stokes equation together with the Stefan-Maxwell equation in this problem for taking into account the influence of walls on diffusion processes in fairly broad channels, it is suggested to experimentally measure diffusion fluxes of a vaporizing component in a diffusion chamber with parallel communicating channels and compare the measured fluxes with the calculation results.     

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.     

Effect of heteroporosity on membrane rejection coefficients

Statistical heteroporosity theory is applied to the case of ultrafiltration across a membrane composed of an array of independent channels or pores. The primary question addressed is: For given values of the reflection coefficient σ, and diffusive permeability P_s, measured independently for a membrane of unknown heteroporosity, how accurately can the rejection coefficient R be predicted by using the Spiegler—Kedem expression for rejection, derived without concern for heteroporosity? For the case of applied pressure much greater than back-osmotic pressure, limits are analytically derived which show a maximum 30% relative error in R at σ → 0, and a maximum absolute error of 0.06 at σ ? 0.5. Experimental regions where heteroporosity effects should be largest are given explicitly. It appears that, except for these regions, the Spiegler—Kedem equation can be used to describe accurately rejection across membranes of arbitrary heteroporosity. p]For the case of appreciable solute concentration in the feedstream, a proposed test of heteroporosity consists of measurements of rejection vs. volume flux, at small and large solute concentrations. For a particular model of membrane structure called the two-pipe model it is shown that, in principle, heteroporosity can result in significant changes of rejection with concentration at a given volume flow, but for three real membranes used as examples the effects are predicted to be relatively small.     

Influence of protein–protein interactions on bulk mass transport during ultrafiltration

Bulk mass transfer limitations can have a significant effect on the flux and selectivity during membrane ultrafiltration. Most previous studies of these phenomena have employed the simple stagnant film analysis, but this model is unable to account for the effects of solute–solute interactions on mass transport. We have developed a generalized framework for multicomponent mass transfer that includes both thermodynamic and hydrodynamic (frictional) interactions. Thermodynamic (virial) coefficients were evaluated from osmotic pressure data for albumin (BSA) and immunoglobulins (IgG), while hydrodynamic interaction parameters were determined from filtrate flux data obtained in a stirred cell using fully retentive membranes. The protein concentration profiles in the bulk solution were evaluated by numerical solution of the governing continuity equations incorporating the multicomponent diffusive flux. This model was used to analyze flux and protein transmission data obtained for the filtration of BSA and IgG mixtures through partially permeable membranes. The model accurately predicted the large reduction in flux and BSA transmission upon addition of IgG. These effects were due to the coupling between BSA and IgG mass transfer caused by protein–protein interactions.     

A hydrodynamic study of the retention of polyethylene glycols by cellulose acetate membranes in the absence and presence of proteins

The ultrafiltration behaviour of polyethylene glycols has been investigated with respect to their partial retention by YM5 and YM10 membranes. Retention coefficients were found to follow a log-normal distribution except at low molecular weights, which exhibited higher than expected values. Increasing the applied pressure resulted in increased retention of PEG, although linear flux-pressure relationships were observed in all cases. Increasing hydrodynamic activity by applying higher stirrer speeds led to reduced permeate concentrations. The mean molecular weights of the permeates were higher under hydrodynamic conditions resulting in lower retention coefficients, which suggests increased permeabilities for the larger PEG component molecules.

The retention behaviour of solutions of PEG with varying concentration of bovine serum albumin (BSA) as the flux rate was altered paralleled the results for pure PEG solutions. However, when the flux rate was kept constant and the concentration varied, there was an initial increase of retention at a BSA concentration of 0.2% w/v and thereafter a reduction up to a tested bulk concentration of 10%. Various proposals were made to explain this behaviour.     

The transport of adsorbate mixtures in porous materials: Basic equations for pores with simple geometry

The basic equations governing the transport of single and binary adsorbate mixtures through single pores are considered. An irreversible thermodynamic formulation is adopted and both viscous and diffusive terms are incorporated following the earlier work of Mason and co-workers. The links between phenomenological coefficients and molecular properties are demonstrated. For single components, the gas phase and high density limits are considered. By using simple hydrodynamic models it is shown that the phenomenological coefficients in the mixture equations can all be expressed as functions of the coefficients for the individual components in the same pore, and the properties of the component adsorption isotherms. Whilst it is appreciated that the hydrodynamic approach will be of limited value in very small pores, it is argued that useful insights can be gained into the feasibility of membrane separation processes from this method. The general equations can be used in future development of network models for porous materials.     

Optimization of asymmetric permeation in heterogeneous composite membranes

Asymmetric permeation in two-phase composite membranes with heterogeneous structures represented by a one-dimensional distribution of composition is treated theoretically on the basis of an irreversible thermodynamic transport equation. It is assumed that the permeability of one of the component phases is a monotone function of the activity of permeant while that of the other phase is constant, and that the permeability of the composite membrane is given by the volume average of the resistance coefficient, which is the inverse of permeability. Under these assumptions, it is shown that the optimal membrane which maximizes the degree of asymmetric permeation reduces to a binary laminate membrane. The condition for constructing the optimal laminate membrane is obtained explicitly. Conversely a condition on a desirable membrane component which realizes an arbitrary degree of asymmetric permeation is presented. These results can be applied to the optimal design of a membrane valve which is a chemical analog of a diode. © 1993 John Wiley & Sons, Inc.     

