Radiopharmaceutical and Hydrometallurgical Separations of

Radiopharmaceutical and Hydrometallurgical Separations of. Perrhenate Using Aqueous Biphasic Systems and the Analogous. Aqueous Biphasic Extraction ...
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Ind. Eng. Chem. Res. 2000, 39, 3173-3180

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Radiopharmaceutical and Hydrometallurgical Separations of Perrhenate Using Aqueous Biphasic Systems and the Analogous Aqueous Biphasic Extraction Chromatographic Resins Scott K. Spear, Scott T. Griffin, Jonathan G. Huddleston, and Robin D. Rogers* Department of Chemistry and Center for Green Manufacturing, The University of Alabama, Tuscaloosa, Alabama 35487

Poly(ethylene glycol) (PEG)-based aqueous biphasic systems (ABS) and the analogous aqueous biphasic extraction chromatographic (ABEC) resins have been investigated for the separation of perrhenate (ReO4-) from tungstate (WO42-) in alkaline tungstate media for radiopharmaceutical application and for the separation of perrhenate from molybdate (MoO42-) in hydrometallurgical application. Salts of the water-structuring anions OH-, WO42-, and MoO42- form ABS by salting-out PEG, while chaotropic anions such as ReO4- and the similar TcO4quantitatively partition to the PEG-rich phase in such ABS. Perrhenate can thus be separated from solutions high in WO42- and OH- concentration, and the separation factors improve with an increase in the tungstate or hydroxide concentration. In addition, the ReO4- anion can be separated from load solutions high in MoO42- and SO42-. Perrhenate can also be separated under similar conditions using ABEC resins in both batch and chromatographic modes. The retained ReO4- can be eluted from the column with a simple water strip. The chromatographic separation of ReO4- from WO42- provided 97% of the loaded ReO4- in 20 mL. Unlike traditional technetium generators which are adversely affected by high parent ion concentrations, higher concentrations of the parent anion improve the retention of ReO4- onto ABEC resins. After exposure of the ABEC resins to radiation doses as high as 500 Mrad, an acceptable level of performance was maintained, although degradation of the resin (monitored using differential scanning calorimetry) was obvious. Introduction Rhenium-188 is emerging as an important radiopharmaceutical for both imaging and therapeutic applications, because of its short half-life (16.9 h) and energetic γ emission (155 keV).1-4 The long half-life of the Re-188 parent has been cited as an advantage, leading to long shelf lives and in-house production capabilities.5 Technetium-99m (t1/2 ) 5.9 h) is the most widely used radiotracer in nuclear medicine, and the close similarities in the chemistry of Re and Tc have led to the development of 188Re-labeled compounds based on 99mTc analogues.3 For most medical applications, the chemically stable +7 oxidation states 188ReO4and 99mTcO4- are isolated from the parent oxides and used directly or after reduction and complexation with an imaging or target ligand. Most in-house Tc generators rely on the adsorption of the parent molybdate salt (99MoO42-) on an alumina column followed by elution of 99mTcO4- salts with physiological saline.6-10 High specific activity 99MoO42is required to lower the contamination of the eluant with the 99Mo parent. Thus, 99Mo is isolated as a fission product of 235U in high specific activity. The 99Mo oxide is converted to molybdate, by digestion in NaOH, and adsorbed onto an alumina column, which is then sent to the receiving clinics. Rhenium-188 is the daughter of β decay of tungsten188 (t1/2 ) 69 days). Because there does not exist a fission-product route to high specific activity 188W, it is currently produced by neutron irradiation of 186WO2, which produces 188WO2 in an n,γ reaction.3,5,11 Adaptation of the 99mTc-generator technology to 188Re requires

the conversion of 188WO2 to WO42- by dissolution in NaOH and adsorption onto an alumina column. In this case, however, the low specific activity of the tungstate load solution causes early breakthrough of the tungstate parent.2,3,11 Technological fixes to this problem have included such remedies as using an additional alumina column to remove parent breakthrough11 or modification of the alumina active sites to improve perrhenate adsorption.12 Gel generators based on low specific activity tungsten have been proposed but require the use of a number of columns in tandem and in series to completely remove tungstate which complicates the systems.13 The separation of 188Re from neutron-irradiated tungsten has been achieved by precipitation of tungsten(VI) oxide; however, this method still uses an alumina column to remove the last traces of the tungsten parent.14 Solventextraction-based generators that use a variety of organic solvents (e.g., methyl ethyl ketone is used in “Instant Technetium” generators) are also known;13,15 however, the requirement of flammable, toxic organic solvents gives rise to a much more complicated process. We have previously demonstrated that the TcO4-/ MoO42- separation can be efficiently carried out even for low specific activity molybdate solutions using poly(ethylene glycol)-based aqueous biphasic systems (PEGABS) and the analogous aqueous biphasic extraction chromatographic (ABEC) resins.16-18 We first became interested in ABS in the context of environmentally benign separations technologies and the ability to carry out liquid/liquid extraction using wholly aqueous systems, eliminating the need for volatile organic com-

10.1021/ie990583p CCC: $19.00 © 2000 American Chemical Society Published on Web 07/11/2000

