Environ. Sci. Technol. 2006, 40, 3070-3074
Removal of Toxic Uranium from Synthetic Nuclear Power Reactor Effluents Using Uranyl Ion Imprinted Polymer Particles CHANDRIKA RAVINDRAN PREETHA, JOSEPH MARY GLADIS, AND TALASILA PRASADA RAO* Regional Research Laboratory (CSIR), Trivandrum - 695 019, India GOPALA VENKATESWARAN Bhabha Atomic Research Centre (BARC), Trombay, Mumbai - 400 085, India
Major quantities of uranium find use as nuclear fuel in nuclear power reactors. In view of the extreme toxicity of uranium and consequent stringent limits fixed by WHO and various national governments, it is essential to remove uranium from nuclear power reactor effluents before discharge into environment. Ion imprinted polymer (IIP) materials have traditionally been used for the recovery of uranium from dilute aqueous solutions prior to detection or from seawater. We now describe the use of IIP materials for selective removal of uranium from a typical synthetic nuclear power reactor effluent. The IIP materials were prepared for uranyl ion (imprint ion) by forming binary salicylaldoxime (SALO) or 4-vinylpyridine (VP) or ternary SALO-VP complexes in 2-methoxyethanol (porogen) and copolymerizing in the presence of styrene (monomer), divinylbenzene (cross-linking monomer), and 2,2′azobisisobutyronitrile (initiator). The resulting materials were then ground and sieved to obtain unleached polymer particles. Leached IIP particles were obtained by leaching the imprint ions with 6.0 M HCl. Control polymer particles were also prepared analogously without the imprint ion. The IIP particles obtained with ternary complex alone gave quantitative removal of uranyl ion in the pH range 3.5-5.0 with as low as 0.08 g. The retention capacity of uranyl IIP particles was found to be 98.50 mg/g of polymer. The present study successfully demonstrates the feasibility of removing uranyl ions selectively in the range 5 µg - 300 mg present in 500 mL of synthetic nuclear power reactor effluent containing a host of other inorganic species.
Introduction Uranium is used as a nuclear fuel in nuclear power reactors, material of high density in the aeronautics industry, as radiation shielding, as an additive for catalysts or steels, and in glass and ceramic industries. However, uranium and its compounds, like lead are highly toxic and result in progressive or irreversible renal injury and in acute cases may lead to * Corresponding author phone: 91-471-2515317/2490674; fax: 91-471-2491712/2490186; e-mail:
[email protected]. 3070
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TABLE 1. Composition of a Typical Nuclear Power Reactor Effluent constituent
concn (mg/L)
La Ce Nd Sm Pr Gd Tb Dy Y Ru Pd Na Ba Ni Cr Mn Co Mo Zr U
263.8 532.5 862.5 163.8 243.8 165.0 5.0 2.0 99.0 463.8 267.5 3000 308.8 100.0 100.0 181.3 127.5 731.3 771.3 2000
kidney failure and death. The tolerable daily intake of uranium established by WHO based on Gilman’s studies is 0.6 µg/kg of body weight/day (1-3). The WHO, Health Canada, and Australian drinking water guidelines fixed the maximum uranium concentration in drinking water to be less than 9, 20, and 20 µg/L (1, 2). The inhalation of uranium compounds results in deposition of uranium in lungs, which reach kidneys through the blood stream (4). Liquid-liquid extraction (LLE) technique is always preferred over ion exchange because of its simplicity, rapid mass transfer kinetics, adaptability to scale-up, and easy recovery of analyte and extractant. However, despite these positive features, the technique does possess inherent limitations such as finite aqueous phase solubility, low enrichment factors (∼10), third