Znd. Eng. Chem. Res. 1991,30, 2234-2237
2234
RESEARCH NOTES Introduction of a High-Density Chelating Group into a Porous Membrane without Lowering the Flux A method by which to introduce a high-density iminodiacetate (IDA) group into a polyethylene microfiltration membrane without lowering the water flux was developed by selecting the solvents for graft polymerization and subsequent chemical modification. Grafting of glycidyl methacrylate (GMA) onto the porous hollow fiber in a methanol solvent enhanced the swelling of the matrix and thus compensated for the decrease in pore volume. Subsequently, the dimethyl sulfoxide (DMSO)/water solvent provided a higher conversion of the produced epoxide group into the IDA group. The resulting IDA-group-containing hollow fiber had a sufficient cobalt-binding capacity and maintained the water flux a t a feasible level. The flux of the chelating hollow fiber, whose degree of GMA grafting was 19790,was kept at about 70%, i.e., 2.0 m3/(m2h) of that of the starting hollow fiber. The cobalt saturation capacity of the hollow fiber was 1.9 mol/kg. When a cobalt-containing solution permeated across the chelating- hollow fiber through its pores, a sharp breakthrough curve of the cobalt ion was observed. Introduction
and a cobalt ion as the chelate-forming group and the metal ion, respectively.
Membrane technology is now targeting the development of novel membranes with high functionality. The introExperimental Section duction of an affinity ligand (Brandt et al., 1988) and an ion-exchange group (Saito et al., 1989) into the microfilMaterials. A commercially available microfiltration membrane (Asahi Chemical Industry Co., Ltd., Tokyo, tration membrane enabled us to separate biomolecules Japan) was used as the trunk polymer for grafting. The from biological fluids and remove a trace of undesirable inner and outer diameters of the porous polyethylene ions from ultrapure water during permeation across the hollow fiber were 1.95 and 3.01 mm, respectively. The membrane. A porous membrane containing functional hollow fiber had a symmetric porous structure with a groups is regarded as a very short bed with large internal nominally 0.36-pm-diameter pore size and 72% porosity. surface areas. Since solutes are convectad to the functional Reagent-grade glycidyl methacrylate (CH2= group driven by the preeeure difference, rather than having CCH3COOCH2CHOCH2,GMA) was used without further to diffuse driven by the concentration difference, diffupurification. Other chemicals were of reagent grade. sional path lengths are minimized. Therefore, the colPreparation of Chelating Hollow Fiber. Figure 1 lection of proteins and ions during permeation of the soshows the preparation scheme of the porous iminodiacetate lution can improve the mass-transfer rate. A membrane (IDA)-group-containing hollow fiber. The 10-cm-long module is a powerful alternative to the bead-packed bed hollow fiber was irradiated by an electron beam at a total conventionally used for their separation (Brandt et al., dose of 200 kGy at ambient temperature in a nitrogen 1988). atmosphere. The irradiated hollow fiber was immersed A hollow fiber membrane is superior to a flat sheet in a 10% (v/v) GMA/methanol solution. GMA was remembrane because of its high surface area/volume ratio. aded with the trapped radicals in the trunk polymer. The A polymer with a functional group cannot always be degree of GMA grafting defined below ranged from 30 to molded into a porous hollow fiber membrane; chemical 300% with varying reaction time at 313 K. modification of existing hollow fibers is therefore effective in adding new properties such as the collection of proteins degree of GMA grafting (dg) = 100[(Wl - Wo)/Wo](1) and ions. Grafting is a useful method by which to modify existing polymers chemically. Initiation methods of graft where W oand W 1are the weights of the starting and polymerization include irradiation with high-energy raGMA-grafted hollow fiber. Subsequently, the GMAdiation (%o y-rays and electron beams), UV radiation, grafted hollow fiber was immersed in 0.425 M disodium and plasma treatment. Of these, radiation-induced graft iminodiacetate (NH(CH2COONa)2,IDA-2Na) dissolved in polymerization and subsequent functionalization have dimethyl sulfoxide (DMSO)/water. The volume ratio of certain advantages