Anisotropic microporous supports impregnated with polymeric ion-exchange materials

Novel ion-exchange media are disclosed, the media comprising polymeric anisotropic microporous supports containing polymeric ion-exchange or ion-complexing materials. The supports are anisotropic, having small exterior pores and larger interior pores, and are preferably in the form of beads, fibers and sheets.

BACKGROUND OF THE INVENTION 
There is currently a large research and development effort aimed at 
producing polymeric ion-exchange and ion-complexing materials (commonly 
known as ion-exchange materials) for a wide variety of applications. For 
the most part, these materials are resins comprising a cross-linked 
polymer (e.g., polystyrene crosslinked with divinylbenzene) that is 
substituted with an ion-exchange group either before or after 
polymerization. There are two major obstacles to the production of many 
different types of these resins with highly varied ion-exchange 
characteristics. First, the number of ion-exchange substituents that can 
be easily attached to reactive sites on the polymeric backbones of resins 
is limited. Secondly, only a limited number of polymers are available that 
exhibit suitable physical characteristics such as insolubility in water, 
resistance to abrasion, and resistance to osmotic swelling, and that also 
have chemically active sites for adding ion-exchange groups. For example, 
polytetrafluoethylene and polypropylene have such favorable physical 
characteristics but are chemically unreactive toward addition of 
ion-exchange groups. 
There are many polymeric materials that exhibit desirable ion-exchange 
properties (e.g., high selectivity and high capacities to extract metal 
ions) but that are not structurally suited for use in ion-exchange 
processes. See, for example, J. Poly. Sci.: Polym. Let. Ed. 20(1982) 291 
and J. Chem Soc. Dalton (1981)1486. Thus, it has been found that certain 
water-soluble polymers such as poly(vinylbenzocrownether)s and 
poly(vinylbenzoglyme)s are highly selective toward one metal ion over 
another, J. Pure Appl. Chem. 54(1982)2129. Other copolymers such as 
bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic anhydride with divinylbenzene 
and acrylic acid or acrylonitrile and p-vinylbenzoylacetone and acrylamide 
or maleic acid, all of which show poor abrasion resistance, are useful for 
separating Ca.sup.+2, Co.sup.+2, Cu.sup.+2 from other metal ions and for 
separating Cu.sup.+2 ions, Plaste Kautsch 29(1982)331, and for separating 
Cu.sup.+2 from other transition metal ions, J. Appl. Pol. Sci. 
27(1982)811. 
There is thus a substantial need to exploit the favorable ion-exchange 
characteristics of such materials by incorporating them into media with 
favorable physical characteristics. To this end, in U.S. Pat. Nos. 
4,014,798, 4,045,352 and 4,187,333 there is disclosed hollow-fiber 
ion-exchange membranes comprising structurally sound porous hollow fibers 
containing a polymeric ion-exchange material within the fiber pores. 
However, because the ion-exchange material was not held firmly within the 
particular porous structure it was lost upon flushing the support with 
water. 
SUMMARY OF THE INVENTION 
According to the present invention, there are provided novel ion-exchange 
media useful for highly efficient extraction of ions from aqueous 
solutions, the media comprising polymeric microporous supports, with an 
anisotropic pore structure of small pores at the surface and large pores 
in the interior, the large pores being filled with polymeric ion-exchange 
or ion-complexing materials, and the small pores being sufficiently small 
to retain the ion-exchange materials, thereby preventing their loss from 
the support. Preferred forms of the microporous supports are beads, sheets 
and fibers.

DETAILED DESCRIPTION OF THE INVENTION 
The novel ion-exchange media of the present invention comprises two 
components. These components may be broadly described as (1) a polymeric 
anisotropic microporous support, and (2) a polymeric ion-exchange or 
ion-complexing material. The second component substantially fills the 
pores of the first component. Unlike the one-component ion-exchange 
materials of the prior art, the physical and chemical properties of each 
component can be varied independently. This allows both the physical and 
ion-complexing properties of the novel ion-exchange media of the present 
invention to be optimized. Thus, the present invention allows ion-exchange 
materials which are highly resistant to damage from heat, abrasion, or 
osmotic swelling to be made by using anisotropic microporous support 
materials made from tough thermoplastics such as polysulfone and 
polyvinylidene fluoride, while at the same time utilizing the high 
selectivity and high ion extraction capacity of polymeric ion-exchangers 
which otherwise lack the structural characteristics necessary to be useful 
as ion-exchangers. Unlike the two-component ion-exchange materials of the 
prior art, the microporous support of the present invention is anisotropic 
with surface pores that are sufficiently small to prevent loss of the 
ion-exchange material held within the larger internal pores. 
