Recent Progress in the Development of Extraction Chromatographic

Chapter 11

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Recent Progress in the Development of Extraction Chromatographic Methods for Radionuclide Separation and Preconcentration Mark L. Dietz Chemistry Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439-4831

Extraction chromatography (EXC), a form of liquid chromatography in which the stationary phase typically consists of an extractant solution sorbed on an inert support, provides a simple and effective means by which the separation and preconcentration of a variety of radionuclides and their isolation from major sample constituents can be achieved. Recent advances in extractant design, particularly the development of extractants capable of metal ion recognition or strong complex formation in acidic solution, have substantially improved the utility of the method. Advances in support design, particularly the introduction of functionalized supports capable of enhancing radionuclide retention or the stability of the chromatographic materials, promise to provide further improvements.

© 2004 American Chemical Society In Radioanalytical Methods in Interdisciplinary Research; Laue, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Background Growing public health and safety concerns over the use of nuclear materials and technology, both in the production of power and the fabrication of nuclear weapons, have made rapid and reliable methods for the determination of various radionuclides in environmental and biological samples increasingly important. Such determinations, however, are rendered difficult by the complicated and variable composition of the samples encountered and by the low levels of the radionuclides typically present. Frequently, the radionuclide of interest must be both separated from the major constituents of the sample and/or possible interferents and preconcentrated prior to determination. In this chapter, we summarize recent progress in extraction chromatography, a separation and preconcentration technique that has attracted increasing attention over the last several years, and briefly describe several promising directions for future research efforts in this area. Extraction chromatography ( E X C ) is a type of reversed-phase chromatography in which the stationary phase typically comprises an extractant or a solution of an extractant supported on an inert substrate (eg., Teflon™) ( i ) . Unlike ordinary partition chromatography, in which a solute undergoes little i f any chemical change upon sorption, the uptake of a metal ion in E X C involves the complex chemical changes associated with the conversion of a hydrated metal ion into a neutral, organophilic metal complex, just as in liquid-liquid extraction. This conversion process often involves a number of interactions and equilibria, many of which can be manipulated to provide systems capable of the efficient and selective separation of any of a number of metal ions (2). Conventional E X C materials are prepared by the physical impregnation of a porous substrate with an extractant, which can be accomplished by any of several techniques (/, 3, 4). Most commonly, the support material is contacted with a solution of the extractant (or extractant-diluent mixture) in a volatile solvent, which after a period of equilibration, is slowly removed by evaporation under vacuum. For very hydrophobic extractants, better results {i.e., more homogeneous distribution of the extractant on the support) can sometimes be obtained by contacting a solution of the extractant in a precalculated amount of diluent with a support until all of the liquid has been absorbed. In the alternative, the extractant can be incorporated into the support during its preparation. Macroporous styrene-divinylbenzene copolymers containing an extractant added to the mixture of monomers during polymerization, for example, have been prepared (J,

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Elution Volume (FCV) Figure 2. Effect of column washing on the elution behavior of Sr-85 on a conventional strontium-selective extraction chromatographic material ("Sr Resin"). (Eluent: 1MHNO3; Flow rate: 1-2 mL/cm /min; Temperature: ca. 23 C; Particle size: 100-150 jim; Filled circles: unwashed resin; Open circles: washed (250 FCV) resin.) 2

9

In Radioanalytical Methods in Interdisciplinary Research; Laue, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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volumes upon column washing is consistent with loss of diluent (here, 1-octanol) from the support, also noted by other workers (33), and suggests that the stability of the resin might be improved by changing or even eliminating the diluent. Previous work, however, has shown that a change to a higher molecular weight (and thus, less water-soluble) alcohol would be expected to lead to a decrease in strontium retention by the resin (21, 34). Complete elimination of the diluent would also seem to be out of the question, as prior studies have clearly established the important role played by the alcohol (in particular, by the water dissolved therein) in promoting anion transfer and thus, strontium extraction, from acidic nitrate media (21).

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Elution Volume (FCV) Figure 3. Comparison of the elution behavior of Sr-85 on a conventional (filled circles) and an octanol-free (open circles) strontium-selective extraction chromatographic material. (Conditions are as noted in Figure 2.)


