Highly selective membranes in protein ultrafiltration

A new ultrafiltration technique based on a multimembrane stack has been developed to fractionate solutes closer in size than conventionally possible. The technique is illustrated here by obtaining a pure protein product from a binary protein mixture. By employing membranes in series without any gaskets or spacers in-between, ultrafiltration is carried out to separate two proteins relatively close in molecular weight or size. Flat YM30 regenerated cellulose membranes, all of the same molecular weight cut-off (MWCO) 30,000, are stacked together in the desired number, and ultrafiltration takes place. The membrane rejection of a protein is amplified with each additional membrane, ultimately resulting in a completely rejected species. Complete purification of the more permeable protein may be achieved regardless of the physicochemical condition that may be optimal or suboptimal for selective separation by a single membrane. Two systems, myoglobin and beta-lactoglobulin, as well as myoglobin and alpha-lactalbumin were studied, under various operating conditions. The solvent flux reduction encountered when each membrane is added may also be avoided, by operating at increased pressure, while still achieving the desired purification. Cleaning in situ is achievable with reproducible experimental results before and after on-line cleaning. The results clearly demonstrate that multimembrane stacks can be used for fractionation of proteins that are quite close in molecular weight/size.     

Effect of protein adsorption on the transport characteristics of asymmetric ultrafiltration membranes.

The effects of bovine serum albumin adsorption on the transport characteristics of asymmetric poly(ether sulfone) ultrafiltration membranes were determined using polydisperse dextrans with gel permeation chromatography. Actual dextran sieving coefficients were evaluated from observed sieving data for both the clean and preadsorbed membranes using a stagnant film model. The flux dependence of the actual dextran sieving coefficients was used to evaluate the intrinsic membrane hindrance factors for convective (i.e., sieving) and diffusive transport for the different molecular weight dextrans using classical membrane transport theory. Protein adsorption caused a reduction in both dextran sieving and diffusion, with the magnitude of the reduction a function of the dextran molecular weight and pore size. The effects of adsorption on the specific pore area and the membrane porosity were then determined using a recent model for solute transport through asymmetric ultrafiltration membranes. The data indicate that protein adsorption occurs preferentially in the larger membrane pores, causing a greater reduction in solute sieving compared to the membrane hydraulic permeability and porosity than would be predicted on the basis of either a simple pore blockage or pore constriction model.     

Self-cleaning membranes for ultrafiltration

In order to reduce the severe flux losses encountered during ultrafiltration of protein solutions, proteases were immobilized on Ultrafiltration membranes to hydrolyze the deposited solute molecules. Over a standard 22 hr run 25 to 78% improvement in cumulative permeate yield was obtained when processing 0.5% albumin or hemoglobin. It was also demonstrated that the flux enhancements were due to the biochemical action of the absorbed protease and not to its physical effect as a prefilter coat. with the aid of a model retardation of gel formation mechanism was demonstrated. Economics of the system were shown to be favorable, improving the rate of return on capital investment up to 50% by reduction of the total membrane area of the plant.     

Performance optimization of polysulfone ultrafiltration membranes for riboflavin separation using design experiments

Polysulfone membranes containing different concentrations of polyethylene glycol (M.Wt: 600 Da) as additive was prepared using N,N-dimethyl formamide as solvent. The parameters chosen for the study of separation of vitamin B₂ were concentrations of additive and feed and solvent evaporation time. Box-Behnken design methods and analysis of variance have been applied to the experimental separation of vitamin B₂ by polysulfone membranes. Box-Behnken design with three levels of additive concentration (2.5, 6.25 and 10 wt%), feed concentration (0.002, 0.005 and 0.008%) and solvent evaporation time (10, 20 and 30 s) is used for the identification of significant effects and interaction for the batch studied. Second order polynomial regression model which was used for analysis of the experiment made significant effect. The experimental values are in good correlation with predicted values, and the correlation coefficient was found to be 0.9194.     