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pounds (VOCs) as diluents. We were able to demonstrate that salts of water-structuring anions (e.g., OHand MoO42-) with large magnitudes of Gibbs free energies of hydration (∆Ghyd) salt-out PEG-2000 and partition to the lower salt-rich phase. Chaotropic anions (e.g., TcO4-), however, partition to the upper PEG-rich phase. The preference of each anion for a particular phase increases with an increase in the phase divergence caused by higher PEG or phase-forming salt concentrations.19 Thus, increasing the concentration of molybdate or hydroxide actually improves the separation of TcO4-. By covalent attachment of PEG to a polystyrene/ divinylbenzene resin bead, it is possible to carry out similar separations in a column chromatographic or batch mode. We have demonstrated that the conditions which allow or improve separations in liquid/liquid ABS application also cause retention on the analogous ABEC. The major advantage to the ABEC approach is that stripping of the columns is accomplished simply by washing with water. Given the similarities between the ∆Ghyd of ReO4-/ TcO4- and WO42-/MoO42- and our previous work with technetium separations,16-18,20,21 we initiated the exploration of both PEG-ABS and ABEC for ReO4separations of use in radiopharmacy. It became clear that the same principles in the separation of Re from W could just as easily be applied to the Re/Mo separation of hydrometallurgical importance. Rhenium is a high-value-added metal because of its small relative abundance and its large number of applications (e.g., electrical materials and catalysis). The major source of rhenium metal is recovery from molybdenite or tungstenite ore in hydrometallurgical processing;22,23 thus, an efficient ReO4-/MoO42- separation would improve the economic viability of rhenium recovery. In this contribution we explore the ReO4-/WO42- and ReO4-/MoO42- separations and develop the key parameters for each system. Batch and chromatographic experiments from solutions containing the matrix ions individually and together reveal that the perrhenate anion can be selectively retained in either the liquid/ liquid ABS or ABEC resins. The recovery of perrhenate can be accomplished simply by washing the resins with water. Both liquid/liquid ABS and chromatographic ABEC separations are presented to illustrate the similarities between the two technologies. Experimental Section Reagents and Tracers. The chemicals PEG-2000, NaOH, K2CO3, Na2WO4, Na2MoO4, NaReO4, and NH4ReO4 were obtained from Aldrich (Milwaukee, WI) in reagent grade and used as received. All water was deionized using commercial deionization and polishing systems. The resins, ABEC-5000, ABEC-5000 XL72, and ABEC2000, were obtained from Eichrom Industries (Darien, IL). The particle sizes for these resins were 100-200 mesh (149-74 µm), 30-60 mesh (590-250 µm), and 30-60 mesh (590-250 µm) for the ABEC-5000, ABEC5000 XL72, and ABEC-2000 resins, respectively. All of these resins use a polystyrene/divinylbenzene support. A more detailed discussion of the resin properties and synthesis has been previously reported.20 The technetium tracer as NH499TcO4 in 0.1 M NH4OH was obtained from Amersham Life Science, Inc. (Arlington Heights, IL), and Na188ReO4 was obtained

Table 1. Initial Composition of a Molybdenum Feed Containing Rhenium (pH ) 1.09) element or ion

concn (mg/L)

element or ion

concn (mg/L)

Na Mg K NH4+ Ca Al P Fe

131 29 12 9230 218 27 5 177

Cu Zn As Se Mo W Re SO42-

174 5 235 35 4081 521 905 48500

from Dr. Ahmad Safavy, University of Alabama at Birmingham (Birmingham, AL), after elution of a “Re generator” loaded with Na2188WO4 on alumina. Both radiotracers were used as received for column studies or diluted with deionized water to an activity of ca. 0.03 µCi/µL (1.110 Bq/µL) for batch uptake runs. A rhenium-bearing molybdenum feed solution was obtained from a commercial rhenium mine and used as received. The analysis provided with the feed solution is given in Table 1. Oxidation state and chemical speciation were not specified in the analysis provided with this sample. Radioanalytical Measurements. Rhenium-188 was analyzed by its characteristic γ-rays with a Packard Cobra II Auto-Gamma counting system (Packard Instrument Co., Inc., Meriden, CT). Technetium-99 activity was followed by β decay analysis using an Ultima Gold scintillation cocktail and a Packard Tri-Carb 1900 TR liquid scintillation analyzer (Packard Instrument Co., Inc., Meriden, CT). The addition of a 100-µL sample to 3 mL of a scintillation cocktail (a 30-fold dilution) was found to be sufficient to alleviate the influence of PEG2000 and salt upon the scintillation signal. However, care must be taken not to allow sufficient time for the sample to disengage from the cocktail before counting is completed. The addition of a 100-µL sample to 5 mL of a scintillation cocktail helps further reduce the occurrence of sample disengagement. Liquid scintillation counter efficiency depends on several variables; therefore, it is imperative to carry out a reliable determination of the counting efficiency. Quenching has by far the most significant effect on the counting efficiency. There are several methods for the determination of the quenching of scintillation; two such methods were investigated here.36 The first method investigated for the determination of quenching effects involves successive dilutions of the sample in a scintillation cocktail. If the sample was being quenched, one would expect successive dilution to result in a reduced quenching agent concentration and a resulting increase in the observed count rate. If the sample was not being quenched, then the radioactivity of the counting sample remains the same, and so the count rate remains the same. We added 100 µL of a sample containing the suspected quenching agent to 3, 6, and 20 mL of a scintillation cocktail. The measured activities for these samples were the same. Another method involves actually checking the count rate versus energy profile. Quenching agents such as PEG-2000 and high salt solutions will shift the energy spectrum toward a lower energy level.36 We observed no change in the profile in the absence or presence of PEG-2000 or high salt solutions for our samples as prepared via the above procedure (a 100-µL sample in 3 mL of a scintillation cocktail). Phase Characterizations. Phase diagrams for PEG2000/salt systems were determined by turbidimetric