phase formation, tedious back extraction (needs the addition of caprylic acid), and cumbersome disposal of organic solvents (5). To overcome these problems, solid phase extraction (SPE) has come to the forefront in recent years. However, most SPE methods are nonselective and do not selectively remove uranium from a host of inorganic species that coexist in nuclear power reactor effluents (see Table 1). Ion imprinted polymer-SPE has recently shown promise for the selective recovery of lanthanides, actinides, and noble metals from dilute aqueous solutions as this technique provides higher enrichment factors and selectivity coefficients as compared to control polymers (6). Dai et al. (7) have prepared mesoporous sorbent materials by the ion imprinting technique for selective recovery of uranyl ions using bifunctional ligands such as amines and sulfonic acids. The work of John et al. (8) relates to the detection and extraction of uranyl ions by polymer imprinting wherein complexable functionality is of the formula CTCOOH where T is a hydrogen or any halogen (preferably chlorine), methyl- and halogensubstituted forms thereof, or CCOOH or PhCOOH. However, the work of Dai et al. (7) and John et al. (8) pertains to recovery of uranium from aqueous solutions only. Bae et al. (9) and Gladis and Rao (10-13) utilize vinyl benzoate and quinoline8-ol or its derivatives-vinyl pyridine (VP) as complexable functionalities during the preparation of ion imprinted polymeric resin and particles, respectively, and these materials have been used for recovery of uranium from seawater. 10.1021/es052195m CCC: $33.50
2006 American Chemical Society Published on Web 03/28/2006
However, these IIPs are not suitable for removal of uranium from nuclear power reactor effluents as they contain free acidity. Recently, Say et al. (14) selectively removed uranium using glutamic acid ion imprinted polymeric beads from weakly acidic solutions (pH 3.5) containing Fe(III), Th(IV), and Mn(II) only. Furthermore, the sorption of uranium requires 75 min and has crucial dependence on the pH of equilibration which makes the process tedious and timeconsuming. Again the desorption experiments indicate a percent recovery of 80.1 to 96.1 with various desorbents. Thus, as seen from the above account, previous researchers have not attempted to selectively remove uranium from a host of coexisting inorganic species from weakly acidic to acidic solutions using IIPs. Oxy-imines, >CdN-OH, more commonly abbreviated as oximes, constitute an important class of chelating agents (15). They have found extensive applications as highly selective reagents for the separation and/or spectrophotometric determination of a diversity of metal ions (16). Amidoxime modified polymers have been employed for recovery of uranium from seawater (17, 18). As the pH for maximum sorption of uranium(VI) with polymeric sorbent containing amidoxime functional group is ∼8.0 (pKa ) 4.99), this cannot be used for the removal of uranium from weakly acidic solutions. Conversely, salicylaldoxime (SALO) has been employed for liquid-liquid extraction of uranium(VI) from weakly acidic solutions (i.e. pH ∼ 3.5) (20). In view of this, it is felt SALO is an ideal candidate in preparing uranyl IIP particles that can be used for selective separation of uranium from a host of other coexisting inorganic species and also from weakly acidic solutions. The objectives of this study are to (i) synthesize uranyl ion imprinted polymer particles using UO22+-SALO-VP and (ii) investigate the possible application of the material for the removal of the uranium from synthetic nuclear power reactor effluents.
