Effect of Crosslinker Chemical Structure and ...

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Effect of Crosslinker Chemical Structure and Monomer Compositions on Adsorption of Uranium (VI) Ions Based on Reactive Crosslinked Acrylamidoxime Acrylic Acid Resins a

a

Ayman M. Atta , Husein I. Al-Shafy & Ashraf M. Elsaaed a

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Egyptian Petroleum Research Institute, Nasr City, Cairo, Egypt

Available online: 24 Aug 2011

To cite this article: Ayman M. Atta, Husein I. Al-Shafy & Ashraf M. Elsaaed (2011): Effect of Crosslinker Chemical Structure and Monomer Compositions on Adsorption of Uranium (VI) Ions Based on Reactive Crosslinked Acrylamidoxime Acrylic Acid Resins, Journal of Dispersion Science and Technology, 32:9, 1219-1229 To link to this article: http://dx.doi.org/10.1080/01932691.2010.497709

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Journal of Dispersion Science and Technology, 32:1219–1229, 2011 Copyright # Taylor & Francis Group, LLC ISSN: 0193-2691 print=1532-2351 online DOI: 10.1080/01932691.2010.497709

Effect of Crosslinker Chemical Structure and Monomer Compositions on Adsorption of Uranium (VI) Ions Based on Reactive Crosslinked Acrylamidoxime Acrylic Acid Resins Ayman M. Atta, Husein I. Al-Shafy, and Ashraf M. Elsaaed

Downloaded by [ayman atta] at 04:51 25 August 2011

Egyptian Petroleum Research Institute, Nasr City, Cairo, Egypt

Adsorption capacities of uranyl ions using crosslinked acrylamidoxime acrylic acid (Am/AA) resins composed from acrylonitrile (AN), acrylic acid (AA), having different molar ratios were determined. The AN/AA molar ratios were, 80/20,50/50,20/80, crosslinked with 10% N,N0 -methylenebisacrylamide (MBA) and divinylbenzene (DVB). Polymerization occurred in the presence of potassium persulfate and sodium metabisulfite as redox initiator. Acrylonitrile acrylic acid resins (AN/AA) were converted to acrylamidoxime acrylic acid (Am/AA) by reaction with hydroxylamine solution. The chemical structure of the AN/AA and Am/AA resins, before and after loaded with uranyl ions and after elution, was confirmed by FTIR analysis. The effect of UO2þ 2 ions sorption on the morphologies of the prepared resins was examined by scanning electron microscope (SEM). Effect of pH, time of loading, type of acid, molar ratio, and type of crosslinker were investigated. Elution of adsorbed ions on resins was investigated by different eluents. The elution efficiency was determined. Keywords

Acrylic acid, divinylbenzene, N,N-methylenebisacrylamid, polyacrylamidoxime acrylic acid, polyacrylonitrile, uranyl ions

INTRODUCTION Among separation processes such as solvent extraction, ion exchange, precipitation, and others, the adsorption of metals on various inorganic and organic adsorbents has many advantages from the viewpoint of environmental protection.[1–4] During the last two decades, there has an increased interest in the synthesis and analytical applications of chelating ion exchangers. These are widely used for the extraction of trace elements from various materials and also for selective chromatographic separation of several metal ions. Solid phase extraction (SPE) has commonly been used as a technique for preconcentration= separation of various inorganic and organic species.[5–10] Chelating polymer adsorbents have received considerable attention in the separation of metal ions owing to their inherent advantages over simple ion-exchange resins, for example, their greater selectivity to bind heavy metal ions. The selective adsorption of metal ions depends on a small difference in the stability constant of the complex between Received 11 April 2010; accepted 27 April 2010. Address correspondence to Ayman M. Atta, Petroleum Application Department, Egyptian Petroleum Research Institute, Nasr City 11727, Cairo, Egypt. E-mail: [email protected]

a polymer ligand and a metal ion.[11] So far, several selective solid phase sorbents for preconcentration of U (VI) from aqueous solutions have been prepared either by physical loading or chemical binding of selected chelating reagents to different supports including polymeric resins,[12,13] crosslinked polystyrene,[14] and activated carbon.[15] For the purpose of separation of metal ions either for purification or enrichment, various kinds of organic chelating resins have been developed. This has been the subject of numerous papers and review papers related to synthesis, characterization and metal ion uptake studies.[16–20] Polymers with specific functionalities can be obtained by either synthesizing new monomers carrying the functional groups capable of interacting with the target metal ions followed by polymerization or copolymers with suitable reactants into the desired functional groups or by converting groups on existing polymers. In the present work, the main target is based on synthesis of a crosslinked acrylonitrile acrylic acid resin. The nitrile group of resin was converted to amidoxime group to produce acrylamidoxime acrylic acid polymer to extract uranyl ions from ores. N,N0 - methylenebisacrylamide (MBA) and divinylbenzene (DVB) were used as crosslinkers to study the effect of crosslinker types on uranyl ions adsorption.