168 Surprisingly, however, as shown i n Figure 3, the elution behavior o f 8 5 on an E X C resin comprising only DtBuCH18C6 dispersed on an appropriate polymeric support is barely distinguishable from that observed on the conventional resin in which the stationary phase consists of an 1-octanol solution of the crown ether. The absence of an appreciable effect on elution behavior extends to other elements as well. Thus, the selectivity of the resin for S r ^ over such cations as N a and Ca^ " is preserved, despite the absence of a diluent (55). Figure 4, which relates the amount of water extracted into 1-octanol by an equimolar mixture of the cis-syn-cis (A) and cis-anti-cis (B) isomers of dicyclohexano-18-crown-6 (DCH18C6) to crown ether concentration provides a partial explanation for this unexpected result. That is, DCH18C6 (and by analogy, DtBuCH18C6), like aliphatic alcohols, is able to extract significant amounts (nearly 2 moles per mole of crown ether) of water. Thus, in the absence of a diluent, DtBuCH18C6 can apparently fulfill the role ordinarily played by the alcohol: bringing water into the stationary phase to facilitate transfer of the metal ion-nitrato-crown ether complex. S r

+


+

4

[DCH18C6, A/B], M

Figure 4. Crown ether concentration dependence of the extraction of water from 1 M nitric acid into 1-octanol by DCH18C6 (mixture of A and B isomers).


169 Figure 5 shows the effect of column washing on the elution behavior of *^Sr on the oetanol-free extraction chromatographic resin. A s shown, a volume of mobile phase that had induced a significant change in the elution behavior of 8 5 r on the original resin (Figure 2) has no discernible effect on strontium elution on this material. Taken together, the enhanced physical stability resulting from the elimination of the diluent and the strong and selective strontium sorption provided by DtBuCH18C6 constitute an excellent illustration of the possibilities afforded by improved extractants and by judicious choice of diluent (or lack thereof) in the design of new E X C materials. Downloaded by STANFORD UNIV GREEN LIBR on July 19, 2012 | http://pubs.acs.org Publication Date: November 4, 2003 | doi: 10.1021/bk-2004-0868.ch011

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Elution Volume (FCV) Figure 5. Effect of column washing on the elution behavior of Sr-85 on the oetanol-free, strontium-selective extraction chromatographic material. (Fluent: IMHNO3; Fl° 1-2 mL/cm /min; Temperature: ca. 23 °C; Particle size: 100-150 fim; Filled circles: unwashed resin; Open circles: washed (261 FCV) resin). w

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Supports As already noted, in conventional extraction chromatography, supports are specifically chosen to be "inert" to the extractant, diluent (if any), and the constituents of the sample. In the last few years, however, there has been growing awareness of the possibility of employing support properties to enhance the performance of E X C materials. It has long been known, of course, that the physical properties of a support influence certain aspects of chromatographic separations. For example, the improved column efficiency resulting from the use of smaller particle size supports is well-established (36). Recently, however, interest has turned to the effect of support chemistry on the behavior of E X C materials. In these systems, hereafter denoted as "active-substrate" E X C materials, the support is chosen or designed specifically to either interact with the extractant through other than the weak adsorptive forces that typify extraction chromatography or to actually participate in metal ion uptake. In the first category are E X C resins prepared by the impregnation of a support capable of acid-base interactions or ion exchange with an extractant bearing an appropriate functional group (e.g., an anionic functionality capable of interaction with an anion-exchanger). Table I summarizes the various studies that have been performed with such systems, extending from early investigations by Akaiwa (57), Tanaka (38-41), and Lee (42) to more recent work by Sarzanini (43), Warshawsky (44, 45), and Khalifa (46). Most frequently, a sulfonated extractant has been sorbed on a strong anion-exchanger and the metal ion sorption properties of the resultant materials determined. Not unexpectedly, these studies have shown that the retention of the extractant is likely due to a combination of adsorption and ion-exchange (with the latter dominating). Moreover, this retention is sometimes sufficiently strong for the resin to withstand contact with acids and bases. Two limitations in these materials have become evident, however. First, the capacity of certain of the resins is less than that expected on the basis of extractant loading, suggesting that not all of the extractant is available for complexation (42). In addition, it appears that the binding ability of certain immobilized extractants is less than that of the free extractant (43). In the absence of a detailed, systematic comparison of the performance of E X C materials based on functionalized supports to that of analogous "inert substrate" materials, it remains unclear i f substrates bearing ion-exchange or acid-base functionalities offer a compelling advantage over conventional ones. For now then, this approach can only be regarded as a possible step toward improved (in particular, more stable) E X C materials, but one that clearly warrants additional investigation.


