In recent old ages, membrane-based procedures have attracted considerable attendings as a valuable engineering. Separation membranes have been applied widely in industrial, biomedical and analytical Fieldss every bit good as in effluent intervention. Nowadays, they constitute basic stuffs which stimulate technological developments and scientific research, and continual betterments are being made.
In general, a membrane is a thin barrier or movie between two stages with discriminatory conveyance of some species over others. Compared to other traditional engineerings, membrane owns the features like easiness 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 heavy membranes applied in gas separation and pervaporation. The porous membranes contain fixed pores, whose way is normally Byzantine and size distribution is seldom monodisperse. The selectivity is chiefly determined by the dimensions of the pores and the dissolver passes through the membrane chiefly by convection or majority fluid flow. Dense membranes have n’t got distinguishable pores. In this type of membrane, the permeableness and selectivity of membranes are depended on the intrinsic belongingss of the stuff. When placed between two aqueous stages, chemical species can travel through the membrane by agencies of a solution-diffusion procedure from high solute concentration into a part of low solute concentration. However, the species can besides be transported across membrane against their ain concentration gradient as a effect of an bing concentration gradient of a 2nd species present in the system ( coupled conveyance ) or in the presence of an extractant or bearer contained within the membrane ( facilitated conveyance ) .
Although all other membrane sectors have encounter a market roar late, practical applications of liquid membranes remain mostly limited. Depending whether a polymeric support is involved, liquid membranes may be divided into two classs: 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 countries and mass transportation rates are the chief disadvantages of BLMs while the emusion breakage is the chief job associated with ELMs. SLMs have hapless stableness job. These factors make liquid membranes impractical for many large-scale applications.
However, over the last two decennaries, the demand for metal ion recovery every bit good as for the extraction of legion little organic compounds in hydrometallurgy, biotechnology and in the intervention of industrial effluent has become stronger and stronger. Significant scientific attempt has been expended to understand and better the stableness of liquid membranes. The Numberss of scientific probes devoted to this subject has been lifting steadily and as a consequence, a fresh type of liquid membranes has been discovered. They are normally 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 besides being used. PIMs are prepared through projecting a solution incorporating a plasticiser, an extractant and a base polymer to organize a thin, flexible and stable movie. They can be used to accomplish the separation of certain solute similar to SLMs.
PIMs exhibit first-class stableness and versatility while retaining most of the advantages of SLMs. Compared to SLMs, the bearer loss during the membrane pull outing procedure can be ignored, the sum of bearer reagent can be greatly reduced and the mechanical belongingss are rather similar to those of filtration membranes. All of these will no uncertainty widen the application of PIMs and enable PIM-based systems to exhibit many advantages such as easiness of operation, lower limit usage of risky chemicals and flexibleness in membrane composing to accomplish the coveted selectivity every bit good as separation efficiency.
Base polymers for membrane readying.
PIMs usually consist of a base polymer, a plasticiser, and a bearer molecule. Base polymer is used to supply mechanical strength for the membrane. To day of the month, despite the fact that assorted polymers are presently used for many technology intents, PVC and CTA have been the merely two major polymers used for most of the PIM probes conducted. This is because both PVC and CTA are easy feasible and readily available. They can be used to fix a thin movie through a comparatively simple process based on disintegration in an organic dissolver. Another factor is the deficiency of information sing the function of base polymers in automatically back uping the membranes. It ‘s indispensable that base polymer demands to heighten the membrane stableness and at the same clip creates a minimum hinderance to the conveyance of metal ions and little organic compounds within the membranes.
Recently, the feasibleness of several cellulose derived functions has been studied. They include cellulose acetate propionate ( CAP ) , cellulose ethanoate butyrate ( CAB ) and cellulose tributyrate ( CTB ) . Several consequences were found: Membrane lastingness increased with replacing of acetyl permutation on the cellulose polymer with propionyl or butyryl ; as longer alkyl ironss were added to the cellulose glucoside units, membrane opposition to hydrolysis increased ; as the alkyl concatenation lengths increased, ion conveyance across the membrane decreased. These fresh types of membranes enable PIMs to treat rough waste solutions without the ester linkages in the cellulose anchor of the polymer.
