Adsorptive Preconcentration of Uranium in Hydrogels from Seawater

Jun 8, 2009 - The change in binding of U(VI) in the AO-hydrogel may influence the kinetics ..... This material is available free of charge via the Int...
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Ind. Eng. Chem. Res. 2009, 48, 6789–6796

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Adsorptive Preconcentration of Uranium in Hydrogels from Seawater and Aqueous Solutions Sadananda Das,†,‡ Ashok K. Pandey,*,† T. Vasudevan,§ Anjali A. Athawale,‡ and Vijay K. Manchanda† Radiochemistry DiVision, BARC, Mumbai-400 085, India, Department of Chemistry, UniVersity of Pune, Pune-411 007, India, Research Reactor SerVices DiVision, BARC, Mumbai-400 085, India

Cross-linked hydrogels containing poly(ethylene glycol methacrylate phosphate) (EGMP), poly(2-acrylamido2-methyl-1-propane sulfonate) (AMPS), poly(acrylamidoxime) (AO), and AO along with different acidic (acrylic acid (AA), methacrylic acid (MAA), AMPS, and EGMP) and basic (3-(acrylamido propyl) trimethylammonium chloride (APTAC)) comonomers were prepared by UV-initiated bulk polymerization. These hydrogels were characterized in terms of their chemical structure and U(VI)-uptake behavior from aqueous and seawater samples. Except AMPS hydrogel, all these hydrogels were found to sorb U(VI) quantitatively (≈90%) from the seawater. The U(VI)-sorption kinetics in EGMP and AO+MAA (60:40) hydrogels were found to be faster than other hydrogels under seawater conditions. The presence of a strong acid (-SO3H) comonomer with AO retards the overall kinetics involve in the U(VI) sorption from seawater. It was also observed that the U(VI) sorption kinetics was highly dependent on the composition of the acidic and AO groups in the hydrogel. The uptake of U(VI) from acidic solution in AO+MAA hydrogel was found to be dependent on the acidity of the feed solution, and there was no uptake of U(VI) in this hydrogel from solution containing 1 mol L-1 HNO3. Contrary to this, EGMP hydrogel could sorb the U(VI) from solution containing high concentration of HNO3. This indicated that EGMP hydrogel can also be used for U(VI) preconcentration from the nuclear waste. U(VI) complexed in EGMP hydrogel and AO+MAA could be desorbed by their equilibration with 0.5 mol L-1 Na2CO3 and 1 mol L-1 HCl/HNO3, respectively. The mechanism of sorption of U(VI) in the hydrogels was studied to understand the factors that control the U(VI) sorption from seawater. Introduction Uranium(VI) concentration in seawater is of the order of 1.4 × 10-8 mol L-1 (3.3 mg m-3), and is found to be present principally as the anionic uranyl tricarbonato species [UO2(CO3)3]4-.1,2 [UO2(CO3)3]4- has a high stability under the prevailing seawater conditions.3,4 As seawater is a lean source of uranium, it is not realistic to process large volumes of seawater in a commercial scale plant. The separation process must be able to operate in situ without any manipulation of chemical conditions of seawater.5,6 These factors limit the choice of functional groups for uranium recovery from seawater.1,2 The poly(acrylamidoxime) (AO) has been found to be chemically viable for uranium recovery from seawater.7-13 However, it is desirable that the uranium species present in seawater should be instantaneously sorbed when it comes in contact with the surface of sorbent. The fast kinetics of uranium sorption is important not only to reduce the sorbent inventory but also to avoid the prolong exposure of sorbent in seawater leading to biofouling. Therefore, the major challenge for making uranium recovery economically viable is to develop a sorbent that has very high uranium sorption rate. The physical and chemical structures of the AO-sorbents play the important role in sorption kinetics of U(VI) from seawater. The diffusion mobility of U(VI), either in [UO2(CO3)3]4- form or UO22+ form, in the sorbent would be dependent on the physical as well as chemical interactions (electrostatic and * To whom correspondence should be addressed. E-mail: ashokk@ barc.gov.in. Fax: +91-22-25505150/25505151. Tel.: +91-22-25590641. † Radiochemistry Division, BARC. ‡ University of Pune. § Research Reactor Services Division, BARC.

