KR20140070540A - Process for fabricating pbi hollow fiber asymmetric membranes for gas separation and liquid separation - Google Patents

Abstract

The present invention provides a method for producing asymmetric hollow fibers, asymmetric hollow fibers produced by the method, and asymmetric hollow fibers. One way is to pass the polymeric solution through the outer annular orifice of the orifice-inner-tube emitter, passing the bore fluid through the inner tube of the emitter, dropping the polymeric solution and the bore fluid through the atmosphere over the dropping distance , And contacting the polymeric solution and the bore fluid in the bath to form an asymmetric hollow fiber.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a PBI hollow yarn asymmetric membrane for gas separation and liquid separation,

Assignee: SAL A International

Inventor: Indiraja Yawera, Gopalanien. Chrysinan, Angel Sanfuhr, Palitaja Yawera and Srinivas Ba'midi

Cross reference of related application

This application claims priority to U.S. Serial No. 61 / 531,448, filed September 6, 2011, the disclosure of which is incorporated herein by reference in its entirety.

Government Support

The invention was made with the support of the United States government under N00014-10-C-0059 granted by the US Navy Research Service and DE-FC26-07NT43090 granted by the US Department of Energy. The US Government has some rights to the invention.

Introduction

Membrane processes are used for separation and purification in many industries, from water treatment to gas separation. These processes usually use polymeric membranes in the form of flat sheets or hollow fibers. Hollow fiber membranes are more widely used than flat sheets due to the ratio of surface area to high volume.

Related arts fields include US 7771518, US 5683584, and US 2011/0266223.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a process for the preparation of (a) an outer annular orifice of an orifice-tube-inorifice spinneret comprising (i) 15-25 wt% of polybenzimidazole; (ii) 1-5 wt% polymeric pore-forming material; And (iii) passing a polymeric solution comprising a solvent associated with polybenzimidazole; (b) in the inner tube of the emitter, (i) a non-solvent 65-99 wt. %; And (ii) solvents 1-35 wt.% Associated with polybenzimidazole. %, Wherein the bore fluid retains the annular polymeric solution; (c) dropping a polymeric solution and a bore fluid into a gap containing a dropping distance of 0.3 to 20 cm and an atmosphere; (d) concentrating the polymeric solution in the bath and the bore fluid to form a first and a second layer of annular and concentric circles, wherein the first layer is nonporous in contact with the second layer, To form a porous asymmetric hollow fiber having a void in the range of 5-250 nm.

In an embodiment:

The method further comprises transporting the fibers into a fiber bundle at a rate of 1-100 meters per minute. The fiber bundle can be used as a hollow fiber membrane in suitable membrane applications as described herein.

Polymeric solutions are stable to chemical degradation for a period of six months or longer at temperatures in the range of 15-25 ° C.

The method further comprises washing and stretching after spinning of the fibers. The post spinning process, for example, increases the mechanical strength of the fibers.

Polybenzimidazole is a sulfonated polybenzimidazole.

The first layer forms the outer surface of the asymmetric hollow fiber, and the second layer forms the inner surface.

The first layer forms the inner surface of the asymmetric hollow fiber and the second layer forms the outer surface.

The thickness of the first and second layers is controlled by the length of the dropping distance and the relative polarity of the solvent and non-solvent.

The first layer has a thickness in the range of 0.1-10 mu m and the second layer has a thickness in the range of 10-500 mu m.

The chemical composition of the first layer and the chemical composition of the second layer are the same.

The polymer precipitate is partially solidified during the loading and is completely solidified during the quench.

The outer annular orifice of the orifice-inner-tube emitter has an outer diameter in the range of 100-2000 μm.

The thickness of the first and second layers is controlled by the length of the dropping distance and the relative polarity of the solvent and non-solvent, wherein the polybenzimidazole is a sulfonated polybenzimidazole.

The first layer has a thickness in the range of 0.1-10 mu m and the second layer has a thickness in the range of 10-500 mu m and the polymer precipitate partially solidifies during the dropping and is completely solidified during the quenching.

In another aspect, there is provided an asymmetric hollow fiber comprising concentric first and second layers forming a wall of fiber, wherein: the asymmetric hollow fiber comprises a polybenzimidazole material; The first layer is non-porous and the second layer is porous having voids in the range of 5-250 nm; The asymmetric hollow fiber has an outer diameter in the range of 100-2000 mu m.

