Metal-Ion Separation and Preconcentration - American Chemical Society

Chapter 13

Design of Novel Polymer-Supported Reagents for Metal Ion Separations

Downloaded by UNIV OF ARIZONA on January 22, 2013 | http://pubs.acs.org Publication Date: February 11, 1999 | doi: 10.1021/bk-1999-0716.ch013

Spiro D. Alexandratos and Latiff A. Hussain Department of Chemistry, University of Tennessee, Knoxville, TN 37996

Polymer-supported reagents are prepared by the immobilization of ligands onto macromolecular matrices. Appropriately chosen ligands with high affinities toward targeted metal ions allow for the application of these reagents to separations science. Ion exchange and chelating resins are the two types of reagents traditionally used in metal ion separations. The latter are more selective than the former but with significantly slower rates of complexation. Examples of both types of resins are presented. Current research has shown that combining both mechanisms within a single polymer support yields selective complexation at rapid rates. Examples will be discussed. The complexation and separation of metal ions is an area where functionalized polymers have been used extensively. It has been estimated that over 25,000 hazardous waste sites exist in the United States alone and the cost of remediation will exceed one trillion dollars (i). For example, at the Hanford nuclear facility, nearly 1.4 billion m of hazardous waste, most of it high level radioactive waste, spread over 560 mi of land needs to be treated (2). Making the problem more difficult is the fact that much of the waste exists in complex mixtures which must be separated prior to transfer to a safe storage area. Radioactive metal ions may be present in low pH solutions which contain high levels of dissolved solids and/or organic compounds, as is the case with uranium (3), as well as in basic solutions, as found with cesium (4). The complexity of such mixtures poses a formidable challenge to existing separation methods. A need exists for the development of improved technologies in order to make remediation of these sites both chemically and economically feasible. 3

2

Ion Exchange Resins Ion exchange is a technique that has found widespread use in both remediation and pollution prevention. Ligands are covalently bound to an insoluble organic or inorganic polymer (5) and the ion-containing aqueous phase is passed through a column containing

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©1999 American Chemical Society

In Metal-Ion Separation and Preconcentration; Bond, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

195 the polymer, usually in the form of beads. An ion exchange resin removes the metal by replacing it with an ion such as Na (if the resin has cation exchange sites with ionically bound sodium ions) or CI" (if the resin has anion exchange sites with ionically bound chloride ions) while a chelating resin removes the metal through complex formation (6). After the polymer has been loaded, the metal is eluted by passing an appropriate solution through the column to regenerate the polymer and produce a solution containing the extracted metal. The regenerability of the polymer and the recoverability of the metal ion are important factors to consider when deciding whether a resin can be used under process conditions. In the case of cation exchange resins, a strongly acidic solution may be used to recover the metal ion, but a more selective resin may require costly régénérants such as EDTA or 1-hydroxyethane-1,1-diphosphonic acid. In general, ion exchange resins can be loaded and regenerated many times without a significant loss of capacity (7). The initial price of ion exchange systems can be high due to the cost of the resin, but the recyclability of the polymer and the recovery of precious metals can make the process economically viable. The kinetic performance of the polymers can also be enhanced by varying the physical properties of the polymer such as the degree of crosslinking and porosity. The organic polymers used as supports for ion exchange resins can be synthetic or naturally occurring. Cellulose has been the most extensively studied natural polymer (8, 9). Unmodified cellulose has a low ion exchange capacity; in most cases, cellulose is used after it has been modified by oxidation, esterification or etherification. Because degradation is possible in very acidic solutions, the application of cellulose is limited to solutions with an acidity no greater than ΙΟ" M (10). The synthetic polymer supports can be either step-growth or chain-growth polymers. The most common crosslinked step-growth polymer used as an ion exchange support is a phenolic made by the condensation of phenol and formaldehyde (11, 12). Chain-growth polymers used as ion exchange supports are prepared by free radical polymerization of vinyl monomers with divinyl monomers added as cross-linking agents. The most important example of this type of support is a copolymer of styrene and divinylbenzene (DVB) (12-14) due to its display of the following properties:


+

4

• stable to a wide range of pH with alkaline and acidic reagents; • resists hydrolytic cleavage; • thermally stable at normal use temperatures; • mechanical stability can be controlled by the degree of cross-linking; • easily prepared at different porosities; • readily functionalized via electrophilic and nucleophilic aromatic substitution. Polystyrene has become the most widely used support not only for ion exchange applications but also for other purposes such as immobilized catalysts. Ion exchange resins with various degrees of cross-linking have different physical and mechanical properties (13, 15). Lightly cross-linked beads can be swollen by contact with good solvents but the resin can become mechanically unstable. The resin volume can change considerably in different solutions and is a disadvantage when column work is considered. Resins with high degrees of cross-linking do not swell as much as lightly cross-linked resins and their volume does not undergo a significant change in different


