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Researchers Invent Novel Plutonium-Selective Anion Exchange Resins for Waste Minimization

How can we minimize the plutonium content of our waste streams? We asked ourselves this question as we reviewed the anion exchange process used for nearly 40 years to recover plutonium from a variety of impurities.

Background

Ion exchange resins are polymers that contain charged functional groups. Such resins have been used for many years in a wide range of applications for recovery of positively charged cations and negatively charged anions from solution. Perhaps the most common application is the removal of impurities from potable water in home water softeners.

Ion exchange resins contain fixed-charge functional groups that require counterions of the opposite charge to maintain an overall electroneutrality. Anions or cations present in solution may exchange with groups of the same charge bound electrostatically to the resin functional groups, hence the term ion exchange for this process. A major advantage of the ion exchange process is the ease of separating the solid resin from the treated solution.

Although ion exchange is a mature technology, the performance characteristics of a specific resin structure and solution composition are determined by complex factors that are still only partially predictable. For this reason, we have directed much effort toward gaining a better theoretical understanding of ion exchange processes.

Plutonium

Pu(IV), the most stable oxidation state of plutonium in acid solutions, has four positive charges, yet it readily bonds with six nitrate ions to form a double-negative complex that is strongly retained on anion exchange resins. Anion exchange is an especially attractive optionfor separating plutonium because (1) the Pu(IV) nitrate complex is very strongly held, and (2) few other metal ions form competing complexes.

The major disadvantage of the nitrate anion exchange system has always been the unusually slow rate at which the Pu(IV) nitrate complex sorbs onto the resin. For this reason, we previously measured the sorption rate of more than 30 commercial and experimental resins. The results of that study led us to replace gel-type polystyrene resin with a macroporous polystyrene resin, whose more porous structure significantly increased the plutonium sorption rate.

Safety Considerations

Although such polystyrene anion exchange resins have been used for many years in the nuclear industry, these polymers can react exothermally with nitric acid under certain conditions. Another polymer, polyvinyl-pyridine, is known to provide greater stability to chemical attack by nitric acid and radiolytic degradation; however, no commercial polyvinylpyridine resin was suitable for plutonium. Consequently, we began a collaborative effort with Reilly Industries, Inc., a manufacturer of vinylpyridine polymers. Our collaboration resulted in ReillexTM HPQ, a new macroporous anion exchange resin that met our objectives.

For the past six years Reillex HPQ has been used for plutonium processing at TA-55. This polyvinylpyridine resin has outperformed all known commercial resins, while providing superior resistance to nitric acid and radiation.

Development of Plutonium-Selective Resins

Although our efforts had significantly improved the plutonium recovery process, we felt we could do better. However, this time we took a more creative approach. We tried to envision the structure of an anion exchange resin that might be an ideal match for the plutonium nitrate complex. What resin structure, we asked, might provide a "glove" into which the plutonium "hand" would best fit?

Since our successful collaboration with Reilly Industries, we have used many powerful structural analysis techniques, including spectrophotometry, nuclear magnetic resonance, and extended x-ray absorption fine structure analysis to identify which plutonium complexes are involved in the anion exchange process. Our investigations revealed that the uncharged tetranitrato complex is most likely to be sorbed on an anion exchange resin, even though this neutral complex acquires two more nitrate groups and a double-negative charge during the sorption process.

Once we had identified the plutonium complex, we attempted to design a bifunctional anion exchange resin structure that would provide two anion exchange sites separated by a fixed distance. We then contracted the research group of Prof. Richard Bartsch of Texas Tech University to synthesize a series of novel bifunctional resins that provide specified spacings between two anion exchange sites. Such resins were prepared with the second exchange site being an alkylam-monium, phosphonium, or pyridinium group.

Performance of Plutonium-Selective Resins

As these new resins were synthesized, we compared their performance by measuring the sorption of Pu(IV) from a range of nitric acid concentrations. We compared the distribution coefficients (Kd values) of Pu(IV) on Reillex HPQ resin and on three of our new bifunctional resins, each with a 5-carbon spacer between the two exchange groups. We measured Kd values for 30 minutes, 2 hours, and 6 hours to obtain information about the rate at which plutonium is removed from solution.

Our tests demonstrated that bifunctional anion exchange resins with the two exchange groups separated by five carbon atoms offer a much faster and more quantitative uptake of plutonium from intermediate concentrations of nitric acid. Moreover, the uptake of plutonium from dilute nitric acid was much lower, which should allow the purified plutonium to be recovered more completely in a smaller liquid volume. Thus, these new bifunctional resins offer the realistic prospect of significantly decreasing the quantity of plutonium in our waste streams while simultaneously decreasing the volume of secondary liquid waste.

Figure 3. Anion exchange polymer structures such as these can be used to remove Pu(IV) from waste streams.


We compared the removal of Pu(IV) from an acidic Hanford waste simulant on the Reillex HPQ and on one of the early versions of our new bifunctional resin. Although some of our later resins significantly outperform this one, we noted that the Kd value on our resin in 30 minutes is more than twice what Reillex HPQ achieves in 6 hours. We noted also that, although our new resin greatly enhances Kd values for plutonium, the Kd values of the other 13 elements included in our study change relatively little, which demonstrates how selectively the new bifunctional resin removes plutonium.

An important advantage of our bifunctional resins is that they can all be prepared as derivatives of the Reillex HP commercial resin. Because these new resins are modifications of an existing resin, they are much simpler to prepare in commercial quantities than a resin that requires complete synthesis. Moreover, the fact that the major starting material is an existing Reillex resin makes Reilly Industries the obvious choice to be our commercial partner. Reilly has expressed a strong interest in manufacturing these new resins under license to the Laboratory.

Los Alamos National Laboratory has submitted a patent application for this new class of anion exchange resins. Although we have tested them primarily for plutonium recovery applications, we anticipate that related resins can be designed to remove other targeted anions selectively from ground water and industrial waste streams.

The principal developers of this project are S. Fredric Marsh, NMT-6 associate, Gordon D. Jarvinen, NMT-6, and Richard A. Bartsch, Texas Tech University.


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