Experimental Section Apparatus. A Shimadzu UV 2401 PC controlled spectrophotometer (Shimadzu, Japan) and an LI-120 digital pH meter (ELICO, India) were used for absorbance and pH measurements, respectively. A Perkin-Elmer A Analyst - 100 flame atomic absorption spectrometer (Perkin-Elmer, USA) was used for determining inorganic species that have the potential to coexist with uranium in nuclear power reactor effluent (Table 1). FTIR spectra were collected over the frequency range 4000-400 cm-1 by the KBr pellet method using a Prestige - 21 IR spectrometer (Shimadzu, Japan). The surface morphology of the IIP particles was studied using a scanning electron microscope (SEM) (JEOL, Model JSM 5600 LV). Materials and Methods. A stock solution (1000 µg mL-1) of uranium(VI) was prepared by dissolving 0.5047 g of uranyl nitrate (UO2(NO3)2‚6H2O) (Aldrich, USA) in 5 mL of concentrated HCl and then diluting to 250 mL with deionized water. Salicylaldoxime (SALO) and Arsenazo III were purchased from Aldrich, USA. The concentration of uranium(VI) was measured spectrophotometrically by forming a binary complex with Arsenazo III (0.1% of Arsenazo III solution was used during the determination step). A 1.0 M sodium acetate-acetic acid buffer (pH 3.5) was used to maintain the pH of the aqueous phase at ∼3.5 during preconcentration. Styrene, 4-vinyl pyridine (VP), divinyl benzene (95%) (DVB), and 2,2′-azobisisobutyronitrile (AIBN) were obtained from Aldrich, USA. All other chemicals were of analytical reagent grade. Preparation of Uranyl Ion Imprinted (IIP) and Control Polymer (CP) Particles. Ion imprinted polymer materials (IIP1-IIP3) were prepared by thermal polymerization under the conditions described in Table 2. The imprint ion (UO22+) (1 mmol) was complexed with SALO (4 mmol) or VP (4 mmol)
TABLE 2. Preparation of Control and Uranyl Ion Imprinted Polymers polymer SALO CP1 IIP1 VP CP2 IIP2 ternary CP3 IIP3
imprint ion UO22+ (mM)
SALO (mM)
VP (mM)
styrene (mM)
DVB (mM)
AIBN (mg)
1
4 4
-
20 20
20 20
50 50
1
-
4 4
20 20
20 20
50 50
1
2 2
2 2
20 20
20 20
50 50
to form a binary complex and with SALO and VP (2 mmol each) to form a ternary complex in 10 mL of 2-methoxy ethanol. These binary/ternary complexes in the porogen were mixed with styrene (monomer) and divinyl benzene (crosslinking monomer) in the presence of 50 mg of AIBN as initiator. The polymerization mixtures were cooled to 0 °C (by keeping in a trough containing ice) and purged with N2 for 10 min, sealed, and thermally polymerized in an oil bath at ∼80 °C while stirring for 3 h. The resulting polymer materials were then dried, ground, and sieved to obtain ion imprinted polymer particles. Control polymer (CP1-CP3) particles were similarly prepared by omitting the imprint ion (see Table 2). Leaching Studies. Experiments were carried out with IIP particles by leaching 2 g of the particles with various leachants, selected based on previous studies (6.0 M HCl or HNO3, 3.0 M H2SO4, and 1.0 M Na2CO3) (10-14). The effect of leachant (HCl) concentration (1.0-10.0 M) and leaching time (3-9 h) for the removal of uranyl imprint ion was also studied. Characterization Studies. The CP/IIP particles were characterized by X-ray diffraction, UV-visible, FTIR, and SEM studies. (The results of first two techniques are not included in this paper as it is described by us elsewhere) (11-13). Optimization Studies. The effect of pH (1.0-6.0), weight of polymer particles (0.02-0.1 g), time of stirring (5-20 min) during the leaching step and elution of bound uranium, aqueous phase volume (25-100 mL), nature of eluent (1 M HCl/1 M HNO3/0.5 M H2SO4), eluent concentration (0.1-2.0 M HCl), and volume (5-20 mL) were studied by checking the removal efficiency of 100 µg of uranyl ion present in 500 mL of aqueous solutions by stirring with 0.1 g of CP3/IIP3 particles. Retention Capacity Studies. The maximum amount of preconcentrated uranyl ion/g (retention/binding capacity of CP3 or IIP3) was calculated by saturating 0.1 g of polymer particles with 40-600 mg/L of initial uranyl ion concentration present in 25 mL of deionized water under optimal conditions. The maximum amount of uranyl ion thus preconcentrated was eluted with 1.0 M HCl and determined spectrophotometrically by using the Arsenazo III procedure. Selectivity Studies. The selectivity of uranyl IIP and CP particles for uranium over other inorganic ions that typically coexist with uranium in nuclear power reactor effluents was determined by stirring 0.1 g of polymer particles with 100 µg of each individual inorganic ion (Table 1) present in 500 mL of diionized water under identical conditions. The selectivity coefficient (S UO22+/Mn+) is defined as
SUO22+/Mn+ )
DUO22+
(1)
DMn+
where DUO22+ and DMn+ are the distribution ratios of the uranyl ion and other inorganic species, respectively, with polymer VOL. 40, NO. 9, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. SEM photograph of leached uranyl IIP particles prepared using the uranium-salicylaldoxime-4-vinyl pyridine template.