PVC and CTA are thermoplastic. They are made up of additive polymer strands and there are no cross-links between these strands. Therefore, they can be dissolved in a suited organic dissolver, where the polymer strands become detached. Intermolecular forces combines with the procedure of web to give the mechanical strength of a thermoplastic thin movie membrane. The intermolecular forces determine the flexibleness of the stuff with high intermolecular forces ensuing in a stiff membrane, while the procedure of web is the consequence of random diffusion of the flexible polymer strands in a colloidal suspension as the dissolver evaporates. As a consequence, a really stable thin movie can be formed without any intermolecular covalent bonds, even though there is a disentanglement procedure happening over a really long clip graduated table. However, the molecular weight of the polymer used should be larger than the critical web molecular weight of that polymer. Above the critical web molecular weight values, fluctuations in the base polymer molecular weight exert small influence on the behaviour of the separation agent within the membrane.
CTA, which is a polar polymer, is frequently extremely crystalline. It has a figure of hydroxyl and ethanoyl group groups which are capable of organizing extremely orientated H bonding. Its crystalline sphere gives CTA the first-class mechanical strength and the features of cellulose enable it infusible. However, all of the cellulose derived functions possess high H2O soaking up. The extent of H2O surface assimilation decreases with increasing grade of permutation. The effects of H2O surface assimilation are diminishing the softening point, tensile strength and modulus, increasing elongation at interruption and impact strength, and the possibility of dimensional alterations in moulded articles. Therefore CTA can be somewhat hydrated which makes it prone to hydrolysis, peculiarly in an acidic environment. PVC is an amorphorous polymer. In PVC, the C-Cl functional group is comparatively polar and non-specific scattering forces dominate the intermolecular interactions. PVC can non be hydrated.
Besides supplying mechanical support to the membrane, base polymers besides exert influences on metal ion conveyance through their majority belongingss. Presents, the thaw temperature and the glass passage temperature are used to depict polymer ‘s microstructural features and the built-in flexibleness. It is found that, below the glass passage temperature, the polymer becomes glassy and stiff. This is unfavourable for metal ion conveyance in membranes. Therefore, it ‘s indispensable to take down the glass passage temperature. Plasticizers are added to accomplish this and a more flexible and less brickle membrane is formed. It ‘s worthy to observe that the glass passage temperature or the liquescent temperature of a pure polymer without a plasticiser is normally much higher than room temperature. This is the ground why plasticisers are needed during PIMs readying unless the bearer itself can besides move as a plasticiser.
Carrier is used to carry through the conveyance across liquid membranes. It is basically a complexing agent or an ion money changer. It reacts with metal ion to organize a complex or ion-pair and facilitates metal ion conveyance across the membrane. The well-known bearers include macrocyclic and macromolecular compounds, and liquid-liquid extraction extractants. Examples of PIMs bearers reported in the literature and their typical mark solutes are shown in Table 1.
Table 1. Examples of PIM bearers reported in the literature and their typical mark solutes.
Macrocyclic and macromolecular compounds.
Crown quintessences are the first group of macrocyclic compounds. They contain sulfur, O and N as giver atoms. The figure of ether giver atoms and the size and form of pit relation to the cation size find the stableness of the crown ether-metal ion composite. There exist three types of crown quintessences: sulfur-containing Crown quintessences, nitrogen-containing Crown quintessences and oxygen-containing Crown quintessences. The polarizability of them is different and thereby they display different ionic selectivities. Through the debut of one or more side weaponries to monocyclic Crown, lariat quintessences could be formed and metal ion binding strength and selectivity would be enhanced. The lariat quintessences possess the possibility of 3-dimensional cation encapsulation and acceptable complexation-decomplexation kineticss. To accomplish a successful separation procedure, a counter anion which is soluble in both the organic and aqueous stages is besides needed. However, the composites formed with common anions have low distribution coefficients between an aqueous stage and an organic stage. Attaching a proton-ionizable sidearm to the Crown ether pealing can work out this job and moreover, it couples metal ions conveyance from the aqueous beginning stage into the aqueous having stage with back-transport of proton cation. As a consequence, a pH gradient offers the potency for metal ions conveyance.