in that the technique makes it easy to DMSO to water ranged from 0.33 to 1. For comparison, control the site of the grafted polymer branches and the 0.425 M IDA-2Na in a 1M Na2C03solution (pH = 11.5) density of the functional group (Okamoto, 1987). In genwas also used. The reaction was performed at 353 K for eral, when the grafting technique is applied to incorporate up to 10 h, which waa sufficient to reach the final confunctionality into a porous membrane, grafted polymer version. After the introduction of the IDA group into the branches are formed on the pore surface, thus inducing a grafted polymer branches, the remaining epoxide group decrease in water permeability. was hydrolyzed into a diol group by soaking the hollow The objectives of our study were 2-fold (1)to suggest fiber for 2 h in 0.5 M sulfuric acid at 353 K. Subsequently, a novel method by which to introduce the high-density the hollow fiber, washed repeatedly with deionized water, chelate-forming group into a commercial microfiltration was was dried under reduced preasure and the weight (W,) membrane without lowering the water flux and (2) to exmeasured. The conversion of the epoxide group into the amine the properties and performance of the resulting IDA group, X,can be related to the weight change as chelating membrane. We selected an iminodiacetate group follows: 0888-5885f 91f 2630-2234$02.5Qf 0 Q 1991 American Chemical Society
< GMA
Ind. Eng. Chem. Res., Vol. 30, No. 9,1991 2235 fCH2CHfn I
fCH2CCH3fm I
COCHeCHCH2
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= Wo + (Wi- Wo)[X(142 + 133) + (1 - X)(142 + 18)]/142 (2)
where the factors 142,133, and 18 correspond to the molecular weights of GMA, iminodiacetic acid, and water, respectively. Thus, the conversion and the density of the IDA group were calculated as conversion (X) = [142(W2 - Wo)/(Wi- Wo)- 160]/115 (3) density of IDA group = X ( W l - W0)/(142W2) (4) The resulting hollow fiber will hereafter be referred to as the IDA-T(dg/X) fiber, where T designates tubular and the former and latter figures in parentheses indicate the degree of GMA grafting and the conversion, respectively. Properties of Chelating Hollow Fiber. The inner and outer diameters and the length of the IDA-T fibers in the wet state were measured with a microscope and a scale, respectively. After the IDA-T fiber was dried under reduced pressure, the pore volume distribution was measured by the mercury intrusion method. The permeability of pure water through the chelating hollow fiber, Le., the pure water flux (PWF), was determined by the established constant-pressure method. Figure 2 shows the experimental apparatus for measuring PWF. The hollow fiber was set in a U-shaped configuration. The pure water, prepared by passing water through an ultrafiltration membrane module, was forced to permeate radially from the inside to the outside under a filtration pressure of 1.0 X lob Pa. The flow rate of the effluent was convertad into the PWF based on the inside area of the hollow fiber. The saturation capacity of the cobalt ion onto the IDA-T fiber was measured by the batchwise method. The IDA-T fiber was equilibrated with 1 M CoC1, (pH = 1.3) at 303 K. The initial molar ratio of the cobalt ion to the IDA group was set at about 50. After a 72-h contact, the hollow fiber was taken out of the solution and washed repeatedly with deionized water. The hollow fiber was then dried, and the weight was measured. The cobalt sorbed to the hollow fiber was eluted with 1 M HC1 for 24 h at 303 K. The saturation capacity of Co was defined as the amount eluted divided by the weight of the IDA-T fiber. Cobalt was determined by atomic absorption spectrometry. Breakthrough Curve. The collection of cobalt during permeation of the solution across the membrane was carried out according to a similar procedure used for the flux measurement. The 13-cm-long chelating hollow fiber, whose degree of GMA grafting and density of the IDA group were 197% and 1.9 mol/kg of dry fiber, respectively, was set in a U-shaped configuration. The inner and outer diameters of the IDA-T fiber were 2.64 and 4.20 mm, respectively. The solution containing 20 g of Co/m3 was
1 Feed Tank 2 Flow Controller 3 Pressure Gauge
4 Hollow Fiber
5 Measuring Cylinder
Figure 2. Experimental apparatus for determining flux and breakthrough curves.