Some comments on the applicability of gas permeation methods to characterize porous membranes based on improved experimental accuracy and data handling

The measurement of the gas permeability coefficient as a function of the mean pressure across a membrane can be used to determine a mean pore radius of the membrane. This method has been applied by several authors to characterize microporous and asymmetric ultrafiltration or hyperfiltration membranes. This paper shows how the data acquisition and handling are improved. Experiments are performed on microporous Millipore membranes with a nominal pore radius of 50 nm and on ultrafiltration merebranes of poly(2,6-dimethyl-1,4-phenyleneoxide) with an expectedly sharp pore-size distribution around 2 nm. For the Millipore membrane an unexpected dependence of the calculated pore radius on the type of gas used in the experiment has been found. Measurements on the ultrafiltration membranes indicate that the viscous flow contribution to the permeability coefficient cannot be determined with sufficient accuracy. It is concluded that application of the gas permeation method has some limitations which were not previously recognized.     

A simple instrument to characterize convective and diffusive transport of single hollow fibers of short length

The use hollow fiber membranes for cell encapsulation is being developed as an experimental transplantation technology in which a permselective membrane physically isolates grafted cells from directly interacting with host cells or tissue. Although laboratory characterization of commercial ultrafiltration and dialysis membranes using multi-fiber test modules is well established, a simple apparatus for characterizing the transport properties of small individual segments of hollow fiber membrane has not been described. In the current study, we describe an instrument for evaluating the diffusive and convective transport characteristics of individual cell encapsulation membranes at size scales typically used in animal and human clinical studies. The performance capabilities of the instrument are described, as well as methods for determining hydropermeability, diffusive permeability, and solute rejection.     

磺化聚醚酰亚胺/聚醚酰亚胺共混膜的结构与性能

将亲水性的磺化聚醚酰亚胺(SPEI)和疏水性的聚醚酰亚胺(PEI)共混,以N,N-二甲基乙酰胺(DMAc)为溶剂,制备了SPEI(Na型)／SPEI中空纤维超滤膜．研究了纺丝过程中内凝固浴组成比例和空气间隙距离变化对膜结构以及膜分离性能的影响。实验结果表明,随着内凝固浴中DMAc含量的提高,纤维内指状孔减少,水通量下降,而截留率则不受影响;随着空气间隙距离的增大,从膜内壁侧出发的指状孔结构前端逐渐向外壁发展,而膜的外壁侧则逐渐变得致密,同时,膜的外表面可能出现较大的微孔结构,导致膜的水通量随着空气间隙距离的增加而迅速增加后随之下降,而截留率则一直保持下降趋势。     

Transport and separation of charged macromolecules under nonlinear electromigration in nanochannels

In this work, we theoretically investigate the implications of nonlinear electrophoretic effects on the transport and size-based separation of charged macromolecules in nanoscale confinements. By employing a regular perturbation analysis, we address certain nontrivial features of interconnection among wall-induced transverse migrative fluxes, electrophoretic and electroosmotic transport, confinement-induced hindered diffusive effects, and hydrodynamic interactions in detail. We demonstrate that there occurs an optimal regime of influence of the nonlinear electrophoretic effects, within which high values of separation resolution may be achieved. This size-based optimal regime, however, can be effectively exploited only for nanochannel flows, as attributed to the strong electric double layer interactions prevalent within the same.     

A semi-continuous laboratory-scale polymer enhanced ultrafiltration process for the recovery of cadmium and lead from aqueous effluents

A semi-continuous process of polymer enhanced ultrafiltration for removal of lead and cadmium has been elaborated. This operation mode would let a better coupling between industrial and laboratory-scale processes. Basically, it includes two stages: (1) metal retention, where we can obtain a permeate stream free of heavy metals; (2) polymer regeneration, where the polymer is regenerated in order to be reused in metal retention stage. In order to work in this way, a control system of permeate and feed stream flows has been installed in a batch laboratory-scale plant. In the first place, more suitable hydrodynamic operating parameters were obtained by ultrafiltration experiments. The influence of pH has been studied to fix the pH for metal retention and polymer regeneration experiments, and the operative polymer binding capacity has been determined to know the metal amount that can be treated. A mathematical model taking into account both conservation equations and competitive reactions which occur in the medium has been established. The development of this mathematical model (which is in good agreement with experimental data) enables to estimate design parameters to dimension pilot and industrial scale installations based on this process.     

Electrostatic exclusion on anionic membranes for ultrafiltration

This work concerns electrostatic processes on ionic membranes for ultrafiltration. The salt retention and the flow-rate of electrolytes are determined. The degree of swelling τ is controlled to deduce the volume fraction of pores. The salt retention varies with the ionic concentration of the solution and nature of the counter-ion; it is correlated with the net charge of the membrane. The influence of the ionic concentration of the electrolyte on the flow-rate is interpreted by introduction of a streaming potential.     

Protein type effects on steady-state crossflow membrane ultrafiltration fluxes and protein transmission

The permeate fluxes and percent protein transmission were evaluated for steady-state crossflow ultrafiltration of two proteins of different composition: bovine serum albumin (BSA), containing fatty acid, and “fatty-acid-poor” BSA, from which most of the fatty acids had been removed (BSA/FAP). The influences of protein concentration up to 6.5 percent w/v, transmembrane pressure, ionic environment and membrane type (i.e. nominal molecular weight cut-off) were investigated. For both BSA and BSA/FAP, the fluxes and the protein transmission were dependent on the amount of salt present. The higher fatty acid content in the BSA apparently enhanced protein-protein interaction, resulting in a more cohesive and resistant fouling layer; permeate fluxes were lower with BSA/FAP than with BSA at otherwise corresponding operating conditions. A hysteresis behaviour of the flux (J)-transmembrane pressure (TMP) relationship was observed whenever the ultrafiltration unit was operated at a TMP less than some higher value to which the membrane previously had been exposed.