The microporous supports may be in virtually any shape with an anistropic 
pore structure, having very small pores on the surface and relatively 
large pores in the interior. Preferred forms of the supports are beads, 
sheets and fibers with and without lumens, although any geometric shape 
will work. It is preferable to have surface pores less than 0.1 micron in 
diameter and interior pores of from about 2 to 200 microns in diameter. 
Exemplary polymeric compounds from which the microporous supports of the 
present invention may be fabricated include polysulfone, polystyrene, 
polyvinylchloride, polyacetonitrile, polyamides, polyphenylene oxide, 
polyvinylacetate, polyetherimides, polyvinylidene fluoride and 
combinations thereof. 
Anisotropic microporous bead supports of the present invention, as shown in 
FIG. 1, can be made by injecting droplets of a solution of the polymer 
through a stainless steel tube into a non-solvent bath where they are 
precipitated, the precipitation occurring more rapidly at the exterior 
surfaces than the interior, causing anisotropy with a graduation of pore 
sizes from very small (less than 0.1 micron) on the exterior to relatively 
large (20 to 200 microns) at the center. Bead size may be varied between 
about 1 to about 5 mm by varying the tube diameter. The preferred bead 
size is 2 to 3 mm in diameter. After precipitation, the beads may be 
washed with water and air-dried. Smaller beads with diameters from 0.001 
to 1.0 mm can be made by substantially the same method, except that the 
polymer solution is dispersed into droplets 0.001 to 1.0 mm in diameter by 
use of a spray atomizer or by breaking a stream of polymer with an air jet 
and then allowing the droplets to fall into a water bath where the polymer 
precipitates. 
Suitable anisotropic supports in the form of fibers with lumens, as shown 
in FIG. 2, are made by using the tubein-orifice solution spinning 
technique. This system utilizies a spinneret that consists of two 
concentric tubes; the annular space between the tubes is the polymer 
solution orifice and the inner tube is the lumen-forming orifice. In 
practice, the polymer solution is forced down the outside of the inner, 
lumen-forming tube and through the polymer solution orifice, while the 
lumen-forming solution of water (or water and solvent) flows from the 
inner orifice of the needle. The fiber falls through an air gap of up to 
30 inches and collects in a water precipitation bath. A hollow fiber forms 
as the polysulfone precipitates from the polymer solution. The 
lumen-forming solution precipitates the inside fiber wall; air and then 
water precipitate the outside fiber wall. Precipitation occurs more 
rapidly on the surfaces of the hollow-fiber wall than on the interior, 
causing anisotropy with a graduation of pore sizes from very small (less 
than 0.1 micron on the exterior to relatively large pores (2 to 20 
microns) on the interior. The fibers can have an outside diameter from 
about 0.2 to about 1.0 mm and an outside diameter of from about 0.10 to 
about 0.95 mm. 
Anisotropic supports in the form of lumenless fibers, as shown in FIG. 3, 
are made by injecting a continuous stream of polymer solution through a 
stainless steel tube into a water bath under conditions substantially 
similar to those used to fabricate anisotropic beads, except that the tube 
is submerged in the water bath. Fibers thus formed have surface pores less 
than 0.1 micron in diameter and interior pores from about 5 to about 100 
microns in diameter. 
Flat sheets are made by conventionally practiced casting procedures used in 
the production of anisotropic microporous polymeric membranes, as 
disclosed, for example, in Adv. Chem. Serv. 38(1962)117, U.S. Pat. No. 
3,651,024 and Polym. Let. 11(1973)102. 
As previously mentioned, the present invention comprises the anisotropic 
microporous supports noted above, the pores of which are substantially 
filled with polymeric ion-exchange or ion-complexing materials. These are 
normally made by first loading the anisotropic microporous support with 
monomers or sufficiently low-molecular-weight prepolymers to permit 
introduction through the small surface pores. These monomers or 
prepolymers are selected to yield a polymeric ion-exchange or 
ion-complexing material under suitable reaction conditions. Polymerization 
within the support is then carried out. Once the polymer is formed in the 
support, it cannot escape through the small surface pores. Upon 
polymerization, the support with its polymeric ion-exchange material may 
be used for the extraction of metal ions from aqueous solutions. Even 
structurally weak gels and poorly crosslinked, non-crosslinked, or low 
molecular weight polymers, including those that are water-soluble can be 
used as ion-exchange material as long as the size of the polymer is 
sufficient to prevent its escape through the small surface pores of the 
support. This is an important aspect of the present invention, since it 
has been observed that highly selective ion-exchange materials can be made 
from structurally weak water-swelled gels, J. Appl. Poly. Sci. 