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 respond with a broad assortment of organic and inorganic substances to organize composites in their hydrophobic pit. They would go less soluble and more stable when they are polymerized. Hence, their derived functions can be used to insulate or take many organic and inorganic substances. Similarly, the derived functions of other macrocyclic compounds can be besides used as ion bearers to divide metal ions from aqueous solutions.
Liquid-liquid extraction extractants.
Liquid-liquid extraction extractants can be divided into four categories: 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, Personal computer 88A ) , aminoalkanes and quaternate ammonium salts ( TOA, Aliquat 336 ) , phosphorous acids esters ( TBP ) , 8-hydroxyqinoline ( Kelex 100, LIX 26 ) , and hydroxyoximes ( LIX 64N, LIX 70, LIX 84, LIX 984 ) .
The influences of bearers on membrane public presentation.
It is known that the chemical reactions that are involved in the extraction and denudation of mark solutes are the same in both PIMs system and the corresponding solvent extraction systems. The lone difference between these two systems is the conveyance of the mark solutes through the membrane, which can be strongly influenced by the bearer molecular construction.
The bearer molecular construction can markedly act upon the conveyance efficiency of membrane. In a survey, PIMs with lariat quintessences carboxylic acids bearers and sym- ( alkyl ) dibenzo-16-crown-5-oxyacetic acids, is used to divide Na cations from equimolar mixtures of alkali metal cations in aqueous solutions. It was found that, the length of alkyl concatenation which is attached to the functional side arm in the lasso ether bearer strongly influenced the entire base metal cation flux. When the alkyl group contained nine C atoms, maximum flux could be obtained. In another survey, it was demonstrated that both membrane selectivity and conveyance efficiency could be improved through a careful combination of pealing size and substituent groups of macrocyclic bearer.
The molecular construction of the bearer can besides act upon the membrane selectivity. Through orienting the bearer molecular constructions, a specific selectivity can be obtained. For illustration, it was found that the less basic bearer gives higher initial fluxes of all metal ions, but the more basic bearer gives higher Cr/Cd and Cr/Zn selectivity coefficients. Diazadibenzocrown quintessences with more hydrophilicity was demonstrated to possess a higher selectivity for Pb over Zn and Cd.
A plasticiser is a stuff which is added to the plastic to increase its flexibleness or distensibility and workability, and lower the temperature of the second-order passage, the thaw viscousness, or the elastic modulus of the plastic. The plasticization procedure is normally composed of 6 stairss: ( 1 ) wetting, surface assimilation ; ( 2 ) redemption and/or incursion of the surface ; ( 3 ) soaking up, diffusion, with initial ( limited ) puffiness ; ( 4 ) disassociation and liberation of polar groups ; disintegration in the formless part ; ( 5 ) Structure dislocation, diffusion and disintegration of some of the crystalline parts ; ( 6 ) reestablishment of construction.
In PIMs, assorted types of attractive forces hold the single molecular ironss together, including new wave der Waals forces and polar interactions. Van der Waals forces are abundant but are nonspecific and weak. Polar interactions are strong but can merely happen at polar centres of the molecule, and frequently organize a stiff non-flexible thin movie with a tree-dimensional construction within its polymeric matrix. This 3-dimensional construction rigidness decreases the diffusing flux of stuff within the polymer matrix. The add-on of plasticisers can work out this job. By perforating between polymer molecules and “ neutralizing ” the polar groups of the polymer or increasing the distance between the polymer molecules, plasticisers cut down the strength of the intermolecular forces.
