Conductive diamond hollow fiber membranes

Boron-doped diamond hollow fiber membrane (BDD–HFM) was fabricated as a novel type of porous conductive diamond. BDD–HFM was obtained by deposition of BDD polycrystalline film onto a quartz filter substrate consisting of quartz fibers, followed by etching of the substrate in HF/HNO₃ aqueous solution. Cross-sectional scanning electron microscope (SEM) observation showed the inner diameter and wall thickness of the BDD hollow fibers were in the range of 0.4–2 and 0.2–2 μm, respectively. The BDD–HFM electrode exhibited a relatively large double-layer capacitance (ca. 13 F g⁻¹) in 0.1 M H₂SO₄. Electrochemical AC impedance properties were simulated using an equivalent circuit model containing a transmission line model, which indicated characteristics of a porous electrode material.     

Oxygen selective ceramic hollow fiber membranes

Oxygen ion conducting Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ hollow fiber membranes with o.d. 1.15 mm and i.d. 0.71 mm were fabricated using a sequence of extrusion, gelation, coating and sintering steps. The starting ceramic powder was synthesized by combined EDTA–citrate complexing followed by thermal treatment at 900 °C. The powder was then dispersed in a polymer solution, and extruded through a spinerette. After gelation, an additional thin coating of the ceramic powder was applied on the fiber, and sintering was carried out at 1190 °C to obtain the final ceramic membrane. The fibers were characterized by SEM, and tested for air separation at ambient pressure and at temperatures between 700 and 950 °C. The maximum oxygen flux measured was 5.1 mL/min/cm² at 950 °C.     

Bromine recovery with hollow fiber gas membranes

Gas membranes supported by microporous hollow fibers have been used to concentrate bromine from a variety of brines similar to seawater. The bromine transport is governed by diffusion in the brine, and hence is almost independent of membrane properties except the surface area per volume. In some cases, this type of membrane can be an alternative to packed towers, simultaneously carrying out both absorption and stripping.     

Gas permeation properties of asymmetric carbon hollow fiber membranes prepared from asymmetric polyimide hollow fiber

Asymmetric carbon hollow fiber membranes were prepared by pyrolysis of an asymmetric polyimide hollow fiber membrane, and their mechanical and permeation properties were investigated. The carbon membrane had higher elastic modulus and lower breaking elongation than the polyimide membrane. Permeation experiments were performed for single gases such as H₂, CO₂, and CH₄, and for mixed gases such as H₂/CH₄ at high feed pressure ranging from 1 to 5 MPa with or without toluene vapor. The permeation properties of the carbon membranes and the polyimide membrane were compared. There was little change in the properties of the carbon membranes with a passage of time. The properties were hardly affected by the feed pressure, whether the feed was accompanied with the toluene vapor or not, because the carbon membranes were not affected by compaction and plasticization.     

Photoinduced electron transport across protein-containing membranes

It is shown in the present work that a protein globule can mediate the donor-to-acceptor electron transport, when the protein is in the solution or incorporated into the membrane.     

Preparation and characterization of inorganic hollow fiber membranes

Inorganic hollow fiber membranes were prepared by spinning a polymer solution containing suspended aluminum oxide (Al₂O₃) powders to a hollow fiber precursor, which is then sintered at elevated temperatures. In spinning these hollow fiber precursors, polyethersulfone (PESf), N-methyl-2-pyrrolidone (NMP), and polyvinyl pyrrolidone (PVP) were used as a polymer binder, a solvent, and an additive, respectively. The inorganic hollow fiber membranes prepared were characterized using scanning electron microscope (SEM), gas permeation techniques Coulter porometer, and gravimetric analysis. Some primary factors affecting the structure and performance of the membranes such as the sintering temperature and the ratio of the aluminum oxide to the PESf polymer binder were studied extensively. The prepared inorganic membranes show an asymmetric structure, which is similar to the conventional polymeric membranes prepared from the same phase-inversion technique. The inorganic hollow fiber membrane with a higher porosity and better mechanical strength could be prepared by blending the spinning solution with a smaller amount of aluminum oxide powder.     

Solvent-induced morphological change of microporous hollow fiber membranes

Microporous polyethylene hollow fiber membranes (EHF-1 and EHF-2) were subjected to solvent treatment, and the effects of this treatment on membrane morphology and permeating properties were studied. Membranes treated with various organic solvents exhibited enhanced permeability, enlarged pore size, and increased shrinkage in the longitudinal direction. These phenomena were found to depend on the surface tension of the solvent: the higher the surface tension of the solvent, the larger the change in morphology and permeation of the membrane. A mechanism to account for the effects of solvent treatment on the morphology of the membrane is proposed taking into consideration the influence of the type of solvent used for treatment. The enhanced morphological and permeation changes are ascribed to the formation of liquid bridges between two microfibrils of the membrane during drying followed by the deformation and adhesion of the adjacent microfibrils based on the surface tension of the solvent.     