196 solutions. The disadvantages of the highly cross-linked resins are their low exchange capacity and restriction of ionic diffusion. Generally, the selectivity of the resins towards metal ions can be improved by increasing the degree of cross-linking (5). Ion exchange resins can be classified by their functional groups (5, 10, 15): • Cation exchange resins - contain acidic functional groups such as -S0 H and -COOH. • Anion exchange resins - contain basic functional groups such as -NR and -N R C1". • Amphoteric ion exchange resins - contain both acidic and basic functional groups. • Chelating resins - contain functional groups that chelate metal ions. 3

+


2

3

At this time, the sulfonic acid and carboxylic acid resins are the two most common cation exchangers (16-21). However, they have disadvantages that limit their application. The sulfonic acid resin is a strong cation exchanger; its most serious drawback is its lack of selectivity towards targeted metal ions when other cations are present in solution. For example, it cannot effectively recover uranium from leach liquor when iron, aluminum, and other metal ions are present along with the U 0 (10). The weakly acidic carboxylic acid resin is more selective than the sulfonic acid resin, especially for the alkaline earth metal ions over monovalent ions (22, 23). However, the exchange capacity of the carboxylic resin is strongly dependent upon the solution pH. The effective pH range is 6 to 14 because of the high affinity of the carboxylic acid resin for H (given dissociation constants between 10" and 10") (10). This property obviates its application to the recovery of metal ions from highly acidic solutions. In order to solve problems for which sulfonic and carboxylic acid resins are not applicable, chelating resins, which can selectively recover specific metal ions from dilute solutions, have been developed (6, 24). Their selectivity is due to the ability of the ligand to form a highly stable complex with a particular metal ion and less stable complexes with other ions. Early work in this field was published by Kennedy (25), who observed that thorium(IV), iron(m) and uranyl cations formed relatively strong inner chelate complexes with partially esterified phosphates and phosphonates as the acids or their corresponding sodium salts, whereas alkaline earth, divalent transition metal ions, and lanthanides formed weak complexes. Moreover, the chelate complexes usually show much higher stability constants than the corresponding monomer complexes (25). For example, the stability constants of the phosphonic acid resin for U 0 and Fe(III) are greater by 10 or more over that of the corresponding monomers. The increased stability of the polymer-supported complexes was attributed to a combination of factors such as the polymer entropy effect and a lower dielectric constant of the resin matrix as compared to an aqueous medium. 2+

2

+

5

9

2+

2

6

Chelating Resins In order to understand the interaction between polymer-supported ligands and metal ions, numerous chelating resins have been synthesized that contain nitrogen and/or oxygen as the donor atoms. The iminodiacetic acid resin (Figure 1), which is commercially available as Dowex A - l (Dow Chemical Co.) and Chelex-100 (Bio-Rad Laboratories), is probably the most extensively studied chelating resin and has been used for selective


197 separations of transition metal ions from alkali and alkaline-earth metal ions in different pH and ionic strength solutions (26-28). The selectivity of Dowex A - l has been determined to be M^ lanthanides(ni)>hydrogen(I)>copper(n)>cobalt(n)> barium(ïï)>sodium(I).


Reactive Ion Exchange The concept of reactive ion exchange (RIEX) was introduced by Helfferich (42) and extended by Janauer (43) as a means of enhancing the separation abilities of traditional ion exchange resins. In conventional ion exchange, the overall process is exclusively a redistribution of counterions and the exchanging ions retain their identity in the resins. However, a reactive ion exchange procedure, by definition, contains at least one ion exchange step and one chemical reaction such as reduction, precipitation, neutralization or complex formation. In general, the chemical reaction involves transformation of an initially present species to a new species. An example of RIEX is the titration of a strong acid cation exchange resin with sodium hydroxide which involves an ion exchange reaction (exchange of the resin proton with the sodium ion) accompanied by a neutralization reaction (water formed by the consumption of hydroxide ions from solution and protons from the resin). Because the AG of ion exchange is only a few kilocalories per mole (44), the values among different metal ions are too small to impart selectivity to the separation. The advantage of employing RIEX to achieve selective separation is to utilize favorable free energy changes of accompanying chemical reactions which yield an overall negative free energy of sufficient magnitude for the desired separation (45,46). Since ion exchange can be coupled with many suitable reactions, RIEX is a very useful tool in the design of new extractants for selectively recovering metal ions. Dual Mechanism Bifunctional Polymers A new category of phosphorus-based metal ion complexing agents, termed dual mechanism bifunctional polymers (DMBPs), has been developed for the selective complexation and recovery of metal ions from aqueous solutions (47). These polymers are synthesized with two different functional groups, each displaying either an access mechanism or a recognition mechanism. The access mechanism is provided by an aspecific ion exchange ligand which mainly enhances ion accessibility and mobility within the polymer network. The recognition mechanism is responsible for the specificity through reduction, coordination or precipitation. During the separation process, the ion exchange group first serves to bring the metal ions into the polymer matrix and the highly specific reaction occurs subsequently between the metal ion and the recognition group. DMBPs are divided into three classes based upon the type of recognition mechanism. The Class I resin is an ion exchange/redox resin which combines ion exchange with a recognition group capable of reducing certain metal ions. This class of resins is exemplified by the phosphinic acid resin (Figure 6). The P-O-H moiety is capable of ion exchange with the metal ions through the acidic hydrogen while the P-H group is capable of metal ion reduction. The ligand is thus oxidized to the phosphonic acid. Both Ag(I) and Hg(II) are rapidly reduced to the zerovalent state (48). This resin



