In recent years, membrane-based processes have attracted considerable attentions as a valuable technology. Separation membranes have been applied widely in industrial, biomedical and analytical fields as well as in wastewater treatment. Nowadays, they constitute basic materials which stimulate technological developments and scientific research, and continual improvements are being made.
In general, a membrane is a thin barrier or film between two phases with preferential transport of some species over others. Compared to other traditional technologies, membrane owns the characteristics like ease of operation, energy and selectivity advantages, and low cost operation factors. Basically, membranes can be divided into two types: one is the porous membranes applied in microfiltration and ultrafiltration; the other one is dense membranes applied in gas separation and pervaporation. The porous membranes contain fixed pores, whose path is usually tortuous and size distribution is rarely monodisperse. The selectivity is mainly determined by the dimensions of the pores and the solvent passes through the membrane mainly by convection or bulk fluid flow. Dense membranes haven't got distinct pores. In this type of membrane, the permeability and selectivity of membranes are depended on the intrinsic properties of the material. When placed between two aqueous phases, chemical species can move through the membrane by means of a solution-diffusion process from high solute concentration into a region of low solute concentration. However, the species can also be transported across membrane against their own concentration gradient as a consequence of an existing concentration gradient of a second species present in the system (coupled transport) or in the presence of an extractant or carrier contained within the membrane (facilitated transport).
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Although all other membrane sectors have encounter a market boom recently, practical applications of liquid membranes remain largely limited. Depending whether a polymeric support is involved, liquid membranes may be divided into two categories: nonsupported liquid membranes and supported liquid membranes (SLM). The most common types of non-SLMs are emulsion liquid membranes (ELM) and bulk liquid membranes (BLM). The low interfacial surface areas and mass transfer rates are the principal disadvantages of BLMs while the emusion breakage is the main problem associated with ELMs. SLMs have poor stability problem. These factors make liquid membranes impractical for many large-scale applications.
Nevertheless, over the last two decades, the need for metal ion recovery as well as for the extraction of numerous small organic compounds in hydrometallurgy, biotechnology and in the treatment of industrial wastewater has become stronger and stronger. Significant scientific effort has been expended to understand and improve the stability of liquid membranes. The numbers of scientific investigations devoted to this topic has been rising steadily and as a result, a novel type of liquid membranes has been discovered. They are commonly called polymer inclusion membranes (PIMs), but other names like polymer liquid membranes (PLMs) , gelled supported liquid membranes (GSLMs) , plasticized polymeric membranes (PPMs) , fixed sites membranes (FSMs) , and solvent polymeric membranes are also being used. PIMs are prepared through casting a solution containing a plasticizer, an extractant and a base polymer to form a thin, flexible and stable film. They can be used to achieve the separation of certain solute similar to SLMs.
PIMs exhibit excellent stability and versatility while retaining most of the advantages of SLMs. Compared to SLMs, the carrier loss during the membrane extracting process can be ignored , the amount of carrier reagent can be greatly reduced and the mechanical properties are quite similar to those of filtration membranes. All of these will no doubt widen the application of PIMs and enable PIM-based systems to exhibit many advantages such as ease of operation, minimum use of hazardous chemicals and flexibility in membrane composition to achieve the desired selectivity as well as separation efficiency.
Base polymers for membrane preparation.
PIMs normally consist of a base polymer, a plasticizer, and a carrier molecule. Base polymer is used to provide mechanical strength for the membrane. To date, despite the fact that various polymers are currently used for many engineering purposes, PVC and CTA have been the only two major polymers used for most of the PIM investigations conducted. This is because both PVC and CTA are easily workable and readily available. They can be used to prepare a thin film through a relatively simple procedure based on dissolution in an organic solvent. Another factor is the lack of information regarding the role of base polymers in mechanically supporting the membranes. It's essential that base polymer needs to enhance the membrane stability and at the same time creates a minimal hindrance to the transport of metal ions and small organic compounds within the membranes.