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titration.24 A solution of known weight percent PEG2000 was weighed into a sample tube. Salt solutions of known concentration were then added dropwise with vortexing until the clear mixture just became turbid, indicating phase formation. The final mass of the system was then determined, and the compositions at the point of phase transition were calculated. The system stability and the mathematical construction of the tie lines have been previously described.25,26 Partitioning Studies. The PEG-2000 stock solutions were prepared on a weight/weight percent basis, while the stock salt solutions were prepared on a molar basis. Equal volumes of the stock solutions were added to form each ABS studied. Unless otherwise specified, the compositions referred to in the text and figures are the final concentrations after mixing to form each ABS. In liquid/liquid partition studies, the metal ion distribution ratios were determined radiometrically using previously described methods.27 For each determination, equal aliquots of 40% (w/w) PEG-2000 and a salt stock solution of varying molar concentrations were mixed. Each system was equilibrated by vortexing for 2 min, followed by 2 min of centrifugation (2000 g). A tracer quantity of NH499TcO4 or Na188ReO4 was then added to the ABS. The system was then twice vortexed for 2 min, followed by 2 min of centrifugation (2000 g). Equal aliquots of each phase were then removed for radioanalysis. The distribution ratio (D) was calculated by the ratio of the activity in the top PEG-rich phase divided by the activity in the lower salt-rich phase. Analysis of the uptake by the ABEC resins was carried out by measurement of weight distribution ratios (Dw) determined by batch contacts of the resin with a radiotracer-spiked salt solution of known concentration. The Dws were calculated as follows:

Dw )

Ai - Af contact volume (mL) Af mass of resin (g) × dwcf

where Ai ) activity of the solution before contact with the resin, Af ) activity of the solution after contact, and dwcf ) dry weight conversion factor relating the mass of the hydrated resin to its dry weight. The dwcf was determined by gravimetric analysis.27 Prior to their use, the resins were conditioned by placing approximately 3 g of resin in a 2-in.-diameter Bu¨chner funnel containing a Whatman #2 qualitative filter disk. The resins were washed by passing several portions of deionized water over the resin and dried with air that was bubbled through deionized water. (The resins were conditioned in this manner to ensure a constant water content.) The batch contacts were carried out following previously described methods.27 The radiotracer was added to 2.2 mL of the solution of interest, the solution was mixed, and a 100-µL aliquot was removed for analysis (Ai). A total of 1 mL of the spiked solution (contact volume) was added to two separate samples of conditioned resin of known mass (typically 15-30 mg). The samples were centrifuged for 2 min and then gently stirred for 30 min, followed by another 2 min of centrifugation (to ensure the resin remained in contact with the solution). The solution was stirred for another 30 min, after which the solution was filtered through a 0.45-µm pipet-tip filter. A 100-µL aliquot was then removed for radioanalysis (Af).

The distribution ratios (D and Dw) reported here are the average of at least two measurements and are typically accurate to (5%. Chromatographic Studies. A small 1 × 30 cm glass column (purchased from Fisher Scientific, Norcross, GA) was slurry packed with 1.0227 g of ABEC-5000 (100200 mesh) resin, yielding a bed volume of 1.36 mL in water. A small piece of glass wool was placed on top of the resin bed to prevent disturbance of the bed from the addition of eluants. The column was preconditioned by eluting 10 mL of 6 M NaOH. A 25 mL solution of 6 M NaOH and 0.1 M Na2WO4 containing 0.0123 M NaReO4 spiked with Na188ReO4 was then eluted through the column. The column was then rinsed with 6 M NaOH before stripping with water. The commercial rhenium-bearing molybdenum feed solution was centrifuged (2000 g) for 5 min, and the supernatant was separated from any precipitates. A 60mL aliquot of the centrifuged feed solution was oxidized with 0.24 g of Na2S2O8 and spiked with 50 µCi NH499TcO4. A total of 50 mL of this feed solution was then loaded onto a column previously packed with 1.758 g of ABEC-2000 (30-60 mesh) resin (bed volume ) 2.24 mL in water). The column was then rinsed with 3.5 M (NH4)2SO4 before stripping with water. Resin Irradiation Experiments. A production-scale ABEC resin, ABEC-5000 XL72 (30-60 mesh), was utilized in the irradiation experiments. For each test, a 2-g sample of the resin was suspended in 4 M NaOH in a borosilicate glass tube. The tube with suspended resin was irradiated with 60Co at 1.4 × 106 rad/h (1.4 × 103 Gy/h) to the total exposure stated. The irradiation experiments were conducted in the hot cell in the Chemistry Division at Argonne National Laboratory. The samples were placed in approximately the same location for each run, and the exposure was monitored utilizing a cobalt glass Fricke dosimeter. After irradiation, the samples were thoroughly washed with deionized water and conditioned. Batch metal ion uptakes on the irradiated resins were performed as mentioned above. The 500 Mrad irradiated resin sample was broken up using a spatula prior to batch metal ion uptake experiments. Differential Scanning Calorimetry. The differential scanning calorimeter (DSC) used was a TA Instruments DSC 2920 (New Castle, DE). The ABEC samples studied were freeze-dried prior to analysis. Approximately 10 mg of the freeze-dried original or irradiated ABEC-5000 XL72 resin was placed into a DSC sample cell. The temperature of the cell was ramped from 0 to 100 °C and the change in heat flux measured. Results and Discussion Batch Studies. The ability of salts to form ABS with PEG can be related to the Gibbs free energy of hydration of the salts’ anion28 and to a lesser extent that of the cation. Anions of water-structuring salts that interact strongly with water (i.e., have large -∆Ghyd) such as PO43- (-2765 kJ/mol), SO42- (-1080 kJ/mol), CO32(-1315 kJ/mol), and even OH-(-439 kJ/mol)29 will saltout PEG to form a biphasic system. On a molar basis, the more negative the anion’s ∆Ghyd, the less salt is required to salt-out a given amount of PEG. Chaotropic ions with small -∆Ghyd partition to the PEG-rich phase of such an ABS. We have previously utilized this feature of ABS to quantitatively partition 99TcO - (-∆G 30 to the PEG-rich 4 hyd ) -251 kJ/mol)