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Figure 3. Conversion of the epoxide group into an iminodiacetate group in various solvent as a function of reaction time. ( 0 ) DMSO/water = 1/1; (A) DMSO/water = 1/2; ( 0 )DMSO/water = 1/3; (0) NalCOB aqueous solution.
applied to the inside of the hollow fiber and forced to permeate radially outward under a constant filtration pressure of 0.5 X lo6 Pa. The effluent from the hollow fiber was collected in measuring cylinders. The change in the concentration of the cobalt ion in the effluent with time, Le., the breakthrough curve, was determined.
Results and Discussion Preparation of Chelating Hollow Fiber. Figure 3 shows the conversion of the epoxide group into the IDA group for various solvents as a function of reaction time. The higher ratio of DMSO to water exhibited a higher conversion. The GMA-grafted hollow fiber having 150% degree of GMA grafting underwent a conversion of about 60% after 20 h. On the other hand, the final conversion in the IDA-2Na/Na&'OS solution adopted by Hemdan and Porath (1985) was 15%. Higher conversions in the DMSO/water solvent can be explained by the following two effects. First, DMSO enhanced the inversion of the epoxide group in the reaction via a SN2mechanism with iminodiacetate (Ichikawa and Inoue, 1980). Second, the DMSO/water solvent induced relaxation of the grafted polymer branches and resulted in a higher conversion. The swelling caused by relaxation was observed from the measurement of the volume change upon immersion of the GMA-grafted hollow fiber in DMSO/water. Figure 4 shows the final conversion as a function of the degree of GMA grafting. The conversion of the epoxide group to the IDA group was increased gradually with an increasing degree of GMA grafting. The conversion of 60% leveled off at 120% degree of GMA grafting.
2236 Ind. Eng. Chem. Res., Vol. 30, No. 9, 1991
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Figure 6. Pore volume distribution of the starting hollow fiber and
Figure 4. Final conversion vs degree of grafting. I
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Figure 5. Swelling ratio of the hollow fiber vs degree of grafting. Swelling ratio was defined RE the volume of the GMA- or IDA-T fiber divided by that of the starting hollow fiber in the wet state.
[XI Figure 7. Cobalt saturation capacity and water flux of IDA-T fibers as a function of the degree of GMA grafting.