In an embodiment:

Polybenzimidazole is a sulfonated polybenzimidazole.

Asymmetric hollow fibers are stable against chemical decomposition below 400 ℃.

The first layer has a thickness in the range of 0.1-10 mu m and the second layer has a thickness in the range of 10-500 mu m.

The first layer forms the outer surface of the asymmetric hollow fiber, and the second layer forms the inner surface.

In another aspect, polybenzimidazole; And an asymmetric hollow fiber comprising a first layer of non-porous and a second layer of porous concentric first and second layers having pores in the range of 5-250 nm, wherein the fibers are 100- Lt; RTI ID = 0.0 > 2000 < / RTI >

 In an embodiment:

The membrane is used in a process for separating H ₂ from a gaseous mixture comprising H ₂ , CO ₂ , CO, and methane, including passing a gas mixture through the membrane.

The membrane is used in a method for removing impurities from an aqueous solution, including passing an aqueous solution through the membrane.

As each effortlessly described individual, the present invention specifically provides all combinations of the aspects described.

Specific Description of Specific Embodiments

In another aspect, the present invention provides a process for preparing an orifice-in-tube emissive layer comprising: (a) providing an outer annular orifice of an orifice-in-tube emissive zone with (i) 15-25 wt% polybenzimidazole; (ii) 1-5 wt% polymeric pore-forming material; And (iii) passing a polymeric solution comprising a solvent associated with polybenzimidazole; (b) In the inner tube of the emitter, (i) a non-solvent 65-99 wt. %; And (ii) solvents 1-35 wt.% Associated with polybenzimidazole. %, Wherein the bore fluid retains the annular polymeric solution; (c) dropping the polymeric solution and the bore fluid into the atmosphere over a dropping distance of 0.3 to 20 cm; (d) concentrating the polymeric solution and the bore fluid in the bath, comprising concentric first and second layers wherein the first layer is non-porous and the second layer is porous with pores in the range of 5-250 nm To form an asymmetric hollow fiber which is asymmetric.

The polymeric solution carries a polymeric material that forms an asymmetric hollow fiber and, in some embodiments, carries at least one additional component such as a void forming material, a salt, a pH modifier, a viscosity modifier, and at least one solvent.

Polymeric solutions include polybenzimidazole (PBI). In some embodiments, the PBI is sulfonated. Sulfonation can be carried out using any conventional method. For example, the sulfonated form of PBI can be readily prepared by treating with sulfuric acid to form a covalently bonded SO ^{3 -} with a proton that forms a stable bond with the nitrogen of the imidazole ring. Sulfonated PBI (SPBI) hollow fibers provide higher chlorine resistance, flow rate and salt removal rates. The PBI may be present in an amount effective to produce an asymmetric hollow fiber according to the method of the invention. In an embodiment, the PBI is present in an amount ranging from 10-30, or 15-25, or 15-20 wt%, or greater than 10,15,17,20, or 25 wt%, or 30, 25, 22, 20, Or less than 18 wt%. If the total weight percent is within a given range, there can be more than one type of PBI.

In an embodiment, the polymeric solution comprises a void-forming material. The void-forming material is a material that causes or assists in the formation of voids in the material of the present invention. For example, void-forming materials assist in the solvent exchange mechanism of void formation. Any suitable void-forming material may be used. An example of a void-forming material is a compound comprising a plurality of hydroxyl groups, such as glycols and polyols. Examples include isopropanol, ethylene glycol, propylene glycol, polyvinyl alcohol, saccharides and polysaccharides. Another example of a void-forming material is PVP. The void-forming material is present in the polymeric solution in an amount sufficient to result in the desired porosity within the resulting asymmetric hollow fiber. In an embodiment, the void-forming material may be present in an amount ranging from 1-5 wt%, or 1-3 wt%, or less than 5, 4, 3, or 2 wt%, or greater than 1, 2, Lt; / RTI > solution.

Polymeric solutions include solvents associated with PBI. Such a solvent can completely dissolve the PBI present in the solution under the conditions used in the method of the invention. Examples of suitable solvents are N, N-dimethylacetamide (DMAc), dimethylsulfoxide (DMSO), NN-dimethylformamide (DMF), N-methyl-2-pyrrolidine (NMP), pyridine and the like. A combination of solvents is also suitable.

