Mixing Commercially Available Extraction Resins for Minor Actinide and Lanthanide Separation

Technical Summary: Mixing Commercially Available Extraction Resins for Minor Actinide and Lanthanide Separation

Problem Statement
Nuclear forensics and the analysis of irradiated materials often require the separation of minor actinides (MAs), specifically Americium (Am) and Curium (Cm), from complex mixtures of lanthanides (Ln) and fission/activation products. While solvent extraction processes like Actinide Lanthanide SEParation (ALSEP) are well-established for bulk-scale reprocessing, they are often impractical for the small sample sizes typical of nuclear forensics (often <0.1 grams). Solvent extraction generates organic waste and requires complex regeneration, whereas resin-based chromatography offers a quicker, more cost-effective alternative for small-scale samples without organic waste. However, translating the ALSEP solvent extraction system—which relies on the tandem action of an acidic cation exchanger (HEH[EHP]) and a neutral diglycolamide (TODGA)—into a solid-phase resin system presents challenges. The authors sought to determine if commercially available resins containing these extractants (Eichrom's LN2 and DGA-n, respectively) could be mixed in a single column to replicate the ALSEP separation performance for rapid Am isolation.

Methodology
The study utilized commercially available Eichrom Technologies resins: DGA-n (containing TODGA) and LN2 (containing HEH[EHP]). The experimental approach involved comparing three configurations:

1. Single Resins: Columns containing only DGA-n or only LN2.
2. Stacked Resins: A column with DGA-n placed directly above an LN2 column.
3. Mixed Resins: DGA-n and LN2 resins physically mixed together within a single column.

The separation was tested using:

• Metal Tracers: Custom cocktails (CC-1 and CC-2) containing various transition metals, alkaline earth metals, and lanthanides (Ce, Eu, Nd, Pr, Sm, Tb, etc.) analyzed via ICP-OES.
• Radioactive Tracers: $^{241}$Am spikes and irradiated Highly Enriched Uranium (HEU) foils to simulate real-world fission and activation product matrices.
• Elution Protocol: Samples were loaded in 3 M HNO$_3$, rinsed, and eluted using sequential pH adjustments (pH 2 and pH 4) of a 0.015 M DTPA/0.2 M ammonium citrate solution.

Key variables investigated included the ratio of resins (1:10 LN2:DGA-n by mass), the necessity of the ammonium citrate buffer, and the dissolution state of the DTPA ligand.

Key Results

• Resin Configuration: Mixing the resins in a single column proved superior to using them alone or in a stacked series. The mixed column yielded over 80% recovery of Am in specific fractions with less than 3% overlap from lanthanides (specifically Ce and Nd).
• Separation Mechanism: The system behaved more like an Advanced TALSPEAK process than a traditional ALSEP solvent extraction. In the mixed resin column, the HEH[EHP] (on LN2) acted as the dominant extractant under the specific elution conditions, facilitating the separation of Am from Ln despite their similar trivalent charge and ionic radii (differing by only ~0.8 pm).
• Buffer Necessity: Contrary to some expectations for TALSPEAK-like systems, the ammonium citrate buffer was found to be unnecessary for the separation. Experiments conducted without citrate (Experiment 7) showed improved resolution, with a significant shift in elution profiles that further separated Am from Nd and Ce.
• Matrix Tolerance: The mixed resin column successfully separated Am from a complex matrix containing irradiated HEU, recovering ~55% of the Am in the initial pH 4 fraction while retaining nearly 100% of non-trivalent elements (Ag, Ba, Co, Ru) in the load/rinse steps.

Significance and Claims
The authors claim that this work demonstrates a viable, rapid technique for separating minor actinides from mixed activation and fission product samples on a radioanalytical scale. By utilizing commercially available resins in a mixed-modal configuration, the study establishes a method that:

1. Avoids the generation of organic solvent waste associated with traditional solvent extraction.
2. Provides a high degree of separation between Am and Ln (specifically Nd) despite their chemical similarities.
3. Simplifies the process by eliminating the need for complex ammonium citrate buffering in this specific resin configuration.

The paper concludes that this mixed-resin approach offers a practical, cost-effective route for the analysis of Am in nuclear forensics, bridging the gap between bulk-scale solvent extraction concepts and small-scale chromatographic analysis.