Characteristics of collodion membranes for ultrafiltration

An apparatus for the production of graded collodion membranes is described. The theoretical considerations of calibration are discussed in relation to the demonstrable structure and statistical characteristics of the membranes. A new definition of an "end-point" is suggested.     

Alginate ultrafiltration membranes

Summary A simple method of producing ultrafiltration membranes, using calcium and barium alginates, has been developed. Although restricted in their flux and rejection characteristics these membranes offer a cheap model system for studying the fouling of hydrophilic membrane surfaces. The method is reproducible, and with suitable modification allows the membranes to be dried and reconstituted with little effect on performance.     

Effect of ion binding on protein transport through ultrafiltration membranes

Electrostatic interactions can have a significant impact on protein transmission through semipermeable membranes. Experimental data for the transport of bovine serum albumin (BSA) through a polyethersulfone ultrafiltration membrane were obtained in different salt solutions over a range of pH and salt concentrations. Net BSA charge under the same conditions was evaluated from mobility data measured by capillary electrophoresis. The results show that specific ionic composition, in addition to solution pH and ionic strength, can strongly affect the rate of protein transport through semipermeable ultrafiltration membranes. The effects of different ions on BSA sieving are due primarily to differences in ion binding to the protein, which leads to significant differences in the net protein charge at a given pH and ionic strength. This effect could be described in terms of an effective protein radius, which accounts for the electrostatic exclusion of the charged protein from the membrane pores. These results provide important insights into the nature of the electrostatic interactions in membrane systems.     

Effect of solution pH on protein transport through ultrafiltration membranes

Although a number of previous studies have demonstrated that solution pH can have a dramatic effect on protein transport through ultrafiltration membranes, the exact origin of this behavior has been unclear. Experimental data were obtained for the transport of a broad range of proteins with different surface charge and molecular weight. The effective hydrodynamic size of the proteins was evaluated using size‐exclusion chromatography. The membrane charge, both before and after exposure to a given protein, was evaluated using streaming potential measurements. In most cases, the electrostatic interactions were dominated by the distortion of the electrical double layer surrounding the protein, leading to a distinct maximum in protein transmission at the protein isoelectric point. Attractive electrostatic interactions did occur when the protein and membrane had a large opposite charge, causing a second maximum in transmission at a pH between the isoelectric points of the protein and membrane. The sieving data were in good agreement with theoretical calculations based on available models for the partitioning of charged solutes in cylindrical pores. © 1999 John Wiley & Sons, Inc. Biotechnol Bioeng 64: 27–37, 1999.     

Effect of membrane morphology and operation on protein deposition in ultrafiltration membranes

Membrane morphology is compared to protein depostion under passive adsorption and ultrafiltration conditions. Solute resistance of protein deposits for membranes of varying roughness, structure, and permeability can vary dramatically with operating conditions. Using Brunauer-Emmett-Teller adsorption isotherm (BET), study of the internal area and accessibility of several uttrafiltration membranes to protein deposition allows better understanding of the fouling mechanisms and interpretation of adsorbed protein quantities. (c) 1995 John Wiley & Sons, Inc.     

Biocatalytic membranes for ultrafiltration treatment of wastewater containing dyes

A possibility to prepare the biofunctional membranes showing the biocatalytic properties and use those in post-treatment of wastewater containing synthetic dyes have been established. Selected Pseudomonas mendocina and Bacillus subtilis cultures were used as biocatalysts for dye destruction. It has been established that cells in spore form are able to survive in N-methylpyrrolidone that allow to use method of polymer solution casting for membrane preparation. The optimal conditions for entrapping of whole cells of microorganisms into the polymer matrix have been determined. Membrane biocatalytic activity has been studied depending on method of casting solution preparation, biocatalyst loading and operating parameters. Dye destruction occurs both in membrane pores and on membrane surface. Membrane obtained provide discolouring of treated solutions (permeate). The dye concentration in retentate depends on the trans-membrane fluxes. The concentration in retentate need not be observed at relatively low fluxes (up to 20 l/m² h).     