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EXPERIMENTAL


Materials Acrylonitrile and acrylic acid were obtained from Fluka, Japan. Divinylbenzene (DVB, 55%) was obtained from Sigma-Aldrich (UK). N,N-methylenebisacrylamide (MBA) was obtained from Fluka, Switzerland. Potassium persulfate and sodium metabisulfite were obtained from Adwec Co. (Egypt). Source of Uranium used was uranyl nitrate Sigma-Aldrich. All other chemicals were Prolabo products and were used as received. Preparation of Crosslinked Acrylonitrile Acrylic Acid Polymerization and crosslinking of acrylonitrile and acrylic acid with DVB and MBA as crosslinking agents with different mol ratios of monomers were carried out in three necked flask at 40 C under nitrogen atmosphere in the presence of water as a solvent. Potassium persulfate and sodium metabisulfite, 0.0576 and 0.0557 wt% with respect to monomers feed, were used as redox initiators. The water to monomer ratios kept as 10. Poly (vinyl alcohol) was added (730 ml to 100 gm monomers) as surfactant. The AN=AA monomers with ratios 80=20, 50=50, and 20= 80 were added slowly to the reaction mixture in 25–30 minutes and polymerization was continued for 90 minutes with good agitation. The polymer was isolated by filtration washed with water and methanol and dried under vacuum at 50 C. Molar ratio of AN=AA, 20=80, was excluded from other steps due to formation of gel. Synthesis of Acrylamidoxime/Acrylic Acid Resin (Am/AA) Preparation of Hydroxylamine Hydroxylamine hydrochloride (40.1 g) was dissolved in 290 ml of methanolic solution (methanol: water 5: 1). The hydroxyl amine was neutralized by sodium hydroxide solution till pH 10 and the precipitated NaCl was removed by filtration. Preparation of Acrylamidoxime/Acrylic Acid (Am/AA) Resin The prepared hydroxyl amine solution was added to 30 gm of each 80=20, 50=50 acrylonitrile acrylic acid resins. The reaction carried out at 70 C for two hours. The prepared acrylamidoxime=acrylic acid (DVB,MBA) resin was separated by filtration and washed several time by methanolic solution then treated with 0.1 N HCl solution for at least 5 minutes. The resin was filtered and washed several times by methanolic solution and dried at 50 C to constant weight. Characterization FTIR spectra of hydrogels loaded with uranyl ions were recorded using JASCO 460 plus FTIR spectrometer.

Scanning electron microscopy (SEM) was used to study the morphological properties of the crosslinked copolymer before and after adsorption of uranium. Specimens were coated with gold in SEM coating equipment and magnified to 1500 for 10.00 mm and scanning electron micrographs were taken with a JEOL–JSM–5400 scanning microscope. Uptake of Uranyl Ions Using Batch Adsorption Method A simple and sensitive spectrophotometric method based on colored complexes with Arsenazo III in aqueous medium was used for determination of uranium.[21] The concentration of UO2þ 2 ions in the solution was determined with a Thermo3000 ultraviolet-visible (UV-visible) spectrophotometer by measuring absorbance at kmax of 650 nm for uranium. All glassware for adsorption experiments was washed with 1.0 M HNO3 and rinsed thoroughly with deionized water. A 0.1 g of dry resin gel was placed in a flask containing 100 ml of uranyl solution at a temperature of 25 C. The contents of the flask were shacked at 1000 rpm for various time intervals. After the equilibration time, 5 ml of the solution were taken. The initial pH of the working solutions was 4.0., when the adsorption equilibrium was reached, it was filtrated to separate hydrogel and the solution. The concentration of the free uranium (VI) ions in the filtrate was determined. The data obtained in batch studies were used to calculate the equilibrium metal uptake capacity for each sample of uranium(VI) by using the following expression: q ¼ (C0-Ce). V=W, q is the amount of UOþ2 2 adsorbed onto unit mass of the copolymer (mg=g); Co and Ce are the concentrations of the uranium in the initial solution and in the aqueous phase after treatment for certain period of time, respectively (mg=l); V is the volume of the aqueous phase (l); and m is the mass of the copolymer used (g). Effect of pH on the Uptake of Uranyl Ions Uptake experiments under controlled pH were carried out by placing 0.1 g of dry gel in uranyl ion solutions 100 (mg=l) having different pH ranges of 2 to 8 at 25 C. The desired pH was adjusted using HCl, HNO3 and H2SO4, and NH4OH. The residual metal ion concentration determined after the resin was reached the equilibrium as represented in the previous section. Effect of Initial Concentration of Uranyl Ions on the Uptake The effect of initial concentration of the metal ion on the resin uptake was carried out by placing 0.1 g of dry resin in different aqueous uranyl ion concentrations flask at 25 C. The residual metal ion uptake was determined as described in the previous section.