covalent) with fixed-sites in the matrix. These interactions retard the mobility of U(VI) species considerably in the matrix of sorbent. Sorption of U(VI) from seawater in the AO-sorbent has been found to be highly dependent on the physical parameters of the sorbent matrix (free volume, pore structure, tortuosity, etc.) as well as its hydrophilicity.14-19 The chemical composition of the sorbent affects the decomplexation of [UO2(CO3)3]4- to UO22+, followed by complexation of UO22+ with functional groups (fixed-sites). The decomplexation of [UO2(CO3)3]4- at the fixed-sites accessible at surface of sorbent may be the rate determining step in U(VI) sorption from seawater. Since decomplexation of [UO2(CO3)3]4- can be catalyzed by H+-ions, the presence of an acidic comonomer with appropriate pKa value may enhance the sorption kinetics of U(VI) from seawater in the AO-sorbent.16-19 The decomplexation of [UO2(CO3)3]4- at the surface of AO-sorbent is also expected to enhance the U(VI) sorption kinetics, as [UO2(CO3)3]4- diffusion would be quite slow in the sorbent matrix as compared to UO22+ ions. Zhang et al. have studied the mechanism and kinetics of U(VI)-sorption in a fibrous amidoxime-functionalized polymer matrix at different pH.20-22 These studies indicated that the sorption of U(VI) in AO-sorbent is a endothermic chemical process, and dependent on the pH of the equilibrating solution. The objective of the present work is to study the effects of chemical compositions of sorbents on the rate of sorption of U(VI) from seawater. The hydrogels can be anchored within pores of a robust microporous host membrane.23,24 The microporous host membrane provides a containment and mechanical strength to the hydrogel and mitigates the impact of osmotic forces. The appropriate chemistry of the hydrogel, the guest component, leads to good separation characteristics of a resulting

10.1021/ie801912n CCC: $40.75  2009 American Chemical Society Published on Web 06/08/2009

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membrane. This “gel in a shell” construct is referred to as a “pore-filled” membrane.23,24 In the present work, the single or two component hydrogels have been prepared by UV-initiated bulk polymerization of the monomers in a desired proportion with a cross-linker. Two component gels consisting of AO groups along with acidic or basic comonomer have been prepared to explore the possibility of enhancing U(VI) sorption kinetics from seawater. The single component hydrogels containing ethylene glycol methacrylate phosphate (EGMP) or 2-acrylamido-2-methyl-1-propane sulfonic acid (AMPS) have been studied to explore the better functionalized hydrogel for U(VI) preconcentration from multicomponent lean feed like seawater. The monomers used in preparation of the two component AO hydrogels are AMPS, acrylic acid (AA), 3-(acrylamido propyl) trimethylammonium chloride (APTAC), methacrylic acid (MAA), and EGMP. These comonomers are ionic (acidic or basic) and are expected to influence the binding of U(VI) with AO-groups in the hydrogels. N-N-Methylene-bis-acrylamide (MBA) and R,R-dimethoxy-Rphenyl acetophenone have been used as the cross-linker and UV-initiator, respectively. All the AO-hydrogels have been conditioned with alkali as described in the literature.25 These hydrogels have been studied in terms of their water uptake capacity, U(VI) uptake capacity, U(VI)-uptake efficiency in presence of large excess of vanadyl ions, and U(VI) sorption kinetics from seawater spiked with natU + 233U. The results obtained in these studies indicated that chemical design of the sorbent can be tuned to enhance the sorption kinetics of U(VI) from seawater in the AO-sorbent, and EGMP hydrogel is a promising alternate to AO-sorbents. EGMP hydrogel has been found to sorb U(VI) from the seawater as well as aqueous solutions having high acidity. Experimental Section Reagents and Apparatus. Analytical reagent grade chemicals and deionized water (18 MΩ/cm) purified by model Quantum from Millipore (Mumbai, India) were used throughout the present studies. Ethylene glycol methacrylate phosphate (EGMP), 2-acrylamido-2-methyl-1-propane sulfonic acid (AMPS), acrylic acid (AA), 3-(acrylamido propyl) trimethylammonium chloride (APTAC), methacrylic acid (MAA), N-N-methylenebis-acrylamide (MBA), and R,R-dimethoxy-R-phenyl acetophenone (DMPA) were obtained from Sigma-Aldrich (Steinheim, Switzerland). Acrylonitrile (AN), tetrahydrofuran (THF), methanol, and N-N-dimethylformamide (DMF) were obtained from Merck (Mumbai, India). Seawater used in the present work was collected from Andaman and Nicobar Islands, and filtered with 5 µm pore size filter. Preparation of Hydrogels. The monomer AN, cross-linker MBA, and UV-initiator (DMPA) were dissolved in DMF and THF mixture (75:25). In this polymerizing solution, the required amount of a comonomer (AA, MAA, AMPS, APTAC or EGMP) was added. AMPS was first dissolved in water-methanol (50:50) mixed solvent, and then added to the polymerizing solution. The amounts of cross-linker and monomer were adjusted in the polymerization solution to get 5 mol % of crosslinking. The amount of UV-initiator was taken as 1 wt % as this was found to be minimum quantity to initiate the bulkpolymerization. Finally, the polymerizing solutions in the 15 mL borosilicate beakers were exposed to 365 nm UV light in a multilamp photoreactor (Heber Scientific, model No. HMLSW-MW-LW-888) for a period of 20 min. The photoreactor contained six UV lamps (8 W each) arranged in a circle. After irradiation in the photoreactor, the hydrogels were washed