27(1982)811, from polymers with low cross-link density, J. Poly. Sci. 
Poly. Chem. Ed., 20(1982)1609, and from low molecular weight water-soluble 
polymers, J. Pure Appl. Chem., 54(1982)2129. 
Ion-exchange or ion-complexing monomers or low molecular weight prepolymers 
suitable for polymerization within the anisotropic microporous supports of 
the present invention include compounds containing the following 
functional groups: amides, amines, beta-diketones, hydroxyoximes, 
alkylphosphate esters, hydroxyquinolines, thiophosphate esters, carboxylic 
acids, and macrocyclic ethers. They also include alkyl-, aryl-, halogen-, 
and amino-substituted derivitives of such compounds and mixtures thereof. 
The monomers should also be chosen such that after polymerization the 
functional groups are available for ion complexation. Acceptable monomers 
or prepolymers also include those that have little or no ion-exchange or 
ion-complexing functionality but that develop this functionality during 
polymerization. Examples include aziridines and epoxides. In the case of 
aziridines, upon polymerization, tertiary amine ion-exchange sites are 
formed. In the case of epoxides, hydroxyl or ether ion-complexing sites 
are formed upon polymerization. The monomers must be chosen such that the 
resulting polymer is sufficiently hydrophilic to allow migration of the 
ionic species into the polymer. 
Polymerization and crosslinking of the monomers within the support are 
normally accomplished by loading the microporous support, via the small 
surface pores, with a solution composed of monomers or prepolymers (about 
10 wt % to about 99 wt %), solvent, and possibly a polymerization 
initiator (about 0.001 wt % to about 1.0 wt %). Loading may be 
accomplished by submerging the support in the monomer solution and drawing 
a vacuum of 5 mmHg or less, and then alternately releasing and applying 
the vacuum until the pores are substantially filled. Polymerization and 
crosslinking of the monomers take place by heating the monomer-filled 
support or by contact with external initiators. 
Polymerization may also be initiated by thermal decomposition of an organic 
free-radical initiator. Suitable free-radical initiators include 
azo-nitriles and other azoderivatives, peroxides and peresters. 
Alternatively, polymerization and crosslinking can be carried out with 
reactive heterocyclic compounds that react by ring-opening ractions upon 
heat; examples include epoxide and aziridine derivatives. 
EXAMPLE 1 
Anisotropic microporous supports in bead form substantially as shown in 
FIG. 1 were prepared by injecting dropwise a solution of 120 g polysulfone 
in 1.0 L dimethylformamide through a stainless steel tube with an inside 
diameter of 0.75 mm through an air gap of 2 inches and into a bath of 
water at 20.degree. C., thereby precipitating beads 2 to 3 mm in diameter 
with surface pores less than 0.1 micron in diameter and interior pores 100 
to 200 microns in diameter. The beads were washed with water and allowed 
to air-dry. 
EXAMPLE 2 
Microporous polysulfone lumen-containing fibers substantially as shown in 
FIG. 2 having surface pores less than 0.1 micron in diameter, interior 
pores of 2 to 20 microns, an average diameter of 0.625 mm, and an 
internal diameter of 0.50 were prepared by using the tube-in-orifice 
solution spinning technique. This system utilizes a spinneret, which 
consists of two concentric tubes; the annular space between the tubes is 
the polymer-solution orifice and the inner tube is the lumen-forming 
orifice. A polymer solution composed of 175 g polysulfone and 200 g 
2-methoxyethanol per liter of N,N-dimethylformamide (DMF) was forced down 
the outside of the inner, lumen-forming tube and through the 
polymer-solution orifice while a lumen-forming solution composed of 60 vol 
% DMF in water flowed through the lumen-forming tube. The two solutions 
fell through an air gap of 20 inches into a water precipitation bath at 
20.degree. C. The hollow fiber thus formed was rinsed with water to remove 
residual DMF and 2-methoxyethanol. The fibers were air-dried for 48 hours 
and were then ready for use. 