Presents, a big figure of plasticisers are commercially available, but few of them have been used in PIMs readying. Amongst them, the most frequent used plasticisers in PIMs surveies are 2-nitrophenyl octyl quintessence ( 2-NPOE ) and 2-nitrophenyl pentyl quintessence ( 2-NPPE ) . Figure 1 shows chemical constructions of several plasticisers normally used in PIMs.
Figure 1. Chemical constructions of plasticisers normally used in PIMs.
It can be seen from Figure 1 that plasticisers are by and large organic compounds. They contain a hydrophobic alkyl anchor with one or several extremely solvating polar groups. The hydrophobic alkyl anchor governs the compatibility of the plasticiser with the membrane, while the solvating polar groups interact with the polar groups of the base polymer to “ neutralize ” them. Hence, it ‘s of import to accomplish a proper balance between the polar and non-polar parts of the plasticiser molecule. This balance was foremost studied by Sugiura. In this survey, membranes made with polyoxyethylene alkyl quintessences of different alkyl concatenation length and different Numberss of polar oxyethylene groups are used to divide lanthanide ions. It was found that the best combination is alkyl concatenation of 12 C atoms with 2 or 3 polar groups. Any farther addition in the length of the alkyl concatenation or the figure of polar groups would impact the plasticiser public presentation. The former 1 increases the hydrophobicity and viscousness of the plasticiser, which suppresses the polar belongingss of the plasticiser. The latter one consequences in a less syrupy and more hydrophilic plasticiser that finally renders it unserviceable.
Facilitated conveyance is a particular procedure carried out in a membrane. It is different from most of the other membrane procedures which are alternate signifiers of filtration or depend on diffusion and solubility in thin polymer movies. It involves specific chemical reactions like those in extraction. Facilitated conveyance has got four features: ( 1 ) It is extremely selective ; ( 2 ) It is easy poisoned ; ( 3 ) A maximal flux can be acquired at high concentration differences ; ( 4 ) It can concentrate and divide the mark solute. These features make it different from other membrane separations, and point 3 and 4 are the most powerful grounds that facilitated conveyance is happening.
Both SLMs and PIMs are based on facilitated conveyance. They use membrane to selective conveyance a mark solute from one aqueous solution to another. This overall procedure consists of two procedures. One is the diffusion of mark solute across the membrane, and the other one is the transportation of the mark solute across the two interfaces. First, the bearer molecule in the membrane picks up metal ion/species from the provender solution to organize a composite. 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 bearer diffuses back across the membrane for usage in another rhythm. The solute conveyance through the two interfaces is similar in both SLMs and PIMs, but the existent majority diffusion within the membrane stage can be rather different. That ‘s because SLMs are wholly different from PIMs in their composing and morphology.
5.1 Interfacial conveyance mechanisms.
The interfacial conveyance mechanisms have been studied by several writers. It has been found that the diffusion procedure through this aqueous stagnant bed is comparatively fast and can be ignored when suited hydrodynamic conditions are maintained. Figure 2 describes the conjugate conveyance of a positively charged ( M+ ) or negatively charged ( M- ) species through a PIM. It can be seen that coupled conveyance can be divided into two types: one is co-transport and the other one is counter-transport.
Figure 2. Conventional description of conjugate conveyance of a positively charged ( M+ ) or negatively charged ( M- ) species through a PIM. C represents the bearer and Ten is an aqueous soluble coupled-transport ion. [ M+ ] , [ M- ] , [ X- ] and [ X+ ] represent the entire analytical concentrations of the several solute in the majority aqueous stages. ( a ) The mark solute is a cation and is at the same time transported with a coupled-transport anion ; ( B ) the mark solute is a cation and is counter-currently transported with a coupled-transport cation ; ( degree Celsius ) the mark solute is an anion and is counter-currently transported with a coupled-transport anion ; ( vitamin D ) the mark solute is an anion and is at the same time transported with a twosome conveyance cation.