CROSSLINKER CHEMICAL STRUCTURE AND MONOMER COMPOSITIONS

Polymer Regeneration Elution of UOþ2 2 ions from Am=AA resins was studied by using three types of eluents such as 4 M HCl and 2 M HNO3 solutions, used as acidic eluents, whereas 1 M Na2CO3 was used as basic eluent. The Am=AA gels loaded with UOþ2 2 were placed in this elution medium and stirred (at a stirring rate of 100 rpm) for 24 hours at room temperature. The final concentration of UOþ2 2 in the aqueous phase was determined spectrophotometrically. The elution ratio was calculated from the amount of UOþ2 2 adsorbed on the hydrogels and the final concentration of UOþ2 2 in the elution medium, by using the following expression:


Efficiency of regeneration ð%Þ total adsorption capacity in the second run 100 ¼ total adsorption capacity in the first run:

RESULTS AND DISCUSSION AN=AA copolymers could be crosslinked with DVB and MBA due to the presence of –C=C groups of DVB and MBA. The crosslinking of mechanism AN=AA with DVB and MBA was illustrated in Scheme 1. It was previously reported[22,23] that the conversion of the copolymerization of AN was increased with the addition of the vinyl acid comonomer. This is probably because the polymer radical with an acid unit at the chain end is considerably more active than is the polyacrylonitrile (PAN) growing radical, which means that the addition of either of the monomers will be more rapid than in the case of a radical terminating in an AN unit.[22] On the other hand, the distribution of monomeric units is random and in no case is the homopolymer formation expected. It was determined that the reactivity of monomers in a system depends on the method of polymerization and the medium and temperature of the polymers. In this respect, r1 and r2 were

SCH. 1. crosslinkers.

Crosslinking of An=AA copolymer with a) DVB and b) MBA

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found to be 0.34 and 3.25 in aqueous suspension polymerization of AN and AA, respectively. These values, rl and r2, were found to be 0.495 and 2.502 in solution DMF. This means that the produced AN=AA copolymer forms random copolymer having high AA content. Crosslinked copolymers were produced in the presence of divinylbenzene or methylenebisacrylamid as crosslinking agent. Potassium persulfate and sodium metabisulfite) was used as redox initiator. Poly (vinyl alcohol) was added as suspending agent. The mechanism of copolymerization can be illustrated as previously reported.[24,25] When AN=AA copolymer was synthesized with the solvent water suspension technique, oligomeric radicals may have formed in the initial stages of polymerization, precipitated out after a certain critical molecular weight was attained, and then acted as primary particles. Propagation then occurred in the water phase depending on the contents of monomers in feed. In the present system because of the insolubility oligomeric radicals, propagation followed the suspension polymerization technique more. A two-locus polymerization mechanism was assumed (i.e., water phase and oligomeric radicals phase). Propagation then mostly occurred in the oligomeric radicals phase. AN units were more easily absorbed by polymer radicals than by AA units. The solubility of AA was greater than that of AN in water. The impacting opportunities between AA units and polymer radicals rose, and this led to a random array of AA units in the copolymer chain. DVB and MBA were used to study the effect of crosslinker chemical structure on crosslinking reaction of AN=AA copolymers. Amidoximation reaction of crosslinked AN=AA copolymeric hydrogels was completed using hydroxyl amine as described in the experimental section. The chemical structure of the produced polymers was confirmed by FTIR analysis. In this respect, FTIR spectra of AN=AA and Am=AA (50=50) crosslinked with 10% of either DVB or MBA were selected and represented in Figures 1a–1d. The spectra of crosslinked AN=AA with either DVB or MBA show the same bands at 1735.8 cm1 for C=O streatching band of carboxylic acid, 2244.4 cm1 for CN streatching band, 3567.5 cm1 for OH streatching band of carboxylic group. New band at 1673 cm1 was observed in Figure 1b for C=O stretching amid of MBA. New bands at 1654.7 cm1 C=N stretching of amidoxime and 3369.6 cm1 NH, OH stretching of amidoxime of Am= AA copolymers Figures 1c and 1d. It was previously reported that,[26] the increase in AN content of gel causes a decrease in both percentage swelling and efficiency of amidoximation conversion. In the present study it was observed that the band 2244.4 cm1 for CN streatching band was completely disappeared upon amidoximation of CN with hydroxyl amine. This band was completely disappeared for copolymers crosslinked with 10 wt% of either DVB or MBA. It was observed that the amidoxime group


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A. M. ATTA ET AL.