200

also shows a significant ability to coordinate non-reducible metal ions through the phosphoryl oxygen in highly acidic solutions wherein ion exchange is not expected to occur (49). Finding that the Class I resins had a strong coordinating ability led directly to the development of Class Π DMBPs: the ion exchange / coordination resins. The application of phosphoryl-containing extractants to metal ion recovery processes is well known. For example, tributyl phosphate and trioctylphosphine oxide have been used in the recovery of actinides and numerous other metal ions (50). Purely coordinating polymer-supported extractants have been prepared and most of these resins display excellent selectivity for different metal ions (51). However, the polymeric coordinating extractants usually show very low metal ion loading capacity due to their limited hydrophilicity and accessibility. The Class Π resins studied most extensively consist of a phosphonic acid ligand as the ion exchange group for enhanced accessibility and either a phosphorus ester or tertiary amine group as the coordinating ligand for enhanced selectivity (Figure 7) (52). In one example, the distribution coefficient (the ratio of milliequivalents M per g (dry weight) polymer to milliequivalents M per mL solution) for Ag(I) ions was 2900 in 4 Ν HN0 for the polymer with monoethyl and diethylphosphonate groups bound to the polymer. Under the same conditions, the monofunctional polymer with diethylphosphonate ligands had a distribution coefficient of490 and the polymer with monoethylphosphonate ligands had a distribution coefficient of 440 (36). The third class of DMBPs are the ion exchange/precipitation resins (53). In many cases, if the metal ions cannot be reduced by the first class or coordinated by the second class of DMBPs, removal of metal ions from aqueous solutions by the formation of insoluble salts is an important alternative. The Class ΠΙ resins combine the phosphonic acid ligand for enhanced ionic accessibility with a quaternary amine group (Figure 8) whose associated anion can react with the targeted metal ion to form an insoluble metal salt. The salt precipitates within the beads and can be recovered through resolubilization into a concentrated solution. For example, this class of resins is useful for the recovery of barium ions which readily form precipitates with certain anions: barium has a reduction potential too low to be reduced by the Class I resin and does not readily coordinate with the phosphoryl ligands of the Class Π resins. The DMBPs operate through the inter-ligand cooperation of two different groups on neighboring sites in complexing a given metal ion. The importance of intraligand cooperation for metal ion chelate formation was quantified through the synthesis of an immobilized gem-diphosphonic acid ligand (Figure 9) (54). In one example, the resulting ion exchange resin (now available as Diphonix® from Eichrom Industries, Inc.) had a distribution coefficient of 7xl0 for U 0 in 1 Ν HN0 as compared to a value of 900 for the analogous monophosphonic acid and 200 for the sulfonic acid resin. Our current research emphasizes the synthesis of other polymers capable of selective intra-ligand cooperation such as the ketophosphonates and will be the subject of a future report. n +

n +

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Conclusion

Ion exchange and chelating resins will continue to play a pivotal role in many applications involving water treatment, the mining industry, and environmental



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Figure 5. Polymer-supported cryptand. Ο II P-OH ι Η Figure 6. Class I Dual Mechanism Bifunctional Polymer.

R = H, Et Figure 7. Class II Dual Mechanism Bifunctional Polymer.

Figure 8. Class III Dual Mechanism Bifunctional Polymer.



Figure 9. Intra-ligand (top) vs. inter-ligand (bottom) cooperation.


203 remediation for the foreseeable future. Their use within well-engineered continuous processes is an important advantage. The key issue in separations science has been selectivity coupled with rapid kinetics. It is now understood that this can be achieved with bifunctional polymers: an access ligand can greatly enhance complexation kinetics without dimimshing the selectivity of the recognition ligand. Further progress is thus expected in the preparation of ion-selective polymer-supported reagents.


Acknowledgment It is a pleasure to acknowledge the Department of Energy, Office of Energy Research, Division of Chemical Sciences, Office of Basic Energy Sciences, for their continued support of this research through grant DE-FG05-86ER13591. Literature Cited (1)

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Metal-Ion Separation and Preconcentration - American Chemical Society

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