Properties of Chelating Hollow Fiber. Figure 5 shows the swelling ratio of the porous hollow fiber caused by the grafting of GMA and introduction of the IDA group. Both this grafting and subsequent functionalization induced swelling of the matrix, Porous polyethylene consists of macropores of about 0.4 Mm in diameter surrounded by polyethylene. Polyethylene has the structure of crystalline lamellae separated by an amorphous domain. When preirradiation graftii is employed, the radicals are located mainly on the surface of the crystalline lamellae. The methanol-diluted monomer, therefore, can react with these and form graft chains both on the surface of the macropores and in the amorphous domain between the individual folded-chain lamelhe. The graft chains thus formed in the amorphous domain cause the hollow fiber to swell. Figure 6 shows the comparison of the cumulative pore volume distribution between the starting hollow fiber and the IDA-T fibers. Chemical modifications resulted in the decrease in the pore volume per kilogram of the chelating hollow fiber. The pore volume of the IDA-T(198/63) fibers, 1.1X lom8ma/kg, decreased by 44% from that of the ma/kg. This value of the starting hollow fiber, 2.6 X IDA-T(198/63) fiber can be converted into 4.7 X m3/kg with respect to the trunk polymer by multiplying W e Therefore, the decrease in porosity caused it by W2/ by the grafted polymer branches is compensated for by the swelling of the matrix. In addition, the uniform distribution of the IDA group throughout the hollow fiber was
confirmed according to a procedure described in our previous work (Saito et al., 1988). Figure 7 shows the cobalt saturation capacity and flux as a function of the degree of GMA grafting. The IDA-T fiber with more than 90% degree of GMA grafting exhibited a higher cobalt saturation capacity, 1.0 mol/kg of the hollow fiber, as compared to that of the commercial chelating beads. For example, a commercial IDA-groupcontaining bead has the density of 0.7 mol/kg of the resin (Mitsubishi Kasei Co., 1975), and thus its cobalt saturation capacity is estimated to be 0.7 mol of Co/kg at maximum, assuming that the complexing ratio of the IDA group to cobalt ion is unity. The flux curve as a function of the degree of GMA grafting showed parabolic behavior. The value for the minimum flux was half that of the starting hollow fiber. Functionalization of the microfiltration membrane may be attained at the expense of the water flux. At over 260% degree of GMA grafting, the chelating hollow fiber had a higher flux than did the starting hollow fiber. The water flux was determined by the balance between the pore volume reduction and the matrix volume swelling accompanied by grafting. In conventional liquid-phase graft polymerization with a 100% monomer liquid, the pore reduction outweighed the matrix swelling (Yamagishi et al., 1988). The graft polymerization onto the porous membrane in the methanol-diluted monomer solution resulted in a higher amount of the polymer branches in the matrix, as compared to conventional liquid grafting in the 100% monomer because the methanol
Ind. Eng. Chem. Res., Vol. 30,No. 9, 1991 2237
1 .o
-
s
ions from pure waters in semiconductor production factories. Moreover, the grafting technique suggested here will be applied to the preparation of functional porous membranes. Furthermore, the separation principle using the resulting membrane without lowering the water flux can be effectively employed.
ldeol breakthrough curve
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i
Acknowledgment I 303K
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Figure 8. Breakthrough curve of cobalt ion using the IDA-T(197/ 62) fiber.
which was employed as an effective solvent of the polyethylene caused it to swell. Thus, grafting in GMA in methanol allowed the membrane to prevent the pore volume reduction and maintain the water flux at a feasible level. The proper selection of preparation conditions, in particular, the solvent, for grafting and functionalization enabled us to incorporate the metal collection capability at a high density into an existing microfiltration membrane without lowering the water flux. Breakthrough Curves. Figure 8 shows a breakthrough curve for the IDA-T(197/62) fiber, i.e., a plot of the cobalt concentration in the effluent against dimensionless time. Dimensionless time is defined as the ratio of the cobalt amount fed from the inside to the amount (qo) of cobalt sorbed in equilibrium with the inlet concentration (Co): dimensionless time = 4diuiCot/(dO2- d?)paqo ( 