FIG. 1. FTIR spectra of a) AN=AA=DVB, b) Am=AA=DVB, c) AN= AA=MBA, and d) Am=AA=MBA resins of molar ratio (50=50). (Color image available online.)

content was increased for Am=AA crosslinked with DVB. This can be attributed to higher reactivity of DVB with AA or AN polymers and copolymers than MBA crosslinker. This indicated that the suspension crosslinking and copolymerization of AN=AA is more effective with DVB than MBA crosslinker. This can be attributed to the difference in the solubility between DVB and MBA crosslinkers in water solvent. DVB units were more easily absorbed by polymer radicals than by MBA units. The solubility of MBA was greater than that of DVB in water. The impacting opportunities between MBA units and polymer radicals rose, and this led to a random array of MBA units in the copolymer chain which increased with increasing of MBA

content. On the other hand, the formation of uniform crosslinked AN=AA=DVB increases the probability to conversion of CN to amidoxime than random AN=AA=MBA resins. The present work deals with synthesis of crosslinked AN=AA network with porous network through suspension polymerization. The suspension polymerization is based on polymerization of two immiscible liquid ikn the presence of suspensing agent such as poly (vinyl alcohol). It is now well understood that a phase separation during the network formation process is mainly responsible for the formation of porous structure in dried state. In order to obtain macroporous structure, a phase separation must occur during the course of the crosslinking process so that the two-phase structure is fixed by the formation of additional crosslinks. Depending on the synthesis parameters, phase separation takes place on a macroscale or on a microscale. In the first case, when the networks start to form crosslinked structure, the network collapses at the critical point for phase separation and becomes a microsphere. The separated liquid phase remains as continuous phase in the reaction system. As the reaction proceeds, new microspheres are continuously generated due to the successive separation of the growing polymers. Agglomeration of microspheres leads to formation of a macroporous network consisting of two continuous phases. In the second state, phase separation results in the formation of a dispersion in the reaction system. Thus, the liquid phase during the gel formation process separates in the form of the small droplets inside the gel and become discontinuous. Due to slowness of the volume change of the gel sample, the initiator of the sample is initially under constant volume condition; further polymerization and crosslinking reactions fix the two phase structure in the final material. In the present system, two crosslinkers DVB and MBA were used to prepare porous crosslinked Am=AA networks. In this respect, the morphology of the prepared networks can be examined with a scanning electron microscope (SEM). The SEM micrographs were selected for crosslinked AN=AA (80= 20 mol %), (50=50) crosslinked with 10 wt% of MBA and were represented in Figures 2a–2c, to study the effect of copolymer composition and type of crosslinker on the porosity of the networks. Indeed, SEM shown in Figure 2 illustrates the development of the heterogeneity in the networks depending on the crosslinker type and AN content. The network consists of large polymer domains; the discontinuities between the domains are also large. If the crosslinker DVB, the morphology changes drastically and a structure consisting of aggregates of spherical domains appears. As the AA content further increases from 20 to 50 mol %, the morphology changes from a structure of large aggregates of poorly defined microspheres to one consisting of aggregates of well-defined microspheres. The structure looks like cauliflowers, typical for a macroporous


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called microspheres of about 0.1–0.5 mm in diameter. The microsphere are nonporous and constitute the highly crosslinked region of the network. The agglomeration of the microspheres during crosslinking polymerization through their peripheral pendant vinyl groups and radical ends leads to the formation of large, unshaped, discrete agglomerates of 10–100 mm in diameter, which are further agglomerated to form the final network. Macropores constitute the interstices between the microspheres while the voids between the agglomerates build the large pores in Am=AA network. SEM photograph of the crosslinked Am=AA=MBA (50=50) (Figure 2b) shows skin layer structure and possess homogenous wall-like texture. The formation of homogenous wall-like texture indicates that the crosslinked polymers cannot form porous structure.

FIG. 2. SEM micrograph of Am=AA having different mol ratios of a) 80=20=MBA, b) 50=50=MBA, and c) 50=50=DVB resins. (Color image available online.)

copolymer network. The SEM photographs of Am=AA crosslinked with MBA is different than that crosslinked with DVB. SEM photographs (Figure 2), indicate that large pores were formed with increasing AN content in copolymer when crosslinked with DVB crosslinker. This can be attributed to the high affinity of copolymer having high content of AN towards crosslinking. Accordingly, we can propose the following scheme for the formation of heterogeneous and microsphere AN=AA networks. When the polymerization is initiated by the decomposition of initiator molecules, the primary radicals formed start to grow by adding the monomers of AN and AA and the crosslinkers either DVB or MBA. Initially, the primary molecules contain AN=AA, AN, and AA units, DVB or MBA units with two unreacted vinyl or acrylate (i.e., with pendant vinyl groups) and MBA or DVB units involved in cycles. As the time goes on, more and more primary molecules are formed so that the intermolecular crosslinking reactions among the primary molecules may also occur during the polymerization. Thus, the cyclization clearly dominates over the intermolecular crosslinking reactions. Since every cycle reduces the coil dimension of the molecule as well as the solvent content inside the coil, the structure of the formed polymers is rather compact and can be considered as clusters. The higher the crosslinker content, the higher is the cyclization density of the clusters. When the cyclization density of the clusters exceeds a critical value, they phase separate and form primary particles