thoroughly with THF, methanol, and distilled water to remove the unpolymerized components, dried for 8-10 h under vacuum, and weighed to obtain the weight of dry hydrogel. The C≡N groups in the hydrogels were converted to AO-groups by treating these in the 3 wt % hydroxylamine solution (methanol/water ) 1:1) at 60 °C for 8 h. After amidoximation, the hydrogels were washed thoroughly with water. All AO containing hydrogels were conditioned by treating them with 0.1 mol L-1 NaOH at 80 °C for 1 h. The EGMP and AMPS hydrogels were also prepared by UV-initiator induced polymerization using a similar procedure, and conditioned with 0.25 mol L-1 NaCl. Characterization of Hydrogels. The conversion of AN to AO groups in the hydrogels was studied by the Fourier Transform Infrared (FTIR) spectrometry and C, H, N analyses. The dried hydrogel samples were powdered in liquid nitrogen, mixed with KBr in 1:50 ratio, and homogenized with a mortar and pestle. The pellets were made by applying a pressure of around 75 kg/cm2. FTIR spectra of the 13 mm diameter pellets were recorded using the Jasco 4100-model. The water uptake capacity of the hydrogel, defined as the amount of water absorbed per unit dry weight of the gel, was obtained from the difference in weight of the hydrated gel and dried gel sample. The uptake studies of U(VI) in the hydrogels from seawater were carried out using 233U radiotracer. The amount of 233U spiked in seawater was 9.54 µg mL-1. This amount of 233U was taken to obtain sufficient R-scintillation counts (∼10000 cpm) in a 50 µL sample of aqueous feed added to the liquid scintillation cocktail. To keep the pH of aqueous feed unaltered, the known volume of 233U radiotracer solution was dried under an IR lamp, and 200 µL of 100 mg L-1 NaHCO3 solution was added to prevent the precipitation of uranium from aqueous feed. The uptake of U(VI) in the hydrogel sample (∼0.3 g) was monitored by liquid scintillation counting of the samples of feed solution (15 mL of seawater or aqueous solutions having different acidity) taken before and after equilibration. The R-radioactivity of 233U in equilibrating aqueous solutions was monitored by removing 50 µL of sample and adding it to a vial containing 5 mL of scintillation cocktail-O. The composition of cocktail-O was 2,5-diphenyl oxazole ) 10 g, 1,4-di-2-(5phenyloxazolyl) benzene ) 0.25 g, and naphthalene ) 100 g in 1000 mL of toluene with 10% v/v bis(2-ethylhexyl) phosphoric acid (HDEHP). The liquid scintillation counting was carried out in home-built liquid scintillation counter with EMI 9514 photomultiplier tube (13 stages) coupled to an amplifier and a single channel analyzer via a discriminator. The equilibration time required for optimum uptake of U(VI) in the hydrogel was determined by spiking a known amount of 233U + natU in 15 mL of seawater/aqueous sample, and equilibrating the known weight of hydrogel with this solution using constant stirring at room temperature. The uptake of 233U in the hydrogel was monitored by R-scintillation counting of 50 µL samples taken from equilibrating solution at regular time intervals. U(VI)loading capacity of the hydrogel was measured by drying a fixed volume of the solution containing known concentration of uranyl nitrate [natUO2(NO3)2] spiked with a required radioactivity of 233 U, and 15 mL of seawater was added to equilibrate the hydrogel sample of a known weight for overnight with a constant stirring. The radioactivity of 233U sorbed in the samples was obtained from the difference in total initial radioactivity of 233 U in solution and residual radioactivity of 233U in solution left after equilibrating the hydrogel sample. U(VI) loading capacity of the hydrogel was calculated from the standard radioactivity comparison method and knowledge of weight of the dried hydrogel sample.25

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Figure 1. FTIR spectra of (a) AO hydrogel, (b) AO+MAA (60:40) hydrogel, (c) AMPS hydrogel, and (d) EGMP hydrogel.