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Recently, the feasibility of several cellulose derivatives has been studied. They include cellulose acetate propionate (CAP), cellulose acetate butyrate (CAB) and cellulose tributyrate (CTB). Several results were found: Membrane durability increased with replacement of acetyl substitution on the cellulose polymer with propionyl or butyryl; as longer alkyl chains were added to the cellulose glucoside units, membrane resistance to hydrolysis increased; as the alkyl chain lengths increased, ion transport across the membrane decreased. These novel types of membranes enable PIMs to process harsh waste solutions without the ester linkages in the cellulose backbone of the polymer.
PVC and CTA are thermoplastic. They are made up of linear polymer strands and there are no cross-links between these strands. Thus, they can be dissolved in a suitable organic solvent, where the polymer strands become separated. Intermolecular forces combines with the process of entanglement to give the mechanical strength of a thermoplastic thin film membrane. The intermolecular forces determine the flexibility of the material with high intermolecular forces resulting in a rigid membrane, while the process of entanglement is the result of random diffusion of the flexible polymer strands in a sol as the solvent evaporates. As a result, a very stable thin film can be formed without any intermolecular covalent bonds, even though there is a disentanglement process occurring over a very long time scale. However, the molecular weight of the polymer used should be larger than the critical entanglement molecular weight of that polymer. Above the critical entanglement molecular weight values, variations in the base polymer molecular weight exert little influence on the behavior of the separation agent within the membrane.
CTA, which is a polar polymer, is often highly crystalline. It has a number of hydroxyl and acetyl groups which are capable of forming highly orientated hydrogen bonding. Its crystalline domain gives CTA the excellent mechanical strength and the characteristics of cellulose enable it infusible. However, all of the cellulose derivatives possess high water absorption. The extent of water adsorption decreases with increasing degree of substitution. The consequences of water adsorption are decreasing the softening point, tensile strength and modulus, increasing elongation at break and impact strength, and the possibility of dimensional changes in moulded articles. Thus CTA can be slightly hydrated which makes it prone to hydrolysis, particularly in an acidic environment. PVC is an amorphorous polymer. In PVC, the C-Cl functional group is relatively polar and non-specific dispersion forces dominate the intermolecular interactions. PVC cannot be hydrated.
Besides providing mechanical support to the membrane, base polymers also exert influences on metal ion transport through their bulk properties. Nowadays, the melting temperature and the glass transition temperature are used to describe polymer's microstructural characteristics and the inherent flexibility. It is found that, below the glass transition temperature, the polymer becomes glassy and rigid. This is unfavorable for metal ion transport in membranes. Thus, it's essential to lower the glass transition temperature. Plasticizers are added to achieve this and a more flexible and less brittle membrane is formed. It's worthy to note that the glass transition temperature or the melting temperature of a pure polymer without a plasticizer is usually much higher than room temperature. This is the reason why plasticizers are needed during PIMs preparation unless the carrier itself can also act as a plasticizer.
Carrier is used to accomplish the transport across liquid membranes. It is essentially a complexing agent or an ion exchanger. It reacts with metal ion to form a complex or ion-pair and facilitates metal ion transport across the membrane. The well-known carriers include macrocyclic and macromolecular compounds, and liquid-liquid extraction extractants. Examples of PIMs carriers reported in the literature and their typical target solutes are shown in Table 1.
Table 1. Examples of PIM carriers reported in the literature and their typical target solutes.
Macrocyclic and macromolecular compounds.