5 ) where di, do, and pa are the inner and outer diameters and the apparent density of the hollow fiber, respectively. ui and t are the flux based on the inside area of the hollow fiber and the permeation time. The amount of cobalt sorbed in equilibrium with the inlet concentration was h a t identical with the corresponding saturation capacity due to a favorable sorption isotherm. The thickness of the chelating hollow fiber corresponded to the bed height of the bed packed with chelating beads. The mean residence time of the solution across the hollow fiber through its pores was calculated as 1.4 s from the following equation: mean residence time = c(d2 - dt)/(4diui) (6) where c is the porosity of the IDA-T fiber and can be determined from measurement of its water content. An ideal breakthrough curve without any mass-transfer resistance (the rectangular line) is depicted in Figure 8. In spite of the short residence time required to permeate across the membrane, a relatively sharp breakthrough curve was observed. This is because the metal ion was directly transported to the chelate-forming group located on the internal pore surface of the membrane. Convection through the pores of the chelating hollow fiber reduced the diffusional resistance of the cobalt ion to the IDA group. Deviation from the ideal breakthrough curve may be ascribed to the residence time distribution caused by the pore length distribution. The sorbed cobalt could be easily eluted with 1 M hydrochloric acid. After washing with deionized water, the cobalt collection conducted during permeation was repeated. The reproducible breakthrough curve thus obtained indicates that the chelating membrane prepared here was chemically stable. This separation principle using the porous membrane containing the chelate-forming group has high potential for use in various fields of separation technology, such as for removal of radioactive metal ions from wastewaters in atomic power plants and for collection of undesirable metal
We thank Kazuo Toyomoto of the Industrial Membrane Division of Asahi Chemical Industry Co., Ltd.,for his help in providing the starting hollow fiber. Helpful discussions with Takehiko Ohtoyo of the Polymer Products & Applications Laboratory of Asahi Chemical Industry Co., Ltd., are also gratefully acknowledged. Nomenclature C = cobalt concentration in the permeate, g m-3 Co = cobalt concentration at the inlet, g m-3 di = inner diameter of the hollow fiber, m do = outer diameter of the hollow fiber, m qo = amount of cobalt sorbed in equilibrium with Cotg kg-l t = time, h ui = flux based on the inside area of the hollow fiber, m h-' Wo = weight of the starting hollow fiber, g W , = weight of the GMA-grafted hollow fiber, g W , = weight of the chelating hollow fiber, g X = conversion defined by (3) Greek Symbols t = porosity of the hollow fiber pa = apparent density of the hollow fiber, kg m-3 Registry No. Co, 7440-48-4.
Literature Cited Brandt, S.; Goffe, R. A.; Kessler, S. B.; O'Connor, J. L.; Zale, S. E. Membrane-Based Affinity Technology for Commercial Scale Purifications. BiolTechnology 1988, 6, 779-782. Hemdan, E. S.; Porath, J. Development of Immobilized Metal Affinity Chromatography I. Comparison of T w o Iminodiacetate Gels. J. Chromatogr. 1985,323,247-254. Ichikawa, K.; Inoue, M. Acid Catalyzed Solvolysis of Epoxide. Yukigosei Kagaku 1980,38,61-78. Mitsubishi Kasei Co. Manual DIAION; 1975; p 192. Okamoto, J. Radiation Synthesis of Functional Polymer. Radiat. Phys. Chem. 1987,29,469-475. Saito, K.; Uezu, K.; Hori, T.; Furusaki, S.; Sugo, T.; Okamoto, J. Recovery of Uranium from Seawater Using Amidoxime Hollow Fibers. AIChE J . 1988,34,411-416. Saito, K.; Ito, M.; Yamagishi, H.; Furusaki, S.; Sugo, T.; Okamoto, J. Novel Hollow Fiber Membrane for the Removal of Metal Ion during Permeation: Preparation by Radiation-Induced Cografting of a Cross-Linking Agent with Reactive Monomer. Ind. Eng. Chem. Res. 1989,28, 1808-1812. Yamagishi, H.; Saito, K.; Furusaki, S.; Sugo, T.; Okamoto, J. Effect of Vapor- and Liquid-Phase Radiation Grafting on Water Permeability of Porous Hollow-Fiber Membrane. Nippon Kagaku Kaishi 1988, 212-216.
* Address correspondence to this author. Hideyuki Yamagishi, Kyoichi Saito,* Shintaro Furusaki Department of Chemical Engineering Faculty of Engineering, University of Tokyo Hongo, Bunkyo-ku, Tokyo 113, Japan Takanobu Sugo, Isao Ishigaki Takasaki Radiation Chemistry Research Establishment Japan Atomic Energy Research Institute Watanuki, Takasaki 370-12, Japan Received f o r review June 4, 1991 Accepted June 25, 1991
