Comparison of Amidoxime Adsorbents Prepared by Cografting

Department of Materials Technology, Faculty of Engineering, Chiba University, Inage, Chiba 263-8522, Japan. Akio Katakai, Noriaki Seko, and Takanobu S...
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MATERIALS AND INTERFACES Comparison of Amidoxime Adsorbents Prepared by Cografting Methacrylic Acid and 2-Hydroxyethyl Methacrylate with Acrylonitrile onto Polyethylene Tomomi Kawai, Kyoichi Saito,* and Kazuyuki Sugita Department of Materials Technology, Faculty of Engineering, Chiba University, Inage, Chiba 263-8522, Japan

Akio Katakai, Noriaki Seko, and Takanobu Sugo Takasaki Radiation Chemistry Research Establishment, Japan Atomic Energy Research Institute, Takasaki, Gunma 370-1292, Japan

Jun-ichi Kanno and Takashi Kawakami Ebara Research Co., Ltd., Hon-Fujisawa, Kanagawa 251-8502, Japan

Methacrylic acid (MAA) and 2-hydroxyethyl methacrylate (HEMA) were cografted with acrylonitrile (AN) onto polyethylene fiber by radiation-induced graft polymerization. The cyano groups produced were converted to amidoxime groups (-C(dNOH)NH2) by reaction with hydroxylamine (NH2OH) to recover uranium in seawater. Various weight ratios of AN/MAA or AN/HEMA in the monomer mixture for cografting generated MAA- and HEMA-cografted amidoxime (AO) fibers with various hydrophilicities. The amidoxime group density and water content were balanced to enhance the uranium adsorption from seawater. MAA-cografted AO fibers exhibited a higher adsorption rate than HEMA-cografted AO fibers. The optimum value of the weight ratio of AN/ MAA ) 60/40 in the monomer mixture was observed both in a submerged mode at an ocean site and in a flow-through mode in the laboratory. The amount of uranium adsorbed was 0.90 g/kg of the MAA-cografted AO fiber at 293-298 K after 20 days of contact at the ocean site. Introduction A recovery system for uranium in seawater using amidoxime (AO) fibers is promising in that the ocean current forces seawater to easily move through an adsorption bed charged with AO fiber.1-11 A research group at the Shikoku National Industrial Research Institute has developed bundled AO fibers based on commercially available acrylonitrile (AN) fibers; the fibers exhibited a uranium adsorption rate of 2 mg of U/g of fiber for 60 days operating in a flow-through mode.8,9 A research group at the Japan Atomic Energy Research Institute has proposed a method of preparing the AO fibers based on polyethylene (PE) and polypropylene (PP) fibers by radiation-induced graft polymerization of AN onto the fiber, followed by amidoximation.6,10,11 Radiation-induced graft polymerization is a powerful technique for preparing functional polymers because various shapes and quantities of trunk polymers can be selected. For example, AN was grafted onto porous polyethylene films12 and hollow fibers13-15 as a trunk * To whom correspondence should be addressed. Tel./Fax: +81-43-290-3439. E-mail: [email protected].

polymer, and subsequently the cyano groups produced were converted into AO groups (-C(dNOH)NH2).16-18 Hydrophilization of AO adsorbents is effective in improving the uranium adsorption rate; the diffusion of the uranyl tricarbonate ion UO2(CO3)34-, which is the predominant species of dissolved uranium in seawater,19,20 governs the overall adsorption rate of uranium. Cografting of methacrylic acid with AN onto PE and PP fibers was suggested as a method to enhance the uranium adsorption of the AO adsorbents.10,11 The objective of our study was to evaluate the monomers, methacrylic acid (MAA) and 2-hydroxyethyl methacrylate (HEMA), suitable for cografting onto PE fibers as the trunk polymer. Here, the uranium adsorption rate on the AO fibers in seawater was evaluated both at an ocean site and in the laboratory. Experimental Section Materials. Polyethylene (PE) fibers about 30 µm in diameter were used as the trunk polymer for grafting. Acrylonitrile (CH2dCHCN, AN), methacrylic acid (CH2d CCH3COOH, MAA), and 2-hydroxyethyl methacrylate (CH2dCCH3COOCH2CH2OH, HEMA) were purchased from Tokyo Chemical Co. and used without further