FIGURE 2. Effect of pH on percent removal of uranium using CP3 and IIP3 particles. particles (CP or IIP). These distribution ratios were calculated using the formula
DMn+ )
CiMn+ - CfMn+ CfMn+
×
v m
FIGURE 3. (A) Effect of concentration of eluent (HCl) on removal of uranium using CP3 and IIP3 particles. (B) Effect of volume of eluent (HCl) on removal of uranium using CP3 and IIP3 particles.
(2)
where CiMn+ and CfMn+ are the concentrations of inorganic ions in aqueous phase before and after extraction, v is the volume of the solution, and m is the mass of the polymer. The percent extraction (%E) of inorganic ion is defined as
%E )
CiMn+ - CfMn+ CiMn+
× 100
(3)
Uranium Removal Studies from Synthetic Nuclear Power Reactor Effluents. Synthetic nuclear power reactor effluents were prepared based on the composition described in Table 1 containing varying amounts of uranyl ion (up to 300 mg of uranium per 0.5 L) were taken, and pH was adjusted to ∼3.5. A total of 0.1 g of leached uranyl IIP particles were added to above effluent samples and stirred for 10 min. The preconcentrated toxic uranium was eluted with 10 mL of 1.0 M HCl by stirring for 10 min. The uranyl ion content was monitored spectrophotometrically using the Arsenazo III method by measuring the absorbance at 656 nm (19).
Results and Discussion Leaching Studies. The results obtained on the selection of leachant showed that the use of HCl resulted in quantitative removal. The percent removal of uranyl ions was quantitative 3072
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FIGURE 4. Effect of uranyl ion concentration on the retention capacities of CP3 and IIP3 particles. in the leachant concentration range 5.0-10.0 M and a leaching time of 4-6 h. Uranyl ions from unleached IIP3 particles were leached by using 6 M HCl for a leaching time of about 6 h. This observation is analogous to similar leaching studies with 5,7-dichloroquinoline-8-ol based IIP particles reported elsewhere (10-13). Studies on the Nature of the Binary/Ternary Complex of Template on Removal of Uranium. The percent removal of uranyl ion was complete in the case of IIP3 corresponding to UO22+-SALO-VP ternary complex. This was in contrast
TABLE 3. Comparison of Retention Capacity of Uranyl IIP Particles Prepared Using Ternary Complex (UO22+-SALO-VP) Imprinted Particles with Other IIP Particles metal ion
retention or binding capacity (mg/g)
ref
Cu(II) Cu(II) Ni(II) Cu(II) Zn(II) Cu(II) Dy(III) U(VI) Pd(II) U(VI)
31.13 0.4 0.17 0.26 0.22 3.28 40.15 30.10 21.5 98.5
22 23 23 24 24 25 26 11 27 present method
to IIP1 (51.3 ( 0.5) and IIP2 (82.1 ( 0.8) synthesized with binary UO22+-SALO and UO22+-VP complexes, respectively. Charaterization Studies. FTIR Studies. The FTIR spectra of CP (leached and unleached) and IIP (leached and unleached) were collected. All had similar IR spectra indicating that all the polymers have a similar backbone. The >CdN stretching vibration is generally observed in the range 1400-1600 cm-1. In the IR spectra of both CP and IIP (leached and unleached), there was a dominant absorption peak over the range 1489-1496 cm-1, indicating the presence of the >CdN group of salicylaldoxime in the polymer matrix before and after leaching. The N-O stretching frequency is typically observed over the range of 1055-870 cm-1 (20). The presence of an absorption peak in 989-991 cm-1 indicated that SALO remained intact in the polymer matrix after leaching. Again, the vibration frequencies due to UO22+ expected at 922 cm-1 (asymmetric) and 831 cm-1 (symmetric) have been observed in unleached IIP particles only. A weak band near 802 cm-1 was present in the spectrum of the unleached ion imprinting polymer only. These observations indicate the bonding between UO22+ and the oxygen of oxime (9). SEM Studies. The morphology of the crushed polymers prepared by thermal polymerization was assessed by SEM (micrograph shown in Figure 1). As seen from the micrograph, the polymer particles are agglomerated in irregular shapes, analogous to the results reported by us elsewhere for thermal polymerization (21). Optimization Studies. Effect of pH. The removal of the uranyl ion was constant and reached a maximum from pH