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Table I. Extraction Chromatographic Systems Employing Functionalized Supports Reference

Target Ion

Extractant

Support

Hg(II),Cu(II), Zn(II), Cd(II), Fe(II), Mn(II)

8-quinolinol-5-sulfonic acid

Diaion SA-100

37

Hg(H)

disodium(4-sulphophenyl)1 -[2-(4-sulphophenyl) hydrazide]diazenecarbothio ate

Amberlite IRA-400

38

Hg(II),Cu(II), Cd(II)

disodium4,4-(4diazenediyl-5-mercapto-3methyl-1,2diazacyclopenta-2,4-diene1 -yl)dibenzenesulphonate

Amberlite IRA-400

39,41

Hg(II), Cu(II)

disodium(4-sulphophenyl)1 -t2-(4-sulphophenyl) hydrazide] diazenecarbothio ate

Amberlite IRA-400

40

tetraphenylporphinetrisulph onic acid Fe(III), Cu(II),Pb(II), Cr(VI)

7-iodo-8-hydroxyquinoline5-sulfonic acid

Dowex 1-X8

42

Al(m)

Pyrocatechol Violet

BioRadAGMP-1

43

Pb(II),Zn(n), Ni(II)

di(2-ethylhexyl) dithiophosphoric acid

Reillex™HP, Reillex™425

44,45

U(VI)

Alizarin Red S

DuoliteA 101

46

Recently, another approach to the stabilization of E X C materials has been proposed, one which also involves some novel support chemistry. In this approach (Figure 6), first described by Horwitz and Dietz (47) and subsequently elaborated by Alexandratos and co-workers (48), macroporous beads of a copolymer of chloromethylstyrene and methylmethacrylate are prepared (using divinylbenzene as a cross-linker), then surface-functionalized to leave unreacted carbon-carbon double bonds. Following impregnation of the support with an extractant, a pair of water-soluble monomers ( N,N* methylenebis(acrylamide /



172 glycidyl methacrylate) are polymerized in the presence of the beads, resulting in the formation of a polymer film anchored to the bead by what had been the surface C=C bonds. The effectiveness of this approach as a means of stabilizing an E X C resin is illustrated by results reported for the uptake of copper ion from a p H 8 buffer by a bis(2-ethylhexyl)phosphoric acid (HDEHP)-loaded resin, both in the presence and absence of a polymer coating. Even after 5 days of contact with the buffer solution, no diminution in the uptake of copper ion by the coated resin is observed. In contrast, prolonged contact of the uncoated resin with the same buffer leads to leaching off of the extractant and a pronounced decrease in copper uptake. One problem that has been identified with this approach is that the polymer film decomposes upon contact with acids to release formaldehyde. If this problem can be addressed and i f it can be demonstrated that the encapsulation process does not lead to unacceptably long equilibration times (as has been the case for E X C materials encapsulated by a Nylon 6-10 film (49% for which 2-3 hours are required for the attainment of sorption equilibrium), the use of "anchored coatings" would seem to offer much promise as a means of producing improved E X C materials.

support

surfacefunctionalization

impregnation

encapsulation

Figure 6. Schematic diagram showing the preparation of an extraction chromatographic resin stabilized via polymer encapsulation. To date, little effort has been devoted to the study of the second type of active- substrate E X C material, in which the support actually participates in the metal ion extraction process. Moyer et al. (50) have examined the uptake of copper ion by E X C materials comprising a neutral extractant, tetrathia-14crown-4, sorbed on a series of polystyrene-divinylbenzene-based strong-acid cation-exchange resins, among them the commercially available Dowex 50WX 8 . While neither unfunctionalized polystyrene-divinylbenzene resin nor the same resin loaded with the macrocycle extracted any detectable copper, impregnation of the cation-exchanger produced a 10-100-fold enhancement in the observed copper distribution ratio vs. the cation-exchanger alone. This enhancement was attributed to a synergistic effect involving coordination of the copper by the macrocycle and cation-exchange by the polymer-bound sulfonic acid functional groups. Although such synergistic effects are common in liquid-


173 liquid extraction systems involving mixtures of crown ethers and liquid cationexchangers (51), Moyer's results represent the first demonstration of synergism in an E X C material involving a functionalized support. Such supported synergistic systems appear to offer a wealth of opportunities for the development of new E X C materials exhibiting enhanced metal ion uptake and selectivity.


Conclusions In recent years, the field of extraction chromatography has evolved in several important respects. Extractants of limited selectivity or complexing ability (particularly in highly acidic media), for example, are being supplanted by those exhibiting high selectivity and/or complexing power. A t the same time, interest in purely "passive" supports, whose chemistry plays no significant role in either metal ion uptake or resin stability, is slowly giving way to recognition of the possibilities offered by functionalized (i.e., "active") supports. Progress in support and extractant chemistry has already yielded notable improvements in the performance of E X C materials, but considerable room for improvement clearly remains, particularly in the area of physical stability. If this limitation can be overcome, the utility of extraction chromatography as a method for the separation and preconcentration of radionuclides is certain to increase.

Acknowledgements This paper is based in part on publications with my colleagues, past and present, in the Chemistry Division at Argonne National Laboratory. Their efforts are gratefully acknowledged. This work was performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, U.S. Department of Energy, under contract number W-31-109-ENG-38.

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Recent Progress in the Development of Extraction Chromatographic

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