Ion transport across biomembranes and model membranes

The milestones formerly achieved in the comprehension of ion transport across biological membranes on the basis of electrochemical concepts and/or instrumentation are briefly summarized. The various types of model membranes presently employed for the investigation of ion transport across biomembranes are reviewed and their requirements for the incorporation and functional investigation of membrane proteins are examined. The potential of model membranes for the elucidation of many problems in molecular membrane biology and for the realization of microarray sensors individually addressable to membrane proteins by electrochemical means is assessed.     

Chemical cleaning of oil contaminated polyethylene hollow fiber microfiltration membranes

The primary aim of this paper was to develop a more effective and economical procedure for cleaning polyethylene hollow fiber microfiltration membranes that have been used for removing oil from contaminated seawater. Alkaline cleaning showed higher recovery of operating cycle time but lower permeate flux recovery than acid cleaning. The combination of both alkaline and acid cleaning agents gave the best operating cycle time and flux recoveries (e.g. 96% and 94%, respectively). As the cleaning agent soaking time was reduced, the actual operating cycle time was reduced. However, the ratio of operating time/chemical cleaning time increased as the soaking time was reduced. The soaking time was recommended to be as short as possible (8–10 h) in the design of small capacity plants and 30 h or higher in case of large capacity plants. SEM analysis showed that in case of alkaline cleaning, most of the pores remained covered with a foulant layer, resulting in low flux recovery. The SEM results of acid cleaned membranes showed more complete removal of the foulant layer from the pores resulting in better flux recovery. Surface analysis of membranes cleaned with combined acid/base agents showed the best results. A membrane surface similar to the original one was obtained. The long-term objective is to increase the understanding of membrane fouling phenomena, preventive means and membrane cleaning processes as it applies to the clean-up and desalination of oil contaminated seawater.     

Fractionation of casein hydrolysates using polysulfone ultrafiltration hollow fiber membranes

The objective of this study was to characterize the fractionation profile of casein hydrolysates obtained with polysulfone hollow fiber ultrafiltration membranes. The two-step ultrafiltration process developed by Turgeon and Gauthier [J. Food Sci., 55 (1990) 106] was used: a caseinate solution was submitted to proteolysis with chymotrypsin or trypsin, and the reaction mixture (RM) was subsequently ultrafiltered using a 30 kDa (MWCO) hollow-fiber polysulfone membrane. The total hydrolysate permeating from this first step was further fractionated using a 1 kDa (MWCO) membrane, producing the mixture of polypeptides (retentate) and the amino acid fraction (permeate). The effect of enzyme specificity and of membrane retentivitiy on the total composition (total nitrogen, fat, lactose, minerals) and amino acid profile of the fractions was studied. The overall composition of the fractions was not significantly affected by the nature of the enzyme but the degree of hydrolysis and the molecular weight distribution profile analyses showed a marked effect of the enzyme specificity, with trypsin giving a larger proportion of small peptides (< 200 Da) in the mixture of polypeptides. Amino acid profile analyses provided useful information on the phenomena governing the fractionation of amino acids with a polysulfone membrane: (1) the target amino acids of the enzyme are concentrated in the permeate as a result of their presence in all peptides produced by hydrolysis, (2) polar amino acids are retained by the membrane, (3) non-polar amino acids are not selectively rejected by the membrane. Our results suggest that the charge/hydrophobicity balance of the peptides produced is the predominant factor determining the fractionation of casein hydrolysates.     

Fabrication of multi-layer composite hollow fiber membranes for gas separation

Using multilayer composite hollow fiber membranes consisting of a sealing layer (silicone rubber), a selective layer (poly(4-vinylpyridine)), and a support substrate (polysulfone), we have determined the key parameters for fabricating high-performance multilayer hollow fiber composite membranes for gas separation. Surface roughness and surface porosity of the support substrate play two crucial roles in successful membrane fabrication. Substrates with smooth surfaces tend to reduce defects in the selective layer to yield composite membranes of better separation performance. Substrates with a high surface porosity can enhance the permeance of composite membranes. However, SEM micrographs show that, when preparing an asymmetric microporous membrane substrate using a phase-inversion process, the higher the surface porosity, the greater the surface roughness. How to optimize and compromise the effect of both factors with respect to permselectivity is a critical issue for the selection of support substrates to fabricate high-performance multilayer composite membranes. For a highly permeable support substrate, pre-wetting shows no significant improvement in membrane performance. Composite hollow fiber membranes made from a composition of silicone rubber/0.1–0.5 wt% poly(4-vinylpyridine)/25 wt% polysulfone show impressive separation performance. Gas permeances of around 100 GPU for H₂, 40 GPU for CO₂, and 8 GPU for O₂ with selectivities of around 100 for H₂/N₂, 50 for CO₂/CH₄, and 7 for O₂/N₂ were obtained.     