The polymeric solution may further comprise one or more additives such as LiCl (e.g., a stabilizer of PBI).

The polymeric solution is stable for chemical degradation under ambient conditions, e.g., within a temperature range of 15-25 DEG C for more than 6 months. In some embodiments, the polymeric solution is stable for 9 or 12 months. Thus, the components of the polymeric solution (especially the PBI component) do not undergo significant degradation over the stability period. For example, if the solution is maintained at a temperature in the range of 15-25 DEG C, the decomposition of the PBI component in the polymeric solution is less than 10, 8, 5, 3, 2, or 1% over the stability period.

The method of the invention involves passing the polymeric solution through the outer annular orifice of the orifice-in-tube emissive zone. The passage may be carried out at an elevated pressure (i.e. the polymeric solution may enter through the orifice), or the solution may be allowed to drop in the orifice under the influence of gravity and at ambient pressure. Oriented under the outer annular orifice is a clearance that can be easily divided into an extension area beneath the outer annular orifice and an extension area below the extension area. Upon exiting from the outer orifice of the emissive ball, the polymeric solution (having an annular orifice annulus) first enters the extended region where the perimeter extends slightly. The polymeric solution travels through the extension region and then into the extension region where the circumference is reduced. In some embodiments, some solvent evaporates from the polymeric solution as the polymer solution passes through the gap. Evaporation increases the concentration of PBI in the polymeric solution and solidification of some PBI can occur within the gap. The polymeric solution travels through the extended region of the gap and enters the bath located below the gap. The bath acts to coagulate the PI in the polymeric solution so that the PBI solidifies completely in the bath. Furthermore, solvent exchange takes place in the bath (i.e., the solvent from the polymeric solution exchanges with the solvent from the bath). Solvent exchange causes the formation of voids within the solidified PBI.

The method of the invention further involves passing a bore fluid through the inner tube of the orifice-in-tube emitter. The inner tube is centrally located in the outer annular orifice (with respect to the center axis of the spinnerette). The bore fluid is used to keep the polymeric solution annular while passing the polymeric solution through the gap and into the bath. Thus, as soon as the polymeric solution exits the outer annular orifice, the bore fluid exits the inner tube.

The Bohr fluid comprises a solvent in relation to the PBI and a mixture of non-solvent in relation to the PBI. In an embodiment, the Bohr fluid has a cost of 65-99 wt%, or a cost of less than 65, 70, 75, 80, 85, or 90 wt%, or 99, 95, 90, 85, 80, 75, or 70 wt% Includes a hawk. In an embodiment, the bore fluid comprises 1-35 wt%, or less than 5,10,15,20,25, or 30 wt%, or less than 35,30,25,20,15,10, or 5 wt% poly Benzimidazole and the like.

The non-solvent associated with PBI is a solvent that does not perceptibly dissolve PBI under the temperature and pressure used in the method of the invention. For example, the non-solvent can dissolve less than 10, 5, 1, 0.5, or 0.1% by weight of the PBI in which a similar volume of solvent can dissolve. Examples of non-solvent for PBI include water and alcohols such as methanol, ethanol, i-propanol, n-propanol and the like.

By applying a liquid such as a bore fluid, phase transitions can be induced and the shape of the fibers near the inner surface can be controlled through phase reversal.

Bass is filled with non-payment related to PBI. The in-bath intolerance may be the same as or different from the intolerance present in the bore fluid. The precipitation of the PBI produced from the polymeric solution entering the bath is referred to herein as? The polymeric solution and the bore of the bore fluid have an annular shape and produce an asymmetric hollow fiber having concentric first and second layers as described herein. In some embodiments, the annulus of the hollow fibers is identical to the annular (i. E., Enclosing the bore fluid) of the polymeric solution passing through the gap. In some embodiments, the swelling or shrinkage of the other minor variations makes the annulus of the hollow fibers not identical to the annulus of the polymeric solution in the gap, but the hollow shape is nonetheless derived from the annular shape of the polymeric solution in the gap .