10.1021/ie990474a CCC: $19.00 © 2000 American Chemical Society Published on Web 06/30/2000

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Figure 1. Preparation of amidoxime adsorbents by cografting acrylonitrile and methacrylic acid or 2-hydroxyethyl methacrylate and subsequent amidoximation.

purification. Other chemicals were of analytical or higher grade. A 3 (w/v)% solution of hydroxylamine hydrochloride (NH2OH‚HCl) was prepared using 50/50 (v/v)% water/methanol as a solvent, the pH of which was adjusted to 7. Seawater sampled at the seashore of Mutsu Sekine-Hama in Aomori Prefecture was used after being filtered through 5-µm pore size paper filters (Toyo Filter Co., Japan). Preparation of Hydrophilic Amidoxime Adsorbents. The preparation of hydrophilic amidoxime (AO) fibers based on PE fibers by radiation-induced cograft polymerization and subsequent chemical modifications is illustrated in Figure 1. First, a combination of MAA or HEMA with AN was cografted onto the PE fiber using a preirradiation technique:10,11 irradiation by an electron beam was performed at a dose of 200 kGy in a nitrogen atmosphere at ambient temperature. The irradiated fiber was immersed in a monomer solution previously deaerated with nitrogen. The total concentration of the two monomers was set at 50 (w/w)% in DMSO as a solvent, where the weight ratio of AN/MAA or AN/ HEMA in the monomer mixture ranged from 100/0 to 50/50. Cografting was performed at 313 K for a reaction time up to 7 h. The fiber, rinsed repeatedly with dimethylformamide and methanol, was dried under reduced pressure and then weighed. The degree of cografting, defined below, was calculated from the weight gain due to cografting,

degree of cografting [%] ) 100(W1 - W0)/W0 (1)

Figure 2. Experimental apparatus for uranium recovery from seawater in the submerged mode of operation at the ocean site.

where W0 and W1 are the weights of the trunk and AN/ MAA- or AN/HEMA-cografted fibers, respectively. The resultant fibers were referred to as AN/MAA(x/y, dg) or AN/HEMA(x/y, dg) fibers, where x/y and dg designate the weight ratio of AN/MAA or AN/HEMA in the monomer mixture and the degree of cografting, respectively. Second, the AN/MAA- and AN/HEMA-cografted fibers were immersed in the hydroxylamine solution at 350 K for 45 min to convert the produced cyano groups to AO

groups. After the reaction, the fiber was repeatedly washed with deionized water. Then, the fiber was dried under reduced pressure, and the weight was measured. The AO group density was evaluated as

AO group density [mol/kg] ) 1000(W2 - W1)/(33W2) (2) where W2 is the weight of the MAA- or HEMA-cografted AO fiber. For example, the fibers obtained through

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Figure 3. Cografting rate of AN/MAA and AN/HEMA for various weight ratios of AN/MAA or AN/HEMA in a monomer mixture.

Figure 4. Amidoxime group density vs amidoximation time.

the amidoximation of AN/MAA(x/y, dg) fibers were referred to as AO/MAA(x/y, dg) fibers. Finally, the AO/MAA and AO/HEMA fibers were immersed in 2.5 (w/v)% KOH solution at 353 K for 1 h. Prior to the uranium adsorption experiments, the fibers were repeatedly washed with deionized water. The water content of the resultant hydrophilic AO fibers was determined as

water content [%] ) 100(Ww - Wd)/Wd

(3)

where Wd and Ww are the weights of the hydrophilic AO fibers in the dry and wet states, respectively. Observation of the dried AO/MAA fiber was performed under a scanning electron microscope (JEOL, JXA-733 model). Uranium Adsorption. The uranium adsorption rate of the hydrophilic AO fibers was evaluated in two modes of operation: (1) a submerged mode in the ocean away from the seashore of Japan and (2) a flow-through mode in the laboratory with samples of seawater. Mode (1): about 0.5 g of the AO/MAA or AO/HEMA fiber was charged in a container made of plastic mesh,