Crown ethers are the first group of macrocyclic compounds. They contain sulfur, oxygen and nitrogen as donor atoms. The number of ether donor atoms and the size and shape of cavity relative to the cation size determine the stability of the crown ether-metal ion complex. There exist three types of crown ethers: sulfur-containing crown ethers, nitrogen-containing crown ethers and oxygen-containing crown ethers. The polarizability of them is different and thereby they display different ionic selectivities. Through the introduction of one or more side arms to monocyclic crown, lariat ethers could be formed and metal ion binding strength and selectivity would be enhanced. The lariat ethers possess the possibility of three-dimensional cation encapsulation and acceptable complexation-decomplexation dynamics. To achieve a successful separation process, a counter anion which is soluble in both the organic and aqueous phases is also needed. However, the complexes formed with common anions have low distribution coefficients between an aqueous phase and an organic phase. Attaching a proton-ionizable sidearm to the crown ether ring can solve this problem and furthermore, it couples metal ions transport from the aqueous source phase into the aqueous receiving phase with back-transport of proton cation. As a result, a pH gradient offers the potential for metal ions transport.
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Calixarenes are another group of macrocyclic compounds. Cyclodextrins are one of them. Cyclodextrins are cyclic oligomers which are composed of six, seven or eight anhydrous glucopyranosyl units. They can react with a wide variety of organic and inorganic substances to form complexes in their hydrophobic cavity. They would become less soluble and more stable when they are polymerized. Hence, their derivatives can be used to isolate or remove many organic and inorganic substances. Similarly, the derivatives of other macrocyclic compounds can be also used as ion carriers to separate metal ions from aqueous solutions.
Liquid-liquid extraction extractants.
Liquid-liquid extraction extractants can be divided into four classes: basic, acidic, solvating and chelating. Most of them are commercially available and some of them are new synthesized, such as phosphonic acids (D2EHPA, Cyanex 272, PC 88A), amines and quaternary ammonium salts (TOA, Aliquat 336), phosphoric acids esters (TBP), 8-hydroxyqinoline (Kelex 100, LIX 26), and hydroxyoximes (LIX 64N, LIX 70, LIX 84, LIX 984).
The influences of carriers on membrane performance.
It is known that the chemical reactions that are involved in the extraction and stripping of target solutes are the same in both PIMs system and the corresponding solvent extraction systems. The only difference between these two systems is the transport of the target solutes through the membrane, which can be strongly influenced by the carrier molecular structure.
The carrier molecular structure can markedly influence the transport efficiency of membrane. In a study, PIMs with lariat ethers carboxylic acids carriers and sym-(alkyl)dibenzo-16-crown-5-oxyacetic acids, is used to separate sodium cations from equimolar mixtures of alkali metal cations in aqueous solutions. It was found that, the length of alkyl chain which is attached to the functional side arm in the lariat ether carrier strongly influenced the total alkali metal cation flux. When the alkyl group contained nine carbon atoms, maximal flux could be obtained. In another study, it was demonstrated that both membrane selectivity and transport efficiency could be improved through a careful combination of ring size and substituent groups of macrocyclic carrier.
The molecular structure of the carrier can also influence the membrane selectivity. Through tailoring the carrier molecular structures, a specific selectivity can be obtained. For example, it was found that the less basic carrier gives higher initial fluxes of all metal ions, but the more basic carrier gives higher Cr/Cd and Cr/Zn selectivity coefficients. Diazadibenzocrown ethers with more hydrophilicity was demonstrated to possess a higher selectivity for Pb over Zn and Cd.
A plasticizer is a material which is added to the plastic to increase its flexibility or distensibility and workability, and lower the temperature of the second-order transition, the melt viscosity, or the elastic modulus of the plastic. The plasticization process is commonly composed of 6 steps: (1) wetting, adsorption; (2) salvation and/or penetration of the surface; (3) absorption, diffusion, with initial (limited) swelling; (4) disassociation and freeing of polar groups; dissolution in the amorphous region; (5) Structure breakdown, diffusion and dissolution of some of the crystalline regions; (6) reestablishment of structure.