The gap includes the atmosphere. The atmosphere may be any desired composition of air, a single gas such as nitrogen or argon or a gas. The length of the gap between the spinnerette and the bath is referred to herein as the drop length. The drop length may be any length in the range of 0.3-20 cm, such as 0.3, 0.5, 1, 3, 5, 10 or 15 cm, or 20, 15, 10, 5, 3 or 1 cm. The relative lengths of the swelling and extension regions may vary depending on a variety of factors, such as solution parameters, the atmosphere in the gap, and the like.

The inventive method results in the formation of asymmetric hollow fibers comprising PBI material. The method may further include a post spinning process. For example, the spinning process includes washing and stretching of the fibers. Washing can be with a non-solvent or a mixture of non-solvent for PBI, such as water, alcohol, glycol or polyol solvent. Stretching may include any method for stretching the fibers, for example, stretching through a double roller, or stretching into a species using any suitable method. In some embodiments, this post spinning process increases the mechanical strength of the fibers. This increase in mechanical strength can be up to at least 100, 150, or 200% and can be referred to as another measure of tensile strength or fiber strength.

Asymmetric hollow fibers have a "donut" shape in cross-section. The fiber thus comprises an annular wall (in cross-section), wherein the wall comprises first and second concentric (and contacting) layers. The difference between the outer diameter of the ring and the inner diameter of the ring is twice the thickness of the fiber wall.

The hollow fibers of the present invention are asymmetric in that they include concentric first and second layers, wherein the first layer is in contact with the second layer and is non-porous and the second layer is porous. In some embodiments, the first layer forms the outer surface of the asymmetric hollow fiber and the second layer forms the inner surface. In another embodiment, the first layer forms the inner surface of the asymmetric hollow fiber and the second layer forms the outer surface. Due to the porosity of the second layer, the first layer is generally denser than the second layer. In an embodiment, the first layer is at least 1.1, 1.3, 1.5, 2, 3, 4, or 5 times tighter than the second layer.

The thickness of the first and second layers is controlled by the length of the dropping distance and the relative polarity of the solvent and non-solvent. In some embodiments, the first layer has a thickness in the range of 0.1 to 10 microns, such as 0.1, 0.5, 1, 2, 3, 5, or 8 microns or greater, or 10, 8, 5, 3, 2, 1, . In some embodiments, the second layer may have a thickness in the range of 10-500 microns, such as 10, 25, 50, 100, 150, 200, 250, or 300 microns or 500, 300, 250, 200, 150, Lt; RTI ID = 0.0 > 25 < / RTI > In some embodiments, the relatively less dense second layer has a thickness of at least 10, 20, 50, 100, or 500 times greater than the relatively denser first layer. The thickness of the various layers is measured as the cross-sectional area of the fibers.

The transition region between the first and second layers can be very pronounced, such as less than 0.5, 0.1, 0.05, or 0.01 times the thickness of the first layer. In the transition region, the fibrous material transitions from porous to non-porous (i.e., at relatively low densities to relatively high densities). In some embodiments, the transition region is thicker, and the porosity is gradually reduced over a region having a thickness of 0.5, 0.8, or more times the thickness of the first layer.

In an embodiment, the porous second layer has interconnected nanometer scale voids. For example, the pores have an average diameter in the range of 5-250 nm, or 5, 25, 50, 100, 150, or more than 200 nm, or 250, 200, 150, 100, 50, or 25 nm. The void may be spherical, partially spherical, or irregular in shape. The degree and size of the voids in the second layer are determined by the polarity of the partially used solvent and non-solvent (which affects the solvent exchange mechanism to form voids). Other factors include bath solvent temperature and pressure, and rate and extent of solvent evaporation within the gap.

The dimensions of the annular spinneret hole, the hollow fiber dimensions, the shear stress in the spinnerette, the dropping flow rate, the polymer to bore volume flow rate ratio, and the carrier initial velocity ratio (draw ratio) .

In an embodiment, the chemical composition of the first layer and the chemical composition of the second layer are the same. Thus, for example, the first and second layers are all made of the same PBI material selected from the materials described herein.

In an embodiment, the asymmetric hollow fibers are stable at < RTI ID = 0.0 > 400 C < / RTI > Thus, there is little or no decomposition of the fibrous material (i. E., Less than 10, 5, 3, or 1 wt%) below 400 캜.