which was attached to the outside of a frame (adsorption unit) made of stainless steel 30 cm in diameter and 10 cm in height, as illustrated in Figure 2. The adsorption units were submerged for 20 days, 15 m below the surface of the sea located about 6 km offshore at Mutsu Sekine-Hama in Aomori Prefecture. Fibers of about 0.5 g were removed from the container and then immersed twice in 1 M HCl of 10 mL at ambient temperature to elute the adsorbed metals.12 Metals in the eluent were determined using an inductively coupled plasma (ICP) spectrometer (Seiko Instruments Inc., SPS-7700 model). Mode (2): the AO/MAA or AO/HEMA fibers were packed in a column with an inner diameter of 0.7 cm and height of 5 cm. Seawater (20 L) was circulated upward through the fiber-charged column at a flow rate of 0.47 mL/s for 24 h at 298 K. The trunk PE fibers were packed at the lower end of the AO fibers to prevent fouling of the AO fibers. After 24-h contact in the flowthrough mode, the AO fibers were removed from the column and immersed in 1 M HCl to elute the adsorbed metals.

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Figure 5. Amidoxime group density and water content as a function of wt % of AN in the monomer mixture.

Figure 6. SEM images of amidoxime fiber.

Results and Discussion Properties of Hydrophilic Amidoxime Fiber. Time course studies of the degree of cografting are shown in Figure 3a,b for various weight ratios of AN/ MAA and of AN/HEMA, respectively, in the monomer mixture for cografting, which was diluted with DMSO at a percentage of 50 (w/w)%. As the weight ratio of AN/ MAA or AN/HEMA increased, the cografting rate also increased. However, the weight ratios of x/y of 80/20 and 100/0 were almost identical with respect to the time course of the degree of cografting. The AN/MAA- and AN/HEMA-cografted fibers prepared at various weight ratios of x/y were reacted with NH2OH to convert the cyano groups into amidoxime (AO) groups. An example of a time course of the AO group density is shown in Figure 4. For both AN/MAA (80/20, 150) and AN/HEMA (80/20, 150) fibers, the AO group density leveled off over 30 min; therefore, the amidoximation time was set at 45 min. The AO group density and water content of the resultant AO fibers are shown in Figure 5 as a function of the value of x/y. For both AO/MAA and AO/HEMA fibers, as the AN

content in the monomer mixture increased, the AO group density in the fibers increased while their water content decreased. At an identical AO group density, the AO/MAA fibers had a higher water content than the AO/HEMA fibers. This difference arises because the potassium carboxylate group (-COOK) of the poly-MMA chain has a higher affinity for water as compared to the hydroxyl group (-OH) of the poly-HEMA chain. The scanning electron microscopy (SEM) image of the AO/MAA (80/20, 85) fiber is shown in Figure 6 along with that of the trunk fiber. After the introduction of the grafted polymer chain and subsequent amidoximation, the fiber diameter of the AO/MAA fiber swelled 2-fold compared to the trunk fiber. Uranium Adsorption Rate. The AO/MAA and AO/ HEMA fibers were attached to the adsorption unit submerged at the ocean site to evaluate the uranium adsorption rate. Time course studies of the amount of uranium adsorbed onto the AO/MAA and AO/HEMA fibers are shown in Figure 7 as a function of contact time with seawater. The AO/MAA fiber exhibited the amount of adsorbed uranium of 0.75 g/kg of the dry

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Figure 7. Time course of changes in the amount of uranium adsorbed in the submerged mode of operation.