In PIMs, various types of attractive forces hold the individual molecular chains together, including van der Waals forces and polar interactions. Van der Waals forces are abundant but are nonspecific and weak. Polar interactions are strong but can only occur at polar centers of the molecule, and often form a rigid non-flexible thin film with a tree-dimensional structure within its polymeric matrix. This three-dimensional structure rigidity decreases the diffusive flux of material within the polymer matrix. The addition of plasticizers can solve this problem. By penetrating between polymer molecules and "neutralizing" the polar groups of the polymer or increasing the distance between the polymer molecules, plasticizers reduce the strength of the intermolecular forces.
Nowadays, a large number of plasticizers are commercially available, but few of them have been used in PIMs preparation. Amongst them, the most frequent used plasticizers in PIMs studies are 2-nitrophenyl octyl ether (2-NPOE) and 2-nitrophenyl pentyl ether (2-NPPE). Figure 1 shows chemical structures of several plasticizers commonly used in PIMs.
Figure 1. Chemical structures of plasticizers commonly used in PIMs.
It can be seen from Figure 1 that plasticizers are generally organic compounds. They contain a hydrophobic alkyl backbone with one or several highly solvating polar groups. The hydrophobic alkyl backbone governs the compatibility of the plasticizer with the membrane, while the solvating polar groups interact with the polar groups of the base polymer to "neutralize" them. Hence, it's important to achieve a proper balance between the polar and non-polar portions of the plasticizer molecule. This balance was first studied by Sugiura. In this study, membranes made with polyoxyethylene alkyl ethers of different alkyl chain length and different numbers of polar oxyethylene groups are used to separate lanthanide ions. It was found that the best combination is alkyl chain of 12 carbon atoms with 2 or 3 polar groups. Any further increase in the length of the alkyl chain or the number of polar groups would affect the plasticizer performance. The former one increases the hydrophobicity and viscosity of the plasticizer, which suppresses the polar properties of the plasticizer. The latter one results in a less viscous and more hydrophilic plasticizer that eventually renders it unusable.
Facilitated transport is a special process carried out in a membrane. It is different from most of the other membrane processes which are alternative forms of filtration or depend on diffusion and solubility in thin polymer films. It involves specific chemical reactions like those in extraction. Facilitated transport has got four characteristics: (1) It is highly selective; (2) It is easily poisoned; (3) A maximum flux can be acquired at high concentration differences; (4) It can concentrate and separate the target solute. These characteristics make it different from other membrane separations, and point 3 and 4 are the most powerful evidence that facilitated transport is occurring.
Both SLMs and PIMs are based on facilitated transport. They use membrane to selective transport a target solute from one aqueous solution to another. This overall process consists of two processes. One is the diffusion of target solute across the membrane, and the other one is the transfer of the target solute across the two interfaces. First, the carrier molecule in the membrane picks up metal ion/species from the feed solution to form a complex. Then, the complex diffuses to the other side of the membrane. Last, the metal ion/species are released into the strip solution through decomplexation and free carrier diffuses back across the membrane for use in another cycle. The solute transport through the two interfaces is similar in both SLMs and PIMs, but the actual bulk diffusion within the membrane phase can be quite different. That's because SLMs are totally different from PIMs in their composition and morphology.
5.1 Interfacial transport mechanisms.
The interfacial transport mechanisms have been studied by several authors. It has been found that the diffusion process through this aqueous stagnant layer is relatively fast and can be ignored when suitable hydrodynamic conditions are maintained. Figure 2 describes the coupled transport of a positively charged (M+) or negatively charged (M-) species through a PIM. It can be seen that coupled transport can be divided into two types: one is co-transport and the other one is counter-transport.
Figure 2. Schematic description of coupled transport of a positively charged (M+) or negatively charged (M-) species through a PIM. C represents the carrier and X is an aqueous soluble coupled-transport ion. [M+], [M-], [X-] and [X+] represent the total analytical concentrations of the respective solute in the bulk aqueous phases. (a) The target solute is a cation and is concurrently transported with a coupled-transport anion; (b) the target solute is a cation and is counter-currently transported with a coupled-transport cation; (c) the target solute is an anion and is counter-currently transported with a coupled-transport anion; (d) the target solute is an anion and is concurrently transported with a couple transport cation.