In co-transport, both metal ions and counter ions are transported from the provender solution through SLM and into the strip solution. First, the metal ion and counter ion react with the bearer C in the membrane to organize a composite. 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 procedure are shown below.
Mn+ + nX- + C ( membrane ) a†’ CMXn ( membrane )
CMXn ( membrane ) a†’ C ( membrane ) + Mn+ + nX-
In counter-transport, an acidic bearer, HC, loses a proton and reacts with the metal ion to organize a complex MC at the provender 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 at the same time. Finally, the regenerated bearer HC diffuses back to the provender solution-membrane interface and repeats the whole procedure. The chemical reactions involved in this conjugate conveyance are shown below.
Mn+ + nHC ( membrane ) a†’ MCn ( membrane ) + nH+
MCn ( membrane ) + nH+ a†’ nHC ( membrane ) + Mn+
In order to accomplish the extraction procedure, the distribution ratio of the mark solute/carrier composite between the organic stage of the membrane and the aqueous solution must be every bit high as possible. In order to accomplish the back extraction of the mark solute from the membrane stage, the distribution ratio of the mark solute/carrier composite at the having site must be every bit low as possible. As a consequence, a concentration gradient of the mark solute/carrier complex or ion-pair exists within the membrane stage to move as a drive force for its conveyance across the membrane. That is to state, acclivitous conveyance is really declivitous conveyance sing the existent chemical species spreading across the membrane, despite the fact that the entire analytical concentration of the mark solute in the beginning solution might be well lower than in the receiving solution. The entire analytical concentration is the amount of the concentrations of all chemical species incorporating this metal ion.
The possible gradient of a coupled-transport ion across the membrane is another driving force for the acclivitous conveyance phenomenon. In a typical PIM procedure, the mark solute is transported together with this ion to keep electroneutrality. The facilitated conveyance 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 bearer, and tris- ( 2-butoxyethyl ) phosphate as plasticiser ( TBEP ) . It was found that the possible gradient of protons can be seen as the drive force for the acclivitous conveyance of a metal cation across the membrane, and it could be maintained by seting the solution pH. It should be noted that the solution pH besides influences the distribution ratio of the mark solute between the aqueous solution and the membrane stage.
In fact, the two drive forces described above can non be distinguished. Both of them belong to a complex interfacial conveyance mechanism. One focuses on the distribution ratio difference, while the other one focuses on the possible gradient of the coupled-transport ion across the membrane.
5.2 Bulk conveyance mechanisms.
In add-on to transport across the two solution/membrane interfaces, the diffusion of the carrier/target composite through the majority membrane is besides involved in the facilitated conveyance across a membrane. In a majority liquid membrane, the bearer plays the function of bird, traveling freely within the membrane. However, when the bearer is immobilized, the majority diffusion of the mark solute is assumed to take topographic point through consecutive resettlements from one reactive site to another. The membrane in PIM is basically a quasi-solid homogenous thin movie non a true liquid stage. The bearers in it are non immobilized but frequently bulky and the mobility of the bearer is more restricted compared to SLMs. Hence, the mechanism of the majority diffusion processes in PIMs is thought to be different from other liquid membranes.
“ Chained bearer ” theory is used to depict the facilitated conveyance procedure in a solid membrane where the bearer is covalently bound to the polymeric anchor construction. In this theory, both chained bearers and solute are assumed indissoluble in the membrane, and the bearer reacts rapidly and selectively with solutes at the membrane interfaces to organize a composite within the membrane. However, unlike nomadic bearers, the composite can non spread across the membrane because of the chemical binding. Some intramolecular diffusion makes the side concatenation which contains this complex gigue around its equilibrium place. In this jigging, the composite may meet a 2nd, uncomplexed bearer where the solute leaves the first bearer and reacts with the 2nd one to organize another composite. The diffusion of the mark solute through the majority membrane can be achieved by reiterating the above procedures. Figure 3 below shows the differences between nomadic bearers and chained bearers.