The apparent acid dissociation constant (pKa) values of watersoluble acidic comonomers AMPS, AA, and MAA were determined by potentiometric titration with standard NaOH. Apparent pKa of a weak acid can be obtained by the equation:26 pKa ) pH + log[HA]/[A-] where [HA] and [A-] are the molar concentration of the acid and dissociated anion, respectively, at equilibrium. When [A-] ) [HA], then [A-]/[HA] ) 1 and log[A-]/[HA] ) 0. Under this condition pKa will be equal to pH. Typically, 50 mL of 0.01 mol L-1 of the acid monomer was titrated with standard 0.1 mol L-1 NaOH. A pH meter (model PICO+, LAB INDIA, Mumbai, India) calibrated with buffers 4.0, 7.0, and 10.0 was used to measure the change in pH during titration. A weight buret was used for adding the increments of known weight of NaOH in the beaker containing a well-stirred aqueous solution of acidic monomer at 25 °C. The end point was obtained by taking the derivative of the titration curve. The volume of alkali corresponding to the end point was calculated. The apparent pKa values were obtained from the pH value corresponding to the half neutralization point. Safety Considerations. Radioactive solutions used in this work present radiological hazards. The physical handling of radioactive material was conducted inside a fume-hood connected to an exhaust system using disposable latex gloves. Results and Discussion Chemical and Physical Characterizations. The presence of required functional groups in the hydrogels (AO, AO + comonomer, AMPS, and EGMP) was confirmed by the characteristic bands of functional groups in the FTIR spectra as shown in Figure 1. The conversion of AN (-CtN) to AO groups (-C(dNOH)NH2) during amidoximation of the hydrogels were confirmed by recording FTIR spectra before and after treating these with hydroxylamine. In FTIR spectra of amidoximated gels, the intensity of band at 2243 cm-1 corresponding to CtN groups reduced significantly. The bands at 1650 cm-1 (CdN stretching vibration), 920 cm-1 (N-O stretching vibration), and multiple peaks broadband at 3000-3600 (N-H, O-H stretching vibrations) were observed in FTIR spectra of the

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amidoximated hydrogels. The conversion of AN to AO groups with hydroxylamine was not complete as characteristics band of CtN did not disappear completely in FTIR spectra of the AO gels. The FTIR spectrum of AMPS hydrogel showed the vibration bands of sulfonyl group at 1040 cm-1 (S-O), 1210 and 1380 cm-1 (SdO), 1455 cm-1 (-SO2-), amide carbonyl group at 1650 cm-1 (CdO), and a broadband in the region of 3000-3600 cm-1. The FTIR spectra of EGMP gel showed bands corresponding to P-O-H vibrations at 1070 and 984 cm-1, and HPO2- vibration at around 1260 cm-1, associated PdO (1170 cm-1), free PdO (1380 cm-1), CdO (1730 cm-1), and a broadband in the region of 3400-3700 cm-1. The bands were assigned on the basis of a comparison of FTIR spectra with the characteristic bands reported in the literature for these groups.12,27-29 The elemental analyses of hydrogels indicated increase in nitrogen content after amidoximation of the hydrogel, which corresponded to 75-80% conversion of -CtN to -C(dNOH)NH2 group. The presence of comonomer did not affect the conversion of AN to AO groups during amidoximation. The chemical structures of different AO-hydrogels are shown in Scheme 1. It is seen from the chemical structures that the hydrogels prepared in the present work contain fixed-sites that can form electrostatic as well as covalent bonds with UO22+ ions. The fixed-sites in the hydrogels may also have significant extent of hydrogen bonding that is not shown in Scheme 1 for the sake of simplicity in representations of chemical structures of the hydrogels. As can be seen from the water uptake capacity given in Table 1, the hydrogels swell more in deionized water as compared to seawater. The water uptake capacities of AO gels with strong acidic groups (-SO3H) and strong basic groups (-N+-(CH3)3) were higher (300 wt %) as compared to other hydrogels (∼100 wt %) when these were equilibrated with the seawater. Sorption and Desorption of U(VI) in Hydrogels. The sorption of U(VI) from seawater in the AO hydrogel involves following chemical reaction: -