Adsorption of UOþ2 2 Ions by Crosslinked Am/AA Copolymeric Resins The Am=AA resins possesses amidoxime and caboxylic chelating properties. The proposed sites of chelation of uranyl ions or any other ions adsorbed on are OH of amidoxime, carboxylic or NH2 of amidoxime. The proposed chemical structure of uranyl ions adsorption on Am=AA resins is represented in Scheme 2. It was reported that the adsorption of uranyl ions can be obtained on amidoxime and carboxylic groups or amidoxime or carboxylic groups.[30,31] The crosslinked copolymers amidoximated under optimal conditions (Am=AN) were left in the uranyl ion solutions, until they reach their maximum adsorption equilibrium. Post-determination of uranyl ion concentrations was carried out as mentioned in the experimental section. Accordingly, due to the incompletion of conversion of all nitrile groups to amidoxime groups, the active sites which is able to interact with UO2þ 2 ions becomes relatively

SCH. 2. Adsorption of uranyl ions on crosslinked Am=AA resins. (Color image available online.)


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A. M. ATTA ET AL.

low causing a subsequent reduction in the adsorption capacities. FTIR was ocurred on loaded Am=AA copolymers with UO2þ 2 ion also to resin after elution with strong acid media as 4 N HCl to elucidate that strong acid not affect on chemical structure of resins used. The FTIR spectra of loaded and eluted Am=AA (50:50) copolymers were selected and represented in Figures 3a–3c. Bands at 1651.1 cm1 streatching C=N produced of amidoxime, 3386.9 cm1 streatching NH group of amidoxime and that of carboxylic acid were observed for loaded Am=AA resins. Due to the interactions with UO2þ 2 ions, there are some shifts on some characteristic bands. The band for C==N at 1600 cm1 shifts approximately 60 cm1. This difference can be better seen in the difference spectrum given in Figure 3a. The increases in the band intensities are due to O=U=O bands, which appear at the same regions with the functional groups.[27] There are also band shifts at 3400 cm1 for the interaction of NH2 with UO2þ ions. 2 The shift at 1500 cm1 belongs to C==N stretching vibrations for amidoxime also proves the interaction of UO2þ 2

ions with COOH group. The most important band, which is linear O=U=O stretching vibration, appears at 937 cm1.[29] Figure 3 also shows that the bands due to COOH, amide carbonyl and hydroxyl groups decrease in transmittance after the chelation of uranyl ions. This is attributed to the decrease in the dipole moment of COOH, amide, and hydroxyl groups as a result of electron donation on the metal ion from the nitrogen and oxygen atoms. In the present system, there are many type of suggested interaction between (amidoxime=carboxylic group) and urany ions that indicated in below schemes. The SEM micrographs of the Am=AA resins loaded with UO2þ 2 ions were illustrated in Figures 4. The SEM micrographs of the Am=AA resins after absorbing UO2þ 2 ions, showed a different morphology in which the pores are not observed. It seems that the introduction of metal ions inside the matrix causes a large disruption to the surface.

FIG. 3. FTIR spectra of crosslinked Am=AA resins crossinked with a)

FIG. 4. SEM micrographs of crosslinked Am=AA resins loaded with uranyl ions having mol ratios a) 80=20=DVB and b) 50=50=DVB. (Color image available online.)

DVB loaded with uranyl ions, b) MBA and loaded Withuranyl ions, and c) MBA eluted with 4 M HCl.

Effect of Copolymer Composition on UO2þ 2 Ions Uptake Polymeric materials having polyfunctional groups not only possess good hydrophilic properties, but also have good ion exchange properties which make them suitable for metal recovery from aqueous solutions. The water uptake and, therefore, the metal-ion uptake increase in proportion to the kind and amount of hydrophilic groups because the diffusion of aqueous solutions in more hydrophilic polymers will be faster than in less hydrophilic polymers, which is the rate-determining step for adsorption.[30] Figure 5 shows the variation of UO2þ 2 ions uptake with AA percentage in the hydrogels. It is clear that the UO2þ 2 ions uptake decrease with the decrease of AA percentage of the copolymer, this indicates that the AA comonomer in the hydrogel structure is primarily responsible for the specific binding of the UO2þ 2 ions due to the coordination between UO2þ ions and the carboxylic acid groups. AA rich compo2 sitions possess high metal uptake that it possess high degree of swelling at UO2þ 2 ions feed solutions, which assist the