The outer annular orifice of the orifice-inner-tube emitter has an outer diameter in the range of 100-2000 [mu] m. Thus, the final asymmetric hollow fibers may have an outer diameter within the range of 100-2000 m, such as 100, 200, 300, 400, 500, 1000, or 1500 m, or 2000, 1500, 1000, 500, 400, 300, Lt; / RTI > The inner diameter (i.e., the diameter of the cavity in the hollow fiber) will be measured by the outer diameter and the thickness of the first and second layers (thus, for example, may be within the range of 90-1990 [mu] m).

The invented asymmetric hollow fibers can be used to form a hollow fiber membrane (HFM). For example, the spinning process described herein can further comprise transporting the fibers at a rate of 1-100 meters per minute to form HFM.

The membrane may be used in a process for the separation of H ₂ from a gas mixture comprising H ₂ , CO ₂ , CO, and methane, the method comprising passing the gas mixture through a membrane.

The membrane can be used in a method for removing impurities from an aqueous solution, the method comprising passing an aqueous solution through the membrane.

The PBI membrane can be sulfonated, for example, after processing of the hollow fibers using a dip-and-dry process.

Unless specifically stated otherwise, the disclosure is not limited to a particular process, material, and so on, and is generally variable. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting.

As used in the specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the content clearly dictates otherwise. Thus, for example, what is referred to as "one solvent" includes not only a single solvent but also a combination or mixture of two or more different solvents.

The present invention includes all combinations of the specific and preferred embodiments presented. It is understood that the illustrations and embodiments described herein are for illustrative purposes only and that various modifications or variations thereof will be suggested to those skilled in the art and should be included within the scope of the present application and the appended claims. All patents, patent applications, and publications cited herein, including those cited therein, are hereby incorporated by reference in their entirety.

Example

The Dope solution was prepared as follows: 18 wt% PBI dope in DMAc and 2 wt% PVP (K16-18, Acros Organics, New Jersey) (8000 Daltons, high molecular weight Pore forming agent).

A Bohr fluid was prepared as follows: 75 to 90 wt% IPA and 5 to 25 wt% DMAc.

The coagulation bath was made to contain 100% IPA.

A strong non-solvent selected from water, isopropyl alcohol, methanol and combinations thereof was used as a bore fluid and solidification bath. The strong non-solvent has the ability to coagulate the polymeric solution at the exit of the spinnerette: therefore, a thin film layer will be formed between the outer polymeric solution, otherwise the fibers will break easily and the polymeric solution will be gravity It will go down under. The inner bore fluid is a mixture of non-solvent and solvent to avoid the formation of a film layer. The spinnerets are manufactured with an outer diameter of 1.2 mm and an inner diameter of 0.4. This dope solution contains 26 wt% PBI and 2 wt.% LiCl in N dimethylacetamide (DMAc). The following specific dope solutions, bore fluids and coagulation baths were used to produce asymmetric PBI hollow fiber membranes with a H ₂ permeation of 300 GPU and a selective gas separation layer without defects on the shell side.

A high magnification of the fiber with a cross-sectional area of 0.5 mm OD was taken. Similarly, a high magnification of the fiber with a cross-sectional area of 0.8 mm OD was taken. The image shows a dense outer layer of porous and non-porous in the medial layer.

The dense layer provides a separation between much permeable H ₂ and less permeable CO ₂ , while the porous layer provides mechanical strength with a low pressure drop for the passage of permeable gas. Testing of the fabric produced shows that H ₂ permeates through the membrane faster than CO ₂ . The permeation of H ₂ increases with increasing temperature, while the permeation of CO ₂ is relatively less sensitive to temperature.

The performance of the fabric produced was evaluated over a period of 50 days and the performance was expressed over 1000 hours. H ₂ / CO ₂ selectivity improved over time, from 35 to 50, exceeding 40 design goals. Term performance evaluation data. H ₂ performance values were maintained at about 80 GPU (Gas Permeation Unit) throughout the test period. At the end of the 1000 hour test period, H ₂ performance was measured at 130 GPU at 250 ° C.