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Figure 1. Phase diagrams for PEG-2000/salt (NaOH, Na2WO4, or Na2MoO4) ABS at 25 °C. The biphasic regions are system compositions to the right of the binodals (solid lines).

phase of several important high ionic strength solutions including simulants of the highly alkaline radioactive waste tanks at the Hanford, WA, site31 and solutions of molybdate in caustic found in Tc generator technologies.18 In the latter study, it was noted that molybdate salts were sufficiently water-structuring to salt-out PEG. The similarity in chemistries of the ReO4-/TcO4- and WO42-/MoO42- congeners prompted us to study the applicability of ABS and ABEC to two important rhenium separations: radiopharmaceutical production of 188Re and hydrometallurgical separations of Re from Mo and W ores. On the basis of the similarity of molybdate and tungstate, we have previously demonstrated that both Na2WO4 and Na2MoO4 salt-out PEG.32 Figure 1 illustrates the binodal region for both salts and for NaOH on a weight percent basis. Combinations of salt and PEG-2000 to the left of the binodal are monophasic, while systems composed of PEG and salt concentrations to the right of the binodal are biphasic. On a molar basis, the salting-out strength increases in the order OH- < WO42- < MoO42-, reflecting the presumed order of -∆Ghyd, although accurate values for WO42- and MoO42- are lacking.30 The tie lines shown for the NaOH binodal illustrate the effect of increasing salt or PEG concentrations. The tie lines pass through the total system composition and have a node on the upper part of the binodal representing the upper PEG-rich phase composition and a node on the bottom part of the curve representing the composition of the lower salt-rich phase. Note how increasing either PEG or salt concentration results in longer tie-line lengths and increasing phase divergence. We have previously shown that the preference of a solute for either phase is enhanced as phase divergence increases; thus, increasing the concentration of Na2MoO4 increases the separation factors for NH4TcO4.18,32 The chemistries of pertechnetate and perrhenate are similar, as one might expect from their relative positions in the periodic table. We, therefore, expected both the partitioning for pertechnetate and perrhenate and their ∆Ghyd to be similar. However, the ∆Ghyd reported in Marcus’ Ion Properties30 for perrhenate is given as -338 kJ/mol. This value for ∆Ghyd is significantly different from that reported for pertechnetate (-251 kJ/mol), and the calculated values of ∆Ghyd for the perrhenate anion. The standard molar ∆Ghyd’s of TcO4- and ReO4- are from a linear correlation expression of the liquid anionexchange equilibrium constants using long-chain qua-

Figure 2. Liquid/liquid distribution ratios (D - ABS) and dry weight distribution ratios (Dw - ABEC) for 188ReO4- and 99TcO4versus increasing concentration of NaOH in the stock solution (Dw) or salt-rich phase (D) at 25 °C.

ternary ammonium cation extraction.33,34 Many of the literature values for ∆Ghyd are based on utilizing a partitioning method. Closer scrutiny of these discrepancies, and further discussion with Marcus, led to the use of the more consistent experimentally determined ∆Ghyd value of -234 kJ/mol for perrhenate. Our results suggest that the actual value of ∆Ghyd for ReO4- may be more negative than that for TcO4-, but further work would be necessary to refine the actual value. The partitioning results here do suggest that ABS and ABEC could be used as an empirical tool for determination of ∆Ghyd. Armed with a more realistic value of ∆Ghyd for ReO4(-234 kJ/mol)33,34 and the knowledge that Na2WO4 will salt-out PEG-2000, we adapted ABS and ABEC separations strategies developed for technetium to rhenium. Because rhenium is often used as a nonradioactive stand-in for the development of the chemistry of technetium, the approach seemed an obvious avenue to pursue. What is perhaps not as obvious is the technological importance and thus economic drivers found for new rhenium separations technologies. Without a high specific activity source for the 188W parent of 188Re and the high cost and demand for Re catalysts, new, efficient, separations strategies may lead to a ready supply of an important radiopharmaceutical or lower costs of rhenium metal. The most important separations involve removal of rhenium(VII) from basic tungstate or molybdate solutions. We therefore investigated the partitioning of 188Re in ABS and ABEC from NaOH, Na2WO4, and Na2MoO4 and from NaOH containing increasing concentrations of Na2WO4 or Na2MoO4. The liquid/liquid partitioning and ABEC uptake data for removal of Na188ReO4 from NaOH is presented in Figure 2. Because the perrhenate anion prefers the PEG-rich phase as expected, the distribution ratios (ABS) and Dw values increase with increasing NaOH concentration. This is a reflection of the increasing phase divergence as the systems move toward longer and longer tie-line lengths (see Figure 1). What is not immediately obvious is why the distribution ratios and Dw values for perrhenate are lower than those observed for pertechnetate. We had hoped to account for any differences in the uptake of Re versus Tc by analysis of the ∆Ghyd of each ion, because we have previously shown that the more negative this value, the lower the partitioning to the PEG-rich phase of an ABS.

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Figure 3. Liquid/liquid distribution ratios (D - ABS) and dry weight distribution ratios (Dw - ABEC) for 188ReO4- and 99TcO4versus increasing concentration of (a) Na2MoO4 or (b) Na2WO4 in the stock solution (Dw) or salt-rich phase (D) at 25 °C.