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diffusion of the metal ions inside the hydrogel, that is, to reach to the chelating functional groups of the hydrogel. As the AA content in the hydrogel increases, the swelling ability of the hydrogel increases. The metal ion uptakes for the various samples are in the order of their hydrophilic character. Higher water uptake of the sorbent shows higher metal ion uptake. This behavior was reported for crosslinked polyacrylamide-hydroxamic acid sorbents[31] and for polyacrylamide-2-Hydroxypropyl methacrylate (HPMA) copolymers.[32] Yetimoglu et al. reported that for AMPS=AA=NVP based hydrogels, the highest water absorption and metal ion uptake was achieved by using the hydrogel that posse‘s highest AMPS content.[33] Rivas et al. reported that the uranyl ions retention of crosslinked poly (1-vinyl imidazole-co-2-acrylamido-2-methyl-1-propanesulfonic acid) is greater than that of (1-vinyl imidazole-co acrylic acid) at the same pH.[34]

FIG. 5. Uranyl ions adsorption of crosslinked Am=AA resins at different times in aqueous solutions of a) nitric acid, b) hydrochloric acid, and c) sulfuric acid at pH ¼ 6 and temperature 25 C.

Effect of Crosslinker Type on UO2þ 2 Ions Uptake Tables 1–3 show the variation of UO2þ 2 ions uptake with crosslinker percentage and type in the hydrogels at pH 6. It is clear that the UO2þ ions uptake decrease with the 2 increment of crosslinker percentage of the copolymer, this indicates that the crosslinker contents in the hydrogel structure are primarily responsible for the specific binding 2þ of the UO2þ 2 ions due to the coordination between UO2 ions and the active functional groups of the networks. It was found that crosslinker rich compositions did not possess high metal uptake that it possess low degree of swelling

TABLE 1 Adsorption of UO2þ on Am=AA resins in nitric acid media as function of time and pH 2 Uranium concentration (mg=g) on Am=AA resin from nitric acid solution at different times (hours) Resins

Mol ratios

Am=AA=MBA

80=20

Am=AA=DVB

80=20

Am=AA=MBA

50=50

Am=AA=DVB

50=50

pH pH pH pH pH pH pH pH pH pH pH pH pH pH pH pH pH

2 4 6 8 2 4 6 8 2 4 6 8 2 4 6 8

1 hours

2 hours

3 hours

4 hours

5 hours

6 hours

20 hours

19 37 69 77 18 38 75 79 19 47 85 77 19 49 90 82

28 64 125 110 28 65 121 110 30 78 135 113 33 72 130 119

51 95 151 181 52 100 153 189 57 121 160 191 62 120 162 201

74 134 190 231 74 139 195 242 104 170 201 281 115 164 211 289

93 144 229 310 95 153 233 331 127 218 251 370 133 211 281 378

115 172 275 401 121 190 280 413 144 253 289 465 160 261 329 465

135 209 307 431 136 213 317 461 172 281 319 482 180 291 361 496

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TABLE 2 Adsorption of UO2þ on Am=AA resins in hydrochloric acid media as function of time and pH 2


Uranium concentration (mg=g) on Am=AA resin from hydrochloric acid solution at different times (hours) Resins

Ratios

Am=AA=MBA

80=20

Am=AA=DVB

80=20

Am=AA=MBA

50=50

Am=AA=DVB

50=50


2 4 6 8 2 4 6 8 2 4 6 8 2 4 6 8

1 hours

2 hours

3 hours

4 hours

5 hours

6 hours

20 hours

14 20 29 50 14 21 24 53 15 22 30 57 18 22 31 61

20 38 67 101 22 41 78 101 23 47 75 110 25 51 81 113

27 49 96 143 30 52 101 165 32 52 111 190 34 66 119 199

35 71 133 191 39 78 141 204 39 78 155 278 40 83 157 281

40 91 155 257 46 103 160 329 51 105 171 345 54 145 177 346

50 100 185 316 55 131 188 403 59 122 190 410 71 157 203 416

61 130 207 398 66 144 210 419 76 140 211 448 88 160 229 457

at UO2þ 2 ions feed solutions, which prevent the diffusion of the metal ions inside the hydrogel. As the crosslinker content decreases, the swelling ability of the hydrogel increases. The metal ion uptakes for the various samples are in the

order of their hydrophilic character. Higher water uptake of the sorbent shows higher metal ion uptake. It was expected that the hydrophilicity of the MBA crosslinker can increase the UO2þ 2 ions uptakes due to hydrophilicity

TABLE 3 Adsorption of UO2þ 2 on Am=AA resins in sulfuric acid media as function of time and pH Uranium concentration (mg=g) on Am=AA resin from sulfuric acid solution at different times (hours) Resins