Transport of strontium cation through hollow fiber supported liquid membranes

Transport of strontium through supported hollow fiber dichlorobenzene liquid membranes has been studied. The possible mechanism of strontium transport with 18-crown-6 ether as a carrier and picrate as co-counter-ion as well as the construction of a pertraction device with on-line radiometric detection of strontium using⁸⁵Sr tracer is described. Preliminary results of strontium pertraction in a recycling and one-pass mode with different concentrations of crown are shown.     

Parameters influencing transport across conducting electroactive polymer membranes

The parameters which influence electrochemically facilitated transport of electroinactive ions across conducting electroactive polymer membranes have been investigated. The design of membranes and the materials used as well as transport cells and systems have been addressed to improve selectivity and flux. Polypyrrole-para-toluenesulfonate (PPy-pTS) was compared with the copolymer of pyrrole with 3-carboxy-4-methylpyrrole (PPy/PCMP-pTS) and their different chemistries resulted in different membrane selectivities for ions. Platinum mesh was found to be the most suitable auxiliary electrode material and its placement in the cell chamber(s) facilitated ion incorporation/expulsion at the membrane working electrode. This enhanced the flux of ion transport. The flux can also further enhanced by narrowing the distance between the membrane working electrode and the platinum mesh auxiliary electrode(s), and/or by stirring to improve the hydrodynamics. An alternative cell design, namely a dual membrane flow through cell, also proved to be more efficient for ion transport. Good connection geometry to the membrane as well as the application of a square wave pulsed potential waveform to the membrane was found to be essential for achieving high and sustainable flux in ion transport.     

Ion transport across membranes prepared by gel crystallization

A one-step procedure for the preparation of ion-selective membranes is described. The method employs the thermally induced gel crystallization of ultrahigh molecular weight poly(ethylene) (UHMW-PE) from a dilute xylene solution. After evaporation of the xylene, a microporous UHMW-PE film remains, which can serve as the support for liquid and polymeric ion-selective membranes. The addition of a membrane solvent and suitable receptor molecules to the xylene solution allows a one-step incorporation of these membrane components into the UHMW-PE support. The influence of the preparation conditions of the UHMW-PE support on the rates of the p-tert-butylcalix[4]arene tetraethylester-mediated transport of NaClO₄ was studied. Two concepts to improve the life-time of the membranes are introduced. In a first approach, the addition of photocrosslinkable nitril-substituted siloxane copolymers to the membrane phase has been evaluated. The enhanced viscosity of the membrane phase reduces leaching of carrier and solvent molecules from the membrane into the aqueous phases. In a second approach, the solvent is omitted and the membrane-phase merely consists of a benzo-15-crown-5 or calix[4]arene modified siloxane-copolymer, which is substituted to such a degree that ion transport no longer has to take place via diffusion of host–guest complexes but by jumping of the cations from one fixed carrier to a neighboring carrier. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36 : 383–394, 1998     

Ionic transport across tailored nanoporous anodic alumina membranes

Monodispersed silica particles with bimodal size distribution were successfully prepared through adding an ethanol (EtOH) solution containing tetraethylorthosilicate (TEOS) dropwise into an ammonia EtOH solution at a constant low rate. The effects of the reaction parameters such as ammonia/ethanol ratio, feeding rate of TEOS solution, reaction temperature, and time on the size and size distribution of the as-obtained particles were investigated. Based on these phenomena, a modified LaMer model of nucleation and growth mechanism was proposed to reasonably explain the formation of the as-obtained silica particles with bimodal size distribution. The as-prepared monodispersed silica particles with bimodal size distribution can be directly fabricated into binary colloidal crystals with small particles surrounding large particles by evaporation-induced cooperative self-assembly. This suggests that the method reported here provides a straightforward and effective route to the in situ fabrication of novel binary colloidal crystals and their replicated patterns in one reaction system.     