-

[UO2(CO3)3]4 + 4HL h [UO2(HL)2(L)2] + 3CO23 + 2H+ (1) where HL is the fixed complexing sites in the hydrogel. This chemical reaction can proceed either by direct ligand exchange of U(VI) or decomplexation of [UO2(CO3)3]4- to UO22+, followed by complexation of UO22+ with fixed-sites in the hydrogels. The ligand-exchange/decomplexation of [UO2(CO3)3]4- may be the rate determining step as this anionic U(VI) species has high stability constant under prevailing seawater conditions.1,2 Since decomplexation of [UO2(CO3)3]4- can be catalyzed by H+-ions, the presence of acidic comonomer like AA and MAA were found to enhance the sorption kinetics of U(VI) in the AOsorbent from seawater.16-19 Contrary to decomplexation of [UO2(CO3)3]4-, eq 1 seems to suggest that the complexation of UO22+ with the fixed-sites in hydrogels may be retarded in the presence of H+ ions. It was reported that the microenvironment around the complexation sites governs the U(VI) sorption process from seawater.30-33 Therefore, the AO gels with different acidic groups (-SO3H, -COOH, -P(dO)(OH)2) and strong basic group (-N+-(CH3)3) were prepared to study the possibility of enhancing overall kinetics involved in the sorption of U(VI) from seawater. The U(VI)-uptake efficiency of different hydrogels, measured by equilibrating hydrogels with well-stirred seawater spiked with 1-5 ppm of 233U, are given in Table 1. U(VI)-uptake efficiency represents sorption of U(VI) in a hydrogel having excess of binding sites for the U(VI)

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Scheme 1. Chemical Structures of Different AO-Hydrogels: (a) AO+MBA, (b) AO+AA+MBA, (c) AO+MAA+MBA, (d) AO+PSA+MBA, (e) AO+EGMP+MBA, and (f) AO+APTAC+MBA

Table 1. The Properties of AO Hydrogels Studied in the Present Work water-uptake capacity (wt %) gel composition (mol %)

deionized water

seawater

U(VI)-uptake efficiency (%)

equilibration time (min)

AO/U(VI) mol ratio

AO AO+AMPS (60:40) AO+AA (60:40) AO+MAA (60:40) AO+APTAC (60:40) AO+EGMP (60:40) AMPS EGMP

152 928 168 220 480 508 1450 360

118 502 76 70 310 116 520 195

89 ( 4 95 ( 3 94 ( 3 95 ( 2 90 ( 3 98 ( 2 35 95

25 ( 2 35 ( 2 20 ( 2 10 ( 1 ∼720 200 ( 10

4.4 1.1 1.2 2.6

complexation. It was observed that the AO gels with different acidic and basic comonomer take up U(VI) from seawater quantitatively (>90%). It was also observed that AMPS and EGMP hydrogels also take up U(VI) from seawater with 35% and 95% efficiency, respectively. Unlike AO-gels, the desorption of U(VI) from EGMP gels was not possible with 1 mol L-1 HCl. The quantitative desorption of U(VI) from EGMP hydrogel could be achieved by equilibrating it with 0.5 mol L-1 Na2CO3. The reloading of U(VI) in the EGMP-hydrogel did not require any treatment or conditioning. In case of AO-hydrogels, the reloading of U(VI) was not possible without alkali treatment.25 The U(VI)-uptake efficiency in AO sorbent as a function of pH was studied by Zhang et al.20 It was observed in this study that U(VI) uptake in AO-sorbent was quantitative only in the pH ranging from 4 to 6. The U(VI) uptake efficiency of AO sorbent decreased sharply below pH 4, and there was no uptake of U(VI) below pH ) 1. In the present work, the uptake efficiency of U(VI) in AO+MAA (60:40) and EGMP hydrogels were studied as a function of acidity in the equilibrating solution. The results thus obtained are shown in Figure 2. As can be seen from this figure, U(VI) uptake in AO+MAA hydrogel is