Mol ratios

Am=AA=MBA

80=20

Am=AA=DVB

80=20

Am=AA=MBA

50=50

Am=AA=DVB

50=50


2 4 6 8 2 4 6 8 2 4 6 8 2 4 6 8

1 hours

2 hours

3 hours

4 hours

5 hours

6 hours

20 hours

13 18 25 40 15 19 24 45 16 20 28 44 17 21 28 51

21 25 40 83 21 25 47 95 21 27 50 99 23 31 53 103

24 35 86 119 24 35 98 155 24 44 103 160 28 49 111 165

29 50 130 169 33 53 130 193 33 55 140 205 34 63 155 201

35 80 141 227 45 88 151 290 46 99 162 291 49 102 167 297

50 91 182 305 52 93 183 381 59 113 185 382 62 131 199 388

59 113 199 339 60 121 203 410 66 125 199 401 73 150 209 413



of MBA. It was also observed that the incorporation of DVB crosslinker instead of MBA in the network structure increases the UO2þ 2 ions uptakes. This can be attributed to the formation of homogeneous networks when DVB used as crosslinker in suspension copolymerization of AA and AN copolymer as illustrate in the previous section. Effect of contact time on the adsorption of UO2þ 2 ions at pH ¼ 6 for Am=AA crosslinked with MBA and DVB hydrogels is illustrated in Figure 5. The percentage of UO2þ ions uptake was observed to increase with time 2 and this trend was observed for all the hydrogels. The plots showed that kinetics of adsorption of UO2þ 2 ions consisted of two phases: an initial rapid phase where adsorption was fast and contributed significantly to equilibrium uptake, and a slower second phase whose contribution to the total metal adsorption was relatively small. The rapid uptake of UO2þ may indicate that most of the active sites of the 2 hydrogels are exposed for interaction with the metal ions.[35] The first phase is interpreted to be the instantaneous adsorption stage or external surface adsorption. The second phase is interpreted to be the gradual adsorption stage where intraparticle diffusion controls the adsorption rate until finally the metal uptake reaches equilibrium. After 360 minutes (6 hours), the change of adsorption capacities for UO2þ 2 ions did not show notable effects. As a consequence, 360 minutes was chosen as the reaction time required to reach pseudo-equilibrium in the present ‘‘equilibrium’’ adsorption experiments. Reported experimental data on sorption kinetics of uranium on polymeric adsorbents have shown a wide range of values. UO2þ 2

Effect of pH on Ions Uptake It is well known that the initial pH value of the solution is a critical parameter that can affect the hydrogel performance by influencing its swelling and ion uptake capability. The pH has two kinds of influence on metal sorption: an effect on the solubility and speciation of metal ions in solution, and on the overall charge of the sorbent. For selective adsorption, besides the use of a specific ligand modified sorbent, selectivity could be achieved by adjusting the pH of the medium to different values.[36] The pH dependence of adsorption values of UO2þ was represented in 2 Figure 6; it is obvious that the adsorption of UO2þ 2 onto the Am=AA hydrogels is pH dependent. The results show that uranium adsorption by the hydrogels is low at pH 3.0, but increased with increasing pH and then reached the maximum at pH 8.0. Uptake of UOþ2 2 by hydrogels at neutral or slightly acidic conditions may be explained to proceed via complex formation between the metal ions and the active sites on the resins.[37,38] As the pH decreases, the active sites become protonated and their ability for interaction with UO2þ decreases because at lower pH, 2 hydrogen ions may be competing with the metal ions.

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FIG. 6. Uranyl ions adsorption of crosslinked Am=AA resins at different pH in aqueous solutions of a) nitric acid, b) hydrochloric acid, and c) sulfuric acid at temperature of 25 C. Many authors reported various species of uranium at different pH values,[39,40] among these species, UO2þ 2 , þ2 UO2(OH)þ, UO2(OH)2(aq), UO2 ðOHÞ , ðUO Þ ðOHÞ 2 3 2 , 2 þ þ were ðUO2 Þ3 ðOHÞ5 , ðUO2 Þ3 ðOHÞ7 , ðUO2 Þ4 ðOHÞ7 þ detected. At pH 5, UOþ2 2 and UO2(OH) predominate and are responsible for uranium uptake by hydrogels.[41] Different researchers indicated complex formation between UO2þ and=or UO2(OH)þ with different functional 2 [42,43] The carboxylic, amidoxime, and amide groups groups. are active sites in the Am=AA hydrogels which may participate in chelation. Decreasing the acidity of the medium causes the active sites to become protonated leading to the depression in the uptake capacity due to the electrostatic repulsion with the positively charged uranyl species. It is well known in adsorption mechanisms, that a decrease in solubility favors an improvement in adsorption performance. The further increase of pH was followed by a decrease in the uptake of uranyl ions, because there is a decrease in dissolved uranyl ion concentration at higher pH due to the formation of solid schoepite (4UO2 9H2O) reducing uranium adsorption. In mildly acidic and neutral pHs 6.0–8.0, Am=AA hydrogels are effective for removal of UO2þ from aqueous solutions. It was also 2

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A. M. ATTA ET AL.