product after 40 days of contact, which amounted to 1.7and 3-times the amount using the AO/HEMA and AO fibers, respectively. The MAA as a cografted monomer was found to be effective in improving the uranium adsorption rate. The enhancement of the uranium adsorption in seawater onto the adsorbent by the cografting of MAA or HEMA with AN can be explained by an increase in the diffusivity and accessibility of the uranyl species to the AO group in the adsorbent: the potassium carboxylate or hydroxyl group coexisting with the AO group attracts water molecules around it to produce pores, which promotes the access of the uranyl tricarbonate ion, UO2(CO3)34-, as the predominant species to the AO group, and increases the diffusion rate of the uranyl

species into the interior of the adsorbent through the pores. In addition, the neighboring MAA or HEMA group can stabilize the complex of the uranyl species with the AO group. At present, a quantitative evaluation of these contributions to the uranium adsorption is difficult. Here, the AN and MAA or HEMA during the preparation of the AO adsorbent should be experimentally balanced to maximize the uranium adsorption rate. The amount of uranium adsorbed in the submerged mode of operation at an ocean site for 20 days of contact is shown in Figure 8a as a function of the weight ratio of AN/MAA or AN/HEMA in the monomer mixture. The AO fibers prepared by cografting MAA with AN at a weight ratio of AN/MAA of 60/40 and subsequent amidoximation exhibited a maximum value of 0.90 g of U/kg. However, the submerged mode of operation includes some uncertainity because the temperature and velocity of the seawater vary according to weather and seawater conditions. The uranium adsorptivity of the fibrous adsorbents was ascertained in the flow-through mode in a laboratory, where the temperature and flow rate of seawater were maintained at prescribed values. A similar dependence of the amount of uranium adsorbed on x/y was observed as shown in Figure 8b. Conclusions To improve the adsorption rate of uranium in seawater, hydrophilic amidoxime (AO) fibers were prepared by cografting methacrylic acid (MAA) and 2-hydroxyethyl methacrylate (HEMA) with acrylonitrile (AN) onto polyethylene fibers and subsequent conversion of the produced cyano groups to amidoxime groups by reaction with hydroxylamine. When x/y was varied, the weight ratio of AN/MAA or AN/HEMA in the monomer mixture, cografted polymers were prepared. The value of x/y governed the AO group density and water content of the resultant fibrous adsorbents. As x/y increased, the AO group density of the fibers increased and its water content decreased. The MAA unit provided a higher

Figure 8. Comparison of the uranium adsorption rate between the submerged and flow-through modes as a function of wt % of AN in the monomer mixture.

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water content than the HEMA unit as a neighboring group to the AO groups in the adsorbents. The AO/MAA adsorbents based on the PE fibers prepared by cografting at an x/y of 60/40 and subsequent amidoximation exhibited the highest uranium adsorption rate both in the submerged mode of operation at the ocean site and in the flow-through mode in the laboratory. Literature Cited (1) Katoh, S.; Sugasaka, K.; Sakane, K.; Takai, N.; Takahashi, H.; Umezawa, Y.; Itagaki, K. Preparation of Fibrous Adsorbent Containing Amidoxime Group and Adsorption Property for Uranium. Nippon Kagaki Kaishi 1982, 1449. (2) Katoh, S.; Sugasaka, K.; Sakane, K.; Takai, N.; Takahashi, H.; Umezawa, Y.; Itagaki, K. Enhancement of the Adsorptive Property of Amidoxime-Group-Containing Fiber by Alkaline Treatment. Nippon Kagaki Kaishi 1982, 1455. (3) Kobuke, Y.; Tabushi, I.; Aoki, T.; Kamaishi, T.; Hagiwara, I. Composite Fiber Adsorbent for Rapid Uptake of Uranyl from Seawater. Ind. Eng. Chem. Res. 1988, 27, 1461. (4) Nobukawa, H.; Tamehiro, M.; Kobayashi, M.; Nakagawa, H.; Sakakibara, J.; Takagi, N. Development of Floating TypeExtraction Systerm of Uranium from Seawater Using Seawater Current and Wave Power. 1. J. Shipbuild. Soc. Jpn. 1989, 165, 281. (5) Kato, T.; Kago, T.; Kusakabe, K.; Morooka, S.; Egawa, H. Preparation of Amidoxime Fibers for Recovery of Uranium from Seawater. J. Chem. Eng. Jpn. 1990, 23, 744. (6) Kabay, N.; Katakai, A.; Sugo, T.; Egawa, H. Preparation of Fibrous Adsorbents Containing Amidoxime Groups by RadiationInduced Grafting and Application to Uranium Recovery from Seawater. J. Appl. Polym. Sci. 1993, 49, 599. (7) Kusakabe, K.; Goto, A.; Morooka, S. Recovery of Uranium from Natural Seawater by Moored Adsorption Bed Packed with Fibrous Amidoxime Adsorbent. Nippon Kaisui Gakkaishi 1994, 48, 22. (8) Takagi, N.; Hirotsu, T.; Sakakibara, J.; Katoh, S. Preparation of Bundle-Shaped Fiber Adsorbents Having Amidoxime Groups and Their Ability to Adsorb Uranium from Seawater. Nippon Kaisui Gakkaishi 1997, 51, 133. (9) Takagi, N.; Hirotsu, T.; Sonoda, A.; Sakakibara, J.; Katoh, S. Characteristics of Bundle-Shaped Fiber Adsorbent Containing Amidoxime Groups with Respect to the Repetitive AdsorptionDesorption of Uranium. Nippon Kaisui Gakkaishi 1998, 52, 177.