Figure 4. Liquid/liquid distribution ratios (D - ABS) and dry weight distribution ratios (Dw - ABEC) for 188ReO4- and 99TcO4versus increasing concentration of (a) Na2MoO4 or (b) Na2WO4 in a 6 M NaOH stock solution at 25 °C.

However, there are sufficient discrepancies in the actual value of ∆Ghyd for perrhenate and pertechnetate to obviate this approach. A second feature of Figure 2, which is immediately obvious, is the degradation in Dw as NaOH concentrations increase above 6 M. Previous work has shown that uptake to ABEC resin may decrease at very high salt concentrations, and it has been suggested that this may represent a dehydration of the resin due to the surface tethering and the high local salt concentration at the surface of the resin.32 The bulk PEG-rich phase in ABS does not approach this dehydration limit and appears to be unaffected at high NaOH concentrations. The working range of 1-6 M NaOH for ABEC-5000 is certainly adequate for the applications suggested here. To look at the possibility of resin degradation in NaOH, a portion of the ABEC resin was refluxed in 4 M NaOH and small portions of the resin were removed at 2 and 32 h. The Dw’s for 99TcO4- were then determined as described in the Experimental Section above. No change in Dw was found after 2 h; however, the Dw for the 32 h sample dropped to 208, which is slightly lower than the initial Dw of 330 without refluxing. As the salt concentration increases, the density difference between the salt solution and resin also increases, which may lead to poor contact. In an ABS, as the salt concentration increases, the phase incompatibility of the two phases increases.24 The greater the phase incompatibility, the more difficult it is to provide good mixing of the resin with the aqueous phase in the batch uptake studies, which may also result in reduced Dw values. Both Na2WO4 and Na2MoO4 salt-out PEG-2000, and thus both could be used to support the ABS for perrhenate and pertechnetate separation. Figure 3 illustrates the uptake of both target ions from increasing concentrations of either Na2MoO4 (Figure 3a) or Na2WO4 (Figure 3b). The distribution ratios in the ABS for Re versus Tc are very close, perhaps reflecting the low salt concentrations used. The trends in Dw in the ABEC studies are more clearly resolved, and again we see higher Dw values for 99TcO4- than for 188ReO4-. The D

and Dw values from either a molybdate or tungstate solution are nearly identical. This is a reflection of the similar salting-out strength of Na2WO4 and Na2MoO4 (Figure 1). Although rhenium can be directly separated from either Na2WO4 or Na2MoO4, the most important systems for production-scale separation are the alkaline solutions of the WO2 targets or the molybdenite or tungstenite ores. We have simulated these conditions with 6 M NaOH containing increasing concentrations of Na2WO4 or Na2MoO4. The results are depicted in Figure 4. At the low concentrations of Na2WO4 or Na2MoO4 added, there is a small but noticeable effect of the added salt on D or Dw as the concentration of added salt increases. Nonetheless, the values of D or Dw are essentially dominated by the presence of 6 M NaOH, with only a small additive effect of additional waterstructuring anions. To begin to understand the capacity of the ABS and ABEC systems, we examined the partitioning behavior of both Tc and Re as the concentration of NaReO4 is increased. Distribution ratios and Dws were measured from 6 M NaOH containing 0.1 M Na2MoO4 (Figure 5a) or 0.1 M Na2WO4 (Figure 5b) as the concentration of NaReO4 was increased. There is a noticeable decline in the uptake performance by the ABEC resins as the perrhenate concentration increases (although even Dw values as low as 100 are acceptable). This decline is likely due to the capacity being reached in the small sample of the ABEC resin used in the batch uptake experiment. There is virtually no affect on DRe in the ABS system until 0.1 M NaReO4 concentrations are reached. The same effect is observed in the absence of the parent ion. Figure 6 plots Dw for pertechnetate from 4 or 2 M NaOH. As the concentration of ReO4- (in this case the NH4+ salt) increases above 10-4 M, Dw values begin to decrease. Higher loading of the PEG-rich phase in an ABS is also indicated by phase behavior. The PEG-rich phase can become so loaded with NaReO4 that phase inversion can occur. An ABS prepared by mixing equal aliquots

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Figure 7. Chromatographic separation of NaReO4 using ABEC5000 (100-200 mesh). The load solution contained 0.0123 M NaReO4, 0.1 M Na2WO4, and 6 M NaOH. The rinse was 6 M NaOH, and the strip was deionized water and contained 26 mg of NaReO4.

Figure 5. Liquid/liquid distribution ratios (D - ABS) and dry weight distribution ratios (Dw - ABEC) for 188ReO4- and 99TcO4versus increasing concentration of (a) NaReO4 in 6 M NaOH/0.1 M Na2MoO4 and (b) NaReO4 in 6 M NaOH/0.1 M Na2WO4 at 25 °C.

Figure 8. Chromatographic separation of NH4TcO4 from a commercial rhenium-bearing molybdenum feed using ABEC-2000 (30-60 mesh). The 50-mL feed solution was oxidized with 0.24 g of Na2S2O8. The rinse was 3.5 M (NH4)2SO4, and the strip was deionized water and contained 15.92 µCi of 99TcO4-.