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similar to that reported for AO hydrogel,20 and not quantitative from aqueous solution having HNO3 concentration higher than 0.01 mol L-1. Unlike AO and AO+MAA hydrogels, the uptake of U(VI) in the EGMP hydrogel was not affected by the concentration of HNO3 as high as 4 mol L-1. This means that EGMP hydrogel can not only be used for preconcentration of U(VI) from seawater but also from a variety aqueous feed. Comparison of U(VI) Sorption Capacity of Hydrogels. To study the U(VI) loading capacity, the known weights of the hydrogel samples were equilibrated with a well-stirred seawater having 10-fold excess of U(VI) than expected total functional groups density in the hydrogel for 24 h. U(VI) was tagged with 233 U for monitoring the uptake in hydrogel. The saturation uptake of U(VI) (loading capacity) of the AO, AO+MAA(60: 40), and EGMP hydrogels were found to be 670, 493, and 550 mg g-1, respectively. Therefore, these sorbents can be used in adsorptive preconcentration of U(VI) because of their high U(VI) loading capacities. The AO to U(VI) mole ratio in AO and AO+MAA (60:40) hydrogels were found to be 4.4 and 2.6, respectively. In case of EGMP hydrogel, the EGMP to U(VI) mole ratio was 1.9. The possible modes of UO22+ binding

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Figure 2. U(VI) uptake efficiency of AO+MAA (60:40) (×) and EGMP (∆) hydrogels as a function of HNO3 concentration in the equilibrating feed solution containing 2-4 µg mL-1 233U(VI).

in EGMP and AO+MAA (60: 40) hydrogel are given in the Supporting Information. These structures are based on the mole ratio of AO, comonomer, and U(VI) in the hydrogels, and FTIR of U(VI)-loaded hydrogels. It is interesting to note that the AO/ U(VI) ratio was changed depending upon the proportions of an acidic comonomer in the hydrogel. For example, AO/U(VI) ratios were found to be 2.1 and 1.2 in AO+AA hydrogels having 80:20 and 60:40 AO/AA mol %, respectively. Similarly, the AO/U(VI) ratio changed from 1.1 to 1.6 on changing the proportion of AO/AMPS from 60:40 to 80:20, respectively. This seems to suggest that U(VI) binding in AO-hydrogels is highly dependent on the nature of comonomer and its proportion with respect to AO groups. The change in binding of U(VI) in the AO-hydrogel may influence the kinetics involved in chemical reaction shown in eq 1 as well as diffusion of U(VI)-species in the sorbent matrix. Effects of Chemical Composition of Hydrogel on Sorption Kinetics. To understand the influence of chemical compositions of the sorbent on overall kinetics of U(VI) sorption from seawater, the known weights of hydrogels were equilibrated with a well-stirred 15 mL of seawater spiked with 10-fold excess of nat+233 U(VI) than expected total functional groups density in the hydrogels to minimize the concentration gradient in the equilibrating solution. The U(VI)-sorption profiles thus obtained only represent the parameters involved in the transfer of U(VI) from equilibrating solution to hydrogel matrix (complexation reaction kinetics at the interface), and subsequent diffusion of U(VI) in the hydrogel matrix. The equilibration time t required for the saturation sorption of U(VI) in different hydrogels, obtained from the sorption profiles, are given in Table 1. It is seen that equilibrium time required for the saturation sorption was slowest in AO+APTAC hydrogel (720 min), and fastest in AO+MAA (10 min) and EGMP hydrogels (7 min) among the different hydrogels listed in Table 1. The comparison of U(VI) sorption kinetics in the EGMP hydrogel and hydrogels having the combination of AO + comonomer is shown in Figure 3. The slow sorption rate of U(VI) in AO+APTAC hydrogel may be due to sorption of [UO2(CO3)3]4- as an ion-pair with fixedpositive sites (quaternary ammonium groups) in the sorbent. Since [UO2(CO3)3]4- is quite bulky, the transfer of this U(VI) species from seawater to hydrogel and its subsequent diffusion in the matrix of hydrogel would be quite slow. The lower equilibration time for saturation sorption of U(VI) in AO+MAA and EGMP hydrogel seems to indicate that the acidic groups

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Figure 3. Comparison of fractional attainment of U(VI)-sorption equilibrium F(t) in AO hudrogels containing 40 mol % of the comonomer. The comonomers used were 2-acrylamido-2-methyl-1-propane sulfonic acid (AMPS), acrylic acid (AA), 3-(acrylamido propyl) trimethylammonium chloride (APTAC), methacrylic acid (MAA), and ethylene glycol methacrylate phosphate (EGMP).

in the hydrogels enhance the ligand-exchange and/or decomplexation of [UO2(CO3)3]4- to UO22+ ions, which forms complex with the fixed-sites in the hydrogels. UO22+ ions would also diffuse faster in the sorbent matrix as compared to bulky [UO2(CO3)3]4-. Therefore, EGMP and AO+MAA are better candidates for U(VI) recovery from seawater as compared to other AO based sorbents. Mechanistic Study of U(VI) Sorption. The most commonly used kinetics models for the analyses of sorption profiles are pseudo-first-order, pseudo-second-order, and diffusion model. Lagergren suggested a rate equation for the sorption of solutes from a liquid solution.34-36 This pseudo-first-order rate equation is ln