(10) Katakai, A.; Seko, N.; Kawakami, T.; Sugo, T.; Saito, K. Adsorption of Uranium in Sea Water Using Amidoxime Adsorbents Prepared by Radiation-Induced Cografting. J. Atom. Energy Soc. Jpn. 1998, 40, 878. (11) Katakai, A.; Seko, N.; Kawakami, T.; Sugo, T.; Saito, K. Adsorption Performance in Sea Water of Amidoxime Nonwoven Fabrics Prepared by Radiation-Induced Cografting of Acrylonitrile and Methacrylic Acid. Nippon Kaisui Gakkaishi 1999, 53, 180. (12) Saito, K.; Hori, T.; Furusaki, S.; Sugo, T.; Okamoto, J. Porous Amidoxime-Group-Containing Membrane for the Recovery of Uranium from Seawater. Ind. Eng. Chem. Res. 1987, 26, 1977. (13) 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. (14) Takeda, T.; Saito, K.; Uezu, K.; Furusaki, S.; Sugo, T.; Okamoto, J. Adsorption and Elution in Hollow-Fiber-Packed Bed for Recovery of Uranium from Seawater. Ind. Eng. Chem. Res. 1991, 30, 185. (15) Sekiguchi, K.; Saito, K.; Konishi, S.; Furusaki, S.; Sugo, T.; Nobukawa, H. Effect of Seawater Temperature on Uranium Recovery from Seawater Using Amidoxime Adsorbents. Ind. Eng. Chem. Res. 1994, 33, 662. (16) Egawa, H.; Harada, H.; Shuto, T. Recovery of Uranium from Sea Water by the Use of Chelating Resins Containing Amidoxime Groups. Nippon Kagaku Kaishi 1980, 1773. (17) Astheimer, L.; Schenk, H. J.; Witte, E. G.; Schwochau, K. Development of Sorbents for the Recovery of Uranium from Seawater. 2: The Accumulation of Uranium from Seawater by Resins Containing Amidoxime and Imidoxime Functional Groups. Sep. Sci. Technol. 1983, 18, 307. (18) Hirotsu, T.; Takagi, N.; Kotoh, S. Recovery of Uranium from Seawater. Nippon Kaisui Gakkaishi 1995, 49, 202. (19) Davies, R. V.; Kennedy, J.; Mcilroy, R. V.; Spence, R.; Hill, K. M. Extraction of Uranium from Sea Water. Nature 1964, 203, 1110. (20) Saito, K.; Miyauchi, T. Diffusivities of Uranium in Artrificial Seawater. Kagaku Kogaku Ronbunshyu 1981, 7, 545.

Received for review June 30, 1999 Revised manuscript received April 4, 2000 Accepted April 29, 2000 IE990474A