Figure 6. Dry weight distribution ratios (Dw) for 99TcO4(simulating ReO4-) in 4 M NaOH and 2 M NaOH versus increasing concentration of NH4ReO4 at 25 °C.

of 40% (w/w) PEG-2000 and a 6 M NaOH/0.5 M NaReO4 salt solution results in a PEG-rich phase which is more dense than the salt-rich phase. This phenomena is currently under investigation in our laboratories. Column Chromatographic Studies. The liquid/ liquid and batch data both suggest that a viable chromatographic separation of perrhenate is possible from either alkaline tungstate or alkaline molybdate solutions. The loading, retention, and stripping of Na188ReO4 from 0.1 M Na2WO4 in 6 M NaOH using ABEC5000 is presented in Figure 7. A total of 25 mL of the load solution containing 0.0123 M NaReO4 spiked with Na188ReO4 was passed through a column prepared with a 1.36-mL bed volume in water. The column was then rinsed with 6 M NaOH and stripped with water. At 8% breakthrough, 33 mg of NaReO4 had been loaded on the column. In the first 4 mL of the strip, 26 mg of NaReO4 or 79% of the load was eluted. The reason for the excessive column bleed is probably related to the absence of the tungstate matrix ions in the rinse. As discussed above, tungstate is a phase-forming anion so the perrhenate anion will be retained better in the 6 M

NaOH/0.1 M Na2WO4 load solution than in the 6 M NaOH rinse. An attempt to use a stronger phaseforming salt in the rinse, K2CO3, resulted in a precipitate that completely plugged the column. Presumably, this precipitate is the less soluble KReO4 salt. A second chromatographic run (Figure 8) was carried out to test the applicability of this method to the separation of Re from an actual rhenium-bearing molybdenum feed. A 60-mL aliquot of the molybdenum feed solution (Table 1) was oxidized with 0.24 g of Na2S2O8 and spiked with 50 µCi of NH499TcO4. A total of 50 mL of the spiked solution was loaded onto an ABEC-2000 column with a bed volume of 2.24 mL in water. At 100% breakthrough, 19.8 µCi of 99TcO4- had been loaded on the column. The column was washed with 3.5 M (NH4)2SO4 and stripped with water. The strip contained 97% of the loaded 99TcO4- (15.92 µCi) in the first 12.3 mL. ABEC-2000 resin was chosen for this study because it was available in a larger particle size (30-60 mesh) than ABEC-5000 resin (100-200 mesh) and has a better column performance (i.e., less shrink/swell, lower pressure drop, better packing, and less channeling). The larger particle size, however, resulted in a reduced capacity compared to ABEC-5000. Resin Irradiation Studies. Application of the ABEC resins in a radiopharmaceutical generator application will require exposure to high levels of radioactivity. Initial studies on the radiation stability of ABEC-5000

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Figure 9. Dry weight distribution ratios for NH499TcO4 versus molality of K2CO3 using nonirradiated and irradiated ABEC-5000 XL72 (30-60 mesh).

indicated no loss of performance after 12 Mrad of 60Co exposure of the resins suspended in water18 or after 131 Mrad of exposure of the resins suspended in 4 M NaOH.35 For the current study, we obtained the large bead size ABEC-5000 (XL72; 30-60 mesh) and exposed it to moderate and large doses of 60Co irradiation. The resin performance was evaluated by comparing batch uptake (Dw) values before and after irradiation. Resin degradation was followed using DSC. At moderate doses of radiation, there is essentially no difference in the uptake characteristics of irradiated and nonirradiated ABEC-5000 XL72. Batch uptake of TcO4- from K2CO3 (a strong salting-out agent) was used to monitor the resin performance as a function of irradiation. The results, depicted in Figure 9, show that the Dw values from increasing concentrations of K2CO3 increase only up to approximately 5 M K2CO3 and then plateau or slightly decrease. After 500 Mrad of exposure, performance degradation is readily apparent, both from the general decrease in Dw and an apparent shift of the inflection point in the Dw curve to lower concentrations of K2CO3. Nonetheless, the uptake parameters (Dws as high as 103) are still within acceptable limits. The uptake performance at low to moderate irradiation notwithstanding, the physical appearance of the resin indicated degradation of the beads. The resin beads appeared to caramelize, with the most pronounced affects found at the higher radiation doses. After 500 Mrad of irradiation, the beads had to be physically separated with a spatula, suggesting some interbead polymerization. DSC was utilized to study resin degradation. PEG is generally a liquid at room temperature below about 800 Da molecular weight and a solid above that. Thus, for molecular weights of 2000 and 5000 Da, of interest here, we exploited the melting point to characterize particular samples and “visualize” PEG. However, PEG must be solid and therefore dry or no phase transition will occur. To characterize the ABEC samples, it was thus necessary to freeze-dry them prior to analysis. Figure 10 illustrates the DSC traces for ABEC-5000 XL72 and the resin irradiated to 54.5, 130, and 500 Mrad. The original ABEC-5000 XL72 resin has at least two populations of the polymer present on the support, one with a phase transition closer to 50.3 °C and the other with a phase transition closer to 40 °C. It is not clear exactly what each represents: different polymer chain lengths which could have been produced during synthesis or different sites of attachment or locations

Figure 10. DSC traces of nonirradiated and irradiated ABEC5000 XL72 after freeze-drying.