EXAMPLE 3 
Microporous polysulfone non-lumen-containing fibers substantially as shown 
in FIG. 3, having surface pores of less than 0.1 micron in diameter and 
interior pores 5 to 100 microns in diameter, were prepared by forcing a 
solution of 175 g polysulfone per liter of N,N-dimethylformamide (DMF) 
through a stainless steel tube 0.75 mm in diameter submerged in a water 
bath at 20.degree. C. The polysulfone precipitated, forming a lumenless 
fiber as polymer solution contacted the water solution. The fibers thus 
formed were rinsed in water and air-dried. 
EXAMPLE 4 
Anisotropic microporous polysulfone fiber supports containing a polymeric 
hydrophilic methacrylate gel with pendant tertiary, amine groups were 
prepared by first immersing the fibers of Example 3 in a solution of 47.5 
wt % N,N-dimethylaminomethacrylate, 2.5 wt % 
tetraethyleneglycoldimethacrylate and 0.25 wt % azo-bis-isobutylnitrile in 
methanol then alternately drawing a vacuum of about 5 mmHg and 
repressurizing to atmospheric pressure until the pores were substantially 
filled. The solution-filled fibers were subjected to a temperature of 
65.degree. C. for 75 minutes, substantial completion of the polymerization 
being indicated by the formation of a gel on the exterior of the fibers. 
The fibers were removed and the methanol solvent exchanged for water by 
soaking the fibers in water at 20.degree. C. for 24 hours. 
EXAMPLE 5 
The metal-ion complexing capacity of the fibers of Example 4 was evaluated 
by contacting 0.85 g of the fibers with 1 L of an aqueous solution 
containing 10 ppm uranium as uranyl sulfate at pH 3.0 (pH adjusted by 
addition of sulfuric acid) at 25.degree. C. After 16 hours the 
concentration of uranium in the aqueous solution was reduced to 3.2 ppm 
uranium. This corresponds to a uranium content of the fibers of 0.80 g of 
uranium per 100 g of fiber. 
EXAMPLE 6 
Anisotropic microporous polysulfone fiber supports containing a polymeric 
ion-exchange material were prepared by first immersing the fibers of 
Example 3 in a solution of 50 wt % polyethyleneimine (CORCAT P-18 sold by 
Cordova Chemical Company of Muskegon, Michigan) and 15 wt % 
epichlorohydrin in a 1:1 butanol-water mixture, then alternately drawing a 
vacuum of about 5 mmHg and repressurizing to atmospheric pressure until 
the pores were substantially filled. The solution-filled fibers were then 
subjected to a temperature of 60.degree. C. for about 2 hours, substantial 
completion of the polymerization being indicated by gellation of the 
exterior solution. The fibers were removed from the gelled solution and 
the butanol solvent exchanged for water by soaking the fibers in water at 
20.degree. C. for 24 hours. 
EXAMPLE 7 
The ion-exchange capacity of the fibers of Example 6 was evaluated by 
cutting them into 10-cm-long bundles and contacting them with an aqueous 
feed solution comprising 10 ppm uranium as uranyl sulfate and 10-g/L 
sulfuric acid (pH 1.0). Extraction of uranium ion was complete after about 
24 hours. Uranium was stripped from the fibers by transferring them to a 
150-g/L sodium carbonate solution (pH 11.5), and stripping was complete in 
about 2 hours. The loading/stripping cycle is shown in FIG. 4. The maximum 
distribution coefficient, defined as the concentration of uranium ions in 
the fibers divided by the concentration of uranium ions in the aqueous 
feed solution, was approximately 750. 
EXAMPLE 8 
Fibers of Example 6 were tested over 40 loading/stripping cycles. During 
each loading/stripping cycle the fibers were contacted with an aqueous 
uranium feed solution comprising 10 ppm uranium as uranyl sulfate and 
10-g/L sulfuric acid at pH 1.0 for 16 hours and then transferred to an 
aqueous stripping solution comprising 150-g/L sodium carbonate (pH 11.5) 
for 8 hours. The amount of uranium transferred from the feed solution to 
the stripping solution in wt % (defined as grams uranium per 100 grams 
fiber) is shown in FIG. 5 over 40 loading/stripping cycles. The results 
demonstrate that the fibers retain their ion-exchange characteristics over 
an extended period of operation. 
The terms and expressions which have been employed in the foregoing 
specification are used therein as terms of description and not of 
limitation, and there is no intention, in the use of such terms and 
expressions, of excluding equivalents of the features shown and described 
or portions thereof, it being recognized that the scope of the invention 
is defined and limited only by the claims which follow.