3.2 to 5.0 (see Figure 2). For comparison, the removal efficiency of uranyl ion with CP3 (see Figure 2) remains almost the same over the pH range of 1-6. As shown in the figure, the imprinting effect is clearly observed at all pH values as the percent removal of uranium with IIP3 is more than CP3. The decrease in percent removal of uranyl ion at pH < 3.2 for IIP3 is attributed to competition of H+ ions with uranyl ions to form a complex with SALO and also due to an increase in the ratio of protonated SALO to free SALO. Therefore, the pH was adjusted to ∼3.5 in all subsequent studies. Effect of Weight of Polymer Particles. The removal efficiency of the uranyl ion was tested with different weights of CP3 and IIP3 particles. Again, as in case of pH studies, imprinting effect was noticed in all samples where 0.08 g of IIP3 particles was adequate for quantitative removal of uranyl ions from dilute aqueous solutions (0.5 L). Therefore, 0.1 g of IIP3 particles was used in all subsequent studies. Optimization of Other Experimental Variables. The effect of stirring time for the removal of uranyl ions at pH ∼ 3.5 using 0.1 g of CP3 or IIP3 particles shows that as low as 10 min was enough. Similar studies conducted on elution of uranyl ion loaded on IIP3 particles showed that 10 min of stirring time is required. Once again, the imprinting effect was noticed in all samples, as the removal efficiency of uranium with IIP3 particles is higher than that with CP3 particles in all conditions. The increase of aqueous phase volume up to 500 mL did not affect the quantitative removal of uranyl ion using 0.1 g of IIP3 particles. Subsequent uranyl ion removal studies were carried out from 500 mL of solutions. Of the various mineral acid eluents evaluated, hydrochloric acid was the most efficient eluent for the quantitative elution of uranium from uranyl ion loaded IIP3 particles. Figure 3A,B shows the effect of concentration/volume of HCl eluent required for elution of uranyl ions bound to IIP3/CP3 particles. A minimum concentration of 1.0 M HCl was required to elute the bound uranyl ion (even with 5 mg of uranium), and this eluent is recommended for subsequent studies. Again, single elution with 10 mL of eluent volume was required for quantitative removal of uranyl ion enriched from a 500 mL of aqueous solution. Retention Capacity Studies. The retention capacity increases with an increase in initial uranyl ion concentration and reaches a plateau at 14.3 (CP3) and 98.5 (IIP3) mg/g of the polymer when the uranyl ion concentration is greater than 480 mg/L (Figure 4). As shown in Figure 4, the retention
TABLE 4. Distribution Ratio (D) and Selectivity Coefficient (SUO22+/Mn+) of CP3 and Uranyl IIP3 Particlesa D (× 5 103)
a
SUO22+/Mn+
metal ion
analysis procedure
CP3
IIP3
CP3
IIP3
UO22+ Th4+ La3+ Nd3+ Sm3+ Gd3+ Ce4+ Pd2+ Au3+ Cu2+ Zn2+ Co2+ Ni2+ Mn2+ Pb2+ Cd2+ Fe3+ CrO42Zr4+
spectrophotometry spectrophotometry spectrophotometry spectrophotometry spectrophotometry spectrophotometry spectrophotometry FAAS FAAS FAAS FAAS FAAS FAAS FAAS FAAS FAAS FAAS FAAS spectrophotometry
0.15 1.10 1.77 0.95 1.81 1.10 0.82 0.003 0.01 0.01 0.16 0.01 0.03 0.04 0.29 0.001 0.74 0.002 0.71
99 0.96 0.40 0.11 0.19 0.14 0.58 0.12 0.003 0.02 0.03 0.01 0.11 0.05 2.49 0.06 1.36 0.08 0.60
0.14 0.08 0.16 0.08 0.14 0.18 50 15 15 0.94 15 1.4 3.0 0.52 150 0.20 751 0.21
103 248 900 521 707 171 825 3.3 × 104 4950 3300 9900 900 1980 40 1650 73 237 165
Average of three determinations. FAAS: flame atomic absorption spectrometry.