producing different constraints on the structure of the solid PEG. After irradiation of the resin sample with 54.5 Mrad of cumulative radiation exposure, a lower melting fraction has become emphasized. The temperature of phase transition is lowered, and the heat required is reduced, which is consistent with shorter PEG chain lengths. After 130 Mrad of exposure, the lower melting peak is dominant and the heat requirement of each transition is once again lower. After 500 Mrad of exposure, the solid/liquid transition has effectively disappeared. The DSC results indicate that the PEG chains are indeed radiolytically cleaved by the exposure, producing shorter chain lengths. It is not clear, however, to what extent the DSC method can resolve this chain breakage. One might expect to see peaks with lower melting temperatures appearing in the DSC scans, but these are not observed in this study. We are currently developing additional procedures that will clarify this issue. Conclusions As we have observed previously, the trends observed for the partitioning of solutes to the PEG-rich phase from the salt-rich phase in ABS are the same trends as those for adsorption to the ABEC resin. Here, we have demonstrated that the separation of rhenium in the form of perrhenate salts from an alkaline molybdenum or tungsten solution can be accomplished using either liquid/liquid ABS approaches or chromatographic ABEC techniques. The chromatographic approach can be adapted to a Re generator technology and has a major advantage over the current alumina columns used for Tc generators: high concentrations of the tungstate parent, likely to be present as a result of low specific activity are not deleterious but should actually improve the separation, as is the case in pertechnetate/molybdate separations using ABS systems.18 (It is noted that separation factors for perrhenate/tungstate still need to be measured to further substantiate the utility of this method.) Similarly, removal of rhenium from dissolved molybdenum ores will be enhanced at high molybdenum concentrations. The studies reported here indicate that the separations are possible, but further study is needed to develop the technologies (e.g., Re generator configuration and optimization). In addition, more detail is needed on the exact nature of all breakdown products of the ABEC resins upon exposure to high radiation doses. Finally, there has been little work on the effects of temperature

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on ABEC adsorption. Because many technologically important separations may require higher temperature separation, we are currently investigating the relationship between the uptake on ABEC resins and the partitioning in ABS at elevated temperatures. Acknowledgment Support of this work by the U.S. National Science Foundation (Grant CTS-9522159) is gratefully acknowledged. We thank Dr. Ahmad Safavy, Assistant Professor of Radiation Oncology, Wallace Tumor Institute, University of Alabama at Birmingham, for his generous donation of Na188ReO4. We also thank Dr. E. Philip Horwitz and Argonne National Laboratory for access to the 60Co source used in the resin irradiation studies. Literature Cited (1) Lide, D. R., Ed. CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL,1991; pp 4-27. (2) Knapp, F. F., Jr.; Beets, A. L.; Guhlke, S.; Zamora, P. O.; Bender, H.; Palmedo, H.; Biersack, H.-J. Availability of Rhenium188 from the Alumina-Based Tungsten-188/Rhenium-188 Generator for Preparation of Rhenium-188-Labeled Radiopharmaceuticals for Cancer Treatment. Anticancer Res. 1997, 17, 1783. (3) Knapp, F. F., Jr. Rhenium-188 - A Generator-Derived Radioisotope for Cancer Therapy. Cancer Biother. Radiopharm. 1998, 13, 337. (4) Lin, W.-Y.; Lin, C.-P.; Yeh, S.-J.; Hsieh, B.-T.; Tsai, Z.-T.; Ting, G.; Yen, T.-C.; Wang, S.-J.; Knapp, F. F., Jr.; Stabin, M. G. Rhenium-188 Hydroxyethylidene Diphosphonate: A New Generator-Produced Radiotherapeutic Drug of Potential Value for the Treatment of Bone Metastases. Eur. J. Nucl. Med. 1997, 24, 590. (5) Bla¨uenstein, P. Rhenium in Nuclear Medicine: General Aspects and Future Goals. New J. Chem. 1990, 14, 405. (6) Tucker, W. D.; Greene, M. W.; Murenhoff, A. P. Production of Carrier-Free Te132, Mo99, and Tc99m from Neutron-Irradiated Uranium by Fractional Sorption on Aluminum Oxide. Atompraxis 1962, 8, 163. (7) Mikheev, N. B.; El Garhy, M.; Moustafa, Z. Generator for Production of 99Tcm from Irradiated Molybdenum. Atompraxis 1964, 10, 263. (8) Boyd, R. E. Molybdenum-99: Technetium-99m Generator. Radiochim. Acta 1982, 30, 123. (9) Molinski, V. J. A Review of 99mTc Generator Technology. Int. J. Appl. Radiat. Isot. 1982, 33, 811. (10) Bremer, K.-H. Large-Scale Production and Distribution of Tc-99m Generators for Medical Use. Radiochim. Acta 1987, 41, 73. (11) Callahan, A. P.; Rice, D. E.; McPherson, D. W.; Mirzadeh, S.; Knapp, F. F., Jr. The Use of Alumina “SepPaks” as a Simple Method for the Removal and Determination of Tungsten-188 Breakthrough from Tungsten-188/Rhenium-188 Generators. Appl. Radiat. Isot. 1992, 43, 801. (12) Mulcahy, F. M.; Houalla, M.; Hercules, D. M. Adsorption of Perrhenate on Modified Aluminas. J. Catal. 1994, 148, 654. (13) Dadachov, M.; Lambrecht, R. M.; Hetheringtion, E. An Improved Tungsten-188/Rhenium-188 Gel Generator Based on Zirconium Tungstate. J. Radioanal. Nucl. Chem., Lett. 1994, 188, 267. (14) Akhtar, M.; Mushtaq, A.; Karim, H. M. A.; Iqbal, M. Z. Separation of No-Carrier-Added 188Re from Neutron-Irradiated Natural Tungsten. Sep. Sci. Technol. 1996, 31, 1997. (15) Rohal, K. M.; Van Seggen, D. M.; Clark, J. F.; McClure, M. K.; Chambliss, C. K.; Strauss, S. H.; Schroeder, N. C. Solvent Extraction of Pertechnetate and Perrhenate Ions From NitrateRich Acidic and Alkaline Aqueous Solutions. Solvent Extr. Ion. Exch. 1996, 14, 401. (16) Rogers, R. D.; Bond, A. H.; Zhang, J.; Bauer, C. B. Polyethylene Glycol Based-Aqueous Biphasic Systems as Technetium-99m Generators. Appl. Radiat. Isot. 1996, 47, 497. (17) Rogers, R. D.; Bond, A. H.; Griffin, S. T.; Horwitz, E. P. New Technologies for Metal Ion Separations: Aqueous Biphasic Extraction Chromatography (ABEC). Part I. Uptake of Pertechnetate. Solvent Extr. Ion. Exch. 1996, 14, 919.