Biotechnical and other applications of nanoporous membranes

Recent advances mean that arrays of nearly uniform cylindrical, conical and pyramidal shaped pores can be produced in several types of substrates. Surface modification of nanopore surfaces can give unique mass transport characteristics that have recently been explored for biomolecule separation, detection and purification. Recent interest has focused on the use of nanoporous membranes for mass transfer diodes that act analogous to solid-state devices based on electron conduction. Asymmetric pores such as conical pores can show superior performance characteristics compared to traditional cylindrical pores in ion rectification. However, many phenomena for membranes with asymmetric pores still remain to be exploited in biomolecular separation, biosensing, microfluidics, logic gates, and energy harvesting and storage.     

Two-dimensional nanopores and nanoporous membranes for ion and molecule transport

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Evaluation of ultrafiltration membranes with biological macromolecules

Eighteen ultrafiltration membranes ranging in molecular weight cutoff ratings from 500 to 300,000 were tested with water, 0.5M NaCl solution, and, in some cases, with macromolecules and urea in a 3-in. stirred filter cell. Approximately half of the membranes showed a significant decrease in filtration rate during the first 24-hr period. The steady-state rates were less than the manufacturers' rating for about two thirds of the membranes, the discrepancy being greater for the membranes with high molecular weight cutoffs. The filtration rates were linearly dependent on applied pressure over the range at least as great as 15 to 55 psig. The rate decreased as the concentration of macromolecules such as transfer ribonucleic acid (tRNA) increased; the rate for a concentration of 3 mg tRNA/per ml was one-fourth of that observed when no tRNA was present. Some increase in rate (～33 to 50%) was obtained by increasing the stirring speed from 100 rpm to 1000 rpm. The membranes were effective for desalting and concentration of macromolecules but not for separation of large molecules from each other, such as tRNA from bovine serum albumin. Easily denatured molecules such as catalase were not deactivated by filtration at 4°C.     

Purification of antibiotics by ultrafiltration through semi-permeable membranes

The process of purification of benzylpenicillin, penicillin V and erythromycin solutions with the use of semi-permeable membranes was studied. Practically complete separation of high-molecular admixtures and microbial colonies and separation of significant amounts of the colored compounds were shown to be possible. The purification process may be used effectively at the early stages of antibiotic production.     

Separation albumin-PEG: transmission of PEG through ultrafiltration membranes

Transmission of polyethylene glycol (PEG) through ultrafiltration membranes has been studied under various operating conditions of pressure, crossflow, and concentration, using different membranes cut-offs and two module designs with the aim of understanding the separation of PEG from BSA. The influence of protein adsorption and fouling of the choice of a membrane has also been considered. Retention depends in general on the molecule to average pore size ratio, as expected, but also on concentration polarization. Accordingly, all operating and design parameters favoring concentration polarization lead to higher transmission. At high fluxes, flexible macromolecules can pass through the membrane, even if the random coil is larger than the apparent average pore. From a process selectivity point of view, the best way to separate PEG from BSA would be to use a membrane totally retaining BSA and to enhance concentration polarization of PEG. Unfortunately, such conditions also increase fouling and concentration polarization by BSA, which limits flux and thus PEG concentration polarization and transmission. Consequences of such conditions on separation efficiency are discussed. (c) 1993 Wiley & Sons, Inc.     

Cleaning of inorganic membranes after whey and milk ultrafiltration

Cleaning of an inorganic ultrafiltration membrane has been quantified through hydraulic, physicochemical, and spectroscopic (infrared and x-photoelectron spectroscopy) analyses. An efficient cleaning sequence of nitric acid followed by sodium hypochlorite has been proposed for cleaning of defatted whey protein concentrate and milk ultrafiltration membranes. The influence of reversed sequence and time reduction are discussed together with the action of both cleaning chemicals. In spite of residual fouling left after every cleaning sequence studied, hydraulic cleanliness of the membrane was achieved, particularly after the standard procedure.