Figure 3. Mobile bearer vs. chained bearers.
In this theory, a chained membrane is described to be a lamellar construction. The bearer is located in beds and the bearer in each bed can travel a distance lo around its equilibrium place. The thickness of each bed is l. No bearer can for good travel from one bed into another and no uncomplexed solute can be in the membrane. When cubic decimeter & gt ; lo, solute can non be passed from one bearer to the following. Therefore, there is no solute flux across the membrane. When cubic decimeter & lt ; lo, a complexed bearer can acquire really near to a 2nd uncomplexed bearer molecule through spreading around its equilibrium place. Then it can continue to the reaction which produces a solute flux.
This theory implies that chained bearer membranes are similar to mobile bearer membranes. At low solute concentration, solute fluxes vary linearly with solute concentration. At high solute concentration, solute fluxes would near an asymptote. Fluxes addition with bearer concentration, and these fluxes will frequently be selective. However, some differences besides exist between chained bearer membranes and nomadic bearer membranes. First, a infiltration threshold is shown in chained bearer membranes. Second, some mobility must be possessed by the chained bearers themselves. Third, the evident diffusion coefficient in chained bearer membranes may reflect chemical dynamicss non diffusion. This theory is non perfect for depicting PIMs. It has got a restriction which is the premise that the bearer sites must be within range of one another and free uncomplexed solute can non come in the membrane so that the transportation of the mark solute can take topographic point.
Fixed-site jumping theoretical account is an drawn-out majority diffusion theoretical account. It is basically an betterment of chained bearer theory. In this theory, the conveyance molecules act as “ stepping rocks ” and the solutes jump from one fix-site to another to travel across the membrane. Similar to chained bearer theory, a infiltration threshold exists. When bearer concentration is below the threshold concentration, the distance between fixed-sites becomes excessively great to let solute jumping and there is no flux. When the bearer concentration exceeds the threshold concentration, flux may increase linearly with it or a higher power which is determined by the experimental conditions. In a survey of facilitated carbohydrate conveyance through plasticized cellulose triacetate membranes, a mobile-site jumping mechanism was proposed. The saccharide diffusion invariables was observed to diminish with increasing size of the carbohydrate, bearer anion and bearer cation. This suggests that ion-pair bearer does non stay as a “ fixed site ” within the membrane. The most consistent account is that the saccharide-ion-pair composite is locally nomadic. When the complex moves near plenty to an unoccupied bearer ion-pair, the carbohydrate leap from ion-pair to ion-pair, and/or the saccharide-anion complex leaps from cation to cation. Thereafter, the conveyance procedure through the membrane returns. Figure 4 below depicts the proposed mobile-site jumping mechanism for saccharide conveyance mediated by ion-pair bearers.
Figure 4. Proposed mobile-sited jumping mechanism for saccharide conveyance mediated by ion-pair bearers.
To day of the month, the fixed-site jumping mechanism was based on the infiltration threshold. However, an addition in the bearer concentration can ensue in a fluctuation in the membrane morphology. Consequently, the nature of the diffusion procedure may be influenced. In fact, the bearer within the PIM is non covalently bound to the base polymer, and hence the existent diffusion mechanism of PIM is thought to be intermediate between nomadic bearer diffusion and fixed-site jumping.
Current research position.