In co-transport, both metal ions and counter ions are transported from the feed solution through SLM and into the strip solution. First, the metal ion and counter ion react with the carrier C in the membrane to form a complex. Then, this complex diffuses across the membrane. Finally, the metal ion and counter ion are released into the strip solution together. The chemical reactions for this separation process are shown below.
Mn+ + nX- + C (membrane) â†’ CMXn (membrane)
CMXn (membrane) â†’ C (membrane) + Mn+ + nX-
In counter-transport, an acidic carrier, HC, loses a proton and reacts with the metal ion to form a complex MC at the feed solution-membrane interface. Then, the complex MC diffuses across the membrane. Thereafter it liberates the metal cation into the strip solution at the membrane-strip solution interface and picks up a proton from the strip solution simultaneously. Finally, the regenerated carrier HC diffuses back to the feed solution-membrane interface and repeats the whole process. The chemical reactions involved in this coupled transport are shown below.
Mn+ + nHC (membrane) â†’ MCn (membrane) + nH+
MCn (membrane) + nH+ â†’ nHC (membrane) + Mn+
In order to achieve the extraction process, the distribution ratio of the target solute/carrier complex between the organic phase of the membrane and the aqueous solution must be as high as possible. In order to achieve the back extraction of the target solute from the membrane phase, the distribution ratio of the target solute/carrier complex at the receiving site must be as low as possible. As a result, a concentration gradient of the target solute/carrier complex or ion-pair exists within the membrane phase to act as a driving force for its transport across the membrane. That is to say, uphill transport is actually downhill transport regarding the actual chemical species diffusing across the membrane, despite the fact that the total analytical concentration of the target solute in the source solution might be substantially lower than in the receiving solution. The total analytical concentration is the sum of the concentrations of all chemical species containing this metal ion.
The potential gradient of a coupled-transport ion across the membrane is another driving force for the uphill transport phenomenon. In a typical PIM process, the target solute is transported together with this ion to maintain electroneutrality. The facilitated transport of lead through polymeric inclusion membranes has been studied. The PIM consists of cellulose triacetate as polymeric support, bis-(2-ethylhexy)-phosphoric acid (D2EHPA) as carrier, and tris-(2-butoxyethyl) phosphate as plasticizer (TBEP). It was found that the potential gradient of protons can be seen as the driving force for the uphill transport of a metal cation across the membrane, and it could be maintained by adjusting the solution pH. It should be noted that the solution pH also influences the distribution ratio of the target solute between the aqueous solution and the membrane phase.
In fact, the two driving forces described above cannot be distinguished. Both of them belong to a complex interfacial transport mechanism. One focuses on the distribution ratio difference, while the other one focuses on the potential gradient of the coupled-transport ion across the membrane.
5.2 Bulk transport mechanisms.
In addition to transport across the two solution/membrane interfaces, the diffusion of the carrier/target complex through the bulk membrane is also involved in the facilitated transport across a membrane. In a bulk liquid membrane, the carrier plays the role of shuttle, moving freely within the membrane. However, when the carrier is immobilized, the bulk diffusion of the target solute is assumed to take place through successive relocations from one reactive site to another. The membrane in PIM is essentially a quasi-solid homogeneous thin film not a true liquid phase. The carriers in it are not immobilized but often bulky and the mobility of the carrier is more restricted compared to SLMs. Hence, the mechanism of the bulk diffusion processes in PIMs is thought to be different from other liquid membranes.