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capacities for IIP3 particles are higher than that for CP3 at all initial concentrations of uranyl ion. The retention capacity determined in the present investigation is higher when compared to IIP particles reported in the literature so far (see Table 3). Selectivity Studies during Removal of Uranyl Ion Using CP3 and IIP3 Particles. Under the conditions of optimum preconcentration of uranyl ion, 100 µg of selected inorganic species (that are likely to coexist with uranium in nuclear power reactor effluents) were equilibrated with 0.1 g of CP3 or IIP3 particles. The concentrations of these elements in solutions were determined by spectrophotometry or flame atomic absorption spectrometry after elution with 10 mL of 1.0 M HCl. The distribution ratio and selectivity coefficients of uranyl ion with respect to other inorganics using CP3 and IIP3 particles are shown in Table 4. The selectivity coefficients for IIP3 particles were greater by 102-104 fold as compared to CP3 particles owing to the size and shape specific cavities created in IIP3 unlike CP3. This observation is attributed to imprinting effect. Based on the results shown in Table 4, it is clear that uranyl ion can be removed selectively from several inorganic species present in dilute aqueous solutions. The high selectivity coefficients obtained on imprinting enabled removal of uranirum in real world nuclear processes where the ratio of uranium concentration over other inorganics is higher than that shown in Table 1. Removal of Uranium from Synthetic Nuclear Power Reactor Effluents. The removal efficiency of uranium from synthetic mixtures of inorganics (La + Nd + Sm + Pr + Gd + Tb + Dy + Y; Ru + Pd; Na + Ba, Co + Ni + Cr + Mo + Zr) have been studied. These studies show that the selective removal of uranium using IIP3 particles is possible. Therefore, the removal of 5 µg -300 mg of uranium present in 500 mL solution of synthetic nuclear power reactor effluent (prepared based on the composition listed in Table 1) was tested using 0.1 g of IIP3 particles under the optimal experimental conditions described above. The results indicate complete removal of uranium selectively from synthetic nuclear power reactor effluents by monitoring with both spectrophotometric and inductively coupled plasma mass spectrometric (ICP-MS) methods. Thus, the above studies on the preparation of uranyl IIP particles using binary (UO22+-SALO or UO22+-VP) and ternary mixed ligand (UO22+-SALO-VP) prepolymer complexes show that the latter complex was more efficient for the selective removal of uranium from dilute aqueous solutions. A comparison of the results obtained for IIP with that for CP particles indicates that there is a significant imprinting effect in pH, weight of polymer, aqueous phase volume, retention capacity, and selectivity coefficient (increase by the order of 102-104) for uranyl ion with respect to other inorganic species) studies. The high selectivity coefficients achieved in this paper enable removal of uranium from real world nuclear processes where the ratio of uranium concentration to other inorganic constituents are much higher than that prepared as per the composition of Table 1.
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Received for review November 2, 2005. Revised manuscript received February 15, 2006. Accepted February 22, 2006. ES052195M