(qe - q) ) -K1t qe

(2)

where q and qe are the amounts of solute sorbed in unit weight of the sorbent at time t and at equilibrium, respectively, and K1 is the rate constant of first-order sorption. Another model reported in the literature for the analysis of sorption kinetics is pseudo-second order.37 The rate law for this system in a linear form can be expressed as 1 t t ) + q qe K2q2e

(3)

where K2 is the pseudo-second-order rate constant of the sorption, and other symbols are the same as for eq 2. The model based on diffusion can be expressed by the Morris-Weber equation as38,39 qe ) Ki√t

(4)

where Ki represents the diffusion rate constant. To understand the rate determining process in the sorption of U(VI) in the AO hydrogels from seawater, the U(VI) sorption profiles of AOhydrogels having different proportion of a acidic comonomer were analyzed. The variations of F(t) as a function of square root of equilibration time (t1/2) are shown in Figures 4-6. It is seen from these analyses that F(t) varies linearly with t1/2 up to 70-80% attainment of U(VI) sorption equilibrium in all the cases except AO+AA and AO+AMPS hydrogels having 40:

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Figure 4. Comparison of fractional attainment of U(VI) uptake equilibrium (F(t)) in AO and AO+AA hydrogels from well-stirred seawater spiked with 10 times excess of 233U + natU as a function of square root of equilibrating time (t1/2). Symbols ∆, +, and O, represent AO, AO+AA (40:60), and AO+AA (60:40), hydrogels respectively.

Figure 5. Comparison of fractional attainment of U(VI) uptake equilibrium (F(t)) in hydrogels having different proportions of AO and MAA from wellstirred seawater spiked with 10 times excess of 233U + natU as a function of square root of equilibrating time (t1/2). Symbols ∆, +, and O, represent AO+MAA (60:40), AO+MAA (80:20), and AO+MAA (40:60) hydrogels, respectively.

60 proportion of AO to comonomer. As expected from eq 4, the slope of linear variation of F(t) as a function of t1/2 represents the Fickian diffusion. However, it is likely that more than one process may be involved in the U(VI)-sorption in hydrogels, which may switch to rate determining process in the different stages of the sorption process. The analyses of U(VI)-sorption profiles shown in Figures 4-6 seem to indicate that the U(VI)sorption process from seawater is delayed initially (lag time) for 4-11 s in all the hydrogels except those containing strongly acidic comonomer AMPS up to 60 mol %. After a period of time (lag time), the experimentally measured U(VI)-sorption kinetics followed the trend predicted by eq 4. Thus, U(VI)sorption profiles can be divided into two parts: (i) initial lag representing reaction kinetics at the seawater-hydrogel interface

Figure 6. Comparison of fractional attainment of U(VI) uptake equilibrium (F(t)) in hydrogels having different proportions of AO and AMPS from well-stirred seawater spiked with 10 times excess of 233U + natU as a function of square root of equilibrating time (t1/2). Symbols ∆, +, and O, represent AO+AMPS (80:20), AO+AMPS (40:60), and AO+AMPS (60:40) hydrogels, respectively.

