One of the more versatile of Eichrom’s analytical products is TEVA Resin. It has been applied on a routine basis to the analysis of technetium, the measurement of the tetravalent actinides, and the separation of americium from lanthanides. It is used alone or can be readily combined with other resins to perform more elaborate separations of multiple analytes.
The active component of the TEVA Resin is an aliphatic quaternary amine. (See Figure 1.) As such it has properties similar to those of typical strong base anion exchange resins. However, because the functional groups are in a liquid form, rather than fixed to a polymer backbone (as with IX resins) these groups have greater flexibility to coordinate around target anions. This means that the uptake of these ions is generally higher and often at lower acid concentrations.
This behavior is shown in Figures 2 and 3, which are the acid dependency curves of TEVA Resin for the actinides and technetium from nitric and hydrochloric acid. Tetravalent plutonium, neptunium and thorium show maximum uptake in the region of 2 M to 4 M nitric acid. In this acid concentration range, hexavalent uranium and trivalent americium are not well retained, and as such, the resin can readily separate the tetravalents from the other actinides. This ability has been widely exploited.
The decrease in k’ for the tetravalent actinides as nitric acid concentrations greater than 2-4M is due to competition from nitrate anions for complexation sites on the resin.
The differences between the uptake curves for nitric and hydrochloric acid can be exploited to separate certain actinides from each other. For example, all the tetravalent actinides can be loaded from 3M nitric acid. Valence adjustment may be required to assure that the actinides are tetravalent. Then, by switching to 6M HCl, Th(IV) can be selectively eluted while Pu(IV) and Np(IV) remain on the column.
The retention of Tc(VII), pertechnetate, is also shown in figures 2 and 3. The resin takes up this anion strongly in solutions of lower nitric or hydrochloric acid concentration. The use of TEVA Resin in the analysis of Tc has become an industry standard. TEVA Resin is available in a disc format which can tolerate flow rates of up to 200mL/min when used for Tc analysis.
Although not shown on Figures 2 and 3, neutral, and even basic solutions, show a strong uptake of technetium. Work performed by Darrin Mann’s laboratory at K-25 in Oak Ridge demonstrated that the TEVA Resin could be used to isolate Tc-99 from a variety of matrices including alkaline solutions and neutralized acids. (See reference MA193.)
Horwitz, et al. reported the data in Figures 2 and 3 from studies performed with experimental batches of TEVA Resin. Eichrom’s commercial product conforms to established specifications that ensure proper performance of Eichrom issued methods. Please refer to our product specificationsfor details.
Figure 4 shows the effect of uranium and thorium content in a sample on update of neptunium by TEVA Resin from 5M HNO3. While the effect of Th is significant and linear, uranium doesn’t have much negative impact on Np retention until the concentration reaches 0.05M. In a 15 mL load solution, for example, this would correspond to about 180 mg.
The matrix effects of various polyatomic anions on Np uptake by TEVA Resin from 2M HNO3 is shown in Figure 5. Oxalate shows the strong effect on neptunium uptake. Oxalato complexes form readily with the tetravalent actinides and these complexes are not extracted by the TEVA Resin. Oxalates are often used in stripping solutions of tetravalent actinides from TEVA Resin.
Phosphate is commonly found in a variety of sample matrices. It’s effect on all tetravalent actinides (Pu, Th, and Np) would be similar, but since Th is the least strongly retained by TEVA Resin, it is most readily affected by high levels of phosphate in a sample. The addition of aluminum to the load solution will reduce this effect. Phosphate will preferentially complex aluminum, leaving the tetravalent actinides free to form extractable nitrato complexes.
Another solution to the problem of anionic matrix interferences is shown in Figure 6. In the case of oxalate, increasing the nitric acid concentration of the load solution from 2M to 3.5M increased the uptake of Np by more than an order of magnitude.
Figure 7 shows the separation of Pu from Np using TEVA Resin. This is accomplished by the selective reduction of plutonium from tetravalent to Pu(III). In this valence state, it behaves like Am, which is not retained at all under any concentration of nitric or hydrochloric acid. In the example in figure 7, plutonium is reduced with hydroquinone, although other reagents such as ammonium iodide and ferrous sulfamate may also be used.
Another interesting use of the TEVA Resin is to separate americium from the rare earth elements. Figure 8 shows that the rare earths are eluted as a group in a load solution which comprises 1.0 M ammonium thiocyanate and 0.1 M formic acid. Americium is retained under these conditions and can be eluted later with hydrochloric acid. Although Figure 8 shows the elution of Am with 0.25M HCl, it has been later shown that 2M HCl is a more effective and reproducible eluent for stripping Am from TEVA Resin. Further examples of this application are shown in the bibliography.
TEVA Resin is manufactured in three particle sizes (20-50µ, 50-100µ, and 100-150µ) and is sold in bottles or ready to use in prepackaged columns (for gravity flow), cartridges (for vacuum assisted flow), or in a disc format for Tc-99 analysis of large volume water samples. Click here for part numbers and descriptions.
Source: Horwitz, E.P., Dietz, M.L., Chiarizia, R., Diamond, H., Maxwell III, S.L., and Nelson, M., “Separation and preconcentration of actinides by extraction chromatography using a supported liquid anion exchanger: Application to the characterization of high-level nuclear waste solutions,” Analytica Chimica Acta, 310 (1995) 63-78. (HP195)