(18) Rogers, R. D.; Bond, A. H.; Zhang, J.; Horwitz, E. P. New Technetium-99m Generator Technologies Utilizing Polyethylene Glycol-Based Aqueous Biphasic Systems. Sep. Sci. Technol. 1997, 32, 867. (19) Rogers, R. D.; Zhang, J. Effects of Increasing Polymer Hydrophobicity on Distribution Ratios of TcO4- in Polyethylene/ Poly(propylene glycol)-Based Aqueous Biphasic Systems. J. Chromatogr. B 1996, 680, 231. (20) Bond, A. H.; Chang, F. W. K.; Thakkar, A. H.; Williamson, J. M.; Gula, M. J.; Harvey, J. T.; Griffin, S. T.; Rogers, R. D.; Horwitz, E. P. Design, Synthesis, and Uptake Performance of ABEC Resins for the Removal of Pertechnetate from Alkaline Radioactive Wastes. Ind. Eng. Chem. Res. 1999, 38, 1676. (21) Bond, A. H.; Gula, M. J.; Harvey, J. T.; Duffey, J. M.; Horwitz, E. P.; Griffin, S. T.; Rogers, R. D.; Collins, J. L. Flowsheet Feasibility Studies Using ABEC Resins for Removal of Pertechnetate from Nuclear Wastes. Ind. Eng. Chem. Res. 1999, 38, 1683. (22) Angelidis, T. N.; Rosopoulou, D.; Tzitzios, V. Selective Rhenium Recovery from Spent Reforming Catalysts. Ind. Eng. Chem. Res. 1999, 38, 1830. (23) Blazy, P.; Jdid, E. A.; Floreancig, A.; Mottet, B. Selective Recovery of Rhenium from Gas-Scrubbing Solutions of Molybdenite Roasting Using Direct Precipitation and Separation on Resins. Sep. Sci. Technol. 1993, 28, 2073. (24) Albertsson, P. A° . Partition of Cell Particles and Macromolecules; John Wiley & Sons: New York, 1986. (25) Asenjo, J. A.; Turner, R. E.; Mistry, S. L.; Kaul, A. Separation and Purification of Recombinant Proteins from Escherichia coli with Aqueous Two-Phase Systems. J. Chromatogr. 1994, 668, 129. (26) Salamanca, M. H.; Merchuk, J. C.; Andrews, B. A.; Asenjo, J. A. On the Kinetics of Phase Separation in Aqueous Two-Phase Systems. J. Chromatogr. B 1998, 711, 319. (27) Huddleston, J. G.; Griffin, S. T.; Zhang, J.; Willauer, H. D.; Rogers, R. D. Metal Ion Separations in Aqueous Biphasic Systems and with ABEC Resins. In Aqueous Two-Phase Systems: Methods and Protocols; Hatti-Kaul, R., Ed.; In Methods in Biotechnology, Vol. 11; Walker, J. M., Ed.; Humana Press, Totowa, NJ, 2000, pp 77-94. (28) Rogers, R. D.; Bond, A. H.; Bauer, C. B.; Zhang, J.; Griffin, S. T. Metal Ion Separations in Polethylene Glycol-Based Aqueous Biphasic Systems: Correlation of Partitioning Behavior with Available Thermodynamic Hydration Data. J. Chromatogr. B 1996, 680, 221. (29) Marcus, Y. Thermodynamics of Solvation of Ions Part 5.Gibbs Free Energy of Hydration at 298.15 K. J. Chem. Soc., Faraday Trans. 1991, 87, 2995. (30) Marcus, Y. Ion Properties; Marcel Dekker: New York, 1997; pp 126-127. (31) Rogers, R. D.; Bond, A. H.; Bauer, C. B.; Zhang, J.; Rein, S. D.; Chomko, R. R.; Roden, D. M. Partitioning Behavior of 99Tc and 129I from Simulated Hanford Tank Wastes Using PolyethyleneGlycol Based Aqueous Biphasic Systems. Solvent Extr. Ion. Exch. 1995, 13, 689. (32) Rogers, R. D.; Zhang, J. New Technologies for Metal Ion Separations Polyethylene Glycol Based-Aqueous Biphasic Systems and Aqueous Biphasic Extraction Chromatography. In Ion Exchange and Solvent Extraction; Marinsky, J. A., Marcus, Y., Eds.; Marcel Dekker: New York, 1997; Chapter 4, Vol. 13, pp 141193. (33) Marcus, Y. Personal communication, 1999. (34) Shmidt, V. S.; Rybakov, K. A.; Rubisov, V. N. The Use of Linear Free Energy Relationships for the Quantitative Description of a New Set of Anion-Exchange Extraction Constants for Some Singly-Charged Anions. Russ. J. Inorg. Chem. 1982, 27, 855. (35) Rogers, R. D.; Griffin, S. T.; Horwitz, E. P.; Diamond, H. Aqueous Biphasic Extraction Chromatography (ABEC): Uptake of Pertechnetate From Simulated Hanford Tank Wastes. Solvent Extr. Ion. Exch. 1997, 15, 547. (36) Wang, C. H.; Willis, D. L.; Loveland, W. D. Radiotracer Methodology in the Biological, Environmental, and Physical Sciences; Prentice-Hall: Englewood Cliffs, NJ, 1975; pp 217-221.

Received for review August 2, 1999 Revised manuscript received March 27, 2000 Accepted March 27, 2000 IE990583P