TABLE 4 Regeneration efficiencies of Am=AA resins after 6 cycles in 4 M HCl Resin information

Efficiency in each cycle (%)

Resins

Mol ratios

2nd

3rd

4th

5th

6th

Am=AA=MBA Am=AA=DVB Am=AA=MBA Am=AA=DVB

80=20=10% 80=20=10% 50=50=10% 50=50:=10%

98 98.3 98.5 99.5

97 97 98 98.2

95.5 96 96.5 97

95 95 96.1 96.1

94 94.5 95.5 95


observed that the uranyl ions uptake for Am=AA crosslinked with DVB in most cases is higher than that crosslinked with MBA crosslinker. This can be attributed to the porosity and homogeneity of Am=AA=DVB networks. Regeneration of Hydrogels To repeatedly reuse the hydrogels for the recovery of uranium, uranium adsorbed on the resin must be easily eluted with a certain kind of eluents. Regeneration calculated after 6 cycles for all crosslinked Am=AA polymers. The data of regeneration of Am=AA in 4 M HCl were selected and represented in Table 4. It was determined that the regeneration efficiency was increased in case of highly crossed resin than that of lower crosslinked resin. The elution was investigated by batch method using 2 M HNO3, 4 M HCl, and 1 M Na2CO3, respectively. UOþ2 2 ions adsorbed on the hydrogels show a higher elution in acidic media than in basic media. UOþ2 2 ions adsorbed on the hydrogels were eluted close to 80% by nitric acid, 90% by hydrochloric acid and 70% by sodium carbonate.

CONCLUSIONS The following conclusions can be extracted from the previous results: . AN=AA copolymer was crosslinked with MBA

and DVB using suspension copolymerization technique. . Adsorption of uranyl ions was increased with increment of AA content in Am=AA copolymers crosslinked either MBA or DVB. . The uranyl ion adsorptions of Copolymers show pH sensitivity. As pH increase the uptake of uranyl ions increase due to increasing of chelating properties of amidoxime and carboxylic acid. . Uranyl ions uptake of copolymers was increased in nitric acid solution higher than hydrochloric acid solution higher than sulfuric acid aqueous solutions.

. Uranyl ions uptake was increased for copolymers

crosslinked with DVB crosslinker r than that crosslinked with MBA. . Elution of adsorbed uranyl ions was increased in case of 4 M HCl > 1 M 1 M H2SO4 > 1 M Na2CO3. . The chemical structure of eluted resins was not affected by strong acid eluent as 4 N HCl. REFERENCES [1] El-Naggar, I.M., El-Absy, M.A., Abdel-Hamid, M.M., and Aly, H.F. (1993) Solvent Extr. Ion Exch., 11: 521. [2] Abbasi, W.A. and Streat, M. (1994) Sep. Sci. Technol., 29: 1217. [3] Egawa, H., Nonaka, T., and Nakayama, M. (1990) Bull. Soc. Sea Water Sci. Jpn., 44: 316. [4] Ghafourian, H., Latifi, A.M., and Malekzadeh, F. (1998) Sci. Bull. At. Energy Organizat. Iran, 17: 44. [5] Mahmoud, M.E., Osman, M.M., and Amer, M.E. (1992) Anal Chim Acta., 415: 33. [6] Pyell, U., Stork, G., and Fresenius, J. (1992) Anal Chem, 343: 576. [7] Thuman, E.M. and Mills, M.S. (1998) Solid Phase Extraction Principles Practice; New York: John Wiley. [8] Caroli, S., Alimonti, A., Petrucci, F., and Horvath, Z. (1991) Anal Chim Acta, 248: 241. [9] World Health Organization. (1998) WHO Guideline forDrinking Water Quality: Health Criteria and Other Supporting Information; vol. 2, 2nd ed.; Geneva, Switzerland: WHO. [10] Rao, T.P., Metilda, P., and Gladias, J.M. (2006) Talanta, 68: 1047. [11] Kaneko, M. and Tsuchida, E. (1981) J. Polym. Sci., 16: 397. [12] Metilda, P., Sanghamitra, K., Gladis, J.M., Naidu, G.R., and Prasada, R.T. (2005) Talanta, 65: 192. [13] Jain, V.K., Handa, A., Sait, S., Shrivastav, P., and Agrawal, Y.K. (2001) Anal Chim Acta., 429: 237. [14] Lee, C.H., Kim, J.S., Suh, M.Y., and Lee, W. (1997) Anal. Chim. Acta., 339: 303. [15] Starvin, A.M. and Prasada, R.T. (2004) Talanta, 63: 225. [16] Kabay, N. and Egawa, H. (1993) Sep. Sci. Technol., 28: 1985. [17] Hancock, R.D. and Martell, A.E. (1989) Chem. Rev., 89: 1875. [18] Kantipuly, C., Kantragadda, S., Chow, A., and Gesser, H.D. (1990) Talanta, 37: 491.


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