and (ii) linear variation of F(t) with t1/2 due to Fickian diffusion of U(VI) in the hydrogel matrix. The slopes of curves F(t) vs t1/2 representing U(VI) diffusion rate constant (Ki) in different gels were found to be in the order EGMP (0.41 min-0.5) > AO+MAA (60:40) (0.36 min-0.5) > AO+AA (60:40) (0.29 min-0.5) > AO (0.24 min-0.5) > AO+AMPS (60:40) (0.20 min-0.5). The diffusion rate of UO22+, formed at surface of the sorbent by decomplexation of [UO2(CO3)3]4-, in the sorbent is affected by its physical and chemical interactions with the matrix. Chemical interactions of UO22+ with matrix are due to its covalent and electrostatic interactions with the fixed sites (AO and/or acidic functional groups) of hydrogels. The retardation of diffusion mobility of UO22+ in the matrices of AO+acid comonomer hydrogels follows the trend of apparent pKa values of acid comonomers, that is, AMPS (2.6) > AA (4.2) > MAA (4.6). This seems to suggest that covalent interactions of UO22+ may be the same in all AO-hydrogels with acid comonomer, but electrostatic interactions of UO22+ with a matrix having a strong acid group like -SO3- would be higher. As initial lag in U(VI)-sorption profiles represents the reaction kinetics at the seawater-hydrogel interface, the experiments were carried out to measure the sorption profiles of EGMP and AO+MAA (60:40) hydrogels within short span (2-3 min). The choice of these two hydrogels was based on the fact that these showed better U(VI) sorption kinetics among different hydrogels studied in the present work. 233U(VI) concentration in seawater was kept at 2 ppm which was not sufficient to saturate the hydrogels. The sorption profiles were analyzed using rate equations of pseudo-first-order (eq 2) and pseudo-second-order (eq 3). As can be seen from Figure 7, the early stages of U(VI) sorption from seawater to EGMP and AO+MAA (60:40) hydrogels follow the pseudo-second-order rate eq 3. The analysis of U(VI)-sorption profiles gave the values of pseudo-secondorder rate constant K2 as 3.20 × 104 and 2.94 × 104 g mol-1 min-1 for EGMP and AO+MAA hydrogels, respectively. This seems to indicate that the decomplexation of [UO2(CO3)3]4- to UO22+ in EGMP and AO+MAA hydrogels are comparable. Selectivity of Hydrogels. The uptake of U(VI) in EGMP gel was not found to be affected by the presence of a large excess of representative ions like Ca2+, Cu2+, Fe3+, and VO22+. The selectivity of AO, AO+MAA, and EGMP hydrogels toward

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aqueous feed containing high acid concentration. The sorption kinetics of U(VI) in EGMP hydrogel from a well-stirred seawater was found to be better than AO+MAA (60:40) hydrogel and follows second order kinetics in both cases. This indicated that EGMP hydrogels can be used for preconcentration of U(VI) not only from seawater but also from a lean acidic waste generated during nuclear fuel processing. The advantages of EGMP-hydrogel would be as follows: (i) one step synthesis using a single monomer, no hydroxylamine and alkali treatment are required; (ii) unlike acrylonitrile, EGMP is neither volatile nor toxic; (iii) EGMP is readily polymerizable; (iv) under identical seawater conditions, the profile of U(VI) sorption kinetics is faster in EGMP hydrogel as compared to AO and AO + acid comonomer hydrogels; and (v) EGMP hydrogel can be used for preconcentration of U(VI) from the seawater as well as aqueous feed having high acidity. Acknowledgment

Figure 7. Analyses of sorption profiles of U(VI) in EGMP and AO+MAA(60: 40) hydrogels in terms of pseudo-second-order rate eq 3. The symbols O and × represent EGMP and AO+MAA (60:40) hydrogels, respectively.

The authors thank the Board of Research in Nuclear Sciences (BRNS), DAE, India for the financial assistance to the project under BARC-University of Pune MoU for the Doctoral studies. Supporting Information Available: Scheme showing possible modes of UO22+ binding with fixed-sites in the hydrogels, and images of hydrogels obtained by Scanning Electron Microscopy. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited

Figure 8. Comparison of U(VI)-uptake efficiencies of AO, AO+MAA (60: 40), and EGMP hydrogels from a well-stirred seawater as a function of V(IV)/U(VI) mol ratio.

U(VI) in the presence of vanadyl ions were compared by adding a constant amount of U(VI) and varying the mole ratio of V(IV)/ U(VI) ions in the seawater, and results are shown in Figure 8. The vanadyl ions interference was studied as it is known to be competing ions in U(VI) recovery from seawater. The AO+MAA and EGMP gels were found to have better selectivity toward U(VI) ions in seawater as compared to AO gel, and both gels could tolerate the presence of more than 1000-fold excess of the vanadyl ions as can be seen from Figure 8. Conclusions The study of chemical compositions of the hydrogels indicated that the presence of weak acid comonomer like MAA with AO enhances the selectivity as well as kinetics of U(VI) sorption from seawater. However, the kinetics of uptake of U(VI) from seawater was found to be highly dependent on the proportion of MAA and AO in the hydrogel. The presence of a strong acid (-SO3H) or strong base (-N+(CH3)3) as a comonomer with AO retards the overall kinetics involved in the U(VI) sorption from seawater. It was observed that EGMP hydrogel sorbed U(VI) quantitatively from seawater as well

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ReceiVed for reView December 12, 2008 ReVised manuscript receiVed May 5, 2009 Accepted May 14, 2009 IE801912N