Extraction and Reductive Stripping of Pertechnetate from Spent Nuclear Fuel Waste Streams

Metallic technetium sequestration in nickel core/shell microstructure during Fe(OH)2 transformation with Ni doping

This study investigates the impacts of Ni doping on technetium-99 (Tc) sequestration in aqueous solutions through transformation of Fe(OH)₂(s) to iron spinel (magnetite) under alkaline conditions. Extensive solid characterization was performed for the mineral phases produced, as well as the Tc/Ni speciation and distribution within these phases. X-ray diffraction results show that iron spinel was the dominant mineral product without detectable Ni incorporation. The doped Ni ions mainly precipitated as fine Fe/Ni oxide/hydroxide particles, including strongly reduced nanometer-sized spheroidal Ni-rich and metallic Ni phases. High-resolution analytical scanning transmission electron microscopy using energy dispersive X-ray spectroscopy and electron energy loss spectroscopy on the produced solid samples (focused ion beam-prepared specimens) revealed three Tc distribution domains dominated by nanocrystals and, especially, a Tc-rich metallic phase. Instances of metallic Tc were specifically found in spheroidal, Ni-rich and metallic nanoparticles exhibiting a core/shell microstructure that suggests strong reduction and sequential precipitation of Ni-Tc-Ni. Mass balance analysis showed nearly 100% Tc removal from the 4.8 × 10^-4 M Tc solutions. The finding of the metallic Tc encapsulation indicates that Tc sequestration through Ni-doped Fe(OH)₂(s)-to-iron spinel transformation process likely provides an alternative treatment pathway for Tc removal and could be combined into further waste treatment approaches.

Syntheses and evaluation of new hydrophilic azacryptands used as masking agents of technetium in solvent extraction processes

The extraction of technetium, present in nitric acid medium as pertechnetate anion, is an issue in solvent extraction processes used to recover uranium and plutonium. In the present study, a complexing agent is added in the aqueous nitric acid solution to bind selectively the pertechnetate anion and prevent its extraction into the organic phase or to back extract it in the aqueous phase. Several azacryptands with the addition of hydrophilic groups were synthesized to improve the solubility of the previously studied cage molecule in nitric acid medium. Solvent extraction tests reveal that all the synthesized ligands have a similar complexation strength towards pertechnetate and are able to maintain this anion in the aqueous phase (0.5 M HNO3). These ligands are able to overcome the Hofmeister bias and selectively bind technetium in nitric acid solution. The azacryptand concentration can be increased by a factor of three in the liquid-liquid extraction conditions compared to our previous work. Coordination studies using microcalorimetry, Single Crystal X-Ray Diffraction (SC-XRD), infrared and Raman spectroscopies show the formation of an inclusion complex with hydrogen bonds stabilizing the oxo-anion within the cavity. This solubility improvement is promising for the introduction of this kind of macrocyclic azacryptands in a solvent extraction process.

Technetium immobilization by materials through sorption and redox-driven processes: A literature review

The objective of this review is to evaluate materials for use as a barrier or other deployed technology to treat technetium-99 (Tc) in the subsurface. To achieve this, Tc interactions with different materials are considered within the context of remediation strategies. Several naturally occurring materials are considered for Tc immobilization, including iron oxides and low solubility sulfide phases. Synthetic materials are also considered, and include tin-based materials, sorbents (resins, activated carbon, modified clays), layered double hydroxides, metal organic frameworks, cationic polymeric networks and aerogels. All of the materials were evaluated for their potential in-situ and ex-situ performance with respect to long-term Tc uptake and immobilization, environmental impacts and deployability. Other factors such as the technology maturity, cost and availability were also considered. Given the difficulty of evaluating materials under different experimental conditions (e.g., solution chemistry, redox conditions, solution to solid ratio, Tc concentration etc.), a subset of these materials will be selected, on the basis of this review, for subsequent standardized batch loading tests.

Perrhenate and pertechnetate complexation by an azacryptand in nitric acid medium

Azacryptand addition in nitric acid medium for the recognition of the pertechnetate anion for extraction studies.

Extraction of pertechnetates from HNO3 solutions into ionic liquids

The extraction of pertechnetate ions from aquous solutions containing various concentrations of nitric acid into hydrophobic ionic liquids (ILs) has been examined at 25, 50 and 70 °C. The results show that the distribution ratio of Tc (D_Tc) between both phases weakly depends on the temperature and HNO₃ concentration when IL’s with relatively short aliphatic chains are used. The D_Tc obtained for all examined ILs, except methyltrioctylammonium bis(trifluoromethylsulfonyl)imide and 1-butyl-3-methylimidasolium hexafluorophosphate, are lower than 1.5. In the case of methyltrioctylammonium bis(trifluoromethylsulfonyl)imide a decrease of Tc concentration in aqueous solutions facilitates pertechnetate extraction into the organic phase.