"Chained carrier" theory is used to describe the facilitated transport process in a solid membrane where the carrier is covalently bound to the polymeric backbone structure. In this theory, both chained carriers and solute are assumed insoluble in the membrane, and the carrier reacts quickly and selectively with solutes at the membrane interfaces to form a complex within the membrane. However, unlike mobile carriers, the complex cannot diffuse across the membrane because of the chemical binding. Some intramolecular diffusion makes the side chain which contains this complex jig around its equilibrium position. In this jigging, the complex may encounter a second, uncomplexed carrier where the solute leaves the first carrier and reacts with the second one to form another complex. The diffusion of the target solute through the bulk membrane can be achieved by repeating the above processes. Figure 3 below shows the differences between mobile carriers and chained carriers.
Figure 3. Mobile carrier vs. chained carriers.
In this theory, a chained membrane is described to be a lamellar structure. The carrier is located in layers and the carrier in each layer can move a distance lo around its equilibrium position. The thickness of each layer is l. No carrier can permanently move from one layer into another and no uncomplexed solute can exist in the membrane. When l > lo, solute cannot be passed from one carrier to the next. Therefore, there is no solute flux across the membrane. When l < lo, a complexed carrier can get very close to a second uncomplexed carrier molecule through diffusing around its equilibrium position. Then it can proceeds to the reaction which produces a solute flux.
This theory implies that chained carrier membranes are similar to mobile carrier membranes. At low solute concentration, solute fluxes vary linearly with solute concentration. At high solute concentration, solute fluxes would approach an asymptote. Fluxes increase with carrier concentration, and these fluxes will often be selective. However, some differences also exist between chained carrier membranes and mobile carrier membranes. First, a percolation threshold is shown in chained carrier membranes. Second, some mobility must be possessed by the chained carriers themselves. Third, the apparent diffusion coefficient in chained carrier membranes may reflect chemical kinetics not diffusion. This theory is not perfect for describing PIMs. It has got a limitation which is the assumption that the carrier sites must be within reach of one another and free uncomplexed solute cannot enter the membrane so that the transfer of the target solute can take place.
Fixed-site jumping model is an extended bulk diffusion model. It is essentially an improvement of chained carrier theory. In this theory, the transport molecules act as "stepping stones" and the solutes jump from one fix-site to another to move across the membrane. Similar to chained carrier theory, a percolation threshold exists. When carrier concentration is below the threshold concentration, the distance between fixed-sites becomes too great to allow solute jumping and there is no flux. When the carrier concentration exceeds the threshold concentration, flux may increase linearly with it or a higher power which is determined by the experimental conditions. In a study of facilitated saccharide transport through plasticized cellulose triacetate membranes, a mobile-site jumping mechanism was proposed. The saccharide diffusion constants was observed to decrease with increasing size of the saccharide, carrier anion and carrier cation. This suggests that ion-pair carrier does not remain as a "fixed site" within the membrane. The most consistent explanation is that the saccharide-ion-pair complex is locally mobile. When the complex moves close enough to an unoccupied carrier ion-pair, the saccharide jumps from ion-pair to ion-pair, and/or the saccharide-anion complex jumps from cation to cation. Thereafter, the transport process through the membrane proceeds. Figure 4 below depicts the proposed mobile-site jumping mechanism for saccharide transport mediated by ion-pair carriers.
Figure 4. Proposed mobile-sited jumping mechanism for saccharide transport mediated by ion-pair carriers.
To date, the fixed-site jumping mechanism was based on the percolation threshold. However, an increase in the carrier concentration can result in a variation in the membrane morphology. Consequently, the nature of the diffusion process may be influenced. In fact, the carrier within the PIM is not covalently bound to the base polymer, and therefore the actual diffusion mechanism of PIM is thought to be intermediate between mobile carrier diffusion and fixed-site jumping.
Current research status.