6.1 Flat-sheet PIM
So far, flat-sheet PIM has been the most common constellation in the PIM research. The method for membrane readying is really easy. It can be prepared by projecting a solution of base polymer in an organic dissolver incorporating bearer and plasticiser. After vaporization of organic dissolver, a flat-sheet PIM is obtained. If the bearer itself can move as a plasticiser, the plasticiser is non needed. Lijuan Wang et Al prepared an Aliquat 336/PVC membrane to pull out Cd and Cu from hydrochloric acerb solutions. The membranes were prepared by fade outing a mixture of Aliquat 336 and PVC in THF. Thereafter THF was allowed to vaporize easy over 12h. As a consequence, a colorless, flexible, crystalline and automatically strong membrane was yielded. Aliquat 336 is reported to move as a plasticiser for PVC, therefore no plasticiser is needed in the membrane readying procedure. The carrier-mediated conveyance of Ce ions utilizing PIMs was investigated by experimentation by Samuel P. Kusumocahyo et Al. In this survey, the PIMs readying method is the same but a plasticiser is needed here. The PIM consists of CTA as a polymer matrix, 2-NPOE as a plasticiser and octyl ( phenyl ) -N, N-diisobutylcarbamoylmethylphosphine oxide ( CMPO ) or N, N, N ‘ , N’-tetraoctyl-3-oxapentanediamide ( TODGA ) as bearer.
6.2 Hollow fibre PIM.
Although the readying of flat-sheet PIMs is pretty easy, flat-sheet PIMs are unfavourable for the industrial applications due to the comparatively thick membrane which contributes to the opposition for conveyance. Therefore the production rate is comparatively low for graduated table up and industrial application. Compared to flat-sheet constellation, hollow-fiber constellation possesses the advantages like high packing denseness and low investing. It is the constellation with the highest wadding denseness and can achieve values of 30,000 m2/m3. Furthermore, the conveyance opposition in hollow-fiber PIMs is less than in flat-sheet PIMs. Normally in flat-sheet PIMs, the thicknesses of dense homogenous polymer movies are 20-200I?m, which leads to really low pervasion rates. Such membranes can non be made farther thin to better pervasion because of no mechanical strength. However, in hollow-fiber PIMs, the thickness of the selective bed is thin plenty ( 0.1-1I?m ) . They are asymmetric construction, where a really thin selective bed is supported by a porous sublayer. Therefore, a PIM in hollow-fiber constellation is desired.
However, so far, few plants has been done to develop hollow fibre PIM. This might be partially due to the trouble of the membrane readying. Recently, alteration of readying method for polymer inclusion membrane ( PIM ) to bring forth hollow fibre PIM was studied by Samuel P. Kusumocahyo. Through a certain post-treatment, a CTA hollow fibre membrane was successfully converted into a hollow fibre PIM. The item of the readying procedure is described as followed: ( 1 ) the hollow fibre CTA membranes were cut off, and the fibres were immersed in a solution incorporating trichloromethane, NPOE, and TODGA for a piece ; ( 2 ) the fibres were taken out from the solution ; ( 3 ) the outer surfaces were wiped utilizing tissue paper, and so the fibres were put in a fume goon to dry at a room temperature for 1 twenty-four hours to vaporize trichloromethane. A hollow fibre PIM prepared by this post-treatment method was proved to be effectual to transport Ce ions from the provender stage to the strip stage.
Besides post-treatment method, other methods to fix hollow fibre PIMs should besides be considered. Dip-coating technique is a promising option, which is a really simple and utile technique to fix a composite membrane with a really thin but heavy toplayer. In this instance, a hollow-fiber membrane is foremost immersed in the coating solution incorporating surfacing stuff and dissolver. Then the membrane is removed from the solution and a thin bed of solution adheres to it. Finally, the dissolver is allowed to vaporize, and crosslinking occurs which consequences in the thin bed going fixed to the porous sublayer. However, to day of the month, no 1 has reported to fix hollow-fiber PIMs by utilizing the dip-coating technique.
Aims of this undertaking.
As mentioned above, a PIM in hollow-fiber constellation is desired for industrial application. Therefore, the intents of this undertaking are:
( 1 ) to fix the polymer inclusion membranes ( PIMs ) through dip coating on microporous hollow fibres ;
( 2 ) to measure the separation public presentation of this hollow fibre PIM ;
( 3 ) to optimise the experiment conditions to obtain the hollow fibre PIMs with the best separation public presentation.