H ₂ / CO ₂ was measured as a function of H ₂ permeation in GPU units at 150, 200, 225, and 250 ° C. Both H ₂ / CO ₂ selectivity and H ₂ permeation increased with increasing temperature at below 225 ° C. The H ₂ / CO ₂ ratio increased at 250 ° C, while H ₂ permeation decreased. This shows a slight increase in the thickness of the dense layer. As the selectivity decreases, the transmission increases. The dense layer thickness can be tested between 1 and 10 μm and as low as 0.1 μm.

The presence of macropores is highly dop-specific and strongly depends on the non-solvent and solvent exchange rate during solidification. H ₂ transmission measured for fibers containing macropores ranges from 100 to 200 GPU at room temperature and H ₂ / CO ₂ selectivity is only 5. The presence of macropores also reduces the mechanical strength of the fibers.

Claims (15)

(a) an outer annular orifice of an orifice-tube-in-orifice emissive zone comprising (i) 15-25 wt% polybenzimidazole; (ii) 1-5 wt% polymeric pore-forming material; And (iii) passing a polymeric solution comprising a solvent associated with polybenzimidazole;
(b) In the inner tube of the emitter, (i) non-solvent for polybenzimidazole 65-99 wt. %; And (ii) 1-35 wt.% Solvent for polybenzimidazole. %, Passing a bore fluid to keep the polymeric solution ring-shaped;
(c) dropping a polymeric solution and a bore fluid into a gap containing a dropping distance of 0.3 to 20 cm and an atmosphere;
(d) concentrating the polymeric solution in the bath and the bore fluid, comprising annular, first and second concentric layers, wherein the first layer is nonporous in contact with the second layer and the second layer comprises 5 Forming a porous asymmetric hollow fiber having pores in the range of -250 nm
≪ / RTI > The method according to claim 1,
(a) taking up the fibers into a fiber bundle at a rate of 1-100 meters per minute to form a hollow fiber membrane;
(b) the polymeric solution is stable to chemical degradation for a period of at least 6 months at a temperature in the range of 15-25 占 폚;
(c) spinning after spinning and stretching of the fibers;
(d) wherein the polybenzimidazole is a sulfonated polybenzimidazole; or
(e) any combination or subcombination of (a), (b), (c) and (d)
Gt; The method of claim 1, wherein the first layer forms the outer surface of the asymmetric hollow fiber and the second layer forms the inner surface. The method of claim 1 wherein the first layer forms the inner surface of the asymmetric hollow fiber and the second layer forms the outer surface. The method according to claim 1, wherein the first layer has a thickness in the range of 0.1-10 mu m and the second layer has a thickness in the range of 10-500 mu m. The method of claim 1, wherein the chemical composition of the first layer and the chemical composition of the second layer are the same. The process according to claim 1, wherein the outer annular orifice of the orifice-inner-tube emitter has an outer diameter in the range of 100-2000 μm. The method according to claim 1, wherein the thickness of the first and second layers is controlled by the length of the dropping distance and the relative polarity of the solvent and non-solvent, and the polybenzimidazole is a sulfonated polybenzimidazole. 3. The method of claim 1, wherein the first layer has a thickness in the range of 0.1-10 mu m, the second layer has a thickness in the range of 10-500 mu m, the polymer precipitate is partially solidified during the dropping and completely solidified ≪ / RTI > Wherein the asymmetric hollow fiber comprises a polybenzimidazole material;
The first layer is non-porous and the second layer is porous, having voids in the range of 5-250 nm,
Asymmetric hollow fibers have an outer diameter in the range of 100-2000 μm,
An asymmetric hollow fiber comprising concentric first and second layers forming a wall of fiber. 11. The method of claim 10,
(a) the polybenzimidazole is a sulfonated polybenzimidazole; or
(b) the asymmetric hollow fibers are stable to chemical decomposition below 400 占 폚;
(c) the first layer has a thickness in the range of 0.1-10 [mu] m and the second layer has a thickness in the range of 10-500 [
Asymmetric hollow fiber. 11. The asymmetric hollow fiber of claim 10 wherein the first layer forms the outer surface of the asymmetric hollow fiber and the second layer forms the inner surface. A membrane comprising the asymmetric hollow fiber of claim 10. A method of separating H ₂ from a gas mixture, comprising passing a gas mixture comprising H ₂ , CO ₂ , CO, and methane through the membrane of claim 13. A method for removing impurities from an aqueous solution, comprising passing an aqueous solution through the membrane of claim 13.