6.1 Flat-sheet PIM
So far, flat-sheet PIM has been the most common configuration in the PIM research. The method for membrane preparation is very easy. It can be prepared by casting a solution of base polymer in an organic solvent containing carrier and plasticizer. After evaporation of organic solvent, a flat-sheet PIM is obtained. If the carrier itself can act as a plasticizer, the plasticizer is not needed. Lijuan Wang et al prepared an Aliquat 336/PVC membrane to extract cadmium and copper from hydrochloric acid solutions. The membranes were prepared by dissolving a mixture of Aliquat 336 and PVC in THF. Thereafter THF was allowed to evaporate slowly over 12h. As a result, a colorless, flexible, transparent and mechanically strong membrane was yielded. Aliquat 336 is reported to act as a plasticizer for PVC, thus no plasticizer is needed in the membrane preparation process. The carrier-mediated transport of cerium ions using PIMs was investigated experimentally by Samuel P. Kusumocahyo et al. In this study, the PIMs preparation method is the same but a plasticizer is needed here. The PIM consists of CTA as a polymer matrix, 2-NPOE as a plasticizer and octyl (phenyl)-N, N-diisobutylcarbamoylmethylphosphine oxide (CMPO) or N, N, N', N'-tetraoctyl-3-oxapentanediamide (TODGA) as carrier.
6.2 Hollow fiber PIM.
Although the preparation of flat-sheet PIMs is pretty easy, flat-sheet PIMs are unfavorable for the industrial applications due to the relatively thick membrane which contributes to the resistance for transport. Therefore the production rate is relatively low for scale up and industrial application. Compared to flat-sheet configuration, hollow-fiber configuration possesses the advantages like high packing density and low investment. It is the configuration with the highest packing density and can attain values of 30,000 m2/m3. Furthermore, the transport resistance in hollow-fiber PIMs is less than in flat-sheet PIMs. Normally in flat-sheet PIMs, the thicknesses of dense homogeneous polymer films are 20-200Î¼m, which leads to very low permeation rates. Such membranes cannot be made further thin to improve permeation because of no mechanical strength. However, in hollow-fiber PIMs, the thickness of the selective layer is thin enough (0.1-1Î¼m). They are asymmetric structure, where a very thin selective layer is supported by a porous sublayer. Thus, a PIM in hollow-fiber configuration is desired.
Nonetheless, so far, few works has been done to develop hollow fiber PIM. This might be partly due to the difficulty of the membrane preparation. Recently, modification of preparation method for polymer inclusion membrane (PIM) to produce hollow fiber PIM was studied by Samuel P. Kusumocahyo. Through a certain post-treatment, a CTA hollow fiber membrane was successfully converted into a hollow fiber PIM. The detail of the preparation process is described as followed: (1) the hollow fiber CTA membranes were cut off, and the fibers were immersed in a solution containing chloroform, NPOE, and TODGA for a while; (2) the fibers were taken out from the solution; (3) the outer surfaces were wiped using tissue paper, and then the fibers were put in a fume hood to dry at a room temperature for 1 day to evaporate chloroform. A hollow fiber PIM prepared by this post-treatment method was proved to be effective to transport cerium ions from the feed phase to the strip phase.
Besides post-treatment method, other methods to prepare hollow fiber PIMs should also be considered. Dip-coating technique is a promising alternative, which is a very simple and useful technique to prepare a composite membrane with a very thin but dense toplayer. In this case, a hollow-fiber membrane is first immersed in the coating solution containing coating material and solvent. Then the membrane is removed from the solution and a thin layer of solution adheres to it. Finally, the solvent is allowed to evaporate, and crosslinking occurs which results in the thin layer becoming fixed to the porous sublayer. However, to date, no one has reported to prepare hollow-fiber PIMs by using the dip-coating technique.
Objectives of this project.
As mentioned above, a PIM in hollow-fiber configuration is desired for industrial application. Thus, the purposes of this project are:
(1) to prepare the polymer inclusion membranes (PIMs) through dip coating on microporous hollow fibers;
(2) to evaluate the separation performance of this hollow fiber PIM;
(3) to optimize the experiment conditions to obtain the hollow fiber PIMs with the best separation performance.