The effects of spinneret dimension and hollow fiber dimension on gas separation performance of ultra-thin defect-free Torlon hollow fiber membranes

A defect-free as-spun hollow fiber membrane with an ultra-thin dense-selective layer is the most desirable configuration in gas separation because it may potentially eliminate post-treatments such as silicone rubber costing, simplify membrane manufacture, and reduce production costs. However, the formation of defect-free as-spun hollow fiber membranes with an ultra-thin dense-selective layer is an extremely challenging task because of the complexity of phase inversion process during the hollow fiber fabrication and the trade-off between the formation of an ultra-thin dense-selective layer and the generation of defects. We have for the first time successfully produced defect-free as-spun Torlon^® hollow fiber membranes with an ultra-thin dense layer of around 540 Å from only a one polymer/one solvent binary system at reasonable take-up speeds of 10–50 m/min. The best O₂/N₂ permselectivity achieved is much higher than the intrinsic value of Torlon^® dense films. This is also a pioneering work systematically studying the effects of spinneret dimension and hollow fiber dimension on gas separation performance. Several interesting and important phenomena have been discovered and never been reported: (1) as the spinneret dimension increases, a higher elongation draw ratio is required to produce defect-free hollow fiber membranes; (2) the bigger the spinneret dimension, the higher the selectivity; (3) the bigger the spinneret dimension, the thinner the dense-selective layer. Mechanisms to explain the above observation have been elaborated. The keys to produce hollow fiber with enhanced permselectivity are to (1) remove die swell effects, (2) achieve finer monodisperse interstitial chain space at the dense-selective layer by an optimal draw ratio, and (3) control membrane formation by varying spinneret dimension.     

Diffusive transport of Cu(II) ions through thin ion imprinted polymeric membranes

A novel ion imprinted polymeric membrane (IIPM) for copper (Cu) ions transport was prepared by a ion imprinting technique via cross-linking of blended chitosan (CS)/polyvinyl alcohol (PVA) using glutaraldehyde (GA) as the cross-linker and Cu ions as the template. The obtained IIPM was characterised and evaluated by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and permeation studies. Cavities of IIPM containing recognition sites for Cu ions were formed in the compact structure of the CS/PVA membrane which was prepared via the solution casting method. Under the optimum conditions, transport factor of the IIPM reached 2 when the permeation time was 18 h. Selective permeation of Cu ions versus nickel ions through the imprinted membrane was confirmed and a selectivity factor of 1.71 was obtained.     

Composite hollow fiber membranes for CO2 capture

Composite hollow fibers membranes were prepared by coating poly(phenylene oxide) (PPO) and polysulfone (PSf) hollow fibers with high molecular polyvinylamine (PVAm). Two procedures of coating hollow fibers outside and respective inside were investigated with respect to intrinsic PVAm solution properties and hollow fibers geometry and material.The influence of operating mode (sweep or vacuum) on the performances of membranes was investigated. Vacuum operating mode gave better results than using sweep because part of the sweep gas permeated into feed and induced an extra resistance to the most permeable gas the CO₂. The composite PVAm/PSf HF membranes having a 0.7–1.5 μm PVAm selective layer, showed CO₂/N₂ selectivity between 100 and 230. The selectivity was attributed to the CO₂ facilitated transport imposed by PVAm selective layer. The CO₂ permeance changed from 0.006 to 0.022 m³(STP)/(m² bar h) in direct correlation with CO₂ permeance and separation mechanism of the individual porous supports used for membrane fabrication. The multilayer PVAm/PPO membrane using as support PPO hollow fibers with a 40 nm PPO dense skin layer, surprisingly presented an increase in selectivity with the increase in CO₂ partial pressure. This trend was opposite to the facilitated transport characteristic behaviour of PVAm/porous PSf. This indicated that PVAm/PPO membrane represents a new membrane, with new properties and a hybrid mechanism, extremely stable at high pressure ratios. The CO₂/N₂ selectivity ranged between 20 and 500 and the CO₂ permeance from 0.11 to 2.3 m³(STP)/(m² bar h) depending on the operating conditions.For both PVAm/PSf and PVAm/PPO membranes, the CO₂ permeance was similar with the CO₂ permeance of uncoated hollow fiber supports, confirming that the CO₂ diffusion rate limiting step resides in the properties of the relatively thick support, not at the level of 1.2 μm thin and water swollen PVAm selective layer. A dynamic transfer of the CO₂ diffusion rate limiting step between PVAm top layer and PPO support was observed by changing the feed relative humidity (RH%). The CO₂ diffusion rate was controlled by the PPO support when using humid feed. At low feed humidity the 1.2 μm PVAm top layer becomes the CO₂ diffusion rate limiting step.